Effects of prolonged caging with fertile males on reproductive functions in aging female rats

Abstract

Previous studies showed that retired breeder female rats ceased to exhibit regular estrous cycles beginning around 10-12 months of age (Lu et al., 1979). Since the onset of reproductive dysfunction occurs in multiparous females soon after being separated from fertile males, the present study examined whether prolonged caging with males could maintain normal reproductive functions in aging female rats. Eight-month-old multiparous female rats (Long-Evans) were housed in groups of 5, either with (mated: n-47) or without (nonmated: n=49) a young fertile male. At 13 months of age, the mated females were separated from the males. Between 8 and 13 months of age, 45 of 47 mated females became pregnant and delivered pups. With advancing age, the number of females which delivered pups decreased markedly, but the average litter size did not change. At 13 months of age, 49% of the mated females exhibited regular estrous cycles, whereas only 20% of the non-mated females were regularly cyclic. At 15 and 17 months of age, the mated group still showed a significantly higher percentage of regularly cyclic animals than the nonmated groups. Both groups of animals showed a progression from regular cyclicity to irregular cyclicity to persistent estrous or repetitive pseudopregnancy with advancing age. The profile of change in reproductive patterns in the mated group during 13-17 months of age was remarkably similar to that which occurred during 9-13 months of age in the nonmated controls. The patterns of periovulatory luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion were similar in 4-month-old regular cyclic rats and 15-month-old mated females displaying regular estrous cycles. These results demonstrate that regular estrous cyclicity and normal gonadotropin secretion can be maintained for an extended period of time in aging female rats by caging with fertile males, and suggest that the cessation of regular cyclicity beginning around 10-12 months of age in multiparous females may be due in part to separation from the males.