Sensorimotor Gating is Associated with CHRNA3 Polymorphisms in Schizophrenia and Healthy Volunteers

Attentional gating deficits, commonly measured by prepulse inhibition (PPI) of the acoustic startle response (ASR), have been established as an endophenotype of schizophrenia. Prepulse inhibition is heritable and has been associated with polymorphisms in serotonin and dopamine system genes. Prepulse inhibition can be enhanced by nicotine, and therefore it has been proposed that schizophrenia patients smoke to ameliorate their early attentional deficits. The PPI-enhancing effects of nicotine in rodents are strain dependent, suggesting a genetic contribution to PPI within the nicotinic acetylcholine receptor (nAChR) system. Recent human genetic studies also imply that tobacco dependence is affected by polymorphisms in the α3/α5 subunits of the nAChR (CHRNA3/CHRNA5) gene cluster. We, therefore, investigated the impact of two common CHRNA3 polymorphisms (rs1051730/rs1317286) on PPI, startle reactivity, and habituation of the ASR in two independent samples of 107 healthy British volunteers and 73 schizophrenia patients hailing from Germany. In both samples, PPI was influenced by both CHRNA3 polymorphisms (combined p-value=0.0027), which were strongly linked. Moreover, CHRNA3 genotype was associated with chronicity, treatment, and negative symptoms in the schizophrenia sample. These results suggest that sensorimotor gating is influenced by variations of the CHRNA3 gene, which might also have an impact on the course and severity of schizophrenia.     

Erratum to: Translational Modeling in Schizophrenia: Predicting Human Dopamine D2 Receptor Occupancy

Pharmaceutical Research -     

Chronic Dopamine D1, Dopamine D2 and Combined Dopamine D1 and D2 Antagonist Treatment in Cebus Apella Monkeys: Antiamphetamine Effects and Extrapyramidal Side Effects

To determine: (1) whether the apparent lack of efficacy of dopamine D1 (D1) antagonists in the clinic might be attributable to development of tolerance to antipsychotic effects; and (2) whether combined D1 and D2 antagonism might contribute to clozapine’s clinical profile, eight Cebus apella monkeys were chronically treated with a D1 antagonist (NNC 756) ((+)-8-chloro-7-hydroxy-3-methyl-5-(7-(2,3-dihydrobenzofuranyl)-2,3,4,5-tetrahydro-1H-3-benzazepine), a D2 antagonist (raclopride) or a combination of the two antagonists. Prior neuroleptic exposure had resulted in oral dyskinesia in seven monkeys and sensitization to dystonia in all, yielding a model for acute and chronic extrapyramidal side effects (EPS). Dextroamphetamine-induced motoric unrest and stereotypies were used as a psychosis model. We found tolerance toward dystonic symptoms during D1 and D1 + D2 treatments but not during D2 treatment. D2 but not D1 or D1 + D2 antagonism caused exacerbation of dyskinesia. Both D1 and D1 + D2 antagonism were superior to D2 antagonism alone in counteracting the amphetamine-induced behaviors, with no tolerance to antiamphetamine effects. These findings suggest: (1) reasons other than tolerance (e.g., differences among antagonists) may explain the lack of efficacy in clinical trials with D1 antagonists; and (2) that D1 antagonism alone or combined and modest D1 and D2 antagonism offers the potential of antipsychotic efficacy with a lower risk of EPS than traditional D2 antagonism. Further clinical trials with D1 or combined D1 and D2 antagonists are, therefore, recommended.     

烟草使用行为对首发精神分裂症患者认知功能及脑源性 神经生长因子水平的影响

目的:烟草使用对首发精神分裂症患者认知功能及脑源性神经生长因子水平的影响.方法:选择新疆精神卫生中心(乌鲁木齐市第四人民医院)自2019年1—12月收治的首发精神分裂症患者187人,根据患者是否吸烟分为使用烟草组(112人)和未使用烟草组(75人).分别测定和对比两组患者的BDNF的血清含量及认知功能;分析烟草使用行为...     

