THE ROLE OF THE HYPOTHALAMUS IN THE TONIC SECRETION OF PROLACTIN INDUCED BY OESTROGENS IN THE RAT

Abstract

To investigate where estrogens act to induce an increase in the tonic release of prolactin, the effect of ovariectomy alone or of ovariectomy plus estradiol benzoate treatment (10 .mu.g/day for 7 days) (EB10) on the morning prolactin serum level in adult female rats was determined after each of the following procedures: frontal hypothalamic deafferentation (FHD) (a semi-lunar cut at the caudal border of the optic chiasma, separating the anterior hypothalamus from the preoptic area); medial hypothalamic deafferentation (MHD) (a similar cut, but separating the anterior from the posterior half of the hypothalamus); sham deafferentation; an electrolytic lesion of the arcuate-median eminence area (Arcuate-ME lesion); hypophysectomy and pituitary autotransplantation beneath the kidney capsule. The rats were ovariectomized 1 wk after the latter procedure, or 3 wk after the others, and EB10 or oil vehicle treatment was begun 1 wk after ovariectomy. FHD, the Arcuate-ME lesion and pituitary autotransplantation markedly increased the basal prolactin level; MHD induced a much smaller increase. The level fell after ovariectomy only in the FHD group. A sharp and equal increase in the prolactin level occurred in the FHD, MHD and sham operation groups in response to EB10; a much smaller and more variable increase occurred in the Arcuate-Me and pituitary autotransplantation groups. Estrogens increase the tonic release of prolactin only in part by a direct action on the pituitary, and more importantly by acting on the posterior half of the hypothalamus or on areas with lateral, superior or posterior afferent connections to this site.