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]

Modulation of Proton-Gated Channels by Antidepressants

The chemical structures of some antidepressants are similar to those of recently described amine-containing ligands of acid-sensing ion channels (ASICs). ASICs are expressed in brain neurons and participate in numerous CNS functions. As such, they can be related to antidepressant action or side effects. We therefore studied the actions of a series of antidepressants on recombinant ASIC1a and ASIC2a and on native ASICs in rat brain neurons. Most of the tested compounds prevented steady-state ASIC1a desensitization evoked by conditioning acidification to pH 7.1. Amitriptyline also potentiated ASIC1a responses evoked by pH drops from 7.4 to 6.5. We conclude that amitriptyline has a twofold effect: it shifts activation to less acidic values while also shifting steady-state desensitization to more acidic values. Chlorpromazine, desipramine, amitriptyline, fluoxetine, and atomoxetine potentiated ASIC2a response. Tianeptine caused strong inhibition of ASIC2a. Both potentiation and inhibition of ASIC2a were accompanied by the slowdown of desensitization, suggesting distinct mechanisms of action on activation and desensitization. In experiments on native heteromeric ASICs, tianeptine and amitriptyline demonstrated the same modes of action as on ASIC2a although with reduced potency.

Nitrogen gas produces less behavioural and neurophysiological excitation than carbon dioxide in mice undergoing euthanasia

Carbon dioxide (CO₂) is one of the most commonly used gas euthanasia agents in mice, despite reports of aversion and nociception. Inert gases such as nitrogen (N₂) may be a viable alternative to carbon dioxide. Here we compared behavioural and electrophysiological reactions to CO₂ or N₂ at either slow fill or rapid fill in C57Bl/6 mice undergoing gas euthanasia. We found that mice euthanised with CO₂ increased locomotor activity compared to baseline, whereas mice exposed to N₂ decreased locomotion. Furthermore, mice exposed to CO₂ showed significantly more vertical jumps and freezing episodes than mice exposed to N₂. We further found that CO₂ exposure resulted in increased theta:delta of the EEG, a measure of excitation, whereas the N₂ decreased theta:delta. Differences in responses were not oxygen-concentration dependent. Taken together, these results demonstrate that CO₂ increases both behavioural and electrophysiological excitation as well as producing a fear response, whereas N₂ reduces behavioural activity and central neurological depression and may be less aversive although still produces a fear response. Further studies are required to evaluate N₂ as a suitable euthanasia agent for mice.

The Application of Humane Endpoints and Humane Killing Methods in Animal Research Proposals: A Retrospective Review

Refinement refers to the use of methods that help to minimise animal suffering in the laboratory. Research in this area has increased significantly over the past two decades. However, the extent to which refinements are applied in practice is uncertain. To provide an indication of the implementation and awareness of refinements, we reviewed the experimental techniques for 684 surgical interventions described in 506 animal research applications sent to the German competent authorities for approval in 2010. In this paper, we describe and discuss the appropriateness of the proposed humane endpoints and killing methods. We found that, when the investigators included humane endpoints in their application, these were often lacking in detail and/or were to be implemented at a late stage of suffering. In addition, the choice of method to kill the animals could be improved in the majority of the applications. We provide recommendations for future improvements, based on the recent literature. To ensure scientific rigour, avoid needless animal suffering and enable an accurate harm–benefit analysis, animal researchers have to be knowledgeable about refinement methods and apply them effectively. To assess compliance and ensure that only those studies in which potential benefits outweigh the harms are carried out, reviews such as ours — as well as retrospective assessments of actual harms and benefits — should be conducted widely and regularly, and the findings should be published.

