Effects of beta-phenylethylamine on the hypothalamo-pituitary-adrenal axis in the male rat

Electrophysiological effects of trace amines on mesencephalic dopaminergic neurons

Trace amines (TAs) are a class of endogenous compounds strictly related to classic monoamine neurotransmitters with regard to their structure, metabolism, and tissue distribution. Although the presence of TAs in mammalian brain has been recognized for decades, until recently they were considered to be by-products of amino acid metabolism or as "false" neurotransmitters. The discovery in 2001 of a new family of G-protein-coupled receptors (GPCRs), namely trace amines receptors, has re-ignited interest in TAs. In particular, two members of the family, trace amine receptor 1 (TA(1)) and trace amine receptor 2 (TA(2)), were shown to be highly sensitive to these endogenous compounds. Experimental evidence suggests that TAs modulate the activity of catecholaminergic neurons and that TA dysregulation may contribute to neuropsychiatric disorders, including schizophrenia, attention deficit hyperactivity disorder, depression and Parkinson's disease, all of which are characterized by altered monoaminergic networks. Here we review recent data concerning the electrophysiological effects of TAs on the activity of mesencephalic dopaminergic neurons. In the context of recent data obtained with TA(1) receptor knockout mice, we also discuss the mechanisms by which the activation of these receptors modulates the activity of these neurons. Three important new aspects of TAs action have recently emerged: (a) inhibition of firing due to increased release of dopamine; (b) reduction of D2 and GABA(B) receptor-mediated inhibitory responses (excitatory effects due to disinhibition); and (c) a direct TA(1) receptor-mediated activation of GIRK channels which produce cell membrane hyperpolarization. While the first two effects have been well documented in our laboratory, the direct activation of GIRK channels by TA(1) receptors has been reported by others, but has not been seen in our laboratory (Geracitano et al., 2004). Further research is needed to address this point, and to further characterize the mechanism of action of TAs on dopaminergic neurons.

Trace amines depress D(2)-autoreceptor-mediated responses on midbrain dopaminergic cells

BACKGROUND AND PURPOSE

Although trace amines (TAs) are historically considered 'false neurotransmitters' on the basis of their ability to induce catecholamine release, there is evidence that they directly affect neuronal activity via TA receptors, ligand-gated receptor channels and/or sigma receptors. Here, we have investigated the effects of two TAs, tyramine (TYR) and beta-phenylethylamine (beta-PEA), on electrophysiological responses of substantia nigra pars compacta (SNpc) dopaminergic cells to the D(2) receptor agonist, quinpirole.

EXPERIMENTAL APPROACH

Electrophysiological recordings of D(2) receptor-activated G-protein-gated inward rectifier K(+) channel (GIRK) currents were performed on dopaminergic cells from midbrain slices of mice and on Xenopus oocytes expressing D(2) receptors and GIRK channels.

KEY RESULTS

TYR and beta-PEA reversibly reduced D(2) receptor-activated GIRK currents in a concentration-dependent manner on SNpc neurones. The inhibitory effect of TAs was still present in transgenic mice with genetically deleted TA(1) receptors and they could not be reproduced by the selective TA(1) agonist, o-phenyl-3-iodotyramine (O-PIT). Pretreatment with antagonists of sigma1 and sigma2 receptors did not block TA-induced effects. In GTPgammaS-loaded neurones, the irreversibly-activated GIRK-current was still reversibly reduced by beta-PEA. Moreover, beta-PEA did not affect basal or dopamine-evoked GIRK-currents in Xenopus oocytes.

CONCLUSIONS AND IMPLICATIONS

TAs reduced dopamine-induced responses on SNpc neurones by acting at sites different from TA(1), sigma-receptors, D(2) receptors or GIRK channels. Although their precise mechanism of action remains to be identified, TAs, by antagonizing the inhibitory effects of dopamine, may render dopaminergic neurones less sensitive to autoreceptor feedback inhibition and hence enhance their sensitivity to stimulation.

Trace amine-associated receptors and their ligands

Classical biogenic amines (adrenaline, noradrenaline, dopamine, serotonin and histamine) interact with specific families of G protein-coupled receptors (GPCRs). The term 'trace amines' is used when referring to p-tyramine, beta-phenylethylamine, tryptamine and octopamine, compounds that are present in mammalian tissues at very low (nanomolar) concentrations. The pharmacological effects of trace amines are usually attributed to their interference with the aminergic pathways, but in 2001 a new gene was identified, that codes for a GPCR responding to p-tyramine and beta-phenylethylamine but not to classical biogenic amines. Several closely related genes were subsequently identified and designated as the trace amine-associated receptors (TAARs). Pharmacological investigations in vitro show that many TAAR subtypes may not respond to p-tyramine, beta-phenylethylamine, tryptamine or octopamine, suggesting the existence of additional endogenous ligands. A novel endogenous thyroid hormone derivative, 3-iodothyronamine, has been found to interact with TAAR1 and possibly other TAAR subtypes. In vivo, micromolar concentrations of 3-iodothyronamine determine functional effects which are opposite to those produced on a longer time scale by thyroid hormones, including reduction in body temperature and decrease in cardiac contractility. Expression of all TAAR subtypes except TAAR1 has been reported in mouse olfactory epithelium, and several volatile amines were shown to interact with specific TAAR subtypes. In addition, there is evidence that TAAR1 is targeted by amphetamines and other psychotropic agents, while genetic linkage studies show a significant association between the TAAR gene family locus and susceptibility to schizophrenia or bipolar affective disorder.