Evidence for the involvement of inositol trisphosphate but not cyclic nucleotides in visual transduction in Hermissenda eye

Although several second messengers are known to be involved in invertebrate photoresponses, the mechanism underlying invertebrate phototransduction remains unclear. In the present study, brief injection of inositol trisphosphate into Hermissenda photoreceptors induced a transient Na⁺ current followed by burst activity, which accurately reproduced the native photoresponse. Injection of Ca²⁺ did not induce a significant change in the membrane potential but potentiated the : native photoresponse. Injection of a Ca²⁺ chelator decreased the response amplitude and increased the response latency. Injection of cGMP induced a Ca²⁺-dependent, transient depolarization with a short latency. cAMP injection evoked Na⁺-dependent action potentials without a rise in membrane potential. Taken together, these results suggest that phototransduction in Hermissenda is mediated by Na⁺ channels that are directly activated by inositol trisphosphate without mobilization of cytosolic Ca²⁺.