Effects of an anti-androgen and 5 alpha-reductase inhibitors on estrus duration in the cycling female rat.

Five experiments examined the role of circulating androgens in the control of sexual behavior (lordosis) in the intact cycling rat. The androgen receptor blocker, flutamide (FLU), was administered daily to cycling rats beginning on the day of estrus, and lordotic responsiveness was measured on the 2nd subsequent proestrus day and on the day of estrus. FLU-treated females showed significantly higher levels of lordosis throughout the end of the period of estrus than controls (Experiment 1). Neither the maximal levels of lordosis seen on the evening of proestrus nor the time of onset of estrous responsiveness during the preceeding afternoon were affected by FLU (Experiment 2). Serum estradiol concentrations seen on the morning of proestrus (Experiment 3) did not differ between FLU- and vehicle-treated animals. The weak 5 alpha-reductase inhibitor, testosterone-17 beta-carboxylic acid (17 beta C), prolonged slightly, but did not significantly lengthen, the period of estrus (Experiment 4), while the highly potent steroidal 5 alpha-reductase inhibitor, 4 MA, significantly increased the rate at which estrous behavior declined on the day of estrus (Experiment 5). Circulating androgens do not appear to affect the maximal level of sexual receptivity displayed nor the time of estrus onset; however, they may govern the duration of the period of estrus by influencing the rate of estrus termination.