Photoreceptor excitation and adaptation by inositol 1,4,5-trisphosphate.

A central question concerning vision is the identity of the biochemical pathway that underlies phototransduction. The large size of the ventral photoreceptors of Limulus polyphemus renders them a favourite preparation for investigating this problem. The fact that a single photon opens approximately 1,000 ionic channels in these photoreceptors suggests the need for an internal transmitter. We have investigated whether inositol 1,4,5-trisphosphate (InsP3) functions as such an internal transmitter, given that InsP3 may act as an intracellular messenger in other cellular processes. Here we report that in Limulus, intracellular pressure injection of InsP3 both excites and adapts ventral photoreceptors in a manner similar to light.