The Journal of Neurophysiology Vol. 82 No. 4 October 1999, pp. 1902-1908
Copyright ©1999 by the American Physiological Society

Effect of Divalent Cations on AMPA-Evoked Extracellular Alkaline Shifts in Rat Hippocampal Slices

Department of Physiology and Neuroscience and Department of Neurosurgery, New York University School of Medicine, New York, New York 10016

Smith, S. E. and M. Chesler. Effect of Divalent Cations on AMPA-Evoked Extracellular Alkaline Shifts in Rat Hippocampal Slices. J. Neurophysiol. 82: 1902-1908, 1999. The generation of activity-evoked extracellular alkaline shifts has been linked to the presence of external Ca²⁺ or Ba²⁺. We further investigated this dependence using pH- and Ca²⁺-selective microelectrodes in the CA1 area of juvenile, rat hippocampal slices. In HEPES-buffered media, alkaline transients evoked by pressure ejection of RS--amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) averaged ~0.07 unit pH and were calculated to arise from the equivalent net addition of ~1 mM strong base to the interstitial space. These alkaline responses were correlated with a mean decrease in [Ca²⁺]_o of ~300 µM. The alkalinizations were abolished reversibly in zero-Ca²⁺ media, becoming indiscernible at a [Ca²⁺]_o of 117 ± 29 µM. Addition of as little as 30-50 µM Ba²⁺ caused the reappearance of an alkaline response. In approximately one-fourth of slices, a persistent alkaline shift of ~0.03 unit pH was observed in zero-Ca²⁺ saline containing EGTA. In HEPES media, addition of 300 µM Cd²⁺, 100 µM Ni²⁺, or 100 µM nimodipine inhibited the alkaline shifts by roughly one-half, one-third, and one-third, respectively, whereas Cd⁺ and Ni²⁺ in combination fully blocked the response. In bicarbonate media, by contrast, Cd⁺ and Ni²⁺ blocked only two-thirds of the response. In the presence of bicarbonate, Ni²⁺ caused an unexpected enhancement of the alkalinization by ~150%. However, when the extracellular carbonic anhydrase was blocked by benzolamide, addition of Ni²⁺ reduced the alkaline shift. These results suggested that Ni²⁺ partially inhibited the interstitial carbonic anhydrase and thereby increased the alkaline responses. These data indicate that an activity-dependent alkaline shift is largely dependent on the entry of Ca²⁺ or Ba²⁺ via voltage-gated calcium channels. However, sizable alkaline transients still can be generated with little or no external presence of these ions. Implications for the mechanism of the activity-dependent alkaline shift are discussed.