Circadian rhythm of [3H]imipramine binding in the rat suprachiasmatic nuclei.

High affinity imipramine binding undergoes circadian variations of ca. 35% amplitude in many rat brain nuclei. The suprachiasmatic nuclei of the anterior hypothalamus (considered to be the circadian pacemaker driving many overt rhythms) has highest imipramine binding at the end of the dark and lowest at the end of the light phase. A similar circadian rhythm has previously been observed for serotonin uptake in the suprachiasmatic nuclei. In conjunction with other findings, these data indicate that serotonergic turnover in the suprachiasmatic nuclei decreases at lights on and increases at lights off.