Role of Dopamine D2 Receptors in Human Reinforcement Learning

Influential neurocomputational models emphasize dopamine (DA) as an electrophysiological and neurochemical correlate of reinforcement learning. However, evidence of a specific causal role of DA receptors in learning has been less forthcoming, especially in humans. Here we combine, in a between-subjects design, administration of a high dose of the selective DA D2/3-receptor antagonist sulpiride with genetic analysis of the DA D2 receptor in a behavioral study of reinforcement learning in a sample of 78 healthy male volunteers. In contrast to predictions of prevailing models emphasizing DA''s pivotal role in learning via prediction errors, we found that sulpiride did not disrupt learning, but rather induced profound impairments in choice performance. The disruption was selective for stimuli indicating reward, whereas loss avoidance performance was unaffected. Effects were driven by volunteers with higher serum levels of the drug, and in those with genetically determined lower density of striatal DA D2 receptors. This is the clearest demonstration to date for a causal modulatory role of the DA D2 receptor in choice performance that might be distinct from learning. Our findings challenge current reward prediction error models of reinforcement learning, and suggest that classical animal models emphasizing a role of postsynaptic DA D2 receptors in motivational aspects of reinforcement learning may apply to humans as well.     

Superoxide Dismutase,BDNF, and Cognitive Improvement in Drug-Naive First-Episode Patients With Schizophrenia: A 12-Week Longitudinal Study

ObjectiveCognitive improvement after antipsychotic agents in patients with schizophrenia (SCZ) appears to involve redox regulation through neurotrophins such as brain derived neurotropic factor (BDNF). This study examined whether cognitive improvement was associated with the increase in superoxide dismutase (SOD) and whether higher levels of BDNF could have a permissive role in allowing SOD to improve cognition.MethodsWe examined this hypothesis in 183 drug-naïve first-episode SCZ patients taking risperidone monotherapy for 12 weeks. We measured total copper-zinc SOD (CuZn-SOD), manganese SOD (Mn-SOD), and SOD activities and BDNF levels in these patients and compared their levels with 152 healthy controls. We assessed cognitive functioning and clinical symptoms at baseline and 12-week follow-up.ResultsAfter treatment with risperidone, CuZn-SOD activity was significantly increased, and BDNF levels were slightly increased. Increased CuZn-SOD activity was associated with the cognitive effectiveness of risperidone monotherapy. The BDNF levels and SOD activities were correlated at baseline but not after 12-week treatment. Furthermore, baseline CuZn-SOD activity positively correlated with improvement on the delayed memory subscale of the Repeatable Battery for the Assessment of Neuropsychological Status only in the high BDNF subgroup.ConclusionsOur longitudinal study suggests that risperidone can enhance SOD activity and that, in combination with higher baseline BDNF levels acting in a permissive role, can improve cognitive impairments in SCZ. Greater baseline CuZn-SOD activity also may have predictive value for cognitive improvement of delayed memory in SCZ patients receiving risperidone treatment.     

Blockade of Spinal Dopamine D2 Receptors Enhances the Pressor Effect of Intravenous Quinpirole in Normotensive,Conscious Rats

Abstract: The present investigation was undertaken to examine whether in conscious intact rats blockade of spinal dopamine D₂ receptors enhances the pressor effect of intravenous quinpirole. In saline‐pretreated rats, intravenous quinpirole (1 mg/kg) induced a significant pressor effect, which reached a maximum (17.71±0.60 mmHg) within the first min. after injection. Pretreatment with intravenous (0.5 mg/kg) or intrathecal (40 μg/rat at T₉‐T₁₀) domperidone, a dopamine D₂ receptor antagonist that does not cross the blood‐brain barrier, significantly enhanced the maximal pressor response to quinpirole (25.60±1.52 and 24.00±1.72 mmHg, respectively). The pressor effect of quinpirole was also significantly enhanced after combined pretreatment with intravenous and intrathecal domperidone, and its maximum (31.60±2.31 mmHg) was significantly higher than that recorded in animals pretreated with intrathecal or intravenous domperidone alone. Intravenous pretreatment with metoclopramide (5 mg/kg) fully abolished the quinpirole‐induced pressor effect. These results show that in conscious intact rats, blockade of spinal dopamine D₂ receptors enhances the pressor response to systemic quinpirole, suggesting that this agonist can decrease blood pressure through a spinal dopaminergic mechanism. Thus, our previous hypothesis that the entire effect of intravenous quinpirole on blood pressure in conscious rats can be composed of a central pressor action, a peripheral sympathoinhibitory depressor effect and also a spinal depressor effect is strongly supported by the present findings.     

