Photoperiodic Properties of Circadian Rhythm in Rat.

Abstract

Animals have intrinsic circadian periods of around 24 hours. In order to synchronize to the 24-hour day, circadian rhythms entrain to photic time cues and establish a consistent daily rhythm. The maintenance of a stable phase relationship between circadian rhythms and the light-dark (LD) cycle is necessary for entrainment to occur. It is commonly thought that there is a fixed relationship between the phase angle of entrainment and the intrinsic free-running period, where a smaller phase angle reflects a shorter period. Furthermore, it was thought that individuals with smaller phase angles would adjust faster to advance shift of the LD cycle, and slower to delay shift of the LD cycle. This dissertation reevaluated these relationships in rats using long-term in vivo pineal microdialysis to carefully monitor melatonin secretion rhythm. The studies presented in this dissertation examined both the interindividual differences between outbred rats, as well as differences in melatonin rhythms of inbred rats entrained to different photoperiods. Under long photoperiod (short night), melatonin rhythm has smaller phase angles and shorter durations; the opposite is true for short photoperiods. The phase angle variation, natural as well as manipulated, did not correlate with the free-running period variation in rats. Furthermore, individuals with smaller phase angles reentrained faster in both the advance and delay direction. To gain a greater understanding of the role of photoperiod in melatonin phase angle, the adjustment of the melatonin rhythm to an expansion or compression of dark period was studied by altering the dark period symmetrically or unidirectionally. While reentrainment to an 8h night resulted in very similar melatonin profiles regardless of how the dark period was altered, reentrainment to a 16h night depend significantly on whether the dark period was expanded symmetrically or by morning or evening expansion. These findings demonstrate that the relationship between period, phase, and pattern of reentrainment are quite different than is currently believed, and new analysis may be necessary to understand the underlying biology of the circadian timing system.