Dopamine receptors (nomenclature as agreed by the NC-IUPHAR Subcommittee on Dopamine Receptors [375]) are commonly divided into D1-like (D1 and D5) and D2-like (D2, D3 and D4) families, where the endogenous agonist is dopamine.

ABSTRACT Secretion of insulin in response to extracellular stimuli, such as elevated glucose levels and small molecules that act on G-protein coupled receptors (GPCRs), is the hallmark of β -cell physiology. Trace amines (TAs) are small aromatic metabolites that were identified as low-abundant ligands of the trace amine-associated receptor 1 (TAAR1) in the central nervous system (CNS), a GPCR that is also expressed by pancreatic β -cells. In the present work, we identify TAs as essential autocrine signaling factors for β -cell activity and insulin secretion. We find that β -cells are producing TAs in significant amounts and that the modulation of endogenous TA levels by the selective inhibition of TA biosynthetic pathways directly translated into changes of oscillations of the intracellular Ca 2+ concentration ([Ca 2+ ] i oscillations) and insulin secretion. Selective TAAR1 agonists or inhibitors of monoamine oxidases increased [Ca 2+ ] i oscillations and insulin secretion. Opposite effects were mediated by selective TAAR1 antagonists, by recombinant monoamine oxidase action and by the inhibition of amino acid decarboxylase. As the modulation of TA biochemical pathways immediately translated into changes of [Ca 2+ ] i oscillations, we inferred high metabolic turnover rates of TAs and autocrine feedback. We found that psychotropic drugs modulate [Ca 2+ ] i oscillations and insulin secretion, either directly acting on TAAR1 or by altering endogenous TA levels. Our combined data support the hypothesis of TAs as essential autocrine signaling factors for β -cell activity and insulin secretion as well as TAAR1 as an important mediator of amine-modulated insulin secretion.

Dopamine receptors (nomenclature as agreed by the NC-IUPHAR Subcommittee on Dopamine Receptors [373]) are commonly divided into D1-like (D1 and D5) and D2-like (D2, D3 and D4) families, where the endogenous agonist is dopamine.

Abnormalities of the D2R gene (DRD2) play a role in the pathogenesis of human essential hypertension; variants of the DRD2 have been reported to be associated with hypertension. Disruption of Drd2 (D2−/−) in mice increases blood pressure. The hypertension of D2−/− mice has been related, in part, to increased sympathetic activity, renal oxidative stress, and renal endothelin B receptor (ETBR) expression. We tested in D2−/− mice the effect of etamicastat, a reversible peripheral inhibitor of dopamine-β-hydroxylase that reduces the biosynthesis of norepinephrine from dopamine and decreases sympathetic nerve activity. Blood pressure was measured in anesthetized D2−/− mice treated with etamicastat by gavage, (10 mg/kg), conscious D2−/− mice, and D2+/+ littermates, and mice with the D2R selectively silenced in the kidney, treated with etamicastat in the drinking water (10 mg/kg per day). Tissue and urinary catecholamines and renal expression of selected G protein-coupled receptors, enzymes related to the production of reactive oxygen species, and sodium transporters were also measured. Etamicastat decreased blood pressure both in anesthetized and conscious D2−/− mice and mice with renal-selective silencing of D2R to levels similar or close to those measured in D2+/+ littermates. Etamicastat decreased cardiac and renal norepinephrine and increased cardiac and urinary dopamine levels in D2−/− mice. It also normalized the increased renal protein expressions of ETBR, NADPH oxidase isoenzymes, and urinary 8-isoprostane, as well as renal NHE3 and NCC, and increased the renal expression of D1R but not D5R in D2−/− mice. In conclusion, etamicastat is effective in normalizing the increased blood pressure and some of the abnormal renal biochemical alterations of D2−/− mice.

Background: In keeping with the free-thinking tradition San Antonians are known for, the Scientific Program Committee of the Behavior, Biology and Chemistry: Translational Research in Addiction Conference chose trace amine-associated receptor I (TAAR1) as the focus of the plenary symposium for its 7th annual meeting held at the University of Texas Health Science Center at San Antonio on March 14 and 15, 2015. The timing of the meeting's plenary session on TAAR1 coincided with the Ides of March, an apt concurrence given the long association of this date with the overthrow of the status quo. And whether aware of the coincidence or not, those in attendance witnessed the plunging of the metaphorical dagger into the heart of the dopamine (DA) transporter (DAT)-centric view of psychostimulant action.Methods: The symposium's four plenary presentations focused on the molecular and cellular biology, genetics, medicinal chemistry and behavioral pharmacology of the TAAR1 system and the experimental use of newly developed selective TAAR1 ligands.Results: The consensus was that TAAR1 is a DA and methamphetamine receptor, interacts with DAT and DA D2 receptors, and is essential in modulating addiction-related effects of psychostimulants.Conclusions: Collectively the findings presented during the symposium constitute a significant challenge to the current view that psychostimulants such as methamphetamine and amphetamine solely target DAT to interfere with normal DA signaling and provide a novel conceptual framework from which a more complete understanding of the molecular mechanisms underlying the actions of DA and METH is likely to emerge. (C) 2015 Elsevier Ireland Ltd. All rights reserved.

