Sex differences in circadian control of LH secretion

Sex differences in the hourly values of serum LH were examined in male and female Syrian hamsters exposed to either long photoperiods (14 h light: 10 darkness; 14L: 10D) or short photoperiods (6L: 18D). In long photoperiods, females exhibited a pro-oestrous surge of LH in response to high levels of circulating oestradiol (1047.36 +/- 90.68 pmol/l), while LH values in male hamsters remained essentially constant. In females housed in short photoperiods oestradiol levels were reduced by fivefold; however, a discrete surge in serum LH, similar to that seen at pro-oestrus in females exposed to long photoperiods, was observed. Although exposure to a short photoperiod eliminates the requirement of a positive feedback of oestrogen for the expression of an afternoon surge in LH, male hamsters housed in short photoperiods showed no indication of an afternoon surge in serum LH. These observations indicate that sex differences in the neural control of LH release are not the result of sex differences in oestrogen sensitivity or responsiveness.