The amygdala is a chemosensor that detects carbon dioxide and acidosis to elicit fear behavior. Cell. 2009;139:1012-1021. [PMID: 19945383 DOI: 10.1016/j.cell.2009.10.029]

Differential behavioral sensitivity to carbon dioxide (CO2) inhalation in rats

Inhalation of carbon dioxide (CO₂) is frequently employed as a biological challenge to evoke intense fear and anxiety. In individuals with panic disorder, CO₂ reliably evokes panic attacks. Sensitivity to CO₂ is highly heterogeneous among individuals, and although a genetic component is implicated, underlying mechanisms are not clear. Preclinical models that can simulate differential responsivity to CO₂ are therefore relevant. In the current study we investigated CO₂-evoked behavioral responses in four different rat strains: Sprague-Dawley (SD), Wistar (W), Long Evans (LE) and Wistar-Kyoto, (WK) rats. We also assessed tryptophan hydroxylase 2 (TPH-2)-positive serotonergic neurons in anxiety/panic regulatory subdivisions of the dorsal raphe nucleus (DR), as well as dopamine β hydroxylase (DβH)-positive noradrenergic neurons in the locus coeruleus, implicated in central CO₂-chemosensitivity. Behavioral responsivity to CO₂ inhalation varied between strains. CO₂-evoked immobility was significantly higher in LE and WK rats as compared with W and SD cohorts. Differences were also observed in CO₂-evoked rearing and grooming behaviors. Exposure to CO₂ did not produce conditioned behavioral responses upon re-exposure to CO₂ context in any strain. Reduced TPH-2-positive cell counts were observed specifically in the panic-regulatory dorsal raphe ventrolateral (DRVL)-ventrolateral periaqueductal gray (VLPAG) subdivision in CO₂-sensitive strains. Conversely, DβH-positive cell counts within the LC were significantly higher in CO₂-sensitive strains. Collectively, our data provide evidence for strain dependent, differential CO₂-sensitivity and potential differences in monoaminergic systems regulating panic and anxiety. Comparative studies between CO₂-vulnerable and resistant strains may facilitate the mechanistic understanding of differential CO₂-sensitivity in the development of panic and anxiety disorders.

Expression of acid-sensing ion channels and selection of reference genes in mouse and naked mole rat

Acid-sensing ion channels (ASICs) are a family of ion channels comprised of six subunits encoded by four genes and they are expressed throughout the peripheral and central nervous systems. ASICs have been implicated in a wide range of physiological and pathophysiological processes: pain, breathing, synaptic plasticity and excitotoxicity. Unlike mice and humans, naked mole-rats do not perceive acid as a noxious stimulus, even though their sensory neurons express functional ASICs, likely an adaptation to living in a hypercapnic subterranean environment. Previous studies of ASIC expression in the mammalian nervous system have often not examined all subunits, or have failed to adequately quantify expression between tissues; to date there has been no attempt to determine ASIC expression in the central nervous system of the naked mole-rat. Here we perform a geNorm study to identify reliable housekeeping genes in both mouse and naked mole-rat and then use quantitative real-time PCR to estimate the relative amounts of ASIC transcripts in different tissues of both species. We identify RPL13A (ribosomal protein L13A) and CANX (calnexin), and β-ACTIN and EIF4A (eukaryotic initiation factor 4a) as being the most stably expressed housekeeping genes in mouse and naked mole-rat, respectively. In both species, ASIC3 was most highly expressed in dorsal root ganglia (DRG), and ASIC1a, ASIC2b and ASIC3 were more highly expressed across all brain regions compared to the other subunits. We also show that ASIC4, a proton-insensitive subunit of relatively unknown function, was highly expressed in all mouse tissues apart from DRG and hippocampus, but was by contrast the lowliest expressed ASIC in all naked mole-rat tissues.

ASIC1a regulates insular long-term depression and is required for the extinction of conditioned taste aversion

Acid-sensing ion channel 1a (ASIC1a) has been shown to play important roles in synaptic plasticity, learning and memory. Here we identify a crucial role for ASIC1a in long-term depression (LTD) at mouse insular synapses. Genetic ablation and pharmacological inhibition of ASIC1a reduced the induction probability of LTD without affecting that of long-term potentiation in the insular cortex. The disruption of ASIC1a also attenuated the extinction of established taste aversion memory without altering the initial associative taste learning or its long-term retention. Extinction of taste aversive memory led to the reduced insular synaptic efficacy, which precluded further LTD induction. The impaired LTD and extinction learning in ASIC1a null mice were restored by virus-mediated expression of wild-type ASIC1a, but not its ion-impermeable mutant, in the insular cortices. Our data demonstrate the involvement of an ASIC1a-mediated insular synaptic depression mechanism in extinction learning, which raises the possibility of targeting ASIC1a to manage adaptive behaviours.

The acid-sensing ion channel, ASIC1a, is known to play a role in synaptic transmission and plasticity. Here, the authors demonstrate a role for ASIC1a in regulating plasticity in the insular cortex and find that extinction of conditioned taste aversion memory is disrupted in the ASIC1a knockout mice.

Acid-Sensing Ion Channel 1a Contributes to Airway Hyperreactivity in Mice

Neurons innervating the airways contribute to airway hyperreactivity (AHR), a hallmark feature of asthma. Several observations suggested that acid-sensing ion channels (ASICs), neuronal cation channels activated by protons, might contribute to AHR. For example, ASICs are found in vagal sensory neurons that innervate airways, and asthmatic airways can become acidic. Moreover, airway acidification activates ASIC currents and depolarizes neurons innervating airways. We found ASIC1a protein in vagal ganglia neurons, but not airway epithelium or smooth muscle. We induced AHR by sensitizing mice to ovalbumin and found that ASIC1a^-/- mice failed to exhibit AHR despite a robust inflammatory response. Loss of ASIC1a also decreased bronchoalveolar lavage fluid levels of substance P, a sensory neuropeptide secreted from vagal sensory neurons that contributes to AHR. These findings suggest that ASIC1a is an important mediator of AHR and raise the possibility that inhibiting ASIC channels might be beneficial in asthma.

Microglial Acid Sensing Regulates Carbon Dioxide-Evoked Fear

BACKGROUND

Carbon dioxide (CO2) inhalation, a biological challenge and pathologic marker in panic disorder, evokes intense fear and panic attacks in susceptible individuals. The molecular identity and anatomic location of CO2-sensing systems that translate CO2-evoked fear remain unclear. We investigated contributions of microglial acid sensor T cell death-associated gene-8 (TDAG8) and microglial proinflammatory responses in CO2-evoked behavioral and physiological responses.

METHODS

CO2-evoked freezing, autonomic, and respiratory responses were assessed in TDAG8-deficient ((-/-)) and wild-type ((+/+)) mice. Involvement of TDAG8-dependent microglial activation and proinflammatory cytokine interleukin (IL)-1β with CO2-evoked responses was investigated using microglial blocker, minocycline, and IL-1β antagonist IL-1RA. CO2-chemosensitive firing responses using single-cell patch clamping were measured in TDAG8(-/-) and TDAG8(+/+) mice to gain functional insights.

RESULTS

TDAG8 expression was localized in microglia enriched within the sensory circumventricular organs. TDAG8(-/-) mice displayed attenuated CO2-evoked freezing and sympathetic responses. TDAG8 deficiency was associated with reduced microglial activation and proinflammatory cytokine IL-1β within the subfornical organ. Central infusion of microglial activation blocker minocycline and IL-1β antagonist IL-1RA attenuated CO2-evoked freezing. Finally, CO2-evoked neuronal firing in patch-clamped subfornical organ neurons was dependent on acid sensor TDAG8 and IL-1β.

CONCLUSIONS

Our data identify TDAG8-dependent microglial acid sensing as a unique chemosensor for detecting and translating hypercapnia to fear-associated behavioral and physiological responses, providing a novel mechanism for homeostatic threat detection of relevance to psychiatric conditions such as panic disorder.

