The effect of naloxone on LHRH secretion from the median eminence of anestrous ewes

Abstract

This study tested the hypothesis that opioid activity at the median eminence suppresses luteinizing hormone-releasing hormone (LHRH) secretion in anestrous ewes. Ewes received each of four treatments at 1 day intervals: (1) intravenous (i.v.) saline infusion (0.154 mol NaCl; 30 ml h −1; (2) i.v. naloxone infusion (0.5 mg kg −1 h −1 in saline); (3) median eminence perfusion with artificial cerebrospinal fluid (CSF, 1.2 ml h −1; (4) median eminence perfusion with naloxone (0.5 μg h −1 in CSF) for 6 h, LH pulses were observed during one of four treatment periods each, for i.v. infusions of saline or naloxone or median eminence perfusion with CSF. On two occasions one ewe had one LH pulse and on one occasion two pulses were noted. Two of these LH pulses were associated with a pulse in LHRH. During median eminence perfusion with naloxone, LH pulses peaked within 20 min of the onset of infusion in all four ewes. These initial LH pulses were temporally associated with LHRH in central perfusate in three of the four ewes. An additional LH pulse was detected in each of three ewes receiving naloxone into the median eminence; only one of these pulses was associated with a pulse in LHRH. The results suggested that naloxone causes near-immediate LH pulses by blocking opioid-suppression of LHRH secretion at the median eminence.