The ionic mechanism of the excitatory action of glutamate upon the membranes of motoneurones of the frog.
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 (alpha K+) in the bathing solution evoked a decrease of intracellular Na+ activity (alpha Na+i); a transient increase of alpha Na+i accompanied by a decrease of alpha Na+e was observed during the depolarization induced by GLUT. Both modes of depolarization led to an increase of alpha Cl-i and a concomitant decrease of alpha Cl-e. An experimental increase of alpha K+e led to a threshold dependent increase of alpha Ca2+i by at least one order of magnitude and to an equally threshold dependent strong decrease of alpha Ca2+e. The threshold of these changes of alpha Ca2+ was at a membrane potential of -25 mV. During a depolarization of half the amplitude induced by GLUT a comparable increase of alpha Ca2+i and a smaller decrease of alpha Ca2+e were observed. The GLUT mediated changes of alpha Ca2+ were not threshold dependent and occurred synchronously with the onset of depolarization. A transient decrease of alpha K+i and a parallel strong increase of alpha K+e occurred during the GLUT induced depolarization. Depolarization evoked by an experimental increase of alpha K+e led to an increase of alpha K+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.
D000818:Animals; D001696:Biomechanical Phenomena; D002118:Calcium / Q000502:physiology; D002462:Cell Membrane / Q000187:drug effects; D002712:Chlorides / Q000502:physiology; D004553:Electric Conductivity; D005971:Glutamates / Q000494:pharmacology*; D007477:Ions / Q000502:physiology*; D009046:Motor Neurons / Q000187:drug effects*; D011188:Potassium / Q000502:physiology; D011893:Rana esculenta / Q000502:physiology*; D012964:Sodium / Q000502:physiology
D002712:Chlorides; D005971:Glutamates; D007477:Ions; D012964:Sodium; D011188:Potassium; D002118:Calcium