Mechanism of Action of Hypothalamic Hormones in the Anterior Pituitary Gland and Specific Modulation of Their Activity by Sex Steroids and Thyroid Hormones

Differential Suppression of Follicle-Stimulating Hormone and Luteinizing Hormone Secretion in vivo by a Gonadotropin-Releasing Hormone Antagonist

“…GnRH antagonists have been shown to have a lower dissociation rate in vitro compared to that of GnRH or GnRH agonists [11,18,19]. A decrease in specific pituitary binding of GnRH, which could be accounted for by contin ued occupancy of the receptors, followed a single injection of a GnRH antagonist to long-term gonadectomized rats [18,21]. A third possibility is that antag caused a down-regu lation of GnRH receptors.…”

“…Second, both LH and FSH secretion occur after the administration of GnRH in vivo [3,4,10,37). Third, application of GnRH to cultured pituitary cells elicits synthesis and secretion of both LH and FSH [21]. Fourth, even the observation that FSH can be sec reted without LH does not necessarily require the existence of a separate releasing hormone for FSH [30], since rapid, high-dose infusion of releasing hormone favors LH secre tion, whereas exposure to low doses over a prolonged pe riod of time results in preferential FSH secretion [37].…”

Effect of oestrogen and an LH-RH agonist on the release of gonadotrophins in ovariectomized ewes deprived of LH-RH

“…Administration of LH-RH agonist was without effect on prolactin secretion while the injection of oestrogen resulted in a clear rise in prolactin levels in both groups. This effect of oestrogen is probably brought about by its ability to block dopamine receptors on the prolactin-secreting cells (Labrie et al, 1978).…”

Effect of thyrotropin-releasing hormone on rat myometrium.

“…This modification of uterine reaction to TRH by estradiol should be noted since the stimulatory effect of TRH on the pituitary secretion of TSH or prolactin is augmented by estradiol also 4 days after the treatment (DeLean, et al, 1977;Labrie, et al, 1978). This coincidence is likely to be more than a fortuitous one since the effect of the estrogen does not become apparent within 2 days of treatment in either case.…”