D1/D3 receptor relationships in brain coexpression, coactivation, and coregulation

Modulation of D3R Splicing, Signaling, and Expression by D1R through PKA→PTB Phosphorylation

The D1R and D3R receptors functionally and synergistically interact in striatonigral neurons. Dopaminergic denervation turns this interaction antagonistic, which is correlated with a decrement in D3nf isoform and an increment in D3R membranal expression. The mechanisms of such changes in D3R are attributed to the dysregulation of the expression of their isoforms. The cause and mechanism of this phenomenon remain unknown. Dopaminergic denervation produces a decrement in D1R and PKA activity; we propose that the lack of phosphorylation of PTB (regulator of alternative splicing) by PKA produces the dysregulation of D3R splicing and changes D3R functionality. By using in silico analysis, we found that D3R mRNA has motifs for PTB binding and, by RIP, co-precipitates with PTB. Moreover, D1R activation via PKA promotes PTB phosphorylation. Acute and 5-day D1R blockade decreases the expression of D3nf mRNA. The 5-day treatment reduces D3R, D3nf, and PTB protein in the cytoplasm and increases D3R in the membrane and PTB in the nucleus. Finally, the blockade of D1R mimics the effect of dopaminergic denervation in D1R and D3R signaling. Thus, our data indicate that through PKA→PTB, D1R modulates D3R splicing, expression, and signaling, which are altered during D1R blockade or the lack of stimulation in dopaminergic denervation.

Coexistence of D3 R typical and atypical signaling in striatonigral neurons during dopaminergic denervation. Correlation with D3 nf expression changes

Dopamine D₃ R are widely expressed in basal ganglia where interact with D₁ R. D₃ R potentiate cAMP accumulation and GABA release stimulated by D₁ R in striatonigral neurons through "atypical" signaling. During dopaminergic denervation, D₃ R signaling changes to a "typical" in which antagonizes the effects of D₁ R, the mechanisms of this switching are unknown. D₃ nf splice variant regulates membrane anchorage and function of D₃ R and decreases in denervation; thus, it is possible that D₃ R signaling switching correlates with changes in D₃ nf expression and increases of membranal D₃ R that mask D₃ R atypical effects. We performed experiments in unilaterally 6-hydroxydopamine lesioned rats and found a decrease in mRNA and protein of D₃ nf, but not of D₃ R in the denervated striatum. Proximity ligation assay showed that D₃ R-D₃ nf interaction decreased after denervation, whereas binding revealed an increased B_max in D₃ R. The new D₃ R antagonized cAMP accumulation and GABA release stimulated by D₁ R; however, in the presence of N-Ethylmaleimide (NEM), to block G_i protein signaling, activation of D₃ R produced its atypical signaling stimulating D₁ R effects. Finally, we investigated if the typical and atypical effects of D₃ R modulating GABA release are capable of influencing motor behavior. Injections of D₃ R agonist into denervated nigra decreased D₁ R agonist-induced turning behavior but potentiated it in the presence of NEM. Our data indicate the coexistence of D₃ R typical and atypical signaling in striatonigral neurons during denervation that correlated with changes in the ratio of expression of D₃ nf and D₃ R isoforms. The coexistence of both atypical and typical signaling during denervation influences motor behavior.

The neurosteroidogenic enzyme 5α-reductase modulates the role of D1 dopamine receptors in rat sensorimotor gating

