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Abstract

It is believed that the most effective method of treatment in schizophrenia is pharmacotherapy, in particular, the use of atypical neuroleptics like aripiprazole (ARI) and olanzapine (OLA). Moreover, studies of many authors have shown that enriched living conditions and tobacco smoke exposure can also affect the cognitive functions that are disturbed in the course of schizophrenia. The aim of the study was to find whether tobacco smoke and enrichment living conditions have the influence on cognitive functions in the newborn offspring of prenatally stressed rats and whether drugs such as ARI (1.5 mg/kg intraperitoneally (i.p.)) and OLA (0.5 mg/kg ip) in single and chronic treatment modify those functions (Morris water maze). The study (in the same conditions) also analyses immobility time (Porsolt test) and motor activity of animals that received ARI and OLA. It has been shown that ARI and OLA as well as enriched environment reduce cognitive function disorders and modify cognitive functions in rats exposed to tobacco smoke. In turn, current research has shown that nicotine has increased cognitive function disorders compared to the previous study (animals without tobacco smoke exposure).

Keywords

Schizophrenia prenatal stress enrichment environment aripiprazole oplanzapine

Introduction

Schizophrenia following depression is one of the most common mental diseases in the global population. The Ohio State University estimates that 2.7 million American citizens suffer from schizophrenia, while the World Health Organization estimates the number of persons suffering from schizophrenia globally at 24 million (1% of global population).¹

Many factors play an important role in the progression of schizophrenia. Study analysis^2,3 demonstrated that the increased risk of schizophrenia and cognitive function disorders occur in both humans and animals whose mothers were exposed to stress during pregnancy. It was found⁴ that disorders of spatial memory occur in the offspring of rats whose mothers were exposed to the same stress sensor during pregnancy. It was also found that excessive stress deregulates the hypothalamic–pituitary–adrenal axis and, in consequence, causes an increased secretion of cortisol/corticosterone which can permeate through the placenta and adversely affect the foetus.⁵ Maternal glucocorticoids easily penetrate the foetus’s brain and can affect the expression of receptors for those hormones. Prolonged elevated cortisol/corticosterone levels in brain can lead to abnormal development of the hippocampus or can initiate neurodegenerative processes.⁶ The authors refer to previously published articles devoted to neurodevelopmental animal models of schizophrenia with the use of both prenatal stress and other effective methods such as the application of methylasoxymethanol actate, maternal deprivaton, isolation rearing, maternal malnutrition or prenatal immune challenge.⁷

Enriched environment (defined as a complex of inanimate objects and social interactions⁸) can increase brain plasticity, which is crucial for acquiring basic skills, for example, related to the survival and the effect on animal reproduction, especially in early life.⁹ It is believed that enriched living conditions can positively affect cognitive functions that are disturbed in the animal model of schizophrenia.^10,11 It has been demonstrated that early exposure to enriched environment is associated with a lower level of antisocial behaviours and prevents the development of schizoid personality in human beings.¹² Numerous studies suggest that neurological and neurochemical changes in brains of rats induced by enriched environment can improve cognitive functions, including spatial memory, and can improve learning processes.^13

–16 Moreover, it has been proven that enriched environment may reduce immobility time in animals studied in the Porsolt test^9,17 or increase the antidepressant effect of drugs used in clinical therapy.¹⁸

Tobacco smoking has serious health consequences both in the healthy population and in persons with schizophrenia.¹⁹ The severity of nicotine addiction and the prevalence of smoking in the population of schizophrenic patients is significantly greater than the general population and in persons with other mental disorders.²⁰ The probability of quitting smoking successfully is much smaller in schizophrenic patients, which can be related to negative symptoms and the adverse effects of the used drugs.²⁰

The treatment of schizophrenia consists in the simultaneous use of pharmacotherapy (neuroleptic drugs) and non-pharmacological treatment (psychotherapy and social therapy). Olanzapine (OLA) and aripiprazole (ARI) used in the treatment are atypical antipsychotic drugs that are more effective than classic drugs in alleviating deficit symptoms and cause fewer adverse effects.²¹

The aims of the study were to find whether tobacco smoke impairs cognitive functions in the newborn offspring of rats whose mothers were exposed to prenatal stress and whether drugs such as ARI (1.5 mg/kg intraperitoneally (i.p.)) and OLA (0.5 mg/kg ip) modify those impaired functions in single and chronic treatment. The study also analyses whether prolonged stress and enriched environment reduce the immobility time and motor activity of animals that received ARI and OLA in the conditions of exposure to tobacco smoke.

