Gonadotropin release as a function of mating and steroid feedback in the female rat

The aim of this investigation was to study possible relationships between mating-induced and steroid-induced luteinizing hormone (LH) release in spayed Long-Evans rats. Large amounts of LH were released approximately 7 hr following progesterone injection in rats primed with estradiol benzoate (EB). The amount of LH release varied widely depending on (1) the interval between the time of the progesterone injection and the EB priming; (2) the progesterone dose; and (3) the time of day when blood samples were collected. These findings provided confirmation of those of Caligaris, Astrada and Taleisnik (1971a). Females, prepared with estrogen-progesterone treatment in a variety of schedules in which the three above-mentioned variables were altered systematically, were allowed to mate with vigorous males. Mating under these various conditions did not significantly increase plasma LH levels even when the females showed high degrees of sexual receptivity. Sodium pentobarbital prevented the afternoon LH rise resulting from progesterone treatment 3 days after EB priming. Pituitary sensitivity to LRF was not enhanced in the afternoon and the mating did not significantly increase plasma LH in these barbiturate-blocked rats. Following administration of 5 large daily doses of EB without progesterone, however, significant increases in LH were produced by mating on the sixth day. Postcopulatory LH release in these circumstances was dependent on a diurnal factor since the effect of mating was greater in the afternoon than in the morning. Thus, although major LH release can be readily induced by mating in estrogen-treated spayed rats, this effect could not be obtained under conditions of progesterone administration to estrogen primed rats.