Dopamine Receptor Subtypes Research Articles

Dopamine receptor D3 deficiency results in chronic depression and anxiety

Over the last decade accumulating evidence suggests that brain dopamine (DA) has a role in depression, particularly given the high comorbidity of depression with Parkinson's Disease (PD) and the antidepressant effects of the DA receptor subtype 3 (D3R) agonist pramipexole. The present study assesses the role of D3R in depression. Here we hypothesized that D3R mediates the antidepressant effects of DA. Thus, genetic deficiency of D3R in D3R knockout (D3RKO) mice would yield animals with chronic depressive symptoms. Whereas D3R deficient mice did not show significant alterations in locomotion when tested in the openfield, these animals showed anxiety-like symptoms measured as a significant increase in thigmotaxis at the openfield and a significantly lower time spent in the lit compartment at the light/dark exploration test. D3RKO animals also showed depressive-like symptoms as measured by increased immobility time in the Porsolt forced swim test and the tail suspension test, as well as anhedonia measured in the non-motor dependent sucrose test. In conclusion, D3R deficiency results in anxiety-like and depressive-like symptoms that cannot be attributed to motor dysfunction.

Differential dopamine receptor subtype regulation of adenylyl cyclases in lipid rafts in human embryonic kidney and renal proximal tubule cells

Dopamine D1-like receptors (D1R and D5R) stimulate adenylyl cyclase (AC) activity, whereas the D2-like receptors (D2, D3 and D4) inhibit AC activity. D1R, but not the D5R, has been reported to regulate AC activity in lipid rafts (LRs). We tested the hypothesis that D1R and D5R differentially regulate AC activity in LRs using human embryonic kidney (HEK) 293 cells heterologously expressing human D1 or D5 receptor (HEK-hD1R or HEK-hD5R) and human renal proximal tubule (hRPT) cells that endogenously express D1R and D5R. Of the AC isoforms expressed in HEK and hRPT cells (AC3, AC5, AC6, AC7, and AC9), AC5/6 was distributed to a greater extent in LRs than non-LRs in HEK-hD1R (84.5±2.3% of total), HEK-hD5R (68.9±3.1% of total), and hRPT cells (66.6±2.2% of total) (P<0.05, n=4/group). In HEK-hD1R cells, the D1-like receptor agonist fenoldopam (1μM/15min) increased AC5/6 protein (+17.2±3.9% of control) in LRs but decreased it in non-LRs (−47.3±5.3% of control) (P<0.05, vs. control, n=4/group). By contrast, in HEK-hD5R cells, fenoldopam increased AC5/6 protein in non-LRs (+67.1±5.3% of control, P<0.006, vs. control, n=4) but had no effect in LRs. In hRPT cells, fenoldopam increased AC5/6 in LRs but had little effect in non-LRs. Disruption of LRs with methyl-β-cyclodextrin decreased basal AC activity in HEK-D1R (−94.5±2.0% of control) and HEK-D5R cells (−87.1±4.6% of control) but increased it in hRPT cells (6.8±0.5-fold). AC6 activity was stimulated to a greater extent by D1R than D5R, in agreement with the greater colocalization of AC5/6 with D1R than D5R in LRs. We conclude that LRs are essential not only for the proper membrane distribution and maintenance of AC5/6 activity but also for the regulation of D1R- and D5R-mediated AC signaling.

Pericyazine for schizophrenia.

