The Retina and other Light Sensitive Ocular Clocks #MMPMID27095816
Besharse JC; McMahon DG
J Biol Rhythms 2016[Jun]; 31 (3): 223-43 PMID27095816show ga
Ocular clocks, first identified in the retina, are also found in the retinal pigment epithelium (RPE), cornea and ciliary body. The retina is a complex tissue of many cell types and considerable effort has gone into determining which cell types exhibit clock properties. Current data suggest that photoreceptors as well as inner retinal neurons exhibit clock properties with photoreceptors dominating in non-mammalian vertebrates and inner retinal neurons dominating in mice. However, these differences may in part reflect the choice of circadian output, and it is likely that clock properties are widely dispersed among many retinal cell types. The phase of the retinal clock can be set directly by light. In non-mammalian vertebrates direct light sensitivity is commonplace among body clocks, but in mice only the retina and cornea retain direct light-dependent phase regulation. This distinguishes the retina and possibly other ocular clocks from peripheral oscillators whose phase depends on the pace-making properties of the hypothalamic central brain clock, the suprachiasmatic nuclei (SCN). However, in mice retinal circadian oscillations dampen quickly in isolation due to weak coupling of its individual cell autonomous oscillators and there is no evidence that retinal clocks are directly controlled through input from other oscillators. Retinal circadian regulation in both mammals and non-mammalian vertebrates uses melatonin and dopamine as dark- and light-adaptive neuromodulators respectively, and light can regulate circadian phase indirectly through dopamine signaling. The melatonin/dopamine system appears to have evolved among non-mammalian vertebrates and retained with modification in mammals. Circadian clocks in the eye are critical for optimum visual function where they play a role fine tuning visual sensitivity, and their disruption can impact diseases such as glaucoma or retinal degeneration syndromes.
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J+Biol+Rhythms 2016 ; 31 (3): 223-43
