Dye-induced 'photo-degeneration' and 'photo-permeabilization' of mammalian neurons in vivo.

Dyes are known to induce neuronal 'photo-degeneration' and 'photo-permeabilization' in fly photoreceptor cells in vivo. In the present study, we attempted to generalize this photodynamic damage to vertebrate neurons, using the rat retina, a brain part which is optically accessible in vivo. After intravitreal injection of the photosensitizing dye Rose Bengal (RB), irradiation of the retina of a living rat with a T-shaped microbeam was found to induce striking 'optograms' which could be observed on the excised retina. The T-shaped pattern which was to be seen in the translucent retina under transmitted light was attributed to neuronal degeneration of the neurons irradiated in the presence of RB, as attested by classical degenerative features such as a cytoplasmic darkening or a drastic swelling. The T-shaped pattern could also be observed on adding the dye Lucifer yellow to the extracellular space of the retina either in vitro or in vivo, showing that the cells irradiated in the presence of RB became permeable. These structural reactions were observed in the cells in the inner nuclear layer (INL) and ganglion cell layer (GCL), in the processes in both plexiform layers, and in the ganglion cell axons crossing this area, whereas the photoreceptors in the outer retina appeared to be undamaged. From these reactions, due to photo-degeneration and photo-permeabilization, it was possible to identify the photodynamic damage to the nervous system histologically at the macroscopic, cellular and ultrastructural levels. In view of its accuracy and reproducibility, the photo-lesion technique holds great potential as a tool for investigating various nervous systems.