Roles of dopamine 2 receptor isoforms and g proteins in ethanol regulated prolactin synthesis and lactotropic cell proliferation

Alcohol consumption has been shown to increase prolactin (PRL) production and cell proliferation of pituitary lactotropes. It also causes a reduction in the lactotrope's response to dopaminergic agents and a differential expression of dopamine 2 receptor short (D2S) and long (D2L) isoforms in the pituitary. However, the role of each of these D2 receptor isoforms and its coupled G protein in mediation of ethanol actions on lactotropes is not known. We have addressed this issue by comparing ethanol effects on the level of PRL production gene transcription rate cellular protein, G proteins and cell proliferation in enriched lactotropes and lactotrope-derived PR1 cells containing various D2 receptor isoforms. Additionally, we determined the effects of G protein blockade on ethanol-induced PRL production and cell proliferation in these cells. We show here that the D2 receptor, primarily the D2S isoform, is critically involved in the regulation of ethanol actions on PRL production and cell proliferation in lactotropes. We also present data to elucidate that the presence of the pertussis toxin (PTX)-sensitive D2S receptor is critical to mediate the ethanol stimulatory action on Gs and the ethanol's inhibitory action on Gi3 protein in lactotropes. Additionally, we provide evidence for the existence of an inhibitory action of Gi3 on Gs that is under the control of the D2S receptor and is inhibited by ethanol. These results suggest that ethanol via the inhibitory action on D2S receptor activity suppresses Gi3 repression of Gs expression resulting in stimulation of PRL synthesis and cell proliferation in lactotropes.

Capturing the angel in "angel dust": twenty years of translational neuroscience studies of NMDA receptor antagonists in animals and humans

Here, we describe our collaborative efforts to use N-methyl-d-aspartate (NMDA) receptor antagonists as a translational tool to advance our understanding of the pathophysiology of schizophrenia and identify potential new targets for treatment of schizophrenia. We began these efforts in the late 1980s with a keen sense that, in both human and animal studies, we needed to move beyond the dopamine hypothesis of schizophrenia; if the dopamine hypothesis were correct, the existing dopamine antagonists should have cured the disease but they have not. We used NMDA receptor antagonists, not to produce schizophrenia, but as a tool to provide insights into effects of disturbances in glutamate synaptic function in schizophrenia. Our work has provided insights into potential mechanisms that may contribute to disrupted cortical function in schizophrenia and has helped identify potential treatment targets for the disorder. The translational nature of this study made the clinical testing of the first of these targets feasible. Advances in systems neuroscience approaches in animals and humans make new types of translational research possible; however, our concern is that the current obstacles facing translational research funding and academia-industry collaborations threaten the future progress in this field.

Free-choice and no-choice high-fat diets affect striatal dopamine D2/3 receptor availability, caloric intake, and adiposity

Different types of high-fat (HF) diets are used to study diet-induced obesity (DIO) in rodents and this has led to different phenotypes. This study assesses whether different HF diets differentially affect striatal dopamine D(2/3) receptor (DRD(2/3)) availability, as decreased striatal DRD(2/3) availability has been implicated in obesity in relation to reward deficiency for food. Thirty rats were randomized to either a free-choice HF diet (HF-choice), a premixed HF diet (HF-no-choice), or a standard chow diet for 28 days. Striatal DRD(2/3) was measured using (123)I-IBZM storage phosphor imaging at day 29. DRD(2/3) availability was significantly decreased in the dorsal striatum in the HF-choice rats compared to chow rats, but not in HF-no-choice rats. Additionally, caloric intake of the HF-choice rats was significantly higher than that of HF-no-choice rats and serum leptin and percentage abdominal fat store weight of total body weight were significantly higher in the HF-choice rats compared to chow rats. These preliminary results suggest that the choice element in HF diets, which is possibly related to the motivational aspects of eating, leads to overconsumption and to a distinct state of obesity. These results are relevant for future studies on DIO when considering choice of diet type.

[Cardiac, respiratory, and motor activity in norm and after activation of catecholaminergic systems in newborn rat pups]