Searchable abstracts of presentations at key conferences in endocrinology ISSN 1470-3947 (print) | ISSN 1479-6848 (online)

In keeping with the free-thinking tradition San Antonians are known for, the Scientific Program Committee of the Behavior, Biology and Chemistry: Translational Research in Addiction Conference chose trace amine-associated receptor 1 (TAAR1) as the focus of the plenary symposium for its 7th annual meeting held at the University of Texas Health Science Center at San Antonio on March 14 and 15, 2015. The timing of the meeting's plenary session on TAAR1 coincided with the Ides of March, an apt concurrence given the long association of this date with the overthrow of the status quo. And whether aware of the coincidence or not, those in attendance witnessed the plunging of the metaphorical dagger into the heart of the dopamine (DA) transporter (DAT)-centric view of psychostimulant action. The symposium's four plenary presentations focused on the molecular and cellular biology, genetics, medicinal chemistry and behavioral pharmacology of the TAAR1 system and the experimental use of newly developed selective TAAR1 ligands. The consensus was that TAAR1 is a DA and methamphetamine receptor, interacts with DAT and DA D2 receptors, and is essential in modulating addiction-related effects of psychostimulants. Collectively the findings presented during the symposium constitute a significant challenge to the current view that psychostimulants such as methamphetamine and amphetamine solely target DAT to interfere with normal DA signaling and provide a novel conceptual framework from which a more complete understanding of the molecular mechanisms underlying the actions of DA and METH is likely to emerge.

Trace amine-associated receptor 1 (TAAR1) shares significant nucleotide and amino acid sequence identity with dopamine, adrenergic, and serotonergic receptors. Heterologously expressed in vitro, TAAR1 couples to cyclic adenosine monophosphate production via Gαs when exposed to molecules containing a phenylethylamine moiety. This pharmacophore is present in the endogenous noncatecholic biogenic “trace amines” β-phenylethylamine and p-tyramine, 3-iodothyronamine, the catecholamines dopamine, norepinephrine, and epinephrine, and the synthetic amines amphetamine, methamphetamine, parahydroxyamphetamine, and methylenedioxymethamphetamine (ecstasy). TAAR1 is expressed in glutamatergic neurons of the prefrontal cortex, dopamine-producing neurons of the ventral tegmental area, insulin-producing β-cells, discrete sections of the gastrointestinal tract, and immune cells. Due to its pharmacologic and anatomic profiles, TAAR1 is a target of commercial drug development with several TAAR1-selective compounds already reported. Behavioral studies involving TAAR1-selective ligands confirm modulating TAAR1-mediated signaling favorably alters rodent responses to amphetamines and cocaine. Consequently, TAAR1 must be considered an essential element of any comprehensive model of psychostimulant action.

Several studies have investigated the reinforcing effects of food in genetically engineered mice lacking dopamine D(2) receptors (DA D(2)Rs); however, behavioral economic analyses quantifying reinforcement have not been conducted.The role of DA D(2)Rs in food reinforcement was examined by comparing responding under various fixed-ratio (FR) schedules of reinforcement, and effects of extinction, satiation, and the DA D(2)R antagonist eticlopride, in mice with and without genetic deletions of the receptor.Response rates of DA D(2)R knockout (KO) mice were generally lower than those of littermate wild-type (WT) and heterozygous (HET) mice. The demand curve (consumption vs. FR value) for KO mice decreased more steeply than that of HET or WT mice, suggesting that reinforcing effectiveness is decreased with DA D(2)R deletion. Prefeeding decreased, whereas extinction increased overall response rates as a proportion of baseline, with no significant genotype differences. Both (+)- and (-)-eticlopride dose-dependently decreased responding in all genotypes with (-)-eticlopride more potent than (+)-eticlopride in all but KO mice. The enantiomers were equipotent in KO mice, and similar in potency to (+)-eticlopride in WT and HET mice.That prefeeding and extinction did not vary across genotypes indicates a lack of involvement of DA D(2)Rs in these processes. Differences between (-)-eticlopride effects and extinction indicate that DA D(2)R blockade does not mimic extinction. The maintenance of responding in KO mice indicates that the DA D(2)R is not necessary for reinforcement. However, the economic analysis indicates that the DA D(2)R contributes substantially to the effectiveness of food reinforcement.

