EFFECTS OF SEROTONERGIC AGONISTS ON FIRING RATES OF PHOTICALLY RESPONSIVE CELLS IN THE HAMSTER SUPRACHIASMATIC NUCLEUS

Serotonergic neurons from the midbrain raphe nuclei innervate the supr achiasmatic nucleus (SCN) of the hypothalamus, which functions as the dominant pacemaker for mammalian circadian rhythms. We investigated th e effects of serotonin (5-HT) on firing rates of light-activated SCN c ells in urethane-anesthetized hamsters. Micro-iontophoretic applicatio n of 5-HT or 5-HT1A agonists (8-OH-DPAT and 5-CT) caused a dose-depend ent inhibition of spontaneous activity and photic responses in the maj ority of SCN cells tested. Application of metergoline alone, a non-sel ective 5-HT antagonist, slightly increased firing rates during darknes s and light exposure, suggesting a tonic serotonergic suppression of S CN activity. Metergoline also effectively attenuated suppression induc ed by the three 5-HT agonists. In addition, the effects of 8-OH-DPAT w ere blocked by a 5-HT1A antagonist, SDZ 216-525. However, other putati ve 5-HT antagonists were weak (propranolol and NAN-190) or ineffective (ketanserin) in blocking the action of 8-OH-DPAT. These results indic ate that serotonin has a potent role in reducing photic effects on ret inally activated SCN cells in hamsters, and that these effects are med iated by a receptor with properties similar to those of the 5-HT1A sub type.