Study of parameters of the cardiac, respiratory, and motor activity (MA) was carried out on newborn rat pups for the first day after birth (P0) and at the 14th day of postnatal development (P14) after change of the level of activity of catecholaminergic systems. The animals were administered with L-DOPA (25-100 mg/kg) and the indirect adrenomimetic isoamine (3 and 10 mg/kg). Additionally there were studied effects of L-DOPA and isoamine after blockade of D1 and D2 dopamine receptors (antagonists SCH-23390 and sulpiride). The L-DOPA administration produced a dose-dependent MA enhancement with its possible transition into the uninterrupted activity. In P0 the release of monoamines was accompanied by development of weak bradycardia. There was noted a tendency for acceleration of respiration at administration of the low dose both of L-DOPA and of isoamine and for its retardation at high doses. In P14 the L-DOPA administration was accompanied by retardation of the heart rate (HR) by 8 % and by acceleration of respiratory rate by 26%. The isoamine administration produced an insignificant decrease of HR and an increase of respiratory rate (RR) by 8% at the low dose and by 21% at the high dose of the agent. At the blockade of D1 receptors, RR remained close to the background values, while at the blockade of D2 - decreased insignificantly. Blockade of D1 and D2 receptors did not cause significant HR changes. Analysis of the HR variability has shown that both after L-DOPA administration and at blockade of dopamine receptors no unidirectional reaction was observed: in 80 % of rat pups the portion of nerve mechanisms of HR regulation increased, while in the rest--of sympathetic and humoral factors at a decrease of parasympathetic effects. In all rat pups the isoamine administration was accompanied by a shift of the specter power into the higher frequency area; in 60% of animals there were enhanced sympathetic influences. In P14 in rat pups after administration both of L-DOPA and of isoamine, the symphathetic nervous influences were predominant. Thus, in P0 both at release of endogenous catecholamines and at their excessive concentration in rat pups there occurs a qualitative change of character of the catecholaminergic influences on functional activity of excitable structures, especially of those connected with regulation of respiration.

Deficient dopamine D2 receptor function causes renal inflammation independently of high blood pressure

Renal dopamine receptors participate in the regulation of blood pressure. Genetic factors, including polymorphisms of the dopamine D(2) receptor gene (DRD2) are associated with essential hypertension, but the mechanisms of their contribution are incompletely understood. Mice lacking Drd2 (D(2)-/-) have elevated blood pressure, increased renal expression of inflammatory factors, and renal injury. We tested the hypothesis that decreased dopamine D(2) receptor (D(2)R) function increases vulnerability to renal inflammation independently of blood pressure, is an immediate cause of renal injury, and contributes to the subsequent development of hypertension. In D(2)-/- mice, treatment with apocynin normalized blood pressure and decreased oxidative stress, but did not affect the expression of inflammatory factors. In mouse RPTCs Drd2 silencing increased the expression of TNFα and MCP-1, while treatment with a D(2)R agonist abolished the angiotensin II-induced increase in TNF-α and MCP-1. In uni-nephrectomized wild-type mice, selective Drd2 silencing by subcapsular infusion of Drd2 siRNA into the remaining kidney produced the same increase in renal cytokines/chemokines that occurs after Drd2 deletion, increased the expression of markers of renal injury, and increased blood pressure. Moreover, in mice with two intact kidneys, short-term Drd2 silencing in one kidney, leaving the other kidney undisturbed, induced inflammatory factors and markers of renal injury in the treated kidney without increasing blood pressure. Our results demonstrate that the impact of decreased D(2)R function on renal inflammation is a primary effect, not necessarily associated with enhanced oxidant activity, or blood pressure; renal damage is the cause, not the result, of hypertension. Deficient renal D(2)R function may be of clinical relevance since common polymorphisms of the human DRD2 gene result in decreased D(2)R expression and function.

Dopamine signaling is essential for precise rates of locomotion by C. elegans

Dopamine is an important neuromodulator in both vertebrates and invertebrates. We have found that reduced dopamine signaling can cause a distinct abnormality in the behavior of the nematode C. elegans, which has only eight dopaminergic neurons. Using an automated particle-tracking system for the analysis of C. elegans locomotion, we observed that individual wild-type animals made small adjustments to their speed to maintain constant rates of locomotion. By contrast, individual mutant animals defective in the synthesis of dopamine made larger adjustments to their speeds, resulting in large fluctuations in their rates of locomotion. Mutants defective in dopamine signaling also frequently exhibited both abnormally high and abnormally low average speeds. The ability to make small adjustments to speed was restored to these mutants by treatment with dopamine. These behaviors depended on the D2-like dopamine receptor DOP-3 and the G-protein subunit GOA-1. We suggest that C. elegans and other animals, including humans, might share mechanisms by which dopamine restricts motor activity levels and coordinates movement.

Postanoxic akinesia with bilateral pallidal lesions: a PET study

A 70-year-old woman developed marked akinesia after an anoxic event related to bronchiectasia. Magnetic resonance imaging studies revealed lesions in the bilateral globus pallidus and, to a lesser extent, in the putamen. Positron emission tomography studies with (18)F-6-fluoro-L-dopa and (11)C-N-methylspiperone showed a decreased pre- and post-synaptic uptake in the striatum. Consistent with previous reports, the present case demonstrated the basal ganglia, particularly the globus pallidus, to be selectively susceptible to anoxic insults. Furthermore, a PET study indicated a disrupted presynaptic integrity of the dopaminergic terminals and decreased dopamine D(2) receptor binding, which together appear to underlie the pathophysiology of post-anoxic akinesia, at least in the present case.

