EFFECTS OF LIGHT-INTENSITY ON THE ACTIVITY RHYTHM OF YOUNG-RATS

We studied the effect of different constant light intensities (0.25 lu x of dim red light and 4 lux, 40 lux and 500 lux of white light) on th e motor activity rhythm of rats. From the day of weaning, rats were ke pt in individual cages and motor activity was recorded every 15 minute s for 50 days, by a system with two crossed infrared beams. Data were analyzed by means of the periodogram of Sokolove and Bushell and by Fo urier analysis. The mean waveform of the rhythm of the adult animals w as also calculated and quantified by defining 3 mathematical indices t hat describe several aspects of the waveform, as noise, stability and abruptness. Under dim red light, the animals show from the beginning o f the record a circadian component. In the groups kept under white lig ht, a large number of ultradian components appear during the first day s after weaning, and a well defined circadian component appears at abo ut day 10. The waveform that the animals acquire in adulthood depends on the light intensity. The effect of light is manifested by a smoothi ng of the waveform of the circadian rhythm and by a change from bimoda l to unimodal. This effect is gradual, the higher the intensity the st ronger the effect. These results are consistent with the hypothesis of a multioscillatory circadian system. in which the coupling between th e different oscillators would depend on the light intensity.