Release of Prolactin in Response to Microinjection of Morphine into Mesencephalic Dorsal Raphe Nucleus

Abstract

Blood samples were collected via jugular catheters from ovariectomized rats at 10 min intervals for 1 h before and 2 h after microinjection of 0.5 .mu.l either saline vehicle or morphine sulfate (10 .mu.g) into the dorsal raphe nucleus (DRN) or adjacent periaqueductal gray by means of chronically implanted guide cannulae. Prolactin was measured by radioimmunoassay, and mean preinjection and mean postinjection values were compared for each rat (t test) as well as for each treatment group (paired t test). Neither saline in DRN nor morphine in the surrounding periaqueductal gray significantly altered circulating prolactin. A significant rise in prolactin was observed following morphine injection into DRN. This effect of morphine was prevented by pretreatment of the animals with the narcotic antagonist naltrexone (10 mg/kg i.v.), indicating the involvement of opiate receptors. Apparently, DRN is 1 site at which systemically adminstered morphine might act, and this mechanism may participate in modulation of prolactin release by endogenous opioids.