CYCLES OF DIM RED-LIGHT CAPABLE OF ENTRAINING CIRCADIAN-RHYTHMS OF RATS AFTER LONG-TERM EXPOSURE TO CONSTANT WHITE-LIGHT

In circadian studies dim red light is often used as a convenient alternative for darkness because it would not affect the pacemaking system. In this study male Wistar rats were first exposed to constant light (LL) for 3 months and next to LD 14:10 cycles of dim red light (> 610 nm; 1 - 5-mu-Watt/cm2) while recording feeding behaviour. All rats displayed entrainment of the circadian feeding rhythm to the red light cycle. A phase reversal of the LD cycle was followed by a gradual reentrainment over 5 - 8 weeks via delaying transients. It was concluded that dim red light acts as a weak zeitgeber and that cone-like photoreceptor surviving long-term LL exposure could mediate entrainment.