Pericyazine is a 3-cyano-10 (3-4'-hydroxypiperidinopropyl) phenothiazine. It is overall pharmacologically similar with chlorpromazine, though particularly sedating. Dopamine receptor subtype analysis has not been performed for pericyazine, but the drug appears to induce greater noradrenergic than dopaminergic blockade. Compared to chlorpromazine, pericyazine reportedly has more potent antiemetic, antiserotonin, and anticholinergic activity. To evaluate the clinical effects and safety of pericyazine in comparison with placebo, typical and atypical antipsychotic agents and standard care for people with schizophrenia. We searched the Cochrane Schizophrenia Group Trials Register (February 2013) which is based on regular searches of CINAHL, EMBASE, MEDLINE and PsycINFO. We inspected references of all identified studies for further trials. All relevant randomised controlled trials focusing on pericyazine for schizophrenia and other types of schizophrenia-like psychoses (schizophreniform and schizoaffective disorders). We excluded quasi-randomised trials. Data were extracted independently from included papers by at least two review authors. Risk ratios (RR) and 95% confidence intervals (CI) of homogeneous dichotomous data were calculated. We assessed risk of bias for included studies and used GRADE to judge quality of evidence. We could only include five studies conducted between 1965 and 1980. Most of the included studies did not report details of randomisation, allocation concealment, details of blinding and we could not assess the impact of attrition due to poor reporting.For the primary outcome of Global state - not improved, the confidence interval was compatible with a small benefit and increased risk of not improving with pericyazine compared with typical antipsychotics (2 RCTs, n = 122, RR 1.24 CI 0.93 to 1.66, very low quality of evidence) or atypical antipsychotics (1 RCT, n = 93, RR 0.97 CI 0.67 to 1.42, very low quality of evidence).When compared with typical antipsychotics relapse was only experienced by one person taking pericyazine (1 RCT, n = 80, RR 2.59 CI 0.11 to 61.75, very low quality of evidence).Pericyazine was associated with more extrapyramidal side effects than typical antipsychotics (3 RCTs, n = 163, RR 0.52 CI 0.34 to 0.80, very low quality of evidence) and atypical antipsychotics (1 RCT, n = 93, RR 2.69 CI 1.35 to 5.36, very low quality of evidence).The estimated risk of leaving the study early for specific reasons was imprecise for the comparisons of pericyazine with typical antipsychotics (2 RCTs, n = 71, RR 0.46 CI 0.11 to 1.90, very low quality of evidence), and with atypical antipsychotics (1 RCT, n = 93, RR 0.13 CI 0.01 to 2.42, very low quality of evidence). On the basis of very low quality evidence we are unable to determine the effects of pericyazine in comparison with typical or atypical antipsychotics for the treatment of schizophrenia. However, there is some evidence that pericyazine may be associated with a higher incidence of extrapyramidal side effects than other antipsychotics, and again this was judged to be very low quality evidence. Large, robust studies are still needed before any firm conclusions can be drawn.

L‐Stepholidine is a potent pan‐dopamine receptor antagonist at G protein and β‐arrestin mediated signaling in vitro (662.5)

Dopamine receptors (DARs) are highly validated drug targets in neuropsychiatric disease and, as such, compounds acting on this receptor family are both scientifically and clinically important. The Chinese herbal extract component l‐stepholidine has been reported to exhibit D2 DAR antagonism and D1 DAR agonist‐like activities in a number of in vivo behavioral analyses. Prior studies have further shown complicated mixed pharmacological responses involving both dopamine and serotonin receptors; however, clear in vitro characterization on individual DAR subtype signaling pathways has not been extensively explored. These data are particularly important given the recent discovery of the potential for signaling bias within the DAR family. In this study we present a broad examination of the biological actions and binding activities of l‐stepholidine on all DAR subtypes. l‐Stepholidine demonstrated relatively high binding affinity for all DAR subtypes, but failed to display any agonist response in either G protein‐ (cAMP assays and GTPγS binding), or β‐arrestin‐ (β‐arrestin translocation assays) mediated signaling at any DAR tested. Furthermore, l‐stepholidine was a full antagonist of dopamine mediated signaling at both D1‐ and D2‐like receptors and did not display signaling bias, thereby demonstrating that it acts as a non‐selective pan‐dopamine receptor antagonist in vitro.Grant Funding Source: NIH Intramural Research Program

