Two components of muscarine‐sensitive membrane current in rat sympathetic neurones.

Abstract

Membrane currents induced by muscarine (Imus) were recorded in voltage-clamped neurons is isolated rat superior cervical ganglia. Two components of Imus were regularly recorded: an inward current resulting from inhibition of the outward K+ current, IM; and an outward current attributable to the reduction of a steady inward current. The presence of these 2 components caused a cross-over in the current-voltage curves at -50 .+-. 3 mV in neurons impaled with KCl-filled micro-electrodes or at -63 .+-. 4 mV in neurons impaled with K-acetate-filled electrodes. Both components of Imus were prevented by atropine. Both persisted in Krebs solution containing tetrodotoxin (1 .mu.M), Cd2+ (200 .mu.M) or 0 Ca2+. When IM was inhibited by external Ba2+ or internal Cs+ only the outward component of Imus could be detected. This component reversed at +3 .+-. 2 mV in cells impaled with CsCl-filled electrodes or at -20 .+-. 3 mV in cells impaled with Cs-acetate-filled electrodes. The reversal potentials agreed with those for the currents induced by .gamma.-aminobutyric acid (+4 .+-. 2 mV and -16 .+-. 3 mV with CsCl and Cs acetate electrodes, respectively). Replacement of external NaCl with Na acetate (so reducing external Cl- concentration ([Cl-]0) from 155 to 22 mM) shifted the reversal potential for Imus by + 25 and + 14.5 mV in 2 cells impaled with CsCl-filled electrodes. A 10-fold reduction of external [Na+] (by glucosamine replacement) did not significantly alter the reversal potential for Imus in KCl or CsCl-impaled cells. Under conditions where Im is already inhibited, the residual outward component of Imus can lead to hyperpolarization and inhibition of neuronal activity in unclamped cells. Both inward and outward components of Imus result from direct activation of muscarinic receptors on the ganglion cells. The inward component results from IM inhibition. The outward component results from inhibition of another, voltage-independent current IX which largely comprises a Cl- current. The inward component induces membrane depolarization and an increased excitability; the outward component can lead to hyperpolarization and reduced excitability.