Acute anxiety and social inference: An experimental manipulation with 7.5% carbon dioxide inhalation

BACKGROUND

Positive self-bias is thought to be protective for mental health. We previously found that the degree of positive bias when learning self-referential social evaluation decreases with increasing social anxiety. It is unclear whether this reduction is driven by differences in state or trait anxiety, as both are elevated in social anxiety; therefore, we examined the effects on the state of anxiety induced by the 7.5% carbon dioxide (CO2) inhalation model of generalised anxiety disorder (GAD) on social evaluation learning.

METHODS

For our study, 48 (24 of female gender) healthy volunteers took two inhalations (medical air and 7.5% CO2, counterbalanced) whilst learning social rules (self-like, self-dislike, other-like and other-dislike) in an instrumental social evaluation learning task. We analysed the outcomes (number of positive responses and errors to criterion) using the random effects Poisson regression.

RESULTS

Participants made fewer and more positive responses when breathing 7.5% CO2 in the other-like and other-dislike rules, respectively (gas × condition × rule interaction p = 0.03). Individuals made fewer errors learning self-like than self-dislike, and this positive self-bias was unaffected by CO2. Breathing 7.5% CO2 increased errors, but only in the other-referential rules (gas × condition × rule interaction p = 0.003).

CONCLUSIONS

Positive self-bias (i.e. fewer errors learning self-like than self-dislike) seemed robust to changes in state anxiety. In contrast, learning other-referential evaluation was impaired as state anxiety increased. This suggested that the previously observed variations in self-bias arise due to trait, rather than state, characteristics.

CO2 exposure as translational cross-species experimental model for panic

The current diagnostic criteria of the Diagnostic and Statistical Manual of Mental Disorders are being challenged by the heterogeneity and the symptom overlap of psychiatric disorders. Therefore, a framework toward a more etiology-based classification has been initiated by the US National Institute of Mental Health, the research domain criteria project. The basic neurobiology of human psychiatric disorders is often studied in rodent models. However, the differences in outcome measurements hamper the translation of knowledge. Here, we aimed to present a translational panic model by using the same stimulus and by quantitatively comparing the same outcome measurements in rodents, healthy human subjects and panic disorder patients within one large project. We measured the behavioral-emotional and bodily response to CO2 exposure in all three samples, allowing for a reliable cross-species comparison. We show that CO2 exposure causes a robust fear response in terms of behavior in mice and panic symptom ratings in healthy volunteers and panic disorder patients. To improve comparability, we next assessed the respiratory and cardiovascular response to CO2, demonstrating corresponding respiratory and cardiovascular effects across both species. This project bridges the gap between basic and human research to improve the translation of knowledge between these disciplines. This will allow significant progress in unraveling the etiological basis of panic disorder and will be highly beneficial for refining the diagnostic categories as well as treatment strategies.

A Good Death? Report of the Second Newcastle Meeting on Laboratory Animal Euthanasia

Simple Summary

Millions of laboratory animals are killed each year worldwide. However, there is a lack of consensus regarding what methods of killing are humane for many species and stages of development. This report summarises research findings and discussions from an international meeting of experts and stakeholders, with recommendations to inform good practice for humane killing of mice, rats and zebrafish. It provides additional guidance and perspectives for researchers designing projects that involve euthanasing animals, researchers studying aspects of humane killing, euthanasia device manufacturers, regulators, and institutional ethics or animal care and use committees that wish to review local practice.

Abstract

Millions of laboratory animals are killed each year worldwide. There is an ethical, and in many countries also a legal, imperative to ensure those deaths cause minimal suffering. However, there is a lack of consensus regarding what methods of killing are humane for many species and stages of development. In 2013, an international group of researchers and stakeholders met at Newcastle University, United Kingdom to discuss the latest research and which methods could currently be considered most humane for the most commonly used laboratory species (mice, rats and zebrafish). They also discussed factors to consider when making decisions about appropriate techniques for particular species and projects, and priorities for further research. This report summarises the research findings and discussions, with recommendations to help inform good practice for humane killing.

The function and regulation of acid-sensing ion channels (ASICs) and the epithelial Na(+) channel (ENaC): IUPHAR Review 19

Acid-sensing ion channels (ASICs) and the epithelial Na(+) channel (ENaC) are both members of the ENaC/degenerin family of amiloride-sensitive Na(+) channels. ASICs act as proton sensors in the nervous system where they contribute, besides other roles, to fear behaviour, learning and pain sensation. ENaC mediates Na(+) reabsorption across epithelia of the distal kidney and colon and of the airways. ENaC is a clinically used drug target in the context of hypertension and cystic fibrosis, while ASIC is an interesting potential target. Following a brief introduction, here we will review selected aspects of ASIC and ENaC function. We discuss the origin and nature of pH changes in the brain and the involvement of ASICs in synaptic signalling. We expose how in the peripheral nervous system, ASICs cover together with other ion channels a wide pH range as proton sensors. We introduce the mechanisms of aldosterone-dependent ENaC regulation and the evidence for an aldosterone-independent control of ENaC activity, such as regulation by dietary K(+) . We then provide an overview of the regulation of ENaC by proteases, a topic of increasing interest over the past few years. In spite of the profound differences in the physiological and pathological roles of ASICs and ENaC, these channels share many basic functional and structural properties. It is likely that further research will identify physiological contexts in which ASICs and ENaC have similar or overlapping roles.

Panic Anxiety in Humans with Bilateral Amygdala Lesions: Pharmacological Induction via Cardiorespiratory Interoceptive Pathways

We previously demonstrated that carbon dioxide inhalation could induce panic anxiety in a group of rare lesion patients with focal bilateral amygdala damage. To further elucidate the amygdala-independent mechanisms leading to aversive emotional experiences, we retested two of these patients (B.G. and A.M.) to examine whether triggering palpitations and dyspnea via stimulation of non-chemosensory interoceptive channels would be sufficient to elicit panic anxiety. Participants rated their affective and sensory experiences following bolus infusions of either isoproterenol, a rapidly acting peripheral β-adrenergic agonist akin to adrenaline, or saline. Infusions were administered during two separate conditions: a panic induction and an assessment of cardiorespiratory interoception. Isoproterenol infusions induced anxiety in both patients, and full-blown panic in one (patient B.G.). Although both patients demonstrated signs of diminished awareness for cardiac sensation, patient A.M., who did not panic, reported a complete lack of awareness for dyspnea, suggestive of impaired respiratory interoception. These findings indicate that the amygdala may play a role in dynamically detecting changes in cardiorespiratory sensation. The induction of panic anxiety provides further evidence that the amygdala is not required for the conscious experience of fear induced via interoceptive sensory channels.

SIGNIFICANCE STATEMENT

We found that monozygotic twins with focal bilateral amygdala lesions report panic anxiety in response to intravenous infusions of isoproterenol, a β-adrenergic agonist similar to adrenaline. Heightened anxiety was evident in both twins, with one twin experiencing a panic attack. The twin who did not panic displayed signs of impaired cardiorespiratory interoception, including a complete absence of dyspnea sensation. These findings highlight that the amygdala is not strictly required for the experience of panic anxiety, and suggest that neural systems beyond the amygdala are also involved. Determining these additional systems could provide key neural modulation targets for future anxiolytic treatments.

