Age-dependent modulation of circadian parameters in the field mouse Mus booduga

The locomotor activity of the field mouse Mus booduga was monitored under continuous darkness (DD) for extended periods of 275 to 388 days. Free-running period (tau), activity time (alpha) and rest time (rho) were computed from the activity records for all the animals (n = 10). Decreases in the circadian period (tau) with age were observed in some of the animals at an early age (< ca. 100 days) and then tau remained relatively constant for rest of their life span. The activity time (alpha), rest time (rho), and their ratio (alpha/rho), however, did not show any consistent trend of increase/decrease. The initial decrease in tau is consistent with previous models suggesting a time-dependent increase in the number of cells generating oscillations in the suprachiasmatic nucleus (SCN) and/or increase in the strength of coupling between these cells. In such models, after an optimal number of cells launch themselves into a state of self-sustained oscillation, tau stabilizes, achieving relative constancy, a pattern similar to the one observed in our study.