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Abstract

Voluntary movement in animals is modulated by a number of subcortical systems. One of these resides in the basal nuclei and their associated projections and utilizes dopamine as a neurotransmitter. Apart from regulating movement, the dopaminergic axis is also involved in the control of goal-oriented behavior, cognition, and mood. Disorders of this system result in common human neurologic disorders such as Parkinson's and Huntington's diseases, as well contributing to a host of behavioral conditions, such as schizophrenia, attention deficit hyperactivity disorder, and addiction. Many individual mouse models of human dopaminergic dysfunction have been described in varying degrees of detail. However, when evaluating this region of the brain, the veterinary pathologist is confronted by a paucity of information summarizing the comparative aspects of the anatomy, physiology, and pathology of the central dopaminergic system. In this review, a systematic approach to anatomic phenotyping of the central dopaminergic system in the mouse is described and illustrated using tyrosine hydroxylase immunohistochemistry. Differences between murine neuroanatomy and comparable regions of the nonhuman primate brain are highlighted. Although the mouse is the focus of this review, conditions in domestic animals characterized by lesions within the basal nuclei and its projections are also briefly described. Murine behavioral and motor tests that accompany abnormalities of specific anatomic regions of the dopaminergic axis are summarized. Finally, we review mouse models of Parkinson's and Huntington's diseases, as well as those genetically altered mice that elucidate aspects of dopamine metabolism and receptor function.

Keywords

Basal nuclei brain dopamine immunohistochemistry mice phenotyping substantia nigra tyrosine hydroxylase

Appropriate and coordinated voluntary movement is essential to the well-being of all animals. Although these signals originate in the cerebral cortex, they are modulated by a number of subcortical systems. One of these is the dopaminergic (DA) axis residing in the ventral brainstem and striatum.³⁹ Dopamine is a highly conserved catecholaminergic neurotransmitter involved in the control of movement, goal-oriented behavior, cognition and mood.^76, ¹⁰¹ It is utilized predominantly in the brain, where it controls the function of the basal nuclei and their associated projections. In the peripheral nervous system, it is used as a neurotransmitter in a few peripheral neuronal groups, but primarily serves as the substrate for the synthesis of norepinephrine and epinephrine in sympathetic neurons and the adrenal medulla.^84, ^85, ¹⁰¹

Most of our knowledge of the role of the basal nuclei in movement has been gained from study of human neuropsychiatric disorders such as Parkinson's and Huntington's diseases.¹⁰² However, dopamine also plays a central role in behavior, particularly in the areas of attention and reward.¹⁰¹ At moderately elevated extracellular levels, dopamine promotes enhanced mood and energy levels, whereas, at higher levels, it is associated with hyperactivity and compulsive and erratic behaviors.³² Abnormal dopaminergic transmission contributes to symptoms of schizophrenia,^38, ^76, ⁹² attention deficit hyperactivity disorder,^55, ⁸⁹ Tourette's syndrome,⁴⁰ and manic-depressive syndromes.¹⁰⁵ Additionally, dopaminergic transmission mediates the reinforcing effect of many addictive substances.^29, ⁵⁶ Dopaminergic pathways controlling voluntary movement are most easily studied, as the relevant neuroanatomy is more clearly defined and the locomotor phenotype easily discernible. Consequently, dopamine-associated movement disorders in humans have been quite comprehensively replicated using mouse models that develop lesions in the relevant anatomical areas^64, ^66, ^68, ^87, ⁹⁰ and that develop gait abnormalities, with a variable behavioral phenotype. In contrast, mice that are genetically altered for genes encoding proteins involved in dopamine synthesis, metabolism, or neurotransmission result in global dopaminergic dysfunction rather than a specific neuroanatomic lesion. These models have minimal pathology, but usually display behavioral changes⁷³ that replicate some features of dopamine-influenced human behavioral syndromes. Although individual mouse models are described in varying degrees of detail, currently no texts exist that describe an overall approach to anatomic phenotyping of the dopaminergic system.

Using tyrosine hydroxylase immunohistochemistry, a systematic approach to anatomic phenotyping of the central dopaminergic system in the mouse is described and illustrated. Differences between murine neuroanatomy and comparable regions of the nonhuman primate brain are highlighted. Behavioral and motor tests that accompany abnormalities of specific anatomic regions of the murine dopaminergic axis are introduced. The mouse models that illustrate dysfunction of specific anatomic regions of the basal nuclei and their attendant pathways as well as those genetically altered mice that elucidate aspects of dopamine metabolism and receptor function are reviewed.

Materials and Methods

Methods described in this section were used to illustrate the functional anatomy of the basal nuclei and its projections. These methods can also be used as a guide to obtain the needed coronal and sagittal sections to screen the major components of the central dopaminergic system.

Animals and tissues

All tissues were obtained from animals used in Institutional Animal Care and Use Committee (IACUC)-approved protocols at the Yale School of Medicine. Brains were obtained from 3-month-old male C57BL/6 mice (Mus musculus) that were used as control animals in an unrelated study. Following euthanasia by placement in a chamber prefilled with 70% CO₂, mice were decapitated, the calvarium was removed, and the head placed in Fekete's acid ethanol for 48 hours. Next, brains were removed from the cranium and sectioned in coronal and sagittal planes as illustrated in Fig. 1. For nonhuman primate sections, the brain was obtained from a healthy 5-year-old male squirrel monkey (Saimiri sciureus) euthanized for unrelated reasons. The brain was provided by the investigator and had been fixed in 10% formalin for 7 days prior to sectioning. The brain was sectioned in the coronal plane at 0.3-cm intervals from the midfrontal gyrus to just caudal to the precentral gyrus. Following routine paraffin embedding, 5 µm hematoxylin and eosin-stained sections were obtained for initial evaluation.

Fig. 1.

Tyrosine hydroxylase immunohistochemistry of the mouse brain. Fig. 1A–H depicts coronal sections from a rostral to caudal direction. The level at which the coronal section was taken is indicated by a vertical line on the adjacent sagittal section. Fig. 1I is an enlarged, labeled version of the sagittal sections in Fig. 1a–h. Tyrosine hydroxylase immunostaining identifies dopamine-producing cells (e.g., A9 and A10) as well as dopaminergic fibers terminating on target neurons (e.g., striatum). Cell groups constituting the origin of the mesencephalic dopaminergic system (A8–A10) are located predominantly in the substantia nigra pars compacta (snc: A9) and ventral tegmentum (VTA: A10) in Fig. 1F–H. Cell groups A11–A14 constitute the diencephalic dopaminergic system (Fig. 1A–F). Neurons of the A9 group project rostrally to the dorsal caudate-putamen (Cpu: A-D) to constitute the dorsal mesostriatal system. Neurons of the A10 group project rostrally to the ventral caudate-putamen, accumbens nucleus, olfactory tubercle, and ventral pallidum to constitute the ventral mesostriatal system. The mesolimbocortical system also originates in the A10 group and projects to diverse cortical areas as well as the olfactory bulb, lateral septal nucleus, habenula, and locus coeruleus. Tyrosine hydroxylase immunohistochemistry; bar = 200 µm.

Sections needed to examine the central dopaminergic system

A list of coronal and sagittal sections needed to initially assess the dopaminergic system is given in Table 1. The sections identify the key functional components of this system and primarily focus on the dorsal and ventral mesostriatal systems. More detailed evaluation of specific regions may require serial sectioning of that segment of the system. The olfactory bulbs are often damaged when the brain is removed. If this region must be examined, fixing the entire head in Bouin's solution may be most appropriate. Bouin's solution will fix and decalcify simultaneously (approximately 7–10 days for an adult mouse) and can be used for sectioning where the skull is retained. Dopaminergic neurons in the retina and olfactory bulb can be examined in the same section. This can be obtained by cutting a coronal section through an intact decalcified head through the middle of both eyes. The olfactory bulbs can be visualized between the eyes.

Table 1.

Histologic sections needed to assess the dopaminergic system in mice.


Level	Structure

Eyes (coronal section across entire head)	Retinal amacrine cells, olfactory bulb
Bregma, 1.34 mm	Caudate-putamen, accumbens nucleus, ventral pallidum, olfactory tubercle
Bregma, −1.34 mm	Caudate-putamen, amygdala, lateral and medial globus pallidus, nigrostriatal bundle, thalamus
Bregma, −2.18 mm	Subthalamic nucleus
Bregma, −3.08 mm and −3.80 mm	Ventral tegmentum, substantia nigra
Lateral 1.20 mm	Caudate-putamen, accumbens nucleus, ventral pallidum, olfactory tubercle, substantia nigra, nigrostriatal bundle

Immunohistochemistry

Tyrosine hydroxylase-stained tissues are best suited to general orientation and identification of dopaminergic anatomy, as well as cell counting and morphometry. As tyrosine hydroxylase is the first enzyme in catecholamine synthesis, immunohistochemical detection identifies all catecholaminergic neurons, i.e., those producing dopamine, norepinephrine, and epinephrine. Catecholaminergic neurons are found in the brainstem (from medulla oblongata to the diencephalon) and in the olfactory bulb and retina. The nomenclature used to describe the distribution of catecholaminergic neurons originated with Dahlstrom and Fuxe,¹⁶ who assigned numbers to catecholaminergic cell groups in rat brain beginning with A1 in the lower medulla oblongata and extending rostrally. The first seven groups are adrenergic and noradrenergic; further identification of these can be done using dopamine beta-hydroxylase as an adrenergic marker.¹⁰¹ The remaining groups constitute dopaminergic neurons. The initial classification system concluded with A12 in the basal hypothalamus.¹⁶ This nomenclature has subsequently been expanded in a rostral direction with groups A13–A15 in the hypothalamus^10, ³¹ group, A16 in the glomerular layer of the olfactory bulb, and last, dopaminergic amacrine cells within the retina.^9, ⁴² Lesions identified at this stage may be examined further using immunohistochemistry and histochemical stains for specific neurotransmitters in affected regions. These tests often require frozen sections. Neurochemical mapping of the dopaminergic system is comprehensively reviewed in Gerfen.³³

In this study, unstained 10-µm sections of appropriate regions were used for immunohistochemistry. Antigen retrieval was performed by boiling sections in a microwave oven for 10 minutes in 10 mM sodium citrate. Sections were allowed to cool for 20 minutes, prior to a 5-minute endogenous peroxidase block in 5% H₂O₂. Immunohistochemistry was performed using a rabbit anti-human polyclonal antibody against tyrosine hydroxylase at a concentration of 1:400. A biotinylated goat anti-rabbit secondary antibody combined with horseradish peroxidase reagent was then applied (DAKO-USA, Carpinteria, CA), followed by development with diaminobenzidine. Negative controls were incubated with normal rabbit serum instead of the primary antibody. Using this protocol, we have been able to obtain comparable results in tissues fixed with Bouin's fixative, Fekete's acid ethanol, or 10% formalin.

Image processing

Slides were examined with a Zeiss Axioplan light microscope and digital images were collected at each location illustrated in Figs. 1 and 2 using an Axiocam digital imaging camera and Zeiss Axiovision software (Carl Zeiss MicroImaging, Inc., Thornwood, NY). For nonhuman primate sections, multiple images were collected and montages assembled in Adobe Photoshop 7.0 (Adobe, San Jose, CA).

Fig. 2.

