High concentrations of extracellular potassium enhance bacterial endotoxin lipopolysaccharide-induced neurotoxicity in glia-neuron mixed cultures

Abstract

A sudden increase in extracellular potassium ions (K+) often occurs in cerebral ischemia and after brain trauma. This increase of extracellular K+ constitutes the basis for spreading depression across the cerebral cortex, resulting in the expansion of neuronal death after ischemic and traumatic brain injuries. Besides spreading depression, it has become clear that cerebral inflammation also is a key factor contributing to secondary brain injury in acute neurological disorders. Experiments to validate the relationship between elevated levels of extracellular K+ and inflammation have not been studied. This study aims to elucidate the roles of high concentrations of extracellular K+ in bacterial endotoxin lipopolysaccharide-induced production of inflammatory factors. Increased concentration of KCl in the medium (20 mM) significantly enhanced neurotoxicity by lipopolysaccharide in glia-neuron mixed cultures. To delineate the underlying mechanisms of increased neurotoxicity, the effects of high extracellular K+ were examined by using mixed glial cultures. KCl at 20 mM significantly enhanced nitrite, an index for nitric oxide, production by about twofold, and was pronounced from 24 to 48 h, depending on the concentration of KCl. Besides nitric oxide production of tumor necrosis factor-α was also enhanced. The augmentative effects of high KCl on the production of inflammatory factors were probably due to the further activation of microglia, since high KCl also enhanced the production of tumor necrosis factor-α in microglia-enriched cultures. The increased production of nitrite by high K+ was eliminated through use of a K+-blocker. Taken together, the results show that increases of extracellular K+ concentrations in spreading depression augment lipopolysaccharide-elicited neurotoxicity, because production of inflammatory factors such as nitric oxide and tumor necrosis factor-α are potentiated. Since spreading depression and cerebral inflammation are important in acute neurological disorders, the present results suggest a biochemical mechanism: elevated extracellular K+ concentrations augment glial inflammatory responses, and thus the neurotoxicity.