Influence of Ovarian Steroids on Pituitary Sensitivity to Luteinizing Hormone-Releasing Hormone in the Ovariectomized Guinea Pig*

Neuroendocrine Control of Gonadotropin Secretion

“…A similar pattern has been reported in cows 136 and guinea pigs, 137 but the stimulatory effects observed in the latter are fairly modest, particularly when compared to the dramatic effects of E 2 on the self-priming action of GnRH. 137 The one exception to this general pattern is the pig, in which E 2 appears to have inhibitory, but no stimulatory, effects on the pituitary. 130 It should be noted that very rapid inhibitory effects can be observed in several of these mammals with intravenous infusion of E 2 , 138-141 although these studies sometimes used supraphysiological E 2 treatments.…”

“…The exact time course varies among species, being shortest in rats (nadir at 4-5 h and peak at 12 h), 131 intermediate in sheep (nadir: 6-10 h; peak: 20-28 h), 129 and longest in primates (nadir: 16-24 h 128,134 ; peak: 3-4 days 128,135 ). A similar pattern has been reported in cows 136 and guinea pigs, 137 but the stimulatory effects observed in the latter are fairly modest, particularly when compared to the dramatic effects of E 2 on the self-priming action of GnRH. 137 The one exception to this general pattern is the pig, in which E 2 appears to have inhibitory, but no stimulatory, effects on the pituitary.…”

Neuroendocrine control of ovarian function in the primate

“…How¬ ever, this effect is accompanied by a lengthening in the duration of the pulse, leaving the total amount of LH that is released unchanged (Wehrenberg & Ferin, 1981 (Goodman & Karsch, 1980). To these authors, this would appear consistent with a direct effect of oestradiol on the pituitary, as they also cite data to support the fact that oestradiol can decrease the LH response to Gn-RH (Terasawa, Bridson, Weishaar & Rubens, 1980) and that an effect of oestradiol at the hypophysial level may account for the negative feedback action of oestradiol (Plant, Nakai, Belchetz, Keogh & Knobil, 1978b (Wardlaw, Wehrenberg, Ferin, Carmel & Frantz, 1980). A hypothalamic rather than hypophysial origin for pituitary portal ß-endorphin is suggested by the fact that portal ß-endorphin concentrations remain unchanged after hypophysectomy and that the elution profile of portal ß-endorphin immunoactivity on Sephadex G-50 is similar to that of hypothalamic but not hypophysial extracts (Wardlaw, Wehrenberg, Ferin, Antunes & Frantz, 1982).…”

Developmental Changes in Luteinizing Hormone Secretion in the Female Guinea Pig. II. Positive Feedback Effects of Ovarian Steroids*

“…EB, on the other hand, probably exerts its effect at the level of the medial basal hypothalamus and/or pituitary to stimulate LH release. This contention is supported by the observation that complete deafferentation of the medial basal hypothalamus does not abolish estrous cyclicity in the female guinea pig (20,21), and that EB, but not P, greatly enhances pituitary sensitivity to LHRH (13). Despite the proposed different sites of action for P and EB in facilitating LH release, our observation that the positive feedback response to both ovarian steroids is established at a similar time as the establishment of the negative feedback system during development suggests that a common mechanism is involved in regulating the release of LH (presumably LHRH release).…”

“…All values were determined in duplicate aliquots of 100 /A using a dried pituitary powder of intact male guinea pigs as a reference standard (9,13). The immunological activity of 1 ng of this reference standard is equivalent to 5 ng NIH LH-S1 (13).…”