Dopamine D2 receptors mapped in rat brain with [3H](+)PHNO

Association of dopamine D2/3 receptor binding potential measured using PET and [11C]-(+)-PHNO with post-mortem DRD2/3 gene expression in the human brain

Open access post-mortem transcriptome atlases such as the Allen Human Brain Atlas (AHBA) can inform us about mRNA expression of numerous proteins of interest across the whole brain, while in vivo protein binding in the human brain can be quantified by means of neuroreceptor positron emission tomography (PET). By combining both modalities, the association between regional gene expression and receptor distribution in the living brain can be approximated. Here, we compare the characteristics of D2 and D3 dopamine receptor distribution by applying the dopamine D2/3 receptor agonist radioligand [11C]-(+)-PHNO and human gene expression data. Since [11C]-(+)-PHNO has a higher affinity for D3 compared to D2 receptors, we hypothesized that there is a stronger relationship between D2/3 non-displaceable binding potentials (BPND) and D3 mRNA expression. To investigate the relationship between D2/3 BPND and mRNA expression of DRD2 and DRD3 we performed [11C]-(+)-PHNO PET scans in 27 healthy subjects (12 females) and extracted gene expression data from the AHBA. We also calculated D2/D3 mRNA expression ratios to imitate the mixed D2/3 signal of [11C]-(+)-PHNO. In accordance with our a priori hypothesis, a strong correlation between [11C]-(+)-PHNO and DRD3 expression was found. However, there was no significant correlation with DRD2 expression. Calculated D2/D3 mRNA expression ratios also showed a positive correlation with [11C]-(+)-PHNO binding, reflecting the mixed D2/3 signal of the radioligand. Our study supports the usefulness of combining gene expression data from open access brain atlases with in vivo imaging data in order to gain more detailed knowledge on neurotransmitter signaling.

D3 dopamine receptors and a missense mutation of fatty acid amide hydrolase linked in mouse and men: implication for addiction

The endocannabinoid and dopaminergic systems have independently been implicated in substance use disorder and obesity. We investigated a potential interaction between genetically inherited variation in fatty acid amide hydrolase (FAAH, C385A), which metabolizes the cannabis-like endocannabinoid anandamide, and dopaminergic system, measured by dopamine receptor levels and mRNA. Binding of the dopamine D3 preferring probe [C-11]-(+)-PHNO was measured with positron emission tomography (PET) in 79 human subjects genotyped for the FAAH C385A polymorphism (36/79 AC + AA). Autoradiography with [H-3]-(+)-PHNO and in situ hybridization with a D3-specific S-35 riboprobe were carried out in 30 knock-in mice with the FAAH C385A polymorphism (20/30 AC + AA). We found that the FAAH genetic variant C385A was associated with significantly higher (+)-PHNO binding in both humans and in knock-in mice, and this effect was restricted to D3 selective brain regions (limbic striatum, globus pallidus, and ventral pallidum (9-14%; p < 0.04) in humans and Islands of Calleja (28%; p = 0.036) in mice). In situ hybridization with a D3-specific S-35 riboprobe in FAAH knock-in C385A mice confirmed significantly increased D3 receptor mRNA across examined regions (7-44%; p < 0.02). The association of reduced FAAH function with higher dopamine D3 receptors in human and mouse brain provide a mechanistic link between two brain systems that have been implicated in addiction-risk. This may explain the greater vulnerability for addiction and obesity in individuals with C385A genetic variant and by extension, suggest that a D3 antagonism strategy in substance use disorders should consider FAAH C385A polymorphism.

Effects of exposure to chronic uncertainty and a sensitizing regimen of amphetamine injections on locomotion, decision-making, and dopamine receptors in rats

Gambling disorder (GD) is a behavioral addiction that may be linked to alterations in dopamine (DA) systems. Gambling involves chronic exposure to uncertain reward, which can sensitize the activity of DA systems. Here we explored how combinations of Pavlovian and instrumental uncertainty impact DA sensitization and risky decision-making. Experiment 1: 40 rats underwent 66 uncertainty exposure (UE) sessions during which they responded for saccharin. Animal responding was reinforced according to a fixed or variable (FR/VR) ratio schedule that turned on a conditioned stimulus (CS; light), which predicted saccharin on 50% or 100% of trials. Animals responded under one of the four conditions: FR-CS100% (no uncertainty), VR-CS100%, FR-CS50%, and VR-CS50% (maximal uncertainty). DA sensitization was inferred from an enhanced locomotor response to d-amphetamine (d-AMPH; 0.5 mg/kg) challenge. The rat gambling task (rGT) was used to assess decision-making. Experiment 2: 24 rats received 5 weeks of sensitizing d-AMPH or saline doses, followed by locomotor activity and rGT testing. Experiment 3: Effects of UE and a sensitizing d-AMPH regimen on DA D1, D2, and D3 receptor binding were assessed in 44 rats using autoradiography. Compared to FR-CS100%, VR-CS100% and VR-CS50% rats displayed a greater locomotor response to d-AMPH, and VR-CS50% rats demonstrated riskier decision-making. Chronic d-AMPH-treated rats mirrored the effects of VR-CS50% groups on these two indices. Both VR-CS50% and d-AMPH-treated groups had increased striatal DA D2 receptor binding. These results suggest that chronic uncertainty exposure, similar to exposure to a sensitizing d-AMPH regimen, sensitized the function of DA systems and increased risky decision-making.

PET radiotracer development for imaging high-affinity state of dopamine D2 and D3 receptors: Binding studies of fluorine-18 labeled aminotetralins in rodents

