Postsynaptic Dopamine D2 Receptors Research Articles

Safety and efficacy of KarXT (Xanomeline Trospium) in Schizophrenia in the Phase 3, Randomized, Double-Blind, Placebo-Controlled EMERGENT-2 Trial

AbstractIntroductionKarXT combines the M1/M4 preferring muscarinic receptor agonist xanomeline and the peripherally restricted anticholinergic trospium. In the phase 2 EMERGENT-1 study, KarXT met the primary endpoint of a significant reduction in Positive and Negative Syndrome Scale (PANSS) total score through week 5 vs placebo, improved other key secondary efficacy measures, and was generally well tolerated.MethodsEMERGENT-2 was a phase 3, randomized, double-blind, placebo-controlled, 5-week trial of KarXT in acutely psychotic patients with schizophrenia in the inpatient setting. Eligible patients were randomized 1:1 to KarXT or matched placebo. Dosing of KarXT (mg xanomeline/mg trospium) started at 50 mg/20 mg BID and increased to a maximum of 125 mg/30 mg BID. The primary efficacy endpoint was change from baseline to week 5 in PANSS total score. Key secondary endpoints included change from baseline to week 5 in PANSS positive subscale, PANSS negative subscale, and PANSS negative Marder factor scores compared with placebo. Efficacy analyses were performed using the modified intent-to-treat population (patients with ≥1 dose of study medication, a baseline PANSS assessment, and ≥1 postbaseline PANSS assessment). All patients receiving ≥1 dose of study drug were included in safety analyses.Results252 US patients were enrolled. KarXT demonstrated a statistically significant and clinically meaningful 9.6-point reduction from baseline to week 5 (effect size=0.61) in PANSS total score vs placebo (p<0.0001); a significant improvement in PANSS total score was demonstrated starting at week 2 (first postbaseline rating) and continued through the study end. KarXT also met key secondary endpoints. Results at week 5 included a 2.9-point reduction in PANSS positive subscale score with KarXT vs placebo (p<0.0001), a 1.8-point reduction in PANSS negative subscale score with KarXT vs placebo (p=0.0055), and a 2.2-point reduction in PANSS negative Marder factor score with KarXT vs placebo (p=0.0022). KarXT was generally well tolerated. Overall discontinuation rates were similar with KarXT (25%) and placebo (21%). The overall treatment-emergent adverse events (TEAEs) rate for KarXT and placebo was 75% and 58%, respectively. Discontinuation rates related to TEAEs were similar between KarXT (7%) and placebo (6%). Rates of serious TEAEs were similar with KarXT and placebo (2%, each group); no serious TEAEs were determined to be drug related. The most common TEAEs (≥5%) with KarXT were all mild to moderate in severity and included constipation, dyspepsia, nausea, vomiting, headache, blood pressure increases, dizziness, gastroesophageal reflux disease, abdominal discomfort, and diarrhea. KarXT was not associated with sedation/somnolence, weight gain, and extrapyramidal symptoms.ConclusionsKarXT has the potential to be the first in a new class of treatments for patients with schizophrenia and a promising alternative to postsynaptic dopamine D2 receptor antagonists.FundingKaruna Therapeutics, Inc.

Partial Ablation of Postsynaptic Dopamine D2 Receptors in the Central Nucleus of the Amygdala Increases Risk Avoidance in Exploratory Tasks.

The central nucleus of the amygdala (CeA) is involved in the expression of fear and has been implicated in several anxiety disorders. This structure is densely innervated by DAergic projections that impinge on amygdalar neurons expressing various dopamine (DA) receptor subtypes, including D2 receptors (D2Rs). Although various pharmacological approaches have assessed the role of D2Rs in the CeA, the actual participation of postsynaptic D2Rs in the CeA to defensive behaviors remains unclear. Here, we investigated the distribution of D2Rs in the CeA and their role in modifying neuronal activity and fear related behaviors in mice. First, using the mouse reporter strain D2R-EGFP, we verified that D2Rs are present both in neurons of the CeA and in A10 dorsocaudal (A10dc) DAergic neurons that innervate the CeA. Moreover, we showed that pharmacological stimulation of D2Rs increases the activity of protein kinase C (PKC)δ cells present in the CeA, a type of neuron previously associated with reduced defensive behaviors. Finally, using a molecular genetics approach that discriminates postsynaptic D2Rs from presynaptic D2 autoreceptors, we demonstrated that mice carrying targeted deletions of postsynaptic D2Rs in the CeA display increased risk avoidance in exploratory tasks. Together, our results indicate that postsynaptic D2Rs in the CeA attenuate behavioral reactions to potential environmental threats.

