Brain PET Oral 3: Miscellaneous Targets

56. Propofol anesthesia alters tissue clearance of leucine but not regional rates of cerebral protein synthesis

S. Bishu¹, K. Schmidt¹, A. Unterman¹, A. Zametkin¹, M. Channing², Z.-H. Liu¹, M. Qin¹, W. Kammerer³, P. Herscovitch², Z. Quezado³ and C. Beebe Smith¹

¹Section on Neuroadaptation and Protein Metabolism, National Institute of Mental Health; ²PET Department; ³Department of Anesthesia, National Institute of Health, Bethesda, Maryland, USA

Objectives: Determine the effect of propofol anesthesia on regional rates of cerebral protein synthesis (rCPS) and leucine kinetics in adult males with the L-[1-¹¹C]leucine PET method (Schmidt et al, JCBFM 2005; 25:617–28; Smith et al, JCBFM 2005;25:629–40; Bishu et al, JCBFM 2008;28:1502–13).

Methods: Subjects (20–24 years; n = 10) were dynamically scanned on the High Resolution Research Tomograph for 90 mins following ¹¹C-leucine injection. Region of interest-based analyses were used to estimate kinetic model rate constants. rCPS, λ (fraction of unlabeled leucine in the tissue precursor pool for protein synthesis derived from arterial plasma), and C_E (concentration of unlabeled leucine in tissue precursor pool) were calculated. Sedation was monitored with the bi-spectral index (BIS) monitor (Gan et al, Anesth 1997;87:808–15).

Results: Mean (±s.d.) plasma leucine concentrations were similar in awake (118±14 nmol/mL) and propofol-anesthetized (109±21) states at baseline as were physiological variables. Clearance curves of arterial plasma ¹¹C-leucine were also similar in both states. Tissue ¹¹C-leucine clearance was slower in the propofol-anesthetized state in 6/8 cortical and 2/6 subcortical regions examined, indicating an increased half-life of unlabeled leucine in tissue. Estimated C_E was 30% higher in most cortical (7/8) and subcortical (4/6) regions. Under propofol anesthesia, λ tended to be slightly higher (NS); rCPS was unchanged (Table). In both awake and anesthetized conditions, rCPS varied regionally, whereas λ was more uniform. The mean BIS score was 34, consistent with anesthesia.

Region	λ		rCPS (nmol g−1min−1)		Half-life of precursor pool (min)
	Awake	Propofol	Awake	Propofol	Awake	Propofol
Cerebellum	0.75±0.02	0.77±0.02	2.39±0.27	2.39±0.23	3.6±0.4	4.0±0.4
Occipital cortex	0.73±0.03	0.75±0.02	2.52±0.34	2.38±0.30	4.3±0.4	5.2±0.3*
Frontal cortex	0.71±0.03	0.73±0.02	2.29±0.26	2.28±0.21	5.4±0.4	6.3±0.5*
Temporal cortex	0.70±0.03	0.72±0.02	2.07±0.23	2.03±0.22	6.0±0.3	7.0±0.6*
Hippocampus a	0.67±0.04	0.68±0.02	1.85±0.24	1.91±0.31	7.5±1.3	8.0±1.0
Thalamus	0.75±0.02	0.77±0.02	1.84±0.18	1.74±0.21	4.4±0.4	4.9±0.4*

Values are means±s.d. for 10 subjects, except were indicated;

a

9 subjects;

*

Statistically significant after bonferroni correction, P<0.05.

Conclusions: Since leucine participates in glutamate trafficking and leucine metabolites are oxidized via the TCA cycle, the longer half-life of unlabeled leucine in tissue is consistent with the effect of anesthetics to decrease neuronal activity and the rate of cerebral energy metabolism. Despite these changes in kinetics, rCPS remained unaffected.

Supported by IRP/NIMH; CC, NIH.

