Associative and limbic regions of monkey striatum express high levels of dopamine D3 receptors: effects of MPTP and dopamine agonist replacement therapies

The dopamine D3 receptor antagonist NGB 2904 increases spontaneous and amphetamine-stimulated locomotion

The dopamine D3 receptor is believed to play an important role in regulation of rodent locomotor behavior, and has been proposed as a therapeutic target for substance abuse, psychotic disorders, and Parkinson's disease. One model of dopamine D3 receptor function, based on studies utilizing D3 receptor knockout mice and D3 receptor-preferring agonists, proposes that D3 receptor stimulation is inhibitory to psychostimulant-induced locomotion, in opposition to the effects of concurrent dopamine D1 and D2 receptor stimulation. Recent progress in medicinal chemistry has led to the development of highly-selective dopamine D3 receptor antagonists. In order to extend our understanding of D3 dopamine receptor's behavioral functions, we determined the effects of the highly-selective dopamine D3 receptor antagonist NGB 2904 on amphetamine-stimulated and spontaneous locomotion in wild-type and dopamine D3 receptor knockout mice. NGB 2904 (26.0 microg/kg s.c.) enhanced amphetamine-stimulated locomotion in wild-type mice, but had no measurable effect in dopamine D3 receptor knockout mice. Of a range of doses (0.026 microg-1.0 mg/kg) given acutely or once daily for seven days, the highest dose of NGB 2904 (1.0 mg/kg) stimulated spontaneous locomotion in wild-type mice, but was without measurable effect in dopamine D3 receptor knockout mice. These behavioral effects of NGB 2904 contrast with those described for other highly D3 receptor-selective antagonists, which have not previously demonstrated an effect on spontaneous locomotor activity. In combination, these data add to the behavioral profile of this novel D3 receptor ligand and provide further support for a role for dopamine D3 receptor inhibitory function in the modulation of rodent locomotion.

Cognitive, Affective, and Psychiatric Features of Parkinson's Disease

Nonmotor symptoms, including cognitive deterioration and dementia, depression and apathy, and psychosis, are common in Parkinson's disease. Their presence is associated with a tremendous burden for the patient and family members. This article reviews the pathophysiololgy, risks, impact, major features, diagnosis, and treatment of these symptoms in Parkinson's disease.

Computational elucidation of the structural basis of ligand binding to the dopamine 3 receptor through docking and homology modeling

The dopamine subtype 3 receptor (D3) is a promising therapeutic target for the treatment of cocaine addiction, schizophrenia, Parkinson's disease, and other disorders, but little is known about the binding of ligands to D3 at the atomic level. In the present study, binding of 29 known ligands to the D3 receptor was modeled computationally using four D3 receptor models which were obtained from homology modeling. The predicted binding models were validated with experimental data from site-directed mutagenesis, structure-activity relationship studies, and affinity labeling studies. Docking scores calculated for these 29 ligands correlate reasonably well with the experimentally determined binding affinities. A pharmacophore model is proposed that describes the binding of ligands at a single D3 receptor binding site and offers insights into the binding of structurally diverse D3 ligands to this receptor.

Striatal neuroadaptation and rescue of locomotor deficit by l-dopa in aphakia mice, a model of Parkinson's disease

Preferential neurodegeneration of dopaminergic neurons in the ventral substantia nigra of the midbrain is a hallmark of Parkinson's disease. The homeobox transcription factor Pitx3 is similarly and selectively expressed in the same neurons. Pitx3 deficiency in a natural mouse mutant, the aphakia mouse, was correlated with the loss of these neurons and with a deficit in locomotor activity. We now report that the locomotor deficit of aphakia mice is established by 40 days of age and that it can be rescued by injection of l-dopa. We further show that downstream striatal correlates of the midbrain neuronal losses in aphakia mice, as assessed by dopamine transporter binding and expression of dopamine receptors, enkephalin, dynorphin and neurotensin, are highly similar to neuroadaptive responses observed following rapid neurodegeneration induced by neurotoxin administration in adult animals or following the progressive neurodegenerative processes as seen in Parkinson patients. Taken collectively, these data support the idea that the aphakia mice represent a selective model of dopaminergic deficiency that closely resembles the midbrain and striatal neuropathology associated with Parkinson's disease, and this suggests that these mice are a good model to assess therapies for Parkinson's disease as well as to understand the susceptibility of these neurons to neurodegeneration.

Lack of effect of reserpine-induced dopamine depletion on the binding of the dopamine-D3 selective radioligand, [11C]RGH-1756

The effect of reserpine induced dopamine depletion on the binding of the putative dopamine-D3 receptor ligand, [(11)C]RGH-1756 was examined in the monkey brain with positron emission tomography (PET). In a previous series of experiments, we have made an attempt to selectively label D3 receptors in the monkey brain using [(11)C]RGH-1756. Despite high selectivity and affinity of RGH-1756 in vitro, [(11)C]RGH-1756 displayed only low specific binding to D3 receptors in vivo. The aim of the present study was to examine whether low specific binding of [(11)C]RGH-1756 is caused by insufficient in vivo affinity of the ligand, or by high physiological occupancy of D3 receptors by endogenous dopamine (DA). PET experiments were performed in three monkeys under baseline conditions and after administration of reserpine (0.5 mg/kg). The results of the baseline measurements corresponded well to our earlier observations with [(11)C]RGH-1756. Reserpine caused no evident change in the regional distribution of [(11)C]RGH-1756 in the monkey brain, and no conspicuous regional accumulation of activity could be observed. After reserpine treatment there was no evident increase of specific binding and binding potential (BP) of [(11)C]RGH-1756. The lack of increased [(11)C]RGH-1756 binding after reserpine treatment indicates that competition with endogenous DA is not the predominant reason for the failure of the radioligand to label D3 receptors. Therefore, the low binding of [(11)C]RGH-1756 could largely be explained by the need for very high affinity of radioligand for D3 receptors in vivo, to obtain a suitable signal for the minute densities of D3 receptors expressed in the primate brain.

Non-uniform blockade of intrastriatal D2/D3 receptors by risperidone and amisulpride

RATIONALE

Atypical antipsychotic drugs have been shown to preferentially affect extrastriatal (mesolimbic) D2/D3 receptors over those within the striatum (nigrostriatal). The striatum does not contain exclusively nigrostriatal dopamine tracts, however. The caudate nucleus and ventral parts of the striatum primarily contain limbic and associative dopamine pathways more relevant to psychosis.

