CIRCADIAN BEHAVIOR AND PLASTICITY OF LIGHT-INDUCED C-FOS EXPRESSION IN SCN OF TAU MUTANT HAMSTERS

In hamsters homozygous for the circadian clock mutation tau, the photi c history dramatically affects the magnitude of Light-induced circadia n phase shifts. The maximum amplitude of phase shifts produced by l-h light pulses presented at CT 14 was less than 2 h in animals that had been in DD for 2 days, whereas animals that had been kept in DD for 49 days could be shifted by more than 8 h. In this study the authors com pared the effect of previous light history on the amplitude of circadi an phase shifts and on c-fos expression in the SCN of tau mutant hamst ers. Although the maximum amplitude of behavioral phase shifts was dra stically different between animals that had been held for either 2 or 49 days in DD, maximal fos induction was not significantly different i n these two groups. However, photic thresholds for light-induced behav ioral phase shifts, c-fos mRNA, and Fos immunoreactivity were closely correlated within both groups, and these thresholds were lower (more s ensitive to Light) after 49 than after 2 days in DD. The correlation b etween phase shifting and Fos induction thresholds, under conditions w here both responses are dramatically altered by the previous light his tory, demonstrates an association between changes in circadian behavio ral phase-shifting responses of tan mutant hamsters and plasticity of Light-induced c-fos expression in SCN. However, because the maximum am plitudes of Fos induction and phase shifting were not correlated in an imals that had been in DD for 2 days, we speculate that the level of c -fos expression does not directly determine phase shift amplitude.