625. Age and gender affect 5-HT₄ receptor binding: a brain pet study in healthy volunteers with [¹¹C]SB207145

K. Madsen^1,2, W.-J. Neumann^1,2, L. Marner^1,2, M. Haahr^1,2, N. Gillings³, W. Baaré^2,4, S.G. Hasselbalch^1,2 and G.M. Knudsen^1,2

¹Neurobiology Research Unit, Rigshospitalet; ²Center for Integrated Molecular Brain Imaging; ³PET and Cyclotron Unit, Rigshospitalet; ⁴Danish Research Center for Magnetic Resonance, Hvidovre Hospital, Copenhagen, Denmark

Objectives: 5-HT₄ receptors are of clinical relevance due to their involvement in memory and learning and their presumed ability to reduce the pathological accumulation of cerebral β-amyloid in Alzheimer's disease (AD). The aim of this study was to investigate the effects of age and gender on 5-HT₄ receptor binding in both the high-binding region striatum, the global neocortex, and 3 cortical regions of importance for memory, learning and AD.

Methods: A bolus injection of [¹¹C]SB207145 was followed by a 2-h dynamic PET scan in a GE Advance scanner in 30 healthy volunteers (16 men, mean age 44.4 years range 19.9 to 86.2). Volumes of interest (VOIs) were delineated automatically on co-registered 3T MRIs. Partial volume (PV) corrected time-activity curves were extracted from the VOIs, including gray matter voxels only, and BP_ND was modeled with SRTM. BP_ND in 5 VOIs were investigated for gender differences and correlation to age.

Results: Although non PV corrected BP_ND correlated negatively with age in some regions, when correcting for atrophy, we found positive correlations in the high-binding region striatum (R = 0.3818, P = 0.0373), in the dorsolateral prefrontal cortex (R = 0.42, P = 0.0208, see Figure), and a tendency was found in the posterior cingulate gyrus (R = 0.3378, P = 0.0679). No correlations were found in the global neocortex or in the hippocampus.

Using age as a covariate, gender differences in PV corrected BP_ND were only found in the striatum, where a tendency for 6.8% higher BP_ND in women was found (P = 0.068). In a post hoc analysis, this difference was driven by putamen, which was 8.3% higher in women (P = 0.0355).

Conclusions: To our knowledge, this is the first time an increase of 5-HT receptors with age has been found. The up-regulation of 5-HT₄ receptors with age could be a compensation for a lower serotonergic tone with age. It is highly interesting that the cortical increase with age is seen in the regions affected by the highest β-amyloid accumulation in studies with ¹¹C-PIB PET in AD.

A localized gender difference of 8.3% is somewhat surprising, as most receptors do not show gender differences. However, depression and anxiety are more prevalent in females.

Figure

Increase of 5HT4 receptors in DLPFC with age.

548. Acute nicotine effects on the distribution of nicotine α4β2 and dopamine D2/D3 receptor pet radiotracers

J. Mukherjee, K. Kasabwala, R. Kant, S. Pandey, C. Constantinescu, R Coleman, M.-L. Pan and P. Parekh

Psychiatry and Human Behavior, University of California-Irvine, Irvine, California, USA

Objectives: Nicotine, a major constituent in tobacco exhibits addictive properties by virtue of its interaction with nicotinic receptors (nAChR) including the α4β2 subtype. This interaction releases dopamine in the striatum. Using PET radiotracers for α4β2 nAChR (¹⁸F-nifene) and dopamine D2/D3 receptors (¹⁸F-fally pride (FP) and ¹⁸F-desmethoxyfallypride (DMFP)) we have evaluated acute effects of nicotine in rodents using MicroPET on the two receptor systems.

Methods: Imaging was done using an Inveon MicroPET scanner. ¹⁸F-Nifene, ¹⁸F-FP and ¹⁸F-DMFP were synthesized as previously reported. Sprague Dawley rats (400 to 600 g) anesthetized with isoflurane were used for PET studies. Radiotracers (0.8 to 1 mCi) were given iv. Test-retest was carried out to evaluate reproducibility. Rats were imaged for 90 mins (for ¹⁸F-nifene and ¹⁸F-DMFP) and 150 mins (for ¹⁸F-FP) and binding potential (BP_ND) were measured. Post-¹⁸F-nifene nicotine challenge (0.3 mg/kg, free base) was used to measure reversibility and nicotine occupancy was measured using various doses (0.02 to 0.50 mg/kg) injected 15 mins before ¹⁸F-nifene. Nicotine (0.3 mg/kg) was coinjected with ¹⁸F-FP and ¹⁸F-DMFP to measure potential dopaminergic effects. Data was analyzed using ASIPRO and PMOD. For ex vivo studies, animals were sacrificed for autoradiographic analysis of brain sections using Optiquant imaging system.