Comparative nigro-striatal neuroanatomy of mouse (Mus musculus) and squirrel monkey (Saimiri sciureus). Differences in neuroanatomical distribution of various elements of the nigrostriatal system is illustrated in Fig. 2, however some reference to structures in Fig. 1 is made in this legend. The caudate nucleus (Cau) and putamen (Pu) are clearly separated by the internal capsule (ic) in primates; in the mouse they are combined. In this species, the internal capsule is never as prominent and appears at a relatively more caudal location, at which point the lateral globus pallidus (lgp) can be visualized (Fig. 2C, D). The globus pallidus in primates is divided into lateral and medial portions (mgp and lgp: D). In rodents, the medial globus pallidus is sometimes referred to as the entopeduncular nucleus. In this species, the medial globus pallidus is smaller and can be visualized within the internal capsule (mgp: Fig. 1C). In primates, the substantia nigra and subthalamic nucleus can be easily visualized in the same section (Fig. 2F). In the mouse, the subthalamic nucleus occupies a slightly more rostral position than the substantia nigra (Fig. 1E and Fig. 1F–H, respectively). Tyrosine hydroxylase immunohistochemistry; bar = 150 µm (A, C, E); 500 µm (B, D, F). Abbreviations: A8-A14, dopaminergic cell groups; acu, accumbens nucleus; amy, amygdala; cc, corpus callosum; Cpu, caudate-putamen; Cau, caudate nucleus; DA, dopaminergic; ec, external capsule; f, fornix; gp, globus pallidus; hyp, hypothalamus; hab, habenular nucleus; ic, internal capsule; iCa, islets of Calleja; lgp, lateral globus pallidus; mfb, medial forebrain bundle; mgp, medial globus pallidus; nsb, nigrostriatal bundle; op, optic tract; ot, olfactory tubercle; po, pontine nuclei; Pu, putamen; nr, red nucleus; sth, subthalamic nucleus; snc, substantia nigra pars compacta; snl, substantia nigra pars lateralis; snr, substantia nigra pars reticulata; vp, ventral pallidum; tha, thalamus; VTA, ventral tegmentum; ∗, hippocampus.

Neuroanatomic reference materials

Nomenclature of murine structures demonstrated in coronal and sagittal sections is based on the system described by Paxinos and Franklin.⁸² Superimposition of dopaminergic components on these sections relies heavily on descriptions of tyrosine hydroxylase immunohistochemistry in the rat.^9, ⁴⁵ Nomenclature of non-human primate sections is based upon the atlas of the squirrel monkey brain by Gergen and MacLean.³⁴ Nomenclature used to describe location of structures will be that used in quadripeds rather than in humans, i.e., rostral-caudal (instead of anterior-posterior), and dorsal-ventral (instead of superior-inferior).

Central Dopaminergic System: Overview of Physical Structures

The basal nuclei constitute key brain structures, the primary function of which is to receive, integrate, and project motor and sensory information. They receive input from two primary sources: the motor cortex (glutaminergic input) and the mesencephalic dopaminergic system (dopaminergic input). Neurons from the basal nuclei project to the thalamus, which in turn project to the motor cortex to modulate voluntary movement.^3, ³⁹ In this section, physical structures comprising the dopaminergic system will be discussed under two main headings: those structures comprising the basal nuclei and related nuclei and the brainstem cell groups, which constitute the origin of the major dopaminergic projections to the basal nuclei. Understanding the anatomy of these circuits is important to accurately attach morphologic to behavioral phenotype in individual mouse models.

Dopaminergic neurons in the brainstem constitute the origin for the extensive mesotelencephalic projection system. These circuits define systems that primarily control motor activity (dorsal mesostriatal system) or primarily influence behavior (ventral mesostriatal system).^9, ⁴⁵ Mouse models of human locomotory disorders typically develop histologic lesions of neuronal degeneration and loss in the substantia nigra and caudate-putamen.^64, ^66, ^68, ^87, ⁹⁰ In contrast, mice that model the dopaminergic constituents of motivated behaviors rarely demonstrate visible pathology. In these models, investigators are more reliant on neurochemistry, immunohistochemistry, and behavioral phenotyping for their characterization.²³

1) The basal nuclei and major associated nuclei

The term basal nuclei refers to a conglomerate of forebrain structures arising from the basal telencephalon.^33, ³⁹ These include the caudate nucleus, putamen, nucleus accumbens, globus pallidus, ventral pallidum, and olfactory tubercle. Major associated nuclei that are functionally integral to striatal circuitry include the subthalamic nucleus, ventral tegmentum, and substantia nigra. The substantia nigra is the origin of the striatal system—the subthalamic nucleus is a relay nucleus, and the ventral tegmentum projects to many regions, including the striatum.^3, ^15, ³⁹

The striatum

The caudate and putamen nuclei are, together, referred to as the striatum and constitute the input side of the basal nuclei.^3, ^4, ¹⁵ Their major inputs are from the cerebral cortex, and they project mainly to the globus pallidus. In the mouse, the striatum (caudate nucleus and putamen) is a large structure occupying the region between the cortex and the lateral ventricles (CPu: Fig. 1A–C).⁸² Comparable sections of this region in the mouse and primate present a slightly different anatomic arrangement (Fig. 2A–D). Whereas the caudate nucleus and putamen are clearly separated by the internal capsule in primates,³⁴ they are combined in the mouse.⁸² In the mouse, the internal capsule is never as prominent and appears at a relatively more caudal location, at which point the globus pallidus can be visualized (Fig. 2C,D). Although the striatum lacks clear cytoarchitectural subdivisions (Fig. 3A), its constituent neuronal subpopulations are organized into distinct functional compartments.³³ Medium spiny neurons constitute the vast majority (90–95%) of neuronal types within the striatum.¹⁰⁹ They receive dense dopaminergic innervation from the ventral tegmentum and substantia nigra (Fig. 3A) and utilize gamma amino butyric acid (GABA) as their neurotransmitter.⁵¹ About 2% of the striatal neurons are cholinergic—these are interneurons, which influence dopamine release.¹¹² Acetylcholinesterase histochemistry applied to the mouse brain clearly delineates the striatum.⁸²

Fig. 3.

Comparative histology of the striatum and substantia nigra in the mouse. Striatal neurons (Fig. 3A) are innervated by densely dopaminergic fibers originating from the mesencephalic cell groups, predominantly A9 and A10. The majority of neurons constituting the striatum consist of medium spiny neurons that utilize GABA as a neurotransmitter—very few striatal neurons produce dopamine. Neurons within the substantia nigra pars compacta (Fig. 3B) constitute the major single dopamine-producing cell group (A9) —these neurons project rostrally to the striatum. Tyrosine hydroxylase immunohistochemistry; bar = 50 µm.

Dorsal and ventral pallidum

The globus pallidus (or dorsal pallidum) is an output region of the basal nuclei: it receives input from the caudate and putamen, participates in other circuits involving components of the basal nuclei, and projects to the thalamus, which projects in turn to the motor and premotor cortex.^3, ^15, ⁴⁰ Through this route, the basal nuclei exert some control over movement by acting through descending pathways originating in the cerebral cortex. Like the striatum, neurons of the globus pallidus are also GABAminergic.³³ The globus pallidus in primates is divided into lateral and medial portions (Fig. 2D).^3, ^15, ³³ In coronal sections of the mouse brain, the lateral globus pallidus (lgp: Fig. 1C) lies ventromedial to the striatum, occupying the region between the striatum and the internal capsule.⁸¹ In rodents, the medial globus pallidus is sometimes referred to as the endopeduncular nucleus and can be visualized within the internal capsule at Bregma −1.06 to −1.58 (mgp: Fig. 1C).⁸¹ The ventral pallidum (vp: Figs. 1A,B and 2D) receives input predominantly from the accumbens nucleus and projects to a diverse array of midbrain and pontine structures.^9, ⁴⁰

The accumbens nucleus

This nucleus lies at the junction of the caudate and putamen at their anteroventral limit.^34, ³⁹ In the mouse, it lies directly ventral to the striatum (acu: Fig. 1A).⁸² The nucleus accumbens and the ventromedial parts of the caudate and putamen are sometimes referred to as the ventral striatum, the part of the basal nuclei that receives input from the limbic system.^3, ^9, ⁴⁵ This region is considered to have a cognitive function rather than a strictly motor function.³

The amygdala nuclear complex

An important component of the limbic system, amygdalar neurons (amy: Figs. 1B-D and 2C,D) project to the basal nuclei in addition to the hypothalamus and other components of the limbic system.³⁹ This dual role of this multinuclear complex is reflected by its location between the basal nuclei and hippocampus. In the mouse, it is located directly ventral to the striatum.⁸²

The substantia nigra and subthalamic nuclei

The substantia nigra constitutes a prominent aggregate of dopaminergic neurons occupying the ventral aspect of the mesencephalon.^9, ^39, ⁴⁵ It consists of two parts: the pars compacta (snc: Fig. 1F,G), which contains the majority of dopaminergic cells projecting to the dorsal striatum (Fig. 3B), and the pars reticulata (snr: Fig. 1F–H), which receives input from the subthalamic nucleus (sth: Figs. 1E and 2F) and projects primarily to the thalamus.^3, ¹⁵ In primates, the substantia nigra and subthalamic nucleus can be easily visualized in the same section (Figure 2F).³⁴ In the mouse, the subthalamic nucleus occupies a slightly more rostral position than the substantia nigra (Fig. 1E).⁸² A prominent tract known as the nigrostriatal bundle (nsb: Fig. 1C–E) carries afferent dopaminergic fibers from the substantia nigra to the striatum.^9, ⁴⁵ The substantia nigra is an important part of the mesencephalic dopaminergic system and constitutes the origin for the extensive afferent dopaminergic projection system, the mesotelencephalic projection system.^9, ⁴⁵ This system is discussed in more detail below.