OBJECTIVES

We tested the hypothesis that two pharmacologically distinct atypical antipsychotic drugs, amisulpride and risperidone, would preferentially occupy of D2/D3 dopamine receptors in limbic and associative regions of the striatum.

METHODS

Eight amisulpride-treated patients, six risperidone-treated patients and six age- and sex-matched healthy controls were recruited. Dynamic SPET studies were performed after bolus injection of [123I]epidepride. Binding potential (BP) images were generated using a modified Logan method and aligned between subjects. Regions of interest (ROIs) were placed around head of caudate and putamen bilaterally on an average BP map derived from aligned control images. These ROIs were then applied user-independently to the BP maps for each subject to calculate BP for head of caudate and putamen. Mean occupancy of D2/D3 receptors in each ROI was determined by reference to the drug-free healthy volunteer group. Occupancy values for head of caudate and putamen were compared using paired Student's t test.

RESULTS

D2/D3 receptor occupancy was 42% in caudate and 31% in putamen for risperidone (t=5.9, df=11, p=0.0001) and 51% in caudate and 37% in putamen for amisulpride (t=11.1, df=15, p<0.0001).

CONCLUSIONS

Amisulpride and risperidone both show selective occupancy for limbic and associative D2/D3 receptors within the striatum.

Synthesis and in vitro binding of N-phenyl piperazine analogs as potential dopamine D3 receptor ligands

A series of N-(2-methoxyphenyl)piperazine and N-(2,3-dichlorophenyl)piperazine analogs were prepared and their affinities for dopamine D(2), D(3), and D(4) receptors were measured in vitro. Binding studies were also conducted to determine if the compounds bound to sigma (sigma(1) and sigma(2)) and serotonin (5-HT(1A), 5-HT(2A), 5-HT(2B), 5-HT(2C), 5-HT(3), 5-HT(4), 5-HT(5), 5-HT(6), and 5-HT(7)) receptors. The results of the current study revealed a number of compounds (12b, 12c, 12e, and 12g) having a high affinity for D(3) (K(i) at D(3) receptors ranging from 0.3 to 0.9 nM) versus D(2) (K(i) at D(2) receptors ranging from 40 to 53 nM) receptors and a log P value indicating that they should readily cross the blood brain barrier (log P = 2.6-3.5). All of the compounds evaluated in this study had a high affinity for serotonin 5-HT(1A) receptors. These compounds may be useful as probes for studying the behavioral pharmacology of the dopamine D(3) receptor, as well as lead compounds for the development of radiotracers for studying D(3) receptor regulation in vivo with the functional imaging technique, positron emission tomography.

L-DOPA reverses the MPTP-induced elevation of the arrestin2 and GRK6 expression and enhanced ERK activation in monkey brain

Dysregulation of dopamine receptors (DARs) is believed to contribute to Parkinson disease (PD) pathology. G protein-coupled receptors (GPCR) undergo desensitization via activation-dependent phosphorylation by G protein-coupled receptor kinases (GRKs) followed by arrestin binding. Using quantitative Western blotting, we detected profound differences in the expression of arrestin2 and GRKs among four experimental groups of nonhuman primates: (1) normal, (2) parkinsonian, (3) parkinsonian treated with levodopa without or (4) with dyskinesia. Arrestin2 and GRK6 expression was significantly elevated in the MPTP-lesioned group in most brain regions; GRK2 was increased in caudal caudate and internal globus pallidus. Neither levodopa-treated group differed significantly from control. The only dyskinesia-specific change was an elevation of GRK3 in the ventral striatum of the dyskinetic group. Changes in arrestin and GRK expression in the MPTP group were accompanied by enhanced ERK activation and elevated total ERK expression, which were also reversed by L-DOPA. The data suggest the involvement of arrestins and GRKs in Parkinson disease pathology and the effects of levodopa treatment.

Mice lacking dopamine D2 and D3 receptors exhibit differential activation of prefrontal cortical neurons during tasks requiring attention

Mice lacking dopamine D2 and D3 receptors and wildtype littermates were tested in a two-choice perceptual discrimination test that is dependent upon optimal functioning of the prefrontal cortex. Both mutants showed no deficits in attending to the perceptual stimuli or in shifting attention between stimulus dimensions. However, the performance of both mutants differed from the wildtype in different test phases. D2 mutants exhibited significant impairment in the first compound discrimination, indicating deficits in the initial acquisition of the task-governing rules. In contrast, D3 mutants performed significantly better in a set-shifting phase that required reversal learning. The higher response accuracy of D3 mutants was also accompanied by significantly increased response latency. A stereological assessment of test-induced expression of the c-fos gene in neurons of the anterior cingulate and prelimbic/infralimbic cortices revealed highest activation in D3 mutants, intermediate activation in wildtype and lowest activation in D2 mutants, indicating that response accuracy in the cognitive test correlates with the magnitude of prefrontal cortical activation regardless of which test phases revealed different performances. The study illustrates that dopamine differentially modulates prefrontal cortical activity during tasks requiring attention depending upon the type of D2-like receptor that is activated.

Effect of the D3 dopamine receptor partial agonist BP897 [N-[4-(4-(2-methoxyphenyl)piperazinyl)butyl]-2-naphthamide] on L-3,4-dihydroxyphenylalanine-induced dyskinesias and parkinsonism in squirrel monkeys

Although l-3,4-dihydroxyphenylalanine (L-dopa) is one of the most effective therapies for Parkinson's disease, continued treatment may result in excessive involuntary movements known as L-dopa-induced dyskinesias (LIDs). Because LIDs can become dose-limiting, there is great interest in finding ways to ameliorate or prevent this troubling side effect of L-dopa therapy. It was recently reported that the D3 receptor partial agonist BP897 [N-[4-(4-(2-methoxyphenyl)piperazinyl)butyl]-2-naphthamide] reduces LIDs without diminishing antiparkinsonian effects of L-dopa in macaques. In the present study, we tested the effects of BP897 on LIDs in squirrel monkeys, a nonhuman primate particularly prone to dyskinesias. Parkinsonism was induced using 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Animals were then gavaged with L-dopa/carbidopa (7.5 or 15 mg/kg/dose) without and with BP897. The effects of BP897 treatment were evaluated on several components of LIDs, including time course, peak dyskinesias, and area under the curve (AUC), a measure that encompasses both peak and duration of the response. Analyses of the time course and overall dyskinetic response (AUC) showed that BP897 significantly reduced LIDs but at the expense of the antiparkinsonian effect of L-dopa. BP897 had no significant effect on peak dyskinesias. Correlation studies showed that beneficial effects of BP897 on dyskinesias were linked to a decline in the antiparkinsonian action of L-dopa. Analyses of a subgroup of animals with mild/moderate parkinsonism yielded comparable results. Thus, in squirrel monkeys in contrast to macaques, BP897 fails to exert an antidyskinetic effect without diminishing the antiparkinsonian effects of L-dopa. These results suggest that BP897 may be less effective than originally anticipated for treating LIDs in Parkinson's disease.

