EFFECTS OF OPIOID-PEPTIDES ON THE ELECTRICAL-ACTIVITY OF PREOPTIC ANDHYPOTHALAMIC NEURONS IN THE QUAIL BRAIN

We have recently isolated three opioid peptides, i.e., Met- and Leu-en kephalins and Met-enkephalin-Arg(6)-Phe(7), from the avian brain. In t he present study, therefore, effects of these endogenous opioid peptid es on the electrical activity of preoptic and hypothalamic neurons of the adult male Japanese quail were examined using a brain slice prepar ation. All of the three opioid peptides inhibited the spontaneous firi ng activities of subsets of neurons in the preoptic area and the parav entricular nucleus in the hypothalamus. Threshold concentrations for t he inhibitory action were between 10(-7) and 10(-6) M in Met- and Leu- enkephalins and approximately 10(-6) M in Met- enkephalin-Arg(6)-Phe(7 ), respectively. In a few cells in these brain areas, however, Leu-enk ephalin rather potentiated the spontaneous activities, resulting in an increase of firing rates or a decrease of interburst intervals. The i nhibitory effect of Met-enkephalin was completely blocked by naloxone, an opioid receptor antagonist, but not affected by bicuculline, a gam ma-aminobutyric acid A (GABAA) receptor antagonist. These results sugg est that there are functional opiate receptors in subsets of preoptic/ hypothalamic neurons and that one of their main physiological function s in these areas is an inhibition of neuronal activities. Because thes e brain regions are considered to be involved with the regulation of a variety of male reproductive behaviors, opioid peptides may regulate some reproductive behavior through the mechanism that provokes such an inhibition. (C) 1995 Wiley-Liss, Inc.