2) Afferent dopaminergic cell systems in the brainstem

The striatal complex receives afferents from two main regions—the mesencephalic dopaminergic system and various regions of the cortex.^9, ⁴⁵ The mesencephalic DA system (groups A8, A9, and A10: Fig. 1F–H) contains the largest concentration of dopaminergic neurons and constitutes the origin for the extensive mesotelencephalic projection system. Components of this system are the most intensively studied, as they control voluntary motor activity (dorsal mesostriatal system) and motivated behaviors (ventral mesostriatal system). The diencephalic DA system (groups A11–A14) is smaller and gives rise to intradiencephalic projections as well as the descending diencephalospinal pathway.^9, ⁴⁵ It will not be discussed in any detail in this review.

i) The mesencephalic dopaminergic cell system

The mesencephalic dopaminergic neurons form an extensive neuronal system in the ventral midbrain.⁹ A distinction is made between nigral (A9) and nonnigral (A8 and A10) neurons. The A8–A10 cell groups give rise to the predominant dopaminergic system in the brain, the mesotelencephalic pathway. The nigral neurons (A9: Fig. 1F,G) are confined to the pars compacta of the substantia nigra and project to the striatum.¹⁶ The nonnigral dopaminergic neurons are located rostral, medial, and caudal to the substantia nigra. The dopaminergic neurons of the A8 group are located in the ventrolateral tegmentum caudal to the substantia nigra and also project to the striatum (A8: Fig. 1H). The medially located neurons (A10: Fig. 1F–H) occupy the ventral tegmental area (VTA: Fig. 1F,G).^9, ^16, ⁴⁵ In addition, several minor dopaminergic pathways have been described, originating in the A11–A14 cell groups (Fig. 1B–H) and constituting the diencephalic dopaminergic system.^9, ³¹

The mesotelencephalic projection system comprises two major subsystems:^9, ⁴⁵ the mesostriatal DA system, which includes projections to the entire striatum, and the mesolimbocortical DA system, which projects to limbic and cortical areas. The mesostriatal system is in turn divided into dorsal and ventral portions, the dorsal and ventral mesostriatal systems.^9, ^36, ^45, ^91, ¹⁰⁰ Neurons constituting the dorsal mesostriatal system (also known as the nigrostriatal system) originate in the A9 region of the substantia nigra (with lesser contributions by the A8 and A10 regions) and project to the dorsal striatum (caudate nucleus and putamen in primates, cats and dogs, or dorsal caudate-putamen in rodents), as well as the subthalamic nucleus. The dorsal striatum projects to the globus pallidus.⁹ The globus pallidus is divided into distinct anatomical and functional portions—the external (lateral) and internal (medial) globus pallidus.³⁹ This pathway modulates voluntary motor activity, and degeneration of distinct components occurs in Parkinson's and Huntington's diseases in humans.^66, ¹⁰⁴ The ventral mesostriatal system (also known as the mesolimbic system)^9, ^36, ⁴⁵ originates predominantly in the A10 group in the VTA and projects to the ventral striatum (ventral caudate-putamen, accumbens nucleus, and part of the olfactory tubercle) and the ventral pallidum (or substantia innominata). This system is thought to influence motivated behaviors, particularly those related to reward.⁵⁴

The mesolimbocortical DA system originates predominantly in the A10 cell group, with lesser contributions by the A8 and A9 groups.^9, ⁴⁵ These neurons project to a diverse array of cortical areas associated with limbic function as well as the olfactory bulb, amygdala, lateral septal nucleus, lateral habenular nucleus, and locus coeruleus. It is involved in aspects of learning and memory.⁶⁰

The terminology describing these three functional networks (dorsal and ventral mesostriatal and mesolimbocortical systems) can be confusing as different (old and new) terms are used interchangeably by different authors. The more recent terminology is that listed above;⁹ however, current publications will refer to the dorsal mesostriatal system as the nigrostriatal system, the ventral mesostriatal system as the mesolimbic system, and the mesolimbocortical system as the mesocortical system.⁶⁷ Nevertheless, the rationale behind the classification of these networks is determined by where their neuronal origins lie, where they project to, and what functions they control. These are described below and summarized in Table 2.

Table 2.

Origins and projections of mesotelencephalic projection system.


System	Origin	Projection	Function

Mesostriatal system
Dorsal (or nigrostriatal system)	Predominantly A9 (snc)	Dorsal caudate-putamen (dorsal striatum)	Predominantly motor activities
Ventral (or mesolimbic system)	Predominantly A10 (VTA)	Accumbens nucleus, olfactory tubercle, ventral caudate-putamen (ventral striatum), ventral pallidum	Motivated behaviors
Mesolimbocortical system
Mesolimbocortical system (or mesocortical system)	Predominantly A10	Diverse cortical areas, olfactory bulb, lateral septal nucleus, habenula, amygdala, locus coeruleus	Memory and learning

Functional Circuitry of the Basal Nuclei

The striatal complex constitutes the interface between cortical and dopaminergic afferents, and efferent projections to associated nuclei, thalamus, and cortex. Information flow through the basal ganglia is separated into five distinct parallel circuits.^3, ^39, ⁷⁷ These are the motor loop, oculomotor loop (control of orientation and gaze), dorsolateral prefontal loop and lateral orbitofrontal loops (both related to cognitive processes), and limbic loop (emotional and visceral functions). The motor loop is involved in somatosensory and somatomotor control and is exclusively the subject of the text in this section. All five circuits are controlled by simultaneous and coordinated modulation of two parallel pathways, the direct and indirect pathways.^3, ^15, ^39, ⁷⁷ These function primarily through disinhibition, i.e., appropriate release of inhibitory tone.^3, ^39, ⁷⁷ The basic circuitry of the direct and indirect pathways constituting the motor loop is illustrated in Fig. 4 and described below.

Fig. 4.

Functional circuitry of the basal ganglia. Inhibitory, GABAminergic signals are indicated by gray lines and excitatory glutaminergic signals by black lines. The thickness of the line denotes the strength of the signal. Fig. 4A. Basal tone: Basal cortical motor tone is controlled by glutaminergic, excitatory projections from thalamic nuclei. Net cortical activity is the product of innate thalamic activity inhibited to varying degrees by the mgp (medial globus pallidus) and snr (substantia nigra pars reticulata). The mgp and snr are in turn modulated by proximal nuclei, whose components constitute the basic direct and indirect circuitry. The direct circuit is mediated by striatum—mgp—thalamus, whereas the indirect circuit includes an additional loop utilizing the lateral globus pallidus (lgp) and subthalamic nucleus. The subthalamic nucleus is excitatory and strongly influences thalamic activity via its projections to the mgp and snr. Fig. 4B. The direct pathway: The net outcome of predominance of the direct pathway is to increase motor tone via disinhibition of thalamic nuclei. The direct pathway begins as an excitatory, glutaminergic projection from the cerebral cortex to the striatum. Striatal neurons utilize GABA and inhibit their targets—they are further subdivided into two primary populations determined by whether they utilize substance P or enkephalin. Striatal neurons in the direct pathway use GABA and substance P and exert an inhibitory effect on the mgp. This results in reduced inhibition of the thalamus by the MGP (thalamic disinhibition), with consequent cortical excitation. Thus, the function of the direct pathway is to release the thalamus from its pallidal inhibition and increase cortical motor activity. Fig. 4C. The indirect pathway: To engage the indirect pathway, striatal neurons that utilize enkephalin and GABA are activated. These project to and inhibit the lgp. As the lgp inhibits activity in the subthalamic nucleus, inhibition of the lgp results in increased subthalamic nuclear activity (subthalamic disinhibition). Consequently, increased glutaminergic stimulation of subthalamic targets in the mgp and snr occurs—these nuclei in turn increase their inhibitory tone, thus reducing cortical stimulation via the thalamus. Therefore, the net effect of the indirect pathway is to decrease activity of the thalamus and, consequently, decrease activity of the cerebral cortex.

Basal motor tone

Basal cortical motor tone is controlled by glutaminergic, excitatory projections from thalamic nuclei (Fig. 4A). The extent of thalamic activity is increased or decreased by proximal components of the basal nuclei, predominantly the medial globus pallidus (mgp) and substantia nigra pars reticulata (snr).^3, ^15, ^39, ⁷⁷ These two nuclei utilize GABA and inhibit thalamic nuclei. Therefore, net cortical activity is the product of innate thalamic activity inhibited to varying degrees by the mgp and snr. The mgp and snr are in turn modulated by proximal nuclei, whose components constitute the basic direct and indirect circuitry. The direct circuit is mediated by striatum—mgp—thalamus, whereas the indirect circuit includes an additional loop utilizing the lateral globus pallidus (lgp) and subthalamic nucleus.³⁹ The subthalamic nucleus is excitatory and strongly influences thalamic activity via its projections to the mgp and snr.^15, ⁷⁷

The direct pathway

The net outcome of predominance of the direct pathway is to increase motor tone via disinhibition of thalamic nuclei.^3, ^15, ⁷⁷ The direct pathway (Fig. 4B) begins as an excitatory, glutaminergic projection from the cerebral cortex to the striatum. Striatal neurons utilize GABA and inhibit their targets—they are further subdivided into two primary populations determined by whether they utilize substance P (for the direct pathway) or enkephalin (for the indirect pathway). Striatal neurons in the direct pathway use substance P and exert an inhibitory effect on the mgp. This results in reduced inhibition of the thalamus by the mgp (thalamic disinhibition), with consequent cortical excitation. Thus, the function of the direct pathway is to release the thalamus from its pallidal inhibition and increase cortical motor activity.^15, ⁷⁷

The indirect pathway

The indirect pathway (Fig. 4C) also begins with excitatory, glutaminergic projections from the cerebral cortex to the striatum.^3, ^15, ⁷⁷ To engage the indirect pathway, striatal neurons that utilize enkephalin and GABA are activated. These project to and inhibit the lgp. As the lgp inhibits activity in the subthalamic nucleus, inhibition of the lgp results in increased subthalamic nuclear activity (subthalamic disinhibition).^39, ⁷⁷ Consequently, increased glutaminergic stimulation of subthalamic targets in the mgp and snr occurs—these nuclei in turn increase their inhibitory tone, thus reducing cortical stimulation via the thalamus. Therefore, the net effect of the indirect pathway is to decrease activity of the thalamus and consequently decrease activity of the cerebral cortex. Dopamine has opposite effects on the direct and indirect pathways.⁷⁷ It is excitatory to cells feeding into the direct pathway and inhibitory to those feeding into the indirect pathway. Consequently, dopamine promotes increased cortical tone and its depletion, as in Parkinson's disease, results in hypokinesis.^77, ⁷⁸

Imbalance of direct and indirect pathways

The motor deficits seen in human and animal basal nuclei syndromes result from neurochemical imbalance within the striatal complex.^3, ^15, ⁷⁷ Imbalance within direct and indirect pathways results in either increased (hyperkinetic) or reduced (hypokinetic) motor activity.^3, ⁷⁷ Although motor disturbances are most easily appreciated in these syndromes, they are accompanied by associative memory and limbic dysfunction as well.⁷⁷

Hyperkinetic disturbances take the form of dyskinesias.⁷⁷ They result from dysfunction of the indirect pathway with resulting dominance of the direct pathway due to reduced inhibition of thalamic neurons and increased cortical activity. Striatal neurons (medium spiny neurons) that project to the lgp are selectively lost in Huntington's disease.¹⁰⁴ This results in reduced activity of the indirect pathway, with relative dominance of the direct pathway and hyperkinetic motor disturbances. These can also be seen in Parkinsonian patients treated with L-DOPA.¹⁴

Hypokinetic disorders are characterized by akinesia (impaired initiation of movement) and bradykinesia (reduced velocity and amplitude of movement). Both are characteristic of Parkinson's disease.^77, ⁷⁸ Hypokinetic disorders originate in lesions resulting in loss of connection between the striatum and mgp, resulting in relative dominance of the indirect pathway. Parkinson's disease results from loss of dopamine-producing neurons in the substantia nigra pars compacta.⁶⁶ As dopamine is excitatory to cells feeding into the direct pathway, its loss results in relative dominance of the indirect pathway. Thus, tonically active mgp neurons continuously inhibit their thalamic targets, resulting in reduced motor cortical tone.⁷⁷

Hypokinetic syndromes resulting from dopaminergic dysfunction have been reported in the mouse and the horse. Hypophagia is a characteristic of the dopamine-deficient mouse.¹¹³ In the horse, nigropallidal encephalomalacia occurs following prolonged ingestion of yellow star thistle (Centaurea solstitialis) or Russian knapweed (Centaurea repens). Lesions are characterized by malacia of the globus pallidus and substantia nigra^30, ^59, ¹¹¹ and result in a hypokinetic syndrome. Affected horses are inactive and somnolent. A typical sign is an inability to take hold of food, while the capacity to swallow is retained.¹¹¹