Methods

Animals

Timed pregnant Wistar female rats (n = 30) were purchased from Sadowski, Poznań, Poland (licensed by Ministry of Agriculture in Warszawa, Poland) and arrived at our animal facility on day 2 of gestation. The pregnant animals were housed individually in a light- (lights on 07.00–19.00 h), temperature- and humidity-controlled animal facility. The dams had free access to rat chow (Labofeed B, Poland) and water.

For behavioural tests, male rats born to mothers subject to prenatal stress during pregnancy were used.

The total number of animals used in the study was 210 (30 female and 180 male). The male rats were divided into two groups – non-stress control group (NSCG) – (90 male rats obtained directly from Sadowski, Poznań, Poland) and prenatally stressed group (PSG) – (90 male rats – offspring of the 30 prenatally stressed females). Animals were selected from different litters (different mothers) and were subsequently distributed randomly into experimental groups: locomotor activity (LA; 60 rats), Porsolt test (60 rats) and Morris water maze (60 rats; Figure 1).

Figure 1.

Experimental design.

In each group, animals were subdivided into those receiving carboxymethylcellulose sodium (CMC; 10 rats) – control subgroup as well as ARI (10 rats) and OLA (10 rats) subgroups for corresponding tests performed in the experiment.

All procedures related to the use of rats in these experiments were conducted with due respect to ethical principles regarding experiments on animals. The study protocol was approved by the Local Ethical Commission for Research on Animals in Poznan.

Drugs

Pure CMC was obtained from Koch-Light Laboratories (London, England), ARI was purchased from Otsuka Pharmaceutical Europe, Bristol-Myers Squibb (Warsaw, Poland) and OLA was from Zyprexa Eli Lilly (Warsaw, Poland).¹⁶

The rats were administered with ARI (1.5 mg/kg i.p.), OLA (0.5 mg/kg i.p.) or the vehicle i.p. 30 min before the test for 7, 14 and 21 days. ARI and OLA were suspended in a 0.5% CMC solution (2 ml, i.p.). The controls were administered with CMC according to the same schedule. Separate groups of animals were used for different tests.

Prenatal stress procedure

Beginning on day 14 of gestation, the pregnant dams were exposed to a repeated variable stress paradigm until delivery of their pups on gestational day 22 or 23 as previously described.²² The rats used in this paradigm were stressed with the following conditions: (1) restraint in metal tube for 1 h, (2) exposure to a cold environment (4°C) for 6 h, (3) overnight food deprivation, (4) 15 min of swim stress in water of ambient temperature, (5) lights on for 24 h and (6) social stress induced by overcrowded housing conditions during the dark phase of the cycle. A typical schedule of the stress application is presented in prenatal stress table (Table 1). Pregnant control dams remained in the animal room from gestational days 14−21 and were only exposed to normal animal room husbandry procedures. All dams delivered their pups vaginally. At weaning, male offspring from each litter were placed with same-sex litter mates per cage with free access to rat chow and water. The animals were exposed to normal animal room procedures from that point onwards until experimental use on postnatal day 90.

Table 1.

Schedule of stress exposures applied in the prenatal stress paradigm.

Gestation day	Morning	Midday	Evening
14	Metal tube; 60 min	Swim; 15 min	Metal tube; 60 min
15	Cold exposure; 6 h		Fast; overnight
16	Swim; 15 min	Metal tube; 60 min	Swim; 15 min
17	Open	Swim; 15 min	Lights on; overnight
18	Social stress; 12 h
19	Metal tube; 60 min	Swim; 15 min	Metal tube; 60 min
20		Cold exposure; 6 h
21	Swim; 15 min	Metal tube; 60 min	Swim; 15 min

Enriched environment

An enriched environment was created in a plastic rectangular container measuring 102 (length) × 55 (width) × 122 (height) cm³.^23,24 The bottom of the environment was covered with the cage bedding that was also used for the plastic cages in our animal housing facility. Several tactile and visual stimuli were provided in the form of animal toys. There was a treadmill and several hiding places. There was also a moving object in the form of a clock with a colourful pendulum and a mirror. Every other day, the arrangement of objects (including food and water) in the environment was changed. When not in the enriched environment, rats were treated in the same manner as the controls. Starting on day 1, rats were placed in the enriched environment in groups of 10 rats at all times for 28 days (excluding the time of behavioural tests). Animals had free access to food and water.

