Retinomotor Movements

Abstract

In lower vertebrates, photoreceptors and pigment granules within the retinal pigment epithelium undergo positional rearrangements, called retinomotor movements, in response to changes in ambient light conditions and to circadian signals. In the light (day), cones shorten, rods elongate, and pigment granules migrate out into the long apical projections of the retinal pigment epithelium (RPE) cells. In the dark (night), these movements are reversed. Force production for photoreceptor elongation and contraction and for pigment migration is autonomous for each cell type. Cyclic adenosine monophosphate is a key intracellular regulator of retinomotor movements, triggering dark-adaptive movements in all three cell types. Action spectra studies suggest that cone and RPE retinomotor movements are not triggered directly by light but by a rod-mediated pathway. Dopamine, acting through D2 family receptors, plays a paracrine role in both light and circadian regulation of cone and RPE retinomotor movements. Found in vertebrates lacking dilatable pupils, retinomotor movements provide an alternative mechanism for adjusting the retina to varying light conditions.