Increases in dopamine D3 receptor binding in rats receiving a cocaine challenge at various time points after cocaine self-administration: implications for cocaine-seeking behavior

Previous research suggests that cocaine dysregulates dopamine D3 receptors. The present study examined the time course of changes in dopamine D3 receptor binding after terminating a cocaine self-administration regimen. [125I]-7-hydroxy-2-[N-propyl-N-(3'-iodo-2'-propenyl)-amino]-tetralin was used to label dopamine D3 receptors in rats that had undergone testing for cocaine-seeking behavior reinstated by a cocaine priming injection (15 mg/kg, i.p.; the behavior results have been previously published), and were killed 24 h after the test at time points that were either 2, 8, or 31-32 days after their last cocaine self-administration session. The results indicated a time-dependent increase in D3 receptor binding relative to controls that received saline yoked to the delivery of cocaine in an experimental animal. Specifically, there was no significant change in D3 receptor binding in cocaine-experienced rats killed at the 2- or 8-day time points relative to controls, but there was an increase in D3 receptor binding in the nucleus accumbens core and ventral caudate-putamen in rats killed at the 31- to 32-day time point. In a subsequent experiment, we replicated the increase in D3 receptor binding in rats that underwent a less extensive self-administration regimen, then were tested for cocaine-primed reinstatement of cocaine-seeking behavior, and then were killed 24 h later at a time point of 22 days after their last self-administration session. Furthermore, the increase in binding was attenuated by repeated 7-hydroxy-N,N-di-n-propyl-2-aminotetralin administration (1 mg/kg/day, s.c. for 14 days), a regimen that also reduces cocaine-seeking behavior in animals when tested in a nondrug state. Collectively, the findings suggest that regulatory responses of D3 receptors may be functionally related to changes in propensity for cocaine-seeking behavior.

Selective blockade of D3 dopamine receptors enhances the anti-parkinsonian properties of ropinirole and levodopa in the MPTP-lesioned primate

To date, the lack of highly selective antagonists at the dopamine D(3) receptor has hampered clarification of their involvement in the actions of currently used therapies in Parkinson's disease. However, the novel benzopyranopyrrole, S33084, displays greater than 100-fold selectivity as an antagonist for D(3) versus D(2) receptors and all other sites tested. S33084 was administered to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmosets previously primed with levodopa to elicit dyskinesia. Administered alone, S33084 exerted a modest, but significant, anti-parkinsonian effect without provoking dyskinesia. At low D(3)-selective doses (0.16 and 0.64 mg/kg), S33084 potentiated, though to different extents and in qualitatively different ways, the anti-parkinsonian actions of both ropinirole and levodopa. At these doses, S33084 did not significantly modify levodopa-induced or ropinirole-induced dyskinesia. These data suggest that ropinirole and levodopa do not exert their anti-parkinsonian or pro-dyskinetic actions via D(3) receptor stimulation. Indeed, stimulation of D(3) receptors may be detrimental to the anti-parkinsonian properties of D(2)/D(3) agonists. Selectivity for stimulation of D(2), over D(3), receptors may therefore be a beneficial property of dopamine receptor agonists in management of motor symptoms of Parkinson's disease patients with established dyskinesia.

Relevance of the MPTP primate model in the study of dyskinesia priming mechanisms

For nearly 20 years, the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) primate model has allowed great strides to be made in our understanding of the maladaptive changes underlying the levodopa-related motor response complications occurring in most parkinsonian patients. Studies indicate that sustained dopamine D2 receptor occupancy can prevent and reverse existing dyskinesias. Recent experiments in levodopa-treated MPTP animals, co-administered either a threshold dose of cabergoline or a glutamate NMDA NR2B-selective antagonist (CI-1041), have afforded protection against dyskinesia, perhaps through presynaptic inhibition of glutamate release and blockade of supersensitive postsynaptic NMDA receptors in the striatum, respectively. Some of the biochemical events that have correlated with dyskinesias, namely upregulated GABA(A) receptors in the internal pallidum, rise in pre-proenkephalin-A gene expression in the striatum, and upregulated striatal glutamate ionotropic receptors and adenosine A(2a) receptors, may be counteracted by these preventive strategies.

S32504, a novel naphtoxazine agonist at dopamine D3/D2 receptors: II. Actions in rodent, primate, and cellular models of antiparkinsonian activity in comparison to ropinirole

These studies evaluated the potential antiparkinsonian properties of the novel dopamine D(3)/D(2) receptor agonist S32504 [(+)-trans-3,4,4a,5,6, 10b-hexahydro-9-carbamoyl-4-propyl-2H-naphth[1,2-b]-1,4-oxazine] in comparison with those of the clinically employed agonist ropinirole. In rats with a unilateral, 6-hydroxydopamine lesion of the substantia nigra, S32504 (0.0025-0.04 mg/kg, s.c.) more potently elicited contralateral rotation than S32601 [(-)-trans-3,4,4a,5,6, 10b-hexahydro-9-carbamoyl-4-propyl-2H-naphth-[1,2-b]-1,4-oxazine (its less active enantiomer)], ropinirole, and l-3,4-dihydroxyphenylalanine (l-DOPA). Rotation elicited by S32504 was blocked by the D(2)/D(3) receptor antagonists haloperidol and raclopride and by the D(2) antagonist L741,626 [4-(4-chlorophenyl)-1-(1H-indol-3-ylmethyl)piperidin-4-ol], but not by the D(3) antagonist S33084 [(3aR,9bS)-N-[4-(8-cyano-1,3a,4,9b-tetrahydro-3H-benzopyrano[3,4-c]pyrrole-2-yl)-butyl]-(4-phenyl)benzamide]. As assessed by dialysis in both lesioned and nonlesioned animals, S32504 (0.04-2.5 mg/kg, s.c.) reduced striatal levels of acetylcholine. This effect was blocked by raclopride, haloperidol, and L741,626 but not S33084. In rats treated with reserpine, hypolocomotion was reversed by S32504 and, less potently, by ropinirole. In "unprimed" marmosets treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, both s.c. (0.01-0.04 mg/kg) and p.o. (0.04-1.25 mg/kg) administration of S32504 dose-dependently and rapidly (within 10 min) increased locomotor activity and reduced disability. Furthermore, S32504 dose-dependently reversed bradykinesia and improved posture in "L-DOPA-primed" animals, whereas eliciting less pronounced dyskinesia than l-DOPA. Finally, in terminally differentiated SH-SY5Y cells presenting a dopaminergic phenotype, S32504, but not S32601, abrogated the neurotoxic effects of 1-methyl-4-phenylpyridinium, an action inhibited by raclopride and S33084 but not L741,626. Ropinirole was weakly neuroprotective in this model. In conclusion, S32504 displays potent and stereospecific activity in rodent, primate, and cellular models of antiparkinsonian properties. Although activation of D(2) receptors is crucial to the motor actions of S32504, engagement of D(3) receptors contributes to its neuroprotective properties.

