An interneurone, the S cell in the central nervous sytem of the leech, regenerates its severed axon and forms an electrical synapse with its target, another S cell, entirely within the ensheathment of two glial cells. After the two glial cells were killed selectively by intracellular injection of protease, axonal regeneration and synapse formation occurred in a normal fashion during the month following nerve injury. Soon after reconnexion of S cells, the conduction of impulses across the non-rectifying electrical junction between the cells was more reliable from the target than into it from the thinner regenerating axon. The distal segments of severed S-cell axons survived for weeks or months after destruction of their glial cells, indicating that the ensheathing glia is not required for long-term survival of axon segments. The distal axon segment of the S cell remained connected to the target axon at the normal region of synapse midway between ganglia within the nerve cord. In about half the cases in which reconnexion between injured S cell and target S cell occurred between 10 and 25 days following nerve crush, the regenerating neurone had formed an electrical synapse with its severed distal axon and had thereby become reconnected, indirectly, with its target. In the other cases, reconnexion was by direct contact. By 4 weeks, the proportion of injured S cells that were coupled and making direct contact with their targets rose to more than 80% of the total population, indicating that regeneration continued until the two S cells contacted one another directly. This is similar to the course of S-cell regeneration in the presence of the ensheathing glia. Microscopy of the regenerating neurone and both its distal axon segment and its target showed that the site of synapse formation in the absence of the usual glial sheath was normal. Fluorescence microscopy following intracellular injection of Lucifer Yellow dye, which crosses between S cells at the electrical synapse, showed that the regenerated synapse formed specifically between S cells. Moreover, the target did not form alternative synapses when regeneration failed.
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