Reproductive senescence in female rats: a longitudinal study of individual differences in estrous cycles and behavior.

The longitudinal pattern of reproductive senescence was described in individual female rats from 4 to 18-22 mo of age. There were two subgroups of rats with different patterns of aging. The Constant Estrus (CE) subgroup progressed through regular cycles, irregular cycles, and constant estrus, followed by a return to irregular cycles, and then persistent diestrus. In contrast, the Irregular subgroup skipped constant estrus, maintaining irregular cycles until they entered persistent diestrus. In both subgroups, irregular cycles were a transition between the major reproductive states, although the type of transition was different in each subgroup. In the CE subgroup, the transition was gradual, continuous, and began with the onset of irregular cycles. In contrast, in the Irregular subgroup, the transition did not begin until the end of irregular cycles, suggesting that the process of aging was delayed. Most rats entered constant lordosis, a state characterized by a strong lordosis reflex that could be elicited by manual palpation on each day. The CE subgroup maintained the state once they entered it, whereas the Irregular subgroup intermittently returned to a lordosis reflex intensity characteristic of young rats. In addition, in the CE subgroup, but not the Irregular subgroup, changes in lordosis reflex intensity during aging were coupled to changes in the proportion of estrogenized vaginal smears during the cycle.