Neurosteroids exert diverse modulatory actions on dopamine neurotransmission and signaling. We previously documented that the enzyme 5α-reductase, which catalyzes the main rate-limiting step in neurosteroid synthesis, is required for the behavioral responses of Sprague-Dawley rats to non-selective dopaminergic agonists, such as the D1-D2 receptor agonist apomorphine. Specifically, systemic and intra-accumbal administrations of the 5α-reductase inhibitor finasteride countered apomorphine-induced deficits of sensorimotor gating, as measured by the prepulse inhibition (PPI) of the startle reflex; the classes of dopamine receptors involved in these effects, however, remain unknown. Prior rodent studies have revealed that the contributions of dopamine receptors to PPI regulation vary depending on the genetic background; thus, we analyzed the effect of finasteride on the PPI deficits induced by selective dopamine receptor agonists in Long-Evans (a strain exhibiting PPI deficits in response to both D1 and D2 receptor agonists) and Sprague-Dawley rats (which display PPI reductions following treatment with D2, and D3, but not D1 receptor agonists). In Long-Evans rats, finasteride opposed the PPI deficits induced by activation of D1, but not D2 receptors; conversely, in Sprague-Dawley rats, finasteride prevented the reductions in %PPI and accumbal dopamine extracellular levels caused by selective stimulation of D3, but not D2 receptors; however, the effects on %PPI were not confirmed by analyses on absolute PPI values. Our findings suggest that 5α-reductase modulates the effects of D1, but not D2 receptor agonists on sensorimotor gating. These data may help elucidate the role of neurosteroids in neuropsychiatric disorders featuring PPI deficits, including schizophrenia and Tourette syndrome.

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.

Evaluation of the D3 dopamine receptor selective agonist/partial agonist PG01042 on L-dopa dependent animal involuntary movements in rats

The substituted 4-phenylpiperazine D3 dopamine receptor selective antagonist PG01037 ((E)-N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)but-2-enyl)-4-(pyridin-2-yl)benzamide) was reported to attenuate L-dopa-associated abnormal involuntary movements (AIMs) in unilaterally lesioned rats, a model of L-dopa-dependent dyskinesia in patients with Parkinson's Disease (Kumar et al., 2009a). We now report that PG01042 (N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)butyl)-4-(pyridin-3-yl)benzamide), which is a D3 dopamine receptor selective agonist for adenylyl cyclase inhibition and a partial agonist for mitogenesis, is also capable of attenuating AIMs scores. The intrinsic activity of PG01037 and PG01042 were determined using a) a forskolin-dependent adenylyl cyclase inhibition assay and b) an assay for agonist-associated mitogenesis. It was observed that the in vivo efficacy of PG01042 increased when administered by intraperitoneal (i.p.) injection simultaneously with L-dopa/benserazide (8 mg/kg each), as compared to a 60 min or 30 min pretreatment. PG01042 was found to attenuate AIM scores in these animals in a dose dependent manner. While PG01042 did not effectively inhibit SKF 81297-dependent AIMs, it inhibited apomorphine-dependent AIM scores. Rotarod studies indicate that PG01042 at a dose of 10 mg/kg did not adversely affect motor coordination of the unilaterally lesioned rats. Evaluation of lesioned rats using a cylinder test behavioral paradigm indicated that PG01042 did not dramatically attenuate the beneficial effects of L-dopa. These studies and previously published studies suggest that both D3 dopamine receptor selective antagonists, partial agonists and agonists, as defined by an adenylyl cyclase inhibition assay and a mitogenic assay, are pharmacotherapeutic candidates for the treatment of L-dopa-associated dyskinesia in patients with Parkinson's Disease.

Evaluation of the D3 dopamine receptor selective antagonist PG01037 on L-dopa-dependent abnormal involuntary movements in rats

