The electrical activity of neurons is associated with rapid changes in the pH of the brain extracellular fluid. Within milliseconds, a rise in extracelluar pH can be detected that is caused by activation of the plasma membrane calcium ATPase (PMCA), as it pumps out calcium in exchange for extracellular hydrogen ions. A significant increase in pH can occur due to the inability of the extracellular fluid to buffer the flux of hydrogen ions in a fast time frame. This is because the enzyme extracellular carbonic anhydrase, which catalyzes the buffering reaction, is limited in activity. The significance of the rapid elevation of extracellular pH lies in the sensitivity of several membrane channels to hydrogen ions. In this application we propose that the activity of the PMCA in a single neuron results in a boost of the ion flux through post-synaptic N-methyl-D-aspartate receptors (NMDARs). The preliminary data presented suggest that both the amplitude and time course of current through NMDARs are modulated by the rise in extracellular pH generated by the very same cell. Additional experiments suggest that hippocampal interneurons are also subject to modulation, due to the pH sensitivity of their post-synaptic kainate receptors. In this application interneurons will be studied for the first time in this context, with emphasis on their kainate and NMDAR-mediated synaptic currents, and the mechanisms that control the pH distribution across their plasma membrane. The consequences of these pH shifts for network activity are addressed in the final aim of this application, which investigates a form of short-term, post-synaptic potentiation induced by brief high frequency synaptic input. This potentiation is especially pronounced in animals with a knock out of the carbonic anhydrase 14 isoform, and is postulated to occur through a transient rise in extracellular pH, mediated via the PMCA. To accomplish these aims, experiments will be performed on acute mouse hippocampal slices, and cultured interneurons. Studies will make use of animals with a knockout of specific carbonic anhydrases, and will utilize transgenic mice with green fluorescence protein expression in specific interneuron populations. The physiological significance of these issues lies in the increased understanding of short-term plasticity in the brain, while its translational impact stems from the importance of rapid enhancements of excitability, which are a feature in the run up to seizure and spreading depression, pathologies that are implicated in the exacerbation tissue damage from stroke and traumatic brain injury.
This project will focus on the mechanisms by which rapid acid-base changes modulate excitatory synaptic transmission in the brain. These physiological processes are relevant to brain pathologies associated with aberrant, excessive excitability, such as epilepsy, other forms of seizure, and cortical spreading depression. As all of these events of hyper-excitability are implicated in the exacerbation of brain damage after stroke and traumatic injury, the basic studies of this project are relevant to the pathophysiology of these disorders.