Anion permeability of motor nerve terminals

Abstract

Motor nerve terminals in mouse and frog display behavior consistent with an appreciable permeability of the nerve terminal membrane to chloride. In mouse diaphragm, in the presence of 15 mM K⁺ and 2 mM or 8 mM Ca²⁺, replacement of Cl⁻ by NO ⁻₃ , Br⁻ or acetate causes a transient increase in the quantal release of acetylcholine, measured as the frequency of spontaneously occurring miniature end plate potentials (FMEPP); a rapid rise in FMEPP is followed by a slow decline, with a half-time of about 4 min, to an equilibration level close to the control level. After equilibration in a solution in which the Cl⁻ is replaced by another anion, return to Cl⁻-containing solution causes a transient decrease in FMEPP with a subsequent slow recovery. The data are consistent with transient nerve terminal depolarization or hyperpolarization, reflecting a nerve terminal permeability to anions in the sequence Cl⁻>Br⁻>NO ⁻₃ >acetate. In 5 mM K⁺, changes in nerve terminal excitability, determined using focal stimulation, are also consistent with alteration of nerve terminal membrane potential as a consequence of anion substitution. The time course of relaxation of FMEPP after a change from Cl⁻ to an anion of lower permeability, or vice versa, is considerably slower than that expected if Cl⁻ permeability of nerve terminals is similar to that of skeletal muscle fibres, and if the nerve terminal behaves as a single compartment. In frog cutaneous pectoris, transient changes in FMEPP produced by substitution of anions in the bathing solution were similar to those produced in mouse diaphragm, but more rapid in time course.