Light-activated retinoid transport in cephalopod photoreceptors

Retinoid transport and chromophore exchange have been investigated in cephalopods using autoradiographic and radiobiochemical techniques. In dark adapted retinas, [3H]-retinoid is concentrated in myeloid bodies present in the photoreceptor inner segments and is bound to the photopigment retinochrome. In retinas exposed to light, there is a shift in the distribution of [3H]-retinoid. The rhabdomes become more heavily labeled than the inner segments, and rhodopsin labeling exceeds that of retinochrome. In animals returned to the dark, another shift in retinoid distribution occurs and the inner segments are again more labeled than the rhabdomes. In these animals [3H]-retinoid is bound primarily to retinochrome. Exposure to light seems to activate a transport mechanism that results in the redistribution of retinoid between the inner segments and rhabdomes and chromophore exchange among the photopigments.