Imaging the high-affinity, functional state (HA) of dopamine D2 and D3 receptors has been pursued in PET imaging studies of various brain functions. We report further evaluation of ¹⁸ F-5-OH-FPPAT, and the newer ¹⁸ F-5-OH-FHXPAT and ¹⁸ F-7-OH-FHXPAT. Syntheses of ¹⁸ F-5-OH-FHXPAT and ¹⁸ F-7-OH-FHXPAT were improved by modifications of our previously reported procedures. Brain slices and brain homogenates from male Sprague-Dawley rats were used with the 3 radiotracers (74-111 kBq/cc). Competition with dopamine (1-100 nM) and Gpp(NH)p (10-50 µM) were carried out to demonstrate binding to dopamine D2 and D3 HA-states and binding kinetics of ¹⁸ F-5-OH-FPPAT measured. Ex vivo brain slice autoradiography was carried out on rats administered with ¹⁸ F-5-OH-FHXPAT to ascertain HA-state binding. PET/CT imaging in rats and wild type (WT) and D2 knock-out mice were carried out using ¹⁸ F-7-OH-FHXPAT (2-37 MBq). Striatum was clearly visualized by the three radiotracers in brain slices and dopamine displaced more than 80% of binding, with dissociation rate in homogenates of 2.2 × 10^-2 min^-1 for ¹⁸ F-5-OH-FPPAT. Treatment with Gpp(NH)p significantly reduced 50-80% striatal binding with faster dissociation rates (5.0 × 10^-2 min^-1 ), suggesting HA-state binding of ¹⁸ F-5-OH-FPPAT and ¹⁸ F-5-OH-FHXPAT. Striatal binding of ¹⁸ F-5-OH-FHXPAT in ex vivo brain slices were sensitive to Gpp(NH)p, suggesting HA-state binding in vivo. PET binding ratios of ¹⁸ F-7-OH-FHXPAT in rat brain were ventral striatum/cerebellum = 2.09 and dorsal striatum/cerebellum = 1.65; similar binding ratios were found in the D2 WT mice. These results suggest that in vivo PET measures of agonists in the brain at least in part reflect binding to the membrane-bound HA-state of the dopamine receptor.

Involvement of dopamine D2 receptor in the diurnal changes of tuberoinfundibular dopaminergic neuron activity and prolactin secretion in female rats

BACKGROUND

An endogenous dopaminergic (DA) tone acting on D3 receptors has been shown to inhibit tuberoinfundibular (TI) DA neuron activity and stimulate prolactin (PRL) surge in the afternoon of estrogen-primed ovariectomized (OVX+E2) rats. Whether D2 receptor (D2R) is also involved in the regulation of TIDA and PRL rhythms was determined in this study.

RESULTS

Intracerebroventricular (icv) injection of PHNO, a D2R agonist, in the morning inhibited TIDA and midbrain DA neurons' activities, and stimulated PRL secretion. The effects of PHNO were significantly reversed by co-administration of raclopride, a D2R antagonist. A single injection of raclopride at 1200 h significantly reversed the lowered TIDA neuron activity and the increased serum PRL level at 1500 h. Dopamine D2R mRNA expression in medial basal hypothalamus (MBH) exhibited a diurnal rhythm, i.e., low in the morning and high in the afternoon, which was opposite to that of TIDA neuron activity. The D2R rhythm was abolished in OVX+E2 rats kept under constant lighting but not in OVX rats with regular lighting exposures. Pretreatment with an antisense oligodeoxynucleotides (AODN, 10 μg/3 μl/day, icv) against D2R mRNA for 2 days significantly reduced D2R mRNAs in central DA neurons, and reversed both lowered TIDA neuron activity and increased serum PRL level in the afternoon on day 3. A diurnal rhythm of D2R mRNA expression was also observed in midbrain DA neurons and the rhythm was significantly knocked down by the AODN pretreatment.

CONCLUSIONS

We conclude that a diurnal change of D2R mRNA expression in MBH may underlie the diurnal rhythms of TIDA neuron activity and PRL secretion in OVX+E2 rats.

Dopamine D3 receptor ligands for drug addiction treatment: update on recent findings

The dopamine D3 receptor is located in the limbic area and apparently mediates selective effects on motivation to take drugs and drug-seeking behaviors, so that there has been considerable interest on the possible use of D3 receptor ligands to treat drug addiction. However, only recently selective tools allowing studying this receptor have been developed. This chapter presents an overview of findings that were presented at a symposium on the conference Dopamine 2013 in Sardinia in May 2013. Novel neurobiological findings indicate that drugs of abuse can lead to significant structural plasticity in rodent brain and that this is dependent on the availability of functional dopamine D3 autoreceptor, whose activation increased phosphorylation in the ERK pathway and in the Akt/mTORC1 pathway indicating the parallel engagement of a series of intracellular signaling pathways all involved in cell growth and survival. Preclinical findings using animal models of drug-seeking behaviors confirm that D3 antagonists have a promising profile to treat drug addiction across drugs of abuse type. Imaging the D3 is now feasible in human subjects. Notably, the development of (+)-4-propyl-9-hydroxynaphthoxazine ligand used in positron emission tomography (PET) studies in humans allows to measure D3 and D2 receptors based on the area of the brain under study. This PET ligand has been used to confirm up-regulation of D3 sites in psychostimulant users and to reveal that tobacco smoking produces elevation of dopamine at the level of D3 sites. There are now novel antagonists being developed, but also old drugs such as buspirone, that are available to test the D3 hypothesis in humans. The first results of clinical investigations are now being provided. Overall, those recent findings support further exploration of D3 ligands to treat drug addiction.

Dopamine D2 and D3 binding in people at clinical high risk for schizophrenia, antipsychotic-naive patients and healthy controls while performing a cognitive task

BACKGROUND

The dopamine (DA) D2 receptors exist in 2 states: a high-affinity state (D2 high) that is linked to second messenger systems, responsible for functional effects, exhibits high affinity for agonists (e.g., DA), and a low-affinity state that is functionally inert exhibits lower affinity for agonists. The DA D3 receptor subtype exhibits high agonist affinity, whereas the existence of the multiple affinity states is controversial. Preclinical studies in animal models of psychosis have shown a selective increase of D2 high as the common factor in psychosis, and the D3 receptor has been suggested to be involved in the pathophysiology of schizophrenia.

METHODS

We studied D2 high and D3 in people at clinical high risk (CHR) for schizophrenia and in antipsychotic-naive patients with schizophrenia using the novel positron emission tomography radiotracer, [11C]-(+)-PHNO. The binding potential nondisplaceable (BP(ND)) was examined in the regions of interest (ROI; caudate, putamen, ventral striatum, globus pallidus, substantia nigra and thalamus) using an ROI and a voxel-wise approach while participants performed a cognitive task.

RESULTS

We recruited 12 CHR individuals and 13 antipsychotic-naive patients with schizophrenia-spectrum disorder, whom we compared with 12 age- and sex-matched healthy controls. The BP(ND) between patients and controls did not differ in any of the ROIs, consistent with the voxel-wise analysis. Correlations between the BP(ND) in D3-rich regions and psychopathology warrant further investigation.

LIMITATIONS

In the absence of resting-state (baseline) BP(ND) data, or following a depletion paradigm (i.e., α-methyl partyrosine), it is not possible to ascertain whether the lack of difference among the groups is owing to different levels of baseline DA or to release during the cognitive task.

