A light-induced decrease of cyclic GMP is involved in the photoresponse of molluscan extraocular photoreceptors.

The depolarizing photoreceptor potential in the molluscan extraocular photoreceptor, A-P-1, results from the light-induced suppression of specific K+ currents. An application of cGMP or IBMX (inhibitor of phosphodiesterase) to A-P-1-evoked light-suppressed currents, similar to the above specific K+ currents. Thus, the light-induced decrease of cGMP levels in A-P-1 may be responsible for the photoreceptor potential, like vertebrate phototransductions.