Serotonergic Modulation of Photic Entrainment in the Syrian Hamster

In this review, we describe six lines of evidence that reveal a modulatory role for serotonin (5-HT) in the regulation of the response of suprachiasmatic nucleus (SCN) neurons to retinal illumination in the Syrian hamster. Electrical stimulation of the median raphe nucleus, sufficient to elicit the release of 5-HT in the SCN, inhibits light-induced phase shifts of the hamster circadian activity rhythm. Two 5-HT receptors capable of mediating the effects of 5-HT on photic responses, the 5-HT7 receptor and the 5-HT1B receptor, are present in the hamster SCN. Light-induced phase shifts are attenuated by systemic and local administration of two 5-HT receptor agonists, 8-OH-DPAT, and TFMPP, and these agents attenuate photic phase shifts by acting on pharmacologically distinct receptors. Furthermore, both compounds also attenuate light-induced Fos expression and photic suppression of pineal melatonin content, indicating that serotonergic modulation of photic signal transduction in the SCN is not limited to the ...