Ovine Brain Areas Sensitive to Naloxone-Induced Stimulation of Luteinizing Hormone Release

Abstract

Previous work established that intravenous administration of the opioid receptor antagonist naloxone abruptly increased release of luteinizing hormone (LH) and decreased release of prolactin (PRL) in suckled anestrous ewes and also increased LH release in cyclic luteal ewes. The goal of the present research was to identify brain sites at which local unilateral infusions of naloxone would consistently duplicate the previously noted effects of intravenous naloxone. Intracerebral guide tubes were surgically implanted into the brain of 13 nonpregnant and 16 pregnant ewes at least 4 weeks prior to experimentation. Intracerebral infusion (20-40 microliters each through an inner cannula) was performed once daily during postpartum anestrus in suckled fall-lambing ewes and during recurring luteal states of the estrous cycle. Naloxone infusion (n = 142) usually consisted of 50 or 100 micrograms naloxone, although 5 ewes received 200 and 400 micrograms per infusion. Control infusions (n = 103) consisted of the vehicle for naloxone (i.e., 0.9% NaCl). Serum concentrations of LH and PRL were quantified at 10-min intervals from 90 min before to 100 min after infusion. Hormone data from individual ewes were grouped for least-squares analysis of variance based upon postmortem neuroanatomical identification of each infusion site. Unilateral intracerebral administration of naloxone consistently induced an increase in LH release within 20 min in the following two neuroanatomical groups:basal forebrain (n = 9 ewes) and chiasmatic area (n = 4 ewes).(ABSTRACT TRUNCATED AT 250 WORDS)