Regional differences in resting extracellular potassium levels of rat brain.

The extracellular potassium concentration in the cerebral cortex of the mammalian brain has been repeatedly reported to be approximately 3.0 mM. We have made detailed measurements with potassium-selective microelectrodes and have found significantly lower extracellular potassium concentrations in unstimulated rat brain caudate and thalamus (1.9-2.5 mM) when compared to cortex and cerebral spinal fluid (3.0-3.5 mM). These regional differences may be caused by variations in spontaneous activity of neurons, regional permeability differences in endothelial cells of brain capillaries to potassium, or caused by variations in uptake by glia.