Regulation of Postsynaptic Ca ²⁺ Influx in Hippocampal CA1 Pyramidal Neurons via Extracellular Carbonic Anhydrase

Abstract

Synchronous neural activity causes rapid changes of extracellular pH (pH _e ) in the nervous system. In the CA1 region of the hippocampus, stimulation of the Schaffer collaterals elicits an alkaline pH _e transient in stratum radiatum that is limited by extracellular carbonic anhydrase (ECA). When interstitial buffering is diminished by inhibition of ECA, the alkalosis is enhanced and NMDA receptor (NMDAR)-mediated postsynaptic currents can be augmented. Accordingly, the dendritic influx of Ca ²⁺ elicited by synaptic excitation may be expected to increase if ECA activity were blocked. We tested this hypothesis in the CA1 stratum radiatum of hippocampal slices from juvenile rats, using extracellular, concentric pH- and Ca ²⁺-selective microelectrodes with response times of a few milliseconds, as well as Fluo-5F imaging of intracellular Ca ²⁺ transients. Brief stimulation of the Schaffer collaterals elicited an alkaline pH _e transient, a transient decrease in free extracellular Ca ²⁺ concentration ([Ca ²⁺] _e), and a corresponding transient rise in free intracellular Ca ²⁺ concentration ([Ca ²⁺] _i). Inhibition of ECA with benzolamide caused a marked amplification and prolonged recovery of the pH _e and [Ca ²⁺] _e responses, as well as the dendritic [Ca ²⁺] _i transients. The increase in amplitude caused by benzolamide did not occur in the presence of the NMDAR antagonist APV, but the decay of the responses was still prolonged. These results indicate that ECA can shape dendritic Ca ²⁺ dynamics governed by NMDARs by virtue of its regulation of concomitant activity-dependent pH _e shifts. The data also suggest that Ca ²⁺ transients are influenced by additional mechanisms sensitive to shifts in pH _e .