HALOTHANE, BUT NOT ALPHA-CHLORALOSE, BLOCKS POTASSIUM-EVOKED CORTICALSPREADING DEPRESSION IN CATS

The effects of two anesthetics, halothane and cy-chloralose, on induct ion of spreading depression and on extracellular glutamate elevation a fter intracortical potassium administration were investigated in artif icially ventilated (30% oxygen/70% nitrous oxide) cats. High potassium concentrations were achieved using either direct KCl injections (7 mu l, 150 mM via a micropipette) or microdialysis by supplementing 100, 300 or 500 mM KCl, respectively, for 10 min to the perfusion solution (Ringer's). Changes of the cortical DC potential were recorded adjacen t (1-2 mm: electrode DC1) and distant (6-7 mm: electrode DC2) to the i njection site. Either under halothane (0.75% in the respiratory gas mi xture) or under alpha-chloralose (60 mg/kg i.v.) anesthesia, prolonged negative shifts of the DC potential reflecting the elevated potassium levels after KCl injection were measured near the injection site (ele ctrode DC1). In contrast, spreading depressions (transient short DC de flections) were almost exclusively observed under alpha-chloralose. Sp reading depressions recorded with electrode DC1 were superimposed on t he prolonged negative DC shifts and they propagated frequently to the more distant site (DC2). Upon KCl administration, dose dependent eleva tions of extracellular glutamate were measured. These elevations were not significantly altered by the type of anesthesia. Our results sugge st that in cats, spreading depression induction is affected by anesthe sia, i.e, spreading depression induction is inhibited by halothane as compared to alpha-chloralose. Furthermore, factors other than glutamat e or high potassium seem to contribute to spreading depression inducti on.