Abstract
The identity of the second messenger that directly activates the light-dependent conductance in invertebrate photoreceptors remains unclear; the available evidence provides some support for cGMP and Ca2+. To resolve this issue we have applied these second messengers to membrane patches excised from the light-sensitive lobe of Limulus ventral photoreceptors. Our results show that these patches contain channels that can be opened by cGMP, but not by Ca2+. These cGMP-activated channels closely resemble the channels activated by light in cell-attached patches. This evidence suggests that cGMP is the messenger that opens the light-dependent channel in invertebrate photoreceptors.