The neuroendocrine control of clock-timed gonadotropin release in the female Syrian hamster: role of serotonin

We hypothesized that rhythms in hypothalamic seroton-ergic activity were permissive to daily and estrous cycle-related rhythms of LH, FSH and prolactin (PRL). In the Syrian hamster, proestrus (PRO) is characterized by a surge of LH, FSH and PRL; diestrus (DIE) by low LH and FSH and a small surge of PRL, while in photoperiod-induced anestrous (PIA) animals there is a surge of LH and FSH and low PRL. Turnover rates of serotonin (5HT) in four brain areas were determined for the three reproduc-tive states at 2-h intervals. Turnover in the preoptic area and arcuate nuclei did not change, indicating that 5HT projections to these regions probably do not control LH, FSH or PRL release. Serotonin turnover in the median eminence (ME) was elevated at 0600 h in PIA females, a