Disruption of the activity-rest cycle by MAOI treatment: dependence on light and a secondary visual pathway to the circadian pacemaker.

The disruptive effects on the activity-rest cycle of the monoamine oxidase inhibitor (MAOI) clorgyline and of continuous light were examined in Syrian hamsters. When administered in dim and moderate light intensities, clorgyline delayed the daily onset of wheel-running. When administered in bright light, it dissociated the circadian rhythm of wheel-running.

C-fos mRNA increases in the ground squirrel suprachiasmatic nucleus during arousal from hibernation.

During hibernation the body temperature of the golden-mantled ground squirrel, Spermophilus lateralis, may drop below 5 degrees C for a few hours to a week or more. Animals cycle between euthermia and deep hibernation many times over the course of the hibernation season. Expression of the transcription factor c-fos increased in the suprachiasmatic nucleus (SCN) of the hypothalamus, the mammalian circadian clock, during deep hibernation and peaked during the arousal from hibernation.

Light-induced gene expression in the suprachiasmatic nucleus of young and aging rats.

We investigated age-related changes in a molecular mechanism associated with synchronization of circadian rhythms to the environment. In mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus contains a circadian pacemaker that regulates a variety of physiological and behavioral rhythms. Recent studies have demonstrated photic induction of immediate early genes (IEGs) such as c-fos in the SCN in a circadian-phase dependent manner, suggesting that IEGs may be part of the pathway for entrainment of circadian rhythms.

Melatonin influences Fos expression in the rat suprachiasmatic.

Administration of the pineal hormone melatonin to rats induces expression of Fos, the protein product of the c-fos proto-oncogene, in the suprachiasmatic nucleus (SCN), the putative biological clock of mammals. Expression of the Fos protein is dependent on circadian phase: injections in the late subjective night (circadian time (CT) 22) induce Fos expression in cells within the ventral SCN whereas injections during the subjective day are ineffective. Since melatonin injections in the late subjective day have previously been shown to phase advance circadian rhythms, these results indicate that phase-advances of the circadian system can occur without increased expression of Fos protein in the SCN, at least at levels detectable by immunohistochemistry.

Circadian and light-induced expression of immediate early gene mRNAs in the rat suprachiasmatic nucleus.

Recent work has suggested that immediate early genes are involved in the phase-shifting response of the circadian pacemaker to light. In mammals, the endogenous pacemaker is located in the suprachiasmatic nucleus (SCN). We report here a systematic examination of the expression of the immediate early genes fos, NGFI-A, and NGFI-B in the rat SCN, in basal and light-stimulated conditions across the circadian cycle.