A chemical may be classified as a neurotransmitter if it meets the following criteria [1]:

1. The chemical is present in the presynaptic cell.
2. Stimulation of the cell results in release of the chemical.
3. It is available in sufficient quantity in the presynaptic neuron to affect the postsynaptic neuron.
4. There are postsynaptic receptors and the chemical is able to bind to them.
5. A biochemical mechanism for inactivation is present.
6. Exogenous application of the chemical must mimic the endogenous response.
7. Blocking the receptor blocks the activity of neurotransmitter.

Studies raise the possibility that extracellular protons might act as a neurotransmitter. #1 is consistent with the presence of protons in presynaptic vesicles [2], although whether those protons reduce synaptic pH is uncertain. #2 and #3 have been tested recently [3] . #4 is consistent with the location of the proton receptor acid-sensing ion channel 1a (ASIC1a) in postsynaptic spines [4]. #5 is consistent with the finding that protons induce transient ASIC1a currents [3][5]. #6 and #7 are supported by our previous data that extracellular application of protons induces long-term potentiation (LTP) in wild-type mice, and LTP is impaired in ASIC1a-/- mice [3]. However, other mechanisms might also alter synaptic pH, including neuron and glia metabolism, Na+/H+ exchanger activity, Cl-/HCO3- exchanger activity, lactate production, etc. Thus, whether or not protons are a neurotransmitter, the data will provide important new insight into how alterations in pH control neural function.

Although overall changes in extracellular pH in the brain are tightly balanced by homeostatic mechanisms, pH fluctuations in specific micro-regions, such as the synaptic cleft, may be dramatic [2]. Changes in interstitial pH have been shown in a variety of preparations. The Chesler lab detected pH increases in CA1 hippocampal slices during the antidromic stimulation of CA1 neuronal population using a concentric pH microelectrode and showed that local alkaline transients lasted for seconds [6]. However, other experimental paradigms revealed that synchronous activation of nerve cells induces a rapid acidification that can precede or preclude early alkaline transients [7]. In these experiments, it was suggested that the acidified synaptic vesicles (pH 5.67) transiently influence local extracellular pH upon vesicle release [2]. Consistent with this, a rapid, apparent acid transient was recorded with the synaptic transmission in hippocampal slices following the stimulation of Schaffer collaterals, although a later study using a fluorescein-dextran probe to measure pH was only able to detect an alkaline shift within the same time frame [8]. In strong support of a model where an acid transient occurs in the synaptic cleft due to the release of protons from synaptic vesicles, a patch clamp study showed that vesicular protons feedback to block nearby presynaptic pH-sensitive Ca2+ channels [9]. The differing observations in these previous studies of how pH changes with synaptic transmission are due in part to the limitations of previous techniques for measuring pH changes that occur in a micro-region and on a rapid time scale. Thus, studies that incorporate new techniques to measure local extracellular pH changes during synaptic transmission are necessary to address this question.

There are two major counteracting processes that control pH in the brain. Briefly, the aerobic and anaerobic utilization of glucose in neuron and glia metabolism generates CO₂, and/or lactic acid, which results in an acidic pH shift. In response to these changes in metabolism, an enhancement of neural activity causes an increase in local blood ?ow that facilitates the clearance of CO₂, which is expired through the respiratory system, leading to local alkaline pH shifts. The rapid dynamics of metabolism and CO2 clearance suggest that physiological changes in brain pH may have significant consequences for behavior, learning, and memory [6]. An intriguing idea that has emerged from studies of pH-dependent alteration in excitability is that highly localized pH transients might play a signaling role in neuronal communication. It is quite possible that changes in neuronal excitability and synaptic plasticity, hitherto solely attributed to intracellular Ca2+ transients, may include a significant component mediated by pH shift [10], and that protons may function as a neurotransmitter to effect these changes.

Acid-sensing ion channels (ASICs) are members of the degenerin/epithelial Na+ channel (DEG/ENaC) family. To date, six proteins of the ASIC family have been identified (ASIC1a, ASIC1b, ASIC2a, ASIC2b, ASIC3 and ASIC4). ASICs assemble as homo- or hetero-trimers to form proton-gated, voltage-insensitive, Na+ and Ca2+ permeable channels that are activated by extracellular protons [11]. ASIC1a is expressed in many areas in the brain, and previous studies in mice showed that it contributes to many brain functions and disorders; these include hippocampal learning and memory, anxiety, depression, stroke, neurodegeneration, seizures, Inflammation, and nerve injury [12]. Recent studies indicated that ASIC1a is particularly abundant in the amygdala and other fear circuit structures and is required for normal responses in tests of both conditioned and unconditioned fear behavior [13]. Also, some studies showed that ASIC1a is located postsynaptically and is required for synaptic plasticity [3].

Keywords:Acid-sensing ion channels, Neurotransmitter, Protons, Synaptic transmission

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