Partial Uncoupling of Neurotransmitter Release From [Ca]i by Membrane Hyperpolarization

Ravin, R., H. Parnas, M. E. Spira, and I. Parnas. Partial uncoupling of neurotransmitter release from [Ca]i by membrane hyperpolarization. J. Neurophysiol. 81: 3044–3053, 1999. The dependence of evoked and asynchronous release on intracellular calcium ([Ca]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, [Ca]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 [Ca]i. When each of the depolarizing pulses was immediately followed by a hyperpolarizing pulse, [Ca]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 [Ca]i. The experimental results are consistent with the hypothesis that the HPP reduces the sensitivity of the release machinery to [Ca]i, thereby not only reducing the quantal content but also terminating the quantal release process sooner.