Extracellular ATP-Induced Alterations in Extracellular H+ Fluxes From Cultured Cortical and Hippocampal Astrocytes

Abstract

Small alterations in the level of extracellular H+ can profoundly alter neuronal activity throughout the nervous system. In this study, self-referencing H+-selective microelectrodes were used to examine extracellular H+ fluxes from individual astrocytes. Activation of astrocytes cultured from mouse hippocampus and rat cortex with extracellular ATP produced a pronounced increase in extracellular H+ flux. The ATP-elicited increase in H+ flux appeared to be independent of bicarbonate transport, as ATP increased H+ flux regardless of whether the primary extracellular pH buffer was 26 mM bicarbonate or 1 mM HEPES, and persisted when atmospheric levels of CO2 were replaced by oxygen. Adenosine failed to elicit any change in extracellular H+ fluxes, and ATP-mediated increases in H+ flux were inhibited by the P2 inhibitors suramin and PPADS suggesting direct activation of ATP receptors. Extracellular ATP also induced an intracellular rise in calcium in cultured astrocytes, and ATP-induced rises in both calcium and H+ efflux were significantly attenuated when calcium re-loading into the endoplasmic reticulum was inhibited by thapsigargin. Replacement of extracellular sodium with choline did not significantly reduce the size of the ATP-induced increases in H+ flux, and the increases in H+ flux were not significantly affected by addition of EIPA, suggesting little involvement of Na+/H+ exchangers in ATP-elicited increases in H+ flux. Given the high sensitivity of voltage-sensitive calcium channels on neurons to small changes in levels of free H+, we hypothesize that the ATP-mediated extrusion of H+ from astrocytes may play a key role in regulating signaling at synapses within the nervous system.

Review and Hypothesis: A Potential Common Link Between Glial Cells, Calcium Changes, Modulation of Synaptic Transmission, Spreading Depression, Migraine, and Epilepsy—H+

“…Extracellular ATP also elicits increases in extracellular H + fluxes from astrocytes cultured from mouse hippocampus and rat cortex (Choi et al, 2021). Cultured cells were identified as astrocytes by high levels of the intermediate filament protein glial fibrillary acidic protein (GFAP).…”

“…A potent but commonly overlooked regulator of synaptic transmission is simple H + -that is, small changes in levels of extracellular acidity around sites of neurotransmitter release. Recent studies have shown that activation of glial cells inducing increased intracellular calcium also promotes the release of H + from glia, and it has been proposed that this may play a key role in regulating synaptic transmission (Tchernookova et al, 2018(Tchernookova et al, , 2021Choi et al, 2021). In this review and hypothesis article, we first review studies demonstrating the potency of extracellular H + as a modulator of synaptic transmission and then describe techniques used and studies conducted to show calcium-dependent extrusion of H + from glial cells activated by extracellular ATP.…”

The circadian regulation of extracellular ATP

“…The eATP regulation was involved in the inhibitory transmission of SCN synapses and mechanical pain perception as well as function recovery of astrocyte after metabolic inhibition [ 39 – 41 ]. The electrical activity and arginine vasopressin secretion rhythms of SCN negatively correlate with eATP level [ 27 , 32 , 42 ]; the eATP level altered extracellular H + flux in astrocytes of the hippocampus and cortex [ 43 ]. Thus, characterized by stable rhythm and wide participation in multiple system functions, eATP can be used as an important output signal of the circadian rhythm to investigate internal clock and circadian regulation.…”