Emerging Treatments in Schizophrenia.

Although antipsychotics have been available for almost 70 years and greatly improved outcomes for individuals with schizophrenia, all currently available options derive their efficacy from blockade of dopaminergic receptors. However, this mechanism of action leaves many symptoms unresolved and is associated with a significant side effect burden. The mechanisms underlying schizophrenia, which were initially thought to be related to excessive presynaptic dopamine in specific areas of the brain, are now understood to be much more complex and involve structural and molecular changes throughout brain circuits. Consequently, drug discovery efforts have sought new targets in the search for safer and more effective medications that can improve symptoms of schizophrenia and psychosis, including trace amine-associated receptors (TAARs), muscarinic receptors, and serotonergic receptors. Positive phase 2 trial results indicating efficacy and safety of the TAAR1 agonist ulotaront (SEP-363856) and of the muscarinic M1/M4 agonist KarXT (xanomeline plus trospium) for total, positive, and negative symptoms in patients with acute exacerbation of schizophrenia, and of the serotonin 5-HT2A agonist/antagonist pimavanserin in patients with schizophrenia and predominant negative symptoms for negative symptom control are encouraging. Taken together, these data indicate in the context of ongoing phase 3 trial programs that patients with schizophrenia may soon have access to the first non-D2 blocking medication, which could drastically change the treatment landscape and improve outcomes for many of the individuals with schizophrenia who do not fully respond to or cannot tolerate currently available antipsychotic agents that currently all act via postsynaptic dopamine D2 receptor blockade.

Emerging Treatments in Schizophrenia.

Although antipsychotics have been available for almost 70 years and greatly improved outcomes for individuals with schizophrenia, all currently available options derive their efficacy from blockade of dopaminergic receptors. However, this mechanism of action leaves many symptoms unresolved and is associated with a significant side effect burden. The mechanisms underlying schizophrenia, which were initially thought to be related to excessive presynaptic dopamine in specific areas of the brain, are now understood to be much more complex and involve structural and molecular changes throughout brain circuits. Consequently, drug discovery efforts have sought new targets in the search for safer and more effective medications that can improve symptoms of schizophrenia and psychosis, including trace amine-associated receptors (TAARs), muscarinic receptors, and serotonergic receptors. Positive phase 2 trial results indicating efficacy and safety of the TAAR1 agonist ulotaront (SEP-363856) and of the muscarinic M1/M4 agonist KarXT (xanomeline plus trospium) for total, positive, and negative symptoms in patients with acute exacerbation of schizophrenia, and of the serotonin 5-HT2A agonist/antagonist pimavanserin in patients with schizophrenia and predominant negative symptoms for negative symptom control are encouraging. Taken together, these data indicate in the context of ongoing phase 3 trial programs that patients with schizophrenia may soon have access to the first non-D2 blocking medication, which could drastically change the treatment landscape and improve outcomes for many of the individuals with schizophrenia who do not fully respond to or cannot tolerate currently available antipsychotic agents that currently all act via postsynaptic dopamine D2 receptor blockade.

Alterations in dopamine system and in its connectivity with serotonin in a rat model of Alzheimer's disease.

