Transient elevation of spontaneous release at the frog neuromuscular junction following acetylcholine iontophoresis

Abstract

Miniature end-plate currents (m.e.p.c.s) were recorded from frog neuromuscular junctions using a two-electrode voltage clamp. The m.e.p.c. frequency was temporarily elevated following 10s iontophoretic applications of acetylcholine (ACh) when the junctions were clamped at 100 mV. This post-ACh "burst" of quanta was also observed at unclamped junctions. At-100 mV, the intensity of the burst was proportional to the amount of current flowing across the end-plate during ACh iontophoresis, but usually did not reach its peak until the end-plate receptor channels were almost completely closed. Addition of 0.5 microM TTX to the Ringer's solution, or total replacement of sodium with guanidine, lithium, or methylamine did not inhibit the burst. No burst was observed in Ca2+-free, EGTA solutions, or in Ca2+-free solutions containing 2 mM Mn2+. Sr2+ effectively substituted for Ca2+. Addition of 2 mM Co2+ or Mn2+ to normal Ringer's did not inhibit the burst. Presynaptic muscarinic receptors did not obviously contribute to the burst, since it was not blocked by atropine, nor produced by oxotremorine or pilocarpine. The ACh analogs carbachol and acetyl-beta-methylcholine also produced the burst. The burst was highly dependent on the muscle membrane potential during the period of ACh iontophoresis, becoming more intense at potentials negative to -100 mV and disappearing at -60 mV. The critical importance of the post-synaptic membrane potential suggests that the burst may be due to an action of the muscle end-plate on the motor nerve terminal, possibly by the movement of an anionic substance through open end-plate receptor channels, but this hypothesis does not account for the delay of the burst until near the end of the ACh-induced end-plate current.