CONCLUSION

To our knowledge, the present study represents the first effort to measure the D2 and D3 receptors under a cognitive challenge in individuals putative/prodromal for schizophrenia using [11C]-(+)-PHNO.

The role of dopamine D(3) receptors in antipsychotic activity and cognitive functions

Dopamine D(3) receptors have a pre- and postsynaptic localization in brain stem nuclei, limbic parts of the striatum, and cortex. Their widespread influence on dopamine release, on dopaminergic function, and on several other neurotransmitters makes them attractive targets for therapeutic intervention. The signaling pathways of D(3) receptors are distinct from those of other members of the D(2)-like receptor family. There is increasing evidence that D(3) receptors can form heteromers with dopamine D(1), D(2), and probably other G-protein-coupled receptors. The functional consequences remain to be characterized in more detail but might open new interesting pharmacological insight and opportunities. In terms of behavioral function, D(3) receptors are involved in cognitive, social, and motor functions, as well as in filtering and sensitization processes. Although the role of D(3) receptor blockade for alleviating positive symptoms is still unsettled, selective D(3) receptor antagonism has therapeutic features for schizophrenia and beyond as demonstrated by several animal models: improved cognitive function, emotional processing, executive function, flexibility, and social behavior. D(3) receptor antagonism seems to contribute to atypicality of clinically used antipsychotics by reducing extrapyramidal motor symptoms; has no direct influence on prolactin release; and does not cause anhedonia, weight gain, or metabolic dysfunctions. Unfortunately, clinical data with new, selective D(3) antagonists are still incomplete; their cognitive effects have only been communicated in part. In vitro, virtually all clinically used antipsychotics are not D(2)-selective but also have affinity for D(3) receptors. The exact D(3) receptor occupancies achieved in patients, particularly in cortical areas, are largely unknown, mainly because only nonselective or agonist PET tracers are currently available. It is unlikely that a degree of D(3) receptor antagonism optimal for antipsychotic and cognitive function can be achieved with existing antipsychotics. Therefore, selective D(3) antagonism represents a promising mechanism still to be fully exploited for the treatment of schizophrenia, cognitive deficits in schizophrenia, and comorbid conditions such as substance abuse.

The dopamine dysfunction in schizophrenia revisited: new insights into topography and course

Schizophrenia has long been associated with an imbalance in dopamine (DA) neurotransmission, and brain imaging has played an important role in advancing our knowledge and providing evidence for the dopaminergic abnormalities. This chapter reviews the evidence for DA dysfunction in different brain regions in schizophrenia, in particular striatal, extrastriatal, and prefrontal regions, with emphasis on recently published findings. As opposed to the traditional view that most striatal dopaminergic excess, associated with the positive symptoms of schizophrenia, involves the dopaminergic mesolimbic pathway, recent evidence points to the nigrostriatal pathway as the area of highest dysregulation. Furthermore, evidence from translational research suggests that dopaminergic excess may be present in the prodromal phase, and may by itself, as suggested by the phenotype observed in transgenic mice with developmental overexpression of dorso-striatal D(2) receptors, be an early pathogenic condition, leading to irreversible cortical dysfunction.

The antipsychotics olanzapine, risperidone, clozapine, and haloperidol are D2-selective ex vivo but not in vitro

In a recent human [(11)C]-(+)-PHNO positron emission tomography study, olanzapine, clozapine, and risperidone occupied D2 receptors in striatum (STR), but, despite their similar in vitro D2 and D3 affinities, failed to occupy D3 receptors in globus pallidus. This study had two aims: (1) to characterize the regional D2/D3 pharmacology of in vitro and ex vivo [(3)H]-(+)-PHNO binding sites in rat brain and (2) to compare, using [(3)H]-(+)-PHNO autoradiography, the ex vivo and in vitro pharmacology of olanzapine, clozapine, risperidone, and haloperidol. Using the D3-selective drug SB277011, we found that ex vivo and in vitro [(3)H]-(+)-PHNO binding in STR is exclusively due to D2, whereas that in cerebellar lobes 9 and 10 is exclusively due to D3. Surprisingly, the D3 contribution to [(3)H]-(+)-PHNO binding in the islands of Calleja, ventral pallidum, substantia nigra, and nucleus accumbens was greater ex vivo than in vitro. Ex vivo, systemically administered olanzapine, risperidone, and haloperidol, at doses occupying approximately 80% D2, did not occupy D3 receptors. Clozapine, which also occupied approximately 80% of D2 receptors ex vivo, occupied a smaller percentage of D3 receptors than predicted by its in vitro pharmacology. Across brain regions, ex vivo occupancy by antipsychotics was inversely related to the D3 contribution to [(3)H]-(+)-PHNO binding. In contrast, in vitro occupancy was similar across brain regions, independent of the regional D3 contribution. These data indicate that at clinically relevant doses, olanzapine, clozapine, risperidone, and haloperidol are D2-selective ex vivo. This unforeseen finding suggests that their clinical effects cannot be attributed to D3 receptor blockade.

Further evaluation of the carbon11-labeled D(2/3) agonist PET radiotracer PHNO: reproducibility in tracer characteristics and characterization of extrastriatal binding

[(11)C]-(+)-PHNO is a new dopamine D(2/3) receptor agonist radiotracer which has been successfully used to measure D(2/3) receptor availability in experimental animals and man. Here we report in vivo evaluation in the rat of the biodistribution, metabolism, specificity, selectivity, and dopamine sensitivity of carbon11-labeled PHNO ([(11)C]-3-PHNO) produced by an alternative radiochemical synthesis method. [(11)C]-3-PHNO showed rapid metabolism and clearance from most peripheral organs and tissues. [(11)C]-3-PHNO, but not its polar metabolite, readily crossed the blood-brain barrier and showed high levels of uptake in the D(2/3)-rich striatum. Pretreatment with unlabeled PHNO and the D(2/3) receptor antagonist raclopride indicated that binding in the striatum was specific and selective to D(2/3) receptors. PET studies in anesthetized rats revealed significant reductions in [(11)C]-3-PHNO binding in the striatum following amphetamine administration, indicating sensitivity to increases in endogenous dopamine concentrations. D(2/3) antagonist pretreatment additionally indicated moderate levels of [(11)C]-3-PHNO specific binding in several extrastriatal brain areas-most notably the olfactory bulbs and tubercles, thalamus, and hypothalamus. Of particular interest, approximately 30% of [(11)C]-3-PHNO signal in the cerebellum-a region often used as a "low-binding" reference region for PET quantification-was attributable to specific signal. These data demonstrate that [(11)C]-3-PHNO shows similar tracer characteristics to [(11)C]-(+)-PHNO, but additionally indicate that radiolabeled PHNO may be used to estimate D(2/3) receptor availability in select extrastriatal brain regions with PET.