Evaluation of Low versus High Volume per Minute Displacement CO₂ Methods of Euthanasia in the Induction and Duration of Panic-Associated Behavior and Physiology

Current recommendations for the use of CO ₂ as a euthanasia agent for rats require the use of gradual fill protocols (such as 10% to 30% volume displacement per minute) in order to render the animal insensible prior to exposure to levels of CO ₂ that are associated with pain. However, exposing rats to CO ₂ , concentrations as low as 7% CO ₂ are reported to cause distress and 10%-20% CO ₂ induces panic-associated behavior and physiology, but loss of consciousness does not occur until CO ₂ concentrations are at least 40%. This suggests that the use of the currently recommended low flow volume per minute displacement rates create a situation where rats are exposed to concentrations of CO ₂ that induce anxiety, panic, and distress for prolonged periods of time. This study first characterized the response of male rats exposed to normoxic 20% CO ₂ for a prolonged period of time as compared to room air controls. It demonstrated that rats exposed to this experimental condition displayed clinical signs consistent with significantly increased panic-associated behavior and physiology during CO ₂ exposure. When atmospheric air was then again delivered, there was a robust increase in respiration rate that coincided with rats moving to the air intake. The rats exposed to CO ₂ also displayed behaviors consistent with increased anxiety in the behavioral testing that followed the exposure. Next, this study assessed the behavioral and physiologic responses of rats that were euthanized with 100% CO ₂ infused at 10%, 30%, or 100% volume per minute displacement rates. Analysis of the concentrations of CO ₂ and oxygen in the euthanasia chamber and the behavioral responses of the rats suggest that the use of the very low flow volume per minute displacement rate (10%) may prolong the duration of panicogenic ranges of ambient CO ₂ , while the use of the higher flow volume per minute displacement rate (100%) increases agitation. Therefore, of the volume displacement per minute rates evaluated, this study suggests that 30% minimizes the potential pain and distress experienced by the animal.

Separation anxiety: at the neurobiological crossroads of adaptation and illness

Physiological and adaptive separation anxiety (SA) is intimately connected with the evolutionary emergence of new brain structures specific of paleomammalians, the growth of neomammalian—and later hominid—brain and skull size, and the appearance of bipedalism. All these evolutionary milestones have contributed to expanding the behavioral repertoire and plasticity of prehuman and human beings, at the cost of more prolonged dependency of the infant and of the child on parental care. Separation anxiety disorder (SAD) can be seen as an exaggerated/inappropriate manifestation of SA that constitutes a gateway to poorer mental and physical health. By blending epidemiological, genetic-epidemiological, endophenotypic, and animal laboratory approaches, it is possible to delineate some of the mechanisms that link childhood-adolescence SA and SAD to health problems later in life. Causal mechanisms include gene-environment interplays and likely differential regulation of genes and functional net-works that simultaneously affect multiple behavioral and physical phenotypes after exposure to early-life adversity, including parental separation/loss.

The DEG/ENaC cation channel protein UNC-8 drives activity-dependent synapse removal in remodeling GABAergic neurons

Genetic programming and neural activity drive synaptic remodeling in developing neural circuits, but the molecular components that link these pathways are poorly understood. Here we show that the C. elegans Degenerin/Epithelial Sodium Channel (DEG/ENaC) protein, UNC-8, is transcriptionally controlled to function as a trigger in an activity-dependent mechanism that removes synapses in remodeling GABAergic neurons. UNC-8 cation channel activity promotes disassembly of presynaptic domains in DD type GABA neurons, but not in VD class GABA neurons where unc-8 expression is blocked by the COUP/TF transcription factor, UNC-55. We propose that the depolarizing effect of UNC-8-dependent sodium import elevates intracellular calcium in a positive feedback loop involving the voltage-gated calcium channel UNC-2 and the calcium-activated phosphatase TAX-6/calcineurin to initiate a caspase-dependent mechanism that disassembles the presynaptic apparatus. Thus, UNC-8 serves as a link between genetic and activity-dependent pathways that function together to promote the elimination of GABA synapses in remodeling neurons.

DOI:http://dx.doi.org/10.7554/eLife.14599.001

The brain contains billions of nerve cells, or neurons, that communicate with one another through connections called synapses. As the brain develops, these circuits are extensively modified as new synapses are created and others are removed. Neurological disorders may emerge if these processes are not regulated correctly. Identifying the biological pathways that control the addition and removal of synapses could therefore provide new insights into how to treat human brain diseases.

To communicate across a synapse, the signaling neuron releases chemicals called neurotransmitters that alter the activity of the receiving neuron. Some neurotransmitters, such as GABA, inhibit the activity of the receiving neuron. The activity of a neuron – and hence how often it releases neurotransmitters – depends on different ions moving into and out of the neuron through proteins called ion channels that are embedded in the cell membrane. For example, the movement of calcium ions into the neuron can trigger the release of neurotransmitters.

The roundworm Caenorhabditis elegans is often used as a model organism to study how the brain develops. During development, the worm nervous system eliminates synapses that release GABA and reassembles them at new locations. However, the nervous system does not eliminate these synapses at random. Miller-Fleming, Petersen et al. now show that a C. elegans protein called UNC-8 is responsible for this effect. UNC-8 forms part of an ion channel that allows sodium ions to enter the neuron and is selectively produced in GABA neurons that are destined for remodeling.

Miller-Fleming, Petersen et al. found that inside GABA-releasing neurons, calcium ions stimulate an enzyme called calcineurin that may in turn activate UNC-8. Sodium ions then enter the neuron through UNC-8 channels. This boosts the activity of the calcium ion channels, which further increases how many calcium ions enter the cell. Ultimately, the amount of calcium inside the neuron becomes high enough to activate an additional pathway that eliminates the synapse. This downstream pathway involves components of a cell-killing (or “apoptotic”) mechanism that is repurposed in this case to remove the GABA release apparatus at the synapse.

Other proteins are likely to help UNC-8 sense the activity of neurons and destroy synapses in response. Further work is required to investigate these additional components and to determine how they work with UNC-8 to remove synapses in the nervous system during development.

DOI:http://dx.doi.org/10.7554/eLife.14599.002

Histone Modifications in a Mouse Model of Early Adversities and Panic Disorder: Role for Asic1 and Neurodevelopmental Genes

Hyperventilation following transient, CO₂-induced acidosis is ubiquitous in mammals and heritable. In humans, respiratory and emotional hypersensitivity to CO₂ marks separation anxiety and panic disorders, and is enhanced by early-life adversities. Mice exposed to the repeated cross-fostering paradigm (RCF) of interference with maternal environment show heightened separation anxiety and hyperventilation to 6% CO₂-enriched air. Gene-environment interactions affect CO₂ hypersensitivity in both humans and mice. We therefore hypothesised that epigenetic modifications and increased expression of genes involved in pH-detection could explain these relationships. Medullae oblongata of RCF- and normally-reared female outbred mice were assessed by ChIP-seq for H3Ac, H3K4me3, H3K27me3 histone modifications, and by SAGE for differential gene expression. Integration of multiple experiments by network analysis revealed an active component of 148 genes pointing to the mTOR signalling pathway and nociception. Among these genes, Asic1 showed heightened mRNA expression, coherent with RCF-mice’s respiratory hypersensitivity to CO₂ and altered nociception. Functional enrichment and mRNA transcript analyses yielded a consistent picture of enhancement for several genes affecting chemoception, neurodevelopment, and emotionality. Particularly, results with Asic1 support recent human findings with panic and CO₂ responses, and provide new perspectives on how early adversities and genes interplay to affect key components of panic and related disorders.

