Triggering and Amplification

The molecular events which link photon absorption to membrane conductance changes in photoreceptor cells begin with light activation of rhodopsin. In vertebrates, activated rhodopsin (Rh*) triggers an enzymatic cascade resulting in hydrolysis of cyclic GMP (cGMP). Some 80–90% of the protein in the photoreceptor rod outer segment is devoted to the execution or regulation of this enzymatic cascade. We may, therefore, specify light-controlled cGMP metabolism as a major function of the vertebrate rod outer segment. Indeed, cGMP has been proposed to serve as the important messenger linking photon absorption to membrane conductance changes. It may well play this role and be involved in other important cellular functions. It has not been our goal to settle this issue. Rather, we set out to examine thoroughly the rhodopsin-triggered mechanisms controlling metabolism at the molecular level. We extended our examination of these mechanisms to invertebrates, drawing on recent evidence and underlying expectations which suggest that initial events in visual transduction will be conserved among diverse species.