Loss of D 3 receptors in the zitter mutant rat is not reversed by l -dopa treatment

In Parkinson's disease (PD) and animal models of parkinsonism the destruction of nigrostriatal (NSB) system results in a marked loss of the dopamine D(3) receptor and mRNA in the islands of Calleja (ICj) and the nucleus accumbens shell (NAS). In animal models, it has been reported that both measures are elevated by repeated intermittent administration of L-dopa. However, a large proportion of PD cases are resistant to L-dopa-induced elevation of D(3) receptor number. The zitter mutant (Zi/Zi) rat replicates the slow progressive degeneration of the NSB observed in PD and also exhibits a loss of D(3) receptor number in the NAS or ICj. To test if this could be reversed with subchronic L-dopa treatment, injections of carbidopa (10 mg/kg i.p.) were followed an hour later with injection of L-dopa (100 mg/kg i.p.) twice a day for 10 days. In control Sprague-Dawley (SD) and zitter heterozygote (Zi/-) rats that do not show a loss of D(3) receptors with vehicle treatment, L-dopa produced no change in D(3) receptor number or in DA terminal density as measured by dopamine transporter (DAT) binding and tyrosine hydroxylase immunoautoradiography (TH-IR). There was a marked loss of DAT and TH-IR in caudate-putamen (CPu) and NA, as well as D(3) receptors in NAS and ICj in Zi/Zi rats but no further change with L-dopa treatment. To determine if the resistance to L-dopa-induced increase in D(3) receptor was due to a deficiency in expression of cortical BDNF or its receptor, TrkB, in CPu and NAS, we examined BDNF mRNA by ISHH in frontal cortex and TrkB mRNA in frontal cortex, CPu, and NA. The loss of the NSB in the Zi/Zi did not alter levels of BDNF or TrkB mRNA, nor did L-dopa administration alter levels BDNF or TrkB mRNA. Thus, unlike in 6-hydroxydopamine-treated rats, in Zi/Zi rats administered L-dopa does not reverse the loss of BDNF mRNA or lead to an elevation of D(3) receptor number.

Methamphetamine-induced loss of striatal dopamine innervation in BDNF heterozygote mice does not further reduce D3 receptor concentrations

Depletion of dopamine (DA) reduces D(3) receptor number, but D(3) receptor expression is also regulated by brain-derived neurotrophic factor (BDNF). We took advantage of transgenic heterozygous BDNF mutant mice (+/-) to determine if reduced BDNF and loss of DA fibers produced by methamphetamine were additive in their impact on D(3) receptor number. We assessed selective markers of the dopaminergic system including caudate-putamen DA concentrations and quantitative autoradiographic measurement of tyrosine hydroxylase (TH) levels, DA transporter (DAT), and DA D(3) receptor binding between vehicle and methamphetamine-treated BDNF +/- and their wildtype (WT) littermate control mice. Caudate-putamen DA concentrations, TH and DAT levels were significantly reduced following methamphetamine treatment in both WT and BDNF +/- mice. The extent of methamphetamine-induced reduction in TH and DAT was greater for the WT than BDNF +/- mice and DAT levels were also decreased to a greater extent in nucleus accumbens of WT as compared to BDNF +/- mice. Lower D(3) receptor existed in caudate-putamen and nucleus accumbens in BDNF +/- mice and these differences were not affected by methamphetamine treatment. Taken together, these results not only substantiate the importance of BDNF in controlling D(3) receptor expression, but also indicate that a methamphetamine-induced depletion of DA fibers fails to produce an additive effect with lowered BDNF for control of D(3) receptor expression. In addition, the reduction of D(3) receptor expression is associated with a decreased neurotoxic response to methamphetamine in BDNF +/- mice.

Conformationally-flexible benzamide analogues as dopamine D3 and σ2 receptor ligands

A series of conformationally-flexible analogues was prepared and their affinities for D2-like dopamine (D2, D3 and D4) were determined using in vitro radioligand binding assays. The results of this structure-activity relationship study identified one compound, 15, that bound with high affinity (K(i) value=2nM) and moderate selectivity (30-fold) for D3 compared to D2 receptors. In addition, this series of compounds were also tested for affinity at sigma1 and sigma2 receptors. We evaluated the affinity of these dopaminergic compounds at sigma receptors because (a) several antipsychotic drugs, which are high affinity antagonists at dopamine D2-like receptors, also bind to sigma receptors and (b) sigma receptors are expressed ubiquitously and at high levels (picomoles per mg proteins). It was observed that a number of analogues displayed high affinity and excellent selectivity for sigma2 versus sigma1 receptors. Consequently, these novel compounds may be useful for characterizing the functional role of sigma2 receptors and for imaging the sigma2 receptor status of tumors in vivo with PET.

Dopamine and glutamate dysfunctions in schizophrenia: role of the dopamine D3 receptor

Symptoms of schizophrenia are improved by dopamine antagonists and exacerbated by dopamine-releasing agents, suggesting hyperactivity of dopamine. However, chronic blockade of glutamate neurotransmission by antagonists at the N-methyl-D-aspartate (NMDA) receptor subtype produces a pathophysiological state resembling schizophrenia. A link between cortical glutamate/NMDA deficiency and subcortical dopamine hyperactivity, particularly in the mesolimbic pathway, has been hypothesized in schizophrenia. Here we show that hyperactivity produced by NMDA receptor blockade is dependent upon stimulation of the dopamine D3 receptor subtype. Since D3 receptor antagonists and antipsychotics produced very similar effects, our results add to the growing evidence suggesting that D3 receptor blockade might produce antipsychotic effects.

