It is now well accepted that peptides are able to modulate sexual behavior. Probably the best known in this respect is gonadotropin-releasing hormone (GnRH) which has been shown by numerous workers to stimulate sexual receptivity in female rats. 1 2 Other peptides, such as prolactin, have inhibitory effects. 3 (3-Endorphin also acts in this way, and there are now numerous reports that this opioid will suppress sexual behavior in both females and males. 4-7 It is thought that (3-endorphin may act by inhibiting the release of GnRH. 6 ·K

Other pro-opiomelanocortin (POMC)-derived peptides will also affect sexual behavior, and it has been reported that adrenocorticotropin (ACTH) will stimulate certain elements of copulatory behavior, such as penile erection and ejaculation in several different species. 9 10

Other workers have, however, failed to confirm this, 11 and in the female rat, ACTH has been shown to inhibit lordosis behavior when administered directly into the brain. 12 ACTH will, however, stimulate receptivity when administered systemically, but this is likely to be an indirect effect due to the release of progesterone from the adrenal glands. 13 In some of these earlier studies, notably those in the male, it was observed that melanotropin (MSH) was able to mimic the effects of ACTH. 9 It is, of course, well known that MSH peptides share many of the properties of ACTH, and there is now a vast literature describing their effects in aversive situations. It is, however, less well recognized that MSH is able to affect sexual behavior, and the purpose of this chapter is to review some of the evidence and to consider, in particular, how a-MSH is able to modulate sexual behavior in the female rat through both central and peripheral mechanisms.