In vivo quantification of regional dopamine-D3 receptor binding potential of (+)-PHNO: Studies in non-human primates and transgenic mice

Examination of dopamine-D3 (D3) receptors with positron emission tomography (PET) have been hampered in the past by the lack of a PET ligand with sufficient selectivity for D3 over dopamine-D2 (D2) receptors. The two types co-localize in the brain, with D2 density significantly higher than D3, hence nonselective PET ligands inform on D2, rather than D3 status. [(11)C]-(+)-PHNO is a novel PET ligand with a preferential affinity for D3 over D2. We used the selective D3 antagonist, SB-277011 to dissect regional fractions of the [(11)C]-(+)-PHNO signal attributable to D3 and D2 in primate brain. The results were compared with quantitative autoradiography with (3)H-(+)-PHNO in wild-type, D2-knock-out, and D3-knock-out mice examined at baseline and following administration of SB-277011. Both sets of results converged to indicate a predominant D3-related component to (+)-PHNO binding in extra-striatal regions, with binding in the midbrain being entirely attributable to D3. The midbrain is thus an excellent target region to examine D3 receptor occupancy with [(11)C]-(+)-PHNO PET in vivo.

Brain region binding of the D2/3 agonist [11C]-(+)-PHNO and the D2/3 antagonist [11C]raclopride in healthy humans

The D(2) receptors exist in either the high- or low-affinity state with respect to agonists, and while agonists bind preferentially to the high-affinity state, antagonists do not distinguish between the two states. [(11)C]-(+)-PHNO is a PET D(2) agonist radioligand and therefore provides a preferential measure of the D(2) (high) receptors. In contrast, [(11)C]raclopride is an antagonist radioligand and thus binds with equal affinity to the D(2) high- and low-affinity states. The aim was to compare the brain uptake, distribution and binding characteristics between [(11)C]-(+)-PHNO and [(11)C]raclopride in volunteers using a within-subject design. Both radioligands accumulated in brain areas rich in D(2)/D(3)-receptors. However, [(11)C]-(+)-PHNO showed preferential uptake in the ventral striatum and globus pallidus, while [(11)C]raclopride showed preferential uptake in the dorsal striatum. Mean binding potentials were higher in the putamen (4.3 vs. 2.8) and caudate (3.4 vs 2.1) for [(11)C]raclopride, equal in the ventral-striatum (3.4 vs. 3.3), and higher in the globus pallidus for [(11)C]-(+)-PHNO (1.8 vs. 3.3). Moreover [(11)C]-(+)-PHNO kinetics in the globus pallidus showed a slower washout than other regions. One explanation for the preferential binding of [(11)C]-(+)-PHNO in the globus pallidus and ventral-striatum could be the presence of a greater proportion of high- vs. low-affinity receptors in these areas. Alternatively, the observed distribution could also be explained by a preferential binding of D(3)-over-D(2) with [(11)C]-(+)-PHNO. This differential binding of agonist vs. antagonist radioligand, especially in the critically important region of the limbic striatum/pallidum, offers new avenues to investigate the role of the dopamine system in health and disease.

Positron emission tomography quantification of [11C]-(+)-PHNO binding in the human brain

The kinetic modeling of [11C]-(+)-PHNO binding to the dopamine D2/3 receptors in six human volunteers using positron emission tomography (PET) is described. [11C]-(+)-PHNO is the first agonist radioligand for the D2/3 in humans and as expected showed high uptake in caudate, putamen, globus pallidus (GP) and ventral striatum, and low uptake in cerebellum. A two-tissue compartment model (2CM) with four parameters was necessary to adequately fit time-activity data in all regions. Although a 2CM provided an excellent estimation of total distribution volumes, which were highly correlated with those obtained with the invasive Logan approach, it provided a poor identification of the k3/k4 ratios. Coupling K1/k2 between brain regions (Method C) or fixing K1/k2 to the value obtained in cerebellum (Method D) enabled more stable estimates of k3/k4 as compared with an unconstrained 2CM. The k3/k4 obtained with Method D ranged from 0.12+/-0.03 in cerebellum to 3.93+/-0.77 in GP and were similar to those obtained when coupling K1/k2. Binding potentials (BPs) obtained using the simplified reference tissue model (BP(SRTM)) ranged from 2.08+/-0.34 in caudate to 3.55+/-0.78 in GP and were highly correlated with k3/k4 estimates obtained with Method D (r=0.98). However, BP(SRTM) were 11%+/-5% lower than values obtained with Method D. BPs derived using the noninvasive Logan approach were slightly lower but not significantly different than BP(SRTM). This study demonstrates that [11C]-(+)-PHNO can be used for the quantitative measurement of D2/3 densities and should enable further studies of potential D2/3 dysregulation in several important psychiatric and neurologic illnesses.

Alterations in dopamine D3 receptors in the circling (ci3) rat mutant

We have previously described a black-hooded mutant rat (BH.7A/Ztm-ci3/ci3) that displays abnormal lateralized circling behavior, but normal auditory and vestibular functions. Neurochemical determination of dopamine and dopamine metabolite levels in striatum, nucleus accumbens and substantia nigra showed that ci3 rats have a significant asymmetry in striatal dopamine in that dopamine levels were significantly lower in the hemisphere contralateral to the preferred direction of turning. Consistent with this finding, immunohistological examination of dopaminergic neurons in substantia nigra and ventral tegmental area yielded a significant laterality in the medial part of substantia nigra pars compacta with a lower density of tyrosine hydroxylase-positive neurons in the contralateral hemisphere of mutant circling rats, while no laterality was seen in unaffected rats of the background strain. In the present study, quantitative autoradiography was used to examine the binding of [(3)H]SCH 23390, [(3)H]raclopride and [(3)H]7-OH-DPAT (7-hydroxy-N,N-di-n-propyl-2-aminotetralin) to dopamine D1, D2, and D3 receptors, respectively, in various brain regions of ci3 rats and unaffected rats of the background strain (BH.7A(LEW)/Won). No significant differences between circling rats and controls were obtained for D1 and D2 receptor binding in any region, but mutant rats differed from controls in dopamine D3 binding in several regions. A significant decrease in D3 binding was seen in the shell of the nucleus accumbens, the islands of Calleja, and the subependymal zone of ci3 mutant rats. Furthermore, a significant laterality in D3 binding was determined in ci3 rats in that binding was lower in the contralateral hemisphere in the shell of the nucleus accumbens and the islands of Calleja. Our data indicate that alterations of dopamine D3 receptors may be involved in the behavioral phenotype of the ci3 rat, thus substantiating the findings from a recent genetic linkage analysis that indicated the D3 receptor gene as a candidate gene in this rat mutant.

