Distribution of rhodopsin and retinochrome in the squid retina.

Abstract

The cephalopod retina contains 2 kinds of photopigments, rhodopsin and retinochrome. For many years retinochrome has been thought to be localized in the inner segments of the visual cells, whereas rhodopsin is in the outer segments. It is now clear that retinochrome can be extracted also from fragments of outer segments. In the dark-adapted retina of Loligo pealei retinochrome is distributed half-and-half in the inner and outer segments. Todarodes pacificus contains much more retinochrome than Loligo, and it is more abundant in the outer than in the inner segments. The outer segments of Loligo contain retinochrome and metarhodopsin in addition to rhodopsin, whether squids are kept in the dark or in the light. There is very little metarhodopsin (about 3% of rhodopsin) even in light-adapted eyes. The inner segments contain only retinochrome; much less in the light than in the dark. Retinochrome in the outer segments increases markedly during light adaptation. Some retinochrome probably moves forward from the inner to the outer segments during light adaptation and reacts with metarhodopsin to promote regeneration of rhodopsin.