Role of dopamine D(2) receptors for antipsychotic activity

This review summarizes the current state of knowledge regarding the proposed mechanisms by which antipsychotic agents reduce the symptoms of schizophrenia while giving rise to adverse side effects. The first part summarizes the contribution of neuroimaging studies to our understanding of the neurochemical substrates of schizophrenia, putting emphasis on direct evidence suggestive of a presynaptic rather than a postsynaptic dysregulation of dopaminergic neurotransmission in this disorder. The second part addresses the role of D(2) and non-D(2) receptor blockade in the treatment of schizophrenia and highlights a preponderant role of D(2) receptors in the mechanism of antipsychotic action. Neuroimaging studies have defined a narrow, but optimal, therapeutic window of 65-78 % D(2) receptor blockade within which most antipsychotics achieve optimal clinical efficacy with minimal side effects. Some antipsychotics though do not conform to that therapeutic window, notably clozapine. The reasons for its unexcelled clinical efficacy despite subthreshold levels of D(2) blockade are unclear and current theories on clozapine's mechanisms of action are discussed, including transiency of its D(2) receptor blocking effects or preferential blockade of limbic D(2) receptors. Evidence is also highlighted to consider the use of extended antipsychotic dosing to achieve transiency of D(2) blockade as a way to optimize functional outcomes in patients. We also present some critical clinical considerations regarding the mechanisms linking dopamine disturbance to the expression of psychosis and its blockade to the progressive resolution of psychosis, keeping in perspective the speed and onset of antipsychotic action. Finally, we discuss potential novel therapeutic strategies for schizophrenia.

Polysialic acid is required for dopamine D2 receptor-mediated plasticity involving inhibitory circuits of the rat medial prefrontal cortex

Decreased expression of dopamine D2 receptors (D2R), dysfunction of inhibitory neurotransmission and impairments in the structure and connectivity of neurons in the medial prefrontal cortex (mPFC) are involved in the pathogenesis of schizophrenia and major depression, but the relationship between these changes remains unclear. The polysialylated form of the neural cell adhesion molecule (PSA-NCAM), a plasticity-related molecule, may serve as a link. This molecule is expressed in cortical interneurons and dopamine, via D2R, modulates its expression in parallel to that of proteins related to synapses and inhibitory neurotransmission, suggesting that D2R-targeted antipsychotics/antidepressants may act by affecting the plasticity of mPFC inhibitory circuits. To understand the role of PSA-NCAM in this plasticity, rats were chronically treated with a D2R agonist (PPHT) after cortical PSA depletion. PPHT-induced increases in GAD67 and synaptophysin (SYN) neuropil expression were blocked when PSA was previously removed, indicating a role for PSA-NCAM in this plasticity. The number of PSA-NCAM expressing interneuron somata also increased after PPHT treatment, but the percentages of these cells belonging to different interneuronal subpopulations did not change. Cortical pyramidal neurons did not express PSA-NCAM, but puncta co-expressing this molecule and parvalbumin could be found surrounding their somata. PPHT treatment increased the number of PSA-NCAM and parvalbumin expressing perisomatic puncta, but decreased the percentage of parvalbumin puncta that co-expressed SYN. PSA depletion did not block these effects on the perisomatic region, but increased further the number of parvalbumin expressing puncta and increased the percentage of puncta co-expressing SYN and parvalbumin, suggesting that the polysialylation of NCAM may regulate perisomatic inhibition of mPFC principal neurons. Summarizing, the present results indicate that dopamine acting on D2R influences structural plasticity of mPFC interneurons and point to PSA-NCAM as a key player in this remodeling.

Novel dopamine D2 receptor signaling through proteins interacting with the third cytoplasmic loop

The diverse activities of dopamine D2-like receptors, including D2, D3, and D4 receptors, are mediated by proteins that interact with the third cytoplasmic loop and regulate receptor signaling, receptor trafficking, and apoptosis. Such interacting proteins include calmodulin, the N-methyl-D: -aspartate receptor 2B subunit, calcium/calmodulin-dependent protein kinase II, prostate apoptosis response-4, and β-arrestins, which regulate receptor signaling and the pharmacological action through D2 receptor. The gene encoding the D2 receptor gives rise to two isoforms, termed the dopamine D2 receptor long isoform (D2L) and the dopamine D2 receptor short isoform; the latter lacks 29 amino acids of the D2L receptor within the third cytoplasmic loop. In this review, we first focus on novel functions of the hetero-oligomeric D1/D2 and D2/adenosine A(2A) receptors. We next discuss novel signaling through proteins interacting with the D2 receptor third cytoplasmic loop and define the function of a novel binding protein, heart-type fatty acid binding protein, which interacts with the D2L third cytoplasmic loop.

