Opioid inhibition of luteinizing hormone secretion compared in developing male and female sheep.

The sheep exhibits a marked sex difference in the timing of the pubertal increase in luteinizing hormone (LH). Male lambs undergo a reduction in sensitivity to inhibitory steroid feedback, leading to an increase in LH by 10 weeks of age, but females remain hypersensitive until 30 weeks of age. Endogenous opioids suppress LH secretion in the female lamb prepubertally and in adult male and female sheep. It has been suggested that a reduction in opioid inhibition of LH secretion is the signal to time puberty. Therefore, if a decrease in opioid tone occurs during sexual maturation, it should begin earlier in the male lamb than in the female. The objective of this study was to compare opioid inhibition of LH secretion in male and female lambs in relation to the timing of puberty. Our approach was to examine the response to the opioid antagonist naloxone at various ages in both sexes. To determine the timing of the pubertal LH rise in the presence of constant inhibitory steroid feedback, male and female lambs (n = 5 each) were gonadectomized at 3 weeks of age and implanted with a Silastic capsule of estradiol. They were then challenged with naloxone at 5, 11, and 23 weeks of age; blood samples were collected every 12 min for 8 hours, and lambs received naloxone (1 mg/kg i.v.) at hours 4, 5, 6, and 7. Mean LH before and during naloxone treatment was compared at each age.(ABSTRACT TRUNCATED AT 250 WORDS)