Dopamine D2 Receptor Relies upon PPM/PP2C Protein Phosphatases to Dephosphorylate Huntingtin Protein

Striatal dopamine D2 receptor (D2R) relies upon G protein- and β-arrestin-dependent signaling pathways to convey its action on motor control and behavior. Considering that D2R activation inhibits Akt in the striatum and that huntingtin physiological functions are affected by Akt phosphorylation, we sought to investigate whether D2R-mediated signaling could regulate huntingtin phosphorylation. We demonstrate that D2R activation decreases huntingtin phosphorylation on its Akt site. This dephosphorylation event depends upon the Gαi-dependent engagement of specific members of the protein phosphatase metallo-dependent (PPM/PP2C) family and is independent of β-arrestin 2. These observations identify the PPM/PP2C family as a mediator of G protein-coupled receptor signaling and thereby suggest a novel mechanism of dopaminergic signaling.

Dopamine D3 receptors contribute to methamphetamine-induced alterations in dopaminergic neuronal function: Role of hyperthermia

Methamphetamine administration causes long-term deficits to dopaminergic systems that, in humans, are thought to be associated with motor slowing and memory impairment. Methamphetamine interacts with the dopamine transporter (DAT) and increases extracellular concentrations of dopamine that, in turn, binds to a number of dopamine receptor subtypes. Although the relative contribution of each receptor subtype to the effects of methamphetamine is not fully known, non-selective dopamine D2/D3 receptor antagonists can attenuate methamphetamine-induced changes to dopamine systems. The present study extended these findings by testing the role of the dopamine D3 receptor subtype in mediating the long-term dopaminergic, and for comparison serotonergic, deficits caused by methamphetamine. Results indicate that the dopamine D3 receptor selective antagonist, PG01037, attenuated methamphetamine-induced decreases in striatal DAT, but not hippocampal serotonin (5HT) transporter (SERT), function, as assessed 7 days after treatment. However, PG01037 also attenuated methamphetamine-induced hyperthermia. When methamphetamine-induced hyperthermia was maintained by treating rats in a warm ambient environment, PG01037 failed to attenuate the effects of methamphetamine on DAT uptake. Furthermore, PG01037 did not attenuate methamphetamine-induced decreases in dopamine and 5HT content. Taken together, the present study demonstrates that dopamine D3 receptors mediate, in part, the long-term deficits in DAT function caused by methamphetamine, and that this effect likely involves an attenuation of methamphetamine-induced hyperthermia.

Pharmacological modulation of abnormal involuntary DOI-induced head twitch response in male DBA/2J mice: I. Effects of D2/D3 and D2 dopamine receptor selective compounds

Because of the complexity and heterogeneity of human neuropsychiatric disorders, it has been difficult to identify animal models that mimic the symptoms of these neuropathologies and can be used to screen for antipsychotic agents. For this study we selected the murine 5HT2A/2C receptor agonist-induced head twitch response (HTR) induced by the administration of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), which has been proposed as an animal model of symptoms associated with a variety of behavioral and psychiatric conditions. We investigated the DOI-induced HTR in male DBA/2J mice using a panel of D2-like (D2, D3 and D4) and D2 dopamine receptor selective compounds. When DBA/2J mice were administered a daily dose of DOI (5 mg/kg), tolerance to the DOI occurs. However, administrations of the same dose of DOI every other day (48 h) or on a weekly basis did not lead to tolerance and the ability to induce tolerance after daily administration of DOI remains intact after repeated weekly administration of DOI. Subsequently, a panel of D2-like dopamine receptor antagonists was found to effectively inhibit the DOI-induced HTR in DBA/2J mice. However, the benzamide eticlopride, which is a high affinity D2-like antagonist, was a notable exception. SV 293, SV-III-130s and N-methylbenperidol, which exhibit a high affinity for D2 versus the D3 dopamine receptor subtypes (60- to 100-fold binding selectivity), were also found to inhibit the HTR in DBA/2J mice. This observation suggests a functional interaction between dopaminergic and serotonergic systems through D2 dopamine receptors and the 5-HT2A serotonin receptors in vivo.

