Synchronizing Effect of Photoperiodicity on Ovulation in Hamsters1

Publisher Website
Google Scholar
Abstract
Hamsters (Misocricetus auratus) exposed daily since birth to light from 6:00 AM to 6:00 PM (6-6 lighting) ovulated within a 2-hr, period (1:00-3:00 AM) up to 4 months of age and within a 4-hr, period (11:00 PM-3:00 AM) at 6 months of age. Six-month-old hamsters exposed daily since birth to light from midnight to noon (12-12 lighting) ovulated 6 hr. earlier (5:00-9:00 PM) than hamsters reared in 6-6 lighting. Six-month-old hamsters representing the 3rd generation reared in constant light (i.e., 24 hr. daily) ovulated at various times of day. This asynchronous ovulation was accompanied by a reduction in 4-day vaginal cytological regularity and by an increase in body weight. Six-month-old hamsters reared in 6-6 lighting except for exposure to constant light from the 3rd to 44th day of age ovulated at the same time (11:00 PM-3:00 AM) as control hamsters reared only in 6-6 lighting. The time of ovulation advanced 6 hr. after post-pubertal hamsters were transferred from 6-6 to 12-12 lighting. The 6-hr. advance was not completed by the 15th day after transfer but was completed within 2 months after transfer. When hamsters were transferred on the day of ovulation from 6-6 lighting to constant light, the next ovulation, which occurred 4 days after transfer, was delayed about 6 hr. Longer exposure to constant light (48-55 days) resulted in asynchronous ovulation, a slight reduction in 4-day vaginal regularity, and an increase in body weight. Data obtained with phenylisopropylhydrazine indicated that release of pituitary ovulating hormone occurred 6 hr. earlier in hamsters reared in 12-12 lighting than in hamsters reared in 6-6 lighting. Present findings suggest that internal factors maintain the basic ovulatory mechanism, including in large measure its 4-day cyclic nature, and that environmental lighting, by controlling the time of release of pituitary ovulating hormone, sets the time of ovulation. Precise timing is maintained from one ovulation to the next by the intervening 24-hr, light-dark cycles, rather than by lighting experienced during development of puberty. Selective advantage to the species might result from synchronization of the estrous period, which precedes ovulation, with night, thereby restricting mating in these nocturnal animals to the safer hours of darkness.