mu Opioid receptor mRNA expression in neuronal nitric oxide synthase-immunopositive preoptic area neurons

Nitric oxide (NO) as well as beta-endorphin are involved in the neuroendocr ine control of gonadotropin-releasing hormone (GnRH) secretion. Recently, m orphological and microdialysis experiments have suggested that beta-endorph in may exert an inhibitory influence on NO release in the preoptic area of rat hypothalamus. The present study determines if the mu. opioid receptor m RNA is expressed in neuronal NO synthase (nNOS)-immunopositive neurons and if this expression varies among the regions of the basal forebrain being ex amined. We found, through the use of immunohistochemical and in situ hybrid ization techniques, that the mu. opioid receptor mRNA is expressed in a rep resentative subpopulation of nNOS-immunoreactive neurons in the rat preopti c area. Interestingly, the mu, opioid receptor mRNA/nNOS-immunoreactive coe xpression is predominant in the rostral and median preoptic area, containin g most of GnRH cell bodies. These results strongly suggest that beta-endorp hin, via an action through mu opioid receptors, may directly participate in the regulation of NO production in the preoptic area. Our results strength en the hypothesis that beta-endorphin may participate in GnRH neuronal modu lation at the cell body level by regulating NO release from the interneuron s of the preoptic area that express nNOS. (C) 2000 Elsevier Science B.V. Al l rights reserved.