Properties of parametric photic entrainment of circadian rhythms in the rat.

Wistar rats were exposed to 24-h cycles of sinusoidal varying light intensity at various amplitudes (A) and mean intensity (I) levels. The cycle minimum was at least 0.1 lux. Food approaches and wheel running activity were continuously recorded. Entrainment was observed to light cycles with amplitudes as low as 2 lux. The success of entrainment increased with A and decreased with I, and loss of entrainment was accompanied by relative coordination. In a separate experiment, we measured the freerunning period (tau) in constant light at various I levels. The tau-illuminance curve was pseudolinear at low I (above 0.1 lux) and saturated at high I (above 100 lux). The intensity dependence of tau corresponded to the intensity response characteristics of light responsive cells in the rat suprachiasmatic nuclei, as published by other authors. It was hypothesized that the light responsive cells mediate the parametric effects of constant light and of sine waves of light.