http://physrev.physiology.org/ Downloaded from ANKER JON HANSEN Volume 65 A. Methodology: Ion-Sensitive Microelectrode Much of the data described in this review stems from experiments with ion-sensitive microelectrodes (ISM). The ISM mechanism and the results of in vivo measurements in brain tissue are briefly described next.

Summary:We have examined the extracellular pH (pH e) during spreading depression and complete cerebral isch emia in rat parietal cortex utilizing double-barrelled H + liquid ion exchanger microelectrodes. The baseline pHe of the parietal cortex was 7.33 at a mean arterial Peoz of 38 mm Hg. Following spreading depression and cerebral ischemia, highly reproducible triphasic changes in pHe occurred, which were intimately related to the negative deflection in tissue potential (Ye). The changes in pHe for spreading depression (n = 23) were a small initial acidic shift, beginning before the rapid change in Ye, followed by a rapid transient alkaline shift of 0.16 pH units, the onset of which coincided with the negative deflection in Ye• A prolonged acidic shift of 0.42 pH units then oc curred. The maximal decrease in pHe was to 6.97 and the mean duration of the triphasic pHe change was 7.8 min. The lactate concentration in brain cortex increased fromThe adaptive response by the brain microenvi ronment to altered extracellular pH (pHe) remains poorly understood, partially due to the difficulty in directly measuring the interstitial hydrogen ion con centration. Now, however, high-fidelity recordings of pH changes in mammalian brain interstitium can be obtained with double-barrelled H+ liquid ion ex changer (LIX) microelectrodes (Ammann et aI., 1981; Kraig et aI., 1983). The use of similar LIX microelectrodes has greatly facilitated study of Dr. Mutch's present address is Department of Anesthesia, St. Boniface General Hospital, 40 9 Ta che Avenue, Winnipeg, Man itoba, Canada R2H 2A6.Address correspondence and reprint requests to Dr. Hansen at Department of Medical Physiology A, University of Copen hagen, The Panum Institute, B1egdamsvej 3, DK-22 00, Copen hagen N, Denmark.Abbreviations used: DIDS, 4,4'-Diisothiocyanostilbene-2,2' disulfonic acid; DNDS, 4, 4' -dinitrostilbene-2,2' -disulfonic acid; EH+, potential derived from H+ activity; LlX, liquid ion ex changer; pHe, extracellular pH; Ve, tissue potential. 17 baseline 1.2 mM to 7.0 mM (n = 6) during the maximal acidic change in spreading depression. In addition, lactate levels correlated well with resolution of the pHe changes during spreading depression. The triphasic pHe changes following complete cerebral ischemia were an initial acidic shift of 0.43 pH units which developed over 2 min, then an alkaline shift of 0.10 pH units coincident with the negative deflection in Ye, and a final acidic shift of 0.26 pH units. The terminal pHe was 6.75. Superfusion of the cortex with inhibitors of carbonic anhydrase (acetazol amide), Na+/H+ counter transport (amiloride), and Cl-/ HC03' countertransport (4,4' -diisothiocyanostilbene-2,2' disulfonic acid) altered the triphasic pHe changes in a similar fashion for both spreading depression and cerebral ischemia, providing insights into the pHe regulatory mechanisms in mammalian brain.

Cerebral blood flow during and after spreading depression (SD) was studied in rat brain by quantitative autoradiography. The rise of cortical blood flow during SD was followed by 20 to 25% reduction below normal after SD, lasting at least an hour. Blood flow in th putamen, thalamus, and hippocampus did not change at any time during or after SD. Previous measurements of cerebral blood flow in humans showed that migraine attacks may be accompanied by wavelike spreading oligemia (Olesen et al, 1981). We speculate that the spreading oligemia of migraine may be a phenomenon physiologically related to the present finding of an oligemia after SD.

Brain ion homeostasis is severely perturbed during spreading depression of Leao and during anoxia. The ionic composition of the extracellular space changes abruptly and approaches the intracellular concentrations owing to an increase in cell permeability. In spreading depression, synchronous transmitter efflux caused by a depolarization of the presynaptic terminals has been implicated as a possible mechanism that would explain the concomitant movement of ions. Anoxia, having many features in common with spreading depression, may follow the same mechanism. We have measured the concentrations of extracellular potassium with ion-selective microelectrodes and dopamine by in vivo voltammetry with carbon fiber microelectrodes during spreading depression and anoxia to compare the temporal relationship between the release of dopamine and ion movements in the striatum. There is a pronounced release of dopamine during both spreading depression and anoxia. In spreading depression, the sharp increase of potassium concentration that follows an initial smaller and slower increase of potassium is accompanied by the release of dopamine. In anoxia, the dopamine release clearly precedes the fast rise of extracellular potassium concentration. We conclude that in striatum, there is a pronounced dopamine release during spreading depression and anoxia, but that the relationships between ionic changes and transmitter release for these two phenomena are different and probably reflect different mechanisms.

Purulent meningitis is a serious disease that often has a lethal outcome or gives lasting complications due to brain damage. The processes causing brain dysfunction or damage are still not uncovered nor are the reasons for the characteristic increase of CSF lactate, or the decrease of glucose levels and of pH. We studied rabbits with experimentally induced purulent meningitis (Streptococcus pneumoniae). Ten hours after the inoculation into cisterna magna the rabbits developed symptoms of meningitis, with stiffness of the neck, tachypnea, and fever. The CSF level of lactate and the number of leukocytes were significantly increased and the glucose level was decreased. Brain interstitial pH, as measured by ion selective microelectrodes, was significantly decreased from the normal level of 7.4 to 6.9. The levels of energy metabolites in brain cortex, including glucose, were not different between controls and infected animals, and the lactate level was not elevated more than could have been explained by passive diffusion from the CSF. This shows that the brain tissue is not the source of CSF lactate nor the sink for glucose in CSF. The marked acidification of brain interstitial space and CSF demonstrates that purulent meningitis causes a significant disturbance of brain ion homeostasis that could be, at least in part, responsible for the brain dysfunction. We suggest that activated leukocytes consume CSF glucose and produce lactic acid and secrete protons, which causes the CSF and interstitial acidosis.