Phototransduction in Limulus photoreceptors and enigmas of the transduction mechanism in photoreceptor cells of invertebrates

The receptor current of the ventral photoreceptor of Limulus is composed of three components which differ in several aspects: Time course of light activation, time course of recovery from Light adaptation, reversal potential, and pharmacology. In addition, two different types of single photon-evoked events (bumps) and three different elementary channel conductances are observed in this photoreceptor cell. These findings indicate that the three receptor current components are controlled by three different enzymatic pathways using three different terminal transmitters. Probably one of the terminal transmitters is cyclic GMP, another one is activated via the IP3 cascade, the third one might be cyclic AMP.