Dopamine and mesotocin neurotransmission during the transition from incubation to brooding in the turkey

We investigated the neuroendocrine changes involved in the transition from incubating eggs to brooding of the young in turkeys. Numbers of mesotocin (MT; the avian analog of mammalian oxytocin) immunoreactive (ir) neurons were higher in the nucleus paraventricularis magnocellularis (PVN) and nucleus supraopticus, pars ventralis (SOv) of late stage incubating hens compared to the layers. When incubating and laying hens were presented with poults, all incubating hens displayed brooding behavior. c-fos mRNA expression was found in several brain areas in brooding hens. The majority of c-fos mRNA expression by MT-ir neurons was observed in the PVN and SOv while the majority of c-fos mRNA expression in dopaminergic (DAergic) neurons was observed in the ventral part of the nucleus preopticus medialis (POM). Following intracerebroventricular injection of DA or oxytocin (OT) receptor antagonists, hens incubating eggs were introduced to poults. Over 80% of those injected with vehicle or the D1 DA receptor antagonist brooded poults, while over 80% of those receiving the D2 DA receptor antagonist or the OT receptor antagonist failed to brood the poults. The D2 DA/OT antagonist groups also displayed less c-fos mRNA in the dorsal part of POM and the medial part of the bed nucleus of the stria terminalis (BSTM) areas than did the D1 DA/vehicle groups. These data indicate that numerous brain areas are activated when incubating hens initially transition to poult brooding behavior. They also indicate that DAergic, through its D2 receptor, and MTergic systems may play a role in regulating brooding behaviors in birds.

The role of dopamine transporter (SLC6A3) and dopamine D2 receptor/ankyrin repeat and kinase domain containing 1 (DRD2/ANKK1) gene polymorphisms in personality traits

Variations in personality traits are caused by interactions between multiple genes of small effect and environmental factors. To date, gender- and ethnicity-specific variations in personality have been established. In the present study, we aimed to test: (1) the effects of four polymorphisms of dopamine system genes: ANKK1/DRD2 Taq1A, DRD2 rs6275, SLC6A3 40-bp VNTR and rs27072, on personality traits; (2) whether these effects differ between men and women and between Russians and Tatars. A sample of 652 healthy individuals (222 men and 430 women) of Caucasian origin (233 Russians and 419 Tatars) from Russia was subjected to personality traits assessment with Eysenck Personality Inventory (EPI) and Temperament and Character Inventory-125 (TCI-125). The associations between each personality trait and polymorphisms were assessed with regression models adjusted for gender and ethnicity. There were significant effects of ANKK1/DRD2 Taq1A on Neuroticism (p=0.016) and of SLC6A3 rs27072 on Persistence (p=0.021) in both genders. The association between ANKK1/DRD2 Taq1A A2/A2-genotype and higher Novelty Seeking and lower Reward Dependence was shown in men only (p for gender interaction=0.018). In women only, there was a significant association between SLC6A3 10R*G-haplotype and higher Persistence (p=0.002). Our findings provide evidence for a modifying effect of gender on the associations between dopamine system genes and approach-related traits (in men) and Persistence (in women).

[Dopamine D2 receptor as a novel target molecule for heart-type fatty acid binding protein]

Essential roles of long-chain polyunsaturated fatty acids (LCPUFAs) have been documented in higher brain functions including emotion, learning and memory. Several clinical studies indicate that oral administration of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) can improve emotional and cognitive dysfunctions in schizophrenic patients. Likewise, arachidonic acid supplementation can improve cognitive dysfunction seen in human neurodegenerative disorders such as Alzheimer's disease. Since LCPUFAs are insoluble in an aqueous cellular environment, fatty acid binding proteins (FABPs) are essential to function as intracellular transport of LCPUFAs to appropriate intracellular compartments. Of various FABPs, heart-type fatty acid binding protein (H-FABP, FABP3) is highly expressed in neurons of mature brain. We previously demonstrated that H-FABP is associated with dopamine D2 receptor long isoform (D2LR) in vitro. Furthermore, we demonstrated that H-FABP knockout mice exhibit dopamine D2 receptor dysfunction. These results indicate that administration of LCPUFAs regulates dopamine D2 receptor functions through H-FABP in the brain.

