Progesterone facilitates thrust in female hamsters implanted with estradiol at the medial preoptic area.

Progesterone (P) facilitates male-like sexual behavior in female hamsters primed with estrogen. Since estrogen plus P activates lordosis in female hamsters and the site of the hormonal action is at the ventromedial hypothalamus (VMH), it is possible that the same hormones act at the same site to induce male-like sexual behavior. However, VMH lesion abolishes lordosis, but not thrust, in ovariectomized female hamsters treated with estrogen plus P. Therefore, we tried another brain site--the medial preoptic area (mPOA). We implanted cannula, 27 gauge and filled with estradiol at the tip, at the mPOA or the VMH bilaterally in ovariectomized female hamsters. The hamsters received subcutaneous injections of P (0.5 mg in 0.05 ml sesame oil) at noon every fourth day. Behavior tests were conducted the same night of the injection day and the day before. The procedure was repeated five times. During a test night, each experimental female hamster was paired with a male hamster first and then with another estrous female. Recorded behavioral events were duration of lordosis and frequency of thrust. Both mPOA and VMH groups displayed lordosis only on the day of P injection, without any significant group difference. As to the male-like sexual behavior, 9 out of 54 mPOA females displayed thrust at least once but none of 27 VMH females ever did. Furthermore, 7 of the 9 females displayed thrust only on the day of the P injection. We conclude that the mPOA was also the site for the activation of male sex behavior in female hamsters. However, the hormones acting at the mPOA to induce thrusts were estrogen plus P.