Augmented synaptic release by one excitatory axon in regions in which a synergistic axon was removed in lobster muscle.

1. In the lobster, every fibre of the lateral abdominal extensor muscle is innervated by two excitatory axons. When one of the excitatory axons (the common excitor) was removed chronically by intracellular injection of pronase, terminals of the remaining axon (the specific L1 excitor) showed augmented transmitter release. 2. Evidence as to the mechanism of this strengthening can be obtained taking advantage of the peculiar innervation pattern of the abdominal extensors. The L1 excitor axon of one segment sends a branch to part of the next posterior segment. The common excitor axon innervates only muscle fibres of its own segment. 3. 10-20 days after removing the common excitor axon of segment II, the quantum content of release of terminals of the L1 excitor axon was measured in segments I, II and III. Terminals of the L1 excitor axon of segment I which innervate segment II released much more transmitter than controls, while the terminals of the same axon innervating segment I remained normal. Similarly, terminals of the L1 excitor axon of segment II became 'stronger' in segment II but remained normal in segment III. 4. It is concluded that only those terminals of one axon which innervate targets with reduced innervation increased the average release rate. It seems that the signal for synaptic strengthening, after removal of a synergistic axon, is generated and acts locally in partially denervated muscle fibres.