Tobacco smoke exposure

The smoke was generated from the Polish ‘Męskie’ brand of cigarettes (0.7 mg of nicotine per one cigarette). In the chamber, the concentration level of carbon monoxide (CO) was 1500 mg/m³ of air. The concentration of tobacco smoke was maintained at CO concentrations with 15% deviation and the oxygen concentration was controlled with an oxygen indicator type 211 and kept at the level of 20.0 ± 0.5% of the volume. The air within the chamber was exchanged 10 times within an hour. The tobacco smoke concentration was controlled indirectly by continuous monitoring of CO concentration with the use of “Infralyt 1110/1210” gas analyser. There were 10 animals in one chamber during passive exposure to tobacco smoke. The animals (PSG and NSCG rats) were exposed to tobacco smoke for 6 h a day, 7 days a week, for 4 weeks. During the exposure, the general condition and behaviour of animals was observed.

Measurement of LA

LA was measured in rats (NSCG and PSG group) using eight 20.5 × 28 × 21 cm³ wire grid cages, each with two horizontal infrared photocell beams along the long axis, 3 cm above the floor. Photocell interruptions were recorded by electromechanical counters in an adjacent room. Before the test, all groups of animals were habituated to a novel cage within 30 min period. Rats were also treated with 1.5 mg/kg ARI (i.p.), 0.5 mg/kg OLA (i.p.) or saline in NSCG and PSG group. Then, photocell activity would be recorded at 10 min intervals for 1 h. This test provided an index of basal LA of animals in a familiar environment, necessary to indicate the presence of a central stimulant or sedating effects of the drug used in the novelty test.

Measurement of immobility time in the FST

To measure immobility time, forced swimming test (FST) was conducted. The procedure follows:

a) Pretest: 24 h prior to the experiments, the rats were individually placed in plexiglass cylinders (height 40 cm and diameter 18 cm) containing water (25°C) up to 17 cm of the cylinder’s height. After 15 min, the rats were removed to a 30°C drying room for 30 min.²⁵

b) Test: ARI (1.5 mg/kg i.p.) and OLA (0.5 mg/kg i.p.) were administered 24 h after the pretest. Thirty minutes after drug administration, the rats were placed in the cylinders and immobility was measured for 5 min. A rat was judged to be immobile when it remained floating in the water in an upright position and only made very small movements necessary to keep its head above water. The total duration of immobility over the 5-min period was recorded by an observer unaware of the treatment applied to the rats.

c) We also examined drug effects after prolonged administration (7, 14 and 21 days). The water was changed after the observation of each rat.

Morris water maze test

The water maze apparatus was a circular basin (height = 50 cm, diameter = 180 cm) filled with water (approximately 22–24°C) to a depth of 24 cm, and pieces of Styrofoam were hiding an escape platform (diameter = 8 cm) that was placed 1 cm below the water surface (learning place and invisible condition).²⁶ Many extra-maze visual cues surrounding the maze were available, and the observer remained in the same location for each trial. The rats were placed in the water facing the midpoint section of the wall at one of four equally spaced locations: north (N), east (E), south (S) and west (W). The pool was divided into four quadrants: NW, NE, SE and SW. The rats were allowed to swim freely until they found and climbed onto the platform. If a rat failed to locate the platform within 60 s, it was placed on the platform for 5 s. Each rat was submitted to six trials per day, and the starting position was changed at each trial (starting on the N side, followed by E, S and W sides, in that order). The interval was 5 min between trials 1–3 and 4–6 and 10 min between trials 3 and 4. For the first 3 days of maze testing, the submerged platform was placed in the NW quadrant. The platform was subsequently placed in the SE quadrant for the following 2 days. After these five testing days, there was a period of 7 days without any testing. On day 6, the rats were retested with the platform located in the same position as it had been on day 5. On day 7, the platform was lifted above the water level and placed in the SW quadrant, and rats were injected with ARI (1.5 mg/kg i.p.) and OLA (0.5 mg/kg i.p.) 30 min before the test. Each rat was subjected to a one-probe trial consisting of six individual trials. The total number of times each rat crossed the probe (crossed quadrants – space exploration) target area and the time of the probe trial (number of escape latencies – space navigation) swim were recorded by the observer. The time of each of the six trials was noted, and a mean value for each rat was calculated. The same procedures were followed in the chronic experiments.

