Mechanisms of suppression of gonadotropins secretion during lactation in the rat.

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Mechanisms responsible for suppression of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) secretion during lactation were investigated in rats. Throught lactation in rats plasma levels of FSH were relatively constant regardless of the litter size though LH levels were consistently lower in dams nursing 8 pups than in dams nursing 2 pups. Changes in follicular degeneration corresponded with the decrease of plasma LH and follicular maturation was noted with an increase in basal levels of LH. It is also true for inhibin activity and plasma levels of estradiol-17 <beta> . Findings in 1985 indicate that the neural stimulus of suckling rather than ovarian inhibin is mainly responsible for suppression of FSH secretion during the first half of lactation in rats nursing 8 pups. However, inhibin from Graafian follicles may play a primary role in the suppression of FSH secretion during the second half of lactation in rats nursing 8 pups and throughout lactation in the rats nursing … More 2 pups.Because endogenous opioid peptides and corticotropin releasing factor (CRF) are activated during suckling and decrease LH levels when injected centrally, I have explored the roles of these peptides in the suckling-induced inhibition of LH and FSH secretion. The acute administration of 10 <micro> g CRF or <beta> -endorphin into the lateral ventricle of ovariectomized lactating rats two days after removal of the litter caused a rapid inhibition of LH secretion within 30 min. On the other hand, levels of FSH were decreased 6 h after the injection and thereafter. Repeated i.c.v. injection of CRF or <beta> -endorphin at 6 h intervals caused not only LH secretion but also FSH secretion in lactating rats. The central administration of <beta> -endorphin caused a dramatic increase in levels of prolactin in the plasma within 30 min, whereas CRF showed no effect. Such observations suggest that CRF and <beta> -endorphin exerts deleterious action on the inhibition of LH and FSH secretion in lactating rats through brain sites of action. Therefore, it is possible that the activation of the central CRF/ <beta> -endorphin pathway may play an important role in the suckling-induced inhibition of LH and FSH secretion. Less