Independence of progesterone blockade of the luteinizing hormone surge in ewes from opioid activity at naloxone-sensitive receptors

摘要

Summary. Fifteen ovariectomized ewes were treated with implants (s.c.) creating circulating luteal progesterone concentrations of 1·6 ± 0·1 ng ml?1 serum. Ten days later, progesterone implants were removed from five ewes which were then infused with saline for 64 h (0·154 mol Nacl 1?1, 20 ml h?1, i.v.). Ewes with progesterone implants remaining were infused with saline (n = 5) or naloxone (0·5 mg kg?1 h?1, n = 5) in saline for 64 h. At 36 h of infusion, all ewes were injected with oestradiol (20 μg in 1 ml groundnut oil, i.m.). During the first 36 h of infusion, serum luteinizing hormone (LH) concentrations were similar in ewes infused with saline after progesterone withdrawal and ewes infused with naloxone, but with progesterone implants remaining (1·23 ± 0·11 and 1·28 ± 0·23 ng ml?1 serum, respectively, mean ± sem, P > 0·05). These values exceeded circulating LH concentrations during the first 36 h of saline infusion of ewes with progesterone implants remaining (0·59 ± 0·09 ng ml?1 serum, P < 0·05). The data suggested that progesterone suppression of tonic LH secretion, before oestradiol injection, was completely antagonized by naloxone. After oestradiol injection, circulating LH concentrations decreased for about 10 h in ewes of all groups. A surge in circulating LH concentrations peaked 24 h after oestradiol injection in ewes infused with saline after progesterone withdrawal (8·16 ± 3·18 ng LH ml?1 serum). At the same time, circulating LH concentrations in ewes infused with saline or naloxone, but with progesterone implants remaining, were similar to concentrations observed before injection (0·91 ± 0·27 and 1·35 ± 0·28 ng LH ml?1 serum, respectively). The results suggested that naloxone antagonism of opioid-dependent progesterone suppression of LH secretion was not equivalent to progesterone withdrawal. It was concluded that progesterone blockade of oestradiol positive feedback on LH secretion in ewes does not depend upon endogenous opioid peptide activity at naloxone-sensitive receptors.