Sex differentiation of t-e circadian system in the golden hamster

The effect of estradiol benzoate (EB) on free-running circadian activity rhythms was studied in gonadectomized hamsters maintained in constant dim illumination. EB shortened the period (tau) of the female, but not of the male circadian activity rhythm. Responsiveness of the circadian system to EB was subject to sexual differentiation. The circadian period of wheel running by female hamsters given a single injection of testosterone propionate on the day of birth did not shorten in response to EB in adulthood. This failure to respond to EB also was observed in normal male hamsters, and was different from the response shown by normal females. Preliminary data suggest that tau of the activity rhythm of males castrated on the day of birth is shortened during EB treatment in adulthood. The differential effects of estradiol on tau are related to anatomic differences between the sexes in neural connections of the substrate for circadian rhythms.