After prolonged administration of ARI and OLA (7, 14 and 21 days), the drug effects were tested as described for day 7 of the Morris water maze test procedure.

Statistical analysis

The data are shown as the mean values ± SEM. The data distribution pattern was not normal (unlike Gaussian function). Statistical analyses for the memory test, LA and antidepressant test were carried out using the nonparametric Kruskal–Wallis H test for unpaired data and Friedman’s two-way analysis of variance test for paired data. Statistical significance was tested using Dunn’s post hoc test.

Results

The effect of a single or chronic treatment of ARI and OLA and enriched environment on the spatial memory of animals tested in the Morris test (number of escaped latency) in the NSCG and the PSG exposed to tobacco smoke.

A single administration of ARI in the dose of 1.5 mg/kg i.p. in the NSCG did not show a statistically significant difference in the number of escape latencies compared with the control group (Table 2). Only chronic treatment (21 days) showed a statistically significant reduction of swimming time in the NSCG (Table 2), which is a sign of improved spatial memory in this group of animals. A single administration of ARI in the dose of 1.5 mg/kg i.p. in the PSG did not show a statistically significant difference in the number of escaped latencies compared with the control group (Table 2). A statistically significant reduction of time in the test was found only after 14 and 21 days of administering ARI to animals from the PSG which is a sign of improved spatial memory (Table 2).

Table 2.

The effect of a single and chronic treatment of ARI and OLA and enriched environment on the spatial memory of animals, tested in the Morris test (time in the test) in the NSCG and PSG exposed to tobacco smoke.^a

Group	Escape latency (s)				Friedman H (3.37)
	Single administration (× ± SEM)	Chronic treatment
	Single administration (× ± SEM)	7 days (× ± SEM)	14 days (× ± SEM)	21 days (× ± SEM)
NSCG
CMC (0.5 ml/rat) control	9.5 ± 2.13	8.12 ± 2.01	6.95 ± 1.41	6.00 ± 0.71	1.9
ARI 1.5 mg/kg i.p. 30 min before the test	8.79 ± 1.51	8.56 ± 2.12	5.47 ± 1.45	3.00 ± 0.37^b	5.4
OLA 0.5 mg/kg i.p. 30 min before the test	6.62 ± 1.01	4.33 ± 0.73^b	4.12 ± 0.49^b	4.22 ± 0.79	1.8
PSG
CMC (0.5 ml/rat) control	11.25 ± 1.61	10.71 ± 1.46	9.37 ± 1.09	7.4 ± 0.50	3.5
ARI 1.5 mg/kg i.p. 30 min before the test	9.86 ± 1.21	9.77 ± 1.55	6.12 ± 1.04^c	5.56 ± 0.88^c	3.4
OLA 0.5 mg/kg i.p. 30 mins before the test	6.70 ± 1.34^c	6.11 ± 1.36^c	5.80 ± 1.01^c	4.68 ± 0.81^c	2.7
Kruskal–Wallis H (3.37)	4.6	9.3	5.1	4.6

ARI: aripiprazole; OLA: olanzapine; CMC: carboxymethylcellulose; NSCG: non-stressed control group; PSG: prenatally stressed group.

^aNumber of animals tested = 10.

^b p < 0.05: statistically significant difference compared with CMC-NSCG.

^c p < 0.05: statistically significant difference compared with CMC-PSG.

A single administration of OLA (0.5 mg/kg i.p.) to animals from the NSCG did not make a statistically significant difference in the number of escaped latencies compared with the control group. Only chronic treatment (7 and 14 days) of OLA (0.5 mg/kg i.p.) to animals from the NSCG caused a statistically significant reduction of the swimming time compared with the control group (improved memory; Table 2).

In PSG animals, the number of escaped latencies was statistically significant shortly after both single and repeated (7, 14, and 21 days) administration of OLA in the dose of 0.5 mg/kg i.p., which is a sign of improved spatial memory (Table 2).

The effect of a single and chronic treatment of ARI and OLA and enriched environment on the spatial memory of animals was tested in the Morris test (number of crossed quadrants) in the NSCG and the PSG exposed to tobacco smoke.

