LET THERE BE LIGHT - SIGNAL-TRANSDUCTION IN A MAMMALIAN CIRCADIAN SYSTEM

Mammalian circadian rhythms are controlled by a biological clock locat ed in the hypothalamic suprachiasmatic nuclei (SCN). This clock is ent rained by light through a retinohypothalamic pathway that interacts wi th the SCN through glutamate neurotransmission. Light pulses during th e subjective night induce phase shifts of behavioral rhythms, and also trigger intracellular changes such as the expression of immediate-ear ly genes and activation of transcription factors. In this review, we p resent a model of the signal transduction pathway leading to photic sy nchronization of the circadian clock, including the activity of specif ic second messenger systems, gene expression, and interaction between potential agents capable of producing phase shifts.