In elucidating the role of pharmacodynamic efficacy at D 3 receptors in therapeutic effectiveness of dopamine receptor agonists, the influence of study system must be understood. Here two compounds with D 3 over D 2 selectivity developed in our earlier work, D-264 and D-301, are compared in dopamine receptor-mediated G-protein activation in striatal regions of wild-type and D 2 receptor knockout mice and in CHO cells expressing D 2 or D 3 receptors. In caudate-putamen of D 2 knockout mice, D-301 was ~3-fold more efficacious than D-264 in activating G-proteins as assessed by [ 35 S]GTPγS binding; in nucleus accumbens, D-301 stimulated G-protein activation whereas D-264 did not. In contrast, the two ligands exerted similar efficacy in both regions of wild-type mice, suggesting both ligands activate D 2 receptors with similar efficacy. In D 2 and D 3 receptor-expressing CHO cells, D-264 and D-301 appeared to act in the [ 35 S]GTPγS assay as full agonists because they produced maximal stimulation equal to dopamine. Competition for [ 3 H]spiperone binding was then performed to determine K i /EC 50 ratios as an index of receptor reserve for each ligand. Action of D-301, but not D-264, showed receptor reserve in D 3 but not in D 2 receptor-expressing cells, whereas dopamine showed receptor reserve in both cell lines. Gα o 1 is highly expressed in brain and is important in D 2 -like receptor-G protein coupling. Transfection of Gα o 1 in D 3 - but not D 2 -expressing CHO cells led to receptor reserve for D-264 without altering receptor expression levels. D-301 and dopamine exhibited receptor reserve in D 3 -expressing cells both with and without transfection of Gα o 1. Altogether, these results indicate that D-301 has greater intrinsic efficacy to activate D 3 receptors than D-264, whereas the two compounds act on D 2 receptors with similar intrinsic efficacy. These findings also suggest caution in interpreting E max values from functional assays in receptor-transfected cell models without accounting for receptor reserve.

Studies have shown that exposure to chronic mild stress decreases ethanol intake and preference in dopamine D2 receptor wild-type mice (Drd2 (+/+)), while it increases intake in heterozygous (Drd2 (+/-)) and knockout (Drd2 (-/-)) mice. Dopaminergic neurotransmission in the basal forebrain plays a major role in the reinforcing actions of ethanol as well as in brain responses to stress. In order to identify neurochemical changes associated with the regulation of ethanol intake, we used in vitro receptor autoradiography to measure the levels and distribution of dopamine D1 and D2 receptors and dopamine transporters (DAT). Receptor levels were measured in the basal forebrain of Drd2 (+/+), Drd2 (+/-), and Drd2 (-/-) mice belonging to one of four groups: control (C), ethanol intake (E), chronic mild stress exposure (S), and ethanol intake under chronic mild stress (ES). D2 receptor levels were higher in the lateral and medial striatum of Drd2 (+/+) ES mice, compared with Drd2 (+/+) E mice. Ethanol intake in Drd2 (+/+) mice was negatively correlated with striatal D2 receptor levels. D2 receptor levels in Drd2(+/-) mice were the same among the four treatment groups. DAT levels were lower in Drd2(+/-) C and Drd2 (-/-) C mice, compared with Drd2 (+/+) C mice. Among Drd2(+/-) mice, S and ES groups had higher DAT levels compared with C and E groups in most regions examined. In Drd2(-/-) mice, ethanol intake was positively correlated with DAT levels in all regions studied. D1 receptor levels were lower in Drd2(+/-) and Drd2(-/-) mice, compared with Drd2(+/+), in all regions examined and remained unaffected by all treatments. The results suggest that in normal mice, ethanol intake is associated with D2 receptor-mediated neurotransmission, which exerts a protective effect against ethanol overconsumption under stress. In mice with low Drd2 expression, where DRD2 levels are not further modulated, ethanol intake is associated with DAT function which is upregulated under stress leading to ethanol overconsumption.

BACKGROUND:The trace amine-associated receptor 1 (Taar1) is one member of the Taar family of G-protein-coupled receptors (GPCR) accepting various biogenic amines as ligands. It has been proposed that Taar1 mediates rapid, membrane-initiated effects of thyronamines, the endogenous decarboxylated and deiodinated relatives of the classical thyroid hormones T4 and T3.OBJECTIVES:Although the physiological actions of thyronamines in general and 3-iodothyronamine (T1AM) in particular are incompletely understood, studies published to date suggest that synthetic T1AM-activated Taar1 signaling antagonizes thyromimetic effects exerted by T3. However, the location of Taar1 is currently unknown.METHODS:To fill this gap in our knowledge we employed immunofluorescence microscopy and a polyclonal antibody to detect Taar1 protein expression in thyroid tissue from Fisher rats, wild-type and taar1-deficient mice, and in the polarized FRT cells.RESULTS:With this approach we found that Taar1 is expressed in the membranes of subcellular compartments of the secretory pathway and on the apical plasma membrane of FRT cells. Three-dimensional analyses further revealed Taar1 immunoreactivity in cilial extensions of postconfluent FRT cell cultures that had formed follicle-like structures.CONCLUSIONS:The results suggest Taar1 transport along the secretory pathway and its accumulation in the primary cilium of thyrocytes. These findings are of significance considering the increasing interest in the role of cilia in harboring functional GPCR. We hypothesize that thyronamines can reach and activate Taar1 in thyroid follicular epithelia by acting from within the thyroid follicle lumen, their potential site of synthesis, as part of a nonclassical mechanism of thyroid autoregulation.