The D3 dopamine receptor selective antagonist PG01037 has been evaluated for the ability to attenuate L-dopa-associated abnormal involuntary movements (AIMs) in unilaterally lesioned male Sprague-Dawley rats, which is a model of L-dopa-dependent dyskinesia in patients with Parkinson's Disease. The intrinsic activity of PG01037 was determined using a) a forskolin-dependent adenylyl cyclase inhibition assay with transfected HEK 293 cells expressing either the human D2Long or D3 dopamine receptor subtype and b) an assay for agonist-associated mitogenesis. For the initial experiments, the 5-HT1A receptor selective partial agonist buspirone was used as a positive control to verify our ability to quantitate changes in total AIMs and AIMs minus locomotor scores. Subcutaneous (s.c.) administration of PG01037 was found to have minimal effect on AIMs score. However, it was observed that the in vivo efficacy of PG01037 increased when administered by intraperitoneal (i.p.) injection 15 min after L-dopa/benserazide administration, as compared to a 60 min, 30 min or 0 min pretreatment. It was also found that i.p. administration of PG01037 could inhibit involuntary movements after they had achieved maximum intensity. PG01037 was found to attenuate AIM scores in these animals in a dose dependent manner with IC(50) value equal to a) 7.4 mg/kg following L-dopa/benserazide administration (8 mg/kg each, i.p.) and b) 18.4 mg/kg following the administration of apomorphine (0.05 mg/kg, s.c.). However, PG01037 did not effectively inhibit SKF 81297-dependent abnormal involuntary movements. Rotarod studies indicate that PG01037 at a dose of 10 mg/kg did not adversely affect motor coordination of the unilaterally lesioned rats. Evaluation of lesioned rats using a cylinder test behavioral paradigm indicated that PG01037 did not dramatically attenuate the beneficial effects of L-dopa. These studies suggest that D3 dopamine receptor selective antagonists are potential pharmacotherapeutic candidates for the treatment of L-dopa-associated dyskinesia in patients with Parkinson's Disease.

Evaluation of D2 and D3 dopamine receptor selective compounds on L-dopa-dependent abnormal involuntary movements in rats

A panel of novel D2 and D3 dopamine receptor selective antagonists, partial agonists and full agonists have been evaluated for the ability to attenuate L-dopa-associated abnormal involuntary movements (AIMs) in 6-hydroxydopamine (6-OHDA) unilaterally lesioned male Sprague Dawley rats, which is an animal model of L-dopa-induced dyskinesia (LID). LID is often observed in patients with Parkinson's Disease following chronic treatment with L-dopa. The intrinsic activity of these dopaminergic compounds was determined using a forskolin-dependent adenylyl cyclase inhibition assay with transfected HEK 293 cells expressing either the human D2Long or D3 dopamine receptor subtype. For the initial experiments the 5-HT1A receptor selective partial agonist buspirone was used to verify our ability to quantitate changes in total AIMs and AIMs minus locomotor scores. Two D2 dopamine receptor selective antagonists, SV 156 and SV 293, were evaluated and found to minimally attenuate AIM scores in these animals. Four members of our WC series of D3 dopamine receptor selective compounds of varying intrinsic activity at the D3 dopamine receptor subtype, WC 10, WC 21, WC 26 and WC 44, were also evaluated and found to attenuate AIM scores in a dose dependent manner. The in vivo efficacy of the compounds increased when they were administered simultaneously with L-dopa, as compared to when the compounds were administered 60 min prior to the L-dopa/benserazide. It was also found that the D3 receptor antagonist WC 10 could inhibit the involuntary movements after they had achieved maximum intensity. Unlike the D1-like dopamine receptor selective agonist SKF 81297 and the D2-like dopamine receptor agonist bromocriptine which can precipitate abnormal involuntary movements in these unilaterally lesioned animals, abnormal involuntary movements were not observed after administration of our D3 receptor selective agonist WC 44. In addition, we evaluated the effect of these four D3 dopamine receptor selective compounds for their effect on a) spontaneous locomotion and b) coordination and agility using a rotarod apparatus. We also used a cylinder test to assess the effect of L-dopa on spontaneous and independent use of each of the rat's forelimbs in the presence or absence of test compound. The results of these studies suggest that substituted phenylpiperazine D3 dopamine receptor selective compounds are potential pharmacotherapeutic agents for the treatment of L-dopa-associated dyskinesia in patients with Parkinson's Disease.

