Strengthening of synaptic contacts of an excitatory axon on elimination of a second excitatory axon innervating the same target.

In the deep abdominal extensor muscles of spiny lobsters (Panulirus-pennicillatus), the common excitor axon of segment II was eliminated by intracellular injection of pronase. At 1 to 23 days after the operation, the quantal content of excitatory postsynaptic currents (EPSCs), elicited by stimulation of the specific excitor of the L1 muscle, was determined in a specific area of the L1 muscle, both in the operated and in the contralateral control side. The EPSCs in the operated muscles had about a 5 times higher quantal content compared to those in the controls, the change developing within 1 to 2 days after operation. In camera lucida drawings of preparations stained with methylene blue, increased branching of the remaining excitatory axon was obvious at more than 4 days after the operation. To investigate the possibility of contribution of central mechanisms (Rotshenker, S. (1979) J. Physiol. (Lond.) 292: 535-547). to this effect, the bundle of five axons to the deep abdominal extensors of segment II was cut immediately after injection of pronase into the common excitor axon. This caused a reduction of the quantal content of EPSCs and shrinking of the field of innervation in the operated L1 muscle as compared to the control. Therefore, axonal continuity or central connections seem to be necessary for the development of an increased innervation by the specific excitor to L1 after eliminating the common excitor axon. Possible postsynaptic effects of the elimination of the common excitor axon were controlled by recording synaptic single channel currents elicited by the excitatory transmitter glutamate, using the patch clamp method. These single current events did not show appreciable changes in operated L1 muscles. Therefore, the presynaptic strengthening effect on the nerve terminals of the specific L1 excitor is predominant after elimination of the common excitor axon.