Hyperpolarizing photoreceptors in the eyes of the giant clam Tridacna: physiological evidence for both spiking and nonspiking cell types.

Intracellular studies on photoreceptors in the eyes of the giant clam Tridacna give evidence for two types of light-sensitive cells, both of which are hyperpolarized by light. These cells are distinguished by the presence or absence of spikes and corresponding characteristics of the receptor potential. In non-spiking (NS) receptors, the average resting potential in the dark is low (-15 mV) and peak receptor potentials are large (to 100 mV) and adapt rapidly to light. Spiking (S) receptors have higher average resting potentials (-45 mV), but receptor potentials do not exceed 20 mV and also do not adapt to light. The spikes in S-receptors are small (3-8 mV), occur spontaneously at low levels of illumination and are inhibited by light. Bursts of spikes arise on the repolarizing off-component of the receptor potential. Light adaptation increases the excitability of S-receptors in terms of a higher frequency and shorter latency of the off response burst. The receptor potential in both cells is due to a light-activated increase in membrane conductance to potassium ions. Membrane conductance decreases in NS-receptors in relation to light adaptation. Unlike the scallop eye, no depolarizing photoreceptors are present.