Multiprobe monitoring of ionic, metabolic, and electrical activities in the awake brain

A new approach for studying brain metabolic, ionic, and electrical activities in the awake animal is described. We developed a multiprobe assembly holding electrodes for extracellular K⁺, pH, DC potential, electrocorticogram, and temperature as well as light guide for monitoring intramitochondrial NADH oxidation-reduction state. The assembly was designed so that because of the type of electrodes used and the protection system around them, the same system could be used in many experiments. The results presented here are typical and show the potential use of the multiprobe approach for studying the effects of hypoxia, anoxia, spreading depression, and ischemia on the awake brain. From the results obtained the following conclusions can be drawn. 1) The DC correction for the K⁺ and H⁺ measurements is necessary, although it was not as good for the H⁺ as for the K⁺ signals. 2) During ischemia (complete or partial) a clear acidification of the brain was found in correlation with the decrease in oxygen availability as evaluated by the NADH fluorescence signal. 3) During brain activation (induced by spreading depression) extracellular K⁺ was elevated and then actively pumped back into the cells. The NADH showed an oxidation response, and the pH response started with an alkalinization followed by a short acidification.