The concentration of cytoplasmic estrogen receptors in the brain of ovariectomized female rats varies during the light-dark cycle. There is no variation in the affinity of the receptors for estradiol, and the rhythm is not due to estrogens from nonovarian sources. Pentobarbital reverses the reduction of receptors that occurs in the dark, and melatonin injection in the light partially mimics the action of darkness in reducing receptor levels. The factors that cause this rhythm in brain estrogen receptors may be one means by which light affects reproductive function.
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