Daytime gating in the Syrian hamster pineal gland.

Melatonin synthesis in rodents is tightly regulated at the transcriptional level by stimulatory and inhibitory transcription factors. Among them, phosphorylated cAMP-related element binding protein (pCREB) and inducible cAMP early repressor (ICER), a strong inhibitor of cAMP-related element-driven genes, have an antagonistic action in activating/inhibiting the transcription of the Aa-nat gene, which is an important enzyme in melatonin synthesis. In the Syrian hamster, a rodent displaying a seasonal control of reproduction, melatonin synthesis is strongly gated to the second part of the night. Indeed, exogenous adrenergic stimulation is unable to stimulate Aa-nat gene transcription and melatonin synthesis during daytime. In the present study, we investigated whether ICER may be the cause of this daytime repression by comparing the dynamic of ICER and the adrenergic regulation of two genes whose expression is rapidly activated by cAMP-dependant mechanisms, c-fos and Icer. Adrenergic induction of c-fos and Icer expression was not possible during daytime, except at early day. ICER immunoreactivity was elevated throughout the daily cycle but reached the highest levels at early day, when gene expression can be induced by adrenergic agonists. Additionally, CREB phosphorylation was subjected to the same daily gating with an adrenergic induction occurring in the early but not in the late day. Taken together, our results indicate that the diurnal gating of pineal activity in the Syrian hamster is not caused by the repressor ICER and that it may occur at the level of noradrenergic receptor signalling.