Presynaptic membrane potential and transmitter release at the crayfish neuromuscular junction

Release of transmitter was evoked at neuromuscular junctions of the crayfish opener muscle by passage of current through an intracellular electrode impaling a branch of the motor axon close to a muscle fiber. Membrane-potential changes in the presynaptic axon branch were monitored, together with postsynaptic potentials. Depolarization of impaled secondary axonal branches by more than 10 mV led to an increase in asynchronous transmitter release. The release was facilitated by prolonged (50-500 ms) depolarizations and it decayed rapidly when depolarization was terminated. Ca2+ was essential for facilitated release; however, no indication of a Ca spike was found at the recording site. Input-output curves for the synapse were obtained by applying depolarizing pulses of varying amplitude to the axon branch. Transmitter output was strongly influenced by both amplitude and duration of the applied depolarization. During normal synaptic transmission, propagated Na+-dependent action potentials were recorded in the secondary axonal branches but there was no evidence for a calcium-dependent component for these action potentials. Evoked release was dependent on Ca2+ and was steeply dependent on the amplitude of the action potential, which could be made variable in size by application of tetrodotoxin (TTX). Prolonged depolarization of axonal branches resulted in enhancement of transmitter release evoked by an action potential. The enhancement occurred in spite of a simultaneous reduction of the amplitude of the action potential. Morphological features of the terminals were investigated after injection of lucifer yellow into the axon. An electrical model incorporating the morphological features suggests that membrane-potential changes set up in the main axon reach the nearest terminals with 30-40% attenuation, while events originating in the terminals would be severely attenuated in the main axon. Comparison of the crayfish synapse with other frequently studied synapses shows both similarities and differences, suggesting that it is not possible to apply findings made in one synapse to all others.