The reaction of Helix photosensitive neurons to light and cyclic GMP

The effect of light and cyclic nucleotides (cGMP and cAMP) on a group of identified photosensitive neurons in Helix pomatia left parietal ganglion was examined. The onset of light induces in these cells a slow inward current shift associated with decrement of slope conductance. The reversal potential (Erev) of the light induced current (IL) and its changes due to altered extracellular concentration of potassium support the assumption that (IL) is due to suppression of K+ conductance. Elevation of intracellular concentration of cyclic GMP by adding 8-bromo-cGMP into the bathing solution or by injecting the nucleotide into the cell mimics, in all cases, the effect of light: it produces an inward current accompanied by decrement of slope conductance. The effect of elevation of cAMP was inconsistent. The I-V relations of IL and IcGMP follow a similar course and have a common Erev. The assumption that 3',5'-cyclic GMP is increased by light is also supported by comparing the effect of light and the cyclic nucleotide on the action potential. Both light and 8-bromo-cGMP broaden the action potential by increasing the voltage-dependent Ca2+ current.