Molecular interactions and mutational impact upon rhodopsin (G90→D90) for hindering dark adaptation of eye: A comparative structural level outlook for signaling mechanism in night blindness.
Delayed recovery of visual sensitivity and night blindness caused by inadequate regeneration of the visual pigment rhodopsin are typical early signs of this condition.
The decrease in the thermal stability of dark state rhodopsin is likely to be associated with higher levels of dark noise that undermine the sensitivity of rhodopsin, potentially accounting for night blindness in the early stages of RP.
In an examination of the effect of three rhodopsinnight blindness mutations on the rate of association of 11-cis-retinal with opsin, one of the mutations (G90D) was found to slow the rate of reaction by more than 80-fold.
The T94I mutant pigment (with a bound 11-cis-retinal chromophore), like the other known rhodopsinnight blindness mutants, is not active in the dark and has wild-type activity upon exposure to light.
In night blindness resulting from defects in rhodopsin, the alpha subunit of rod transducin, or the beta subunit of rod cGMP phosphodiesterase, rod photoreceptors respond only to light intensities far brighter than normal, and the sensitivity of rods to light is similar to that of normal individuals who are not dark adapted.
The apparent preservation of functioning rods despite extensive and lifelong night-blindness in this kindred is inconsistent with one current hypothesis that chronic rod activation from constitutively active mutant rhodopsin necessarily contributes significantly to photoreceptor demise in human retinal dystrophies.