Epithelial sodium channel (ENaC) family: Phylogeny, structure-function, tissue distribution, and associated inherited diseases

The epithelial sodium channel (ENaC) is composed of three homologous subunits and allows the flow of Na(+) ions across high resistance epithelia, maintaining body salt and water homeostasis. ENaC dependent reabsorption of Na(+) in the kidney tubules regulates extracellular fluid (ECF) volume and blood pressure by modulating osmolarity. In multi-ciliated cells, ENaC is located in cilia and plays an essential role in the regulation of epithelial surface liquid volume necessary for cilial transport of mucus and gametes in the respiratory and reproductive tracts respectively. The subunits that form ENaC (named as alpha, beta, gamma and delta, encoded by genes SCNN1A, SCNN1B, SCNN1G, and SCNN1D) are members of the ENaC/Degenerin superfamily. The earliest appearance of ENaC orthologs is in the genomes of the most ancient vertebrate taxon, Cyclostomata (jawless vertebrates) including lampreys, followed by earliest representatives of Gnathostomata (jawed vertebrates) including cartilaginous sharks. Among Euteleostomi (bony vertebrates), Actinopterygii (ray finned-fishes) branch has lost ENaC genes. Yet, most animals in the Sarcopterygii (lobe-finned fish) branch including Tetrapoda, amphibians and amniotes (lizards, crocodiles, birds, and mammals), have four ENaC paralogs. We compared the sequences of ENaC orthologs from 20 species and established criteria for the identification of ENaC orthologs and paralogs, and their distinction from other members of the ENaC/Degenerin superfamily, especially ASIC family. Differences between ENaCs and ASICs are summarized in view of their physiological functions and tissue distributions. Structural motifs that are conserved throughout vertebrate ENaCs are highlighted. We also present a comparative overview of the genotype-phenotype relationships in inherited diseases associated with ENaC mutations, including multisystem pseudohypoaldosteronism (PHA1B), Liddle syndrome, cystic fibrosis-like disease and essential hypertension.

The Thumb Domain Mediates Acid-sensing Ion Channel Desensitization

Acid-sensing ion channels (ASICs) are cation-selective proton-gated channels expressed in neurons that participate in diverse physiological processes, including nociception, synaptic plasticity, learning, and memory. ASIC subunits contain intracellular N and C termini, two transmembrane domains that constitute the pore, and a large extracellular loop with defined domains termed the finger, β-ball, thumb, palm, and knuckle. Here we examined the contribution of the finger, β-ball, and thumb domains to activation and desensitization through the analysis of chimeras and the assessment of the effect of covalent modification of introduced Cys at the domain-domain interfaces. Our studies with ASIC1a-ASIC2a chimeras showed that swapping the thumb domain between subunits results in faster channel desensitization. Likewise, the covalent modification of Cys residues at selected positions in the β-ball-thumb interface accelerates the desensitization of the mutant channels. Studies of accessibility with thiol-reactive reagents revealed that the β-ball and thumb domains reside apart in the resting state but that they become closer to each other in response to extracellular acidification. We propose that the thumb domain moves upon continuous exposure to an acidic extracellular milieu, assisting with the closing of the pore during channel desensitization.

Acid-sensing ion channel 1a contributes to hippocampal LTP inducibility through multiple mechanisms

The exact roles of acid-sensing ion channels (ASICs) in synaptic plasticity remain elusive. Here, we address the contribution of ASIC1a to five forms of synaptic plasticity in the mouse hippocampus using an in vitro multi-electrode array recording system. We found that genetic deletion or pharmacological blockade of ASIC1a greatly reduced, but did not fully abolish, the probability of long-term potentiation (LTP) induction by either single or repeated high frequency stimulation or theta burst stimulation in the CA1 region. However, these treatments did not affect hippocampal long-term depression induced by low frequency electrical stimulation or (RS)-3,5-dihydroxyphenylglycine. We also show that ASIC1a exerts its action in hippocampal LTP through multiple mechanisms that include but are not limited to augmentation of NMDA receptor function. Taken together, these results reveal new insights into the role of ASIC1a in hippocampal synaptic plasticity and the underlying mechanisms. This unbiased study also demonstrates a novel and objective way to assay synaptic plasticity mechanisms in the brain.

The Genetics of Stress-Related Disorders: PTSD, Depression, and Anxiety Disorders

Research into the causes of psychopathology has largely focused on two broad etiologic factors: genetic vulnerability and environmental stressors. An important role for familial/heritable factors in the etiology of a broad range of psychiatric disorders was established well before the modern era of genomic research. This review focuses on the genetic basis of three disorder categories-posttraumatic stress disorder (PTSD), major depressive disorder (MDD), and the anxiety disorders-for which environmental stressors and stress responses are understood to be central to pathogenesis. Each of these disorders aggregates in families and is moderately heritable. More recently, molecular genetic approaches, including genome-wide studies of genetic variation, have been applied to identify specific risk variants. In this review, I summarize evidence for genetic contributions to PTSD, MDD, and the anxiety disorders including genetic epidemiology, the role of common genetic variation, the role of rare and structural variation, and the role of gene-environment interaction. Available data suggest that stress-related disorders are highly complex and polygenic and, despite substantial progress in other areas of psychiatric genetics, few risk loci have been identified for these disorders. Progress in this area will likely require analysis of much larger sample sizes than have been reported to date. The phenotypic complexity and genetic overlap among these disorders present further challenges. The review concludes with a discussion of prospects for clinical translation of genetic findings and future directions for research.

Novel Insights into Acid-Sensing Ion Channels: Implications for Degenerative Diseases

Degenerative diseases often strike older adults and are characterized by progressive deterioration of cells, eventually leading to tissue and organ degeneration for which limited effective treatment options are currently available. Acid-sensing ion channels (ASICs), a family of extracellular H(+)-activated ligand-gated ion channels, play critical roles in physiological and pathological conditions. Aberrant activation of ASICs is reported to regulate cell apoptosis, differentiation and autophagy. Accumulating evidence has highlighted a dramatic increase and activation of ASICs in degenerative disorders, including multiple sclerosis, Parkinson's disease, Huntington's disease, intervertebral disc degeneration and arthritis. In this review, we have comprehensively discussed the critical roles of ASICs and their potential utility as therapeutic targets in degenerative diseases.

Euthanasia using gaseous agents in laboratory rodents

Several questions have been raised in recent years about the euthanasia of laboratory rodents. Euthanasia using inhaled agents is considered to be a suitable aesthetic method for use with a large number of animals simultaneously. Nevertheless, its aversive potential has been criticized in terms of animal welfare. The data available regarding the use of carbon dioxide (CO2), inhaled anaesthetics (such as isoflurane, sevoflurane, halothane and enflurane), as well as carbon monoxide and inert gases are discussed throughout this review. Euthanasia of fetuses and neonates is also addressed. A table listing currently available information to ease access to data regarding euthanasia techniques using gaseous agents in laboratory rodents was compiled. Regarding better animal welfare, there is currently insufficient evidence to advocate banning or replacing CO2 in the euthanasia of rodents; however, there are hints that alternative gases are more humane. The exposure to a volatile anaesthetic gas before loss of consciousness has been proposed by some scientific studies to minimize distress; however, the impact of such a measure is not clear. Areas of inconsistency within the euthanasia literature have been highlighted recently and stem from insufficient knowledge, especially regarding the advantages of the administration of isoflurane or sevoflurane over CO2, or other methods, before loss of consciousness. Alternative methods to minimize distress may include the development of techniques aimed at inducing death in the home cage of animals. Scientific outcomes have to be considered before choosing the most suitable euthanasia method to obtain the best results and accomplish the 3Rs (replacement, reduction and refinement).

Fluorescence-Based Automated Screening Assay for the Study of the pH-Sensitive Channel ASIC1a

Acid-sensing ion channel 1a (ASIC1a) is involved in several pathologies, including neurodegenerative and neuroinflammatory disorders, stroke, epilepsy, and inflammatory pain. ASIC1a has been the subject of intense drug discovery programs devoted to the development of new pharmacological tools for its modulation. However, these efforts to generate new compounds have faced the lack of an efficient screening procedure. In the past decades, improvements in screening technologies and fluorescent sensors for the study of ion channels have provided new opportunities in this field. Unfortunately, ASIC1a is mainly a Na+ permeable channel and undergoes desensitization after its activation, two features that make the use of the available screening procedures problematic. We propose here a novel screening approach for the study of ASIC1a activity in full automation. Our method is based on the stimulation of ASIC1a-expressing cells by protons and the use of electrochromic fluorescent voltage sensors as a readout of ion channel activation. This method will prove to be useful for drug screening programs aimed at ASIC1a modulation.

