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The roles of corticotrophin-releasing factor (CRF) and β-endorphin in the suppression of LH and FSH secretion during lactation were investigated using ovariectomized lactating rats separated from their litters overnight. Within 1 h of returning the pups to their mothers a marked fall in plasma LH concentration and a large increase in plasma prolactin were noted. However, resuckling caused no significant change in plasma concentration of FSH until 12 h after the return of the litter but a significant decline occurred thereafter. Twenty-four hours after removal of the litter, a single i.v. injection of 200 µl anti-LHRH serum caused similar changes in plasma concentrations of LH and FSH observed in nursing rats during suckling. These results suggest that the suckling stimulus itself is responsible for the suppression of LH as well as FSH, via inhibition of the secretion of LHRH. Twenty-four hours after removal of the litter, a single intracerebroventricular (i.c.v.) injection of either 10 µg CRF or β-endorphin resulted in a rapid decrease in plasma LH. Only β-endorphin caused a marked increase in plasma levels of prolactin within 1 h whereas FSH was less affected by either hormone. Repeated i.c.v. administration of 10 µg CRF or β-endorphin at 6-h intervals caused a prolonged inhibition of LH as well as FSH secretion during 48 h, with β-endorphin being less effective than CRF.

These results demonstrate that the suckling stimulus alone suppressed the secretion of both LH and FSH, and suggest that this effect may be mediated by the inhibition of LHRH secretion from the hypothalamus. They also suggest that the suckling-induced inhibition of LHRH may be primarily mediated by endogenous CRF and opioid peptides.

Journal of Endocrinology (1989) 120, 509–515