A single and chronic treatment of ARI in the dose of 1.5 mg/kg i.p. 30 min before the test in NSCG and PSG did not show statistically significant difference in the number of crossed quadrants in the Morris test (Table 3).

Table 3.

The effect of a single and chronic treatment of ARI and OLA and enriched environment on the spatial memory of animals, tested in the Morris test (number of crossed quadrants) in the NSCG and PSG exposed to tobacco smoke.^a

Group	Crossed quadrants				Friedman H (3.37)
	Single administration (× ± SEM)	Chronic treatment
	Single administration (× ± SEM)	7 days (× ± SEM)	14 days (× ± SEM)	21 days (× ± SEM)
NSCG
CMC (0.5 ml/rat) control	2.79 ± 0.81	2.57 ± 0.81	2.50 ± 0.54	1.91 ± 0.42	1.2
ARI 1.5 mg/kg i.p. 30 min before the test	3.52 ± 1.25	2.00 ± 1.00	1.98 ± 0.70	1.15 ± 0.23	2.4
OLA 0.5 mg/kg i.p. 30 min before the test	2.5 ± 0.67	2.21 ± 0.37	2.04 ± 0.48	1.66 ± 0.41	1.7
PSG
CMC (0.5 ml/rat) control	3.75 ± 1.08	3.12 ± 1.12	2.43 ± 0.49	1.99 ± 0.38	1.9
ARI 1.5 mg/kg i.p. 30 min before the test	4.25 ± 0.85	2.14 ± 1.01	2.10 ± 0.58	1.5 ± 0.48	2.8
OLA 0.5 mg/kg i.p. 30 min before the test	1.43 ± 0.45^b	2.01 ± 0.56	2.68 ± 0.82	0.68 ± 0.17^b	3.1
Kruskal–Wallis H (3.37)	4.1	1.3	1.6	3.2

ARI: aripiprazole; OLA: olanzapine; CMC: carboxymethylcellulose; NSCG: non-stressed control group; PSG: prenatally stressed group.

^aNumber of animals tested = 10.

^b p < 0.05: statistically significant difference compared with CMC-PSG.

A single and chronic treatment of OLA (0.5 mg/kg i.p.) to NSCG animals 30 min before the test did not cause a statistically significant difference in the number of crossed quadrants compared with the control group (Table 3).

The number of crossed quadrants was statistically reduced only in the PSG after a single and repeated (21 days) administration of OLA compared with the control group, which is a sign of improved spatial memory (Table 3).

The effect of a single and chronic treatment of ARI and OLA and enriched environment on immobility time of animals tested in the Porsolt test in the NSCG and PSG exposed to tobacco smoke.

A single and repeated (7 days) administration of ARI in the dose of 1.5 mg/kg i.p. in the NSCG caused a statistically significant reduction of immobility time of animals compared with the controlled group (Table 4).

Table 4.

The effect of a single and chronic treatment of ARI and OLA and enriched environment on immobility time of animals, tested in the Porsolt test in the NSCG and PSG of rats exposed to tobacco smoke.^a

Group	IT (s)				Friedman H (3.37)
	Single administration (× ± SEM)	Chronic treatment
	Single administration (× ± SEM)	7 days (× ± SEM)	14 days (× ± SEM)	21 days (× ± SEM)
NSCG
CMC (0.5 ml/rat) control	230.0 ± 6.0	223.8 ± 5.4	225.5 ± 6.3	243.5 ± 4.3	2.1
ARI 1.5 mg/kg i.p. 30 min before the test	159.7 ± 9.4^b	192.3 ± 9.6^b	232.3 ± 9.9	249.8 ± 5.6	4.7
OLA 0.5 mg/kg i.p. for 30 min before the test	214.1 ± 4.8^b	227.5 ± 5.6	229.8 ± 8.6	255.3 ± 5.3	2.8
PSG
CMC (0.5 ml/rat) control	261.5 ± 5.0^b	241.8 ± 5.8^b	247.8 ± 6.3^b	245.3 ± 5.6	1.9
ARI 1.5 mg/kg i.p. 30 min before the test	219.1 ± 4.6^c	204.3 ± 7.2^c	221.5 ± 8.4^c	228.2 ± 7.7	2.3
OLA 0.5 mg/kg i.p. 30 min before the test	209.3 ± 4.8^c	245.0 ± 6.8	255.8 ± 6.8	251.2 ± 4.2	2.0
Kruskal–Wallis H (3.37)	4.7	5.1	5.2	1.7

ARI: aripiprazole; OLA: olanzapine; CMC: carboxymethylcellulose; NSCG: non-stressed control group; PSG: prenatally stressed group.