Activation of Nociceptin/Orphanin FQ Peptide Receptors Disrupts Visual but Not Auditory Sensorimotor Gating in BALB/cByJ Mice: Comparison to Dopamine Receptor Agonists

Nociceptin/orphanin FQ (N/OFQ) peptide and its receptor (NOP receptor) have been implicated in a host of brain functions and diseases, but the contribution of this neuropeptide system to behavioral processes of relevance to psychosis has not been investigated. We examined the effect of the NOP receptor antagonists, Compound 24 and J-113397, and the synthetic agonist, Ro64-6198, on time function (2–2000 ms prepulse–pulse intervals) of acoustic (80 dB/10 ms prepulse) and visual (1000 Lux/20 ms prepulse) prepulse inhibition of startle reflex (PPI), a preattentive sensory filtering mechanism that is central to perceptual and mental integration. The effects of the dopamine D1-like receptor agonist, SKF-81297, the D2-like receptor agonist, quinelorane, and the mixed D1/D2 agonist, apomorphine, were studied for comparison. When acoustic stimulus was used as prepulse, BALB/cByJ mice displayed a monotonic time function of PPI, and consistent with previous studies, apomorphine and SKF-81279 induced PPI impairment, whereas quinelorane had no effect. None of the NOP receptor ligands was effective on acoustic PPI. When flash light was used as prepulse, BALB/cByJ mice displayed a bell-shaped time function of PPI and all dopamine agonists were active. Ro64-6198 was also effective in reducing visual PPI. NOP receptor antagonists showed no activity but blocked disruptive effect of Ro64-6198. Finally, coadministration of the typical antipsychotic, haloperidol, attenuated PPI impairment induced by Ro64-6198, revealing involvement of a dopaminergic component. These findings show that pharmacological stimulation of NOP or dopamine D2-like receptors is more potent in disrupting visual than acoustic PPI in mice, whereas D1-like receptor activation disrupts both. They further suggest that dysfunction of N/OFQ transmission may be implicated in the pathogenesis of psychotic manifestations.     

Dopamine D2/D3 receptors modulate cocaine's reinforcing and discriminative stimulus effects in rhesus monkeys

Numerous studies have suggested that dopamine (DA) D2 and D3 receptors are involved in the behavioral effects of cocaine. The present experiments evaluated the reinforcing and cocaine-like discriminative stimulus effects of several D2/D3 agonists in rhesus monkeys. In the first experiment, animals (n = 4) were trained to self-administer 0.03 mg/kg/inj cocaine under a fixed-interval (FI) 5-min schedule. When substituted for cocaine, the D2/D3 agonist quinpirole (0.003-0.03 mg/kg/inj) functioned as a reinforcer in all monkeys. In two cocaine-naive monkeys trained to respond under an FI 3-min schedule of food presentation, quinpirole maintained low rates of responding in one subject, while at the highest dose (0.03 mg/kg/inj) it functioned as a reinforcer in the second monkey. In this animal, increased activity was observed at this dose, which may have contributed to the overall rate of responding. In the second experiment, monkeys (n = 4) were trained to discriminate cocaine from saline using a two-lever, food-reinforced, drug discrimination procedure. The D2/D3 agonists quinpirole, (+/-)-7-OH-DPAT, and R-( + )-7-OH-DPAT fully substituted for cocaine. However, the time-course of substitution differed between quinpirole, which substituted for cocaine 10 min after administration, and (+/-)- and R-(+)-7-OH-DPAT, which required 60-min pretreatments. The behavioral potencies, as determined from ED50, values, correlated with previously reported in vitro binding affinity and functional activity at the D3 receptor [R-(+ )-7-OH-DPAT > (+/-)-7-OH-DPAT > quinpirole]. These results further indicate that direct-acting D2/D3 agonists can function as reinforcers and produce cocaine-like discriminative stimulus effects, and support the idea that D3 receptors should continue to be a valuable target for future behavioral studies evaluating cocaine's mechanisms of action.     