Syntheses and in vitro evaluation of fluorinated naphthoxazines as dopamine D2/D3 receptor agonists: radiosynthesis, ex vivo biodistribution and autoradiography of [18F]F-PHNO

INTRODUCTION

Carbon-11-labeled (+)-4-propyl-3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol ([(11)C]-(+)-PHNO) is a dopamine D2/D3 agonist radioligand that is currently used to image the high-affinity state of dopamine receptors in humans with positron emission tomography (PET). The present study reports the preparation and evaluation of fluorinated (+)-PHNO derivatives.

METHODS

Five fluorinated (+)-PHNO derivatives were synthesized and tested in vitro for inhibition of binding of [(3)H]domperidone in homogenates of rat striatum and inhibition of binding to [(3)H]-(+)-PHNO in homogenates of human-cloned D2Long receptors in Chinese hamster ovary cells and rat striatum. Radiolabeling with fluorine-18 was carried out for the most promising candidate, N-fluoropropyl-(+)-HNO (F-PHNO), and ex vivo biodistribution and autoradiography studies with this radiopharmaceutical were performed in rodents.

RESULTS

(+)-PHNO and the fluorinated analogs inhibited binding of [(3)H]domperidone and [(3)H]-(+)-PHNO to the high- and low-affinity states of dopamine D2 receptors, consistent with D2 agonist behavior. The average dissociation constant at the high-affinity state of D2, K(i)(High), was 0.4 nM for F-PHNO and proved to be equipotent with (+)-PHNO (0.7 nM). All other fluorinated derivatives were significantly less potent (K(i)(High)=2-102 nM). The most promising candidate, F-PHNO, was labeled with fluorine-18 in 5% uncorrected radiochemical yield, with respect to starting fluoride. Ex vivo biodistribution and autoradiography studies in rodents revealed that [(18)F]F-PHNO rapidly enters the rodent brain. However, this radiotracer does not reveal specific binding in the brain and is rapidly cleared.

CONCLUSIONS

Five novel dopamine D2/D3 agonists based on (+)-PHNO were synthesized and evaluated in vitro. F-PHNO was shown to behave as a potent D2 agonist in vitro and was therefore radiolabeled with fluorine-18. Despite the promising in vitro pharmacological profile, [(18)F]F-PHNO did not display in vivo behavior suitable to image dopaminergic receptor expression using PET.

Increased dopamine D2High receptors in amphetamine-sensitized rats, measured by the agonist [3H](+)PHNO

Repeated injections of amphetamine causes animals to become sensitized and supersensitive to DA. Previous work showed that the striata from such sensitized rats revealed a 3.5-fold increase in the density of D2(High) DA receptors, as measured by the guanine-nucleotide-sensitive component of [(3)H]raclopride binding. The present study was done to confirm these earlier findings by different methods and different ligands. The striata from amphetamine-sensitized rats showed an increase of 2.2-fold in the density of guanine-nucleotide-sensitive D2 receptors labeled by saturation experiments with [(3)H](+)PHNO. The proportion of D2(High) receptors was also found to increase 2.5-fold using the method of competition between DA and [(3)H]domperidone. The overall 2.2-3.5-fold increase of DA D2(High) receptors may explain why amphetamine-sensitized animals are much more sensitive to DA agonists, even though the total density of D2 receptors may apparently be unchanged or even decreased.

Binding characteristics and sensitivity to endogenous dopamine of [11C]-(+)-PHNO, a new agonist radiotracer for imaging the high-affinity state of D2 receptors in vivo using positron emission tomography

[11C]-(+)-PHNO (4-propyl-9-hydroxynaphthoxazine) is a new agonist radioligand that provides a unique opportunity to measure the high-affinity states of the D2 receptors (D2-high) using positron emission tomography (PET). Here we report on the distribution, displaceablity, specificity and modeling of [11C]-(+)-PHNO and compare it with the well characterized antagonist D2 radioligand, [11C]raclopride, in cat. [11C]-(+)-PHNO displayed high uptake in striatum with a mean striatal binding potential (BP) of 3.95 +/- 0.85. Pre-treatment with specific D1 (SCH23390), D2 (raclopride, haloperidol) and D3 receptor (SB-277011) antagonists indicated that [11C]-(+)-PHNO binding in striatum is specific to D2 receptors. Within-subject comparisons showed that [11C]-(+)-PHNO BP in striatum was almost 2.5-fold higher than that measured with [11C]-(-)-NPA ([11C]-(-)-N-propyl-norapomorphine). Comparison of the dose-effect of amphetamine (0.1, 0.5 and 2 mg/kg; i.v.) showed that [11C]-(+)-PHNO was more sensitive to the dopamine releasing effect of amphetamine than [11C]raclopride. Amphetamine induced up to 83 +/- 4% inhibition of [11C]-(+)-PHNO BP and only up to 56 +/- 8% inhibition of [11C]raclopride BP. Scatchard analyses of [11C]-(+)-PHNO and [11C]raclopride bindings in two cats showed that the Bmax obtained with the agonist (29.6 and 32.9 pmol/mL) equalled that obtained with the antagonist (30.6 and 33.4 pmol/mL). The high penetration of [11C]-(+)-PHNO in brain, its high signal-to-noise ratio, its favorable in vivo kinetics and its high sensitivity to amphetamine shows that [11C]-(+)-PHNO has highly suitable characteristics for probing the D2-high with PET.

High-affinity states of human brain dopamine D2/3 receptors imaged by the agonist [11C]-(+)-PHNO

BACKGROUND

The high-affinity states of dopamine D2-receptors (D2(high)) are postulated to be functionally responsible for signal transduction. At present, no useful in vivo method exists to selectively measure D2(high) in humans, as current D2 radioligands for positron emission tomography (PET) are either not D2-selective or do not differentiate between D2 high- and low-affinity states.

METHODS

The D2-agonist (+)-PHNO [(+)4-propyl-9-hydroxynaphthoxazine] was labeled with carbon-11 and studied with PET. Eight [11C]-(+)-PHNO scans were acquired in four healthy volunteers.

