Wavelength dependence of light-induced phase shifts and period changes in hamsters

The wavelength dependence of three different circadian responses to light was compared in Syrian hamsters: phase advances and phase delays induced by 15-min light pulses at CT18 (circadian time 18) and CT13, respectively, and changes in free-running period under different levels of constant illumination (LL) intensity. Fluorescent lamps with transmission maxima at 436, 514, and 658 nm were used for delivering blue, green, and red light, respectively. White fluorescent light was also used in the LL portion of the study. The magnitude of each circadian response was plotted as a function of light intensity expressed in photons cm-2 s-1, and intensity-response relationships for the different wavelengths were quantified by fitting the data with a 4-parameter sigmoidal function using a least-squares curve-fitting routine. For each response type, the curves for the different wavelengths were identical but displaced along the horizontal axis [i.e., the functions differed only in the intensity (sigma) required for producing a half-maximal response]. The values of sigma for green and blue light were similar, while that for red light was several times greater. These results indicate that a single class of photoreceptors with peak sensitivity in the blue-green region mediates light-induced phase advances, phase delays, and changes in the period of the hamster circadian pacemaker.