Light-dark-shift stress, with special reference to spontaneous tumor incidence in female BN rats.

As a way to induce mild chronic stress, light-dark (L-D)-shift stress was applied to inbred BN virgin female rats during their whole life-span (group I, 100 animals); the incidences of spontaneous tumor and nontumor processes were recorded. A group of rats (group II, 100 animals) exposed to a standard lighting system served as the control group. Total tumors of 128 in group I and of 154 in group II were found in 74 and 86 animals, respectively. Neither were these differences nor was the pattern of spontaneous tumors statistically significant. Although in earlier studies L-D-shift stress had proved to be effective, especially with regard to its capacity to induce a substantial decrease in cellular immune response, apparently such alterations did not unfavorably affect longevity of BN female rats. Although as a side issue of this study, a strong predisposition for tumor incidence appeared to exist, in particular for the incidence of Langerhans' islet tumors, in fat animals at weaning.