Results: ¹⁸F-Nifene binding to thalamic and other brain regions was consistent with the α4β2 nAChR distribution. Test-retest values for the thalamus varied less than 5% (BP_ND = 1.30, n = 3 rats, using reference cerebellum (CB)), confirming reliability of ¹⁸F-nifene binding. Post-injection of nicotine (0.3 mg/kg, +30 mins) rapidly displaced specifically bound ¹⁸F-nifene indicating >90% occupancy. Low dose nicotine (0.02 mg) reached about 50% occupancy. ¹⁸F-FP binding to D2/D3 receptors gave BP_ND values of dorsal striata (DST) = 15; ventral striata (VST) = 11; and with nicotine values were 20 and 12.5, respectively. ¹⁸F-DMFP binding to D2/D3 receptors in striata (ST) gave BP_ND of 1.8 (without nicotine) and 2.9 (with nicotine). Ex vivo studies revealed similar effects in the case of ¹⁸F-FP, while ¹⁸F-DMFP binding exhibited ≈20% reduction (0.5 mg/kg nicotine) in VST when internal capsule as reference.

Conclusions: Nicotine dose-dependently occupies α4β2 nAChR and saturates receptors at 0.3 mg/kg. Although little or no effect on cerebral blood volume or flow to the ST by nicotine has been reported^1,3 delivery of dopamine D2/D3 receptor tracers, ¹⁸F-FP and ¹⁸F-DMFP to ST was significantly increased, with a net increase in measured BP_ND. Nicotine has been shown to marginally increase dopamine levels in the nucleus accumbens and other brain regions ² and therefore the increase in delivery and BP_ND is unlikely due to greater D2/D3 receptor availability. Recently, ¹⁸F-FP has been used in heavy smokers and a small decrease in D2/D3 receptor availability was observed in the putamen. Further analyses are underway to delineate the effect of nicotine on ¹⁸F-FP and ¹⁸F-DMFP.

Research Supported by NIH R01 AG029479 and R01EB006110.

28. The effect of cAMP dependent protein kinase activator and inhibitor on [¹¹C]rolipram binding to phosphodiesterase 4 in conscious rats

T. Itoh^1,2, K. Abe^2,3, O. Inoue³, J. Hong¹, V.W. Pike¹, R.B. Innis¹ and M. Fujita¹

¹National Institute of Mental Health, Bethesda, Maryland, USA; ²Shionogi & Co., Ltd.; ³Osaka University Graduate School of Medicine, Osaka, Japan

Background and aims: Phosphodiesterase-4 (PDE4), a major phosphodiesterase isozyme hydrolyzing the second messenger cAMP, may play a pivotal role in brain diseases and the effects of centrally acting drug. Rolipram is a selective inhibitor of PDE4 and the binding study using [¹¹C]rolipram with PET should allow the function of the cAMP second messenger cascade to be explored. cAMP Dependent protein kinase (PKA) has been reported to phosphorylate PDE4 and affect the enzyme activity. We recently found that both in vivo B_max and K_D values of [¹¹C](〈i〉R〈/i〉)-rolipram were significantly greater in conscious rats than in anesthetized rats. In the present PET study, we examined effects of PKA modulators on the binding of [¹¹C](〈i〉R〈/i〉)-rolipram and [¹¹C](〈i〉S〈/i〉)-rolipram (affinity of (〈i〉S) is 1/20 of that of (〈i〉R)) using conscious rats.

Methods: For PET scans using conscious rats (n = 26), an acrylic plate was permanently attached to the skull. The animals were trained to acclimate to the scanning holder for at least ten days before the PET experiment. On the day of PET scan, 10 nmol of db-cAMP (cAMP analogue and PKA activator) or 100 nmol of Rp-cAMPS (PKA inhibitor) was injected into the left striatum using 30-gauge needle three times before the scan. At the same time, saline was administered into the right striatum. The animals were given an intravenous bolus injection of [¹¹C](〈i〉R〈/i〉)-rolipram (61.1±18.3 MBq) or [¹¹C](〈i〉S〈/i〉)-rolipram (46.3±9.0 MBq) and were imaged for 100 mins. The binding activity of [¹¹C](〈i〉R)- and (〈i〉S〈/i〉)-rolipram in striatum was measured and expressed as percent standardized uptake value (%SUV), which is normalized to injected activity and body weight. The time-activity curves were drawn and their area under the curves (AUC) were compared between left and right-side striatum for each of [¹¹C](〈i〉R)- and (〈i〉S〈/i〉)-rolipram.

