Abstract
The illuminance threshold for maintenance of testicular function was found to be considerably higher in Syrian hamsters kept in continuous light (LL) than in hamsters on long-day (14-hr) photoperiods (LD 14:10), or in a similar-length skeleton photoperiod (LDSK); the threshold lay between 3 and 30 lux in LL and at approximately 0.3 lux in LD 14:10 or LDSK. The threshold for testicular maintenance in LL was related to the capacity of LL to suppress nocturnal melatonin secretion: 400 lux totally suppressed, 30 or 3 lux partially suppressed, and 0.3 lux failed to suppress melatonin secretion. Hamsters in the LD and LDSK groups, whose locomotion was entrained into a pattern characteristic of long-day exposure, maintained full testicular function; those whose locomotion free-ran or assumed a pattern of entrainment characteristic of short-day exposure underwent testicular regression. These results suggest that light signals entrain the circadian rhythms of locomotion and melatonin secretion in a similar manner, and that LL is less effective than LD or LDSK in shortening the duration of melatonin secretion. For hamsters in LL, a direct relationship was seen between the free-running period (τ) of locomotion and log 10 illuminance at 0.3, 3.0, and 30 lux, but τ at 400 lux was no longer than τ at 30 lux. Splitting of locomotion did not occur at 0.3 or 3.0 lux, and occurred in 43% and 62% of hamsters in 30 and 400 lux, respectively.