Abstract
Nitric oxide (NO) as well as β-endorphin are involved in the neuroendocrine control of gonadotropin-releasing hormone (GnRH) secretion. Recently, morphological and microdialysis experiments have suggested that β-endorphin may exert an inhibitory influence on NO release in the preoptic area of rat hypothalamus. The present study determines if the μ opioid receptor mRNA is expressed in neuronal NO synthase (nNOS)-immunopositive neurons and if this expression varies among the regions of the basal forebrain being examined. We found, through the use of immunohistochemical and in situ hybridization techniques, that the μ opioid receptor mRNA is expressed in a representative subpopulation of nNOS-immunoreactive neurons in the rat preoptic area. Interestingly, the μ opioid receptor mRNA/nNOS-immunoreactive coexpression is predominant in the rostral and median preoptic area, containing most of GnRH cell bodies. These results strongly suggest that β-endorphin, via an action through μ opioid receptors, may directly participate in the regulation of NO production in the preoptic area. Our results strengthen the hypothesis that β-endorphin may participate in GnRH neuronal modulation at the cell body level by regulating NO release from the interneurons of the preoptic area that express nNOS.
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