Modulation of spinal cord transmission by changes in extracellular K+ activity and extracellular volume.

The neuronal activity in spinal cord in response to electrical or adequate stimulation of afferent fibres increases extracellular K+ activity. The increase during a stimulation can reach 9-10 mM (so-called ceiling level) and persists for some time even when a stimulation is discontinued. The activation of a neuronal Na-K pump is a limiting factor in stimulation-evoked increase in extracellular K+ activity and in the time course of its recovery to the resting level. Drugs that affect either the neuronal activity (picrotoxin, strychnine, GABA, 5-HT) or activity of Na-K ATPase (oubain, naloxone, morphine, enkephalins) substantially change the K+ transience. Repetitive electrical stimulation of low threshold cutaneous afferents at frequency 1-100 Hz induced transient shrinkage of extracellular space in spinal dorsal horns by 5-75%. The increase in extracellular K+ activity depolarizes the membranes of neurones, glial cells, and primary afferent fibres and may eventually lead to either facilitation or inhibition of synaptic transmission. It is also suggested that the transient poststimulation changes in extracellular volume may alter synaptic potency in spinal cord.