Facilitation and inhibition of sexual receptivity in the female rat by α-MSH

Abstract

The effect of α-MSH on sexual behaviour was examined in female rats. Ovariectomized rats received 2 μg oestradiol benzoate on Day 1 and on Day 3 an injection of progesterone. Four hours later the females were placed with sexually experienced male rats and the lordosis quotient (LQ) noted. A maximal lordotic reponse was seen in those rats that received 0.5 mg or higher doses of progesterone, although extremely high doses tended to decrease the LQ. After the submaximal doses of 0.1 and 0.2 mg progesterone a number of rats displayed LQ's of less than 50% and these were considered to be non-receptive and those that displayed LQ's greater than 50% were considered to be receptive. Treatment with 20 μg α-MSH on Days 1, 2 and 3, 2 and 3 or as single injections on Days 1, 2 and 3 stimulated lordosis in the non-receptive rats but inhibited lordosis in the receptive rats. α-MSH, however, failed to inhibit the lordotic response in the females that were receptive after 0.5 mg progesterone and had no effect in the absence of progesterone. Thus it would appear that α-MSH has two effects on sexual behaviour in female rats; in rats showing poor receptivity after progesterone α-MSH stimulates lordosis and in rats that are receptive after progesterone α-MSH inhibits lordosis. It is concluded that while α-MSH has no specific effect on lordosis it is able to modify sexual behaviour in the female rat possibly through an effect on arousal.