Mouse Models of Dopaminergic Physiology and Dysfunction

Genetic alterations resulting in neuropathology of the dopaminergic system induce clinical abnormalities in mice that can be appreciated by simple observation. Easily noted changes include alterations in body weight, posture, gait, and spontaneous activities. Motor abnormalities can be quantitated using tests of motor function, such as the rotarod, balance-beam test, vertical pole test, and painted paw test.¹³ In contrast, those genetically altered mice in which components of dopaminergic neurotransmission (dopamine synthesis, transport, reuptake, and receptor binding) have been targeted exhibit more pronounced behavioral changes and less profound gait abnormalities.⁹⁵ These behaviors can be quantitated by assessing baseline open-field locomotion, rearing, and stereotypy.¹³ Some models^6, ^22, ^106, ^110, ¹¹³ also demonstrate the role of dopamine in the vegetative functions, such as feeding, drinking, and blood pressure. In genetic models characterized by neuropathology, bilaterally symmetrical lesions result in bilateral gait deficits. In contrast, unilateral loss of the substantia nigra and nigrostriatal bundle using toxic agents such as 6-hydroxydopamine results in circling toward the lesioned side.^75, ¹⁰⁰ Treatment with dopaminergic agonists, such as dopamine, apomorphine, and amphetamine, induce contralateral circling due to receptor denervation supersensitivity after a unilateral lesion.^26, ⁹⁹

I. Mouse models with neuropathology of the dopaminergic axis

The majority of these models were developed to investigate the two most common human disorders of the dopaminergic axis, namely Parkinson's and Huntington's diseases. In addition, spontaneous mutants (the weaver mouse)⁹⁴ and mice genetically altered for genes originally not suspected to result in striatal pathology have been described.^27, ⁷⁰ The specific neuropathology in these models is relatively subtle and ranges from loss of specific cell groups^27, ^70, ⁸⁷ neurodegeneration⁶⁶ and accumulation of intracytoplasmic or intranuclear inclusions.^18, ^44, ^66, ⁹⁰ These are best visualized using antibodies specific for the relevant protein, e.g., alpha-synuclein,⁶⁶ Huntington,^44, ⁹⁰ or ubiquitin.¹⁸ Electron microscopy may be required to illustrate dendritic lesions.⁵⁷

1) Parkinson's disease

Parkinson's disease is caused by loss of dopamine-containing neurons in the substantia nigra pars compacta that project to the striatum. Several lines of evidence suggest its mixed environmental and genetic origin.⁶⁶ It usually affects patients in midlife (over 50) and is characterized by tremor at rest, increased muscular tone, akinesia, bradykinesia, disturbances of eye movements (reduced blinking), and a flexed posture.^77, ⁷⁸ At autopsy, the substantia nigra pars compacta is devoid of dopaminergic neurons, accompanied by intracytoplasmic inclusions known as Lewy bodies and gliosis.⁷⁹ Lewy bodies are composed of aggregated proteins, notably alpha-synuclein and ubiquitin. Alpha synuclein misfolding appears to be a central feature of the variously triggered forms of Parkinson's disease.⁶⁶ In a small fraction of cases, dominantly inherited mutations in the human alpha-synuclein or parkin genes result in early-onset Parkinson's disease.⁵² Interaction between environmental neurotoxicants (herbicides, other pesticides, and industrial chemicals) and genetic susceptibility is likely to result in presynaptic damage to nigral cells, resulting in cell injury and abnormal protein folding.^12, ^69, ¹⁰³ Progressive injury induced by disposal of possibly toxic abnormally folded proteins is believed to evolve over years. In addition to familial forms caused by mutations in alpha-synuclein itself, mutations in two additional proteins involved in proteasomal degradation of misfolded proteins support the notion that accumulation of aberrantly folded proteins plays a role in the disease.⁶²

Mouse models

Both toxin-induced and genetic mouse models of Parkinsonism exist.⁷⁹ The latter group focus primarily on the role of alpha synuclein in the pathogenesis of the disease. Mouse models of Parkinson's disease typically display hypokinesia and bradykinesia, similar to their human counterparts.

a) Toxin-induced models

The toxin 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP) is the most common chemical agent used to mimic Parkinson's disease in the mouse⁸⁷ as well as in nonhuman primates,⁷⁹ sheep,⁸ and pigs.⁷² MPTP toxicity was first recognized in a group of patients who developed Parkinsonian symptoms at an early age (twenties) after taking heroin contaminated with this compound.⁵⁸ MPTP is metabolized via monoamine oxidase B to the active agent MPP+, which inhibits mitochondrial complex I. This is taken up into dopaminergic neurons via the dopamine transporter, leading to malfunction or loss of dopaminergic neurons, without, however, the Lewy body formation characteristic of Parkinson's disease.³⁵ MPTP given to mice selectively kills dopaminergic neurons, particularly in the substantia nigra and ventral tegmentum.³⁶ Strain-specific differences in susceptibility to MPTP exist, with C57/BL6 mice being more susceptible to toxin-induced neuronal death than other strains.^43, ⁹¹ These findings may be related to higher intrinsic dopamine levels in the striatum of C57/BL6 mice.^36, ^43, ⁹¹ MPTP administration in nonhuman primates results in tremors, rigidity, and akinesia characteristic of Parkinson's disease. Cell loss is limited to the substantia nigra pars compacta and, like MPTP-intoxicated mice, no Lewy bodies occur.⁷⁹ In mice, MPTP-induced hypokinesia is reflected by reduced locomotor activity in the open field test⁹¹ and reduced capacity to remain on a rotarod.⁸⁷ In both mice and nonhuman primates, motor deficits are relieved by administration of L-DOPA.^79, ⁸⁷

b) Transgenic models

Transgenic models have been used to overexpress wild-type or mutant human alpha-synuclein.⁶⁶ Lesions depend largely on the promoter used—promoters such as Thy1,⁴⁹ platelet-derived growth factor-beta,⁷¹ and the prion promoter,⁶¹ which drive widespread expression results in alpha-synuclein accumulation and neuronal damage (accompanied by motor deficits) in multiple brain regions. A more appropriate model limits wild-type or mutant human alpha-synuclein overexpression to the dopaminergic system by using the tyrosine hydroxylase promoter.⁸³ Mice expressing the mutated form develop age-related bradykinesia and hypokinesia, consistent with excessive or inappropriate function of the mutated protein and resulting in cellular toxicity. Although intraneuronal inclusions containing alpha-synuclein can be seen in some models,^49, ^61, ⁷¹ these lack the fibrillar morphology characteristic of Lewy bodies.

c) Alpha synuclein knockout mice

Data from alpha synuclein knockout mice¹ suggest that hereditary Parkinsonism resulting from alpha-synuclein mutations⁵² results from a toxic gain-of-function mechanism rather than simple loss of gene function. Alpha synuclein knockout mice¹ are viable and fertile, exhibit intact brain architecture, and possess a normal complement of dopaminergic cell bodies and projection systems. Consistent with the converse findings in alpha-synuclein overexpressing mice,⁸³ knockout mice display striking resistance to MPTP-induced degeneration of dopaminergic neurons.¹⁷

2) Huntington's disease

Huntington's disease (HD) is an autosomal-dominant, progressive neurodegenerative condition that is caused by expansion of CAG repeats near the 5′ end of the IT15 gene, which encodes a ubiquitously expressed protein of unknown function called huntington.^5, ⁸⁸ The normal huntington gene allele size is between 6 and 37 CAG repeats. Most Huntington's disease occurs in midlife due to polyglutamine stretches of approximately 50 residues; stretches >57–60 residues trigger symptoms in childhood.⁴¹ Huntington's disease displays selectivity for GABAminergic medium spiny neurons in the striatum—these are selectively vulnerable to a toxic property conferred by the polyglutamine segment.¹⁰⁷ The indirect pathway neurons, which express enkephalin and are enriched in D2 dopamine receptors, are affected earlier than the direct pathway neurons coexpressing substance P and D1 dopamine receptors.⁹³ Consequently, overactivity of the direct pathway with consequent hyperkinesis characterizes Huntington's disease. Striatal and cerebral cortical neurons contain characteristic neuronal nuclear inclusions (NII).^21, ^65, ⁹⁷ These are rarely present within other subcortical structures, such as the globus pallidus and thalamus, and are generally absent elsewhere in the brain.^21, ⁶⁵ NII are immunoreactive with antibodies against the N-terminus of huntington (but not the internal segment or C-terminus) and ubiquitin.⁹⁷ Similar aggregates are also present in the neuropil, within the dendrites, and have been termed neuropil aggregates or dystrophic neurites.²¹ The means by which mutant huntington causes neuropathology is not clear; however, it is thought to arise from the interaction of huntington with a large number of endogenous transcription factors, signaling molecules, and trafficking proteins.⁶³

In the early stages, Huntington's disease is characterized by absent-mindedness, irritability, depression, and clumsiness. Over time, choreiform movements develop and worsen until the patient is bedridden. Progressive decline in cognitive function and speech occur until severe dementia results in the terminal stages of the disease.⁴¹ The neuropathology of Huntington's disease can be classified into five grades.¹⁰⁴ Grade-0 patients display no gross or microscopic lesions despite Huntington's disease symptomatology.²⁰ In grades 1–4, shrinkage of the cerebral cortex, white matter, hippocampus, amygdala, and thalamus occurs early, is not progressive, and is caused by loss of both grey and white matter. Grades 1–3 are characterized by progressive atrophy of the caudate nucleus and, to a lesser extent, the putamen and globus pallidus. The most severe grade, 4, is usually found in younger patients and often displays extension of lesions to regions such as the cerebellum not usually affected in adults.⁹³

Mouse models

Like humans with Huntington's disease, genetically altered mice with striatal pathology display an array of dyskinesias, which are not as clearly defined as those in humans. Most mouse models of Huntington's disease^44, ^57, ^64, ^68, ^90, ^107, ¹⁰⁸ are smaller than their littermates and display progressively worsening foot clasping when suspended by the tail. Most are hypoactive, but in some cases,⁵⁷ this can coexist with hyperactivity. Spontaneous abnormalities include stereotypic involuntary movements or tremors,⁶⁸ circling, or slow, wide-based or ataxic gait.^44, ⁹⁰ Gait abnormalities are often quantitated using the painted paw test. Affected mice have a shorter uneven stride.^64, ^107, ¹⁰⁸ Invariably, these mice display a reduced capacity to remain on a rotarod.^64, ^68, ^90, ^107, ¹⁰⁹

A. Transgenic models expressing human huntington gene fragments

Initial transgenic models for Huntington's disease were generated by expressing either the full-length or 5′ fragment of the human huntington gene under direction of a variety of promoters, most of which resulted in widespread expression of the transgene. These models typically display an earlier onset, more severe phenotype than later targeted huntington knock-in models.

