Abstract

The circadian clock in the hypothalamic suprachiasmatic nucleus (SCN) can be entrained by light cycles longer than the normal 24-h light cycle, but little is known about the effect of such exotic light cycles on circadian clocks outside the SCN. The present thesis sought to examine the effect of exposure to a 26-h light-dark (LD) T-cycle (T26; 1-h-25-h LD) on the expression of the clock protein PERIOD2 (PER2) in the SCN and four regions of the limbic forebrain known to exhibit circadian oscillations in PER2 expression, the oval nucleus of the bed nucleus of the stria terminalis (BNSTov), central nucleus of the amygdala (CEA), basolateral amygdala (BLA), and dentate gyrus (DG). Control rats housed under a 24h T-cycle (T24; 1-h-23-h LD) and rats housed under the T26 cycle showed stable entrainment of wheel-running activity rhythms. As previously shown, PER2 expression in the SCN was also stably entrained in all T24 and T26-housed rats, peaking at the beginning of the active phase of the cycle. In contrast, exposure to the T26 LD cycle for 30 and 60 days, uncoupled the rhythm of PER2 expression in the BNSTov and CEA from the SCN, whereas PER2 rhythms in the BLA and DG were mostly unaffected. These results show that exposure to exotic light cycles can uncouple circadian oscillators in the limbic forebrain from the master SCN clock and suggest that such cycles may be used to study the functional consequences of disruptions in the phase relationship between the SCN and subordinate oscillators.