In vivo evidence of D3 dopamine receptor sensitization in parkinsonian primates and rodents with l-DOPA-induced dyskinesias

A growing body of evidence indicates a role for D(3) receptors in l-DOPA-induced dyskinesias. This involvement could be amenable to non-invasive in vivo analysis using functional neuroimaging. With this goal, we examined the hemodynamic response to the dopamine D(3)-preferring agonist 7-hydroxy-N,N-di-n-propyl-2 aminotetralin (7-OHDPAT) in naïve, parkinsonian and l-DOPA-treated, dyskinetic rodents and primates using pharmacological MRI (phMRI) and relative cerebral blood volume (rCBV) mapping. Administration of 7-OHDPAT induced minor negative changes of rCBV in the basal ganglia in naïve and parkinsonian animals. Remarkably, the hemodynamic response was reversed (increased rCBV) in the striatum of parkinsonian animals rendered dyskinetic by repeated l-DOPA treatment. Such increase in rCBV is consistent with D(1) receptor-like signaling occurring in response to D(3) stimulation, demonstrates a dysregulation of dopamine receptor function in dyskinesia and provides a potentially novel means for the characterization and treatment of l-DOPA-induced dyskinesia in patients.

Characterization of the desensitization properties of five dopamine receptor subtypes and alternatively spliced variants of dopamine D2 and D4 receptors

Proper regulation of brain dopaminergic activity is essential for maintaining normal mental functions. In this study, the regulatory properties of five different dopamine receptor subtypes and alternative splicing variants of dopamine D2 and D4 were examined. The stimulation of D1R, D2R, D5R but not D3R, D4R caused the robust translocation of beta-arrestin to the plasma membrane. When D1R or D3R were co-expressed with D2R, D1R significantly inhibited the sequestration of D2R, suggesting that the inhibitory effects of D1R on the D2R sequestration could explain the synergistic activity between two receptors. The sequestration of alternatively spliced isoforms of D2R was differently regulated by GRKs and beta-arrestins. Three alternative splicing variants of D4R produced a similar level of beta-arrestin translocation, and the studies with the deletion mutants of D4R within the third cytoplasmic loop revealed that the regions containing the SH3-binding domains are responsible for the beta-arrestin translocation.

[Central dopaminergic receptors (Part II): pathophysiological and therapeutic considerations]

The five subtypes of dopaminergic receptors exhibit different transduction, cerebral localization, regulation, pharmacological, and physiological roles, explaining their multiple pathophysiological implications in different neuropsychiatric conditions which result, at least in part from anomalous dopaminergic neurotransmission: Parkinson's disease, schizophrenia, addiction, migraine, mode disorders, Gilles de la Tourette disease, hyperactivity syndrome with attention deficit. The wide range of pharmacological implications explains the diversity of the therapeutic approaches perspectives for development of new drugs for these neuropsychiatric conditions.

7-OH-DPAT selectively reduces intake of both chow and high fat diets in different food intake regimens

Mesolimbic dopaminergic system activation correlates with ingestive behavior in numerous feeding regimens. DA release is enhanced by food intake following deprivation, amount of food consumed, and the palatability of the food consumed. The dopamine-3 receptor (D3-R) has a limited expression pattern that is restricted largely to the mesolimbic dopaminergic system. The D3-R has been hypothesized to inhibit DA-mediated reward, locomotion and motivation. To test the potential for an inhibitory role of the D3-R on food intake, we administered the D3-R agonist 7-OH-DPAT (5, 10 and 50 microg/kg ip) to rats that had ad libitum access to standard rodent chow (3.41 kcal/gm, 0.51 kcal/gm from fat) or a preferable, high fat (HF) (4.4 kcal/gm, 1.71 kcal/gm from fat). In the second set of experiments we administered 7-OH-DPAT (10, 50 and 100 microg/kg) to rats that had access to chow or HF diet for only 3 h per day (meal fed). In the third set of experiments we administered 7-OH-DPAT (10 and 50 microg/kg) to rats that had access to chow or HF diet after a 21-h food restriction. The 10 and 50 microg/kg doses significantly, but equally reduced intake of chow and HF diet in animals that were ad libitum fed. In animals that were meal-fed the dose response was effectively shifted to the right and the 10 microg/kg dose was ineffective at reducing intake. The 50 and 100 microg/kg doses significantly but equally reduced intake of both diets. In animals that were 21-h restricted and had access to chow both the 10 and 50 microg/kg doses were ineffective at reducing intake. However, in animals that had access to HF diet, 7-OH-DPAT dose-dependently reduced intake. These results support a potential role for the D3-R in ingestive behavior particularly in situations that involve a significant learned component.

