Dampened circadian rhythms in reproductively senescent female rats

Abstract

Circadian rhythms in locomotor activity and drinking behavior were monitored in reproductively senescent female rats (Long-Evans, hooded) in both entrained (light:dark, 14:10) and free-running (constant dim illumination) conditions and compared to rhythms in young (3- to 4-month-old) rats of the same strain displaying regular estrous cycles. Aged rats were divided into two groups on the basis of age and pattern of daily vaginal smears; old constant estrous (16–18 months) and old repetitive pseudopregnant (24–26 months). The results indicate that circadian rhythmicity in both behaviors declines with increasing age. In diurnal lighting, young cycling, old constant estrous, and old repetitive pseudopregnant rats restrict 72.7 ± 2.4%, 67.1 ± 1.7%, and 58.9 ± 2.2%, respectively, of their drinking behavior to the 10 hr of darkness. This percentage decrease in aged rats is primarily the result of fewer drinking events during the dark phase. Activity and drinking records of young rats in constant dim light reveal marked free-running rhythms which contrast to the more random distribution of activity and drinking events in the aged groups. Power spectral analyses of drinking events in both entrained and free-running conditions confirm a progressive flattening of the spectrum and a diminished circadian peak with advanced age. Circadian rhythms were also monitored in young rats (3–4 months) which had been androgen-sterilized (100 μg testosterone propionate) on the day after birth since they exhibit a permanent constant estrous state similar to the old constant estrous rat and have been viewed as a model of early reproductive senescence. In both entrained and free-running conditions, their activity and drinking rhythms were comparable to rhythms displayed by young cycling rats.