Acid-sensing ion channel 1a is required for mGlu receptor dependent long-term depression in the hippocampus

Acute inhibition of acid sensing ion channel 1a after spinal cord injury selectively affects excitatory synaptic transmission, but not intrinsic membrane properties, in deep dorsal horn interneurons

Following a spinal cord injury (SCI), secondary damage mechanisms are triggered that cause inflammation and cell death. A key component of this secondary damage is a reduction in local blood flow that initiates a well-characterised ischemic cascade. Downstream hypoxia and acidosis activate acid sensing ion channel 1a (ASIC1a) to trigger cell death. We recently showed that administration of a potent venom-derived inhibitor of ASIC1a, Hi1a, leads to tissue sparing and improved functional recovery when delivered up to 8 h after ischemic stroke. Here, we use whole-cell patch-clamp electrophysiology in a spinal cord slice preparation to assess the effect of acute ASIC1a inhibition, via a single dose of Hi1a, on intrinsic membrane properties and excitatory synaptic transmission long-term after a spinal cord hemisection injury. We focus on a population of interneurons (INs) in the deep dorsal horn (DDH) that play a key role in relaying sensory information to downstream motoneurons. DDH INs in mice treated with Hi1a 1 h after a spinal cord hemisection showed no change in active or passive intrinsic membrane properties measured 4 weeks after SCI. DDH INs, however, exhibit significant changes in the kinetics of spontaneous excitatory postsynaptic currents after a single dose of Hi1a, when compared to naive animals (unlike SCI mice). Our data suggest that acute ASIC1a inhibition exerts selective effects on excitatory synaptic transmission in DDH INs after SCI via specific ligand-gated receptor channels, and has no effect on other voltage-activated channels long-term after SCI.

Possible Compensatory Role of ASICs in Glutamatergic Synapses

Proton-gated channels of the ASIC family are widely distributed in central neurons, suggesting their role in common neurophysiological functions. They are involved in glutamatergic neurotransmission and synaptic plasticity; however, the exact function of these channels remains unclear. One problem is that acidification of the synaptic cleft due to the acidic content of synaptic vesicles has opposite effects on ionotropic glutamate receptors and ASICs. Thus, the pH values required to activate ASICs strongly inhibit AMPA receptors and almost completely inhibit NMDA receptors. This, in turn, suggests that ASICs can provide compensation for post-synaptic responses in the case of significant acidifications. We tested this hypothesis by patch-clamp recordings of rat brain neuron responses to acidifications and glutamate receptor agonists at different pH values. Hippocampal pyramidal neurons have much lower ASICs than glutamate receptor responses, whereas striatal interneurons show the opposite ratio. Cortical pyramidal neurons and hippocampal interneurons show similar amplitudes in their responses to acidification and glutamate. Consequently, the total response to glutamate agonists at different pH levels remains rather stable up to pH 6.2. Besides these pH effects, the relationship between the responses mediated by glutamate receptors and ASICs depends on the presence of Mg2+ and the membrane voltage. Together, these factors create a complex picture that provides a framework for understanding the role of ASICs in synaptic transmission and synaptic plasticity.

Updates on the Physiopathology of Group I Metabotropic Glutamate Receptors (mGluRI)-Dependent Long-Term Depression

Group I metabotropic glutamate receptors (mGluRI), including mGluR1 and mGluR5 subtypes, modulate essential brain functions by affecting neuronal excitability, intracellular calcium dynamics, protein synthesis, dendritic spine formation, and synaptic transmission and plasticity. Nowadays, it is well appreciated that the mGluRI-dependent long-term depression (LTD) of glutamatergic synaptic transmission (mGluRI-LTD) is a key mechanism by which mGluRI shapes connectivity in various cerebral circuitries, directing complex brain functions and behaviors, and that it is deranged in several neurological and psychiatric illnesses, including neurodevelopmental disorders, neurodegenerative diseases, and psychopathologies. Here, we will provide an updated overview of the physiopathology of mGluRI-LTD, by describing mechanisms of induction and regulation by endogenous mGluRI interactors, as well as functional physiological implications and pathological deviations.