Enhancement of dopamine-induced signaling responses in the forebrain of mice lacking dopamine D3 receptor

It is well known that the dopamine D(3) receptor plays a critical role in several psychological disorders, such as drug dependence. The present study was designed to investigate the influence of lacking dopamine D(3) receptors in dopamine-induced G-protein activation and Ca(2+) influx in the mouse forebrain. The deletion of dopamine D(3) receptor gene caused the enhancement of dopamine-induced G-protein activation in the limbic forebrain of dopamine D(3) receptor knockout (D(3)KO) mice. Furthermore, the dopamine-induced Ca(2+) influx was enhanced in the coculture of neuron/glia cells obtained from the forebrain of D(3)KO mice. The present data provide direct evidence that a deletion of central dopamine D(3) receptor enhances the dopamine D(1)/D(2) receptor-mediated intracellular signaling.

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.

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.

Cellular colocalization of dopamine D1 mRNA and D2 receptor in rat brain using a D2 dopamine receptor specific polyclonal antibody

1. The main objective of this work was to investigate the extent of cellular colocalization of dopamine D1 and D2 receptors in the rat brain. A double labeling technique, that combined immunocytochemical labeling of the D2 receptor using polyclonal antibodies raised against the third intracellular loop of the short isoform of the human D2 receptor in combination with in situ hybridization detecting D1 mRNA expression, was designed to accomplish this goal. 2. The specificity of the antisera obtained was confirmed by immunoprecipitation assay, Western blot analysis, and immunocytochemistry on D2R transfected cells and murine brain tissue. Western blot using the D2 receptor antibody revealed a specific broad band centered at 67 kDa in transfected cells and a major protein of 88 kDa corresponding to D2R expressed in the caudate-putamen, to a lesser extent in the cortex, and not at all detected in the hypothalamic region. 3. The content of neurons double-labeled for D1/D2 receptors was observed at in differing intensities in the dorsal endopiroform nucleus, the intercalated nucleus of amygdala, the anterior part of the cortical nucleus amygdala, the nucleus of the lateral olfactory tract, the piriform cortex, the parabrachial nucleus, the supraoptic nucleus and the parabigeminal nucleus. All other regions of the brain revealed neurons expressing either D1 or D2 dopamine receptors but not both at that same time. 4. These results clearly demonstrated that specific neurons expressed both receptors D1 and D2, and that this colocalization was restricted to particular regions of the rat brain.

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.

The antisense strategy applied to the study of dopamine D3 receptor functions in rat forebrain

1. The authors have investigated the effects of a dopamine D3 receptor antisense oligodeoxynucleotide (ODN), on neuropeptides (neurotensin and dynorphin) and transcription factor (c-fos) mRNA levels in rat forebrain. 2. Intracerebroventricular injections of ODNs were made into the lateral ventricle (5 and 10 micrograms/h, for 5 days). Effect of antisense administration on dopamine D2 and D3 receptor binding were measured by means of receptor autoradiography. Neuropeptides and c-fos mRNA levels were evaluated by in situ hybridization using specific complementary RNA probes. 3. Dopamine D3 receptor densities were dose-dependently reduced in the shell of nucleus accumbens of rats that received the D3 antisense ODN. Sense and missense controls remained without effect. No significant effect was observed on D2 receptor binding in any of the ODN groups studied, as measured with [3H]raclopride binding. Concomitant reductions of dynorphin and neurotensin mRNA levels were observed in the shell of nucleus accumbens after D3 antisense ODN administration. Interestingly, the D3 antisense administration also reduced c-fos mRNA levels in the cingulate cortex of these animals. 4. The results show that D3 receptors may tonically regulate basal transcription factor, as well as neuropeptides, gene expression in the rat forebrain. These results clearly demonstrate that an antisense strategy could be useful to identify molecular targets under control of specific dopamine receptor subtypes.