Na+ and K+ concentrations, extra- and intracellular voltages, and the effect of TTX in hypoxic rat hippocampal slices

Severe hypoxia causes rapid depolarization of CAI neurons and glial cells t hat resembles spreading depression (SD). In brain slices in vitro, the SD-l ike depolarization and the associated irreversible loss of function can be postponed, but not prevented, by blockade of Na+ currents by tetrodotoxin ( TTX). To investigate the role of Na+ flux, we made recordings from the CA1 region in hippocampal slices in the presence and absence of TTX. We measure d membrane changes in single CAI pyramidal neurons simultaneously with extr acellular DC potential (V-o) and either extracellular [K+] or [Na+]; altern atively, we simultaneously recorded [Na+](o), [K+](o), and V-o. Confirming previous reports, early during hypoxia, before SD onset, [K+], began to ris e, whereas [Na+](o) still remained normal and V-o showed a slight, gradual, negative shift; neurons first hyperpolarized and then ben,an to gradually depolarize. The SD-like abrupt negative Delta V-o corresponded to a near co mplete depolarization of pyramidal neurons and an 89% decrease in input res istance. [K+], increased by 47 mM and [Na+](o) dropped by 91 mM. Changes in intracellular Na+ and K+ concentrations, estimated on the basis of the mea sured extracellular ion levels and the relative volume fractions of the neu ronal, glial, and extracellular compartment, were much more moderate. Becau se [Na+], dropped more than [K+], increased, simple exchange of Na+ for Kcannot account for these ionic changes. The apparent imbalance of charge co uld be made up by Cl- influx into neurons paralleling Na+ flux and release of Mg2+ from cells. The hypoxia-induced changes in interneurons resembled t hose observed in pyramidal neurons. Astrocytes responded with an initial sl ow depolarization as [K+](o) rose. It was followed by a rapid but incomplet e depolarization as soon as SD occurred, which could be accounted for by th e reduced ratio, [K+](i)/ [K+](o), TTX (1 mu M) markedly postponed SD, but the SD-related changes in [K+](o) and [Na+](o) were only reduced by 23 and 12%, respectively. In TTX-treated pyramidal neurons, the delayed SD-like de polarization took off from a more positive level, but the final depolarized intracellular potential and input resistance were not different from contr ol. We conclude that TTX-sensitive channels mediate only a fraction of the Na+ influx, and that some of the K+ is released in exchange for Na+. Even t hough TTX-sensitive Na+ currents are not essential for the self-regenerativ e membrane changes during hypoxic SD, in control solutions their activation may trigger the transition from gradual to rapid depolarization of neurons , thereby synchronizing the SD-Like event.