Behavioral effects of dopamine agonists and antagonists in MPTP-lesioned D3 receptor knockout mice

To test the modulatory role of D(3) receptors in normal and dopamine-depleted mice, D(3) receptor KO mice and wild-type (WT) littermates were administered saline, L-dopa/carbidopa (20/2 mg/kg ip), a preferential D(3)>D(2) agonist S32504, a D1+D(2)/D(3) agonist apomorphine, a selective D(3) antagonist S33084, or apomorphine with S33084 prior to and after administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). We monitored lines crossed in a 55-min session, average number of rears, and average number of grooming bouts. MPTP treatment produced equivalent 70% losses of dopamine fibers in the caudate putamen (CPu) and nucleus accumbens (NAC) of WT and D(3) KO mice as compared to their control (vehicle injected) counterparts. D(3) receptors were absent in KO mice, and the number of D(3) receptors was unaffected by MPTP-induced loss of DA terminals in WT mice. The results support a lack of involvement of the D(3) receptor for D1:D2 receptor-mediated behavioral activity (synergy). First, S32504 inhibited all behaviors and to a similar degree in D(3) KO and WT mice. Second, S33084 at the higher concentration increased number of lines crossed in response to high dose apomorphine in both D(3) KO and WT mice. Third, in nonlesioned mice, apomorphine-induced gnawing stereotypies were inhibited by S33084 in both D(3) KO and WT mice. Interestingly, the inhibition of apomorphine-induced gnawing was not apparent in MPTP-lesioned mice, and this stereotypy was elevated in D(3) KO-MPTP-lesioned mice. Thus, the suppressive effects of S32504 could be via D2 autoreceptor inhibition of DA release, and D2 receptor blockade by S33084 leads to release of that inhibition. This may be more apparent in MPTP-lesioned partially DA denervated mice.

Potent activation of dopamine D3/D2 heterodimers by the antiparkinsonian agents, S32504, pramipexole and ropinirole

Recombinant, human dopamine D3 and D2 receptors form functional heterodimers upon co-expression in COS-7 cells. Herein, actions of the antiparkinsonian agents, S32504, ropinirole and pramipexole, at D3/D2L heterodimers were compared to their effects at the respective monomers and at split, chimeric D3trunk/D2tail and D2trunk/D3tail receptors: the trunk incorporated transmembrane domains (TDs) I-V and the tail TDs VI and VII. In binding assays with the antagonist [3H]nemonapride, all agonists were potent ligands of D3 receptors showing, respectively, 100-, 18- and 56-fold lower affinity at D2L receptors, mimicking the selective D3 receptor antagonist, S33084 (100-fold). At D3trunk/D2tail receptors, except for ropinirole, all drugs showed lower affinities than at D3 sites, whereas for D2trunk/D3tail receptors, affinities of all drugs were higher than at D2L sites. The proportion of high affinity binding sites recognized by S32504, pramipexole and ropinirole in membranes derived from cells co-expressing D3 and D2L sites was higher than in an equivalent mixture of membranes from cells expressing D3 or D2L sites, consistent with the promotion of heterodimer formation. In contrast, the percentage of high and low affinity sites (biphasic isotherms) recognized by S33084 was identical. Functional actions were determined by co-transfection of a chimeric adenylyl cyclase (AC)-V/VI insensitive to D3 receptors. Accordingly, D3 receptor-transfected cells were irresponsive whereas, in D2L receptor-transfected cells, agonists suppressed forskolin-stimulated cAMP production with modest potencies. In cells co-transfected with D3 and D2L receptors, S32504, ropinirole and pramipexole potently suppressed AC-V/VI with EC50s 33-, 19- and 11-fold lower than at D2L receptors, respectively. S32504 also suppressed AC-V/VI activity at split D3trunk/D2tail and D2trunk/D3tail chimeras transfected into COS-7 cells. In conclusion, antiparkinson agents behave as potent agonists at D3/D2'heterodimers', though any role in their actions in vivo remains to be demonstrated.

Attenuation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxicity by the novel selective dopamine D3-receptor partial agonist FAUC 329 predominantly in the nucleus accumbens of mice

We previously synthesised a novel dopamine (DA) partial agonist FAUC 329 with high affinity and selectivity for the DA D(3) receptor. This is the first in vivo study to investigate the protective effects of FAUC 329 in a MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) mouse model of Parkinson's disease. Adult male C57bl/6 mice were injected with FAUC 329 (0, 0.1, 0.5, 0.75, or 1 mg/kg) 30 min before MPTP (2 x 30 mg/kg, 4 hr apart). One week later, accumbal and striatal tissue was processed for DA and metabolite HPLC determination as well as immunohistochemical analysis of DA transporter positive neurons in the substantia nigra pars compacta and ventral tegmental area was carried out. FAUC 329 showed a significant attenuation of MPTP-induced DA reduction in the nucleus accumbens (0.5, 0.75 and 1 mg/kg) in a dose-dependent manner. FAUC 329 (0.75 mg/kg) partly protected against DA depletion in the dorsal striatum as well as protected against loss of DA transporter immunoreactivity in the substantia nigra pars compacta. The highest dose of FAUC 329 (1 mg/kg), however, showed a non-significant tendency to augment the MPTP-induced striatal DA reduction. The protective effect of FAUC 329 against MPTP-induced DA depletion was most pronounced in the nucleus accumbens and appears to be linked to the preferential abundance of D(3) receptors in this region. Targeting the mesolimbic DA system may have implications for improvement of impaired motor behaviour and particularly non-motor functions related to the nucleus accumbens.

