Effect of bleached rhodopsin on signal amplification in rod visual receptors

Abstract

BLEACHING of rhodopsin markedly desensitizes the vertebrate visual system during a subsequent period of dark adaptation^1–6. Previous studies have indicated an origin of bleaching desensitization in the visual pigment itself, but have not identified the mechanism of action. A candidate for the site at which densensitization is initially expressed is the activation of transducin (formation of T^*) on the rod disk membranes; this reaction directly involves rhodopsin in its photoactivated (R^*) form and mediates initial amplification of the visual signal (reviewed in refs 7–9). We have analysed the effect of bleaching on the sensitivity of a flash-induced light-scattering signal known to monitor the disk-based amplifier^10–12, and which has been established as specifically monitoring transducin activation¹³. We have recorded this signal from functioning retinal rods in situ ('ATR' signal)¹⁴ and find that bleaches inducing a pronounced, sustained loss in rod electro-physiological sensitivity do not alter the sensitivity of the ATR response after correction for reduced quantum catch. Our results indicate that the biochemical gain of the R^*→T^* transduction stage remains unchanged in the presence of bleached pigment and implicate a subsequent reaction as the first to show a sustained, bleaching-dependent gain reduction.