Light-induced phase shifts of circadian activity rhythms and immediate early gene expression in the suprachiasmatic nucleus are attenuated in old C3H/HeN mice

Alterations in the mechanisms of entrainment and/or response of the circadian pacemaker to zeitgebers may contribute to age related changes in sleep/wake rhythms. This study examined the effect of age on light-induced phase shifts of circadian activity rhythms and on the expression of the immediate early genes c-fos and jun-B in the suprachiasmatic nucleus (SCN) of young and old C3H/HeN mice. Mice (4 months or 16 months at the beginning of the experiment) were housed in constant darkness with circadian rhythms assessed by running wheel activity. Mice were exposed to light pulses of 30, 100, 300 or 1000 lux and steady state phase shifts of circadian activity rhythms determined. In young mice exposed to light at circadian time (CT) 14, light pulses of 30, 100, 300 or 1000 lux induced phase delays of circadian activity rhythms of similar magnitude (averaging 2.8 h). Phase delays following photic stimulation were reduced in the old mice at all light levels (averaging 1.1 h, P < 0.001). Following behavioral testing, mice were exposed to light (1000 lux) at CT 14 for determination of the light-induced expression of c-fos and jun-B mRNA in the SCN by in situ hybridization histochemistry. Immediate early gene expression following light exposure was reduced by 42% (c-fos) and 48% (jun-B) in the SCN of old mice compared to young controls (P < 0.001). Together, these results suggest an age related reduction in responsiveness to light by the circadian pacemaker.