Dye-induced photopermeabilization and photodegeneration: a lesion technique useful for neuronal tracing.

Using as a neural system the fly retina, which is visually accessible in vivo, we describe a lesion technique that takes advantage of the photodynamic damage produced by extrinsic dyes. Contrary to the photo-inactivation technique described by Miller and Selverston (1979), this technique does not involve intracellular injection, since the dye is applied to the extracellular space of the tissue. This treatment was found to trigger neuronal degeneration and cell permeabilization in fly photoreceptor neurones. We coined the names 'photodegeneration' and 'photopermeabilization' for these two phenomena. While the technique can be used to delete given neurones from the neural circuit after several days' survival time, it was found to produce adequate cytoplasmic labelling for anatomical studies with both light and electron microscopy. Since the area occupied by the degenerating cells is restricted to the light spot imaged onto the nervous tissue, the resolution with this lesion technique can range from single cells to whole neuronal populations. The remarkable precision of the 'photolesions' produced in this way makes this technique a powerful tool for physiological and anatomical investigations on real neural networks, whenever these can be made optically accessible in vivo or in situ.