^aNumber of animals tested = 10.

^b p < 0.05: statistically significant difference compared with CMC-NSCG.

^c p < 0.05: statistically significant difference compared with CMC-PSG.

In the PSG, the reduction of immobility time was statistically significant after both single and repeated (7 and 14 days) administration of ARI in the dose of 1.5 mg/kg i.p. (Table 4).

Both in the NSCG and PSG of rats, only administering OLA (0.5 mg/kg i.p.) caused a statistically significant reduction of immobility time in rats placed in the test compared with the control group (Table 4).

Moreover, a statistically significant increase of immobility time (1, 7 and 14 days) of animals in the CMC-PSG control group compared with the CMC-NSCG control group was found (Table 4).

The effect of a single and chronic treatment of ARI and OLA and enriched environment on motor activity of animals in the NSCG and PSG exposed to tobacco smoke.

In prenatally stressed animals (PSG), a statistically significant increase of the motor activity (1, 7, 14 and 21 days) compared with the CMC-NSCG control group was observed, which is a sign that the animal model of schizophrenia was used correctly (Table 5).

Table 5.

The effect of single and chronic treatment of ARI and OLA and enriched environment on the motor activity of animals in the NSCG and PSG exposed to tobacco smoke.^a

Group	Activity counts/mean				Friedman H (3.37)
	Single administration (× ± SEM)	Chronic treatment
	Single administration (× ± SEM)	7 days (× ± SEM)	14 days (× ± SEM)	21 days (× ± SEM)
NSCG
CMC (0.5 ml/rat) control	135.3 ± 7.1	127.2 ± 4.6	99.5 ± 3.6	102.3 ± 1.7	4.3
ARI 1.5 mg/kg i.p. 30 min before the test	99.8 ± 5.0^b	96.3 ± 5.2^b	69.3 ± 5.7^b	65.8 ± 2.7^b	5.7
OLA 0.5 mg/kg i.p. 30 min before the test	109.2 ± 10.0^b	109.8 ± 5.3^b	110.0 ± 12.4	98.8 ± 2.2	3.1
PSG
CMC (0.5 ml/rat) control	187.5 ± 8.3^b	181.5 ± 11.3^b	135.0 ± 5.5^b	134.0 ± 6.9^b	4.0
ARI 1.5 mg/kg i.p. 30 min before the test	98.5 ± 4.4^c	114.5 ± 4.8^c	78.8 ± 6.3^c	70.7 ± 11.3^c	9.3
OLA 0.5 mg/kg i.p. 30 min before the test	63.8 ± 6.6^c	110.3 ± 3.4^c	86.0 ± 3.8^c	80.0 ± 7.9^c	6.9
Kruskal–Wallis H (3.37)	9.1	8.3	7.0	9.9

ARI: aripiprazole; OLA: olanzapine; CMC: carboxymethylcellulose; NSCG: non-stressed control group; PSG: prenatally stressed group.

^aNumber of animals tested = 10.

^b p < 0.05: statistically significant difference compared with CMC-NSCG.

^c p < 0.05: statistically significant difference compared with CMC-PSG.

A single and repeated (7, 14 and 21 days) administration of ARI in the dose of 1.5 mg/kg i.p. in the NSCG and the PSG caused a statistically significant reduction of animal mobility compared with the control group (Table 5).

Following a single and repeated (7 days) administration of OLA (0.5 mg/kg i.p.) in the NSCG, decreased animal mobility was observed compared with the control group (Table 5). In the PSG group, a statistically significant reduction of animal mobility was observed after a single and repeated administration (Table 5).