i) The R6 transgenic mouse lines

These models were generated by using exon 1 of the human huntington gene containing 115–156 CAG repeat units driven by huntington promoter sequences in the 5′UTR.⁶⁸ Clinically, the mice develop a progressive neurologic phenotype at 2–3 months of age. This includes a resting tremor, stereotypic involuntary movements, hind-limb clasping, choreiform movements (rapid abrupt shuddering of the trunk), mild dysmetria, handling-associated seizures, and abnormal vocalization. The brain of these mice is cytoarchitecturally normal but demonstrates uniform diminution due to reduced size of all central nervous system (CNS) structures. These mice develop pronounced neuronal intranuclear inclusions, containing the proteins huntington and ubiquitin, prior to developing a neurological phenotype.¹⁸

ii) N-terminal huntington driven by mouse prion promoter

This model uses the prion promoter to drive expression of the N-terminal fragment (171 amino acids) of huntington with 82 (juvenile onset HD), 44 (adult onset HD), or 18 (normal) glutamines in virtually every neuron of the CNS.⁹⁰ Mice expressing huntington with 82 glutamine repeats (N171–82Q) develop behavioral abnormalities, including loss of coordination, tremors, hypokinesis, hind-limb clasping, abnormal gait, and inability to stay on a rotarod, before dying prematurely. In mice exhibiting these abnormalities, diffuse nuclear labeling, intranuclear inclusions, and neuritic aggregates, all immunoreactive with an antibody to the N-terminus (amino acids 1–17) of huntington (AP194), were found in multiple populations of neurons.⁹⁰ Although slightly smaller, brains of these mice are grossly normal, with no sign of abnormal development.

iii) N-terminal huntington driven by rat neuron-specific enolase promoter

Broad CNS expression of the 5′ region of human huntington with 18, 46, and 100 CAG repeats was obtained using the rat neuron-specific enolase promoter.⁵⁷ Clinically, mice with 46 and 100 repeats display a spectrum of neurologic signs, including poor rotarod performance, hind-limb clasping, hyperactive stereotypic behavior, hypoactivity, wide-based gait, or slow gait with tremulousness. Neuropathology of these is characterized by cortical cytoplasmic huntington protein accumulation and dysmorphic dendrites. Although Huntington's disease is considered primarily a disease of striatal degeneration, evidence suggests that cortical changes are as important in creating the neurologic phenotype.

iv) Huntington's disease yeast artificial chromosome (YAC) transgenic

YACs containing human genomic DNA spanning the full-length gene, including all regulatory elements, were used ⁴⁴ to express huntington with either 46 (to replicate adult-onset HD) or 72 (juvenile onset) CAG repeats. Both normal and mutant human huntington are expressed in the same tissues as the endogenous mouse protein, with highest levels seen in brain and testes. The YAC72 mouse exhibits the most severe clinical signs, including circling, choreiform movements of the head and neck, as well as gait ataxia and foot clasping. In the striatum, neuronal intranuclear inclusion immunoreactive for N-terminal huntington is detected only in medium spiny neurons, accompanied by striatal degeneration.

B. Targeted models containing the endogenous mouse Hdh gene

i) Huntington knock-in mice

The murine wild-type Hdh gene contains 7 CAG N-terminal repeats. Expanded polyglutamine repeats of 50, 92, 111, and 150 repeats were inserted in the endogenous mouse Hdh gene, with consequent expression of mutant proteins in a distribution most similar to that seen in human patients.^64, ^107, ¹⁰⁸ Expanded repeats of lengths causing adult onset (50 repeats) do not rapidly provoke abnormalities, suggesting that the mouse's short lifespan precludes the accumulation of disease-causing pathology seen in humans.¹⁰⁸ Longer stretches of 92 and 111 CAG repeats induce cytoplasmic accumulation followed by nuclear translocation of mutant huntington protein to striatal neurons.¹⁰⁷ The larger the repeat, the earlier this occurs. No behavioral abnormalities were noted in mice with 92 and 111 repeats.¹⁰⁷ Double immunolabeling studies using calbindin D antibody to identify medium spiny neurons confirmed that nuclear relocation in the striatum targeted this cell population.¹⁰⁷ Insertion of 150 CAG repeats⁶⁴ resulted in earlier onset neuropathology accompanied by behavioral and gait abnormalities.

3) Additional mouse models with striatal pathology

Both spontaneous mutants (the weaver mouse)⁹⁴ and mice genetically altered for genes originally not suspected to result in striatal pathology have been described.^27, ⁷⁰

a) The weaver mouse

The weaver (wv) mouse is a spontaneous autosomal recessive mutant with a predominantly cerebellar phenotype caused by failure of granular cells to migrate normally and their subsequent death.⁹⁴ In addition, wv is also dopamine deficient in the forebrain. This deficiency is profound in the dorsal striatum, whereas the accumbens nucleus, ventral caudate-putamen, and medial olfactory bulb are much less severely affected.^67, ⁸⁶ Although no human homologue of the weaver mouse exists, the combined cerebellar-striatal phenotype is reminiscent of hereditary striatonigral and cerebello-olivary degeneration in the Kerry Blue Terrier.^19, ⁷⁴ Although cerebellar abiotrophy prevails in these dogs, they also demonstrate marked degeneration of the caudate nuclei and the substantia nigra.

b) Genetically altered mice

Mice in which the transcription factors Creb1 and Crem are disrupted in the postnatal forebrain show progressive neurodegeneration in the hippocampus and in the dorsolateral striatum.⁷⁰ The striatal phenotype is reminiscent of Huntington disease and is consistent with the postulated role of CREB-mediated signaling in polyglutamine-triggered diseases.^80, ¹¹⁵ Ablation of the murine ataxia-telangiectasia gene (Atm) results in adult-onset loss of dopaminergic neurons in the A9 and A10 areas of the mesencephalon.²⁷ These lesions are accompanied by locomotor abnormalities manifested as stride-length asymmetry, which could be corrected by peripheral application of the dopaminergic precursor L-dopa.²⁸

II. Mouse models of dopaminergic neurotransmission

Aspects of dopaminergic synthesis, secretion, and receptor activity have been explored by ablating genes controlling these functions. The mice resulting from these experiments have no human counterparts. In addition, their phenotype is locomotor, behavioral, or vegetative rather than neuropathologic. Consequently, they have been used to explore the role of dopaminergic neurotransmission in motivational^22, ¹¹³ or hyperactivity behaviors^2, ⁶ as well as gait.⁴⁸

1) Dopamine synthesis, metabolism, and transport

Dopamine is synthesized from the amino acid tyrosine^85, ¹⁰¹ within the neuronal cytoplasm. Tyrosine is transported into the neuron via membrane-bound amino acid carrier and is converted to l-dihydroxyphenyalanine (L-DOPA) by tyrosine hydroxylase (TH). This is the rate-limiting step in catecholamine synthesis. L-DOPA is converted to dopamine by l -amino acid decarboxylase.²³ Dopamine is transported into and concentrated within the synaptic vesicle by the vesicular monoamine transporter 2 (VMAT2).^85, ¹⁰¹ In noradrenergic and adrenergic neurons, dopamine within vesicles is converted to norepinephrine by dopamine-beta hydroxylase. Norepinephrine is further converted to epinephrine by phenylethanolamine-N-methyltransferase.¹⁰¹ To effect synaptic transmission, vesicles release dopamine through calcium-activated fusion with the presynaptic neuronal membrane. Dopamine in the synaptic cleft is removed by diffusion and reuptake into presynaptic terminals—reuptake is mediated by the dopamine transporter (DAT).^85, ⁹⁸ Cocaine, amphetamine, and many other psychostimulants are believed to produce their stimulating and addictive effects mainly by increasing the extracellular levels of dopamine in the brain reward areas through the interaction with DAT.^4, ²³ In addition, methyphenidate, used to treat attention deficit hyperactive disorder interacts with the DAT¹⁰⁰ (as well as related transporters for serotonin and norepinephrine).

Mouse models

a) Tyrosine hydroxylase deficient mice

Ablation of TH^53, ¹¹⁴ eliminates synthesis of both dopamine and norepinephrine. The majority of TH−/− embryos die between embryonic days 11.5 and 15.5 and display cardiac dilation and bradycardia. Those that are born fail to breathe. Administration of L-DOPA to pregnant females results in survival and birth of TH−/− pups,¹¹⁴ indicating that catecholamines are essential for mouse fetal development. To isolate the role of dopamine alone, Zhou and Palmiter¹¹³ restored norepinephrine synthesis in sympathetic neurons by crossing TH−/− mice with transgenic mice expressing TH under control of the dopamine-beta hydroxylase promoter. Dopamine-deficient pups were born at the normal Mendelian frequency but became hypoactive and aphagic a few weeks after birth, indicating that dopamine is essential for movement and feeding but that the neural networks that utilize it can develop in its absence. The postnatal syndrome in dopamine-deficient mice can be alleviated by administration of L-DOPA.^96, ¹¹³ An alternate pathway for catecholamine synthesis was identified by Rios et al.,⁸⁴ who detected catecholamines in sympathetic nuclei, adrenal medulla, and brain of pigmented, but not albino, TH−/− mice. They concluded that tyrosinase in melanocytes provides an alternate pathway for the synthesis of L-DOPA and catecholamines.

b) Vesicular monoamine transporter 2 knockout mice

Vesicular monoamine transporter 2 (VMAT2) is the predominant dopamine transporter expressed in the brain.^98, ¹⁰¹ VMAT2−/− mice¹⁰⁶ die within a few days after birth, manifesting severely impaired dopamine storage and vesicular release. Heterozygous VMAT2 knockouts are viable into adult life but display VMAT2 levels one half that of wild-type values, accompanied by smaller changes in monoaminergic markers, heart rate, and blood pressure.¹⁰⁶

c) Dopamine transporter knockout mice

Ablation of the DAT results in increased levels of dopamine into extracellular space and enhanced dopaminergic neurotransmission. Deletion of the DAT⁶ results in increased dopaminergic tone, rostral pituitary hypoplasia, dwarfism, and an inability to lactate, thus revealing an unexpected and important role for dopamine in the control of developmental events in the pituitary gland. Behaviorally, DAT knockout mice demonstrate marked hyperactivity characterized by increased locomotion, rearing, and stereotypic behaviors. These behaviors are completely reversed by dopamine antagonists.³²

2) Dopamine receptors

The central actions of dopamine are mediated by 5 distinct dopamine-receptor classes that are expressed on dopamine synthesizing neurons and exert their effects via activation of heterotrimeric guanine nucleotide regulatory proteins.^37, ⁷³ The D1 dopamine receptor is the most widely distributed central dopamine receptor. It is present in the cortex and rostral olfactory nuclei, accumbens nuclei, and amygdaloid nuclei.¹⁰¹ Both D1 and D2 dopamine receptor types are expressed by medium spiny neurons, which constitute over 90% of the striatal population.³³ Medium spiny neurons can be divided into two primary populations determined by whether they utilize substance P or enkephalin. These two populations also possess distinct dopamine-receptor types. Striatal neurons projecting directly to the medial globus pallidus express the D1 dopamine receptor and the neuropeptides substance P and dynorphin. Striatal neurons giving rise to the indirect pathway (projecting to the medial globus pallidus via the lateral globus pallidus and subthalamic nucleus) express the D2 dopamine receptor and enkephalin.^22, ^37, ¹⁰¹ Ablation of D1 or D2 receptors results in phenotypes that reflect an imbalance between the direct and indirect pathways.^7, ²²

