Photoresponses of an extraocular photoreceptor associated with a decrease in membrane conductance in an opisthobranch mollusc.

The photoresponse of an extraocular photoreceptor, the photoresponsive neuron (A-P-1) in the abdominal ganglion of Onchidium verruculatum, was studied by using a voltage-clamp with two micropipettes and a monochromatic light. When the A-P-1 was voltage-clamped at resting membrane potential levels, light induced a slowly developing inward current which peaked at about 20 s. A decrease in membrane conductance accompanied this light-induced current which corresponded to the depolarizing photoreceptor potential in the unclamped A-P-1. The relationship between the peak of the current response and light intensity could be predicted by using the modified Michaelis-Menten equation. The spectral sensitivity for the photoresponse had a peak at 490 nm. The steady-state light-induced current was a non-linear function of the membrane potential. The current-voltage relationship for the instantaneous light-induced current was almost linear. In normal (10 mM K+) saline, the polarity of the instantaneous light current reversed from inward to outward at about -67 mV, and doubling the external K+ from 10 to 20 mM shifted the reversal potential to about-50 mV, similar to that predicted by a K+-electrode. These results suggest that the light-induced current or the depolarizing receptor potential of A-P-1 is due to the light suppression of a voltage- and time-dependent K+ current.