Circadian rhythms in firing rate of individual suprachiasmatic nucleus neurons from adult and middle-aged mice

The suprachiasmatic nucleus contains a biological clock that drives circadian rhythms in vivo and in vitro. It has been suggested that the suprachiasmatic nucleus is a primary target of the aging process, because age-related changes in behavioral rhythms are mirrored in alterations in circadian pacemaker function. Using long-term, single-cell recording, we assessed the effect of age on firing-rate patterns of individual suprachiasmatic nucleus neurons of young adult (2–4 months) and middle-aged (9–11 months) C3H mice. Individual suprachiasmatic nucleus neurons from adult mice maintained in culture for at least one week exhibited robust circadian rhythms in spontaneous activity that were similar in the free-running period (23.7±0.3 h mean±S.E.M.) to recordings from neurons dispersed from neonatal tissue, and showed evidence of entrainment to prior light cycles by exhibiting peak activity, in vitro, approximately 4.0±0.3 h (mean±S.E.M.) after the time of expected light onset. Aging led to a decreased amplitude of impulse activity in dispersed suprachiasmatic nucleus neurons and increased variability in the circadian waveform. From these results we suggest that age-related deterioration in circadian clock function occurs at the level of individual cells, which may account for some of the age-related deficits observed in the expression of behavioral rhythmicity.