Mouse models

a) D1A dopamine receptor knockout mouse

D1A−/− mutants are small and exhibit postnatal growth retardation.^22, ¹¹⁰ These findings are consistent with the role of dopamine in motivational and reward behaviors mediated by the ventral striatum. Neurologically, D1A−/− mice exhibit normal coordination and locomotion, but display conflicting results for tests of exploratory activity in novel environments.^22, ¹¹⁰ Decreased rearing in an open field was identified by Drago et al.,²² suggesting reduced exploratory phenotype, while increased activity based on interruption rate of a photocell beam was reported by Xu et al.¹¹⁰ Results such as these highlight the potential of different test protocols to affect results of behavioral testing.⁵⁰ Using a double transgenic Cre-lox strategy, Drago et al.²⁴ expressed diphtheria toxin gene in D1A-positive cells. This resulted in death of all D1A-positive neurons and a much more severe motor phenotype. Double mutant mice display bradykinesia (a Parkinsonian phenotype), dystonia, and myoclonia (more consistent with signs seen in Huntington's disease). In addition, these mutants, like their predecessors,¹¹⁰ are hypophagic and usually die in the first 2 postnatal weeks.

b) Dopamine D2 receptor knockout

Mice lacking D2 dopamine receptors are smaller, with reduced postnatal growth and reduced fertility.^7, ⁴⁸ Motor signs appear between PN 30 and 45, consistent with maturation of the D2 receptor system.⁴⁸ They exhibit postural abnormalities, including a hunched posture, paw flattening, and sprawling of hind limbs, that are more pronounced when voluntary movement is initiated. Absence of D2 receptors results in animals that are akinetic and bradykinetic in behavioral tests and which show significantly reduced spontaneous movements. This phenotype presents analogies with symptoms characteristic of Parkinson's disease.^7, ²³

c) Dopamine D3 receptor knockout

Homozygous mice lacking D3 receptors display increased locomotor activity and rearing behavior.² Those heterozygous for the D3 receptor mutation show similar, albeit less pronounced, behavioral alterations, suggesting that D3 receptors play an inhibitory role in the control of certain behaviors.²

d) Double D1/D3 receptor knockout

Combined deletion of both receptors abolishes the exploratory hyperactivity of D3 dopamine-receptor mutant mice and further attenuates the low exploratory phenotype of some D1 receptor knockout mice,^22, ⁵⁰ suggesting that dopamine D1 and D3 receptors may interact in an opposing or synergistic fashion.

e) Double D2/D3 receptor knockout

D2/D3 double mutants develop motor phenotypes that, although qualitatively similar to those seen in D2 single mutants, are significantly more severe and persist into adulthood.⁴⁸ These results suggest that D3 receptors compensate for some of the lacking D2 receptor functions and that these functional properties of D3 receptors, detected in mice with a D2 mutant genetic background, remain masked when the abundant D2 receptor is expressed.⁴⁸

f) Dopamine D4 receptor knockout

The human dopamine D4 receptor (D4R) has received considerable attention because of its polymorphic nature and possible role in the phenomenon of novelty seeking.⁸¹ While knockout mouse data support the role of the D4 dopamine receptor in modulating responses to novelty, a recent analysis of the studies linking D4 dopamine receptor variants to novelty seeking, alcoholism, drug abuse, and attention deficit hyperactivity disorder determined that many of these studies had methodologic concerns.⁸¹ D4 dopamine receptor knockout mice display reduced novelty-induced activity.²⁵

g) Dopamine D5 receptor knockout mice

D5 dopamine receptor-deficient mice are viable and fertile but develop hypertension and exhibit significantly elevated blood pressure (BP) by 3 months of age.⁴⁶ Behavioral changes are minimal.⁴⁷

Summary

The intent of this review is to provide an overview of the anatomy and physiology of the dopaminergic system as well as the mouse mutants that describe major categories of dopaminergic function and dysfunction. The requirements of each study (mouse model used, extent of pathologic examination, or choices of behavioral testing paradigms) are heavily dependent on the research question. Human movement disorders, such as Huntington's and Parkinson's diseases, have been quite comprehensively replicated using mouse models that develop lesions in the relevant anatomical areas. The specific neuropathology in these models is relatively subtle and ranges from loss of specific cell groups, neurodegeneration, and accumulation of intracytoplasmic or intranuclear inclusions. Genetically altered mice with striatal pathology display an array of dyskinesias, which are not as clearly defined as those in humans. Easily noted changes include alterations in body weight, posture, gait, and spontaneous activities. Motor abnormalities can be quantitated using tests of motor function, such as the rotarod, balance-beam test, vertical pole test, and painted paw test. Aspects of dopaminergic synthesis, secretion, and receptor activity have been explored by ablating genes controlling these functions. The mice resulting from these experiments have no human counterparts. In addition, their phenotype is locomotor, behavioral, or vegetative rather than neuropathologic. Consequently, they have been used to explore the role of dopaminergic neurotransmission in motivational or hyperactivity behaviors as well as gait.

Footnotes

Acknowledgements

The author gratefully acknowledges Dr Alexander deLahunta for his review of the anatomical portion of this manuscript.

References

1 Abeliovich

Schmitz

Farinas

Choi-Lundberg

Castillo

Shinsky, Verdugo

Armanini

Ryan

Hynes

Phillips

Sulzer

Rosenthal

: Mice lacking alpha-synuclein display functional deficits in the nigrostriatal dopa-mine system. Neuron 25(1): 239–252, 2000

2 Accili

Fishburn

Drago

Steiner

Lachowicz

Park

Gauda

Lee

Cool

Sibley

Gerfen

Westphal

Fuchs

: A targeted mutation of the D3 dopamine receptor gene is associated with hyperactivity in mice. Proc Natl Acad Sci USA 93(5): 1945–1949, 1996

3 Albin

Young

Penney

: The functional anatomy of disorders of the basal nuclei. Trends Neurosci 18(2): 63–64, 1995

4 Amara

Kuhar

: Neurotransmitter transporters: recent progress. Annu Rev Neurosci 16:73–93, 1993

5 Ambrose

Duyao

Barnes

Bates

Lin

Srinidhi

Baxendale

Hummerich

Lehrach

Altherr

: Structure and expression of the Huntington's disease gene: evidence against simple inactivation due to an expanded CAG repeat. Somat Cell Mol Genet 20(1): 27–38, 1994

6 Bosse

Fumagalli

Jaber

Giros

Gainet-dinov

Wetsel

Missale

Caron

: Rostral pituitary hypoplasia and dwarfism in mice lacking the dopamine transporter. Neuron 19(1): 127–138, 1997

7 Baik

Picetti

Saiardi

Thiriet

Dierich

Depaulis

Le Meur

Borrelli

: Parkinsonian-like locomotor impairment in mice lacking dopamine D2 receptors. Nature 377(6548): 424–428, 1995

8 Beale

Higgins

Work

Bailey

Smith

Shinka

Hammock

: MPTP-induced Parkinson-like disease in sheep: clinical and pathologic findings. J Environ Pathol Toxicol Oncol 9(5-6): 417–428, 1989

9 Bjorkland

Lindevall

: Dopamine-containing systems in the CNS. In: Handbook of Chemical Neuroanatomy, vol. 2: Classical Transmitters in the CNS, Part 1, ed. Bjorkland

Hokfelt

, pp. 55–108. Elsevier, New York, NY, 1984

10.

10 Bjorklund

Nobin

: Fluorescence histochemical and microspectrofluorometric mapping of dopamine and norepinephrinee cell groups in the rat diencephalon. Brain Res 51:193–205, 1973

11.

11 Bronson

: Pathologic Characterization of neurological mutants. In: Pathology of Genetically Engineered Mice, ed. Ward

Mahler

Maronpot

Sundberg

, pp. 239–252. Iowa State University Press, Ames, IA, 2000

12.

12 Brooks

Chadwick

Gelbard

Cory-Slechta

Federoff

: Paraquat elicited neuro-behavioral syndrome caused by dopaminergic neuron loss. Brain Rev 823(1-2): 1–10, 1999

13.

13 Crawley

: Motor functions. In: What's Wrong with My Mouse? Behavioral Phenotyping for Transgenic and Knockout Mice, ed. Crawley

, pp. 47–63. Wiley-Liss, New York, NY, 2000

14.

14 Crossman

: A hypothesis on the pathophysiological mechanisms that underlie levodopa- or dopamine agonist-induced dyskinesia in Parkinson's disease: implications for future strategies in treatment. Mov Disord 5(2): 100–108, 1990

15.

15 Crossman

: Functional anatomy of movement disorders. J Anat 196(Pt 4): 519–525, 2000

16.

16 Dahlstrom

Fuxe

: Localization of monoamines in the lower brain stem. Experientia 20(7): 398–399, 1964

17.

17 Dauer

Kholodilov

Vila

Trillat

Goodchild

Larsen

Staal

Tieu

Schmitz

Yuan

Rocha

Jackson-Lewis

Hersch

Sulzer

Przedborski

Burke

Hen

: Resistance of alpha -synuclein null mice to the Parkinsonian neurotoxin MPTP. Proc Natl Acad Sci USA 99(22): 14524–14529, 2002

18.

18 Davies

Turmaine

Cozens

DiFiglia

Sharp

Ross

Scherzinger

Wanker

Mangiarini

Bates

: Formation of neuronal intranuclear inclusions underlies the neurological dysfunction in mice transgenic for the HD mutation. Cell 90(3): 537–548, 1997

19.

19 deLahunta

Averill

Jr : Hereditary cerebellar cortical and extrapyramidal nuclear abiotrophy in Kerry Blue Terriers. J Am Vet Med Assoc 168(12): 1119–1124, 1976

20.

20 de la Monte

Vonsattel

Richardson

Jr : Morphometric demonstration of atrophic changes in the cerebral cortex, white matter, and neostriatum in Huntington's disease. J Neuropathol Exp Neurol 47(5): 516–525, 1988

21.

21 DiFiglia

Sapp

Chase

Davies

Bates

Vonsattel

Aronin

: Aggregation of huntington in neuronal intranuclear inclusions, dystrophic neurites in brain. Science 277:1990–1993, 1997

22.

22 Drago

Gerfen

Lachowicz

Steiner

Hollon

Love

Ooi

Grinberg

Lee

Huang

: Altered striatal function in a mutant mouse lacking D1A dopamine receptors. Proc Natl Acad Sci USA 91(26): 12564–12568, 1994

23.

23 Drago

Padungchaichot

Accili

Fuchs

: Dopamine receptors and dopamine transporter in brain function and addictive behaviors: insights from targeted mouse mutants. Dev Neurosci 20(2-3): 188–203, 1998

24.

24 Drago

Padungchaichot

Wong

Lawrence

McManus

Sumarsono

Natoli

Lakso

Wreford

Westphal

Kola

Finkelstein

: Targeted expression of a toxin gene to D1 dopamine receptor neurons by cre-mediated site-specific recombination. J Neurosci 18(23): 9845–9857, 1998

25.

25 Dulawa

Grandy

Low

Paulus

Geyer

: Dopamine D4 receptor-knock-out mice exhibit reduced exploration of novel stimuli. J Neurosci 19(21): 9550–9556, 1999

26.

26 Dziewczapolski

Menalled

Garcia

Mora

Gershanik

Rubinstein

: Opposite roles of D1 and D5 dopamine receptors in locomotion revealed by selective antisense oligonucleotides. Neuroreport 9(1): 1–5, 1998

27.

27 Eilam

Peter

Elson

Rotman

Shiloh

Groner

Segal

: Selective loss of dopaminergic nigro-striatal neurons in brains of Atm-deficient mice. Proc Natl Acad Sci USA 95(21): 12653–12656, 1998

28.