Results: When using [¹¹C](〈i〉R〈/i〉)-rolipram, db-cAMP significantly increased the AUC in left striatum (P<0.002). In contrast, a significant decrease of AUC was found by pretreatment of Rp-cAMPS (P<0.002). On the other hand, these effects of neither of these PKA modulators were not observed in [¹¹C](〈i〉S〈/i〉)-rolipram PET experiments.

Conclusions: Significant increase and decrease of [¹¹C](〈i〉R〈/i〉)-rolipram binding were observed by PKA activator and inhibitor, respectively. These effects were not caused by changes in brain regional blood flow, since there were no effects when [¹¹C](〈i〉S〈/i〉)-rolipram was used. These results support the value of measuring [¹¹C]rolipram binding in brain for assessing responses to physiological or pharmacological challenges to the cAMP second messenger system.

Table

Effects of db-cAMP and Rp-cAMPS on [¹¹C]-rolipram binding in conscious rats

	AUC 0–100 mins (%SUV*mins)
	[ 11 C](〈i〉R〈/i〉)-rolipram		[ 11 C](〈i〉S〈/i〉)-rolipram
	Left striatum	Right striatum a	Left striatum	Right striatum a
db-cAMP	17,518±4233	15,804±3784	3636±1350	3561±1395
Rp-cAMPS	14,508±2346	16,311±3002	3633±517	3658±592

a

Control saline was administered into the right striatum.

335. Regional differences in P-Glycoprotein function at the human blood-brain barrier

M. Bauer¹, C. Wagner¹, R. Karch², T. Feurstein¹, K. Kletter³, M. Zeitlinger¹, M. Müller¹ and O. Langer¹

¹Clinical Pharmacology; ²Medical Computer Sciences; ³Nuclear Medicine, Medical University of Vienna, Vienna, Austria

Background and aims: P-glycoprotein (P-gp) acts at the blood-brain barrier (BBB) as an active cell membrane efflux pump for several endogenous and exogenous compounds. Recent animal data suggest regional differences of cerebral P-gp activity. ¹ PET with the P-gp substrate (R)-[¹¹C]verapamil (VPM) can be used to measure cerebral P-gp function, but low brain uptake of VPM hampers the mapping of regional differences in P-gp function. This limitation can be overcome by P-gp modulation with moderate doses of P-gp inhibitors. The aim of this study was to assess regional P-gp function in humans before and after administration of the third-generation P-gp inhibitor tariquidar (TQD).

Methods: 5 healthy volunteers underwent paired VPM PET scans and arterial blood sampling, before and at 3 h after i.v. administration of TQD (2 mg/kg body weight). VPM distribution volumes (DV) before and after TQD administration were calculated using a 4-rate-constant-2-tissue-compartment model. Parametric images depicting distribution volume (DV) differences between the 2 PET scans were generated using the pixel-wise Logan plot and analyzed by statistical parametric mapping (SPM5). TQD concentrations in venous plasma were quantified using liquid chromatography/mass spectrometry.

Purpose: The purpose of this study was to investigate P-glycoprotein function at the human blood-brain barrier in vivo.

Results: TQD administration resulted in significant increases (paired t-test) in whole-brain grey matter DVs and influx rate constants (K₁) of VPM across the BBB (DV = 0.64±0.12 and 0.79±0.07, P = 0.012; K₁ = 0.034±0.01 and 0.049±0.01, P = 0.024, before and after TQD, respectively). A strong correlation was observed between TQD exposure in plasma and change in brain DV after administration of TQD (r = 0.932, P = 0.021). SPM analysis revealed significantly smaller DV increases in cerebellum, hippocampus, parahippocampus, enthorinal cortex, inferior temporal lobe, brain stem and cerebellum as compared to other brain regions, which points to increased P-gp expression and function in these regions.

Conclusion: TQD significantly increased brain distribution of VPM, due to increased influx. Regional differences in VPM DV changes in response to TQD treatment suggest regional variability of cerebral P-gp function, which might make some brain areas more resistant and others more vulnerable to the accumulation of P-gp substrates.

The research leading to these results has received funding from the European Community's Seventh Framework Programme under grant agreement no. 201380 (EURIPIDES).