Light‐induced c‐Fos expression in suprachiasmatic nuclei neurons targeting the paraventricular nucleus of the hamster hypothalamus: Phase dependence and immunochemical identification

Jan 5, 2002The Journal of comparative neurology

Light-triggered activity in the brain’s daily rhythm center cells connected to the hormone control area in hamsters: timing and chemical identification

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Abstract

Light stimulation at ZT 19 resulted in 40.7% of SCN output neurons expressing c-Fos, compared to 26.6% at ZT 14.

Circadian rhythms in mammals are influenced by the suprachiasmatic nuclei (SCN), which respond to light signals.
The expression of the immediate early gene c-fos in SCN neurons varies depending on the timing of light exposure.
At ZT 19, more SCN neurons showed c-Fos expression in response to light than at ZT 14.
Light exposure at different zeitgeber times indicates a phase-dependent response in SCN output neurons targeting the paraventricular nucleus.
Light-activated SCN output neurons contain gastrin-releasing peptide, vasoactive intestinal polypeptide, and some vasopressin.

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The suprachiasmatic nuclei (SCN) contain a master clock driving the majority of circadian rhythms in mammals. It is believed that the SCN confers circadian rhythmicity as well as light responsiveness to pineal melatonin secretion via a direct projection to the paraventricular nucleus of the hypothalamus (PVN). Neurons in the SCN respond to light during subjective night with an expression of the immediate early gene c-fos. The number and distribution of c-Fos protein-containing neurons depend on the zeitgeber time (ZT) at 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 with c-Fos immunohistochemistry. Male golden hamsters were injected iontophoretically 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 stimulation 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 increased this fraction to 40.7% +/- 1.9%. This demonstrates the presence of a phase-dependent c-Fos induction in the suprachiasmatic-paraventricular projection system. Triple immunohistochemistry showed that light-activated output neurons contained both gastrin-releasing peptide and vasoactive intestinal polypeptide and to a lesser extent vasopressin. The present findings provide functional evidence of light activation of central pathways involved in the regulation of circadian output rhythms.

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Light‐induced c‐Fos expression in suprachiasmatic nuclei neurons targeting the paraventricular nucleus of the hamster hypothalamus: Phase dependence and immunochemical identification

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