Light-induced c-fos expression in suprachiasmatic nuclei neurons targetingthe paraventricular nucleus of the hamster hypothalamus: Phase dependence and immunochemical identification

The suprachiasmatic nuclei (SCN) contain a master clock driving the majorit y of circadian rhythms in mammals. It is believed that the SCN confers circ adian rhythmicity as well as light responsiveness to pineal melatonin secre tion via a direct projection to the paraventricular nucleus of the hypothal amus (PVN). Neurons in the SCN respond to light during subjective night wit h an expression of the immediate early gene c-fos. The number and distribut ion of c-Fos protein-containing neurons depend on the zeitgeber time (ZT) a t which the light stimulus is presented. To investigate whether this phase- dependent activity is present in the SCN output neurons targeting the PVN, we combined retrograde cholera toxin subunit B (ChB) tracing from the PVN w ith c-Fos immunohistochemistry. Male golden hamsters were injected iontopho retically with ChB into the PVN area and 7 days later given a 1.5-hr light stimulus at either ZT 14 or ZT 19 followed by vascular fixation. Light stim ulation at ZT 19 gave rise to more c-Fos containing neurons in the SCN than light presented at ZT 14. Double immunostaining for ChB and c-Fos revealed that light stimulation at ZT 14 induced c-Fos expression in 26.6% +/- 2.8% of the retrogradely filled perikarya, whereas light-stimulation at ZT 19 i ncreased this fraction to 40.7% +/- 1.9%. This demonstrates the presence of a phase-dependent c-Fos induction in the suprachiasmatic-paraventricular p rojection system. Triple immunohistochemistry showed that light-activated o utput neurons contained both gastrin-releasing peptide and vasoactive intes tinal. peptide and to a lesser extent vasopressin. The present findings prm , ide functional evidence poly of light activation of central pathways invo lved in the regulation of circadian output rhythms. (C) 2002 Wiley-Liss, In c.