The response of suprachiasmatic neurons of the rat hypothalamus to photic and serotonergic stimulation.

Single-unit activity of the rat suprachiasmatic nucleus and surrounding regions was recorded in vivo. Photically responsive neurons were typically inhibited by low intravenous doses of the serotonin (5-HT) agonist quipazine. The 5-HT antagonist, metergoline, administered in a low intravenous dose, reversed the effects of quipazine. The subsequent response to photic stimulation was essentially identical to the initial photic response. Thus the photic response of neurons in the suprachiasmatic nucleus (SCN) and adjacent regions is relatively uninfluenced by 5-HT blockade.