Modification of circadian sleep rhythms by gonadal steroids and the neural mechanisms involved

The effects of gonadectomy and gonadal hormone treatment of castrated rats or ovariectomized (OVX) rats bearing brain lesions on the circadian rhythms of slow wave sleep (SWS) and paradoxical sleep (PS) have been studied under a 14/10 light-dark schedule. Cortical EEGs and dorsal neck EMG were used to monitor SWS, PS and alertness. Intact female rats showed two daytime SWS peaks, one daytime PS peak and a small night PS peak except during proestrus. In intact male rats, the morning SWS peak and night PS peak were variable and SWS and PS peaks in daytime were dissociated. Orchidectomized (ORX) rats showed the morning SWS peak and disrupted the dissociation of SWS and PS peaks. Furthermore, gonadectomy increased the night PS peak. Posterior deafferentation of the hypothalamus (PDM) eliminated the night PS peak. Estradiol (E₂B) injection to long term OVX rats eliminated the night PS peak from the first day of injection. However, E₂B injection into androgenized OVX rats, ORX rats and OVX rats bearing septal lesion or MPO roof cut did not eliminate night PS peak. E₂B injection to short term OVX rats or OVX rats with PDM lesions delayed the E₂B-induced elimination of night PS peak. From these results, it is suggested that: (1) sexual dimorphism exists in the circadian sleep rhythm itself, and this difference partly depends on the hormonal environment produced by sex steroids; (2) the rise and fall of night PS peak reflects the neurohumoral environment in female rats; (3) the appearance of night PS peak involves the abolition of negative feedback of sex steroids and the posterior neural input into hypothalamus; and (4) the elimination of night PS peak on natural proestrus and following E₂B treatment of OVX rats requires the intact positive feedback system of estradiol.