Contrary to other non-photic cues, acute melatonin injection does not induce immediate changes of clock gene mRNA expression in the rat suprachiasmatic nuclei.

The suprachiasmatic nuclei (SCN) contain the main clock of the mammalian circadian system. The endogenous oscillation machinery involves interactive positive and negative transcriptional and posttranslational feedback loops involving the clock genes Per1, Per2, Per3, Clock, Bmal1, Cry1 and Cry2. The SCN endogenous oscillation is entrained to 24 h by the light/dark cycle. Light induced regulation of Per1 and Per2 mRNA expression have been suggested to take part in the clock resetting. However, other factors have chronobiotic and synchronizing effects on SCN activity. Especially, the nocturnal pineal gland hormone, melatonin, which is involved in the regulation of both circadian and seasonal rhythms, is known to feedback on the SCN. Melatonin applied on SCN slices immediately phase-shifts their neuronal electrical activity, while daily injections of melatonin to free running rodents resynchronize their locomotor activity to 24 h. To determine whether melatonin feedback control on SCN activity implicates transcriptional regulation of the clock genes, we monitored the expression pattern of Per 1, 2, 3, Bmal1, Cry1 and AVP mRNAs after a single melatonin injection at the end of the subjective day. Results showed that melatonin injection affected none of the mRNA expression pattern during the first circadian night. Per1, Per3, Bmal1 and AVP expression patterns were, however, significantly but differentially affected, during the second subjective night after the melatonin injection. The present results strongly suggest that the immediate phase shifting effect of melatonin on the SCN molecular loop implicates rather post-translational than transcriptional mechanisms.