Modulation of α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid Receptor Desensitization by Extracellular Protons

Abstract

The interstitial milieu of the brain is buffered to an average pH of 7.3, but synaptic activation produces a temporal sequence of events that can affect pH in the synaptic cleft. Furthermore, pathophysiological processes such as ischemia and seizures produce global and prolonged acidification of interstitial pH. Changes in pH, in turn, can affect neuronal excitability by modulating receptors and channels. Patch-clamp recordings were made from cultured rat hippocampal neurons to determine whether physiologically relevant changes in interstitial pH (6.5-7.8) significantly affect AMPA receptor function. Acidic pH, such as that typically associated with ischemia (pH 6.5), significantly inhibited AMPA receptor-mediated responses in neurons. The effect of pH was agonist-dependent, with 2-fold greater inhibition of responses evoked by the strongly desensitizing agonists glutamate and quisqualate than the weakly desensitizing agonist kainate. Additional experiments tested the hypothesis that protons modulate AMPA receptor desensitization. In the presence of drugs that block AMPA receptor desensitization, pH 6. 5 had no effect on glutamate-evoked responses. In neuronal macropatches, protons increased equilibrium desensitization without affecting macroscopic desensitization or deactivation kinetics. The mechanisms and molecular determinants of pH-mediated effects were further investigated using human embryonic kidney 293 cells expressing recombinant AMPA receptors. Inhibition of kainate-evoked responses varied with subunit and isoform composition, ranging from 10% to >40%. Flop isoforms, which exhibit faster and more extensive desensitization, were most strongly inhibited. These findings suggest that interstitial acidification can modulate AMPA receptor-mediated synaptic transmission and that differences in receptor sensitivity to proton modulation may underlie the selective vulnerability of certain neuronal populations to ischemia.

Glutamate Receptor Ion Channels: Structure, Regulation, and Function

“…Mutations at the same LBD dimer interface residues that alter Zn 2ϩ inhibition also strongly influence proton inhibition (Gielen et al, 2008), supporting a functional link between these two forms of modulation and confirming the hypothesis that Zn 2ϩ binding enhances proton inhibition. The potential link between proton inhibition and NMDA receptor LBD dimer interface stability fits well with previous observations that protons modify GluA receptor desensitization (Ihle and Patneau, 2000;Lei et al, 2001), a process unequivocally shown to involve rearrangement at the LBD dimer interface (see sections II.D and VII.E). first described a rapid component of desensitization in the presence of extracellular Zn 2ϩ and postulated that Zn 2ϩ accelerated receptor desensitization.…”

“…Protons inhibit all glutamate receptors without changing ionization of the agonist (Christensen and Hida, 1990;Giffard et al, 1990;Tang et al, 1990;Traynelis and Cull-Candy, 1990;Vyklický et al, 1990; Ihle and Patneau, 2000;Lei et al, 2001;Mott et al, 2003). Among NMDA receptors, proton IC 50 for inhibition varies with the GluN2 subunit, with IC 50 values near physiological pH for GluN2A, GluN2B, and GluN2D (7.0 -7.4), leading to the idea that these receptors are under tonic inhibition Gielen et al, 2009).…”

“…Extracellular protons inhibit AMPA receptors in a voltage-independent manner by enhancing receptor desensitization and lowering channel open probability (Christensen and Hida, 1990; Ihle and Patneau, 2000;Lei et al, 2001). Proton inhibition varies with receptor subunit and flip/flop isoform, because GluA4 flop receptors are most sensitive to proton inhibition (Ihle and Patneau, 2000).…”

Deactivation kinetics of acid-sensing ion channel 1a are strongly pH-sensitive

“…The synaptic alkalization that follows brief stimulus bouts would likely increase the amplitude of subsequent ASIC EPSCs, as well as speeding their decay kinetics and possibly allowing higher frequency signaling (20). However, such conditions also promote the gating of other excitatory NGICs (68)(69)(70), potentially masking any contribution from ASICs. More acidic circumstances would prolong the subsequent ASIC EPSC but would also lead to steady-state inactivation, producing a small-amplitude, long-duration inward current lasting several hundred milliseconds.…”

Stargazin Modulates Native AMPA Receptor Functional Properties by Two Distinct Mechanisms

“…Primary cultures of hippocampal neurons were prepared according to established methods (Ihle and Patneau, 2000). Briefly, hippocampal tissue dissected from the brains of newborn [postnatal day 0 (P0) to P1] Sprague Dawley rats was incubated in papain and dissociated by trituration with a glass pipette.…”