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AASERT95 Control of Circadian Behavior by Light and Transplanted Supercharimatic Nuclei

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Final rept. 1 Jun 1995-31 May 1998

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A circadian clock is of no use unless biological time is adjusted to local environmental time, and most organisms use the changes in the quantity and quality of light at twilight as their primary zeitgeber to effect photoentrainment. The sensory demands of photoentrainment have imposed a unique set of selection pressures, which has led to the evolution of specialized photoreceptor systems. In the non-mammalian vertebrates, pineal and deep brain photoreceptors play an important, but poorly defined, role in circadian organization. By contrast, photoentrainment in the mammals relies exclusively upon ocular photoreceptors. Although superficially very different, the entraining photoreceptor inputs of mammals and non-mammals appear to both specialized employing novel photopigments, and complex utilizing multiple photopigments. Why there should be this multiplicity of photic inputs to the circadian system remains unclear, but must surely be related to the sensory task of twilight detection. During twilight, the quality of light changes in three important respects 1 the amount of light 2 the spectral composition of light 3 and the position of the sum. In theory all of these parameters could be used by the circadian system to detect the phase of twilight, but each would be subject to considerable variation or noise Furthermore, the impact of this noise will depend upon the organism and the environment in which it inhabits. Thus the task of twilight detection is likely to be very complex and show considerable variation between species. If we are to place the molecular dissections of the circadian system into a functional context, then the ecology of photoentrainment must be given serious consideration.

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  • Anatomy and Physiology

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