Serotonin, excitatory amino acids and the photic control of melatonin rhythms and SCN c-FOS in the rat

There is a growing acceptance that serotonergic pathways to the suprachiasm atic nucleus play an important role in the mediation and modulation of ligh t entrainment of rhythms. In this study administration of the 5-HT2A/2C ago nist (+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane (DOI, 0.5 mg/kg) a t mid dark caused a phase shift in the onset of the urinary excretion of 6- sulphatoxymelatonin in rats that was sustained for at least 8 days and was blocked by the specific 5-HT2C antagonist SB-242084. Administration of DOI (2 mg/kg) across the night resulted in the appearance of c-FOS in the nucle us of cells in the suprachiasmatic nucleus during subjective darkness, but did not cause induction at the time of expected lights on (CT0). By contras t light exposure induced c-fos throughout the night including CT0. Administ ration of the NMDA receptor antagonist MR-801 (3 mg/kg) prior to light puls es had no effect on c-fos in the first part of the night, but towards the e xpected time of lights on, became progressively more potent, such that by C T0, light induction of c-fos was almost completely inhibited. These results provide further evidence that serotonin plays a role in the mediation of l ight effects on rhythms in the rat. (C) 2001 Elsevier Science B.V. All righ ts reserved.