title: 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.
mesh_terms: D000818:Animals; D001931:Brain Mapping; D004958:Estradiol / Q000502:physiology; D004971:Estrus / Q000502:physiology*; D005260:Female; D011374:Progesterone / Q000502:physiology*; D051381:Rats; D011919:Rats, Inbred Strains; D011980:Receptors, Progesterone / Q000502:physiology; D012726:Sexual Behavior, Animal / Q000502:physiology*; D014697:Ventromedial Hypothalamic Nucleus / Q000502:physiology*
chemical_list: D011980:Receptors, Progesterone; D011374:Progesterone; D004958:Estradiol