Melatonin Inhibits the PACAP-lnduced Phosphorylation of the Transcription Factor CREB in the Rat Suprachiasmatic Nucleus

Abstract

The mammalian hypothalamic suprachiasmatic nucleus (SCN) is an endogenous pacemaker generating circadian rhythms. SCN activity is synchronized with environmental light/dark cycles by photic information primarily transmitted via the retinohypothalamic tract (RHT). One putative neurotransmitter/neuromodulator of the RHT is the pituitary adenylate cyclase-activating polypeptide (PACAP), which was shown to phase-shift the activity of SCN neurons during the day. In the SCN, the transcription factor cAMP response element binding protein (CREB) seems to be an important mediator between photic input and downstream transcriptional events. The pineal hormone melatonin, whose synthesis and release is under the control of the pacemaker, feeds back to the SCN, acting on high-affinity receptors. Like PACAP, melatonin is capable of affecting the biological clock by the phase-shifting of circadian rhythms. Using a brain slice technique, immunocytochemistry and immunochemistry we demonstrate that in the rat suprachiasmatic nucleus (1) CREB phosphorylation is induced during late subjective day by forskolin (10 μM) or depolarization with 60 mM KCl, resulting in an increase of intracellular concentrations of the second messengers cAMP or Ca2+, (2) PACAP can induce CREB phosphorylation during this time, and (3) melatonin inhibits the PACAP-induced phosphorylation. Our data show the possibility of CREB involvement in processing of light (or darkness) information even during subjective day. The potency of PACAP, one potential neuromodulator of the RHT, to affect gene expression in the SCN via activation of a transcription factor may give an insight into the rhythm entrainment of the oscillator, whereas the antagonistic effect of melatonin may mirror a feedback loop within the circadian system.