OPIOID-GLUTAMATE NITRIC-OXIDE CONNECTION IN THE REGULATION OF LUTEINIZING-HORMONE SECRETION IN THE RAT

Opioid neurons are recognized to be an important component of the inhi bitory ''brake'' in the CNS that restrains LHRH secretion. Opioid inhi bition could be exerted directly on LHRH neurons, or it could be achie ved via indirect mechanisms involving restrainment of excitatory ''acc elerator'' neurons that facilitate LHRH release. The purpose of the pr esent study was to explore the second hypothesis by investigating whet her removal of opioid inhibition by administering the opioid antagonis t, naloxone leads to enhanced activation of glutamate and nitric oxide (NO) neurons, which are known to be important excitatory ''accelerato r'' components for the control of LHRH secretion. Naloxone administrat ion (2.5 mg/kg) to adult male rats induced a significant elevation of serum LH levels at 20 min post injection. NOS activity in preoptic are a (POA) and medial basal hypothalamic (MBH) fragments was demonstrated to be significantly elevated 20 min post naloxone injection. Administ ration of a glutamate (NMDA) receptor antagonist (MK-801, 0.2 mg/kg) a bolished the naloxone-induced increase in NOS activity in the POA and MBH, with a corresponding block of the naloxone-induced LH release. Gl utamate appears to only be involved in LH surge generation and not to regulate basal LH levels, as MK-801 had no effect on basal LH release. Because previous work by our laboratory and others have provided evid ence that NO is a mediator of glutamate effects in the hypothalamus, t hese findings are interpreted to mean that opioid inhibition is mediat ed on glutamate neurons that are upstream of NO neurons. In support of this contention, we found that NMDA treatment enhanced NOS activity i n the male rat POA and MBH fragments in vitro, an effect that was spec ific as it was completely blocked by the NMDA receptor antagonist, MK- 801. Additionally, in vivo microdialysis studies revealed that naloxon e treatment significantly enhances glutamate release in the preoptic a rea (POA) at 15 min post injection in conscious, unanesthetized, freel y moving male rats. Release rates of the control amino acid, serine di d not change significantly following naloxone injection. Taken as a wh ole, these findings provide evidence for an opioid-glutamate-NO pathwa y in the control of LHRH secretion, and they demonstrate the importanc e of ''brake-accelerator'' interactions in the control of LHRH and LH secretion.