Cyclic GMP is involved in the excitation of invertebrate photoreceptors.

The hyperpolarizing receptor potential in vertebrate rod photoreceptors appears to be mediated by the second messenger, cyclic GMP. Injection of cGMP into rods or application of cGMP to inside-out membrane patches activates a conductance resembling that produced by light. Light produces a rapid reduction of cGMP in living rods, leading to closure of sodium channels and membrane hyperpolarization. In most invertebrate photoreceptors the response to light is depolarizing. We have investigated whether cGMP is involved in controlling the increase in sodium conductance that underlies this depolarization. We show here that injection of cGMP into Limulus photoreceptors produces a depolarization that mimics the receptor potential. We also show that the cGMP concentration of the squid retina increases rapidly during exposure to light. These results support the hypothesis that cGMP mediates the light-induced depolarization in invertebrate photoreceptors and suggests that vertebrate and invertebrate phototransduction may be more similar than previously thought.