Triggering of Major Brain Disorders by Protons and ATP: The Role of ASICs and P2X Receptors

Adenosine triphosphate (ATP) is well-known as a universal source of energy in living cells. Less known is that this molecule has a variety of important signaling functions: it activates a variety of specific metabotropic (P2Y) and ionotropic (P2X) receptors in neuronal and non-neuronal cell membranes. So, a wide variety of signaling functions well fits the ubiquitous presence of ATP in the tissues. Even more ubiquitous are protons. Apart from the unspecific interaction of protons with any protein, many physiological processes are affected by protons acting on specific ionotropic receptors-acid-sensing ion channels (ASICs). Both protons (acidification) and ATP are locally elevated in various pathological states. Using these fundamentally important molecules as agonists, ASICs and P2X receptors signal a variety of major brain pathologies. Here we briefly outline the physiological roles of ASICs and P2X receptors, focusing on the brain pathologies involving these receptors.

Modulators of ASIC1a and its potential as a therapeutic target for age-related diseases

Age-related diseases have become more common with the advancing age of the worldwide population. Such diseases involve multiple organs, with tissue degeneration and cellular apoptosis. To date, there is a general lack of effective drugs for treatment of most age-related diseases and there is therefore an urgent need to identify novel drug targets for improved treatment. Acid-sensing ion channel 1a (ASIC1a) is a degenerin/epithelial sodium channel family member, which is activated in an acidic environment to regulate pathophysiological processes such as acidosis, inflammation, hypoxia, and ischemia. A large body of evidence suggests that ASIC1a plays an important role in the development of age-related diseases (e.g., stroke, rheumatoid arthritis, Huntington's disease, and Parkinson's disease.). Herein we present: 1) a review of ASIC1a channel properties, distribution, and physiological function; 2) a summary of the pharmacological properties of ASIC1a; 3) and a consideration of ASIC1a as a potential therapeutic target for treatment of age-related disease.

Interactive role of acid sensing ion channels and glutamatergic system in opioid dependence

Dysregulation in glutamatergic receptors and transporters has been found to mediate drugs of abuse, including morphine. Among glutamate receptors, ionotropic glutamate receptors (iGluRs) are altered with exposure to drugs of abuse. Acid-sensing ion channels (ASICs) are ligand (H+)-gated channels, which are expressed at the excitatory synaptic clefts and play a role in drug dependence. Overexpression of a specific ASIC subtype, ASIC1a, attenuated reinstatement of cocaine. ASICs are revealed to be involved in cocaine and morphine seeking behaviors, and these effects are mediated through modulation of glutamatergic receptors. In this review, we discussed the interactive role of ASICs and glutamate receptors, mainly iGluRs, in opioid dependence. ASICs are also expressed in astrocytes and are suggested to be involved on regulating glutamate uptake. However, little is known about the coupling between ASICs and the astroglial glutamate transporters. In addition, this review discussed the role of nitric oxide in the modulation of ASIC function and potentially opioid dependence. We also discussed the role of ASICs in the modulation of the function of both glutamatergic receptors in post-synaptic neurons and glutamatergic transporters in astrocytes in animals exposed to drugs of abuse.

Neurodegenerative Disease: What Potential Therapeutic Role of Acid-Sensing Ion Channels?

Acidic pH shift occurs in many physiological neuronal activities such as synaptic transmission and synaptic plasticity but also represents a characteristic feature of many pathological conditions including inflammation and ischemia. Neuroinflammation is a complex process that occurs in various neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, and Huntington’s disease. Acid-sensing ion channels (ASICs) represent a widely expressed pH sensor in the brain that play a key role in neuroinflammation. On this basis, acid-sensing ion channel blockers are able to exert neuroprotective effects in different neurodegenerative diseases. In this review, we discuss the multifaceted roles of ASICs in brain physiology and pathology and highlight ASIC1a as a potential pharmacological target in neurodegenerative diseases.

