AGE-RELATED-CHANGES IN THE PHOTOPERIODIC RESPONSE OF SIBERIAN HAMSTERS

Previous studies indicate that as Siberian hamsters (Phodopus sungorus ) age, they may lose their ability to show gonadal regression in respo nse to short days. In one study, hamsters that regressed on short days early in life failed to regress when exposed to short days a second t ime later in life. Thus, Siberian hamsters may experience age-related deficits in photoresponsiveness or may be incapable of regressing twic e. In the present study, we attempted to discriminate between these po ssibilities by examining patterns of gonadal regression in hamsters tr ansferred back and forth from long (16L:8D) to short days (6L:18D) eve ry 6.5, 13, or 26 wk for a 2-yr period. A control group was maintained on long days and had enlarged gonads throughout the entire study. Ham sters alternating between 26 wk of long and short days exhibited compl ete gonadal regression during their initial but not during their secon d exposure to short days. Hamsters alternated between long and short d ays every 13 or 6.5 wk showed regression two to three times, respectiv ely. After about 52 wk of age, the majority of animals in both groups did not regress when exposed to short days. Taken together, the result s of this experiment indicate that male Siberian hamsters 1) can exhib it at least two rounds of short-day-induced gonadal regression and 2) fail to regress on short days after about 1 yr of age.