Contrasting roles for dopamine D1 and D2 receptor subtypes in the dorsomedial striatum but not the nucleus accumbens core during behavioral inhibition in the stop-signal task in rats

Dopamine and dopamine-receptor function are often implicated in behavioral inhibition, and deficiencies within behavioral inhibition processes linked to attention deficit/hyperactivity disorder (ADHD), schizophrenia, obsessive-compulsive disorder, and drug addiction. In the stop-signal task, which measures the speed of the process of inhibition [stop-signal reaction time (SSRT)], psychostimulant-related improvement of SSRT in ADHD is linked with dopamine function. However, the precise nature of dopaminergic control over SSRT remains unclear. This study examined region- and receptor-specific modulation of SSRT in the rat using direct infusions of the dopamine D1 receptor (DRD1) antagonist SCH 23390 or dopamine D2 receptor (DRD2) antagonist sulpiride into the dorsomedial striatum (DMStr) or nucleus accumbens core (NAcbC). DRD1 and DRD2 antagonists had contrasting effects on SSRT that were specific to the DMStr. SCH 23390 decreased SSRT with little effect on the go response. Conversely, sulpiride increased SSRT but also increased go-trial reaction time and reduced trial completion at the highest doses. These results suggest that DRD1 and DRD2 function within the DMStr, but not the NAcbC, may act to balance behavioral inhibition in a manner that is independent of behavioral activation.

Implications from addiction research towards the understanding and treatment of obesity

Recent research indicates similarities between obesity and addictive disorders on both the phenomenological and neurobiological level. In particular, neuroendocrine and imaging studies suggest a close link between the homeostatic regulation of appetite on the on hand, and motivation and reward expectancy on the other. In addition, findings from neuropsychological studies additionally demonstrate alterations of cognitive function in both obesity and addictive disorders that possibly contribute to a lack of control in resisting consumption. In this review, recent findings on overlapping neurobiological and phenomenological pathways are summarized and the impact with regard to new treatment approaches for obesity is discussed.

Dopamine-nociceptin/orphanin FQ interactions in the substantia nigra reticulata of hemiparkinsonian rats: involvement of D2/D3 receptors and impact on nigro-thalamic neurons and motor activity

Nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor antagonists proved to be effective in alleviating experimental parkinsonism. Nonetheless, loss of effectiveness or even worsening of parkinsonian symptoms have been observed at high doses. With the aim of clarifying the circuitry underlying the dual action of NOP receptor antagonists and the role of endogenous dopamine, the NOP receptor antagonist 1-benzyl-N-[3-[spiroisobenzofuran-1(3H),4'-piperidin-1-yl]propyl]pyrrolidine-2-carboxamide (Compound 24) and the D(2)/D(3) receptor antagonist raclopride were used in 6-hydroxydopamine hemilesioned rats. Systemically administered Compound 24 improved motor activity in the 0.1-10mg/kg dose range being ineffective at 30 mg/kg. To confirm NOP selectivity, Compound 24 improved motor performance in wild-type mice at 1 and 10mg/kg and inhibited it at 60 mg/kg, being ineffective in NOP receptor knockout mice. To prove that the bell-shaped profile was mediated by nigral NOP receptors, reverse dialysis of Compound 24 (0.03 μM) in substantia nigra reticulata ameliorated akinesia whereas Compound 24 (3 μM) was ineffective. To demonstrate that motor responses were mediated by tuning inhibitory and excitatory inputs to nigro-thalamic neurons, the low concentration elevated GABA and reduced glutamate in substantia nigra, simultaneously reducing GABA levels in ventro-medial thalamus. Conversely, the higher concentration reduced nigral and elevated thalamic GABA, without affecting nigral glutamate levels. Co-perfusion with raclopride (1 μM) abolished the antiakinetic action of Compound 24 (0.03 μM) and turned the ineffectiveness of Compound 24 (3 μM) into an antiakinetic effect. The low concentration reduced nigral but did not affect thalamic GABA whereas the higher concentration elevated nigral and reduced thalamic GABA. Neither concentration affected nigral glutamate. We conclude that dual motor effects of Compound 24 in hemiparkinsonian rats are accomplished through blockade of nigral NOP receptors resulting in opposite modulation of nigro-thalamic neurons. Endogenous dopamine contributes to these responses affecting the level of GABAergic inhibition of the nigral output via D(2)/D(3) receptors.

