Specific second messengers activate the caudal photoreceptor of crayfish

The caudal photoreceptor (CPR) found in the last abdominal ganglion of crayfish is a well-known example of a non-retinal photosensitive element. In addition to light sensitivity, this cell has been assigned a command role for a walking behavior. The molecular mechanism of transduction in this cell has not been previously studied. The involvement of an intermediate messenger substance is suggested by its long latency to response, its prolonged afterdischarge, and by the requirement for an amplification process for the efficient transduction of light. We tested the effect of some putative second messengers by pressure injecting them into the CPR and noting the physiological response. Here we report that intracellular injection of inositol 1,4,5-trisphosphate (IP3), calcium, and the guanosine nucleotide GTP mimics the light response, while cAMP, IP1 and IP2 have no effect on the firing rate. The key intermediate in transduction in vertebrate photoreceptors, cGMP, was ineffective in this system. This work adds to the growing body of evidence that IP3 plays a role in invertebrate phototransduction.