Effects of oligonucleotide antisense to dopamine D3 receptor mRNA in a rodent model of behavioural sensitization to levodopa

Levodopa-induced dyskinesias are abnormal involuntary movements that develop as a side-effect of long-term treatment with levodopa for Parkinson's disease. The mechanisms underlying such effects are unclear but may include abnormal stimulation of dopamine D(3) receptors. Elevations in dopamine D(3) receptor mRNA and binding are seen in the denervated striatum of hemiparkinsonian rats treated chronically with levodopa, and these changes correlate well with behavioural sensitization in this model. Further investigation of dopamine D(3) receptor involvement in levodopa-induced dyskinesias is hampered by the lack of appropriately selective ligands for this receptor. Here, in vivo administration of an antisense oligonucleotide designed to reduce striatal dopamine D(3) receptor expression provides a level of specificity not available through traditional pharmacological approaches. Following chronic treatment with levodopa, hemiparkinsonian rats received intrastriatal infusion of oligonucleotide antisense to dopamine D(3) receptor mRNA for 5 days. Antisense treatment effectively and selectively reduced striatal dopamine D(3) receptor binding and blocked behavioural sensitization to the effects of repeated levodopa. These findings confirm the importance of the D3 receptor in the expression of behavioural sensitization to levodopa in animals with dopaminergic denervation and contribute to our limited understanding of the functional significance of this receptor. In that sensitization to the effects of repeated levodopa in this setting may be analogous to medication-induced dyskinesias in humans, our findings furthermore suggest that drugs which block D(3) function may be helpful in the treatment of dyskinesias, without necessarily exacerbating Parkinsonism.

Attenuation of levodopa-induced dyskinesia by normalizing dopamine D3 receptor function

In monkeys rendered parkinsonian with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), expression of the dopamine D3 receptor was decreased. However, levodopa-induced dyskinesia (LID), similar to the debilitating and pharmacoresistant involuntary movements elicited after long-term treatment with levodopa in patients with Parkinson disease (PD), was associated with overexpression of this receptor. Administration of a D3 receptor-selective partial agonist strongly attenuated levodopa-induced dyskinesia, but left unaffected the therapeutic effect of levodopa. In contrast, attenuation of dyskinesia by D3 receptor antagonists was accompanied by the reappearance of PD-like symptoms. These results indicated that the D3 receptor participated in both dyskinesia and the therapeutic action of levodopa, and that partial agonists may normalize D3 receptor function and correct side effects of levodopa therapy in patients with PD.

Brain-derived neurotrophic factor controls dopamine D3 receptor expression: implications for neurodevelopmental psychiatric disorders

Brain-derived neurotrophic factor (BDNF) belongs to a family of proteins related to nerve growth factor, which are responsible for neuron proliferation, survival and differentiation. A more diverse role for BDNF as a neuronal extracellular transmitter has, nevertheless, been proposed. The dopamine D(3) receptor has been implicated in neuropsychiatric disorders including schizophrenia, drug addiction, depression and Parkinson's disease. Its expression during development and in adulthood is highly dependent on dopaminergic innervation. Here we show that BDNF synthesized by dopamine neurons is responsible for the appearance of the D(3) receptor during development and maintains D(3) receptor expression in adults. Moreover, BDNF triggers D(3) receptor overexpression and behavioral sensitization to levodopa in denervated animals. These results suggest that BDNF, by controlling the expression of specific genes such as the D(3) receptor gene, may be an important factor in neurodevelopmental psychiatric diseases.

Functional imaging studies of dopamine system and cognition in normal aging and Parkinson's disease

Modern functional imaging methods, such as positron emission tomography (PET) and single photon emission computed tomography (SPECT), provide non-invasive, quantitative tools for the direct measurement of neurotransmitter function in the living human brain. The dopamine system has been of key interest; first, because it has a prominant role in several cognitive and motor processes, and secondly because the tracers currently available for the dopamine system enable an effective investigation of various pre, post- and intra-synaptic processes. Recent functional imaging findings indicate that certain cognitive deficits associated with both normal aging and Parkinson's disease are modulated by changes in the brain dopamine system. This review covers the literature related to age-associated phenomena in the dopamine system studied with in vivo imaging. In particular, the focus is on describing and discussing the relationships between aging, cognition and the dopaminergic system in healthy subjects and in patients with Parkinson's disease.

Clinical presentation of the "depression-executive dysfunction syndrome" of late life

It has been proposed that a "depression-executive dysfunction (DED) syndrome" occurs in late life. This assertion was based on clinical, neuropathological, and neuroimaging findings suggesting that frontostriatal dysfunctions contribute to the development of both depression and executive dysfunction and influence the course of depression. The authors describe the clinical presentation of DED and its relationship to disability, studying 126 elderly subjects with major depression and evaluating depressive symptoms, cognitive functioning, disability, and personality dimensions. Patients with the DED syndrome had reduced fluency, impaired visual naming, paranoia, loss of interest in activities, and psychomotor retardation, but showed a rather mild vegetative syndrome. Depressive symptomatology, and especially psychomotor retardation and loss of interest in activities, contributed to disability in DED patients, whereas paranoia was associated with disability independently of executive dysfunction. These findings may aid clinicians in identifying patients needing vigilant follow-up, because depression with executive dysfunction was found to be associated with disability, poor treatment response, relapse, and recurrence.

Ventral striatal D(3) receptors and Parkinson's Disease

Antiparkinsonian drugs are thought to act largely through the D2 receptor family that includes the D(2) and D(3) receptors. D(2) and D(3) receptors exhibit both complementary and overlapping expression at the macro and cellular level. The D(3) receptor appears to be a primary target of the mesolimbic dopamine system, is highly enriched in expression within the "limbic" striato-pallidal-thalamic loop, and is recognized as being regulated by dopaminergic activity in distinctly different ways from the D(2) receptor. In Parkinson's Disease it has been determined that loss of dopaminergic innervation results in elevation of the D(2) receptor but reduced levels of the D(3) receptor. In many late-stage Parkinson's Disease patients there is a loss of antiparkinsonian response to L-dopa and other antiparkinsonian drugs that is often correlated with clinical signs for dementia. We have determined that the reduction of D(3) receptor, and not that of the D(2) receptor, is associated with the loss of response to L-dopa and other antiparkinsonian drugs. The reduction of D(3) receptor is also related to the presence of dementia. An elevation of D(3) receptors was evident in those Parkinson's Disease cases with continued good response to L-dopa. Thus, we believe that reduced D(3) receptor number is correlated with certain subgroups of Parkinson's Disease and may also be related to a further diminishment in the mesolimbic DA system.

