There is considerable evidence that circadian rhythms in mammals can be modulated by emotional state, but how emotional state modulates specific circadian outputs is poorly understood. We analyzed the expression of the circadian clock protein Period2 (PER2) in three regions of the limbic forebrain known to play key roles in emotional regulation, the central nucleus of the amygdala (CEA), the basolateral amygdala (BLA), and the dentate gyrus (DG). We report here that cells in all three regions exhibit daily rhythms in expression of PER2 that are under the control of the master clock, the suprachiasmatic nucleus (SCN). The rhythm in the CEA and the rhythms in the BLA and DG are diametrically opposite in phase and are differentially affected by adrenalectomy. Adrenalectomy completely abolished the PER2 rhythm in the CEA but had no effect on the PER2 rhythms in the BLA and DG. We previously reported a rhythm in PER2 expression in the oval nucleus of the bed nucleus of the stria terminalis that is identical in phase and sensitivity to adrenalectomy to that found in the CEA. Together, these findings show that key structures of the limbic forebrain exhibit daily oscillations in clock gene expression that are controlled not only by input from the SCN but, importantly, by hormonal and neurochemical changes that normally accompany motivational and emotional states. Thus, cells within these areas are strategically positioned to integrate the inputs from the SCN and emotional states to modulate circadian rhythms downstream from the SCN clock.hippocampus ͉ suprachiasmatic nucleus ͉ oval nucleus ͉ circadian clock I n mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus is recognized as a master clock responsible for the regulation of all behavioral and physiological circadian rhythms (1). The rhythm of the SCN, although sensitive to the daily cycle of light, is relatively invulnerable to environmental challenges and changes in physiology (2, 3). Yet it is recognized that stressors and changes in emotional states can have disruptive effects on behavioral and physiological rhythms (4-8). Furthermore, disruptions of circadian rhythms have negative effects on mood and well being, as observed during jet lag or shift work (9, 10).Although the SCN is the only clock structure required for the generation of circadian rhythmicity, recent studies show that circadian rhythms in expression of clock genes, such as Per1 and Per2, can be found in other brain regions that participate in the control of specific behavioral and physiological outputs. These include, but are not limited to, the striatum, paraventricular hypothalamic nucleus, arcuate nucleus, preoptic area, and olfactory bulbs (11-17). The study of clock genes in these and other functionally defined brain regions may help to determine how the SCN clock controls specific circadian rhythms and, importantly, may contribute to an understanding of how diverse experiences and pathological conditions can affect circadian rhythms downstream from the SCN clock.We found recently th...

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SUMMARY Protein synthesis is critical for circadian clock function, but little is known on how translational regulation controls the master pacemaker in mammals, the suprachiasmatic nucleus (SCN). Here we demonstrate that the pivotal translational repressor, the eukaryotic translational initiation factor 4E binding protein 1(4E-BP1) is rhythmically regulated via the mechanistic target of rapamycin (mTOR) signaling in the SCN and preferentially represses vasoactive intestinal peptide (Vip) mRNA translation. Knockout (KO) of Eif4ebp1 (gene encoding 4E-BP1) leads to upregulation of VIP and higher amplitude of molecular rhythms in the SCN. Consequently, the 4E-BP1 null mice exhibit accelerated re-entrainment to a shifted light/dark cycle and are more resistant to the rhythm-disruptive effects of constant light. Conversely, in Mtor+/− mice VIP expression is decreased and susceptibility to the effects of constant light is increased. These results reveal a novel role for mTOR/4E-BP1-mediated translational control in regulating entrainment and synchrony of the master clock.

In mammals, a light-entrainable clock located in the suprachiasmatic nucleus (SCN) regulates circadian rhythms by synchronizing oscillators throughout the brain and body. Notably, the nature of the relation between the SCN clock and subordinate oscillators in the rest of the brain is not well defined. We performed a high temporal resolution analysis of the expression of the circadian clock protein PERIOD2 (PER2) in the rat forebrain to characterize the distribution, amplitude and phase of PER2 rhythms across different regions. Eighty-four LEW/Crl male rats were entrained to a 12-h: 12-h light/dark cycle, and subsequently perfused every 30 min across the 24-h day for a total of 48 time-points. PER2 expression was assessed with immunohistochemistry and analyzed using automated cell counts. We report the presence of PER2 expression in 20 forebrain areas important for a wide range of motivated and appetitive behaviors including the SCN, bed nucleus, and several regions of the amygdala, hippocampus, striatum, and cortex. Eighteen areas displayed significant PER2 rhythms, which peaked at different times of day. Our data demonstrate a previously uncharacterized regional distribution of rhythms of a clock protein expression in the brain that provides a sound basis for future studies of circadian clock function in animal models of disease.