Acid-Sensing Ion Channels as Potential Pharmacological Targets in Peripheral and Central Nervous System Diseases

Acid-sensing ion channels (ASICs) are widely expressed in the body and represent good sensors for detecting protons. The pH drop in the nervous system is equivalent to ischemia and acidosis, and ASICs are very good detectors in discriminating slight changes in acidity. ASICs are important pharmacological targets being involved in a variety of pathophysiological processes affecting both the peripheral nervous system (e.g., peripheral pain, diabetic neuropathy) and the central nervous system (e.g., stroke, epilepsy, migraine, anxiety, fear, depression, neurodegenerative diseases, etc.). This review discusses the role played by ASICs in different pathologies and the pharmacological agents acting on ASICs that might represent promising drugs. As the majority of above-mentioned pathologies involve not only neuronal dysfunctions but also microvascular alterations, in the next future, ASICs may be also considered as potential pharmacological targets at the vasculature level. Perspectives and limitations in the use of ASICs antagonists and modulators as pharmaceutical agents are also discussed.

The RS685012 Polymorphism of ACCN2, the Human Ortholog of Murine Acid-Sensing Ion Channel (ASIC1) Gene, is Highly Represented in Patients with Panic Disorder

Panic disorder (PD) is a disabling anxiety disorder that is characterized by unexpected, recurrent panic attacks, associated with fear of dying and worrying about possible future attacks or other behavioral changes as a consequence of the attacks. The acid-sensing ion channels (ASICs) are a family of proton-sensing channels expressed throughout the nervous system. Their activity is linked to a variety of behaviors including fear, anxiety, pain, depression, learning, and memory. The human analog of ASIC1a is the amiloride-sensitive cation channel 2 (ACCN2). Adenosine A2A receptors are suggested to play an important role in different brain circuits and pathways involved in anxiety reactions. In this work we aimed to evaluate the distribution of ACCN2 rs685012 and ADORA2A rs2298383 polymorphisms in PD patients compared with healthy subjects. We found no association between ADORA2A polymorphism and PD. Instead, the C mutated allele for ACCN2 rs685012 polymorphism was significantly more frequent in patients than in controls. On the contrary, the TT homozygous wild-type genotype and also the ACCN2 TT/ADORA2A CT diplotype were significantly more represented in controls. These results are suggestive for a role of ACCN2 rs685012 polymorphism in PD development in Caucasian people.

The basolateral amygdala γ-aminobutyric acidergic system in health and disease

The brain comprises an excitatory/inhibitory neuronal network that maintains a finely tuned balance of activity critical for normal functioning. Excitatory activity in the basolateral amygdala (BLA), a brain region that plays a central role in emotion and motivational processing, is tightly regulated by a relatively small population of γ-aminobutyric acid (GABA) inhibitory neurons. Disruption in GABAergic inhibition in the BLA can occur when there is a loss of local GABAergic interneurons, an alteration in GABAA receptor activation, or a dysregulation of mechanisms that modulate BLA GABAergic inhibition. Disruptions in GABAergic control of the BLA emerge during development, in aging populations, or after trauma, ultimately resulting in hyperexcitability. BLA hyperexcitability manifests behaviorally as an increase in anxiety, emotional dysregulation, or development of seizure activity. This Review discusses the anatomy, development, and physiology of the GABAergic system in the BLA and circuits that modulate GABAergic inhibition, including the dopaminergic, serotonergic, noradrenergic, and cholinergic systems. We highlight how alterations in various neurotransmitter receptors, including the acid-sensing ion channel 1a, cannabinoid receptor 1, and glutamate receptor subtypes, expressed on BLA interneurons, modulate GABAergic transmission and how defects of these systems affect inhibitory tonus within the BLA. Finally, we discuss alterations in the BLA GABAergic system in neurodevelopmental (autism/fragile X syndrome) and neurodegenerative (Alzheimer's disease) diseases and after the development of epilepsy, anxiety, and traumatic brain injury. A more complete understanding of the intrinsic excitatory/inhibitory circuit balance of the amygdala and how imbalances in inhibitory control contribute to excessive BLA excitability will guide the development of novel therapeutic approaches in neuropsychiatric diseases.

Acid-sensing ion channels: trafficking and pathophysiology

Acid-sensing ion channels (ASICs) are proton-gated cation channels that are widely expressed in both the peripheral and central nervous systems. ASICs contribute to a variety of pathophysiological conditions that involve tissue acidosis, such as ischemic stroke, epileptic seizures and multiple sclerosis. Although much progress has been made in researching the structure-function relationship and pharmacology of ASICs, little is known about the trafficking of ASICs and its contribution to ASIC function. The recent identification of the mechanism of membrane insertion and endocytosis of ASIC1a highlights the emerging role of ASIC trafficking in regulating its pathophysiological functions. In this review, we summarize the recent advances and discuss future directions on this topic.

Acid-sensing ion channels and their modulators

According to a modern look acid-sensing ion channels (ASICs) are one of the most important receptors that perceive pH change in the body. ASICs represent proton-gated Na+-selective channels, which are expressed in neurons of the central and peripheral nervous system. These channels are attracting attention of researchers around the world, as they are involved in various physiological processes in the body. Drop of pH may occur in tissues in norm (e.g. the accumulation of lactic acid, the release of protons upon ATP hydrolysis) and pathology (inflammation, ischemic stroke, tissue damage and seizure). These processes are accompanied by unpleasant pain sensations, which may be short-lived or can lead to chronic inflammatory diseases. Modulators of ASIC channels activity are potential candidates for new effective analgesic and neuroprotection drugs. This review summarizes available information about structure, function, and physiological role of ASIC channels. In addition a description of all known ligands of these channels and their practical relevance is provided.

Activation of acid-sensing ion channels by localized proton transient reveals their role in proton signaling

Extracellular transients of pH alterations likely mediate signal transduction in the nervous system. Neuronal acid-sensing ion channels (ASICs) act as sensors for extracellular protons, but the mechanism underlying ASIC activation remains largely unknown. Here, we show that, following activation of a light-activated proton pump, Archaerhodopsin-3 (Arch), proton transients induced ASIC currents in both neurons and HEK293T cells co-expressing ASIC1a channels. Using chimera proteins that bridge Arch and ASIC1a by a glycine/serine linker, we found that successful coupling occurred within 15 nm distance. Furthermore, two-cell sniffer patch recording revealed that regulated release of protons through either Arch or voltage-gated proton channel Hv1 activated neighbouring cells expressing ASIC1a channels. Finally, computational modelling predicted the peak proton concentration at the intercellular interface to be at pH 6.7, which is acidic enough to activate ASICs in vivo. Our results highlight the pathophysiological role of proton signalling in the nervous system.

OREXIN 1 AND 2 RECEPTOR INVOLVEMENT IN CO2 -INDUCED PANIC-ASSOCIATED BEHAVIOR AND AUTONOMIC RESPONSES

BACKGROUND

The neuropeptides orexin A and B play a role in reward and feeding and are critical for arousal. However, it was not initially appreciated that most prepro-orexin synthesizing neurons are almost exclusively concentrated in the perifornical hypothalamus, which when stimulated elicits panic-associated behavior and cardiovascular responses in rodents and self-reported "panic attacks" and "fear of dying" in humans. More recent studies support a role for the orexin system in coordinating an integrative stress response. For instance, orexin neurons are highly reactive to anxiogenic stimuli, are hyperactive in anxiety pathology, and have strong projections to anxiety and panic-associated circuitry. Although the two cognate orexin receptors are colocalized in many brain regions, the orexin 2 receptor (OX2R) most robustly maps to the histaminergic wake-promoting region, while the orexin 1 receptor (OX1R) distribution is more exclusive and dense in anxiety and panic circuitry regions, such as the locus ceruleus. Overall, this suggests that OX1Rs play a critical role in mobilizing anxiety and panic responses.

METHODS

Here, we used a CO2 -panic provocation model to screen a dual OX1/2R antagonist (DORA-12) to globally inhibit orexin activity, then a highly selective OX1R antagonist (SORA1, Compound 56) or OX2R antagonist (SORA2, JnJ10397049) to assess OX1R and OX2R involvement.