Dopamine pathways alterations are reported in Alzheimer’s disease. However, it is difficult in humans to establish when these deficits appear and their impact in the course of Alzheimer’s disease. In the TgF344-Alzheimer’s disease rat model at the age of 6 months, we showed a reduction in in vivo release of striatal dopamine due to serotonin 5HT2A-receptor blockade, in the absence of alterations in 5HT2A-receptor binding, suggesting a reduction in 5HT2A-receptor-dopamine system connectivity. In addition, a functional hypersensitivity of postsynaptic dopamine D2-receptors and D2-autoreceptors was also reported without any change in D2-receptor density and in the absence of amyloid plaques or overexpression of the 18 kDa translocator protein (an inflammatory marker) in areas of the dopamine system. Citalopram, a selective serotonin reuptake inhibitor, induced functional 5HT2A-receptor−D2-receptor connectivity changes but had no effect on D2-autoreceptor hypersensitivity. In older rats, dopamine cell bodies overexpressed translocator protein and dopamine projection sites accumulated amyloid. Interestingly, the 5HT2A-receptor density is decreased in the accumbens subdivisions and the substantia nigra pars compacta. This reduction in the striatum is related to the astrocytic expression of 5HT2A-receptor. Our results indicate that both serotonin/dopamine connectivity and dopamine signalling pathways are dysregulated and potentially represent novel early diagnostic and therapeutic avenues.

Brain disease and network reorganization

Neuroscience Parkinson's disease (PD) offers an opportunity to study the interplay between compensatory functional and molecular mechanisms. Although postsynaptic dopamine D2 receptors are up-regulated in PD, their importance in neural activity has never been identified. To investigate the relationship between reorganization of dopaminergic networks and functional reorganization of brain activity patterns, Rebelo et al. combined PET and fMRI scans of the brain areas involved in the execution of saccades. They found a tight link between functional activation and synaptic changes at the molecular level, reflecting network reorganization in PD. The association between D2 receptor binding and reorganization of the saccadic cortical network reflects parietostriatal rerouting in response to a progressive frontostriatal dopamine deficit as the disease develops. Proc. Natl. Acad. Sci. U.S.A. 118 , e2013962118 (2020).

Expanding Role of Dopaminergic Inhibition in Hypercapnic Responses of Cultured Rat Carotid Body Cells: Involvement of Type II Glial Cells.

Dopamine (DA) is a well-studied neurochemical in the mammalian carotid body (CB), a chemosensory organ involved in O2 and CO2/H+ homeostasis. DA released from receptor (type I) cells during chemostimulation is predominantly inhibitory, acting via pre- and post-synaptic dopamine D2 receptors (D2R) on type I cells and afferent (petrosal) terminals respectively. By contrast, co-released ATP is excitatory at postsynaptic P2X2/3R, though paracrine P2Y2R activation of neighboring glial-like type II cells may boost further ATP release. Here, we tested the hypothesis that DA may also inhibit type II cell function. When applied alone, DA (10 μM) had negligible effects on basal [Ca2+]i in isolated rat type II cells. However, DA strongly inhibited [Ca2+]i elevations (Δ[Ca2+]i) evoked by the P2Y2R agonist UTP (100 μM), an effect opposed by the D2/3R antagonist, sulpiride (1–10 μM). As expected, acute hypercapnia (10% CO2; pH 7.4), or high K+ (30 mM) caused Δ[Ca2+]i in type I cells. However, these stimuli sometimes triggered a secondary, delayed Δ[Ca2+]i in nearby type II cells, attributable to crosstalk involving ATP-P2Y2R interactions. Interestingly sulpiride, or DA store-depletion using reserpine, potentiated both the frequency and magnitude of the secondary Δ[Ca2+]i in type II cells. In functional CB-petrosal neuron cocultures, sulpiride potentiated hypercapnia-induced Δ[Ca2+]i in type I cells, type II cells, and petrosal neurons. Moreover, stimulation of type II cells with UTP could directly evoke Δ[Ca2+]i in nearby petrosal neurons. Thus, dopaminergic inhibition of purinergic signalling in type II cells may help control the integrated sensory output of the CB during hypercapnia.

Dopamine D2 up-regulation in psychosis patients after antipsychotic drug treatment.

