The selective antagonist EPPTB reveals TAAR1-mediated regulatory mechanisms in dopaminergic neurons of the mesolimbic system

Abstract

Trace amine-associated receptor 1 (TAAR1) is a G protein-coupled receptor (GPCR) that is nonselectively activated by endogenous metabolites of amino acids. TAAR1 is considered a promising drug target for the treatment of psychiatric and neurodegenerative disorders. However, no selective ligand to identify TAAR1-specific signaling mechanisms is available yet. Here we report a selective TAAR1 antagonist, EPPTB, and characterize its physiological effects at dopamine (DA) neurons of the ventral tegmental area (VTA). We show that EPPTB prevents the reduction of the firing frequency of DA neurons induced by p-tyramine (p-tyr), a nonselective TAAR1 agonist. When applied alone, EPPTB increases the firing frequency of DA neurons, suggesting that TAAR1 either exhibits constitutive activity or is tonically activated by ambient levels of endogenous agonist(s). We further show that EPPTB blocks the TAAR1-mediated activation of an inwardly rectifying K ؉ current. When applied alone, EPPTB induces an apparent inward current, suggesting the closure of tonically activated K ؉ channels. Importantly, these EPPTB effects were absent in Taar1 knockout mice, ruling out off-target effects. We additionally found that both the acute application of EPPTB and the constitutive genetic lack of TAAR1 increase the potency of DA at D2 receptors in DA neurons. In summary, our data support that TAAR1 tonically activates inwardly rectifying K ؉ channels, which reduces the basal firing frequency of DA neurons in the VTA. We hypothesize that the EPPTB-induced increase in the potency of DA at D2 receptors is part of a homeostatic feedback mechanism compensating for the lack of inhibitory TAAR1 tone.desensitization ͉ dopamine supersensitivity ͉ Kir3 ͉ trace amines ͉ VTA T race amines (TAs) such as p-tyr, ␤-phenylethylamine, octopamine, and tryptamine are metabolites of amino acids that are found at low concentrations in the brain (1). Because of their structural similarity to classical biogenic amines, TAs were for a long time believed to modulate neurotransmission by displacing biogenic amines from vesicular stores or by acting on transporters in an amphetamine-like manner. It was not until TAs were found to bind to members of a family of GPCRs, the TAassociated receptors (TAARs), that receptor-mediated mechanisms were evoked (2-5). While several TAARs were identified, only TAAR1 and, to a lesser extent, TAAR4 respond to typical TAs (5). TAs such as p-tyr and ␤-phenylethylamine activate human, mouse, and rat TAAR1 with EC 50 values of 0.2-1.7 M. Other TAs (octopamine, tryptamine), classical biogenic amines, and amphetamine-related psychostimulants have much reduced potency and efficacy at TAAR1.TA binding to TAAR1 engages G s -type G proteins that activate adenylyl cyclases (1). However, because TAs not only activate TAAR1 but also influence the activity of TAAR4, DA transporters, adrenergic, as well as serotonin receptors it was difficult to assign specific physiological functions to TAAR1 (1, 6). With the availability of Taar1 knockout mice (7,8) ...

Why D-neuron? Importance in schizophrenia research

“…The reduction of TAAR1 stimulation on DA terminals of VTA DA neurons, caused by decrease of striatal trace amine synthesis, may increase the firing frequency of VTA DA neurons [30,32]. This may lead to the increase of DA release in the nucleus accumbens, that is, mesolimbic DA hyperactivity.…”

“…It was clarified that increased stimulation of TAAR1 receptors on cell membranes of DA neurons in the midbrain ventral tegmental area (VTA) reduces firing frequency of VTA DA neurons [30][31][32].…”

“…The trace amine-associated receptor, type 1 (TAAR1) has a large number of ligands, including tyramine, β-phenylethylamine (PEA)and psychostimulants, for example, methamphetamine, 3,4-methylenedioxymethamphetamine (MDMA) and lysergic acid diethylamide (LSD) [23,25,29] (Table 1), and is now a target receptor for exploring novel neuroleptics [30,31].…”

Are D-Neurons and Trace Amine-Associated Receptor, Type 1 Involved in Mesolimbic Dopamine Hyperactivity of Schizophrenia?

“…It has been shown that TAAR1 has a thermoregulatory function [33]. It was clarified that increased stimulation of TAAR1 receptors on cell membranes of DA neurons in the midbrain ventral tegmental area (VTA) reduced firing frequency of VTA DA neurons [30][31][32].…”

“…The trace amine-associated receptor, type 1 (TAAR1) having a large number of ligands, including tyramine, β-phenylethylamine (PEA) and psychostimulants, for example methamphetamine, 3,4-methylenedioxymethamphetamine (MDMA) and lysergic acid diethylamide (LSD) [23,25,29], has become a target receptor for exploring novel neuroleptics [30,31]. TAAR1 knockout mice showed schizophrenia-like behaviors with a deficit in prepulse inhibition [32].…”