In vivo electrophysiological effects of methylphenidate in the prefrontal cortex: involvement of dopamine D1 and alpha 2 adrenergic receptors

Attention deficit hyperactivity disorder (ADHD) is the most commonly diagnosed psychiatric disorder in children. Psychostimulants such as methylphenidate (MPH) are used as first line treatment. The prefrontal cortex (PFC) has a proven role in the expression of ADHD. Previous studies from our laboratory have demonstrated that MPH activates the firing activity of medial PFC neurones in anaesthetised rats. The aim of the present study was to determine the respective contribution and location of the different types of catecholamine receptors in mediating these excitatory effects and to compare these effects with those induced by other selective dopamine or noradrenaline uptake blockers. Single unit activity of presumed pyramidal PFC neurones was recorded in rats anaesthetised with urethane. The activation of firing elicited by an iv administration of MPH (1 or 3mg/kg) was partially reduced or prevented by the selective D1 receptor antagonist SCH 23390 administered systemically (0.5mg/kg, iv), or locally by passive diffusion through the recording electrode. On the other hand, administration of the alpha 2 receptor antagonist yohimbine (1mg/kg, iv) significantly potentiated the excitatory effect of MPH and activated PFC neurones previously treated with a low inactive dose of MPH (0.3mg/kg, iv). Local administration of MPH (1mM through the recording electrode) significantly increased the firing of PFC neurones in a D1 receptor-dependent manner. In addition, the response of PFC neurones to MPH, administered at a low dose (0.3mg/kg, iv), is greatly potentiated by dopamine (1mM), but not by noradrenaline (1mM), diffusing passively through the recording electrode, and this effect is reversed by D1 receptor blockade. Finally, the selective dopamine uptake inhibitor GBR 12909 (6 mg/kg, iv) and desipramine (6 mg/kg, iv) only activate a subset of PFC neurones. These results demonstrate the involvement of cortical dopamine D1 and noradrenergic alpha 2 receptors in the in vivo electrophysiological effects of MPH on PFC neurones.

Differential patterns of alcohol consumption and dopamine-2 receptor binding in Wistar-Kyoto and Wistar rats

The Wistar-Kyoto (WKY) rat strain has been described as an animal model of depressive behavior that consumes significantly greater amounts of alcohol compared to the Wistar (WIS) rat strain. Since the mesolimbic dopamine (DA) type-2 (D2) receptors mediate reward-related behaviors, the present study measured the binding of [(125)I]-Iodosulpiride to D2 receptors in the brains of WKY versus WIS rats following 24 days of voluntary alcohol or water consumption. Alcohol consuming WKY rats showed a significant increase in D2 receptor binding in several regions of the mesolimbic and nigrostriatal systems. In contrast, alcohol consuming WIS rats showed a reduction in D2 receptor binding in DA cell body areas. The differential regulation of D2 receptors by voluntary alcohol consumption in the two rat strains suggests that D2 receptor mediated neurotransmission may be playing a role in the increased alcohol drinking behavior reported in WKY rats.

Conditioned fear is modulated by D2 receptor pathway connecting the ventral tegmental area and basolateral amygdala

Excitation of the mesocorticolimbic pathway, originating from dopaminergic neurons in the ventral tegmental area (VTA), may be important for the development of exaggerated fear responding. Among the forebrain regions innervated by this pathway, the amygdala is an essential component of the neural circuitry of conditioned fear. The functional role of the dopaminergic pathway connecting the VTA to the basolateral amygdala (BLA) in fear and anxiety has received little attention. In vivo microdialysis was performed to measure dopamine levels in the BLA of Wistar rats that received the dopamine D(2) agonist quinpirole (1 μg/0.2 μl) into the VTA and were subjected to a fear conditioning test using a light as the conditioned stimulus (CS). The effects of intra-BLA injections of the D(1) antagonist SCH 23390 (1 and 2 μg/0.2 μl) and D(2) antagonist sulpiride (1 and 2 μg/0.2 μl) on fear-potentiated startle (FPS) to a light-CS were also assessed. Locomotor performance was evaluated by use of open-field and rotarod tests. Freezing and increased dopamine levels in the BLA in response to the CS were both inhibited by intra-VTA quinpirole. Whereas intra-BLA SCH 23390 did not affect FPS, intra-BLA sulpiride (2 μg) inhibited FPS. Sulpiride's ability to decrease FPS cannot be attributed to nonspecific effects because this drug did not affect motor performance. These findings indicate that the dopamine D(2) receptor pathway connecting the ventral tegmental area and the basolateral amygdala modulates fear and anxiety and may be a novel pharmacological target for the treatment of anxiety.

Differential region-specific regulation of α4β2* nAChRs by self-administered and non-contingent nicotine in C57BL/6J mice

