PROLONGED TIME-COURSE OF GLUTAMATE RELEASE FROM NERVE-TERMINALS - RELATIONSHIP BETWEEN STIMULUS-DURATION AND THE SECRETORY EVENT

The kinetics of synaptosomal [H-3]glutamate release were measured on a subsecond time scale to study the relationship between the length of depolarization and the duration of the secretory event. The time cours e of release evoked by elevated K+ was complex, proceeding for several seconds after a 200-ms depolarization. We developed a protocol for de polarizing excitable membranes on a millisecond time scale to deliver brief depolarizations, termed the synthetic action potential, by using batrachotoxin to activate Na+ channels. Depolarization is achieved by superfusing with solutions containing elevated concentrations of Na+, and the duration of the depolarization is limited by including tetrod etoxin (TTX) in the superfusion solution to block Na+ entry. Direct me asurements of the time courses of Na+ current and membrane depolarizat ions were made in batrachotoxin-treated sensory neurons using patch cl amp recording methods. Rapid increases in Na+ and mt concentrations pr oduced transient increases in inward Na+ current that decayed with a t ime course proportional to me concentration. current clamp measurement s indicated that, with 10 mu M TTX, depolarizations last similar to 30 ms. Nonetheless, synaptosomal release of [H-3]glutamate triggered by the synthetic action potential remained prolonged. Brief neuronal acti on potentials at some synapses may trigger transmitter release that pe rsists for several seconds.