Recently, it has been questioned whether the re-emergence of psychotic symptoms following antipsychotic discontinuation or dose reduction is attributable to underlying psychotic vulnerability or to rebound effects of chronic use of antipsychotic medication. It was repeatedly shown that relapse rates are high after discontinuation of maintenance treatment. A potential contributing factor could be the increase in density of postsynaptic dopamine D2 receptors in the striatum and the higher affinity of D2 receptors for dopamine after chronic blockade. To date, little clinical evidence is available for the mechanisms involved in postsynaptic striatal D2 receptor up-regulation after use of antipsychotic medication, and most knowledge comes from animal studies. Further research is needed to investigate whether antipsychotic medication causes neuroadaptations leading to a dopamine supersensitive state in humans, how long such hypersensitive states may last and what differences exist between high and low D2 affinity antipsychotic drugs. Further, information is needed on discontinuation schedules that provide optimal protection for relapse during hypersensitive periods.

Plasma monoamines change under dopamine supersensitivity psychosis in patients with schizophrenia: A comparison with first-episode psychosis.

Patients with first-episode psychosis respond well to initial antipsychotic treatment, but among patients experiencing a relapse of psychosis, the response rate falls to approximately 30%. The mechanism of this discrepancy has not been clarified, but the development of dopamine supersensitivity psychosis with the underlying up-regulation of post-synaptic dopamine D2 receptors could be involved in this lesser response. It is uncertain whether elevated dopamine synthesis and release occurs in patients with dopamine supersensitivity psychosis, in contrast to those with first-episode psychosis. We examined a first-episode psychosis group (n=6) and a chronic schizophrenia group, i.e. patients experiencing relapse (n=23) including those who relapsed due to dopamine supersensitivity psychosis (n=18). Following the initiation of treatment, we measured the patients' blood concentrations of homovanillic acid and 3-methoxy-4-hydroxyphenylglycol at two weeks and four weeks after the baseline measurements. The first-episode psychosis group tended to show decreased homovanillic acid, accompanied by an improvement of symptoms. The chronic schizophrenia group showed no alteration of homovanillic acid or 3-methoxy-4-hydroxyphenylglycol over the treatment period. These results were the same in the dopamine supersensitivity psychosis patients alone. Our findings suggest that unlike first-episode psychosis, the release of dopamine from presynaptic neurons did not increase in relapse episodes in the patients with dopamine supersensitivity psychosis. This indirectly indicates that the development of supersensitivity of post-synapse dopamine D2 receptor is involved in relapse in dopamine supersensitivity psychosis patients.

Use of antipsychotic drugs in dermatology

Antipsychotic drugs can be beneficial in dermatology because of their both central nervous system and peripheral effects. All antipsychotic drugs have a central postsynaptic dopamine D2 receptor blocking effect, which underlies their antipsychotic action. The antipsychotic drugs have varying degrees of histamine H1-receptor, cholinergic muscarinic receptor, and α1-adrenergic receptor blocking effects, which can affect cutaneous perception and the autonomic reactivity of the skin and can be potentially beneficial in the management of certain histamine or sympathetically mediated dermatologic manifestations (eg, urticaria, pruritus, hyperhidrosis). In addition to their antipsychotic effect, antipsychotic drugs also have a general anxiolytic effect related in part to their α1-adrenergic receptor blocking action, which can be of benefit in many dermatologic conditions, including pruritus. The antipsychotic drugs are most commonly used in dermatology for the management of a delusional disorder, somatic type, manifesting as delusional infestation, and as monotherapy or as augmentation therapy of selective serotonin reuptake inhibitor (SSRI) antidepressants, and for management of trichotillomania and skin-picking or excoriation disorder. There is earlier literature (1) on the possible beneficial effect of the phenothiazine antipsychotics in a wide range of pruritic dermatoses, and (2) the efficacy of pimozide as adjunctive therapy for metastatic melanoma, which both warrant further investigation.

Effect of MPTP on Serotonergic Neuronal Systems and Mitochondrial Complex I Activity in the Living Brain: A PET Study on Conscious Rhesus Monkeys

