Inhibition of pulsatile luteinizing hormone release by morphine microinjected into mesencephalic dorsal raphe

Blood samples were collected via jugular catheters from ovariectomized rats at 10-minute intervals for one hour before and two hours after microinjection of -.5 microliter of either saline vehicle or morphine sulfate (10 micrograms) into the dorsal raphe nucleus (DRN) or adjacent periaqueductal gray by means of chronically-implanted guide cannulae. LH was measured by radioimmunoassay and mean pre-injection and mean post-injection values were compared for each rat (t test) as well as for each treatment group (paired t test). Neither saline in DRN nor morphine at other sites significantly altered circulating LH. A significant decrease in LH was observed following injection of morphine into DRN. This effect of morphine was prevented by pre treatment of the animals with the narcotic antagonist naltrexone (10 mg/kg i.v.), indicating the involvement of opiate receptors. These results indicate that DRN is one site at which systemically-administered morphine might act, and suggest the possibility of participation of this mechanism in modulation of LH release by endogenous opioids.