Extracellular potassium accumulation and transmission in frog spinal cord

Abstract

In the isolated frog spinal cord, intracellular recordings from motoneurons and neuroglia and extracellular recordings from dorsal and ventral roots were used to compare electrophysiological changes produced by tetanic stimulation of dorsal roots with those resulting from increasing extracellular potassium concentration, [K +] 0. Many of the after-effects of tetanic dorsal root stimulation could be mimicked by increasing [K +] 0. These include the following: depolarization of motoneurons and neuroglia, prolongation and depression of excitatory postsynaptic potentials, depression of dorsal root potentials, facilitation and depression of ventral root potentials, as well as increases in spontaneous synaptic activity and depression of antidromic spikes recorded from motoneurons. The levels of [K +] 0 necessary to produce effects comparable to those following maximal dorsal root stimulation are about twice those estimated from measurements with K-selective microelectrodes or glial depolarization, suggesting that both these methods underestimate the amount of potassium which accumulates in the narrow intercellular spaces between neurons and glia in the intermediate region of the spinal cord. It is concluded that, at least within the isolated frog spinal cord, K + accumulation is a significant factor modifying transmission following dorsal root tetanic stimulation and that its distribution is inhomogeneous.