The objective of the present PET study was to compare the effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on serotonergic neuronal systems and mitochondrial complex I (MC-I) activity with that of dopamine in conscious rhesus monkeys (Macaca mulatta). Methods: A Parkinson disease monkey model was prepared by repeated administration of MPTP. For the PET measurements, normal and MPTP-treated conscious monkeys received an intravenous injection of 11C-DASB for serotonin transporter, 18F-MPPF for serotonin 1A receptor, 11C-PE2I for dopamine transporter, 11C-6MemTyr for dopamine synthesis, 11C-raclopride for dopamine D2 receptor, or 18F-BCPP-EF for MC-I. Serotonin and dopamine parameters were calculated using time-activity curves in the cerebellum as the input function. The total distribution volume of 18F-BCPP-EF was assessed using Logan plot graphical analysis with metabolite-corrected plasma as the input function. Results: MPTP-induced diffuse reductions in MC-I activity were observed throughout the brain, except the cerebellum. Significant reductions in the presynaptic dopamine parameters-dopamine transporter and dopamine synthesis-were detected in the striatum and substantia nigra pars compacta of MPTP-treated monkeys, whereas no significant differences in postsynaptic dopamine D2 receptor binding were observed. Serotonin transporter binding was reduced by MPTP not only in striatal regions but also in extrastriatal regions. In contrast, serotonin 1A receptor binding was unaffected by MPTP anywhere in the brain. In the cortex, the reduction of serotonin transporter binding correlated with that of MC-I. Conclusion: The results obtained by multiparametric PET measurements in a Parkinson disease monkey model demonstrated that chronic MPTP treatment induced reductions not only in the dopaminergic system in the nigrostriatal pathway but also in serotonin transporter in the cortical and subcortical regions. These results suggest that the neurotoxicity of MPTP is not exclusive to the nigrostriatal pathway, as predicted from MC-I damage in the extrastriatal regions of the brain.

Concomitant behavioral and PFC neuronal activity recorded following dose-response protocol of MPD in adult male rats.

The use of methylphenidate (MPD), a commonly prescribed drug to treat attention-deficit hyperactivity disorder (ADHD), has steadily increased over the past 25 years. This trend has been accompanied by more MPD abuse by ordinary individuals for its cognitive enhancing effects. Therefore, understanding the effects of MPD on the prefrontal cortex (PFC), a brain area involved in higher cortical processing such as executive function, language, planning, and attention regulation, is of particular importance. The goal of this study is to investigate the effects of acute and chronic dose-response characteristics following MPD exposure on both the PFC neuronal population and behavioral activity in freely behaving animals implanted previously with permanent electrodes within the PFC. Four groups of animals were used: saline (control), 0.6, 2.5, and 10.0mg/kg MPD. It was observed that the same dose of either 0.6, 2.5, or 10.0mg/kg repetitive (chronic) MPD exposure elicited behavioral sensitization in some animals and behavioral tolerance in others, and that the majority of PFC units recorded from animals expressing behavioral sensitization to chronic MPD exposure responded to MPD by increasing their neuronal firing rate, whereas the majority of PFC neurons recorded from animals expressing behavioral tolerance in response to chronic MPD responded by decreasing their neuronal firing rate. This data suggests that in animals that display behavioral sensitization, chronic MPD exposure causes an increase in the number of post-synaptic D1 dopamine receptors leading to an increase in behavioral and neuronal firing rate, while in animals that display behavioral tolerance, chronic MPD exposure causes an increase in the number of post-synaptic D2 dopamine receptors leading to a decrease in behavioral and neuronal firing rate. This dichotomy needs to be further investigated.

Dopaminergic abnormalities in Hdac6-deficient mice

Histone deacetylase 6 (Hdac6), a multifunctional cytoplasmic deacetylase, is abundant in brain. We previously demonstrated that global Hdac6 depletion causes aberrant emotional behaviors in mice. Identification of affected brain systems and its molecular basis will lead to new insights into relations between protein acetylation events and psychiatric disorders.Here we report the dopaminergic abnormalities in Hdac6 KO mice. The dopamine transmission mediated by D1-like and D2-like G protein-coupled dopamine receptors is known to play roles in controlling movement, cognition, and motivational processes, and its dysfunction causes psychiatric disorders. We found that Hdac6 KO mice showed significantly increased locomotor response to novel, but not to habituated environment. In addition, Hdac6 KO mice showed a long-lasting sensitivity to psychostimulants, increased locomotor response to D2-like, but not D1 dopamine receptor agonists, and rapid locomotor response to apomorphine, a direct dopamine agonist, in dopamine-depleted condition. Hdac6 protein was expressed in dopaminergic neurons and their terminals in adult mice brain, and Hdac6-depletion augmented acetylation levels of dopamine-enriched synaptosomal proteins. In Hdac6 KO mice, the striatal content of dopamine and its metabolites was normal in basal condition, but mRNA level of D2 dopamine receptor in the striatum was decreased by 30%. Taken together, our results provide evidence that Hdac6 deficiency leads to aberrant dopamine-dependent behaviors by enhancing postsynaptic dopamine D2 receptor response. This study points out the possibility that Hdac6 and reversible-acetylation events play a regulatory role in D2 dopamine receptor signaling, and thus participate in the pathology of the dopamine-related psychiatric disorders such as schizophrenia.