D2 and D3 dopamine receptor affinity predicts effectiveness of antipsychotic drugs in obsessive-compulsive disorders: a metaregression analysis

The relationship between clinically effective antipsychotic drugs in obsessive-compulsive disorders (OCD) and binding affinities to cloned dopamine and serotonin receptor subtypes was analyzed in an effort to clarify the contribution of individual receptor subtypes to medication response. Meta-analysis was used to update previous meta-analyses of effectiveness data of add-on antipsychotic drugs to selective serotonin reuptake inhibitors (SSRIs) in OCD. Twelve previously analyzed randomized controlled trials (RCTs) and one new RCT were included. We performed a metaregression using a mixed-effect model to examine the association between antipsychotic's effectiveness and receptor affinity. A total of 5 treatment arms obtained from 13 RCTs (431 patients) were included in our study. The results of our metaregression showed a significant association between D2 and D3 dopamine receptor affinities and effectiveness in OCD (respectively, slope = -0.36, p = 0.01; and slope = -0.50, p = 0.01) whereas other dopamine receptors and serotonin receptors were not significantly associated. These observations suggest that increasing D2 and D3 dopamine receptor binding affinities enhances antipsychotics' effectiveness in obsessive-compulsive disorders.

Dopamine D3 Receptor Is Necessary for Ethanol Consumption: An Approach with Buspirone

Mesolimbic dopamine (DA) controls drug- and alcohol-seeking behavior, but the role of specific DA receptor subtypes is unclear. We tested the hypothesis that D3R gene deletion or the D3R pharmacological blockade inhibits ethanol preference in mice. D3R-deficient mice (D3R−/−) and their wild-type (WT) littermates, treated or not with the D3R antagonists SB277011A and U99194A, were tested in a long-term free choice ethanol-drinking (two-bottle choice) and in a binge-like ethanol-drinking paradigm (drinking in the dark, DID). The selectivity of the D3R antagonists was further assessed by molecular modeling. Ethanol intake was negligible in D3R−/− and robust in WT both in the two-bottle choice and DID paradigms. Treatment with D3R antagonists inhibited ethanol intake in WT but was ineffective in D3R−/− mice. Ethanol intake increased the expression of RACK1 and brain-derived neurotrophic factor (BDNF) in both WT and D3R−/−; in WT there was also a robust overexpression of D3R. Thus, increased expression of D3R associated with activation of RACK1/BDNF seems to operate as a reinforcing mechanism in voluntary ethanol intake. Indeed, blockade of the BDNF pathway by the TrkB selective antagonist ANA-12 reversed chronic stable ethanol intake and strongly decreased the striatal expression of D3R. Finally, we evaluated buspirone, an approved drug for anxiety disorders endowed with D3R antagonist activity (confirmed by molecular modeling analysis), that resulted effective in inhibiting ethanol intake. Thus, DA signaling via D3R is essential for ethanol-related reward and consumption and may represent a novel therapeutic target for weaning.

Effects of dopamine D2-like receptor agonists in mice trained to discriminate cocaine from saline: Influence of feeding condition

