Studies in montane voles (Microtus montanus) first revealed that the photope-riod in effect during gestation can influence the photoperiodic responses of the developing animals (26). Similar studies were performed in Siberian hamsters, revealing that in this species as well the dam is able to transmit a photoperiodic message to her fetuses. Thus, when male Siberian hamsters were raised from birth in 14 L they exhibited larger testes at 1 month of age if their mothers had been exposed to a shorter day length during gestation than if the mother had experienced only photoperiods >14 L during pregnancy (36,39). This mechanism may enable juvenile hamsters to rapidly assess the significance of a moderately long photoperiod (e.g., 14L) that occurs in both early and late summer. Thus, if photoperiod has increased between the time of gestation and postpartum exposure to 14L, this would be indicative of the early part of the breeding season. Under these conditions, hamsters would rapidly achieve puberty and would reproduce during the latter part of the summer season. By contrast, hamsters born in late summer would experience a decrease in day length between gestation and post-partum life; this might serve as an accurate cue that would lead to withholding reproduction until the following spring.
The ability of the dam to influence the photoperiod responses of her offspring is dependent on some action of her pineal gland; pups born to pinealectomized dams failed to exhibit differential responses to a postpartum 14 L photoperiod in relation to the day length in effect during gestation. Further, when pinealectomized dams were administered daily MEL infusions during late pregnancy, the photoperiodic responses of their male pups were related to the duration of the MEL infusions: Mothers receiving long duration MEL infusions (to simulate a short day gestational photoperiod) gave birth to pups that showed relatively rapid testis growth in 14 L; testis growth was slower in males born to mothers that had received long duration MEL infusions (simulating a long day gestational photoperiod). These results may indicate that the fetuses are able to use the mother's MEL signal as an indicator of the gestational photoperiod (48). This hypothesis is supported by the appearance of MEL binding sites in the brain and pars tuberalis of late fetal hamsters (8).
Further studies were performed to explore how the photoperiodic mechanism of the pups is modified by the photoperiod experience of the dam during gestation. The results indicated that the responsiveness of juvenile male hamsters to daily MEL infusions, administered at 14-32 days after birth, was not altered by the gestational photoperiod (37). However, the PMR of the male pups was influenced by gestational photoperiod. When raised from birth in 14 L and examined 18 days postpartum, males whose mothers had been exposed to shorter day lengths produced longer duration MEL peaks as compared to males whose mothers were exposed to long days during pregnancy. This effect of gestation photoperiod on the PMR of the juvenile males could explain the differences in rate of postnatal testis development between pups gestated in long and short days, respectively. It seems likely that the maternal influence could be exerted via an action of the dam's PMR on the circadian system of the fetus—perhaps involving MEL receptors in the fetal SCN (8). Interestingly, no comparable effect of gestation photoperiod was observed for the PMR of female hamster pups (38).
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