Regulation of the vgf gene in the golden hamster suprachiasmatic nucleus by light and by the circadian clock

By using in situ hybridization in the golden hamster brain, we have found that vgf mRNA levels are induced as a response to light stimulation in the suprachiasmatic nuclei (SCN), the site of the mammalian circadian pacemaker. The induction exhibits delayed kinetics relative to known light- induced immediate early genes: induction of vgf mRNA occurs over a period of 3 to 9 hours after light exposure. Photic induction of vgf expression does not occur in the paraventricular nucleus (PVN) of the hypothalamus, though this nucleus expresses vgf at the mRNA and protein levels. Photic induction of vgf in the SCN occurs only at circadian times when light also causes a phase shift of the circadian system. The irradiance threshold of vgf induction in the SCN closely matches that of the behavioral phase shifting response. In addition, basal expression of vgf in the SCN, but not in the PVN, exhibits a circadian rhythm in constant darkness. The photic induction and circadian rhythm of vgf expression are anatomically separated in the caudal and rostral portions of the SCN, respectively. These results represent the first example of a delayed response to light relative to light-induced immediate early genes at the mRNA level in the SCN.