RESULTS

All compounds except the SORA2 attenuated CO2 -induced anxiety-like behaviors, and all but the SORA2 and DORA attenuated CO2 -induced cardiovascular responses.

CONCLUSIONS

SORA1s may represent a novel method of treating anxiety disorders, with no apparent sedative effects that were present with a benzodiazepine.

ASIC1A in the bed nucleus of the stria terminalis mediates TMT-evoked freezing

Mice display an unconditioned freezing response to TMT, a predator odor isolated from fox feces. Here we found that in addition to freezing, TMT caused mice to decrease breathing rate, perhaps because of the aversive smell. Consistent with this possibility, olfactory bulb lesions attenuated this effect of TMT, as well as freezing. Interestingly, butyric acid, another foul odor, also caused mice to reduce breathing rate. However, unlike TMT, butyric acid did not induce freezing. Thus, although these aversive odors may affect breathing, the unpleasant smell and suppression of breathing by themselves are insufficient to cause freezing. Because the acid-sensing ion channel-1A (ASIC1A) has been previously implicated in TMT-evoked freezing, we tested whether Asic1a disruption also altered breathing. We found that TMT reduced breathing rate in both Asic1a^+/+ and Asic1a^−/− mice, suggesting that ASIC1A is not required for TMT to inhibit breathing and that the absence of TMT-evoked freezing in the Asic1a^−/− mice is not due to an inability to detect TMT. These observations further indicate that ASIC1A must affect TMT freezing in another way. Because the bed nucleus of the stria terminalis (BNST) has been critically implicated in TMT-evoked freezing and robustly expresses ASIC1A, we tested whether ASIC1A in the BNST plays a role in TMT-evoked freezing. We disrupted ASIC1A in the BNST of Asic1a^loxP/loxP mice by delivering Cre recombinase to the BNST with an adeno-associated virus (AAV) vector. We found that disrupting ASIC1A in the BNST reduced TMT-evoked freezing relative to control mice in which a virus expressing eGFP was injected. To test whether ASIC1A in the BNST was sufficient to increase TMT-evoked freezing, we used another AAV vector to express ASIC1A in the BNST of Asic1a^−/− mice. We found region-restricted expression of ASIC1A in the BNST increased TMT-elicited freezing. Together, these data suggest that the BNST is a key site of ASIC1A action in TMT-evoked freezing.

ASICs as therapeutic targets for migraine

Migraine is the most common neurological disorder and one of the most common chronic pain conditions. Despite its prevalence, the pathophysiology leading to migraine is poorly understood and the identification of new therapeutic targets has been slow. Several processes are currently thought to contribute to migraine including altered activity in the hypothalamus, cortical-spreading depression (CSD), and afferent sensory input from the cranial meninges. Decreased extracellular pH and subsequent activation of acid-sensing ion channels (ASICs) may contribute to each of these processes and may thus play a role in migraine pathophysiology. Although few studies have directly examined a role of ASICs in migraine, studies directly examining a connection have generated promising results including efficacy of ASIC blockers in both preclinical migraine models and in human migraine patients. The purpose of this review is to discuss the pathophysiology thought to contribute to migraine and findings that implicate decreased pH and/or ASICs in these events, as well as propose issues to be resolved in future studies of ASICs and migraine. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

Environmental CO2 inhibits Caenorhabditis elegans egg-laying by modulating olfactory neurons and evokes widespread changes in neural activity

Carbon dioxide (CO2) gradients are ubiquitous and provide animals with information about their environment, such as the potential presence of prey or predators. The nematode Caenorhabditis elegans avoids elevated CO2, and previous work identified three neuron pairs called "BAG," "AFD," and "ASE" that respond to CO2 stimuli. Using in vivo Ca(2+) imaging and behavioral analysis, we show that C. elegans can detect CO2 independently of these sensory pathways. Many of the C. elegans sensory neurons we examined, including the AWC olfactory neurons, the ASJ and ASK gustatory neurons, and the ASH and ADL nociceptors, respond to a rise in CO2 with a rise in Ca(2+). In contrast, glial sheath cells harboring the sensory endings of C. elegans' major chemosensory neurons exhibit strong and sustained decreases in Ca(2+) in response to high CO2. Some of these CO2 responses appear to be cell intrinsic. Worms therefore may couple detection of CO2 to that of other cues at the earliest stages of sensory processing. We show that C. elegans persistently suppresses oviposition at high CO2. Hermaphrodite-specific neurons (HSNs), the executive neurons driving egg-laying, are tonically inhibited when CO2 is elevated. CO2 modulates the egg-laying system partly through the AWC olfactory neurons: High CO2 tonically activates AWC by a cGMP-dependent mechanism, and AWC output inhibits the HSNs. Our work shows that CO2 is a more complex sensory cue for C. elegans than previously thought, both in terms of behavior and neural circuitry.

Novel Potent Orthosteric Antagonist of ASIC1a Prevents NMDAR-Dependent LTP Induction

Acid sensing ion channels 1a (ASIC1a) are of crucial importance in numerous physiological and pathological processes in the brain. Here we demonstrate that novel 2-oxo-2H-chromene-3-carboxamidine derivative 5b, designed with molecular modeling approach, inhibits ASIC1a currents with an apparent IC50 of 27 nM when measured at pH 6.7. Acidification to 5.0 decreases the inhibition efficacy by up to 3 orders of magnitude. The 5b molecule not only shifts pH dependence of ASIC1a activation but also inhibits its maximal evoked response. These findings suggest that compound 5b binds to pH sensor of ASIC1a acting as orthosteric noncompetitive antagonist. At 100 nM, compound 5b completely inhibits induction of long-term potentiation (LTP) in CA3-CA1 but not in MF-CA3 synapses. These findings support the knockout data indicating the crucial modulatory role of ASIC1a channels in the NMDAR-dependent LTP and introduce a novel type of ASIC1a antagonists.

Genetic mapping of ASIC4 and contrasting phenotype to ASIC1a in modulating innate fear and anxiety

Although ASIC4 is a member of the acid-sensing ion channel (ASIC) family, we have limited knowledge of its expression and physiological function in vivo. To trace the expression of this ion channel, we generated the ASIC4-knockout/CreERT(2)-knockin (Asic4(Cre) (ERT) (2)) mouse line. After tamoxifen induction in the Asic4(Cre) (ERT)(2)::CAG-STOP(floxed)-Td-tomato double transgenic mice, we mapped the expression of ASIC4 at the cellular level in the central nervous system (CNS). ASIC4 was expressed in many brain regions, including the olfactory bulb, cerebral cortex, striatum, hippocampus, amygdala, thalamus, hypothalamus, brain stem, cerebellum, spinal cord and pituitary gland. Colocalisation studies further revealed that ASIC4 was expressed mainly in three types of cells in the CNS: (i) calretinin (CR)-positive and/or vasoactive intestine peptide (VIP)-positive interneurons; (ii) neural/glial antigen 2 (NG2)-positive glia, also known as oligodendrocyte precursor cells; and (iii) cerebellar granule cells. To probe the possible role of ASIC4, we hypothesised that ASIC4 could modulate the membrane expression of ASIC1a and thus ASIC1a signaling in vivo. We conducted behavioral phenotyping of Asic4(Cre) (ERT)(2) mice by screening many of the known behavioral phenotypes found in Asic1a knockouts and found ASIC4 not involved in shock-evoked fear learning and memory, seizure termination or psychostimulant-induced locomotion/rewarding effects. In contrast, ASIC4 might play an important role in modulating the innate fear response to predator odor and anxious state because ASIC4-mutant mice showed increased freezing response to 2,4,5-trimethylthiazoline and elevated anxiety-like behavior in both the open-field and elevated-plus maze. ASIC4 may modulate fear and anxiety by counteracting ASIC1a activity in the brain.

