NOVEL SYNAPSES COMPENSATE FOR A NEURON ABLATED IN EMBRYOS

In leeches, as well as mammals, neuronal death in adults produces last ing deficits, whereas the embryonic nervous system is believed to be m ore plastic. Killing the single S interneuron in an adult leech gangli on permanently interrupts the chain of S cells linked by electrical sy napses along the entire animal. Axons that synapsed with the ablated n euron do not change length in response to cell ablation, but they will grow if another axon of the same neuron is injured. In the present ex periments, the S cell and surrounding cells in one ganglion were ablat ed with a fine pin during embryogenesis (day 8-11). Effects were evalu ated 1-4 months later. Cell-specific monoclonal antibody confirmed S c ell deletions. Intracellular injection of horseradish peroxidase and 6 -carboxyfluorescein dye showed that intact S cells' axons projected tw ice their usual length into the lesioned ganglion and formed electrica l synapses with homologues of their usual synaptic targets. Conduction was often restored by these connections, which replaced those of the deleted S cell. Therefore, in both adults and embryos, growing S inter neurons respond to loss of a target by greater growth. However, only o n the small scale of the embryo is growth sufficient to reach suitable targets.