Perinatal influences of melatonin on testicular development and photoperiodic memory in Siberian hamsters.

We assessed the influence of perinatal melatonin on reproductive development and adult responsiveness to melatonin. Testicular growth in an intermediate day length (14 : 10 h light/dark cycle) was substantially reduced in Siberian hamsters gestated by pinealectomised compared to pineal-intact females; gonadal development was normalised in offspring of pinealectomised dams that were pinealectomised at 3-4 days of age. Hamsters deprived of melatonin only during gestation, or both pre- and postnatally, underwent testicular involution during treatment with melatonin in adulthood. Photoperiodic histories acquired prenatally did not endure as long as those acquired by adult hamsters. Hamsters first exposed to melatonin in adulthood were not more proficient in acquiring photoperiodic histories than were normal males. These findings indicate that pre- versus postnatal differences in melatonin signal duration determine rates of testicular development. Exposure to melatonin perinatally does not appear to organise the neuroendocrine substrate that mediates effects of day length and melatonin on the gonads of adult hamsters.

Long-term reproductive effects of a single long day in the Siberian hamster (Phodopus sungorus).

Testicular regression was prevented or attenuated in Siberian hamsters exposed to a single 1- to 4-h extension of the 16-h photophase at 18 days of age and subsequently maintained in a short photoperiod (8L:16D) through Day 35. Testicular weights on Day 35 were not correlated with the duration of the active phase of wheel running or with the time of activity onset after transfer to the 8L:16D photoperiod. Wheel-running activity was not stably entrained to the light-dark cycle by 35 days of age. Progonadal effects of a single 33-h light pulse were greatest at 18 days of age, still evident at 30 days, but undetectable in older hamsters. In female hamsters, a single longer day at weaning was associated with increased fecundity several weeks later. Long photoperiods accelerated development of antral ovarian follicles, but exposure to males was necessary to induce ovulation before 60 days of age. The interval beginning shortly after weaning is one of heightened responsiveness to changes in day length (DL); exposure to increasing DL at this time may prolong the breeding season when DL decreases after the summer solstice. We suggest that the long-term effects of acute light treatments on reproduction are mediated by sustained changes in melatonin secretion induced by reprogramming of circadian oscillators.

Sustained reproductive responses in Djungarian hamsters (Phodopus sungorus) exposed to a single long day.

Male and female Djungarian hamsters maintained from birth in a short photoperiod (8 h light per day; 8L:16D) showed substantial testicular and uterine growth in response to a single long photoperiod or a 15-min light pulse that interrupted the 16-h dark period at 18 days of age. These light regimens resulted in heavier testes and uteri at 30 and 35 days of age when compared with those of control animals. Similar results were obtained in hamsters maintained from birth to Day 18 in a long photoperiod (16L:8D), given a single longer day (20L:4D) or constant light on Day 18 and then transferred to a short photoperiod (8L:16D) on Day 19. At 35 days of age animals that received extended light treatment on Day 18 had significantly more developed reproductive structures than did control hamsters. The marked effects of brief light treatment in producing long-term changes in the reproductive axis provide a convenient mammalian model system in which to study neuroendocrine events that underlie photoperiodism.

Maternal melatonin treatment influences rates of neonatal development of meadow vole pups.

Meadow vole dams, housed in a 14L:10D photoperiod were injected daily 3 h before onset of darkness with 10 micrograms melatonin. Treatment during gestation or lactation produced offspring that exhibited altered somatic, testicular, and pelage growth. Gestational melatonin treatment decreased preweaning weight gain, delayed testicular development, and increased pelage growth in offspring, whereas melatonin treatment during lactation increased pelage depth at weaning and increased post-weaning somatic growth. These results suggest that pre- and postnatal maternal melatonin secretory patterns influence postnatal development of photosensitive traits in offspring.