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 photic history dramatically affects the magnitude of Light-induced circadian 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 compared the effect of previous light history on the amplitude of circadian phase shifts and on c-fos expression in the SCN of tau mutant hamsters. Although the maximum amplitude of behavioral phase shifts was drastically different between animals that had been held for either 2 or 49 days in DD, maximal fos induction was not significantly different in these two groups. However, photic thresholds for light-induced behavioral phase shifts, c-fos mRNA, and Fos immunoreactivity were closely correlated within both groups, and these thresholds were lower (more sensitive to Light) after 49 than after 2 days in DD. The correlation between phase shifting and Fos induction thresholds, under conditions where both responses are dramatically altered by the previous light history, demonstrates an association between changes in circadian behavioral phase-shifting responses of tan mutant hamsters and plasticity of Light-induced c-fos expression in SCN. However, because the maximum amplitudes of Fos induction and phase shifting were not correlated in animals that had been in DD for 2 days, we speculate that the level of c-fos expression does not directly determine phase shift amplitude.