Acid-base dysregulation and chemosensory mechanisms in panic disorder: a translational update

Panic disorder (PD), a complex anxiety disorder characterized by recurrent panic attacks, represents a poorly understood psychiatric condition which is associated with significant morbidity and an increased risk of suicide attempts and completed suicide. Recently however, neuroimaging and panic provocation challenge studies have provided insights into the pathoetiology of panic phenomena and have begun to elucidate potential neural mechanisms that may underlie panic attacks. In this regard, accumulating evidence suggests that acidosis may be a contributing factor in induction of panic. Challenge studies in patients with PD reveal that panic attacks may be reliably provoked by agents that lead to acid-base dysbalance such as CO2 inhalation and sodium lactate infusion. Chemosensory mechanisms that translate pH into panic-relevant fear, autonomic, and respiratory responses are therefore of high relevance to the understanding of panic pathophysiology. Herein, we provide a current update on clinical and preclinical studies supporting how acid-base imbalance and diverse chemosensory mechanisms may be associated with PD and discuss future implications of these findings.

Validation of candidate anxiety disorder genes using a carbon dioxide challenge task

Few replicable genetic variants have been identified in the etiology of heritable anxiety disorders such as panic disorder. Endophenotypic measures that have reduced heterogeneity may provide more powerful targets for gene identification. We assessed hypersensitivity to carbon dioxide (a reliable endophenotype of panic and anxiety) in 174 Caucasian college students, who were genotyped on 26 polymorphic markers from 11 genes previously associated with panic/anxiety. Individual trajectories of respiratory and subjective anxiety response to carbon dioxide were measured and tested for association with these genetic markers. One marker in the acid-sensing ion channel 1 (ASIC1) gene, rs1108923, had a significant association with respiratory rate. No genes had a significant association with subjective anxiety response. Our findings support previously reported associations between ASIC1 and panic/anxiety, but not other genes previously associated with anxiety disorders. The use of endophenotypic markers is a promising avenue for gene identification in anxiety and other complex disorders.

Basic residue at position 14 is not required for fast assembly and disassembly kinetics in neural cadherin

In spite of their structural similarities, epithelial (E-) and neural (N-) cadherin are expressed at different types of synapses and differ significantly in their dimerization kinetics. Recent studies proposed a transient intermediate in E-cadherin as the key requirement for rapid disassembly kinetics of the adhesive dimer. This E-cadherin intermediate comprises four intermolecular ionic and H-bonding interactions between adhesive partners. These interactions are not preserved in N-cadherin except for a basic residue at the 14th position, which could stabilize the intermediate through either H-bonding or ionic interactions with the partner protomer. To investigate the origin of the rapid dimerization kinetics of N-cadherin in the presence of calcium, studies reported here systematically test the role of ionic and H-bonding interactions in dimerization kinetics using R14S, R14A, and R14E mutants of N-cadherin. Analytical size-exclusion chromatographic and bead aggregation studies showed two primary results. First, N-cadherin/R14S and N-cadherin/R14A mutants showed fast assembly and disassembly kinetics in the calcium-saturated state similar to that of wild-type N-cadherin. These results indicate that the fast disassembly of the calcium-saturated dimer of N-cadherin does not require a basic residue at the 14th position. Second, the dimerization kinetics of N-cadherin/R14E were slow in the calcium-saturated state, indicating that negative charge destabilizes the intermediate state. Taken together, these results indicate that the basic residue at the 14th position does not promote rapid dimerization kinetics but that an acidic amino acid in that position significantly impairs dimerization kinetics.

ASICs and cardiovascular homeostasis

In this review we address primarily the role of ASICs in determining sensory signals from arterial baroreceptors, peripheral chemoreceptors, and cardiopulmonary and somatic afferents. Alterations in these sensory signals during acute cardiovascular stresses result in changes in sympathetic and parasympathetic activities that restore cardiovascular homeostasis. In pathological states, however, chronic dysfunctions of these afferents result in serious sympatho-vagal imbalances with significant increases in mortality and morbidity. We identified a role for ASIC2 in the mechano-sensitivity of aortic baroreceptors and of ASIC3 in the pH sensitivity of carotid bodies. In spontaneously hypertensive rats, we reported decreased expression of ASIC2 in nodose ganglia neurons and overexpression of ASIC3 in carotid bodies. This reciprocal expression of ASIC2 and ASIC3 results in reciprocal changes in sensory sensitivity of baro- and chemoreceptors and a consequential synergistic exaggeration sympathetic nerve activity. A similar reciprocal sensory dysautonomia prevails in heart failure and increases the risk of mortality. There is also evidence that ASIC heteromers in skeletal muscle afferents contribute significantly to the exercise pressor reflex. In cardiac muscle afferents of the dorsal root ganglia, they contribute to nociception and to the detrimental sympathetic activation during ischemia. Finally, we report that an inhibitory influence of ASIC2-mediated baroreceptor activity suppresses the sympatho-excitatory reflexes of the chemoreceptors and skeletal muscle afferents, as well as the ASIC1a-mediated excitation of central neurons during fear, threat, or panic. The translational potential of activation of ASIC2 in cardiovascular disease states may be a beneficial sympatho-inhibition and parasympathetic activation. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

ASICs and neuropeptides

The acid sensing ion channels (ASICs) are proton-gated cation channels expressed throughout the nervous system. ASICs are activated during acidic pH fluctuations, and recent work suggests that they are involved in excitatory synaptic transmission. ASICs can also induce neuronal degeneration and death during pathological extracellular acidosis caused by ischemia, autoimmune inflammation, and traumatic injury. Many endogenous neuromodulators target ASICs to affect their biophysical characteristics and contributions to neuronal activity. One of the most unconventional types of modulation occurs with the interaction of ASICs and neuropeptides. Collectively, FMRFamide-related peptides and dynorphins potentiate ASIC activity by decreasing the proton-sensitivity of steady state desensitization independent of G protein-coupled receptor activation. By decreasing the proton-sensitivity of steady state desensitization, the FMRFamide-related peptides and dynorphins permit ASICs to remain active at more acidic basal pH. Unlike the dynorphins, some FMRFamide-related peptides also potentiate ASIC activity by slowing inactivation and increasing the sustained current. Through mechanistic studies, the modulation of ASICs by FMRFamide-related peptides and dynorphins appears to be through distinct interactions with the extracellular domain of ASICs. Dynorphins are expressed throughout the nervous system and can increase neuronal death during prolonged extracellular acidosis, suggesting that the interaction between dynorphins and ASICs may have important consequences for the prevention of neurological injury. The overlap in expression of FMRFamide-related peptides with ASICs in the dorsal horn of the spinal cord suggests that their interaction may have important consequences for the treatment of pain during injury and inflammation. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

Two aspects of ASIC function: Synaptic plasticity and neuronal injury

Extracellular brain pH fluctuates in both physiological and disease conditions. The main postsynaptic proton receptor is the acid-sensing ion channels (ASICs). During the past decade, much progress has been made on protons, ASICs, and neurological disease. This review summarizes the recent progress on synaptic role of protons and our current understanding of how ASICs contribute to various types of neuronal injury in the brain. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

Genetic exploration of the role of acid-sensing ion channels

Advanced gene targeting technology and related tools in mice have been incorporated into studies of acid-sensing ion channels (ASICs). A single ASIC subtype can be knocked out specifically and screened thoroughly for expression in the nervous system at the cellular level. Mapping studies have further shed light on the initiation and identification of related behavioral phenotypes. Here we review studies involving genetically engineered mouse models used to investigate the physiological function of individual ASIC subtypes: ASIC1 (and ASIC1a), ASIC2, ASIC3 and ASIC4. We discuss the detailed expression studies and significant phenotypes revealed with gene knockout for most known Asic subtypes. Each strategy designed to manipulate mouse genetics has advantages and disadvantages. We discuss the limitations of these Asic-knockout models and propose future directions to solve the genetic issues. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

International Union of Basic and Clinical Pharmacology. XCI. structure, function, and pharmacology of acid-sensing ion channels and the epithelial Na+ channel

The epithelial Na(+) channel (ENaC) and the acid-sensing ion channels (ASICs) form subfamilies within the ENaC/degenerin family of Na(+) channels. ENaC mediates transepithelial Na(+) transport, thereby contributing to Na(+) homeostasis and the maintenance of blood pressure and the airway surface liquid level. ASICs are H(+)-activated channels found in central and peripheral neurons, where their activation induces neuronal depolarization. ASICs are involved in pain sensation, the expression of fear, and neurodegeneration after ischemia, making them potentially interesting drug targets. This review summarizes the biophysical properties, cellular functions, and physiologic and pathologic roles of the ASIC and ENaC subfamilies. The analysis of the homologies between ENaC and ASICs and the relation between functional and structural information shows many parallels between these channels, suggesting that some mechanisms that control channel activity are shared between ASICs and ENaC. The available crystal structures and the discovery of animal toxins acting on ASICs provide a unique opportunity to address the molecular mechanisms of ENaC and ASIC function to identify novel strategies for the modulation of these channels by pharmacologic ligands.

