Bicuculline infused into the hamster ventral tegmentum inhibits, while sodium valproate facilitates, sexual receptivity

Progesterone's (P) actions on both the ventral medial nucleus of the hypothalamus (VMH) and the ventral tegmental area (VTA) are essential for sexual receptivity in female hamsters. Evidence suggests that progesterone's actions in the hamster VMH may be genomic while those in the VTA may be mediated nongenomically, via GABA_A. Ovariectomized female hamsters were bilaterally implanted with cannulae aimed toward the VTA. One week after surgery, animals were SC injected with 10 μg estradiol benzoate (EB) and 40 h later with 200 or 500 μg P. At hour 43.5, 50 ng bicuculline, a GABA_A antagonist, was infused into each available cannula. Control animals received 0.5 μl sterile saline vehicle, or no infusion. At hour 44, animals were tested for sexual receptivity in an observation arena with a sexually experienced male. Histology revealed that only animals with bicuculline infused into the VTA had reduced lordosis durations compared to controls. Other animals, primed with EB and 200 μg progesterone, showed a facilitation of sexual receptivity after infusion into the VTA of 50 μg sodium valproate, a GABA_A transaminase inhibitor. These results suggest that GABA_A plays a necessary role in the mechanism of progesterone's actions on sexual receptivity in hamster VTA.