Transduction in vertebrate photoreceptors: the roles of cyclic GMP and calcium

For much of the past decade, two theories of transduction have dominated the field of photoreception. In one, light causes calcium ions to be released into the rod cytoplasm, thereby blocking sodium channels in the plasma membrane. Recently a variety of experimental approaches have converged to overturn this idea and to support the rival cyclic nucleotide hypothesis, in which cyclic GMP acts as an internal messenger holding channels open in darkness and allowing them to close when it is hydrolysed by light. Contrary to previous ideas, cytoplasmic calcium levels appear to drop in the light, and this is thought to contribute to light-adaptation.