RESULTS

We observed greatest [11C]-(+)-PHNO accumulation in caudate, putamen, and globus pallidus [binding potentials (BPs): 3.00 +/- .4, 3.10 +/- .2, and 4.17 +/- 1.2]. Small but detectable binding was identified in the substantia nigra/ventral tegmental area. Preliminary test-retest data in two subjects suggests BP-estimates to be reliable. Pre-treatment with haloperidol reduced BPs in regions showing specific binding with no detectable changes in cerebellum. Parallel imaging with [11C]-raclopride showed substantial differences in the globus pallidus.

CONCLUSIONS

[11C]-(+)-PHNO proved to be a D2/3-receptor agonist-radioligand with good brain uptake and favorable kinetics for PET in humans. [11C]-(+)-PHNO delineated D2/3-receptor rich brain regions with high signal-to-noise ratio. This is the first demonstration of a viable agonist-radioligand for D2 receptors in humans and opens the door for investigating D2(high) in health and disease.

In vivo characterization of the pharmacokinetics and pharmacological properties of [11C]-(+)-PHNO in rats using an intracerebral beta-sensitive system

This study reports on the binding kinetics and pharmacological characterization of [11C]-(+)-PHNO ((+)-4-propyl-3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol), a promising agonist radiotracer for in vivo evaluation of the D2-receptor. Its in vivo kinetics were monitored in rat striatum and cerebellum using a beta-sensitive Microprobe system. Control studies showed that [11C]-(+)-PHNO binding was reversible and reached a peak time equilibrium of specific binding in striatum 30 min after radiotracer injection. The binding potential (BP) calculated by the simplified reference tissue model was 3-fold higher than that measured with [11C]-(-)-NPA (2.14 +/- 0.50 vs. 0.66 +/- 0.01, respectively). In contrast, the methyl analog of (+)-PHNO, [11C]-(+)-MHNO, which displayed promising D2-agonist properties in vitro, showed no specific binding in the striatum in vivo. [11C]-(+)-PHNO binding was totally blocked by raclopride (1 mg/kg; i.v.) and 97% displaced by NPA (2 mg/kg; i.v.) suggesting that [11C]-(+)-PHNO was specific for the high affinity states of D2/D3-receptors. However, (+)-PHNO (1 mg/kg; i.v.) totally blocked and displaced [11C]-raclopride binding in striatum. Thus, (+)-PHNO at high concentrations might be able to bind to the low affinity states of D2/D3-receptors. After an amphetamine pretreatment (2 mg/kg; i.v.), a 69% decrease in BP value (P < 0.05) was observed for [11C]-(+)-PHNO indicating that its binding was highly sensitive to variations of endogenous DA. These results substantiate the use of [11C]-(+)-PHNO as an agonist radiotracer for D2-imaging. The sensitivity of its binding to competition with endogenous DA suggests an association with the subset of high affinity state D2-receptors.

Radiosynthesis and evaluation of [11C]-(+)-4-propyl-3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol as a potential radiotracer for in vivo imaging of the dopamine D2 high-affinity state with positron emission tomography

In vivo imaging of dopamine D2 receptors with agonist (as opposed to the more commonly employed antagonist) radiotracers could provide important information on the high-affinity (functional) state of the D2 receptor in illnesses such as schizophrenia, movement disorders, and addictions. We report here the radiosynthesis and evaluation of the potent D2 agonist (+)-4-propyl-3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol, (+)-3, labeled with carbon-11, as a potential radiotracer for imaging the high-affinity state of dopamine D2 receptors with positron emission tomography (PET). [(11)C]-(+)-3 was reliably synthesized in the quantities and at the specific activities and radiochemical purities required for human PET studies. Ex vivo biodistribution studies in rat brain demonstrated that [(11)C]-(+)-3 crossed the blood-brain barrier readily and had an appropriate regional brain distribution for a radiotracer that maps dopamine D2 receptors. The binding of [(11)C]-(+)-3 was saturable and demonstrated an excellent signal-to-noise ratio as measured by its striatum-to-cerebellum ratio of 5.6, 60 min postinjection. The binding was highly stereospecific, and blocking and displacement studies were consistent with selective and specific binding to the dopamine D2 receptors. Further, [(11)C]-(+)-3 showed marked and appropriate sensitivity to both increases and decreases in the levels of endogenous dopamine. Brain radioactive metabolite and physicochemical measurements are in full accord with the desired properties of a neuroreceptor imaging agent for PET. All of the above, coupled with the documented full D2 agonistic properties of (+)-3, strongly indicate that [(11)C]-(+)-3 is a leading candidate radiotracer for the imaging of the dopamine D2 high-affinity state using PET in human subjects.

Binding characteristics of high-affinity dopamine D2/D3 receptor agonists, 11C-PPHT and 11C-ZYY-339 in rodents and imaging in non-human primates by PET

We have evaluated the in vitro autoradiographic binding characteristics and in vivo brain distribution of two high-affinity dopamine D2/D3 receptor agonists, (+/-)-2-(N-phenethyl-N-1'-11C-propyl)amino-5-hydroxytetralin (11C-PPHT) and (+/-)-2-(N-cyclohexylethyl-N-1'-11C-propyl)amino-5-hydroxytetralin (11C-ZYY-339) in rodents and in monkeys using positron emission tomography (PET). In vitro autoradiograms in rat brain slices with (11)C-PPHT and 11C-ZYY-339 revealed binding to dopaminergic regions in the striata, which was substantially (>90%) displaced by 10 microM sulpiride. Striatal binding was also removed in the presence of 5-guanylylimidophosphate (Gpp(NH)p), indicative of binding of these radiotracers to the high-affinity (HA) state. The results of in vivo studies in rats exhibited binding of the two radiotracers to the striata (striata/cerebellum approached 2 in 30 min). The regional selectivity to the striata was reduced by preadministration of haloperidol. PET studies in male rhesus monkeys using an ECAT EXACT HR+ scanner indicated localization of 11C-PPHT and 11C-ZYY-339 in the striata and thalamus. Striata to cerebellum and thalamus to cerebellum ratios were low (1.5 and 1.3, respectively, at 30 min postinjection) for both 11C-PPHT and 11C-ZYY-339, apparently due to the slower nonspecific clearance from cerebellum. These findings with 11C-PPHT and 11C-ZYY-339 indicate the possibility of in vivo imaging of high-affinity state of dopamine D2/D3 receptors in both the striata and the thalamus.

