Molecular Pharmacology, Vol 12, 778-788, Copyright © 1976 by the American Society for Pharmacology and Experimental Therapeutics

The Synaptic Vesicle: Calcium Ion Binding to the Vesicle Membrane and Its Modification by Drug Action

¹ Department of Pharmacology, University of Cambridge, Cambridge, England

The ionogenic nature of the synaptic vesicle surface and its calcium adsorption characteristics have been investigated by microelectrophoretic measurements on vesicles isolated from the guinea pig cerebral cortex. The electrophoretic mobility of synaptic vesicles determined in 0.16 M KCl was -1.31 µm sec^-1 cm V^-1, corresponding to a surface potential of -18.2 mV and surface charge density of 5451 esu cm^-2. Whereas the monovalent cation species of the suspending medium had little effect upon the electrokinetic properties of synaptic vesicles, addition of Ca²⁺, 1-10 mM, markedly decreased the surface charge. From the monomolecular adsorption isotherm the number of Ca²⁺ binding sites was found to be 4.17 x 10¹² cm^-2, with an electrochemical free energy of calcium ion adsorption, G_Ca²⁺, of -5.00 kcal mole^-1, increasing to -5.64 kcal mole^-1 at low (0.02) ionic strength. Adenine and pyridine nucleotides (1 mM) were without effect on synaptic vesicle mobility. In contrast, the neuroactive agents trifluoperazine, benzyl alcohol, and amylobarbitone decreased the surface charge of the synaptic vesicle per se, irrespective of any effect on Ca²⁺ binding. These results are discussed in relation to the ionogenic properties of the synaptic vesicle surface and the molecular mechanism of drug action on the neunotransmitter release process.

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ACKNOWLEDGMENTS We are indebted to Dr. P. M. Dean and Mrs. G. A. Currell for their expert help in this study.