Continous electrophysiological measurements of changes in cell volume of motoneurons in the isolated frog spinal cord

One type of ion-sensitive micro-electrode (K⁺ ligand Corning 477317) is sensitive to large quaternary ammonium ions such as choline or tetramethylammonium (TMA⁺). We have now used such electrodes for continuous electrophysiological measurements of changes in cell volume of motoneurons in the isolated frog spinal cord. The electrodes were double-barrelled with tip diameters of 1 m. The reference barrel was filled with 100 mM choline or 100 mM TMA⁺ in 1 M Mg²⁺-acetate, the sensitive barrel contained the Corning K⁺ ligand. After the impalement of a motoneuron, choline or TMA⁺ diffused into the cell and about 1 h later, a steady-state concentration of these ions in the range of 10–20 mM was reached. Following this period, the motoneurons were activated by repetitive electrical stimulation or by application of amino acids via the bathing solution. All these stimuli led to a transient rise of the intracellular concentrations of choline or TMA⁺ (indicating a cell shrinkage of 3–10% difference to control volume).