In rats, the discriminative stimulus effects of direct- and indirect-acting dopamine receptor agonists are mediated by multiple dopamine receptor subtypes and the relative contribution of dopamine D2 and D3 receptors to these effects varies as a function of feeding condition. In these studies, free-fed and food-restricted mice were trained to discriminate 10.0mg/kg cocaine using a two-lever discrimination procedure in which responding was maintained by food. Both groups of mice acquired the discrimination; however, free-fed mice responded at lower rates than food-restricted mice. Dopamine D3 receptor agonists, pramipexole and quinpirole, increased cocaine-appropriate responding (>85%) in food-restricted, but not in free-fed mice. The dopamine D2 receptor agonist, sumanirole, and the nonselective dopamine receptor agonist, apomorphine, failed to increase cocaine-appropriate responding in either group. Free-fed mice were more sensitive than food-restricted mice to the rate-decreasing effects of dopamine receptor agonists and these effects could not be overcome by increasing the magnitude of reinforcement. Because feeding condition did not alter quinpirole-induced hypothermia, it is unlikely that differences in the discriminative stimulus or rate-decreasing effects of dopamine D2-like receptor agonists were due to differences in the pharmacokinetic properties of the drugs. Although these results suggest that the discriminative stimulus effects of cocaine are mediated by both dopamine D2 and D3 receptors in food-restricted mice, the increased sensitivity of free-fed mice to the rate-decreasing effects of dopamine D2-like receptor agonists limited conclusions about the impact of feeding conditions on the relative contribution of dopamine D2 and D3 receptors to the discriminative stimulus effects of cocaine.

ChemInform Abstract: Synthesis and Binding Affinity of New 1,4‐Disubstituted Triazoles as Potential Dopamine D3 Receptor Ligands.

AbstractA series of new 1,4‐disubstituted triazoles (V) are synthesized using click chemistry methodology and evaluated as potential ligands on several subtypes of dopamine receptors.

Polymorphisms of DRD2 and DRD3 genes and Parkinson’s disease: A meta-analysis

Parkinson's disease (PD) is the second most common neurodegenerative disorder that affects ~2% of the population aged ≥65 years. The degeneration of dopamine neurons in the substantia nigra contributes to the pathogenesis of PD. Dopamine receptor D2 (DRD2) and dopamine receptor D3 (DRD3) are two key subtypes of dopamine receptors. The aim of our study was to evaluate the association between the polymorphisms of DRD2 and DRD3 genes and PD. Meta-analyses were conducted from 16 studies (46 stages) among 4,279 cases and 5,661 controls between PD and 9 polymorphisms (DRD2: rs1800497, rs1079597, rs6278, rs6279, rs273482, rs1799732 and rs1076563; DRD3: rs6280 and rs2134655). A significant association was observed between DRD3 rs2134655 polymorphism and PD [P=0.01, odds ratio (OR)=1.17, 95% confidence interval (CI): 1.03-1.32] and a borderline association was observed between DRD2 rs1800497 polymorphism and PD in Europeans (P=0.05, OR=1.13, 95% CI: 1.00-1.27). Findings of the current meta-analysis suggested that DRD3 rs2134655 polymorphism was associated with a 17% increased risk of PD and that DRD2 rs1800497 polymorphism had a potential to increase the risk of PD by 13% in Europeans. Future large-scale studies are required to confirm the ethnic difference of DRD2 rs1800497 polymorphism and to determine whether there were significant associations of PD with other polymorphisms in DRD2 and DRD3 genes.

Variability in Quantitative Expression of Receptors in Nonfunctioning Pituitary Macroadenomas an Opportunity for Targeted Medical Therapy

The surgical removal of a nonfunctioning pituitary macroadenoma (NFP-Mac) is often incomplete. The appropriate treatment of recurrent/residual NFP-Macs is not well established. Our objective was to detect and quantify receptors that may serve as potential targets for medical therapy for NFP-Macs with postsurgical residuals. Several classes of pituitary receptors were analyzed by quantitative reverse transcriptase-polymerase chain reaction in 17 adult NFP-Mac patients who underwent surgery. The median age was 50 years, and 76% of patients were male. On magnetic resonance imaging, the mean NFP-Mac diameter was 3.3 ± 1.02 cm. Somatostatin receptor (SSTR) and dopamine receptor (DR) subtypes were found in almost all tumors. Based on previous studies, we postulated a cutoff of ≥ 2,000 receptor copies at which a response to therapy may occur. This cutoff was found in SSTR3 in 3 patients, SSTR2 in 2 patients, SSTR1 and SSTR5 in 1 patient each, DR(2_total) in 13 patients, DR(2_short) (considered the most responsive to dopamine agonists) in 10 patients, and DR(2_long), DR5, DR4, and DR1 in 7, 3, 2, and 1 patient, respectively. Tumor size, invasiveness score, immunochemistry, gender, age, clinical symptoms, and postoperative residual tumor growth did not correlate with the type or copy number of receptor mRNAs. NFP-Macs with significant postsurgical tumor residuals contain several DR and SSTR subtypes, some with high copy numbers. The receptor composition of NFP-Macs may guide future clinical research into targeted treatment strategies to reduce residual tumor volume. Such studies would determine the potential threshold of receptor levels for response to therapy for existing dopaminergic agonists and somatostatin analogs.

