Age-related changes in circadian responses to dark pulses

Aging involves many alterations in circadian rhythms, including a loss of s ensitivity to both photic and nonphotic time signals. This study investigat ed the sensitivity of young and old hamsters to the phase advancing effect of a 6-h dark pulse on the locomotor activity rhythm. Each hamster was test ed four times during a period of similar to 9 mo; periods of exposure to a 14-h photoperiod were alternated with the periods of exposure to constant l ight (20-80 lx), during which the dark pulses were administered. There was no significant difference in the phase shifts exhibited by the young (4-10 mo) and old hamsters (19-25 mo) or in the amount of wheel running activity displayed during each dark pulse. However, young hamsters had a significant ly greater propensity to exhibit split rhythms immediately after the dark p ulses. These results suggest that, although aging does not reduce the sensi tivity of the circadian pacemaker to this nonphotic signal, it alters one p roperty of the pacemaker, i.e., the flexibility of the coupling of its comp onent oscillators.