The central and basolateral nuclei of the amygdala exhibit opposite diurnal rhythms of expression of the clock protein Period2

Abstract

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.

Fig. 1.

Expression profiles of PER2 immunoreactivity in the CEA, BLA, and CA3 regions of the DG of rats killed at ZT1 or ZT13. (Scale bars: 200 μm.) CP, caudoputamen; LA, lateral nucleus of the amygdala; SI, substantia innominata; CEAl, central nucleus of the amygdala, lateral part; CEAm, central nucleus of the amygdala, medial part; BMAa, basomedial nucleus of the amygdala, anterior part; sm, molecular cell layer; sg, granular cell layer; h, hilus; sp, pyramidal cell layer.

Fig. 2.

Mean (± SEM) number of PER2-immunoreactive nuclei in the CEA, BLA, and CA3 regions of the DG of rats killed at different ZTs (n = 4 per time point). Cells immunopositive for PER2 were counted manually by using a 400 × 400-μm template for the CEA and BLA and a 200 × 400-μm template for the DG. The number of PER2 immunoreactive cells per region was calculated for each animal from the counts of 10 images showing the highest number of labeled nuclei.

Fig. 3.

Mean (± SEM) number of PER2-immunoreactive nuclei in the CEA, BLA, and CA3 regions of the DG of SCN-lesioned rhythmic and SCN-lesioned arrhythmic rats as a function of ZT (n = 6–8 per group).

Fig. 4.

Mean (± SEM) number of PER2-immunoreactive nuclei in the CEA, BLA, and CA3 regions of the DG of adrenalectomized and sham operated rats as a function of ZT (n = 5–7 per group per time point).