D1 but not D4 Dopamine Receptors are Critical for MDMA-Induced Neurotoxicity in Mice

MDMA, an addictive psychostimulant-consumed worldwide, has the ability to induce neurotoxic effects and addiction in laboratory animals and in humans through its effects on monoaminergic systems. MDMA-induced neurotoxicity in mice occurs primarily in dopaminergic neurons and does not significantly affect the serotonergic system. As the neurotoxic effects of MDMA in mice involve excessive dopamine (DA) release, DA receptors are highly likely to play a role in MDMA neurotoxicity, but the specific dopamine receptor subtypes involved have not previously been determined definitively. In this study, dopamine D1 and D4 receptor knock-out mice (D1R(-/-) and D4R(-/-)) were used to determine whether these receptors are involved in MDMA neurotoxicity. D1R inactivation attenuated MDMA-induced hyperthermia, decreased the reduction of dopamine and dopamine metabolite levels, and protected against dopamine terminal loss and reactive astrogliosis as determined in the striatum, 7days after MDMA treatment. In sharp contrast, inactivation of D4R did not prevent hyperthermia or the neurotoxic effects of MDMA. Altogether, these results indicate that D1R, but not D4R, plays a significant role in the dopaminergic striatal neurotoxicity observed after exposure to MDMA.

Differential Role of D1 and D2 Receptors in the Perifornical Lateral Hypothalamus in Controlling Ethanol Drinking and Food Intake: Possible Interaction with Local Orexin Neurons

The neurotransmitter dopamine (DA), acting in various mesolimbic brain regions, is well known for its role in promoting motivated behaviors, including ethanol (EtOH) drinking. Indirect evidence, however, suggests that DA in the perifornical lateral hypothalamus (PF/LH) has differential effects on EtOH consumption, depending on whether it acts on the DA 1 (D1) or DA 2 (D2) receptor subtype, and that these effects are mediated in part by local peptide systems, such as orexin/hypocretin (OX) and melanin-concentrating hormone (MCH), known to stimulate the consumption of EtOH. The present study in brain-cannulated Sprague-Dawley rats measured the effects of dopaminergic compounds in the PF/LH on drinking behavior in animals trained to consume 7% EtOH and also on local peptide mRNA expression using digoxigenin-labeled in situ hybridization in EtOH-naïve animals. Experiments 1 and 2 showed that the D1 agonist SKF81297 (10.8nmol/side) in the PF/LH significantly increased food intake, while tending to increase EtOH intake, and the D1 antagonist SCH23390 significantly decreased EtOH intake without affecting food intake. In contrast, the D2 agonist quinelorane (6.2nmol/side) in the PF/LH significantly reduced EtOH consumption, while the D2 antagonist sulpiride increased it. Experiments 3 and 4 revealed differential effects of PF/LH injection of the DA agonists on local OX mRNA, which was increased by the D1 agonist and decreased by the D2 agonist. These DA agonists had no impact on MCH expression. These results support a stimulatory role of the PF/LH D1 receptor in promoting the consumption of both EtOH and food, in contrast to a suppressive effect of the D2 receptor on EtOH drinking. They further suggest that these receptors affect consumption, in part, through local OX-expressing neurons. These findings provide new evidence for the function of PF/LH DA receptor subtypes in controlling EtOH and food intake.

