'Zap axotomy': Localized fluorescent excitation of single dye-filled neurons induces growth by selective axotomy

The response of populations of neurons to axotomy has traditionally been studied by crushing or sectioning whole nerve trunks. The present communication describes a technique by which single neurons can be reliably and selectively axotomized in the absence of damage to other axons and non-neuronal cells within the nerve. To obtain selective axotomy, identified neurons of the buccal ganglia of the snail, Helisoma, were first filled with fluorescent dye. Next, the preparation was positioned in a restricted beam of blue light using low light video fluorescence microscopy. Finally, the selected region of axon was briefly exposed to light levels normally employed for fluorescence microscopy. Shortly after irradiation of the identified neuron 5, antidromic action potentials no longer propagated past the region of exposure in the dye-filled cell, whereas adjacent axons were physiologically intact. Several days after exposure, profuse neurite outgrowth was observed from the proximal region of axon of neuron 5, but never in neighboring axons which were not filled with dye at the time of irradiation. When the axons of both neurons 5R and 5L were spot irradiated neurite outgrowth resulted in the formation of a novel electrical connection between these cells. These changes in growth and connectivity which were induced by selective axotomy of single axons were indistinguishable from the changes which are produced by crushing entire nerve trunks.