A Neural Mechanism for Processing Colour Information in Molluscan Extra-ocular Photoreceptors

This study was conducted to clarify the electrical and spectral properties of the combined photoresponses of two extra-ocular photoreceptor neurones, A-P-l and Es-1, in the abdominal ganglion of the marine mollusc Onchidium verruculatum. The depolarizing receptor potential or inward receptor current of Es-1 appeared to result from a decrease in K conductance, in the same way as in A-P-l. The direct photoresponse of Es-1 had a peak at 580 nm, while A-P-l had its peak at 490nm. Under normal conditions, Es-1 was hyperpolarized by blue-green light (490nm) and was depolarized by yellow light (580nm), indicating that its responses were wavelength-dependent. Slow inhibitory synaptic potentials in Es-1 occurred during a sustained depolarization of A-P-l, suggesting that Es-1 received some inhibitory synaptic input from A-P-l. The amplitude of these slow synaptic hyperpolarizations was dependent on graded changes in the depolarization of A-P-l. However, presynaptic spikes in A-P-l were not followed by discrete synaptic potentials in Es-1. These results suggest that graded photoresponses of A-P-l could produce the slow hyperpolarization of Es-1 and that the photoresponse of A-P-l is thereby transmitted to Es-1 with reversed polarity. The differing responses of Es-1 to light of different wavelengths may have a role in colour discrimination.