Regulation of gonadotropin secretion during lactation

Lactation in the rat is characterized by an inhibition of ovarian cyclicity which correlates with a suppression of basal gonadotropin secretion, a decrease in the magnitude of estrogen-induced LH surges and a decrease in the postcastration rise en LH and FSH. Furthermore, pulsatile administration of GnRH (0.4 ng GnRH every 50 min) fails to elicit LH pulses in females nursing 8 pups but produces normal basal LH pulses in females nursing 2 pups. There is a good correlation during lactation among pituitary GnRH receptor content, pituitary responsiveness to GnRH and basal LH secretion. All of these parameters are greatly decreased in the presence of an 8-pup suckling stimulus but are generally normal in the presence of a 2-pup suckling stimulus. Taken together with data from other laboratories suggesting that GnRH positively regulates its own receptors, these data suggest that a large suckling stimulus decreases GnRH release from the hypothalamus. This hypothesis is supported by studies in which GnRH release in lactating rats was indirectly assessed by examining the pulsatile pattern of LH secretion. In intact rats suckling 2 or 8 pups on day 10 postpartum, the pattern of LH secretion was uniformly nonpulsatile. The suppression of pulsatile GnRH release in intact females suckling 2 pups does not appear to be due to the small suckling stimulus itself, because ovariectomized females suckling 2 pups exhibited pulsatile LH secretion on day 10 postpartum, but may be due to some ovarian factor acting within the CNS to inhibit GnRH release. In contrast, in ovariectomized females suckling 8 pups, pulsatile LH secretion was completely suppressed.(ABSTRACT TRUNCATED AT 250 WORDS)