Circadian and Estrous Rhythms in Pineal and Brain Serotonin

Abstract

Differential extraction and spectrophotofluorometry of pineal serotonin in rats demonstrated a 9-fold circadian rhythm, which was shown to be dependent in its timing on the periods of light and darkness and to be slightly modified by the estrous cycle. A circadian change in the serotonin content of anterior and posterior areas of the hypothalamus could not be demonstrated. Available information suggests that the circadian rhythm in pineal serotonin may be an intrinsic one, because serotonin injection does not significantly increase pineal serotonin and because a circadian rhythm, at least of this amplitude and relation to photoperiod, has not been found in the central nervous system. The interpretation of the dynamics of serotonin level in pineal, and possibly some areas of the central nervous system, in terms either of metabolism or binding, must take into account the occurrence or possibility of a circadian rhythm.