The ionic mechanism of the excitatory action of glutamate upon the membranes of motoneurones of the frog

Abstract

Simultaneous intra-and extracellular recordings with K⁺, Na⁺, Ca²⁺, and Cl⁻ sensitive microelectrodes were performed in motoneurones of the spinal cord of the frog during depolarizations mediated by glutamate (GLUT) and by experimentally increased extracellular K⁺. Depolarization resulting from increased K⁺ activity (aK⁺) in the bathing solution evoked a decrease of intracellular Na⁺ activity (aNa _i ⁺ ); a transient increase ofaNa _i ⁺ accompanied by a decrease ofaNa _e ⁺ was observed during the depolarization induced by GLUT. Both modes of depolarization led to an increase ofaCl _i ⁻ and a concomitant decrease ofaCl _e ⁻ . An experimental increase ofaK _e ⁺ led to a threshold dependent increase ofaCa _i ²⁺ by at least one order of magnitude and to an equally threshold dependent strong decrease ofaCa _e ²⁺ . The threshold of these changes ofaCa²⁺ was at a membrane potential of −25 mV. During a depolarization of half the amplitude induced by GLUT a comparable increase ofaCa _i ²⁺ and a smaller decrease ofaCa _e ²⁺ were observed. The GLUT mediated changes ofaCa²⁺ were not threshold dependent and occurred synchronously with the onset of depolarization. A transient decrease ofaK _i ⁺ and a parallel strong increase ofaK _e ⁺ occurred during the GLUT induced depolarization. Depolarization evoked by an experimental increase ofaK _e ⁺ led to an increase ofaK _i ⁺ . The observed changes in the ionic composition of the intra-and extracellular fluids indicate that GLUT evokes an increase in membrane permeability to Na⁺ and Ca²⁺ and a subsequent influx of these ions into motoneurones, while the inward shift of Cl⁻ and the outward shift of K⁺ are presumably passive. A voltage dependent Ca²⁺ influx is triggered at −25 mV membrane potential.