Fluctuation in responsiveness of LH and lack of responsiveness of FSH to prolonged infusion of morphine and naloxone in the ewe

Abstract

In ewes in the mid-luteal phase, LH pulse frequency (P less than 0.01) and amplitude (P less than 0.05) increased during a 24 h infusion of naloxone (0.5 mg/kg/h) compared to a 24 h infusion of vehicle (mean +/- s.e.m.; 0.25 +/- 0.03 vs 0.14 +/- 0.01 pulses/h and 0.84 +/- 0.08 vs 0.55 +/- 0.08 ng/ml serum, respectively). The increase in pulse amplitude was immediate, but was less (P less than 0.05) during the second 12 h, compared to the first 12 h, of naloxone infusion (0.52 +/- 0.14 vs 0.98 +/- 0.08 ng/ml serum). Oestradiol concentrations were higher (P less than 0.01) during naloxone than during control infusion (5.63 +/- 0.26 vs 4.13 +/- 0.15 pg/ml serum). In ovariectomized ewes in the breeding season, LH pulse frequency was lower (P less than 0.01) during a 24 h infusion of morphine (0.5 mg/kg/h) than during a 24 h infusion of vehicle (mean +/- s.e.m.; 1.17 +/- 0.08 vs 1.71 +/- 0.06 pulses/h). We conclude that long-term infusion of naloxone results in a sustained increase in LH pulse frequency but only a transient elevation in pulse amplitude. No effects on FSH secretion were noted. LH secretion was sensitive to morphine in the absence of ovarian steroids, suggesting that ovarian steroids are not required for the presence of functional opioid receptors capable of modulating LH release.