Hypothalamic and suprachiasmatic uptake of serotonin in vitro: twenty-four-hour changes in male and proestrous female rats.

Twenty-four-hour changes in the in vitro serotonin (5-HT) uptake capacity of hypothalamic homogenates and of "Vibratome" slices of the suprachiasmatic nuclear region (SNR) of the hypothalamus were studied in 60-90-day-old Holtzman (albino) rats. Animals acclimated to artificially illuminated (fluorescent, LD 12:12) and temperature controlled (22 +/- 2 C) rooms were killed 6% each of 8 time points. Synaptosomal fractions from homogenates of whole hypothalamus, and slices of the SNR were incubated for 20 min in Krebs-Henseleit buffer with [3H]5-HT. Males showed a single daily peak in SNR uptake at the start of darkness, and a minimum near the onset of light, while a more complex pattern containing 3 peaks and minima occurred in uptake by hypothalamic homogenates. Proestrous females showed a single high amplitude peak SNR uptake during the critical period, just prior to the plasma LH peak determined in the same animals by radioimmunoassay. It is suggested that this short-term and 4-fold increase in SNR uptake of 5-HT may serve to limit free 5-HT and its inhibitory or other effects on the gonadotropin release hormone system and thereby on LH release and ovulation.