Feedback Effect of Oestrogen on Luteinizing Hormone Secretion by the Rat Pituitary Gland

Abstract

Ovariectomized rats with neural deafferentation at the level of the posterior border of the anterior hypothalamic area (AC rats) were used to re-evaluate the direct feedback effect of oestrogen on the regulation of LH secretion by the pituitary gland. Synthetic LH releasing hormone (LH-RH; 300 ng/kg), injected at 30 min intervals into AC rats with undetectable basal LH, induced pulsatile increase of serum LH concentrations. Oestradiol-7Beta (5 micrograms), administered i.v. just before the first LH-RH injection, significantly decreased the LH response to a second injection of LH-RH given 30 min later and to subsequent injections. Maximal inhibition was 58%. Oestradiol 17 beta (5 micrograms) given i.v. to control ovariectomized rats decreased serum LH concentrations 40 min after administration; the maximum reduction being 52%. An s.c. injection of oestradiol benzoate (5 micrograms) increased pituitary responsiveness to LH-RH by the next day in AC rats but decreased serum LH levels in control ovariectomized rats. These results indicate that acute inhibitory and chronic facilitatory effects of oestrogen on LH secretion are exerted at the pituitary gland, without a change in LH-RH secretion. The prolonged inhibitory effect of oestrogen is at the level of the hypothalamus and causes a reduction in LH-RH secretion.

Luteinizing hormone responses to repeated injections of luteinizing hormone releasing hormone in the rat during the oestrous cycle and after ovariectomy with or without oestrogen treatment

“…Since we used intact rats in this experiment, it is possible that increased sex steroid hormone concentrations, especially oestrogen, induced by increased serum LH levels, sensitized the pituitary gland to succeeding LH-RH doses. This possibility, however, seems unlikely because our results were in general agreement with in-vitro results obtained using a pituitary perifusion technique by Waring & Turgeon (1980), and oestrogen is known to inhibit LH responsiveness at the pituitary gland to LH-RH a few hours after injection (Libertun et al 1974;Higuchi & Kawakami, 1982). Response of LH to repeated injections of LH releasing hormone (LH-RH; 300ng/kg body wt; shown by arrows) at 30-min intervals in rats ovariectomized for 120 days.…”

“…It is generally accepted that oestrogen is the main factor which modulates pituitary responsiveness to LH-RH. Oestrogen acts directly on the pituitary gland to inhibit or facilitate the LH response to LH-RH, depending on the duration of oestrogen action on the pituitary gland (Libertun, Orias & McCann, 1974;Higuchi & Kawakami, 1982). In the rat, the LH response to LH-RH is highest at pro-oestrus (Cooper, Fawcett & McCann, 1973;Pickering & Fink, 1979), mainly due to the increased levels of circulating oestrogen at late dioestrus and early pro-oestrus (Aiyer & Fink, 1974;Butcher, Collins & Fugo, 1974).…”

Changes in the characteristics of pulsatile luteinizing hormone secretion during the oestrous cycle and after ovariectomy and oestrogen treatment in female rats

“…Treatment with a long-lasting oestrogen, oestradiol benzoate (Table 3) or s.c. implantation of oestradiol-17ß (Weick, 1977) in ovariectomized rats inhibited the LH pulsatile rhythm. In addition, chronic inhibitory effects of oestrogen on LH secretion seems to be induced by inhibiting hypothalamic LH-RH release rather than by inhibiting pituitary responsiveness to LH-RH (Aiyer, Sood & Brown-Grant, 1976;Higuchi & Kawakami, 1982). Thus, oestrogen might be responsible for maintaining the pulse frequency at a low level in cyclic rats.…”

“…Our previous report (Higuchi & Kawakami, 1982) indicated that the acute inhibitory effect of oestrogen on LH release could be explained by an acute suppressive action of oestrogen on pituitary responsiveness to LH-RH, because a directly inhibitory effect of oestrogen on the pituitary gland could operate in ovariectomized rats with little endogen¬ ous LH-RH after anterior neural deafferentation. An increase of pituitary responsiveness to LH-RH rather than an increase in LH-RH at each pulsatile discharge may play the major role in this enhancement of LH pulse amplitude after ovariectomy.…”

Regional Specificity in the Effect of Estrogen Implantation within the Forebrain on the Frequency of Pulsatile Luteinizing Hormone Secretion in the Ovariectomized Rat

“…However, some other investigators have re ported that systemically applied estrogen decreased serum LH levels and/or LH pulse amplitudes but failed to pro duce a modification of the pulse frequency in ovariectom ized rats [12,21,22] and sheep [10], The reason for this discrepancy is not clear. However, it must be taken into ac count that estrogen can act at the level of the pituitary to suppress LH release [11,17]. Inhibition of the pituitary LH output by estrogen would make it difficult to examine the steroid effect on the hypothalamic pulse generator unless pulsatile patterns of LHRH, rather than LH, were mea sured in a physiological condition.…”