Histamine and Corticosterone Modulate Acid Sensing Ion Channels (ASICs) Dependent Long-term Potentiation at the Mouse Anterior Cingulate Cortex

Increase in proton concentration [H⁺] or decrease in local and global extracellular pH occurs in both physiological and pathological conditions. Acid-sensing ion channels (ASICs), belonging to the ENaC/Deg superfamily, play an important role in signal transduction as proton sensor. ASICs and in particular ASIC1a (one of the six ASICs subunits) which is permeable to Ca²⁺, are involved in many physiological processes including synaptic plasticity and neurodegenerative diseases. Activity-dependent long-term potentiation (LTP) is a major type of long-lasting synaptic plasticity in the CNS, associated with learning, memory, development, fear and persistent pain. Neurons in the anterior cingulate cortex (ACC) play critical roles in pain perception and chronic pain and express ASIC1a channels. During synaptic transmission, acidification of the synaptic cleft presumably due to the co-release of neurotransmitter and H⁺ from synaptic vesicles activates postsynaptic ASIC1a channels in ACC of mice. This generates ASIC1a synaptic currents that add to the glutamatergic excitatory postsynaptic currents (EPSCs). Here we report that modulators like histamine and corticosterone, acting through ASIC1a regulate synaptic plasticity, reducing the threshold for LTP induction of glutamatergic EPSCs. Our findings suggest a new role for ASIC1a mediating the neuromodulator action of histamine and corticosterone regulating specific forms of synaptic plasticity in the mouse ACC.

Role of ASIC1a in Normal and Pathological Synaptic Plasticity

Acid-sensing ion channels (ASICs), members of the degenerin/epithelial Na+ channel superfamily, are broadly distributed in the mammalian nervous system where they play important roles in a variety of physiological processes, including neurotransmission and memory-related behaviors. In the last few years, we and others have investigated the role of ASIC1a in different forms of synaptic plasticity especially in the CA1 area of the hippocampus. This review summarizes the latest research linking ASIC1a to synaptic function either in physiological or pathological conditions. A better understanding of how these channels are regulated in brain circuitries relevant to synaptic plasticity and memory may offer novel targets for pharmacological intervention in neuropsychiatric and neurological disorders.

Acid-Sensing Ion Channel 1a Is Involved in N-Methyl D-Aspartate Receptor-Dependent Long-Term Depression in the Hippocampus

Acid-sensing ion channels (ASICs), members of the degenerin/epithelial Na⁺ channel superfamily, are largely expressed in the mammalian nervous system. ASIC1a is highly permeable to Ca²⁺ and are involved in many physiological processes, including synaptic plasticity, learning, and memory. To clarify the role of ASIC1a in synaptic transmission and plasticity, we investigated N-methyl D-aspartate (NMDA) receptor-dependent long-term depression (LTD) in the CA1 region of the hippocampus. We found that: (1) ASIC1a mediates a component of ASIC1a excitatory postsynaptic currents (EPSCs); (2) ASIC1a plays a role in electrical LTD induced by LFS protocol both in P13-18 and P30-40 animals; (3) ASIC1a is involved in chemical LTD induced by brief bath application of NMDA both in P13-18 and P30-40 animals; and finally (4) a functional interaction between ASIC1a and NMDA receptors occurs during LTD. These findings suggest a new role for ASIC1a in specific forms of synaptic plasticity in the mouse hippocampus.

