Reversal potential for noradrenaline-induced hyperpolarization of spinal motoneurons.

By using two separate electrodes with tips inside a single feline motoneuron, current-voltage characteristics were studied during extracellular iontophoresis of noradrenaline. The usually observed hyperpolarization was accompanied by an increase in membrane resistance and became larger with polarizing and smaller with depolarizing currents. During large depolarizing current injections, the noradrenaline-induced potential reversed its direction, usually at a membrane potential of about -20 millivolts. These data are compatible with the concept that noradrenaline hyperpolarizes nerve cells by decreasing resting membrane conductances to sodium and potassium ions. The observation could also be explained by a nonspecific decrease in ion permeability that is associated with a hyperpolarization due to sodium pump activation.