Effect of light during lactation on the phasic and tonic responses of the rat pacemaker

The circadian system in mammals generates endogenous circadian rhythms and entrains them to external cycles. Here, we examine whether the lighting conditions under which rats are reared affect the properties of the circadian pacemaker. We maintained three groups of rats under constant darkness (DD-rats), constant bright light (LL-rats) or light-dark cycles of 24 hours (LD-rats) during lactation. We then studied motor activity rhythm under constant light of four intensities, and under seven light-dark cycles with periods ranging between 22 and 27 hours. Results show that neither the tau nor the phase angle to the external cycle differed between groups. Differences were found in the amplitude of the circadian rhythm and in the number of rats that became arrhythmic under LL. We conclude that the light received during lactation affects the strength of the circadian pacemaker and its sensitivity to light.