Home Cage Compared with Induction Chamber for Euthanasia of Laboratory Rats

This study compared behavioral and physiologic changes in Sprague-Dawley and Brown Norway rats that were euthanized by using a 30% volume displacement rate of CO₂ in either their home cage or an induction chamber; rats euthanized in the home cage were hypothesized to demonstrate a higher level of animal wellbeing. No significant differences were detected in the physiologic responses to home cage versus induction chamber euthanasia groups. A few strain-related behavioral differences occurred. The number of digs per second was higher in Brown Norway compared with Sprague-Dawley rats when in the home cage, where a digging substrate was present. Rearing frequency was higher in both Brown Norway and Sprague-Dawley rats in the induction chamber compared with the home cage. This study demonstrated that although strainspecific differences were associated with the process of euthanasia, there were no significant differences between the treatment groups of home cage compared with induction chamber. This finding suggests that-from the perspective of a rat-either the home cage or an induction chamber can be used for euthanasia, with likely extension of this conclusion to use of either method to the induction of anesthesia.

Interpreting Neuroendocrine Hormones, Corticosterone, and Blood Glucose to Assess the Wellbeing of Anesthetized Rats during Euthanasia

Current recommendations for assessing animal wellbeing during euthanasia suggest that measuring neuroendocrine hormones-such as ACTH, noradrenaline, and adrenaline-is preferable to measuring corticosterone and blood glucose because of the sensitivity of neuroendocrine hormones to the acute stress associated with rapid methods of euthanasia. However, these neuroendocrine hormones can be stimulated in ways that confound interpretation of welfare assessment in euthanasia studies. Although this property does not negate the usefulness of neuroendocrine hormones as tools of assessment, it is important to differentiate the stress associated with the induction of anesthesia before the loss of consciousness (an animal wellbeing concern) with the physiologic responses that occur after the loss of consciousness (not an animal wellbeing concern). In this study, rats were anesthetized by using a ketamine-xylazine combination. Once the rats achieved a surgical plane of anesthesia, they were exposed to O₂, CO₂, or isoflurane, followed by terminal blood collection to assess concentrations of ACTH, noradrenaline, corticosterone, and blood glucose. Compared with animals exposed to O₂ or isoflurane, rats exposed to CO₂ had significant increases in their serum concentrations of ACTH and noradrenaline, but blood glucose and corticosterone did not differ between groups. These findings indicate that noradrenaline and ACTH should be used with caution to assess animal wellbeing when the method of euthanasia might confound that assessment.

Assessing Panic: Bridging the Gap Between Fundamental Mechanisms and Daily Life Experience

Panic disorder (PD) is one of the most common psychiatric disorders. Recurrent, unexpected panic attacks (PAs) are the primary symptom and strongly impact patients’ quality of life. Clinical manifestations are very heterogeneous between patients, emphasizing the need for a dimensional classification integrating various aspects of neurobiological and psychological circuits in line with the Research Domain Criteria (RDoC) proposed by the US National Institute of Mental Health. To go beyond data that can be collected in the daily clinical situation, experimental panic provocation is widely used, which has led to important insights into involved brain regions and systems. Genetic variants can determine the sensitivity to experimental models such as carbon dioxide (CO₂) exposure and can increase the risk to develop PD. Recent developments now allow to better assess the dynamic course of PAs outside the laboratory in patients’ natural environment. This can provide novel insights into the underlying mechanisms and the influence of environmental factors that can alter gene regulation by changing DNA methylation. In this mini review, we discuss assessment of PAs in the clinic, in the laboratory using CO₂ exposure, genetic associations, and the benefits of real-life assessment and epigenetic research.

Protons as Messengers of Intercellular Communication in the Nervous System

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

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.

Euthanasia of laboratory mice: Are isoflurane and sevoflurane real alternatives to carbon dioxide?