Postsynaptic D2 dopamine receptor supersensitivity in the striatum of mice lacking TAAR1

Trace Amine-Associated Receptor 1 (TAAR1) is a G protein-coupled receptor (GPCR) known to modulate dopaminergic system through several mechanisms. Mice lacking this receptor show a higher sensitivity to dopaminergic stimuli, such as amphetamine; however, it is not clear whether D1 or D2 dopamine receptors and which associated intracellular signaling events are involved in this modulation. In the striatum of TAAR1 knock out (TAAR1-KO mice) we found that D2, but not D1, dopamine receptors were over-expressed, both in terms of mRNA and protein levels. Moreover, the D2 dopamine receptor-related G protein-independent AKT/GSK3 signaling pathway was selectively activated, as indicated by the decrease of phosphorylation of AKT and GSK3β. The decrease in phospho-AKT levels, suggesting an increase in D2 dopamine receptor activity in basal conditions, was associated with an increase of AKT/PP2A complex, as revealed by co-immunoprecipitation experiments. Finally, we found that the locomotor activation induced by the D2 dopamine receptor agonist quinpirole, but not by the full D1 dopamine receptor agonist SKF-82958, was increased in TAAR1-KO mice. These data demonstrate pronounced supersensitivity of postsynaptic D2 dopamine receptors in the striatum of TAAR1-KO mice and indicate that a close interaction of TAAR1 and D2 dopamine receptors at the level of postsynaptic structures has important functional consequences.

Activation of postsynaptic D2 dopamine receptors in the rat dorsolateral striatum prevents the amnestic effect of systemically administered neuroleptics

Systemically administered antipsychotics bind to dopamine (DA) D2 receptors expressed in both pre- and postsynaptic neurons of different striatal sites and present an amnestic effect on learning and memory of conditioned avoidance responses (CAR). The aim of this study was to test whether blockade of the pre- or post-synaptic D2 receptors of the dorsolateral striatum of rats is the mechanism by which systemically administered antipsychotics present this amnestic effect. CAR learning and memory was evaluated in rats that received i.p. administrations of pre- or postsynaptic doses of the antipsychotic sulpiride combined with intra-DLS infusion of the D2 agonist quinpirole. Intra-DLS quinpirole itself was not amnestic and this effect was prevented by co-administration of presynaptic dose of sulpiride. However, sulpiride was amnestic when administered systemically in a post- but not presynaptic dose. This amnestic effect of sulpiride was prevented by the co-administration of quinpirole into the DLS. These results show that a blockade of postsynaptic D2 receptors in the DLS is necessary and sufficient to produce the amnestic effect of neuroleptics on CARs.

Late-onset Quetiapine-related Tardive Dyskinesia Side Effects in a Patient with Psychotic Depression.

The atypical antipsychotics were believed to induce less extrapyramidal syndrome, including tardive dyskinesia (TD). Since the introduction of the quetiapine, it is also reported with less TD side effects. It even can relieve the symptoms of severe TD and reduce the risk of TD. The quetiapine's low affinity and fast dissociation from postsynaptic dopamine D2 receptors should give the least risk of producing the symptoms of TD. The quetiapine even can reduce the TD side effects related to clozapine, which has the lowest risk for TD. However, since the first case report of TD side effects related to quetiapine published on 1999, the safety of quetiapine in TD aspect has been questioned. Therefore, we want to share this case report, which was written to describe the severe late-onset TD side effects after long-term use of quetiapine in a patient with psychotic depression. The patient had no significant findings after concurrent comprehensive neurological examinations, magnetic resonance imaging of brain and electroencephalogram since the onset of TD.

