Transduction Gain in Light Adaptation of Rod Photoreceptors

Nikonov et al. 2000 recently reported a study of light adaptation in salamander rod photoreceptors. Analysis of their experimental data has led these investigators to conclude that elevation of the level of activated cGMP phosphodiesterase (PDE*) by background light, with a resulting decrease in the effective lifetime of cGMP, is the primary mechanism responsible for noncompressive desensitization of the flash response. They further conclude that this desensitization involves little if any reduction in signal amplification within the early, activating reactions of the transduction cascade. One aim of the present Letter is to note the primary dependence of background desensitization on a process, termed “gain” reduction below, that is distinct from PDE* elevation (reduced cGMP lifetime). A second aim is to point out that the Nikonov et al. 2000 data leave open the possibility that the desensitization depends strongly on...