External sodium ions are required for the light response in pineal photoreceptors.

Photoreceptor cells of the pineal organ of river lamprey, Lampetra japonica, showed hyperpolarizing light responses associated with a decrease in membrane resistance. Effects of external ion concentrations (Na+, K+, Li+, Cl-) on the light response and the resting membrane potentials were investigated. The average resting membrane potential in the standard solution was -29.7 +/- 7.3 mV (mean +/- SD, n = 125) and the amplitude of the response to white light (1 sec, 1.4 X 10(-8) W/cm2) was -20.4 +/- 6.3 mV (n = 125). The amplitude depended largely on external Na+ concentration. The change was 18 mV/10-fold increase of Na+ concentration, and the external Na+ was essential for generation of the response.