EVIDENCE SHOWING THAT BETA-ENDORPHIN REGULATES CYCLIC GUANOSINE 3',5'-MONOPHOSPHATE (CGMP) EFFLUX - ANATOMICAL AND FUNCTIONAL SUPPORT FOR AN INTERACTION BETWEEN OPIATES AND NITRIC-OXIDE

Nitric oxide (NO) is now recognized as a diffusible messenger molecule that normally augments intercellular communication in the central ner vous system, but is neurotoxic if released in excessive amounts. NO is synthesized from L-arginine by the Ca2+/calmodulin-dependent neuronal isoform NO synthase (NOS) localized in subpopulations of neurons thro ughout the brain, including the hypothalamus. In the hypothalamus, NO stimulates the release of GnRH, the primary neurohormone governing rep roduction in mammals. Although the excitatory amino acid, glutamate, a cting through the N-methyl-D-aspartate (NMDA) receptor is believed to be responsible for stimulation of NO release, the neuronal system(s) t hat inhibits NO efflux is unknown. As the endogenous opioids, primaril y beta-endorphin (beta END), exert a tonic restraint on GnRH secretion , we sought evidence for a possible functional link between beta END a nd NOS pathways in the hypothalamus. We observed that restraining the opioid influence with the opiate receptor antagonist, naloxone, in int act, but not in castrated, rats rapidly augmented extracellular cGMP/N O efflux in the medial preoptic area, where GnRH, NOS, and beta END im munoreactive pathways are coextensive. Pituitary LH secretion increase d in conjunction with this augmented cGMP/NO response and pretreatment with the mu opiate receptor agonist, morphine, suppressed these nalox one-induced responses. Further, visualization of hypothalamic sections immunostained for both beta END and NOS revealed beta END-immunoreact ive axon terminals in close proximity to NOS-positive cell bodies and dendrites in a number of hypothalamic subdivisions, including the medi al preoptic area. These close appositions represented conventional syn apses between beta END nerve terminals and NOS-positive perikarya and dendrites under the electron microscope. Clearly, the experimental dat a, corroborated by morphological evidence, point to a direct inhibitor y control of beta END on NOS-immunopositive neurons in monitoring sGMP /NO release. These findings together with the previous observations th at the glutamate neurotransmitter acting through NMDA receptors locate d on NOS-immunopositive cells stimulates cGMP/NO efflux and plasma LH selectively in intact rats document the existence of a dual control co mprised of the excitatory NMDA and the inhibitory mu opiate receptors in modulating cGMP/NO release, a response also directed by gonadal ste roids. This new knowledge of an inhibitory opioid influence on cGMP/NO release is probably extremely important both in the generation of per iodicites in GnRH secretion that underlie hypothalamic control of repr oduction and in protecting against neurotoxic overstimulation of NO re lease by excitatory amino acids.