The Effect of Hydrophobic Monoamines on Acid-Sensing Ion Channels ASIC1B

Acid-sensing ion channels (ASICs) are widely distributed in both the central and peripheral nervous systems of vertebrates. The pharmacology of these receptors remains poorly investigated, while the search for new ASIC modulators is very important. Recently, we found that some monoamines, which are blockers of NMDA receptors, inhibit and/or potentiate acid-sensing ion channels, depending on the subunit composition of the channels. The effect of 9-aminoacridine, IEM-1921, IEM-2117, and memantine both on native receptors and on recombinant ASIC1a, ASIC2a, and ASIC3 homomers was studied. In the present study, we have investigated the effect of these four compounds on homomeric ASIC1b channels. Experiments were performed on recombinant receptors expressed in CHO cells using the whole-cell patch clamp technique. Only two compounds, 9-aminoacridine and memantine, inhibited ASIC1b channels. IEM-1921 and IEM-2117 were inactive even at a 1000 μM concentration. In most aspects, the effect of the compounds on ASIC1b was similar to their effect on ASIC1a. The distinguishing feature of homomeric ASIC1b channels is a steep activation-dependence, indicating cooperative activation by protons. In our experiments, the curve of the concentration dependence of ASIC1b inhibition by 9-aminoacridine also had a slope (Hill coefficient) of 3.8, unlike ASIC1a homomers, for which the Hill coefficient was close to 1. This finding indicates that the inhibitory effect of 9-aminoacridine is associated with changes in the activation properties of acid-sensing ion channels.

Expression and activity of acid-sensing ion channels in the mouse anterior pituitary

Acid sensing ion channels (ASICs) are proton-gated cation channels that are expressed in the nervous system and play an important role in fear learning and memory. The function of ASICs in the pituitary, an endocrine gland that contributes to emotions, is unknown. We sought to investigate which ASIC subunits were present in the pituitary and found mRNA expression for all ASIC isoforms, including ASIC1a, ASIC1b, ASIC2a, ASIC2b, ASIC3 and ASIC4. We also observed acid-evoked ASIC-like currents in isolated anterior pituitary cells that were absent in mice lacking ASIC1a. The biophysical properties and the responses to PcTx1, amiloride, Ca2+ and Zn2+ suggested that ASIC currents were mediated predominantly by heteromultimeric channels that contained ASIC1a and ASIC2a or ASIC2b. ASIC currents were also sensitive to FMRFamide (Phe-Met-Arg-Phe amide), suggesting that FMRFamide-like compounds might endogenously regulate pituitary ASICs. To determine whether ASICs might regulate pituitary cell function, we applied low pH and found that it increased the intracellular Ca2+ concentration. These data suggest that ASIC channels are present and functionally active in anterior pituitary cells and may therefore influence their function.

The human ortholog of acid-sensing ion channel gene ASIC1a is associated with panic disorder and amygdala structure and function

BACKGROUND

Individuals with panic disorder (PD) exhibit a hypersensitivity to inhaled carbon dioxide, possibly reflecting a lowered threshold for sensing signals of suffocation. Animal studies have shown that carbon dioxide-mediated fear behavior depends on chemosensing of acidosis in the amygdala via the acid-sensing ion channel ASIC1a. We examined whether the human ortholog of the ASIC1a gene, ACCN2, is associated with the presence of PD and with amygdala structure and function.

METHODS

We conducted a case-control analysis (n = 414 PD cases and 846 healthy controls) of ACCN2 single nucleotide polymorphisms and PD. We then tested whether variants showing significant association with PD are also associated with amygdala volume (n = 1048) or task-evoked reactivity to emotional stimuli (n = 103) in healthy individuals.

RESULTS

Two single nucleotide polymorphisms at the ACCN2 locus showed evidence of association with PD: rs685012 (odds ratio = 1.32, gene-wise corrected p = .011) and rs10875995 (odds ratio = 1.26, gene-wise corrected p = .046). The association appeared to be stronger when early-onset (age ≤ 20 years) PD cases and when PD cases with prominent respiratory symptoms were compared with controls. The PD risk allele at rs10875995 was associated with increased amygdala volume (p = .035) as well as task-evoked amygdala reactivity to fearful and angry faces (p = .0048).

CONCLUSIONS

Genetic variation at ACCN2 appears to be associated with PD and with amygdala phenotypes that have been linked to proneness to anxiety. These results support the possibility that modulation of acid-sensing ion channels may have therapeutic potential for PD.

Identification of Ppk26, a DEG/ENaC Channel Functioning with Ppk1 in a Mutually Dependent Manner to Guide Locomotion Behavior in Drosophila

A major gap in our understanding of sensation is how a single sensory neuron can differentially respond to a multitude of different stimuli (polymodality), such as propio- or nocisensation. The prevailing hypothesis is that different stimuli are transduced through ion channels with diverse properties and subunit composition. In a screen for ion channel genes expressed in polymodal nociceptive neurons, we identified Ppk26, a member of the trimeric degenerin/epithelial sodium channel (DEG/ENaC) family, as being necessary for proper locomotion behavior in Drosophila larvae in a mutually dependent fashion with coexpressed Ppk1, another member of the same family. Mutants lacking Ppk1 and Ppk26 were defective in mechanical, but not thermal, nociception behavior. Mutants of Piezo, a channel involved in mechanical nociception in the same neurons, did not show a defect in locomotion, suggesting distinct molecular machinery for mediating locomotor feedback and mechanical nociception.

Impact of pH on the structure and function of neural cadherin

Neural (N-) cadherin is a transmembrane protein within adherens junctions that mediates cell-cell adhesion. It has 5 modular extracellular domains (EC1-EC5) that bind 3 calcium ions between each of the modules. Calcium binding is required for dimerization. N-Cadherin is involved in diverse processes including tissue morphogenesis, excitatory synapse formation and dynamics, and metastasis of cancer. During neurotransmission and tumorigenesis, fluctuations in extracellular pH occur, causing tissue acidosis with associated physiological consequences. Studies reported here aim to determine the effect of pH on the dimerization properties of a truncated construct of N-cadherin containing EC1-EC2. Since N-cadherin is an anionic protein, we hypothesized that acidification of solution would cause an increase in stability of the apo protein, a decrease in the calcium-binding affinity, and a concomitant decrease in the formation of adhesive dimer. The stability of the apo monomer was increased and the calcium-binding affinity was decreased at reduced pH, consistent with our hypothesis. Surprisingly, analytical SEC studies showed an increase in calcium-induced dimerization as solution pH decreased from 7.4 to 5.0. Salt-dependent dimerization studies indicated that electrostatic repulsion attenuates dimerization affinity. These results point to a possible electrostatic mechanism for moderating dimerization affinity of the Type I cadherin family. Extrapolating these results to cell adhesion in vivo leads to the assertion that decreased pH promotes adhesion by N-cadherin, thereby stabilizing synaptic junctions.