Facilitation of sexual receptivity in hamsters by simultaneous progesterone implants into the VMH and ventral mesencephalon

Intracranial implantation experiments have shown that the ventromedial hypothalamus (VMH) is the most sensitive site for the facilitation of female sexual behavior by progesterone in estrogen-primed rats. However, similar implantation techniques have been much less successful in hamsters. Several lines of evidence indicate that both hypothalamic and midbrain structures are important for hamster lordosis. Therefore we compared the effect of progesterone (P) implants administered simultaneously to VMH and ventral midbrain on opposite sides of the brain to the effects of bilateral implants to each of these sites separately. Ovariectomized female hamsters were stereotaxically implanted with 24-gauge thin-wall guide tubes according to one of five patterns. Bilaterally symmetrical cannulae were aimed at VMH or ventral mesencephalon (vMES) or asymmetrical implants were aimed at one of the following pairs of sites, on opposite sides of the brain: VMH-vMES, VMH-preoptic area (VMH-POA), or anterior hypothalamus-anterior mesencephalon (AH-aMES). After recovery from surgery, females were primed with 10 micrograms estradiol benzoate and given pellets of P or cholesterol through a 30-gauge injector in the targeted sites. Latency, frequency, and duration of lordosis were recorded in 10-min tests with sexually active male hamsters. Sexual receptivity was significantly facilitated by simultaneous contralateral P implants into the VMH-vMES. P implants in any other combination of sites did not significantly facilitate lordosis compared to cholesterol control implants, nor did bilateral administration of this dose of P in either VMH or vMES have a reliable effect. The results support the hypothesis that P action is required in both VMH and vMES to reliably stimulate receptivity in hamsters.