Physiology of Simple Photoreceptors in the Abdominal Ganglion of Onchidium

Abstract

Simple photoreceptors without microvilli or cilia, the photorescponsive neurons, designated as A-P-1, Es-1, Ip-2, and Ip-1, exist in the abdominal ganglion of sea slug Onchidium. Of these, A-P-1 and Es-1 respond to light with a depolarizing receptor potential, caused by the closing of light-dependent, cGMP-gated K⁺ channels, whereas Ip-2 and Ip-1 are hyperpolarized by light, owing to the opening of the same K⁺ channels. Studies show the first demonstration of a new type of cGMP cascade, in which Ip-2 and Ip-1 cells are hyperpolarized when light activates GC through a Go-type G-protein. This new cascade thus contrasts with the well-known phototransduction cGMP cascade mediated by a Gt-type G-protein, seen in rods and cones as well as A-P-1 and Es-1 cells. Studies also suggest that the Onchidium simple photoreceptors and vertebrate simple photoreceptors, called ipRGCs, might be different from the conventional eye photoreceptors, which function as the pattern vision system and that they may be involved in a new sensory modality, the non-visual photoreceptive system, which functions as encoding of ambient light intensities, instead of spatial and temporal resolution. Finally, it is suggested that the Onchidium simple photoreceptors operate in the general regulation by light and dark of synaptic transmission of sensory inputs and subsequent behavioral responses.