Retinomotor Movements

Abstract

In lower vertebrates, photoreceptors and pigment granules within the retinal pigment epithelium (RPE) undergo positional rearrangements, called retinomotor movements, in response to changes in ambient light conditions and to circadian signals. Found in vertebrates lacking dilatable pupils, retinomotor movements provide an alternative mechanism for adjusting the retina to changes in light condition. In the light (day), cones shorten, rods elongate, and pigment granules migrate out into the long apical projections of RPE cells. In the dark (night), these movements are reversed: cones elongate, rods shorten, and pigment granules aggregate into the RPE cell body. In all three cell types force production for retinomotor movement depends upon the cytoskeleton of the cell itself and does not require forces generated by other cells. Cyclic AMP is a key intracellular regulator of retinomotor movements, triggering dark-adaptive movements in all three cell types. Action spectra studies have suggested that cone and RPE retinomotor movements are not triggered directly by light but by a rod-mediated pathway. Dopamine, acting through D2 family receptors, acts as a paracrine messenger that mimics the effect of light or circadian day in both photoreceptors and RPE.