Alcohol-withdrawn animals have a prolonged increase in dopamine D2high receptors, reversed by general anesthesia: relation to relapse?

The biochemical basis for alcohol addiction and relapse is not known. Although ethanol promotes the release of dopamine like other drugs of abuse, many unknown factors remain to be investigated concerning the biochemical abnormalities which persist after ethanol drinking and which contribute to alcohol relapse. Although ethanol withdrawal is associated with enhanced sensitivity to dopamine in animals and humans, only minor changes in the striatal density of dopamine D2 receptors have been found in humans, and animals show a small reduction in striatal D2 receptors. But how can dopamine-related functions be increased in ethanol withdrawal in the face of an unchanged or reduced density of dopamine D2 receptors? Considering that ethanol sensitizes rats to amphetamine, and that the high-affinity state of D2, or D2High, is markedly increased in striata from amphetamine-sensitized rats, we measured the density of D2High in striata from rats withdrawn from ethanol. These sites were elevated by 360% (7.2 pmol/g) for at least 8 days after stopping ethanol and returned to normal levels of 2 pmol/g after 2 weeks of ethanol withdrawal. In addition, 1 h of deep general anesthesia given 5 days into withdrawal resulted in a normal level of D2High within 24 h. Because the D2High states are the functional form of D2, their elevated density in ethanol withdrawal may be related to ethanol relapse in humans. General anesthesia may alleviate aspects of alcohol or amphetamine abuse or psychosis associated with elevated D2High.

Synthesis and biological evaluation of the binding of dopamine D2/D3 receptor agonist, (R,S)-5-hydroxy-2-(N-propyl-N-(5′-18F-fluoropentyl)aminotetralin (18F-5-OH-FPPAT) in rodents and nonhuman primates

We have synthesized a new fluorinated dopamine D2 receptor agonist, (R,S)-2-(N-propyl-N-5'-fluoropentyl)amino-5-hydroxytetralin (5-OH-FPPAT). The radiosynthesis of the fluorine-18 analog, (18)F-5-OH-FPPAT was achieved in decay corrected yields of 10 to 15% in specific activities of approx. 1.5 to 2 Ci/micromol. In vitro binding and autoradiographic studies of this new radiotracer have been investigated. Using rat striatal homogenate binding assay, 5-OH-FPPAT exhibited an affinity of IC(50) = 6.95 nM. The octanol-buffer partition coefficient, Log P was found to be 1.60. In vitro autoradiographs in rat brain slices with (18)F-5-OH-FPPAT revealed selective binding to the dopaminergic regions in the striata that was displaceable by sulpiride. This selective binding to the striata was also removed in the presence of the GTP analog, 5'-guanylylimidodiphosphate, indicative of predominant binding of (18)F-5-OH-FPPAT to the high-affinity state of the D2 receptor. In vivo regional distribution of (18)F-5-OH-FPPAT in rat brains revealed selective localization in the striata with striata/cortex ratio of 1.5 and striata/cerebellum ratio of 1.8 to 2.0. The binding of (18)F-5-OH-FPPAT in the striata was reduced upon pretreatment with the antagonist, risperidone and the agonist, PPHT. A PET study in rhesus monkeys showed selective localization of (18)F-5-OH-FPPAT in the striata and the ratio between striata and cerebellum approached approximately 2 at 40 min post-injection.

Anesthetics inhibit high-affinity states of dopamine D2 and other G-linked receptors

The high-affinity states of dopamine D2 and D3 receptors, serotonin 5HT-2A receptors, beta-2-adrenoceptors, alpha-1 and alpha-2 adrenoceptors, opiate receptors, and muscarinic receptors were inhibited by clinical concentrations of anesthetics, including isoflurane, halothane, chloral hydrate, ketamine, and ethanol. The inhibition occurred not only in vitro, but also in vivo in rats anesthetized with isoflurane, with the high-affinity states recovering at different rates. Because the high-affinity states of G-protein-linked receptors are physiologically functional, their general inhibition by clinical concentrations of anesthetics may underlie general anesthesia and may explain some of the side effects of anesthetics. Subanesthetic concentrations of the anesthetics, including ketamine, stimulated the incorporation of GTP into the cloned dopamine D2 receptors. It is possible that the classical stage 2 excitement phase which occurs with subanesthetic concentrations of general anesthetics and ketamine may be associated with this general stimulation of a variety of G-protein-linked receptors, as found in the present study, while the stage 3 level of surgical anesthesia may be associated with the inhibition of the high-affinity states of several receptors.

Kinetics of the uptake and distribution of the dopamine D(2,3) agonist (R)-N-[1-(11)C]n-propylnorapomorphine in brain of healthy and MPTP-treated Göttingen miniature pigs

The binding of radioligand agonists to dopamine receptors in living brain can be informative about the abundance of receptors which are coupled to intracellular second messenger systems. Therefore, we developed a radiosynthesis for the dopamine D(2,3) partial agonist (R)-N- [1-(11)C]n-propylnorapomorphine ([(11)C]NPA). The uptake of this tracer in brain of anesthetized Göttingen miniature pigs was recorded by positron emission tomography (PET) and analyzed by compartmental analysis using the metabolite-corrected arterial input, and using reference tissue methods. [(11)C]NPA had a blood-brain unidirectional clearance of approximately 0.35 ml g(-1) min(-1) and an apparent distribution volume of 6 ml g(-1) in cerebellum. The ligand had a binding potential of 1.5 in striatum, comparable to that reported previously for the receptor antagonist [(11)C]raclopride in the same strain of animals. Significant binding was detected in the hypophysis, thalamus, and medial forebrain bundle. The binding in striatum was of comparable magnitude in normal pigs and in pigs with a documented 50% dopamine depletion produced by MPTP-intoxication. Deep brain stimulation of the subthalamus was without conspicuous effect on the binding of [(11)C]NPA in vivo. Results of this preliminary study indicate that this tracer meets many requirements for assaying dopamine agonist binding sites by PET.