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.

P‐Aminosalicylic acid prevents the loss of immunofluorescence emissions of postsynaptic dopamine D2 receptors due to manganese treatment (796.1)

Manganese (Mn) a neurotoxin causing Manganism, a Parkinsons‐like disease, disrupts dopamine (DA) neurons. The neurotoxic mechanism is not fully resolved. It’s postulated more related to dysfunction of DA D2 receptors than degeneration of DA neurons. Crassostrea virginica gill lateral cells are innervated by DA nerves and have D2 post‐synaptic receptors. Mn blocks cilio‐inhibition of DA. PAS (p‐aminosalicylic acid) blocks the effects of Mn. It’s questioned if Mn decreases the number of DA D2 receptors in brain. We used 1̊ antibodies against D2 receptors and FITC‐linked 2̊ antibodies to quantify DA D2 receptors to test if Mn decreases the number of receptors in gill of C. virginica and if PAS prevents the decrease. We treated animals with 500 µM of Mn for up to 6 days. Sections were viewed on a fluorescence microscope, intensity was quantified with ImageJ software. Control gills had bright fluorescence corresponding to lateral and other gill cells. Intensity from animals treated up to 6 days with Mn showed a decrease up to 36% of controls. Animals co‐treated with Mn and PAS did not show reduced fluorescence. The study shows negative correlation between fluorescence intensity of DA D2 receptors in Mn treated animals vs controls and PAS negates effects of Mn. Whether intensity loss is due to decrease in D2 receptor number or altering of protein conformation and ligand binding site of D2 receptors needs to be further explored.Grant Funding Source: NIH‐ 2R25GM06003, NSF‐ 0622197, NYSED‐ 0516041071

Dual Control of Dopamine Synthesis and Release by Presynaptic and Postsynaptic Dopamine D2 Receptors

Dysfunctions of dopaminergic homeostasis leading to either low or high dopamine (DA) levels are causally linked to Parkinson's disease, schizophrenia, and addiction. Major sites of DA synthesis are the mesencephalic neurons originating in the substantia nigra and ventral tegmental area; these structures send major projections to the dorsal striatum (DSt) and nucleus accumbens (NAcc), respectively. DA finely tunes its own synthesis and release by activating DA D2 receptors (D2R). To date, this critical D2R-dependent function was thought to be solely due to activation of D2Rs on dopaminergic neurons (D2 autoreceptors); instead, using site-specific D2R knock-out mice, we uncover that D2 heteroreceptors located on non-DAergic medium spiny neurons participate in the control of DA levels. This D2 heteroreceptor-mediated mechanism is more efficient in the DSt than in NAcc, indicating that D2R signaling differentially regulates mesolimbic- versus nigrostriatal-mediated functions. This study reveals previously unappreciated control of DA signaling, shedding new light on region-specific regulation of DA-mediated effects.

A Possible Biological Mechanism for the B Vitamins Altering Behaviour in Attention-Deficit/Hyperactivity Disorder

There is a growing body of recent evidence showing that micronutrients (combinations of minerals, vitamins and amino acids) improve the symptoms of attention-deficit/hyperactivity disorder (ADHD). Dopamine agonists, such as methylphenidate, have long been identified as effective in treating ADHD symptoms, by inhibiting dopamine transporter (DAT) function. This article explores the role that B vitamins might have in the treatment of ADHD symptoms by investigating the structural similarities between B vitamins and methylphenidate. We suggest that the presence of B vitamins and their postulated structure activity relationships (SARs) with dopamine may be responsible for the observed pharmacological effect. This pharmacological activity is likely to be via their competitive binding to the DAT dopamine binding site with a concomitant increase in synaptic dopamine concentration, which in turn might activate the postsynaptic dopamine D2 receptor and thus ameliorate the symptoms ofADHD.Further research is required to assess the validity of the intriguing possibility that B vitamins and methylphenidate share a common neurochemical mechanism of action.