Facilitation of sexual receptivity by hypothalamic and midbrain implants of progesterone in female hamsters.

In the first experiment, ovariectomized female hamsters were stereotaxically implanted with bilateral guide cannulae aimed at the medial preoptic area (POA), ventromedial hypothalamus (VMH), or ventral tegmentum (VTA). The following week these females were injected SC with 10 micrograms estradiol benzoate (EB) and then had 27-gauge cannulae containing crystalline progesterone inserted through the guide tubes. Sexual receptivity was observed in 3 of 11 animals with VMH implants of progesterone, in 2 of 10 with VTA progesterone, but in none with POA implants. In the second experiment, the amount of intracranial progesterone was increased by mechanically expelling a 1.5 micrograms progesterone pellet from the tip of each cannula insert. This treatment facilitated receptivity in 10 of 20 hamsters with VTA implants and in 9 of 32 VMH-implanted animals. This induction of receptivity required approximately 2 hr. Progesterone pellets in the POA, mammillary region, and lateral mesencephalon were generally ineffective. In hamsters, progesterone into either the VMH or the VTA is sufficient to facilitate receptivity, although neither site is highly sensitive to progesterone. These results differ from those in recent studies in rats and this difference may reflect important species differences in the control of lordosis.