Dopamine D1/5 and D2/3 agonists differentially attenuate somatic signs of nicotine withdrawal in rats

Abrupt tobacco/nicotine cessation after chronic use causes various withdrawal symptoms/signs. There is evidence that dysfunction of brain dopaminergic system might be responsible for some nicotine withdrawal symptoms. The hypothesis for the present study was that different dopaminergic agonists would relieve different nicotine withdrawal signs. Adult male Sprague-Dawley rats were used. (−)-Nicotine bitartrate (9 mg/kg/day, salt content) or equimolar sodium tartrate was infused into each rat via a subcutaneous (s.c.) osmotic minipump for 7 days. To assess nicotine withdrawal signs, several somatic abstinence signs including teeth-chattering/chews, stretches/gasps, ptosis, shakes, and yawns were counted one day after removal of pumps. These signs were attenuated by the s.c. injection of 0.4 mg/kg nicotine bitartrate. Both a dopamine D_1/5 agonist (SKF81297) and a D_2/3 agonist (pramipexole) relieved abstinence signs dose-dependently but differentially. SKF81297 (0.32 mg/kg, s.c.) reduced teeth-chattering/chews but not shakes. Pramipexole (1 mg/kg, s.c.) decreased both teeth-chattering/chews and shakes. A low dose of pramipexole (0.1 mg/kg, s.c.) significantly increased yawns, consistent with previous studies that the stimulation of D₃ receptors induces yawning. These results indicate that a D₂-selective agonist should be considered a candidate to relieve nicotine withdrawal symptoms.     

Distribution of Dopamine D3 Receptor Expressing Neurons in the Human Forebrain: Comparison with D2 Receptor Expressing Neurons

The dopamine D₂ and D₃ receptors are members of the D₂ subfamily that includes the D₂, D₃ and D₄ receptor. In the rat, the D₃ receptor exhibits a distribution restricted to mesolimbic regions with little overlap with the D₂ receptor. Receptor binding and nonisotopic in situ hybridization were used to study the distribution of the D₃ receptors and neurons positive for D₃ mRNA in comparison to the D₂ receptor/mRNA in subcortical regions of the human brain. D₂ binding sites were detected in all brain areas studied, with the highest concentration found in the striatum followed by the nucleus accumbens, external segment of the globus pallidus, substantia nigra and ventral tegmental area, medial preoptic area and tuberomammillary nucleus of the hypothalamus. In most areas the presence of D₂ receptor sites coincided with the presence of neurons positive for its mRNA. D₃ binding sites and D₃ mRNA positive neurons were most abundant in the limbic striatum and efferent structures, such as the nucleus accumbens, ventral striatum, substantia nigra, internal segment of the globus pallidus, anteroventral nucleus of the thalamus, and rostral pars reticulata of the substantia nigra. One important difference from the rat is that D₃ receptors were virtually absent in the ventral tegmental area. D₃ receptor and D₃ mRNA positive neurons were observed in sensory, hormonal, and association regions such as the nucleus basalis, anteroventral, mediodorsal, and geniculate nuclei of the thalamus, mammillary nuclei, the basolateral, basomedial, and cortical nuclei of the amygdala. As revealed by simultaneous labeling for D₃ and D₂ mRNA, D₃ mRNA was often expressed in D₂ mRNA positive neurons. Neurons that solely expressed D₂ mRNA were numerous and regionally widespread, whereas only occasional D₃-positive-D₂-negative cells were observed. The regions of relatively higher expression of the D₃ receptor and its mRNA appeared linked through functional circuits, but co-expression of D₂ and D₃ mRNA suggests a functional convergence in many regions of the signals mediated by the two receptor subtypes.     

