pH transients evoked by excitatory synaptic transmission are increased by inhibition of extracellular carbonic anhydrase.

Abstract

December 19, 1991) ABSTRACT Excitatory synaptic transmission has been associated with a rapid alkalinition of the brain extracellular space. These pH shifts are markedly increased by acetazolamide, an inhibitor of carbonic anhydrase. Although this effect can be readily explained by inhibition of extracellular carbonic anhydrase, this enzyme has been considered strictly intracellular in the central nervous system. To determine whether these alkaline shifts are regulated by extracellular carbonic anhydrase, we studied the effects of a membrane impermeant, dextran-bound inhibitor of this enzyme. Extraceflular alkaline transients, measured with pH-sensitive microelectrodes, were generated in the CAl region of rat hippocampal slices by repetitive electrical stimulation of Schaeffer collateral fibers or by local ejection of glutamate. More direct lkalinizations were elicited by focal ejection of NaOH in the vicinity of a pH microelectrode. These pH transients were reversibly enhanced by addition of the dextran-bound inhibitor. We conclude that there is significant carbonic anhydrase activity in the extracellular space of the brain. We postulate that this enzyme functions in the regulation and modulation of extracellular pH transients associated with neuronal activity.The enzyme carbonic anhydrase (CA) catalyzes the reversible hydration of CO2 and plays an important role in the function of erythrocytes and secretory epithelia (1). In the central nervous system, CA is abundant (2) and has been localized predominantly within glia (3-5) and choroid plexus (6). However, apart from its role in the secretion of cerebral spinal fluid (7,8), the functions of CA in the brain have remained obscure.While most forms of CA are intracellular, an extracellular isoform exists in several tissues, including kidney, lung, and muscle (9). Although there is no morphological evidence for extracellular CA in the brain, inhibitors of CA have been shown to have a large effect on the behavior of extracellular pH. Sulfonamide derivatives, such as acetazolamide, greatly increased the alkaline shifts associated with synaptic transmission (10-13). These effects could not be explained by enhancement of electrophysiological responses, since corresponding field potentials were unaffected. However, it remained unclear whether the action of acetazolamide was principally intracellular or extracellular, as this agent can readily penetrate cell membranes (1).It was subsequently noted that the effects of acetazolamide could be simply explained by a decrease in the extracellular buffering capacity (14). Experimental manipulation of the extracellular buffering power supported this view: synaptically evoked alkaline shifts were enhanced when bicarbonatebuffered saline was replaced by poorly buffered solutions containing Hepes (15,16). Although a role of metabolically generated CO2 could not be ruled out, the amplification and persistence of these alkalinizations in Hepes-buffered media suggested that shifts in CO2 and bicarbonate were not immediately respo...

GPI-anchored carbonic anhydrase IV displays both intra- and extracellular activity in cRNA-injected oocytes and in mouse neurons

“…4) (39). Functional extracellular CA activity was found in hippocampal slices, where the enzyme was implicated in the regulation of excitatory transmission (40), and again in hippocampal neurons, where the anion exchanger AE3 activity was enhanced by enzymatic activity (29), and for H + buffering (41). None of these studies addressed the intracellular activity of CA IV.…”

Synaptic Glutamate Release Is Modulated by the Na+-Driven Cl−/HCO3−Exchanger Slc4a8

“…Likewise, mice with a targeted disruption of Slc4a10 were protected from seizure-related mortality (Jacobs et al, 2008), whereas the sensitivity to seizure-inducing agents was increased in mice with a disruption of the Na ϩ -independent Cl Ϫ /HCO 3 Ϫ exchanger Slc4a3, which imposes an intracellular acid load (Hentschke et al, 2006). Some anticonvulsants, such as acetazolamide, reduce extracellular pH in the brain (Chen and Chesler, 1992), suggesting that acidosis may contribute to their anti-epileptic effects. The increased seizure threshold in KO mice suggests that inhibition of Slc4a8 may be a target for anti-epileptic therapy.…”

Expression, Assay, and Structure of the Extracellular Domain of Murine Carbonic Anhydrase XIV

“…CA XIV is found in regions of liver cells distinct from the location of CA IV (10), although certain regions of the kidney show positive immunostaining for both of these isozymes, suggesting redundant function (8). Intriguingly, the presence of an extracellular carbonic anhydrase has long been suspected in mammalian brain (12,13). Known CA inhibitors, including compounds that are impermeable to cells, were shown to enhance the extracellular alkaline shift observed in slices of hippocampus after synaptic transmission (12)(13)(14).…”

“…Intriguingly, the presence of an extracellular carbonic anhydrase has long been suspected in mammalian brain (12,13). Known CA inhibitors, including compounds that are impermeable to cells, were shown to enhance the extracellular alkaline shift observed in slices of hippocampus after synaptic transmission (12)(13)(14). Recent immunostaining results identify CA XIV on neurons and axons in both mouse and human brain, suggesting that this isozyme is responsible for modulating pH shifts during excitatory synaptic transmission (9).…”