Rat and Syrian hamster: two models for the regulation of AANAT gene expression.

The Syrian hamster is a rodent species in which the photoperiodic change in the melatonin peak duration is pivotal for the synchronization of annual functions, like reproduction. In this species, the activity of arylalkylamine N-acetyltransferase (AANAT), the key enzyme for the rhythmic synthesis of melatonin, is precisely controlled and time-gated, suggesting regulatory mechanisms different from those in the rat or mouse. At the beginning of the night, norepinephrine (NE) elicits a rapid and sustained phosphorylation of CREB into pCREB and a transient synthesis of the immediate early gene products c-FOS and c-JUN that peak 3 h after dark onset. c-FOS synthesis requires both pCREB and the pERK1/2 pathways. Interestingly, injection of the protein synthesis inhibitor cycloheximide before, but not after, the c-FOS/c-JUN peak markedly reduces Aanat mRNA levels. This finding suggests that the c-FOS/c-JUN dimer is required for transcriptional activation of the Aanat gene. During daylight, exogenous noradrenergic stimulation cannot stimulate Aanat expression and, therefore, melatonin synthesis. The inhibitory transcription factor ICER is present in the pineal gland but with highest values when AANAT may be activated, suggesting the blockade takes place upstream of Aanat expression. Preliminary experiments indicate that the diurnal inhibition of AANAT occurs at the level of the adrenergic receptor signalling pathway, but it is not known whether this is sufficient to explain the pineal resistance to NE during the daytime. Together, these findings demonstrate that AANAT regulation in the Syrian hamster requires a complex intracellular signalling cascade, different from that described in laboratory rodents like mice and rats.