Neuronal nAChR upregulation is the hallmark of chronic nicotine exposure. Neuroplasticity to abused drugs, however, depends on whether their administration is forced by the experimenter or is under the control of the experimental animal. Neuroadaptation to chronic nicotine self-administration was examined with a yoked-control paradigm, using nose-poking as the operating procedure. Freely moving C57BL/6J mice that responded for 0.03 mg/kg/infusion of intravenous nicotine under a continuous schedule of reinforcement (FR-1), had control over the rate and amount of drug intake that a yoked littermate passively received (n = 11). The impact of response dependency on neurobiological changes in nicotinic and dopaminergic systems was subsequently assessed using quantitative autoradiography. Cytisine-sensitive [(125)I]epibatidine binding, [³H]SCH23390, [³H]raclopride and [³H]mazindol were used to label nAChRs with α4β2* subtype properties, D1 and D2 dopaminergic receptors, and dopamine transporters, respectively. During a period of 12 days, self-administration was reliably initiated and maintained in animals receiving response-contingent nicotine. Region specific changes in the density of α4β2* nAChRs were found to be dependent on the contingency of nicotine treatment. Higher levels of α4β2* receptor binding were observed in the dorsal lateral geniculate nucleus and the ventral tegmental area of self-administering mice, compared to non-contingent animals. Moreover, response-independent increases in D2 binding were observed following chronic nicotine administration. No change in D1 and DAT binding was observed among groups. These findings indicate regional specific alterations in the regulation of the nicotinic cholinergic system following contingent and non-contingent nicotine exposure, and underline the importance of response dependency on the development of nicotine addiction.

Dopamine activity in the lateral anterior hypothalamus modulates AAS-induced aggression through D2 but not D5 receptors

Treatment with anabolic-androgenic steroids (AAS) throughout adolescence facilitates offensive aggression in Syrian hamsters. In the anterior hypothalamus (AH), the dopaminergic neural system undergoes alterations after repeated exposure to AAS, producing elevated aggression. Previously, systemic administration of selective dopamine receptor antagonists has been shown to reduce aggression in various species and animal models. However, these reductions in aggression occur with concomitant alterations in general arousal and mobility. Therefore, to control for these systemic effects, the current studies utilized microinjection techniques to determine the effects of local antagonism of D2 and D5 receptors in the AH on adolescent AAS-induced aggression. Male Syrian hamsters were treated with AAS throughout adolescence and tested for aggression after local infusion of the D2 antagonist eticlopride, or the D5 antagonist SCH-23390, into the AH. Treatment with eticlopride showed dose-dependent suppression of aggressive behavior in the absence of changes in mobility. Conversely, while injection of SCH-23390 suppressed aggressive behavior, these reductions were met with alterations in social interest and locomotor behavior. To elucidate a plausible mechanism for the observed D5 receptor mediation of AAS-induced aggression, brains of AAS and sesame oil-treated animals were processed for double-label immunofluorescence of GAD₆₇ (a marker for GABA production) and D5 receptors in the lateral subdivision of the AH (LAH). Results indicate a sparse distribution of GAD₆₇ neurons colocalized with D5 receptors in the LAH. Together, these results indicate that D5 receptors in the LAH modulate non-GABAergic pathways that indirectly influence aggression control, while D2 receptors have a direct influence on AAS-induced aggression.

Tetrahydroberberine blocks ATP-sensitive potassium channels in dopamine neurons acutely-dissociated from rat substantia nigra pars compacta

Tetrahydroberberine (THB) exhibits neuroprotective effects but its targets and underlying mechanisms are largely unknown. Emerging evidence indicates that ATP-sensitive potassium (K(ATP)) channels in the substantia nigra pars compacta (SNc) promote Parkinson disease (PD) pathogenesis, thus blocking K(ATP) channels may protect neurons against neuronal degeneration. In the present study, we tested a hypothesis that THB blocks K(ATP) channels in dopaminergic (DA) neurons acutely dissociated from rat SNc. Using perforated patch-clamp recording in current-clamp mode, the functional K(ATP) channels can be opened by persistent perfusion of rotenone, an inhibitor of complex I of the mitochondrial respiratory chain. Bath-application of THB reversibly blocks opened K(ATP) channels in a concentration-dependent manner, which is comparable to a classical K(ATP) channel blocker, Tol. Compared to THB analogs, l-stepholidine (l-SPD) or l-tetrahydropalmatine (l-THP), THB exhibits more profound blockade in K(ATP) channels. In addition, exposure of THB alone to the recorded neuron increases action potential firing, and THB also restores rotenone-induced membrane hyperpolarization in the presence of dopamine D2 receptor antagonist (sulpiride), suggesting that THB exhibits an excitatory effect on SNc DA neurons through the block of K(ATP) channels. Collectively, the blockade of neuronal K(ATP) channels by THB in SNc DA neurons is a novel pharmacological mechanism of THB, which may contribute to its neuroprotective effects in PD.