ABSTRACTThe dopamine D4 receptor has been postulated to play a role in the pathophysiology of alcoholism. This study examined how varying levels of D4 expression and their associated behaviors in male and female mice correlate with future alcohol intake. We hypothesized that: (1) mice with low (Drd4+/−) or deficient (Drd4−/−) in D4 receptors would show enhanced ethanol consumption compared with control mice (Drd4+/+), and (2) a specific phenotype in these mice is associated with future vulnerability for alcohol consumption. Individually housed mice were allowed free access to ethanol (20% vv) in the dark (DID). The behaviors measured in male and female mice were: novel object recognition, open‐field locomotor activity, and social interaction. Correlation analyses showed that in male Drd4−/− mice (relative to Drd4+/+ controls), anxiolytic behavior was significantly correlated with increased alcohol consumption. Also, in male Drd4−/− mice, there was a significant positive correlation between increased exploratory behavior and increased alcohol consumption. These findings were not observed in females. In conclusion, our data suggest that the dopamine D4 receptor gene has an important role in increased exploratory and anxiolytic behavior only in males and these behaviors were positively correlated with increased alcohol consumption. This interaction between sex hormones and dopamine D4 receptor genotype/function predicting future alcohol abuse and correlation with anxiolytic and exploratory behavior in male mice could have important implications for better understanding of vulnerabilities associated with addiction. Synapse 69:356–364, 2015. © 2015 Wiley Periodicals, Inc.

Previous studies suggest dopamine (DA) D-2-like receptor involvement in the reinforcing effects of food. To determine contributions of the three D-2-like receptor subtypes, knockout (KO) mice completely lacking DA D-2, D-3, or D-4 receptors (D2R, D3R, or D4R KO mice) and their wild-type (WT) littermates were exposed to a series of fixed-ratio (FR) food-reinforcement schedules in two contexts: an open economy with additional food provided outside the experimental setting and a closed economy with all food earned within the experimental setting. A behavioral economic model was used to quantify reinforcer effectiveness with food pellets obtained as a function of price (FR schedule value) plotted to assess elasticity of demand. Under both economies, as price increased, food pellets obtained decreased more rapidly (ie, food demand was more elastic) in DA D2R KO mice compared with WT littermates. Extinction of responding was studied in two contexts: by eliminating food deliveries and by delivering food independently of responding. A hyperbolic model quantified rates of extinction. Extinction in DA D2R KO mice occurred less rapidly compared with WT mice in both contexts. Elasticity of food demand was higher in DA D4R KOthan WT mice in the open, but not closed, economy. Extinction of responding in DA D4R KO mice was not different from that in WT littermates in either context. No differences in elasticity of food demand or extinction rate were obtained in D3R KO mice and WT littermates. These results indicate that the D2R is the primary DA D-2-like receptor subtype mediating the reinforcing effectiveness of food.

A previous study showed that dopamine (DA) D2 receptors (D2Rs) are involved in the reinforcing effectiveness of food, but the specific involvement of DA D2Rs in choice among food reinforcers remains unclear.

Since the cloning of the D4 receptor in the 1990s, interest has been building in the role of this receptor in drug addiction, given the importance of dopamine in addiction. Like the D3 receptor, the D4 receptor has limited distribution within the brain, suggesting it may have a unique role in drug abuse. However, compared to the D3 receptor, few studies have evaluated the importance of the D4 receptor. This may be due, in part, to the relative lack of compounds selective for the D4 receptor; the early studies were mainly conducted in mice lacking the D4 receptor. In this review, we summarize the literature on the structure and localization of the D4 receptor before reviewing the data from D4 knockout mice that used behavioral models relevant to the understanding of stimulant use. We also present evidence from more recent pharmacological studies using selective D4 agonists and antagonists and animal models of drug-seeking and drug-taking. The data summarized here suggest a role for D4 receptors in relapse to stimulant use. Therefore, treatments based on antagonism of the D4 receptor may be useful treatments for relapse to nicotine, cocaine, and amphetamine use.