Long-term effects of cariprazine exposure on dopamine receptor subtypes

All clinically effective antipsychotics are known to act on the dopaminergic system, and previous studies have demonstrated that repeated treatment with antipsychotics produced region-specific changes in dopamine receptor levels. Cariprazine is a dopamine D₃ and D₂ receptor partial agonist with preferential binding to D₃ receptors. We examined the effects of chronic cariprazine administration on dopamine receptor levels. Rats were administered either vehicle or cariprazine (0.06, 0.2, or 0.6 mg/kg) for 28 days. Dopamine receptor levels were quantitated using autoradiographic assays on brain tissue sections from the medial prefrontal cortex (mPFC), nucleus accumbens (NAc), caudate putamen (CPu), hippocampus (HIPP), olfactory tubercle (OT), and islands of Calleja (ICj). Chronic treatment with cariprazine did not alter D₁ receptor levels in any brain region tested. Cariprazine increased D₂ receptor levels in mPFC (27%-43%), NAc (40%-45%), medial (41%-53%) and lateral (52%-63%) CPu, and HIPP (38%). Cariprazine dose-dependently upregulated D₃ receptor levels in ICj (32%-57%), OT (27%-67%), and NAc shell (31%-48%). Repeated cariprazine treatment increased D₄ receptor in NAc (53%-82%), medial (54%-98%) and lateral (58%-74%) CPu, and HIPP (38%-98%). Similar to other antipsychotics, cariprazine upregulated D₂ and D₄ receptor levels in various brain regions. Cariprazine was unique among antipsychotics in increasing D₃ receptor levels, which may support its unique psychopharmacologic properties.

Initial D2 Dopamine Receptor Sensitivity Predicts Cocaine Sensitivity and Reward in Rats

The activation of dopamine receptors within the mesolimbic dopamine system is known to be involved in the initiation and maintenance of cocaine use. Expression of the D2 dopamine receptor subtype has been implicated as both a predisposing factor and consequence of chronic cocaine use. It is unclear whether there is a predictive relationship between D2 dopamine receptor function and cocaine sensitivity that would enable cocaine abuse. Therefore, we exploited individual differences in behavioral responses to D2 dopamine receptor stimulation to test its relationship with cocaine-mediated behaviors. Outbred, male Sprague-Dawley rats were initially characterized by their locomotor responsiveness to the D2 dopamine receptor agonist, quinpirole, in a within-session ascending dose-response regimen (0, 0.1, 0.3 & 1.0 mg/kg, sc). Rats were classified as high or low quinpirole responders (HD2 and LD2, respectively) by a median split of their quinpirole-induced locomotor activity. Rats were subsequently tested for differences in the psychostimulant effects of cocaine by measuring changes in cocaine-induced locomotor activity (5 and 15 mg/kg, ip). Rats were also tested for differences in the development of conditioned place preference to a low dose of cocaine (7.5 mg/kg, ip) that does not reliably produce a cocaine conditioned place preference. Finally, rats were tested for acquisition of cocaine self-administration and maintenance responding on fixed ratio 1 and 5 schedules of reinforcement, respectively. Results demonstrate that HD2 rats have enhanced sensitivity to the locomotor stimulating properties of cocaine, display greater cocaine conditioned place preference, and self-administer more cocaine compared to LD2 animals. These findings suggest that individual differences in D2 dopamine receptor sensitivity may be predictive of cocaine sensitivity and reward.

