Abstract
During lactation the suckling stimulus exerts profound influences on the structures and functions of the central nervous system of nursing rats. We have examined the acute effect of pup removal on estrogen-induced LH surge. Lactating and nonlactaing, cycling female rats were given an estradiol-containing capsule after ovariectomy and were bled with an indwelling cather for serum LH determinations. In ovariectomized, lactating rats with pups, estrogen treatment induced an afternoon LH surge with an amplitude and timing comparable to those seen in nonlactating rats. Removal of pups from the lactating rats at 0900, 1100, or 1300 h, but not at 1500 h, abolished the estrogen-induced surge that would occur in the following afternoon. The inhibitory effect of pup removal at 0900 h was reversed when the animals were resuckled by 1400 h. However, pup removal was ineffective in abolishing the enhancing effects of progesterone at low and high doses on estrogen-induced LH surge. Double immunohistochemical staining for gonadotropin-releasing hormone (GnRH) and c-Fos, a marker for neuronal activation, revealed a concomitant decrease in the number of c-Fos-immunoreactive GnRH neurons in the preoptic region with the blockade of LH surges in pup-removed lactating rats.These results suggest that neural mechanism for the GnRH surge generator is altered during lactation, and that GnRH neurons require an additional stimulus that is not involved in the surge in nonlactating, cycling female rats. [Jpn J Physiol 55 Suppl:S212 (2005)]