Discussion

It is believed that the enriched environment can positively affect memory processes and other cognitive functions disturbed in the course of schizophrenia. Previous studies by Van Praag et al. and Melendez et al. reported^8,11 an increase in neuron lifetime and the number of glial cells, the creation of new neurons in the dentate band in the group of rodents living in enriched environment; moreover, an increase in the mass and size of animals’ brains has been observed.⁷ It was also shown that the level of brain-derived neurotrophic factor in brains of mice living in the enriched environment increases.²⁷ Our results are confirmed by other authors who observed improved memory in the Morris test in rats living in the enriched environment.^28,29 The improvement of memory can be explained by the stimulating effect of the enriched environment on cell proliferation in the hippocampus, brain structure of paramount importance in terms of cognitive function disorders including spatial memory.³⁰ Exposure to the enriched environment stimulates neuroplasticity and neuron production and increases the lifespan of nervous cells,^27,31,32 that is, deficits created during the prenatal period, which confirms that the enriched environment can eliminate behavioural and emotional deficits observed in schizophrenia.^27,31
–33

The results obtained suggest that memory improvement was observed in animals living in the enriched environment exposed to tobacco smoke following chronic treatment of ARI, both in the NSCG and the PSG. Memory improvement was also observed following chronic treatment of OLA in the NSCG and a single and chronic treatment of OLA in the PSG.

It can be concluded that ARI (a dopamine (DA) partial agonist) by stimulating DA release can cause a pleasant sensation in a patient (ARI can affect the reward system).³⁴ This effect is also observed during smoking tobacco, which can explain more frequent smoking in schizophrenic patients, in whom experiencing pleasure is disturbed due to a low concentration of dopamine in brain.³⁵ Neuroimaging methods have confirmed that the activity of prefrontal cortex in schizophrenic patients decreases during activities requiring the involvement of operating memory, whilst in tested healthy persons its activity increases.³⁶ The case of a patient, a smoker in deep depression, treated with ARI during hospitalization has been described. It was noted that he stopped smoking during the treatment with ARI and started again after the drug was stopped. It suggests that nicotine strengthens the effect of ARI related to DA secretion, hence the drug can be used in the therapy of nicotine addiction.³⁷ Other studies³⁸ conducted on patients diagnosed with schizophrenia and tobacco addiction showed that ARI reduces both the need for nicotine and the urge to smoke a cigarette.

MK-801 is an N-methyl-d-aspartate (NMDA) receptor antagonist which is widely used as an animal model of psychosis and induces a variety of cognitive disturbances related to schizophrenia. In the work of Enomoto et al., they reported that MK-801 can disrupt normal Morris water maze performance similar to cognitive deficits seen in schizophrenic patients. It impairs learning and memory functions that depend on the hippocampus and the amygdala.

Improved spatial memory observed following OLA administration can be related to the antagonistic effect of this drug on serotonin receptors, especially 5-HT_2A,³⁹ and the stimulating effect of nicotine present in tobacco smoke on α₇ nicotine receptors. This hypothesis can be confirmed by studies of Mutlu et al.⁴⁰ carried out on mice with a model of schizophrenia artificially triggered by administering MK-801.⁴¹ MK-801 as an antagonist of NMDA receptors generates numerous cognitive disorders including spatial memory observed in animal model of schizophrenia (disorders typical for clinical form of this disease). Similarly to neurodevelopmental models, in a model based on the use of MK-801 (models created by pharmacological intervention), memory disorders and learning abilities depend mainly on hippocampus and nucleus accumbens disorders. Studies showed that administering OLA in the doses of 1.25, 2.5 and 5 mg/kg caused a statistically significant reduction in the distance covered by animals in the Morris test compared with the control group, which indicates that it improved cognitive functions. Considering the results of studies of Levin et al.⁴² suggesting an improvement of spatial memory after adding nicotine to OLA therapy, it can be concluded that the concomitant administration of those substances has a positive effect on cognitive functions studied in an animal experimental model of schizophrenia.

The results obtained prove the positive effect of the enriched environment on spatial memory and a possible compensating action regarding symptoms triggered by prenatal stress and tobacco smoke. Moreover, the enriched environment can affect the functions of prefrontal and medial cortex, which are responsible for regulating the hypothalamic–pituitary–adrenal axis but are disordered in animals stressed prenatally.⁴³ Study results corroborate with the results of other authors.^40,41

ARI administered once or repeatedly to rats exposed to tobacco smoke had an antidepressant effect both in the NSCG and the PSG. On the other hand, OLA had an antidepressant effect only after a single administration both in the NSCG and the PSG.

