Rhodopsin photoproducts and the visual response of vertebrate rods

Flash responses of the photoreceptors in the isolated, all-rod retina of the skate were recorded extracellularly during exposure of the retina to relatively weak background light of fixed intensity. Under conditions expected to yield approximately constant levels of (previously) bleached visual pigment and transient bleaching intermediates, (quasi-)steady-state, increment receptor thresholds were examined in relation to the prevailing extent of rhodopsin bleaching. The results indicate that, at least for moderate bleaches, the change in log threshold associated with prior bleaching becomes smaller with increasing value of background intensity. This behavior of the increment threshold data is discussed in the context of a recent model (Pepperberg, 1984; Vision Res. 24, 357-366) linking values of photoreceptor threshold during light and dark adaptation with states of the visual pigment molecule.