Protons as Messengers of Intercellular Communication in the Nervous System

Abstract

In this review, evidence demonstrating that protons (H⁺) constitute a complex, regulated intercellular signaling mechanisms are presented. Given that pH is a strictly regulated variable in multicellular organisms, localized extracellular pH changes may constitute significant signals of cellular processes that occur in a cell or a group of cells. Several studies have demonstrated that the low pH of synaptic vesicles implies that neurotransmitter release is always accompanied by the co-release of H⁺ into the synaptic cleft, leading to transient extracellular pH shifts. Also, evidence has accumulated indicating that extracellular H⁺ concentration regulation is complex and implies a source of protons in a network of transporters, ion exchangers, and buffer capacity of the media that may finally establish the extracellular proton concentration. The activation of membrane transporters, increased production of CO₂ and of metabolites, such as lactate, produce significant extracellular pH shifts in nano- and micro-domains in the central nervous system (CNS), constituting a reliable signal for intercellular communication. The acid sensing ion channels (ASIC) function as specific signal sensors of proton signaling mechanism, detecting subtle variations of extracellular H⁺ in a range varying from pH 5 to 8. The main question in relation to this signaling system is whether it is only synaptically restricted, or a volume modulator of neuron excitability. This signaling system may have evolved from a metabolic activity detection mechanism to a highly localized extracellular proton dependent communication mechanism. In this study, evidence showing the mechanisms of regulation of extracellular pH shifts and of the ASICs and its function in modulating the excitability in various systems is reviewed, including data and its role in synaptic neurotransmission, volume transmission and even segregated neurotransmission, leading to a reliable extracellular signaling mechanism.

Keywords: ASIC; amygdala; cochlea; fear; vestibule labyrinth.

FIGURE 1

Extracellular proton homeostasis. Presynaptic cells express pumps and transporters that contribute to control pHi, including monocarboxylate transporters (MCT), anion exchanger (AE), Na⁺ -H⁺ exchangers (NHE), coupled sodium bicarbonate transporters (NBC/NCBTs) and the V-ATPase. During neurotransmission, the extracellular acidification of the synaptic cleft can take place with both rapid or slow kinetics. Synaptic vesicles co-release protons and neurotransmitters, also the transient incorporation of V-ATPase in the plasma membrane contribute to the extrusion of protons, producing rapid acidification of the synaptic cleft. Also intensive activity increases the energy demand of astrocytes, which increases the production of lactate and CO₂, this can diffuse freely while the lactate is transported from the astrocyte to the extracellular space by MCT, which leads to a slow extracellular acidification. Lactate may also be taken from the presynaptic neuron as a source of energy. TWIK-related K⁺ channels (TREK) are modulated both by pHe and pHi, the intracellular H⁺ increase the open probability of TREK-1, hyperpolarizing the cells while extracellular H⁺ inhibits TREK-1 and also voltage gated calcium channels (VGCC). The NBC transporters are widely expressed in neurons and the astrocytes and their action by introducing HCO₃ from the extracellular mediums, producing gradual extracellular acidification. Acid pHe can also activate postsynaptic channels like ASIC or TRPV1 while reducing the open probability of NMDAR. The regulation and actions of pHi and pHe at the synapse are discussed in more detail in Proton Homeostasis and Proton Accumulation.

FIGURE 2

ASIC structure and properties. In (A) scheme of the ASIC channel trimer in the closed and open states. Current is activated by H⁺ and carried by Na⁺ and in lower proportion by Ca²⁺. Activation of the ASIC led to a significant expansion of the central pore, due to a complex modification of the channel structure. The three lateral fenestrations would significantly contribute to ion passage into the extracellular vestibule. In (B) pH dependence of ASIC activation in DRG neurons. The current showed typical sigmoidal pH dependence with a pH₅₀ of 6.1. In (C) typical ASIC current in voltage clamp from a DRG neuron produced by pH 6.1 solution perfusion. Current reached a peak and then desensitized during the first second to a plateau of sustained current. In the lower panel in current clamp condition, the perfusion of pH 6.1 to a DRG neuron induced a series of action potentials followed by a large sustained depolarization, coinciding with the recording in voltage clamp.

FIGURE 3

Sensitivity to pH of different neuronal ion channels. (A) Inhibition by pH of different neural ionic channels span from a pH of about 6.0 to 8.0, which indicates that at normal pH (7.4) a certain percentage of receptors such as KAR, AMPAR, or NMDAR are partially blocked. (B) ASICs are activated from a range of pHs from <7.8 to 4.0; in contrast the TRPV1 channels are much less sensitive to pH. Other channels such as voltage -gated K⁺, Na⁺, and Ca²⁺ channels can increase their activity at more alkaline pHs. The potentiation or activation of different channels has a much higher range (pH 4–8.0) than inhibition.

FIGURE 4

Extracellular protons have been shown to modulate voltage-activated ionic channels in hair cell receptoneural junctions. Presynaptic K⁺ and Ca²⁺ currents are modulated by H⁺, suggesting that they may function as a synaptic feedback mechanism in hair cells. A shift in the voltage dependence of the Ca²⁺ current to a more positive membrane potential was achieved at pH < 6.8. Extracellular pH also modulates the NMDA and AMPA receptors response to afferent transmitters and interacts with ASICs located at the synaptic endings, contributing to EPSC. The end result of H⁺ interactions with ionic channels may boost the postsynaptic response and restrict the release of neurotransmitters.