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-release boutons of the crayfish opener neuromuscular junction. When a single bouton 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 that both increased in a sigmoidal manner as a function of [Ca²⁺](i). When each of the depolarizing pulses was immediately followed by a hyperpolarizing pulse. [Ca²⁺](i) was elevated to a lesser degree than in the control experiments, and the rate of asynchronous release and the quantal content were reduced; most importantly, evoked quantal release terminated sooner. The diminution of neurotransmitter release by the hyperpolarizing postpulse (HPP) could not be entirely accounted for by the reduction in [Ca²⁺](i). The experimental results are consistent with the hypothesis that the HPP reduces the sensitivity of the release machinery to [Ca²⁺](i), thereby not only reducing the quantal content but also terminating the quantal release process sooner.