Temporal aspects of ventromedial hypothalamic progesterone action in the facilitation of estrous behavior in the female rat.

Abstract

Progesterone (P) action following estrogen priming is required normally for the facilitation of estrous behavior in female rats. Although mechanisms by which P exerts its influence on estrous responsiveness have not been elucidated, the primary site of action of P has been shown to be the ventromedial nucleus of the hypothalamus (VMN). The objective of the present series of experiments was to describe the temporal parameters of P action in the VMN in the facilitation of estrous behavior in estrogen-primed female rats. Subjects were female Long-Evans rats stereotaxically outfitted with 23-ga. guide cannulae directed towards the VMN. Crystalline P was applied directly to the brain tissue via bilateral 28-ga. insert cannulae, which could be inserted and removed easily through the guide cannulae. Animals were ovariectomized and estrogen primed with 5% estradiol Silastic capsules. They received a counterbalanced series of two experimental tests: one involving a manipulation with a P-filled implant, and another with a blank implant. In the first experiment, a significant increase in estrous responsiveness occurred only after 2 hr exposure of the VMN to P, whereas 4 hr were required for a full display of estrous behavior, including solicitation. In Experiment 2, P was lowered into the brain for either 1, 2, or 4 hr, and testing took place 4 hr after the lowering of the implant. It was found that 2 hr of P exposure was sufficient to facilitate full estrous responsiveness at 4 hr. In Experiment 3, it was revealed that the duration of estrous responsiveness was directly related to the time the P implant remained in the brain. In the fourth experiment, the time course of P retention in brain tissue, revealed by determination of 3H-progesterone levels in hypothalamus, agreed with the behavioral findings. Progesterone levels in the region of the VMN remained high while a P implant was in place, but declined rapidly after removal. A dual mechanistic hypothesis for P action in the facilitation of estrous behavior is presented.