The Journal of Neurophysiology Vol. 81 No. 6 June 1999, pp. 3044-3053
Copyright ©1999 by the American Physiological Society

Partial Uncoupling of Neurotransmitter Release From [Ca²⁺]_i by Membrane Hyperpolarization

The Otto Loewi Minerva Center for Cellular and Molecular Neurobiology, Department of Neurobiology, The Hebrew University, Jerusalem 91904, Israel

Ravin, R., H. Parnas, M. E. Spira, and I. Parnas. Partial Uncoupling of Neurotransmitter Release From [Ca²⁺]_i by Membrane Hyperpolarization. J. Neurophysiol. 81: 3044-3053, 1999.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.