Fear extinction requires ASIC1a-dependent regulation of hippocampal-prefrontal correlates

Extinction of conditioned fear necessitates the dynamic involvement of hippocampus, medial prefrontal cortex (mPFC), and basolateral amygdala (BLA), but key molecular players that regulate these circuits to achieve fear extinction remain largely unknown. Here, we report that acid-sensing ion channel 1a (ASIC1a) is a crucial molecular regulator of fear extinction, and that this function requires ASIC1a in ventral hippocampus (vHPC), but not dorsal hippocampus, mPFC, or BLA. While genetic disruption or pharmacological inhibition of ASIC1a in vHPC attenuated the extinction of conditioned fear, overexpression of the channel in this area promoted fear extinction. Channelrhodopsin-2-assisted circuit mapping revealed that fear extinction involved an ASIC1a-dependent modification of the long-range hippocampal-prefrontal correlates in a projection-specific manner. Gene expression profiling analysis and validating experiments identified several neuronal activity-regulated and memory-related genes, including Fos, Npas4, and Bdnf, as the potential mediators of ASIC1a regulation of fear extinction. Mechanistically, genetic overexpression of brain-derived neurotrophic factor (BDNF) in vHPC or supplement of BDNF protein in mPFC both rescued the deficiency in fear extinction and the deficits on extinction-driven adaptations of hippocampal-prefrontal correlates caused by the Asic1a gene inactivation in vHPC. Together, these results establish ASIC1a as a critical constituent in fear extinction circuits and thus a promising target for managing adaptive behaviors.

Naked mole-rat cortical neurons are resistant to acid-induced cell death

Regulation of brain pH is a critical homeostatic process and changes in brain pH modulate various ion channels and receptors and thus neuronal excitability. Tissue acidosis, resulting from hypoxia or hypercapnia, can activate various proteins and ion channels, among which acid-sensing ion channels (ASICs) a family of primarily Na⁺ permeable ion channels, which alongside classical excitotoxicity causes neuronal death. Naked mole-rats (NMRs, Heterocephalus glaber) are long-lived, fossorial, eusocial rodents that display remarkable behavioral/cellular hypoxia and hypercapnia resistance. In the central nervous system, ASIC subunit expression is similar between mouse and NMR with the exception of much lower expression of ASIC4 throughout the NMR brain. However, ASIC function and neuronal sensitivity to sustained acidosis has not been examined in the NMR brain. Here, we show with whole-cell patch-clamp electrophysiology of cultured NMR and mouse cortical and hippocampal neurons that NMR neurons have smaller voltage-gated Na⁺ channel currents and more hyperpolarized resting membrane potentials. We further demonstrate that acid-mediated currents in NMR neurons are of smaller magnitude than in mouse, and that all currents in both species are reversibly blocked by the ASIC antagonist benzamil. We further demonstrate that NMR neurons show greater resistance to acid-induced cell death than mouse neurons. In summary, NMR neurons show significant cellular resistance to acidotoxicity compared to mouse neurons, contributing factors likely to be smaller ASIC-mediated currents and reduced NaV activity.

Role of ASIC1a in Aβ-induced synaptic alterations in the hippocampus

Acid-sensing ion channels (ASICs) are widely expressed in the mammalian central nervous system where they play a key role in synaptic transmission and in specific forms of memory. On the other hand, ASICs can be persistently active under pathological conditions contributing to neuronal damage in ischemic stroke, brain trauma, epilepsy and Parkinson's disease. However, to date no experimental evidence has linked ASICs to Alzheimer's disease (AD). Aim of the present work was to investigate, in CA1 pyramidal neurons, the possible involvement of ASIC1a in the Aβ-mediated effect on metabotropic glutamate (mGlu) receptor dependent transmission. We found that, in slices pretreated with Aβ, the pharmacological blockade of ASIC1a restored the increased intrinsic excitability following group I mGlu receptor activation. This suggests that, under certain conditions, ASIC1a might further contribute to the Aβ-related depolarizing response. We have recently demonstrated that ASIC1a is also involved long-term depression (LTD) induced either by low-frequency stimulation or by application of the group I mGlu receptor agonist DHPG. Here, we have shown that psalmotoxin-1, a selective blocker of ASIC1a, rescued the DHPG-LTD facilitation associated with genetic and non-genetic models of AD. Overall, these results suggest that a functional coupling between ASIC1a and mGlu receptors occurs and might contribute to the synaptic alterations associated with AD.