Striatal Dopamine D2 Receptor Occupancy Induced by Daily Application of Blonanserin Transdermal Patches: Phase II Study in Japanese Patients With Schizophrenia

BackgroundTransdermal antipsychotic patch formulations offer potential benefits, including improved adherence. This study investigated the striatal dopamine D₂ receptor occupancy with daily blonanserin transdermal patch application.MethodsThis open-label, phase II study enrolled 18 Japanese outpatients (20 to <65 years) with schizophrenia (DSM-IV-TR criteria; total Positive and Negative Syndrome Scale score <120 at screening) treated with blonanserin 8-mg or 16-mg tablets. Patients continued tablets for 2–4 weeks at their current dose and were then assigned to once-daily blonanserin patches (10/20/40/60/80 mg daily) for 2–4 weeks based on the oral dose. [¹¹C]raclopride positron emission tomography scanning determined blonanserin striatal dopamine D₂ receptor occupancy (primary endpoint). Secondary endpoints included assessment of receptor occupancy by dose, changes in Positive and Negative Syndrome Scale and Clinical Global Impressions-Severity of Illness-Severity scores, patient attitudes towards adherence, and patch adhesiveness.ResultsOf 18 patients who started the blonanserin tablet treatment period, 14 patients completed treatment. Mean D₂ receptor occupancy for blonanserin tablets 8 mg/d (59.2%, n = 5) and 16 mg/d (66.3%, n = 9) was within the values for blonanserin patches: 10 mg/d (33.3%, n = 3), 20 mg/d (29.9%, n = 2), 40 mg/d (61.2%, n = 3), 60 mg/d (59.0%, n = 3), and 80 mg/d (69.9%, n = 3). Occupancy generally increased with increasing blonanserin dose for both formulations with the half maximal receptor occupancy for tablets and patches associated with doses of 6.9 mg/d and 31.9 mg/d, respectively. Diurnal variability in occupancy was lower during transdermal patch treatment than during tablet treatment. Blonanserin transdermal patches were well tolerated with no major safety concerns.ConclusionsBlonanserin patches (40/80 mg/d) have lower diurnal variability in occupancy than blonanserin tablets (8/16 mg/d), and patches at doses of 40 mg/d and 80 mg/d appear to be a suitable alternative for blonanserin tablets at doses of 8 mg/d and 16 mg/d, respectively. Blonanserin patches represent a potential new treatment option for patients with schizophrenia.Trial registryJAPIC Clinical Trials Information registry (www.clinicaltrials.jp; JapicCTI-No: JapicCTI-121914).     

Higher Levels of Glutamate in the Associative-Striatum of Subjects with Prodromal Symptoms of Schizophrenia and Patients with First-Episode Psychosis

The glutamatergic and dopaminergic systems are thought to be involved in the pathophysiology of schizophrenia. Their interaction has been widely documented and may have a role in the neurobiological basis of the disease. The aim of this study was to compare, using proton magnetic resonance spectroscopy (¹H-MRS), glutamate levels in the precommissural dorsal-caudate (a dopamine-rich region) and the cerebellar cortex (negligible for dopamine) in the following: (1) 18 antipsychotic-naïve subjects with prodromal symptoms and considered to be at ultra high-risk for schizophrenia (UHR), (2) 18 antipsychotic-naïve first- episode psychosis patients (FEP), and (3) 40 age- and sex- matched healthy controls. All subjects underwent a ¹H-MRS study using a 3Tesla scanner. Glutamate levels were quantified and corrected for the proportion of cerebrospinal fluid and percentage of gray matter in the voxel. The UHR and FEP groups showed higher levels of glutamate than controls, without differences between UHR and FEP. In the cerebellum, no differences were seen between the three groups. The higher glutamate level in the precommissural dorsal-caudate and not in the cerebellum of UHR and FEP suggests that a high glutamate level (a) precedes the onset of schizophrenia, and (b) is present in a dopamine-rich region previously implicated in the pathophysiology of schizophrenia.     

Effects of Remoxipride's Metabolites on Dopamine D2 Receptors and Receptor Functions in the Rat

