Plasma levels of dihydrotestosterone in the cycling rat: implications for the regulation of lordosis behavior.

Exogenously administered dihydrotestosterone (DHT) is a potent inhibitor of sexual behavior in ovariectomized, estrogen-primed rats and endogenous DHT has been implicated as an inhibitor of sexual behavior in cycling rats. To determine the temporal relationship between DHT levels and the expression of sexual behavior, females were tested at midnight of each day of the 4-day estrous cycle for lordosis response to male mounts and subsequently bled. Plasma DHT levels were highest (177 +/- 10 pg/ml) at proestrus (P), when lordosis was fully expressed, fell to significantly lower levels (136 +/- 8, p less than 0.025) at estrus (E), and rose at diestrus I and diestrus II (DII) (152 +/- 8 and 154 +/- 10 respectively). Other female rats were bled from a jugular cannula at 4-hr intervals between midnight of DII and midnight of E. Plasma DHT was elevated from 1200 of P to 0400 of E and fell rapidly through 0800 of E. This elevation of endogenous DHT coincides with the expression of lordosis behavior, and the magnitude of this peak is many times lower than the concentrations reached by the minimal exogenous dose that inhibits lordosis behavior. This suggests that cyclic changes of DHT in the peripheral circulation do not inhibit lordosis behavior during the estrous cycle.