Electrical responses and K+ activity changes to light in the ocellus of the planarian Dugesia japonica

The extracellular electrical response (ocellar potential, OP) and K+ activity changes to light were measured in the planarian ocellus by conventional and double-barreled ion-selective microelectrodes. The OP evoked by a 0.5 sec light flash is a simple monophasic potential, which is sustained during long-term illumination. The spectral sensitivity of the OP has the characteristics of a rhodopsin with lambda max 505 nm. K+ activity in the extracellular space of the ocellus in the dark was 1.1 +/- 0.2 mM (mean +/- SE, n = 6). A 0.5 sec light flash and a 120 sec long-term illumination evoked increases in the K+ activity. The maximum K+ activity change occurred at 503 nm. These results suggest that the efflux of K+ from the planarian photoreceptor into the extracellular space is caused by light, and that the OP and the increase of K+ activity are mediated by a rhodopsin.