Abstract: The main metabolites of remoxipride formed in rat and man were examined for their affinities for the [³H] SCH 23390-labelled DA D₁ and [³H]-raclopride-labelled D₂ receptors in rat striatal homogenates. In addition, their effectiveness in blocking postsynaptic DA receptor activity in vivo was measured by the use of several different test models in the male rat. Phenolic metabolites formed mainly in the rat retained (similar to remoxipride) their selectivity for the D₂ receptor with very low affinities for the D₁ receptor. The pyrrolidone metabolites formed mainly in man showed very low affinities for both the D₁ and D₂ receptors. The ability of the metabolites to block postsynaptic DA receptor activity in vivo correlated with their affinities for the D₂ receptor. Among the metabolites tested, the phenolic compounds FLA 797 (–) and FLA 908 (–) were much more effective than remoxipride in inducing catalepsy, which is consistent with a higher affinity for [³H] raclopride labelled striatal D₂ receptors. However, analysis of the effectiveness of the DA receptor blockade (blockade of d-amphetamine locomotion or DA agonist hypothermia) after intraperitoneal or subcutaneous administration suggested that FLA 797 (–)/FLA 908 (–) may only contribute marginally to the D₂ receptor-blocking activity of remoxipride in the rat. This conclusion was further supported by the observation that the atypical antipsychotic profile of remoxipride was not mimicked by the active metabolites. The weak DA D₂ blocking effect of the pyrrolidone metabolites indicated that remoxipride is responsible for the clinical action.     

细胞色素P450酶1A2、2D6与多巴胺D2受体基因多态性对奥氮平治疗精神分裂症PANSS减分率的影响

目的 研究细胞色素P450酶（cytochrome P450,CYP）1A2、2D6以及多巴胺D2受体（dopamine receptor D2,DRD2）的基因多态性对奥氮平治疗精神分裂症阳性和阴性症状量表（positive and negative syndrome scale,PANSS）减分率的影响及其程度。方法 入组只用奥氮平治疗的精神分裂症住院患者178例,评定治疗前以及治疗4周后的PANSS量表得分,计算减分率。同时,收集血液,测定CYP1A2*1F、CYP2D6*10、DRD2-141C Ins/Del、DRD2-241 A>G、DRD2 Taq1A位点的基因多态性。通过方差分析,比较各基因型PANSS减分率的差异;通过多元线性回归分析,得出减分率的回归方程,并计算决定系数。结果 PANSS减分率在CYP1A2*1F（CC：65.68±11.22;CA：55.59±15.40;AA：43.75±15.20）、CYP2D6*10（CC：44.36±16.67;CT：51.78±17.81;TT：56.14±17.13）、DRD2-141C Ins/Del （Ins/Ins：55.11±17.39;Ins/Del：39.16±14.28）和DRD2-241 A>G （AA：45.47±17.52;GA：61.82±10.55;GG：75.43±17.71）不同基因型之间均有统计学显著差异（P均<0.01）,而DRD2 Taq1A位点的基因型间PANSS减分率差异无统计学意义。PANSS减分率=58.041-10.703×CYP1A2*1F+4.272×CYP2D6*10-11.921×DRD2-141C Ins/Del+13.443×DRD2-241 A>G （决定系数R²=0.517,P<0.05）。结论 CYP1A2*1F、CYP2D6*10、DRD2-141 C Ins/Del、DRD2-241A>G位点基因多态性影响奥氮平治疗精神分裂症疗效,但只解释了减分率的51.7%,有待纳入更多的影响因素进行分析。     

Effects of Dopamine D1 and D2 Receptor Agonists and Antagonists on Seizures Induced by Chemoconvulsants in Mice

Abstract: Dopamine agonists and antagonists with different affinities for D₁ and D₂ receptors in the brain were assessed for their ability to affect clonic seizures in mice induced by chemoconvulsants. The dopamine D₂ antagonists remoxipride (5-20 mg/kg) and raclopride (5-20 mg/kg), haloperidol (2.5 and 5 mg/kg) and the D₁ antagonist SCH 23390 (0.3, 1.5 mg/kg) did not markedly modify seizures induced by pentylenetetrazole, picrotoxin or bicuculline. The dopamine D₂ agonist quinpirole only weakly blocked the action of pentylenetetrazole while the D₁ agonist SKF 38393 (1-10 mg/kg subcutaneously) caused a dose-dependent blockade of pentylenetetrazole-induced seizures. The D₁/D₂ agonist apomorphine given at “postsynaptic” doses (1 and 2 mg/kg) blocked pentylenetetrazole-induced seizures. The protection afforded by apomorphine against pentylenetetrazole seizures appeared to be associated with its activation of both D₁ and D₂ receptors since both raclopride and SCH 23390 blocked the action of apomorphine. Reserpine and the two partial dopamine autoreceptor agonists, (—)3-PPP and HW-165, at high (non-autoreceptor selective) doses induced seizures in animals treated with the subconvulsive dose of pentylenetetrazole. The overall results suggest that dopamine receptor blockade has a minor or limited effect on seizures caused by GABA inhibition. The anticonvulsant effect of dopamine agonists such as apomorphine appears to be mediated by postsynaptic dopamine D₁ and D₂ receptors. Stimulation of dopamine D₁ receptors can reduce seizure activity caused by GABA receptor blockade possibly by facilitation of GABA transmission in the striatum and substantia nigra.     

