The physiology of dark adaptation: Progress and future directions

Exposure of the eye to bright bleaching light produces a large decrease in photoreceptor sensitivity, followed by a slow return during adaptation to darkness. Although much progress has been made understanding the nature of this phenomenon, particularly its biochemistry, less is known about the physiology of dark adaptation. In this review, we summarize the evidence for desensitization produced by photoproducts of bleaching, especially apo-opsin, that is opsin without bound chromophore. We describe the relationship between these studies and diseases such as vitamin A deprivation and congenital stationary night blindness; the effects of analogs of chromophore on photoreceptor sensitivity; and the roles of transducin, rhodopsin kinase, and arrestin. We review many specialized features of dark adaptation in cones, including the role of retinal G protein-coupled receptor (RGR) opsin. For both rod and cone dark adaptation, we summarize some of the principal uncertainties in our understanding. We hope our review will provide a guide to past work as well as an indicator of many possible areas of future research.