Specific binding of [(11)C]raclopride and N-[(3)H]propyl-norapomorphine to dopamine receptors in living mouse striatum: occupancy by endogenous dopamine and guanosine triphosphate-free G protein

According to the ternary complex model of G-protein linkage to receptors, agonists increase the affinity of the receptors for the G protein. The model predicts that an endogenous agonist's constant of inhibition toward an agonist radioligand is lower than that toward an antagonistic radioligand. The authors hypothesized that competition from endogenous dopamine in striatum of living mice should have a greater effect on the binding of the D2,3 partial agonist N-[3H]propylnorapomorphine than on the binding of the D2,3 antagonist [(11)C]raclopride. The baseline binding potential (pB(0)), defined as the ratio of bound-to-unbound ligand in the absence of competition from endogenous dopamine, was simultaneously measured in mouse striatum for [(11)C]raclopride (pB(0) = 8.5) and N-[(3)H]propylnorapomorphine (p'B(0) = 5.3). The baseline was established by treatment with alpha-methyl-p-tyrosine and reserpine. Relative to these baseline values in saline-treated mice, the pB of N-[(3)H]propylnorapomorphine decreased 52% whereas the pB of [(11)C]raclopride decreased only 30%, indicating greater sensitivity of the former compound to inhibition by synaptic dopamine. Furthermore, amphetamine decreased the pB of N-[(3)H]propylnorapomorphine to a greater extent (73%) than that of [(11)C]raclopride (43%) relative to the reserpine condition. For both radioligands, the occupancy of the dopamine receptors by endogenous agonist obeyed Michaelis-Menten kinetics over a wide range of agonist concentrations established by the pharmacologic treatments. The apparent inhibition constant of endogenous dopamine depended on the dopamine occupancy and decreased to a value 1.66 times greater for N-[(3)H]propylnorapomorphine than for [(11)C]raclopride at its highest occupancies. The results are consistent with the hypothesis that agonist binding is more sensitive than antagonist binding to competition from endogenous dopamine. Therefore, dopamine agonist ligands may be superior to benzamide antagonist ligands for the estimation of dopamine receptor occupancy by endogenous synaptic dopamine. The analysis of the effect of dopamine occupancy on the inhibition of N-[(3)H]propylnorapomorphine binding indicated a limited supply of G protein with a maximum ternary complex fraction of 40% of maximum agonist binding capacity.

In vitro and in vivo evaluation of the binding of the dopamine D2 receptor agonist (11)C-(R,S)-5-hydroxy-2-(di-n-propylamino)tetralin in rodents and nonhuman primate

The in vitro autoradiographic binding characteristics as well as in vivo imaging characteristics of a dopamine D2 receptor agonist, (R, S)-2-(N-propyl-N-1'-(11)C-propyl)amino-5-hydroxytetralin ((11)C-5-OH-DPAT), were studied. In (3)H-spiperone assays using rat striata, 5-OH-DPAT exhibited an affinity of IC(50) = 2.5 nM. In vitro autoradiographs in rat brain slices with (11)C-5-OH-DPAT revealed selective binding to the dopaminergic regions in the striata which was displaceable by sulpiride. Varying concentrations of dopamine displaced this selective binding of (11)C-5-OH-DPAT to the striata in rat brain slices. This selective binding to the striata was also removed in the presence of the GTP analog, 5'-guanylylimidodiphosphate, indicative of the binding of (11)C-5-OH-DPAT to the high-affinity state of the D2 receptor. Ex vivo autoradiographic study in rats exhibited selective binding of (11)C-5-OH-DPAT to the striata. A PET study in a rhesus monkey showed selective localization of (11)C-5-OH-DPAT in the striata and the ratio between striata and cerebellum approached approximately 2 at 40 min postinjection.

Radiosynthesis and in vitro evaluation of 2-(N-alkyl-N-1'-11C-propyl)amino-5-hydroxytetralin analogs as high affinity agonists for dopamine D-2 receptors

We have developed radiotracers based on agonists that may potentially allow the in vivo assessment of the high affinity (HA) state of the dopamine D-2 receptors. The population of HA state, which is likely the functional state of the receptor, may be altered in certain diseases. We carried out radiosyntheses and evaluated the binding affinities, lipophilicity, and in vitro autoradiographic binding characteristics of three dopamine D-2 receptor agonists: (+/-)-2-(N,N-dipropyl)amino-5-hydroxytetralin (5-OH-DPAT), (+/-)-2-(N-phenethyl-N-propyl)amino-5-hydroxytetralin (PPHT), and (+/-)-2-(N-cyclohexylethyl-N-propyl)amino-5-hydroxytetralin (ZYY-339). In 3H-spiperone assays using rat striata, ZYY-339 exhibited subnanomolar affinity for D-2 receptor sites (IC50 = 0.010 nM), PPHT was somewhat weaker (IC50 = 0.65 nM), and 5-OH-DPAT exhibited the weakest affinity (IC50 = 2.5 nM) of the three compounds. Radiosynthesis of these derivatives, 2-(N-propyl-N-1'-11C-propyl)amino-5-hydroxytetralin (11C-5-OH-DPAT), 2-(N-phenethyl-N-1'-11C-propyl)amino-5-hydroxytetralin (11C-PPHT), and 2-(N-cyclohexylethyl-N-1'-11C-propyl)amino-5-hydroxytetralin (11C-ZYY-339) was achieved by first synthesizing 11C-1-propionyl chloride and subsequent coupling with the appropriate secondary amine precursor to form the respective amide, which was then reduced to provide the desired tertiary amine products. The final products were obtained by reverse-phase high performance liquid chromatography (HPLC) purification in radiochemical yields of 5-10% after 60-75 min from the end of 11CO2 trapping and with specific activities in the range of 250-1,000 Ci/mmol. In vitro autoradiographs in rat brain slices with 11C-5-OH-DPAT, 11C-PPHT, and 11C-ZYY-339 revealed selective binding of the three radiotracers to the dopamine D-2 receptors in the striata.

Synthesis and autoradiographic localization of the dopamine D-1 agonists [11C]SKF 75670 and [11C]SKF 82957 as potential PET radioligands

The high affinity benzazepine D1 agonists SKF 75670 and SKF 82957 were labeled with 11C by N-[11C]methylation of SKF 38393 and SKF 81297, respectively, using [11C]methyl iodide in the presence of N-ethyldiisopropylamine. Both radiotracers were purified using a semi-preparative cation exchange HPLC column. Radiochemical yields of 20-75% were obtained (from [11C]methyl iodide, decay-corrected) with a synthesis time of 30-35 min from EOB. The specific activities were 700-2500 Ci/mmol (25.9-92.5 GBq/mumol) at EOS, and the radiochemical purities were > 99%. Autoradiographic studies showed selective binding for both tracers in rat brain regions rich in D1 receptors such as the caudate-putamen, nucleus accumbens, olfactory tubercles and substantia nigra. [11C]SKF 75670 and [11C]SKF 82957 are thus potential PET radioligands for the functional high-affinity state of D1 receptors.