28 Eilam

Peter

Groner

Segal

: Late degeneration of nigro-striatal neurons in ATM–/– mice. Neuroscience 121(1): 83–98, 2003

29.

29 Ferrari

Le Novere

Picciotto

Changeux

Zoli

: Acute and long-term changes in the mesolimbic dopamine pathway after systemic or local single nicotine injections. Eur J Neurosci 15(11): 1810–1818, 2002

30.

30 Fowler

: Nigropallidal encephalomalacia in the horse. J Am Vet Med Assoc 147(6): 607–616, 1965

31.

31 Fuxe

Hokfelt

Nilsson

: Castration, sex hormones, and tubero-infundibular dopamine neurons. Neuroendocrinology 5(1): 107–120, 1969

32.

32 Gainetdinov

Jones

Caron

: Functional hyperdopaminergia in dopamine transporter knockout mice. Biol Psychiatr 46(3): 303–311, 1999

33.

33 Gerfen

: The neostriatal mosaic: multiple levels of compartmental organization in the basal nuclei. Annu Rev Neurosci 15:285–320, 1992

34.

34 Gergen

: MacLean: A stereotaxic atlas of the squirrel monkey's brain (Saimiri sciureus), Public Health Service Publication 933, Washington, DC, 1962

35.

35 Gerlach

Riederer

: Animal models of Parkinson's disease: an empirical comparison with the phenomenology of the disease in man. J Neural Transm 103(8-9): 987–1041, 1996

36.

36 German

Nelson

Liang

Speciale

Sinton

Sonsalla

: The neurotoxin MPTP causes degeneration of specific nucleus A8, A9 and A10 dopaminergic neurons in the mouse. Neuro-degeneration 5(4): 299–312, 1996

37.

37 Glickstein

Schmauss

: Dopamine receptor functions: lessons from knockout mice. Pharmacol Ther 91(1): 63–83, 2001

38.

38 Goldman-Rakic

Castner

Svensson

Siever

Williams

: Targeting the dopamine D(1) receptor in schizophrenia: insights for cognitive dysfunction. Psychopharmacology (Berl) 174(1): 3–16, 2004

39.

39 Groenewegen

: The basal nuclei and motor control. Neural Plast 10(1-2): 107–120, 2003

40.

40 Groenewegen

van den Heuvel

Cath

Voorn

Veltman

: Does an imbalance between the dorsal and ventral striatopallidal systems play a role in Tourette's syndrome? A neuronal circuit approach. Brain Dev Suppl 1:S3–S14, 2003

41.

41 Gusella

MacDonald

: Molecular genetics: unmasking polyglutamine triggers in neurodegenerative disease. Nat Rev Neurosci 1(2): 109–115, 2000

42.

42 Halasz

Ljungdahl

Hokfelt

Johansson

Goldstein

Park

Biberfeld

: Transmitter histochemistry of the rat olfactory bulb. I. Immunohistochemical localization of monoamine synthesizing enzymes. Support for intrabulbar, periglomerular dopamine neurons. Brain Res 126(3): 455–474, 1977

43.

43 Hamre

Tharp

Poon

Xiong

Smeyne

: Differential strain susceptibility following 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration acts in an autosomal dominant fashion: quantitative analysis in seven strains of Mus musculus. Brain Res 828(1-2): 91–103, 1999

44.

44 Hodgson

Agopyan

Gutekunst

Leavitt

LePiane

Singaraja

Smith

Bissada

McCutcheon

Nasir

Jamot

Stevens

Rosemond

Roder

Phillips

Rubin

Hersch

Hayden

: A YAC mouse model for Huntington's disease with full-length mutant huntington, cytoplasmic toxicity, and selective striatal neurodegeneration. Neuron 23(1): 181–192, 1999

45.

45 Hokfelt

Martensson

Bjorklund

Kleinau

Goldstein

: Distributional maps of tyrosine-hydroxylase-immmunoreactive neurons in the rat brain. In: Handbook of Chemical Neuroanatomy, vol. 2: Classical Transmitters in the CNS, Part 1. ed. Bjorkland

Hokfelt

, pp. 277–289. Elsevier, New York, NY, 1984

46.

46 Hollon

Bek

Lachowicz

Ariano

Mezey

Ramachandran

Wersinger

Soares-da-Silva

Liu

Grinberg

Drago

Young

3rd Westphal

Jose

Sibley

: Mice lacking D5 dopamine receptors have increased sympathetic tone and are hypertensive. J Neurosci 22(24): 10801–10810, 2002

47.

47 Holmes

Hollon

Gleason

Liu

Dreiling

Sibley

Crawley

: Behavioral characterization of dopamine D5 receptor null mutant mice. Behav Neurosci 115(5): 1129–1144, 2001

48.

48 Jung

Skryabin

Arai

Abbondanzo

Brosius

Robakis

Polites

Pintar

Schmauss

: Potentiation of the D2 mutant motor phenotype in mice lacking dopamine D2 and D3 receptors. Neuroscience 91(3): 911–924, 1999

49.

49 Kahle

Neumann

Ozmen

Muller

Jacobsen

Schindzielorz

Okochi

Leimer

van Der Putten

Probst

Kremmer

Kretzschmar

Haass

: Subcellular localization of wild-type and Parkinson's disease-associated mutant alpha-synuclein in human and transgenic mouse brain. J Neurosci 20(17): 6365–6373, 2000

50.

50 Karasinska

George

El-Ghundi

Fletcher

O'Dowd

: Modification of dopamine D(1) receptor knockout phenotype in mice lacking both dopamine D(1) and D(3) receptors. Eur J Pharmacol 399(2-3): 171–181, 2000

51.

51 Kita

Kitai

: Glutamate decarboxylase immunoreactive neurons in rat neostriatum: their morphological types and populations. Brain Res 447(2): 346–352, 1988

52.

52 Kitada

Asakawa

Hattori

Matsumine

Yamamura

Minoshima

Yokochi

Mizuno

Shimizu

: Mutations in the parkin gene cause autosomal recessive juvenile Parkinsonism. Nature 392(6676): 605–608, 1998

53.

53 Kobayashi

Morita

Sawada

Mizuguchi

Yamada

Nagatsu

Hata

Watanabe

Fujita

Nagatsu

: Targeted disruption of the tyrosine hydroxylase locus results in severe catecholamine depletion and perinatal lethality in mice. J Biol Chem 270(45): 27235–27243, 1995

54.

54 Koob

Nestler

: The neurobiology of drug addiction. J Neuropsychiatr Clin Neurosci 9(3): 482–497, 1997

55.

55 Krause

Dresel

Krause

la Fougere

Ackenheil

: The dopamine transporter and neuroimaging in attention deficit hyperactivity disorder. Neurosci Biobehav Rev 27(7): 605–613, 2003

56.

56 Kuhar

: Molecular pharmacology of cocaine: a dopamine hypothesis and its implications. Ciba Found Symp 166:81–89, 1992

57.

57 Laforet

Sapp

Chase

McIntyre

Boyce

Campbell

Cadigan

Warzecki

Tagle

Reddy

Cepeda

Calvert

Jokel

Klapstein

Ariano

Levine

DiFiglia

Aronin

: Changes in cortical and striatal neurons predict behavioral and electrophysiological abnormalities in a transgenic murine model of Huntington's disease. J Neurosci 21(23): 9112–9123, 2001

58.

58 Langston

Ballard

Tetrud

Irwin

: Chronic Parkinsonism in humans due to a product of meperidine-analog synthesis. Science 219(4587): 979–980, 1983

59.

59 Larson

Young

: Nigropallidal encephalomalacia in horses in Colorado. J Am Vet Med Assoc 156(5): 626–628, 1970

60.

60 Le Moal

Simon

: Mesocorticolimbic dopaminergic network: functional and regulatory roles. Physiol Rev 71(1): 155–234, 1991

61.

61 Lee

Stirling

Qui

Mandir

Dawson

Copeland

Jenkins

Price

: Human alpha-synuclein-harboring familial Parkinson's disease-linked Ala-53 → Thr mutation causes neurodegenerative disease with alpha-synuclein aggregation in transgenic mice. Proc Natl Acad Sci USA 99(13): 8968–8673, 2002

62.

62 Leroy

Boyer

Auburger

Leube

Ulm

Mezey

Harta

Brownstein

Jonnalagada

Chernova

Dehejia

Lavedan

Gasser

Steinbach

Wilkinson

Polymeropoulos

: The ubiquitin pathway in Parkinson's disease. Nature 395 (6701): 451–452, 1998

63.

63 Li

: Huntingtin-protein interactions and the pathogenesis of Huntington's disease. Trends Genet 20(3): 146–154, 2004

64.

64 Lin

Tallaksen-Greene

Chien

Cearley

Jackson

Crouse

Ren

Albin

Detloff

: Neurological abnormalities in a knock-in mouse model of Huntington's disease. Hum Mol Genet 10(2): 137–144, 2001

65.

65 Maat-Schieman

Dorsman

Smoor

Siesling

Van Duinen

Verschuuren

den Dunnen

Van Ommen

Roos

: Distribution of inclusions in neuronal nuclei and dystrophic neurites in Huntington disease brain. J Neuropathol Exp Neurol 58:129–137, 1999

66.

66 Maguire-Zeiss

Federoff

: Convergent pathobiologic model of Parkinson's disease. Ann NY Acad Sci 991:152–166, 2003

67.

67 Maharajan

Maharajan

Ravagnan

Paino

: The weaver mutant mouse: a model to study the ontogeny of dopamine transmission systems and their role in drug addiction. Prog Neurobiol 64(3): 269–276, 2001

68.

68 Mangiarini

Sathasivam

Seller

Cozens

Harper

Hetherington

Lawton

Trottier

Lehrach

Davies

Bates

: Exon 1 of the HD gene with an expanded CAG repeat is sufficient to cause a progressive neurological phenotype in transgenic mice. Cell 87(3): 493–506, 1996

69.

69 Manning-Bog

McCormack

Uversky

Fink

Di Monte

: The herbicide paraquat causes up-regulation and aggregation of alpha-synuclein in mice: paraquat and alpha-synuclein. J Biol Chem 277(3): 1641–1644, 2002

70.

70 Mantamadiotis

Lemberger

Bleckmann

Kern

Kretz

Martin Villalba

Tronche

Kellendonk

Gau

Kapfhammer

Otto

Schmid

Schutz

: Disruption of CREB function in brain leads to neurodegeneration. Nat Genet 31(1): 47–54, 2002

71.

71 Masliah

Rockenstein

Veinbergs

Mallory

Hashimoto

Takeda

Sagara

Sisk

Mucke

: Dopaminergic loss and inclusion body formation in alpha-synuclein mice: implications for neurode-generative disorders. Science 287(5456): 1265–1269, 2000

72.

72 Mikkelsen

Moller

Jensen

Pedersen

Harajehi

Pakkenberg

: MPTP-induced Parkinsonism in minipigs: a behavioral, biochemical, and histological study. Neurotoxicol Teratol 21(2): 169–175, 1999

73.

73 Missale

Nash

Robinson

Jaber

Caron

: Dopamine receptors: from structure to function. Physiol Rev 78(1): 189–225, 1998

74.

74 Montgomery

Storts

: Hereditary striatonigral and cerebello-olivary degeneration of the Kerry Blue Terrier. I. Gross and light microscopic central nervous system lesions. Vet Pathol 20(2): 143–159, 1983

75.

