Short Latency Induction of Estrous Behavior with Intracerebral Gonadal Hormones in Ovariectomized Rats

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Abstract
In an attempt to localize the site of action of progesterone in facilitating sexual receptivity, ovariectomized rats were primed sc with 2 μg estradiol benzoate (EB) for 3 days, and on the fourth day progesterone crystals were applied directly to the brain by inserting a hormone-containing tube into chronic cannulae implanted unilaterally in the lateral and medial preoptic area (MPOA), anterior hypothalamus, ventromedial-arcuate area, or the caudal mesencephalic reticular formation (MRF). Also, in similarly primed animals bearing chronic cannulae in the MPOA or the MRF, EB was applied directly to the brain. Lordosis behavior was quantified 15, 30, 60, 90 and 120 min after progesterone application and compared to pretest values. Progesterone in the MRF significantly increased lordosis behavior in 15 min, while in the other regions progesterone had no effect even after 2 hr. To test the specificity of this response an empty tube, tubes containing cholesterol, procaine, or serotonin were similarly implanted in the MRF, but none of these procedures facilitated lordosis behavior within the 2-hr test period. When the total amount of progesterone estimated to be in the implants was injected sc in oil, lordosis was not facilitated. The latency to lordosis behavior following MRF application of progesterone was found to be about ¼ that following the sc injection of 500 μg progesterone in oil. These observations suggest that the MRF may contain progesterone-sensitive elements which participate in the neural regulation of lordosis behavior, and that the action of progesterone can be expressed rapidly. Lordosis behavior was quantified at similar time intervals following intracerebral EB application, and, as well, at 24-hr intervals for 5 additional days. Estrogen in both the MPOA and the MRF significantly increased lordosis behavior within 2 hr. Twenty-four hr later, the lordosis frequency had returned to the pre-implantation level; by the fourth day, however, lordosis frequency had again increased significantly. Because these rats had been primed initially with EB sc, it was not possible to evaluate the diffusion of intracerebral EB several days after implantation. The initial and rapid facilitation of lordosis, however, is consistent with the view that estrogen, as well as progesterone, can act at the level of the MRF. (Endocrinology89: 32, 1971)

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