Identifying the photoreceptive inputs to the mammalian circadian system using transgenic and retinally degenerate mice

The endogenous circadian clock of mammals retains synchrony with the extern al light:dark cycle through ocular photoreceptors. To date the identity of the photoreceptors responsible for mediating this response is unknown, This review outlines attempts using transgenic mouse models to address this def icit. Mice bearing specific inherited lesions of both rod and cone photorec eptors retain circadian photosensitivity as assessed by photoentrainment of behavioural rhythms and the light-induced suppression of pineal melatonin. These findings indicate that as yet Unidentified non-rod, non-cone ocular photoreceptors are capable of contributing to circadian light responses. Ne vertheless, the possibility that circadian photosensitivity is the responsi bility of multiple photoreceptor classes including both rod/cone and novel photopigments remains. There is some indirect evidence in favour of this hy pothesis. A definitive resolution of this issue is likely to employ compari sons of circadian action spectra in wild type and retinally degenerate mice . (C) 2001 Elsevier Science B.V. All rights reserved.