Dopamine D2/D3 receptor agonist quinpirole impairs spatial reversal learning in rats: investigation of D3 receptor involvement in persistent behavior

Rationale  Dopamine is strongly implicated in the ability to shift behavior in response to changing stimulus-reward contingencies. Objectives  We investigated the effects of systemic administration of the D2/D3 receptor agonist quinpirole (0.1, 0.3 mg/kg), the D2/D3 receptor antagonist raclopride (0.1, 0.3 mg/kg), the selective D3 antagonist nafadotride (0.3, 1.0 mg/kg), and combined administration of raclopride (0.1 mg/kg) or nafadotride (1.0 mg/kg) with quinpirole (0.3 mg/kg) on spatial discrimination and reversal learning. Materials and methods  Rats were trained on an instrumental two-lever spatial discrimination and reversal learning task. Both levers were presented, only one of which was reinforced. The rat was required to respond on the reinforced lever under a fixed ratio 3 schedule of reinforcement. Following attainment of criterion, a reversal was introduced. Results  None of the drugs altered performance during retention of the previously reinforced contingencies. Quinpirole (0.3 mg/kg) significantly impaired reversal learning by increasing both trials and incorrect responses to criterion in reversal phase, a pattern of behavior manifested as increased perseverative responding on the previously reinforced lever. In contrast, neither raclopride nor nafadotride when administered alone altered reversal performance. However, raclopride blocked the quinpirole-induced reversal deficit, whereas combined administration of nafadotride and quinpirole affected not only performance during the reversal but also the retention phase. The reversal impairment resulting from co-administration of nafadotride and quinpirole was associated with both perseverative and learning errors. Conclusions  Our data indicate distinct roles for D2 and D3 receptors in the capacity to modify behavior flexibly in the face of environmental change.     

Effects of repeated treatment with the dopamine D2/D3 receptor partial agonist aripiprazole on striatal D2/D3 receptor availability in monkeys

Rationale

Chronic treatment with dopamine (DA) receptor agonists and antagonists can differentially affect measures of DA D2/D3 receptor number and function, but the effects of chronic treatment with a partial D2/D3 receptor agonist are not clear.

Objective

We used a within-subjects design in male cynomolgus monkeys to determine the effects of repeated (17-day) treatment with the D2/D3 receptor partial agonist aripiprazole (ARI; 0.03 mg/kg and 0.1 mg/kg i.m.) on food-reinforced behavior (n?=?5) and on D2/D3 receptor availability as measured with positron emission tomography (PET; n?=?9).

Methods

Five monkeys responded under a fixed-ratio 50 schedule of food reinforcement and D2/D3 receptor availability was measured before and 4 days after ARI treatment using PET and the D2/D3 receptor-selective radioligand [¹⁸F]fluoroclebopride (FCP). Four additional monkeys were studied using [¹¹C]raclopride and treated sequentially with each dose of ARI for 17 days.

Results

ARI decreased food-maintained responding with minimal evidence of tolerance. Repeated ARI administration increased FCP and raclopride distribution volume ratios (DVRs) in the caudate nucleus and putamen in most monkeys, but decreases were observed in monkeys with the highest baseline DVRs.

Conclusions

The results indicate that repeated treatment with a low-efficacy DA receptor partial agonist produces effects on brain D2/D3 receptor availability that are qualitatively different from those of both high-efficacy receptor agonists and antagonists, and suggest that the observed individual differences in response to ARI treatment may reflect its partial agonist activity.