EVIDENCE THAT THE CIRCADIAN SYSTEM MEDIATES PHOTOPERIODIC NONRESPONSIVENESS IN SIBERIAN HAMSTERS - THE EFFECT OF RUNNING WHEEL ACCESS ON PHOTOPERIODIC RESPONSIVENESS

Juvenile male Siberian hamsters from a line of hamsters selected for n onresponsiveness to short photoperiod (PNRj) and animals from the gene ral colony (UNS) were separated at weaning into two groups. Group 1 ma les were moved into short days (10 h light:14 h dark [10L:14D]) with f ree access to running wheels (RW). Group 2 animals were the male sibli ngs of Group 1 hamsters; they were moved at the same time into the sam e room, but were housed in cages without access to RW. Group 2 hamster s only had access to RW for the final week of short-day exposure (Week 8). Animals were blood sampled at the time of sacrifice for analysis of serum prolactin (PRL) and follicle-stimulating hormone (FSH) concen trations. At sacrifice, paired testis weights were obtained and pelage color was scored. Animals from the UNS line showed the expected decli nes in testis weight, body weight, and serum concentrations of both PR L and FSH, regardless of the presence or absence of RW. These animals also exhibited a high proportion of individuals molting to winter-type pelage. By contrast, a marked difference was noted between siblings f rom the PNRj line depending on whether RW access was provided at the t ime of weaning. Animals with access to RW exhibited identical response s to those of the UNS responder animals, whereas PNRj animals without access to RW showed no adjustments to short days (i.e., testis regress ion, pelage molt, expansion of alpha). In a second experiment, PNRj an d UNS males were placed in constant darkness (DD), with or without RW access. The results of this experiment indicated that PNRj animals res pond to DD regardless of the presence or absence of RW. In DD, PNRj ha msters also exhibited significantly longer free-running period lengths (taus) than did UNS hamsters; all the PNRj hamsters had taus > 24 h, whereas none of the UNS hamsters had a tau > 24 h. These results indic ate that PNRj hamsters retain the proper neural pathways for respondin g to short day lengths and establish a role for locomotor activity fee dback in modulating the circadian system and, subsequently, photoperio dic responsiveness in PNRj hamsters.