75 Mura

Feldon

Mintz

: Reevaluation of the striatal role in the expression of turning behavior in the rat model of Parkinson's disease. Brain Rev 808(1): 48–55, 1998

76.

76 Nieoullon

Coquerel

: Dopamine: a key regulator to adapt action, emotion, motivation and cognition. Curr Opin Neurol Suppl 2:S3–9, 2003

77.

77 Obeso

Rodriguez-Oroz

Rodriguez

Arbizu

Gimenez-Amaya

: The basal nuclei and disorders of movement: pathophysiological mechanisms. News Physiol Sci 17:51–55, 2002

78.

78 Obeso

Rodriguez-Oroz

Rodriguez

Macias

Alvarez

Guridi

Vitek

DeLong

: Pathophysiologic basis of surgery for Parkinson's disease. Neurology 55(12 Suppl 6):S7–12, 2000

79.

79 Orth

Tabrizi

: Models of Parkinson's disease. Mov Disord 18(7): 729–737, 2003

80.

80 Pandey

Mittal

Silva

: Blockade of cyclic AMP-responsive element DNA binding in the brain of CREB delta/alpha mutant mice. Neuroreport 11(11): 2577–2580, 2000

81.

81 Paterson

Sunohara

Kennedy

: Dopamine D4 receptor gene: novelty or nonsense? Neuropsychopharmacology 21(1): 3–16, 1999

82.

82 Paxinos

Franklin

KBJ

: The Mouse Brain in Stereotaxic Coordinates 2nd ed. Academic Press, San Diego, CA, 2001

83.

83 Richfield

Thiruchelvam

Cory-Slechta

Wuertzer

Gainetdinov

Caron

Di Monte

Federoff

: Behavioral and neuro-chemical effects of wild-type and mutated human alpha-synuclein in transgenic mice. Exp Neurol 175(1): 35–48, 2002

84.

84 Rios

Habecker

Sasaoka

Eisenhofer

Tian

Landis

Chikaraishi

Roffler-Tarlov

: Catecholamine synthesis is mediated by tyrosinase in the absence of tyrosine hydroxylase. J Neurosci 19(9): 3519–3526, 1999

85.

85 Rockhold

: The Chemical Basis for Neuronal Communication. In: Fundamental Neuroscience, ed. Haines

, pp. 62–63. Churchill Livingstone Inc, New York, NY, 1997

86.

86 Roffler-Tarlov

Graybiel

: The postnatal development of the dopamine-containing innervation of dorsal and ventral striatum: effects of the weaver gene. J Neurosci 7(8): 2364–2372, 1987

87.

87 Rozas

Lopez-Martin

Guerra

Labandeira-Garcia

: The overall rod performance test in the MPTP-treated-mouse model of Parkinsonism. J Neurosci Methods 83(2): 165–175, 1998

88.

88 Rubinsztein

Amos

Leggo

Goodburn

Ramesar

Old

Bontrop

McMahon

Barton

Ferguson-Smith

: Mutational bias provides a model for the evolution of Huntington's disease and predicts a general increase in disease prevalence. Nat Genet 7(4): 525–530, 1994

89.

89 Sagvolden

Sergeant

: Attention deficit/hyper-activity disorder—from brain dysfunctions to behaviour. Behav Brain Rev 94(1): 1–10, 1998

90.

90 Schilling

Becher

Sharp

Jinnah

Duan

Kotzuk

Slunt

Ratovitski

Cooper

Jenkins

Copeland

Price

Ross

Borchelt

: Intranuclear inclusions and neuritic aggregates in transgenic mice expressing a mutant N-terminal fragment of huntington. Hum Mol Genet 8(3): 397–407, 1999

91.

91 Sedelis

Hofele

Auburger

Morgan

Huston

Schwarting

: MPTP susceptibility in the mouse: behavioral, neurochemical, and histological analysis of gender and strain differences. Behav Genet 30(3): 171–182, 2000

92.

92 Siever

Davis

: The pathophysiology of schizophrenia disorders: perspectives from the spectrum. Am J Psychiatr 161(3): 398–413, 2004

93.

93 Sieradzan

Mechan

Persichetti

, et al.: Huntington's disease intranuclear inclusions contain truncated, ubiquitinated huntington protein. Exp Neurol 156:92–9, 1999

94.

94 Sieradzan

Mann

: The selective vulnerability of nerve cells in Huntington's disease. Neuropathol Appl Neurobiol 27(1): 1–21, 2001

95.

95 Smeyne

Goldowitz

: Development and death of external granular layer cells in the weaver mouse cerebellum: a quantitative study. J Neurosci 9(5): 1608–1620, 1989

96.

96 Smith

Striplin

Geller

Mailman

Drago

Lawler

Gallagher

: Behavioural assessment of mice lacking D1A dopamine receptors. Neuroscience 86(1): 135–146, 1998

97.

97 Szczypka

Rainey

Kim

Alaynick

Marck

Matsumoto

Palmiter

: Feeding behavior in dopamine-deficient mice. Proc Natl Acad Sci USA 96(21): 12138–12143, 1999

98.

98 Uhl

: Dopamine transporter: basic science and human variation of a key molecule for dopaminergic function, locomotion, and Parkinsonism. Mov Disord 18(Suppl 7):S71–80, 2003

99.

99 Ungerstedt

: Postsynaptic supersensitivity after 6-hydroxy-dopamine induced degeneration of the nigro-striatal dopamine system. Acta Physiol Scand Suppl 367:69–93, 1971

100.

100 Ungerstedt

: 6-Hydroxydopamine-induced degeneration of the nigrostriatal dopamine pathway: the turning syndrome. Pharmacol Ther B2(1): 37–40, 1976

101.

101 Viggiano

Ruocco

Sadile

: Dopamine phenotype and behaviour in animal models: in relation to attention deficit hyperactivity disorder. Neurosci Biobehav Rev 27(7): 623–637, 2003

102.

102 Vila

Przedborski

: Targeting programmed cell death in neurodegenerative diseases. Nat Rev Neurosci 4(5): 365–375, 2003

103.

103 Vila

Vukosavic

Jackson-Lewis

Neystat

Jakowec

Przedborski

: Alpha-synuclein upregulation in substantia nigra dopaminergic neurons following administration of the Parkinsonian toxin MPTP. J Neurochem 74(2): 721–729, 2000

104.

104 Vonsattel

Myers

Stevens

Ferrante

Bird

Richardson

Jr : Neuropathological classification of Huntington's disease. J Neuropathol Exp Neurol 44(6): 559–577, 1985

105.

105 Walker

Curtis

Murray

: Schizophrenia and bipolar disorder: similarities in pathogenic mechanisms but differences in neurodevelopment. Int Clin Psychopharmacol 17(Suppl 3):S11–19, 2002

106.

106 Wang

Gainetdinov

Fumagalli

Jones

Bock

Miller

Wightman

Caron

: Knockout of the vesicular monoamine transporter 2 gene results in neonatal death and supersensitivity to cocaine and amphetamine. Neuron 19(6): 1285–1296, 1997

107.

107 Wheeler

White

Gutekunst

Vrbanac

Weaver

Vonsattel

Gusella

Hersch

Auerbach

Joyner

MacDonald

: Long glutamine tracts cause nuclear localization of a novel form of huntington in medium spiny striatal neurons in HdhQ92 and HdhQ111 knock-in mice. Hum Mol Genet 9(4): 503–513, 2000

108.

108 White

Auerbach

Duyao

Vonsattel

Gusella

Joyner

MacDonald

: Huntington is required for neurogenesis and is not impaired by the Huntington's disease CAG expansion. Nat Genet 17(4): 404–410, 1997

109.

109 Wilson

Groves

: Fine structure and synaptic connections of the common spiny neuron of the rat neostriatum: a study employing intracellular injection of horseradish peroxidase. J Comp Neurol 194(3): 599–615, 1980

110.

110 Xu

Moratalla

Gold

Hiroi

Koob

Graybiel

Tonegawa

: Dopamine D1 receptor mutant mice are deficient in striatal expression of dynorphin and in dopamine-mediated behavioral responses. Cell 79(4): 729–742, 1994

111.

111 Young

Brown

Klinger

: Nigropallidal encephalomalacia in horses fed Russian knap-weed—Centaurea repens L. Am J Vet Rev 31(8): 1393–1404, 1970

112.

112 Zhou

Wilson

Dani

: Cholinergic interneuron characteristics and nicotinic properties in the striatum. J Neurobiol 53(4): 590–605, 2002

113.

113 Zhou

Palmiter

: Dopamine-deficient mice are severely hypoactive, adipsic, and aphagic. Cell 83(7): 1197–1209, 1995

114.

114 Zhou

Quaife

Palmiter

: Targeted disruption of the tyrosine hydroxylase gene reveals that catecholamines are required for mouse fetal development. Nature 374(6523): 640–643, 1995

115.

115 Zuccato

Ciammola

Rigamonti

Leavitt

Goffredo

Conti

MacDonald

Friedlander

Silani

Hayden

Timmusk

Sipione

Cattaneo

: Loss of huntington-mediated BDNF gene transcription in Huntington's disease. Science 293(5529): 493–498, 2001

Neuroanatomical Phenotyping in the Mouse: The Dopaminergic System

Abstract

Keywords

Materials and Methods

Animals and tissues

Sections needed to examine the central dopaminergic system

Immunohistochemistry

Image processing

Neuroanatomic reference materials

Central Dopaminergic System: Overview of Physical Structures

1) The basal nuclei and major associated nuclei

The striatum

Dorsal and ventral pallidum

The accumbens nucleus

The amygdala nuclear complex

The substantia nigra and subthalamic nuclei

2) Afferent dopaminergic cell systems in the brainstem

i) The mesencephalic dopaminergic cell system

Functional Circuitry of the Basal Nuclei

Basal motor tone

The direct pathway

The indirect pathway

Imbalance of direct and indirect pathways

Mouse Models of Dopaminergic Physiology and Dysfunction

I. Mouse models with neuropathology of the dopaminergic axis

1) Parkinson's disease

Mouse models

a) Toxin-induced models

b) Transgenic models

c) Alpha synuclein knockout mice

2) Huntington's disease

Mouse models

A. Transgenic models expressing human huntington gene fragments

i) The R6 transgenic mouse lines

ii) N-terminal huntington driven by mouse prion promoter

iii) N-terminal huntington driven by rat neuron-specific enolase promoter

iv) Huntington's disease yeast artificial chromosome (YAC) transgenic

B. Targeted models containing the endogenous mouse Hdh gene

i) Huntington knock-in mice

3) Additional mouse models with striatal pathology

a) The weaver mouse

b) Genetically altered mice

II. Mouse models of dopaminergic neurotransmission

1) Dopamine synthesis, metabolism, and transport

Mouse models

a) Tyrosine hydroxylase deficient mice

b) Vesicular monoamine transporter 2 knockout mice

c) Dopamine transporter knockout mice

2) Dopamine receptors

Mouse models

a) D1A dopamine receptor knockout mouse

b) Dopamine D2 receptor knockout

c) Dopamine D3 receptor knockout

d) Double D1/D3 receptor knockout

e) Double D2/D3 receptor knockout

f) Dopamine D4 receptor knockout

g) Dopamine D5 receptor knockout mice

Summary

Footnotes

Acknowledgements

References