Hypothalamic progesterone implants and facilitation of lordosis behavior in estrogen-primed ovariectomized guinea pigs

Adult ovariectomized Hartley guinea pigs were bilaterally implanted in the basal hypothalamus or anterior hypothalamic-preoptic area with stainless steel cannulae with removable inserts. In experiment 1, animals received inserts containing crystalline progesterone, 17α-hydroxyprogesterone or cholesterol 36 h after subcutaneous injection of 3.3 μg estradiol benzoate. Regardless of cannula content, about 43% of animals with implants aimed at the basal hypothalamus displayed heat. Lesions or irritation associated with implants in this region may have increased behavioral responsiveness to estrogen. However, control implanted animals (17α-hydroxyprogesterone and cholesterol) tended to have shorter heat durations and more scattered facilitatory implant sites than progesterone implanted animals, suggesting that progesterone might be acting in basal hypothalamus to facilitate lordosis. Progesterone placed in basal anterior hypothalamic-preoptic area did not facilitate lordosis. Progesterone in basal hypothalamus or basal anterior hypothalamic-preoptic area did not produce inhibitory effects on behavior.In experiment 2, a revised procedure was used to control for sexual behavior resulting from estrogen treatment alone. The control procedures revealed that most of the animals with cannulae in the ventromedial-arcuate area displayed lordosis after estradiol benzoate alone. When animals not displaying estrus in response to estradiol benzoate alone were utilized for further implantation, it was clear that progesterone placed in the ventromedial-arcuate-premammillary area facilitated the expression of short latency, long duration heat. However, animals with cannulae in the basal hypothalamus, regardless of cannula content, had short maximum lordoses. No inhibitory effects of progesterone implants (in ventromedial-arcuate-premammillary area) on lordosis were observed. These results suggest that brain sites selectively responsive to the facilitatory actions of progesterone exist in the ventromedial-arcuate-premammillary region and that the whole basal hypothalamus participates in the normal, full expression of guinea pig behavioral estrus.