Depression can lead to excessive tobacco smoking⁴⁴ and attempts to quit can cause a recurrence of depression.^45,46 Nicotine participates in the balance of many neurotransmitters. This substance activates neurons, especially in the limbic system and the hippocampus.⁴⁷ Moreover, it increases mesocorticolimbic activity in the nucleus accumbens and prefrontal cortex.⁴⁸ In a smoker’s brain, the activity of monoamine oxidases (MAOs) is reduced (MAO B by approximately 40% and MAO A by approximately 28%),⁴⁹ which significantly reduces DA metabolism and is responsible for the neuroprotective effect, as it reduces smokers’ proneness to neuroleptic-induced Parkinsonism.^50,51 The antidepressant effect of nicotine in animals is observed, for example, in the FST [Porsolt], and this effect is probably related to the modulating influence of nicotine on α₄β₂ nicotine receptors. It is suggested that nicotine increases glutaminergic transmission in the cortex, causing an increase of DA levels in the striatum⁴⁴ and the nucleus accumbens.^52,53 Hattori et al.⁵⁴ observed that mice living in the enriched environment showed greater mobility in the Porsolt test. Results of other studies⁵⁵ carried out on male Sprague Dawley rats living in standard and the enriched environment indicate that rats living in the enriched environment remain immobile for shorter periods compared with the group living in standard environment.⁵⁵ It is believed that ARI’s antidepressant effect in this study is due to the agonistic effect on 5-HT_1A receptors and the stabilizing effect on the dopaminergic system.⁵⁶ On the other hand, the antidepressant effect of OLA can be related to the antagonistic effect of the drug on the 5-HT_2A receptor.^57,58

A single and chronic treatment of ARI and OLA to rats living in the enriched environment exposed to tobacco smoke caused a statistically significant decrease of animal mobility both in the NSCG and the PSG.¹⁶ However, the results of our studies carried out earlier in a standard environment without exposure to tobacco smoke indicate that the administration of ARI and OLA caused an increase of animal mobility in the NSCG, whilst the administration of those drugs in the PSG caused a decrease of LA.¹⁶ Moreover, during studies on rats living in a standard environment exposed to tobacco smoke (results not published), this regularity was confirmed. However, the results obtained in this study with the use of the enriched environment (with simultaneous exposure to tobacco smoke) suggest a significant effect of the enriched environment of animals on LA. This is confirmed by the results of other authors showing that enriched environment reduces the activity of amphetamine and nicotine, which may affect animal mobility.⁵⁹ Moreover, increased brain weight and cortex thickness,⁶⁰ more dendrocyte branches, synapses^31,61
–63 and more oligodendrocytes in the cortex and the upper layers of occipital cortex are observed in animals living in the enriched environment.⁶⁴ A higher level of acetylcholine, acethylcholinesterase, choline acetyltransferase and monoamines is also observed in many areas of the brain.⁶⁰ It was shown that the level of corticosteroid receptors in the hippocampus was also higher in rats stimulated by the enriched environment.³¹

Schizophrenic patients present with a limited experience of pleasure, whilst nicotine makes it possible to recreate these sensations, which can be the cause of those patients’ addiction to nicotine. Moreover, nicotine has a positive effect on negative symptoms of schizophrenia, such as apathy, lack of motivation or anhedonia. Hypofrontality is said to be the main cause of the negative disorders and the intense excretion of DA in the prefrontal region and the nucleus accumbens can lead to reduction of disorders in the function of frontal regions and reduction, for example, of motor activity.¹⁰

In summary, it has been shown that atypical antipsychotics, ARI and ORI, as well as the enriched environment, reduce cognitive function disorders and modify cognitive functions in the course of animal model of schizophrenia. As results indicate the enriched environment may be considered to be one of the most effective therapeutic tools and may be used to treat many neurodegenerative diseases.⁶⁵ In turn, current research have shown that nicotine increases cognitive function disorders observed in the experiment compared to our previous study.⁶⁶

Footnotes

Conflict of interest

The authors declared no conflicts of interest.

Funding

Project was funded by the National Science Centre – Krakow, Poland.

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Neuroleptics and enrichment environment treatment in memory disorders and other central nervous system function observed in prenatally stressed rats

Abstract

Keywords

Introduction

Methods

Animals

Drugs

Prenatal stress procedure

Enriched environment

Tobacco smoke exposure

Measurement of LA

Measurement of immobility time in the FST

Morris water maze test

Statistical analysis

Results

Discussion

Footnotes

Conflict of interest

Funding

References