Changes in striatal dopamine D3 receptor regulation during expression of and recovery from MPTP-induced parkinsonism

Striatal dopamine (DA) D3 receptor density (measured by quantitative receptor autoradiography) and mRNA expression (measured by reverse transcriptase-polymerase chain reaction) were analyzed in cats symptomatic for and recovered from 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonism. In symptomatic cats, D3 receptor density was significantly decreased in all regions of the caudate nucleus (CD) (66--77%), the nucleus accumbens (NACC) (52--83%) and the islands of calleja (IC) (67%), all of which returned to normal values in recovered cats. In contrast, D3 receptor mRNA expression was slightly elevated in symptomatic cats, and significantly increased above normal in recovered cats (45% increase in the CD and 91% in the NACC). Thus, reduction of parkinsonian signs was related to normalization of striatal D3 receptor number. These alterations in D3 receptor expression may play an important role in the recovery process observed in this model of parkinsonism.

D2 but not D3 receptors are elevated after 9 or 11 months chronic haloperidol treatment: influence of withdrawal period

Previous postmortem studies have identified divergent alterations in D2 and D3 receptors in schizophrenia but those results cannot be interpreted without further understanding of whether antipsychotic regulation of the D3 receptor is different from that of the D2 receptor. Depot parenteral administration of haloperidol decanoate was utilized to achieve consistent high levels in rat brain for 9 months with 2-month withdrawal or 11 months with 48-h withdrawal and compared to vehicle control and acute haloperidol (48-h) treatment groups. Autoradiographic means for measuring levels of D2 ([(3)H]-spiperone) and D3 receptors ([(125)I]trans 7-OH-PIPAT) and of D3 mRNA by in situ hybridization histochemistry in rat caudate-putamen, nucleus accumbens, islands of Calleja, and olfactory tubercle determined that there were significant group differences for regulation of D2 receptor. Chronic haloperidol for 9 or 11 months elevated D2 but not D3 receptors or D3 mRNA in all regions measured. Acute haloperidol treatment had no significant effects for any measure. Treatment for 9 months with a 2-month withdrawal resulted in a persistent increase in D2 receptors that was greater than that observed in the 11 months with 48-h withdrawal. This effect was most noticeable in the olfactory tubercle. These data confirm previous findings that short- or long-term haloperidol treatment leads to elevations in D2 but not D3 receptors or D3 mRNA, and long-term withdrawal from chronic haloperidol does not lead to elevations in D3 receptors or D3 mRNA. This suggests that an elevation in D3 receptors identified at postmortem in schizophrenics withdrawn from antipsychotics is not the result of the previous drug history [Gurevich et al. (1997) Arch Gen Psychiatry 54:225-232].

Dopamine D3 receptor as a therapeutic target for antipsychotic and antiparkinsonian drugs

The cloning of the gene for the D3 receptor and subsequent identification of its distribution in brain and pharmacology allowed for serious consideration of the possibility that it might be a target for drugs used to treat schizophrenia and Parkinson's disease (PD). That is because it is highly expressed in limbic regions of the brain, exhibits low expression in motor divisions, and has pharmacologic similarity to the D2 receptor. Thus, antipsychotics that were presumed to block D2 receptors also had high affinity for the D3 receptor. Dopamine agonists used to treat the clinical symptoms of PD also have high affinity for the D3 receptor, and two D3 receptor-preferring agonists were found to be effective for treatment of PD. Many compounds achieving high potency and selectivity are now available, but few have reached clinical testing. Recent findings with respect to the anatomy of this receptor in human brain, altered expression in schizophrenia and PD, and biological models to study its function support the proposal that it is a target for development of drugs to alleviate symptoms in neuropsychiatric and neurologic disorders. Because of distinct aspects of regulation of the D3 receptor, it represents a unique target for therapeutic intervention in schizophrenia without high potential for unintended side effects such as tardive dyskinesia. It may also be that D3 receptor agonists can provide neuroprotective effects in PD and can modify clinical symptoms that D2 receptor-preferring agonists cannot provide.

Expression of D(3) receptor messenger RNA and binding sites in monkey striatum and substantia nigra after nigrostriatal degeneration: effect of levodopa treatment

D(3) receptors are prominently localized in the primate caudate-putamen, and D(3) receptor agonist properties may offer an advantage in Parkinson's disease therapy. In the present experiments, we investigated the relationship between D(3) receptor mRNA, D(3) receptor sites and the dopamine transporter in monkey basal ganglia by comparing their distribution in the brain of control and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys (Samirai sciureus). In control monkeys, D(3) receptor mRNA appears to be widely expressed throughout the brain, with a distribution similar to that observed in both man and rodent. D(3) receptors are present in areas which express mRNA but also in some which do not, an observation which suggests they may be both pre- and postsynaptic in the monkey brain. Chronic MPTP administration, which selectively destroys the nigrostriatal system, resulted in a 70 to 99% depletion of the dopamine transporter in the basal ganglia. Autoradiographic analysis showed that after MPTP treatment there was a significant decline in D(3) receptors in the caudate, but not putamen, globus pallidus, substantia nigra or other dopaminergic regions. D(3) receptor mRNA expression was not changed in any region after nigrostriatal lesioning. Two weeks of L-3,4-dihydroxyphenylalanine (levodopa, L-DOPA) treatment, which alleviated Parkinsonism but also induced dyskinesias, reversed the MPTP-induced decline in caudate D(3) receptors. These results show that there is a selective decline in D(3) receptors in the caudate after nigrostriatal degeneration, which is reversed by L-DOPA treatment. Since the majority of dopaminergic nerve terminals were destroyed after MPTP lesioning, the reversal in D(3) receptors after L-DOPA treatment may represent an increase in caudate postsynaptic receptors, which could conceivably contribute to an imbalance in striatal circuitry and the development of dyskinesias.

Involvement of the direct striatonigral pathway in levodopa-induced sensitization in 6-hydroxydopamine-lesioned rats

Induction of dopamine D3 receptor gene expression in 6-hydroxydopamine-lesioned rats by repeated administration of levodopa had been suggested to be responsible for behavioural sensitization developing in these animals. Using double in situ hybridization techniques, we show that D3 receptor mRNA induction after repeated administration of levodopa took place mainly in dynorphin/substance P-expressing neurons of the direct striatonigral pathway. In agreement, induction of D3 receptor binding sites was evidenced, using 7-[3H]hydroxy-N,N-di-propyl-2-aminotetralin ([3H]7-OH-DPAT), in substantia nigra pars reticulata, the projection area of the direct nigrostriatonigral pathway. Changes in D3 receptor binding and behavioural sensitization during intermittent administration of levodopa paralleled changes in prodynorphin/preprotachykinin rather than preproenkephalin/prodynorphin and preproenkephalin/preprotachykinin mRNA ratios. Behavioural sensitization, induction of D3 receptor binding and changes in prodynorphin/preprotachykinin ratio were all prevented together when levodopa was continuously delivered or intermittently delivered in combination with R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine (SCH 23390), a selective D1 receptor antagonist. Our results indicate that functional changes of the direct striatal output pathway, possibly through an interaction between D1 and D3 receptors at the level of terminals in the substantia nigra pars reticulata, are important for the development of behavioural sensitization.