Case–control association analysis of Dopamine receptor polymorphisms in alcohol dependence: a pilot study in Indian males

BackgroundBrain imaging studies and knock-out animal models have derived substantial abetment for dopamine receptor (DR) subtypes as potential candidates in susceptibility to addictive disorders, including alcohol dependence (AD). Various association studies that compared the frequencies of alleles of the dopamine D1, D2, D3 and D4 receptor genes between alcohol dependent and control subjects have produced suggestive results, though some of them are discordant in nature. In the absence of genetic data from Indian population, we evaluated genetic association of three polymorphisms namely rs4532 in DRD1, rs6280 in DRD3 and 120 bp duplication in 1.2 kb upstream region of DRD4 with AD.MethodsA total of 90 cases (alcohol dependent males) and 122 age and ethnicity matched healthy male controls were recruited in the study by following DSM-IV criteria. Three polymorphisms, namely rs4532 in DRD1, rs6280 in DRD3 and 120 bp duplication in 1.2 kb upstream region of DRD4 were selected (based on minor allele frequency and available literature) for genotyping by PCR-RFLP/LP method. Allele and genotype frequencies of these genetic markers were compared using Pearson’s χ2 test followed by risk assessment using odds ratio. Statistical analysis of clinical parameters such as AUDIT scores of case subjects was also performed.ResultsStatistically significant associations of polymorphisms in DRD1 and DRD4 with alcoholism were found.ConclusionsOur results underscore that genetic variations in dopamine receptors D1 and D4 may influence genetic predisposition to alcoholism. Unavailability of comparative data from Indian population and small sample size necessitate replication of results in an independent cohort.

Methamphetamine Increases Locomotion and Dopamine Transporter Activity in Dopamine D5 Receptor-Deficient Mice

Dopamine regulates the psychomotor stimulant activities of amphetamine-like substances in the brain. The effects of dopamine are mediated through five known dopamine receptor subtypes in mammals. The functional relevance of D5 dopamine receptors in the central nervous system is not well understood. To determine the functional relevance of D5 dopamine receptors, we created D5 dopamine receptor-deficient mice and then used these mice to assess the roles of D5 dopamine receptors in the behavioral response to methamphetamine. Interestingly, D5 dopamine receptor-deficient mice displayed increased ambulation in response to methamphetamine. Furthermore, dopamine transporter threonine phosphorylation levels, which regulate amphetamine-induced dopamine release, were elevated in D5 dopamine receptor-deficient mice. The increase in methamphetamine-induced locomotor activity was eliminated by pretreatment with the dopamine transporter blocker GBR12909. Taken together, these results suggest that dopamine transporter activity and threonine phosphorylation levels are regulated by D5 dopamine receptors.

Influences of Intrauterine Ethanol on the Maturation of the Monoaminergic Systems in the Developing Rat Brain

The aim of the present work was to conduct a simultaneous study of the levels of the major transmitters of the brain monoaminergic system, their metabolites, catechol-O-methyltransferase (COMT) activity, and the state of different dopamine receptor subtypes (D1 ,D 2L ,D 2S ,D 4, and D5) in the developing brains of the offspring of maternal rats alcoholized during pregnancy and lactation. Alcoholization of rats during pregnancy decreased the activities of the brain monoaminergic systems, which was manifest as decreases in the noradrenaline and dopamine (DA) levels in alcoholized fetuses, along with decreases in mRNA for the catecholamine-metabolizing enzyme COMT in forebrain structures on day 17 but not on day 13 of prenatal development. There were parallel increases in the contents of mRNA for the long and short splice variants of the type D2 dopamine receptor. During the postnatal period (days 4, 10, and 17), there was a further reduction in the activity of the DA system, and particularly reductions in the dihydroxyphenylacetic acid (DHPAA) level and the DHPAA/DA ratio in rat pups whose mothers had consumed alcohol during pregnancy but transferred to water after birth of their offspring. The activity of the serotonin system during the postnatal period of development in alcoholized rat pups was also decreased, which was apparent at the earliest stages of postnatal development (day 4 of life). Measures of serotonin metabolism then normalized. Thus, the activity of the serotonin system showed greater changes at the early periods of postnatal development (day 4), while suppression of the activity of the DA system was more marked at later stages (day 10 of life).