Masking by light in hamsters with SCN lesions

Inhibition of wheel running by light (masking) was investigated in Syrian h amsters with suprachiasmatic nucleus or sham lesions. Approximately 90% of the wheel revolutions made by hamsters with complete suprachiasmatic nucleu s lesions, as judged by histology and power spectrum analysis of their whee l running, occurred during the dark phases of an ultradian light-dark cycle (3.5 h light, 3.5 h dark). This was demonstrated for two illumination leve ls (380 1x and 6 1x). Similar results were obtained with sham-operated anim als. In further tests, the hamsters with lesions also retained a strong pre ference for the dark side of a box divided into dark and light sides. These results demonstrate that the suprachiasmatic nucleus is not necessary for masking by light or the preference for a dark over a light compartment. Evi dently the direct effects of light can substitute for the endogenous contro l by the suprachiasmatic nucleus to maintain appropriate behaviour in time and space.