Possible implications of the dopamine D(3) receptor in schizophrenia and in antipsychotic drug actions

The D(3) receptor may represent an important target for antipsychotic drugs which all bind with high affinity and do not induce upon repeated administration either tolerance or receptor upregulation. The D(3) receptor is localized in brain areas, namely the nucleus accumbens and cerebral cortex, implicated in neural circuits believed to display defective functioning in schizophrenia. Overexpression of the D(3) receptor, which accounts for the behavioral sensitization to levodopa in a rodent model of Parkinson's disease, might also be responsible for the sensitization to dopamine agonists observed in schizophrenia. The appearance of the D(3) receptor during brain development, early in proliferating neuroepithelia and later in neurons from limbic areas, suggests further studies to assess its participation in the neurodevelopmental disorders of schizophrenia. Finally, meta-analysis of approximately 30 studies comprising over 2500 patients indicate that a polymorphism in the coding sequence of the D(3) receptor is associated with a small but significant enhancement of vulnerability to the disease.

Striatal dopaminergic markers in dementia with Lewy bodies, Alzheimer's and Parkinson's diseases: rostrocaudal distribution

Dementia with Lewy bodies (DLB) is a neuropsychiatric disease associated with extrapyramidal features which differ from those of Parkinson's disease, including reduced effectiveness of L-dopa and severe sensitivity reactions to neuroleptic drugs. Distinguishing Alzheimer's disease from DLB is clinically relevant in terms of prognosis and appropriate treatment. Dopaminergic activities have been investigated at coronal levels along the rostrocaudal striatal axis from a post-mortem series of 25 DLB, 14 Parkinson's disease and 17 Alzheimer's disease patients and 20 elderly controls. [(3)H]Mazindol binding to the dopamine uptake site was significantly reduced in the caudal putamen in DLB compared with controls (57%), but not as extensively as in Parkinson's disease (75%), and was unchanged in Alzheimer's disease. Among three dopamine receptors measured (D1, D2 and D3), the most striking changes were apparent in relation to D2. In DLB, [(3)H]raclopride binding to D2 receptors was significantly reduced in the caudal putamen (17%) compared with controls, and was significantly lower than in Parkinson's disease at all levels. D2 binding was significantly elevated at all coronal levels in Parkinson's disease compared with controls, most extensively in the rostral putamen (71%). There was no change from the normal pattern of D2 binding in Alzheimer's disease. The only significant alteration in D1 binding ([(3)H]SCH23390) in the groups examined was an elevation (30%) in the caudal striatum in Parkinson's disease. There were no differences in D3 binding, measured using [(3)H]7-OH-DPAT, in DLB compared with controls. A slight, significant decrease in D3 binding in the caudal striatum of Parkinson's disease (13%) patients and an increase in Alzheimer's disease (20%) in the dorsal striatum at the level of the nucleus accumbens were found. The concentration and distribution of dopamine were disrupted in both DLB and Parkinson's disease, although in the caudate nucleus the loss of dopamine in DLB was uniform whereas in Parkinson's disease the loss was greater caudally. In the caudal putamen, dopamine was reduced by 72% in DLB and by 90% in Parkinson's disease. The homovanillic acid : dopamine ratio, a metabolic index, indicated compensatory increased turnover in Parkinson's disease, which was absent in DLB despite the loss of substantia nigra neurons (49%), dopamine and uptake sites. These differences between DLB, Parkinson's disease and Alzheimer's disease may explain some characteristics of the extrapyramidal features of DLB and its limited response to L-dopa and severe neuroleptic sensitivity. The distinct changes in the rostrocaudal pattern of expression of dopaminergic parameters are relevant to the interpretation of the in vivo imaging and diagnosis of DLB.

Chronic D1 and D2 dopaminomimetic treatment of MPTP-denervated monkeys: effects on basal ganglia GABA(A)/benzodiazepine receptor complex and GABA content

The effect of various chronic dopaminergic treatments in 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkeys on the brain gamma-aminobutyric acid type A (GABA(A)) /benzodiazepine receptor complex and GABA content was investigated in order to assess the GABAergic involvement in dopaminomimetic-induced dyskinesia. Three MPTP monkeys received for one month pulsatile administrations of the D1 dopamine (DA) receptor agonist SKF 82958 whereas three others received the same dose of SKF 82958 by continuous infusion. A long acting D2 DA receptor agonist, cabergoline, was given to another three animals. Untreated MPTP as well as naive control animals were also included. Pulsatile SKF 82958 relieved parkinsonian symptoms but was also associated with dyskinesia in two of the three animals whereas animals treated continuously with SKF 82958 remained as untreated MPTP monkeys. Chronic cabergoline administration improved motor response with no persistent dyskinesia. MPTP treatment induced a decrease of 3H-flunitrazepam binding in the medial anterior part of caudate-putamen and an increase in the internal segment of globus pallidus (GPi) which was in general unchanged by pulsatile or continuous SKF 82958 administration. Throughout the striatum, binding of 3H-flunitrazepam remained reduced in MPTP monkeys treated with cabergoline but was not significantly lower than untreated MPTP monkeys. Moreover, cabergoline treatment reversed the MPTP-induced increase in 3H-flunitrazepam binding in the GPi. GABA concentrations remained unchanged in the striatum, external segment of globus pallidus and GPi following MPTP denervation. Pulsatile but not continuous SKF 82958 administration decreased putamen GABA content whereas cabergoline treatment decreased caudate GABA. No alteration in GABA levels were observed in the GPe and GPi following the experimental treatments. These results suggest that: (1) D2-like receptor stimulation with cabergoline modulates GABA(A) receptor density in striatal subregions anatomically related to associative cortical afferent and (2) the absence of dyskinesia in dopaminomimetic-treated monkeys might be associated with the reversal of the MPTP-induced upregulation of the GABA(A)/benzodiazepine receptor complex in the Gpi.