Dopamine D2 receptor signaling modulates mutant ataxin-1 S776 phosphorylation and aggregation

Spinocerebellar ataxia 1 (SCA1) is a dominantly inherited neurodegenerative disease associated with progressive ataxia resulting from the loss of cerebellar Purkinje cells (PCs) and neurons in the brainstem. In PCs of SCA1 transgenic mice, the disease causing ataxin-1 protein mediates the formation of S100B containing cytoplasmic vacuoles and further self-aggregates to form intranuclear inclusions. The exact function of the ataxin-1 protein is not fully understood. However, the aggregation and neurotoxicity of the mutant ataxin-1 protein is dependent on the phosphorylation at serine 776 (S776). Although protein kinase A (PKA) has been implicated as the S776 kinase, the mechanism of PKA/ataxin-1 regulation in SCA1 is still not clear. We propose that a dopamine D(2) receptor (D2R)/S100B pathway may be involved in modulating PKA activity in PCs. Using a D2R/S100B HEK stable cell line transiently transfected with GFP-ataxin-1[82Q], we demonstrate that stimulation of the D2R/S100B pathway caused a reduction in mutant ataxin-1 S776 phosphorylation and ataxin-1 aggregation. Activation of PKA by forskolin resulted in an enhanced S776 phosphorylation and increased ataxin-1 nuclear aggregation, which was suppressed by treatment with D2R agonist bromocriptine and PKA inhibitor H89. Furthermore, treating SCA1 transgenic PC slice cultures with forskolin induced neurodegenerative morphological abnormalities in PC dendrites consistent with those observed in vivo. Taken together our data support a mechanism where PKA dependent mutant ataxin-1 phosphorylation and aggregation can be regulated by D2R/S100B signaling.

From the cell to the clinic: a comparative review of the partial D₂/D₃receptor agonist and α2-adrenoceptor antagonist, piribedil, in the treatment of Parkinson's disease

Though L-3,4-dihydroxyphenylalanine (L-DOPA) is universally employed for alleviation of motor dysfunction in Parkinson's disease (PD), it is poorly-effective against co-morbid symptoms like cognitive impairment and depression. Further, it elicits dyskinesia, its pharmacokinetics are highly variable, and efficacy wanes upon long-term administration. Accordingly, "dopaminergic agonists" are increasingly employed both as adjuncts to L-DOPA and as monotherapy. While all recognize dopamine D(2) receptors, they display contrasting patterns of interaction with other classes of monoaminergic receptor. For example, pramipexole and ropinirole are high efficacy agonists at D(2) and D(3) receptors, while pergolide recognizes D(1), D(2) and D(3) receptors and a broad suite of serotonergic receptors. Interestingly, several antiparkinson drugs display modest efficacy at D(2) receptors. Of these, piribedil displays the unique cellular signature of: 1), signal-specific partial agonist actions at dopamine D(2)and D(3) receptors; 2), antagonist properties at α(2)-adrenoceptors and 3), minimal interaction with serotonergic receptors. Dopamine-deprived striatal D(2) receptors are supersensitive in PD, so partial agonism is sufficient for relief of motor dysfunction while limiting undesirable effects due to "over-dosage" of "normosensitive" D(2) receptors elsewhere. Further, α(2)-adrenoceptor antagonism reinforces adrenergic, dopaminergic and cholinergic transmission to favourably influence motor function, cognition, mood and the integrity of dopaminergic neurones. In reviewing the above issues, the present paper focuses on the distinctive cellular, preclinical and therapeutic profile of piribedil, comparisons to pramipexole, ropinirole and pergolide, and the core triad of symptoms that characterises PD-motor dysfunction, depressed mood and cognitive impairment. The article concludes by highlighting perspectives for clarifying the mechanisms of action of piribedil and other antiparkinson agents, and for optimizing their clinical exploitation.

Truth, lies or self-deception? Striatal D(2/3) receptor availability predicts individual differences in social conformity

Previous positron emission tomography (PET) studies have consistently shown a negative association between striatal D(2/3) receptor availability and socially desirable responding (SDR). However, as SDR is a complex personality trait, the functional significance of this relationship is unclear. The aim of the present study was to determine whether the relationship between D(2/3) receptor availability and SDR reflects a tendency to present oneself positively to others, consistent with social conformity (impression management, IM), or the tendency to view one's own behavior positively (self-deceptive enhancement, SDE). Striatal D(2/3) receptor availability was assessed in 23 healthy volunteers using [(11)C]raclopride PET. SDR was assessed using the Lie scale of the revised Eysenck Personality Questionnaire, and IM and SDE were measured using the Paulhus Deception Scales. Analysis of personality variables revealed a positive relationship between Lie and log IM (r=0.64, p=0.01) but not Lie and SDE (r=-0.36, ns). Consistent with previous findings, Lie was negatively associated with D(2/3) receptor availability in the sensorimotor striatum (r=- 0.55, p=0.05), and a similar trend-level relationship was observed for log IM (r=-0.54 p=0.06) but not SDE (r=0.23, ns). Whilst these associations are modest, results suggest that striatal D(2/3) receptor availability may be particularly associated with social conformity, rather than self-deception.