Partial uncoupling of neurotransmitter release from [Ca2+](i) by membrane hyperpolarization

The dependence of evoked and asynchronous release on intracellular calcium ([Ca2+](i)) and presynaptic membrane potential was examined in single-relea se boutons of the crayfish opener neuromuscular junction. When a single bou ton was depolarized by a train of pulses, [Ca2+](i) increased to different levels according to the frequency of stimulation. Concomitant measurements of evoked release and asynchronous release, from the same bouton, showed th at both increased in a sigmoidal manner as a function of [Ca2+](i). When ea ch of the depolarizing pulses was immediately followed by a hyperpolarizing pulse, [Ca2+](i) was elevated to a lesser degree than in the control exper iments, and the rate of asynchronous release and the quantal content were r educed; most importantly, evoked quantal release terminated sooner. The dim inution of neurotransmitter release by the hyperpolarizing postpulse (HPP) could not be entirely accounted for by the reduction in [Ca2+](i). The expe rimental results are consistent with the hypothesis that the HPP reduces th e sensitivity of the release machinery to [Ca2+](i), thereby not only reduc ing the quantal content but also terminating the quantal release process so oner.