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Volume 17, Number 9, Issue of May 1, 1997 pp. 2990-3001
Copyright ©1997 Society for Neuroscience

Direct Measurements of Presynaptic Calcium and Calcium-Activated Potassium Currents Regulating Neurotransmitter Release at Cultured Xenopus Nerve-Muscle Synapses

Received Nov. 26, 1996; revised Feb. 12, 1997; accepted Feb. 20, 1997.

Bruce Yazejian¹, David A. DiGregorio¹, Julio L. Vergara¹, Robert E. Poage², Stephen D. Meriney², and Alan D. Grinnell¹

¹ Department of Physiology, Jerry Lewis Neuromuscular Research Center, University of California Los Angeles School of Medicine, Los Angeles, California 90095, and ² Department of Neuroscience, University of Pittsburgh, Pittsburgh, Pennsylvania 15260

The understanding of neurotransmitter release at vertebrate synapses has been hampered by the paucity of preparations in which presynaptic ionic currents and postsynaptic responses can be monitored directly. We used cultured embryonic Xenopus neuromuscular junctions and simultaneous pre- and postsynaptic patch-clamp current-recording procedures to identify the major presynaptic conductances underlying the initiation of neurotransmitter release. Step depolarizations and action potential waveforms elicited Na and K currents along with Ca and Ca-activated K (K_Ca) currents. The onset of K_Ca current preceded the peak of the action potential. The predominantly -CgTX GVIA-sensitive Ca current occurred primarily during the falling phase, but there was also significant Ca²⁺ entry during the rising phase of the action potential. The postsynaptic current began a mean of 0.7 msec after the time of maximum rate of rise of the Ca current. -CgTX also blocked K_Ca currents and transmitter release during an action potential, suggesting that Ca and K_Ca channels are colocalized at presynaptic active zones. In double-ramp voltage-clamp experiments, K_Ca channel activation is enhanced during the second ramp. The 1 msec time constant of decay of enhancement with increasing interpulse interval may reflect the time course of either the deactivation of K_Ca channels or the diffusion/removal of Ca²⁺ from sites of neurotransmitter release after an action potential.

Key words: Key word: neuromuscular junction; nerve terminal; calcium channel; charybdotoxin; conotoxin; synaptic delay

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