Histochemical localization of retinochrome and rhodopsin studied by fluorescence microscopy

Retinochrome is readily reduced by sodium borohydride into an N-retinyl protein that emits visible fluorescence upon irradiation with near-ultraviolet light. Rhodopsin is also converted to a similar fluorescent product, but only when denatured with formaldehyde before reduction. Based upon this difference, retinochrome was discriminated from rhodopsin on frozen sections. The distribution of these two photopigments in various photosensitive tissues was examined by means of epifluorescence microscopy. In the octopus retina (Octopus vulgaris), the yellow-green fluorescence of reduced retinochrome was observed in both the basal regions of the outer segments and throughout the inner segments of the visual cells, while the fluorescence of reduced rhodopsin was restricted to within the rhabdomal layer of the outer segments. In the squid parolfactory vesicles (Todarodes pacificus), rhodopsin was present in the central lumen, which contains the distal processes of the photoreceptor cells, while retinochrome was detected in the myeloid bodies scattered within the vesicular wall. In the slug retina (Limax flavus), rhodopsin was found in the microvilli, and retinochrome appeared to be concentrated in the photic vesicles of the visual cells.