In the European Union (EU) millions of laboratory mice are used and killed for experimental and other scientific purposes each year. Although controversially discussed, the use of carbon dioxide (CO₂) is still permitted for killing rodents according to the Directive 2010/63/EU. Within the scope of refinement, our aim was to investigate if isoflurane and sevoflurane are an appropriate alternative killing method to CO₂ in mice. Different concentrations of CO₂ (filling rates of 20%, 60%, 100%; CO₂ 20, 60, 100), isoflurane (Iso 2%, 5%) and sevoflurane (Sevo 4.8%, 8%) were compared in two mouse strains (NMRI, C57Bl/6J) using a broad spectrum of behavioral parameters, including the approach-avoidance test, and analyzing blood for stress parameters (glucose, adrenaline, noradrenaline). We focused in our study on the period from the beginning of the gas inlet to loss of consciousness, as during this period animals are able to perceive pain and distress. Our results show that only higher concentrations of CO₂ (CO₂ 60, 100) and isoflurane (5%) induced surgical tolerance within 300 s in both strains, with CO₂ 100 being the fastest acting inhalant anesthetic. The potency of halogenated ethers depended on the mouse strain, with C57Bl/6J being more susceptible than NMRI mice. Behavioral analysis revealed no specific signs of distress, e. g. stress-induced grooming, and pain, i. e. audible vocalizations, for all inhalant gases. However, adrenaline and noradrenaline plasma concentrations were increased, especially in NMRI mice exposed to CO₂ in high concentrations, whereas we did not observe such increase in animals exposed to isoflurane or sevoflurane. Escape latencies in the approach-avoidance test using C57Bl/6J mice did not differ between the three inhalant gases, however, some animals became recumbent during isoflurane and sevoflurane but not during CO₂ exposure. The rise in catecholamine concentrations suggests that CO₂ exposure might be linked to a higher stress response compared to isoflurane and sevoflurane exposure, although we did not observe a behavioral correlate for that. Follow-up studies investigating other fast-acting stress hormones and central anxiety circuits are needed to confirm our findings.

The role of acid-sensitive ion channels in panic disorder: a systematic review of animal studies and meta-analysis of human studies

Acid-sensitive ion channels, such as amiloride-sensitive cation channel (ACCN), transient receptor potential vanilloid-1 (TRPV1), and T-cell death-associated gene 8 (TDAG8) are highly related to the expression of fear and are expressed in several regions of the brain. These molecules can detect acidosis and maintain brain homeostasis. An important role of pH homeostasis has been suggested in the physiology of panic disorder (PD), with acidosis as an interoceptive trigger for panic attacks. To examine the effect of acid-sensitive channels on PD symptoms, we conducted a systematic review and meta-analysis of these chemosensors in rodents and humans. Following PRISMA guidelines, we systematically searched the Web of Science, Medline/Pubmed, Scopus, Science Direct, and SciELO databases. The review included original research in PD patients and animal models of PD that investigated acid-sensitive channels and PD symptoms. Studies without a control group, studies involving patients with a comorbid psychiatric diagnosis, and in vitro studies were excluded. Eleven articles met the inclusion criteria for the systematic review. The majority of the studies showed an association between panic symptoms and acid-sensitive channels. PD patients appear to display polymorphisms in the ACCN gene and elevated levels of TDAG8 mRNA. The results showed a decrease in panic-like symptoms after acid channel blockade in animal models. Despite the relatively limited data on this topic in the literature, our review identified evidence linking acid-sensitive channels to PD in humans and preclinical models. Future research should explore possible underlying mechanisms of this association, attempt to replicate the existing findings in larger populations, and develop new therapeutic strategies based on these biological features.

Amygdala Represents Diverse Forms of Intangible Knowledge, That Illuminate Social Processing and Major Clinical Disorders

Amygdala is an intensively researched brain structure involved in social processing and multiple major clinical disorders, but its functions are not well understood. The functions of a brain structure are best hypothesized on the basis of neuroanatomical connectivity findings, and of behavioral, neuroimaging, neuropsychological and physiological findings. Among the heaviest neuroanatomical interconnections of amygdala are those with perirhinal cortex (PRC), but these are little considered in the theoretical literature. PRC integrates complex, multimodal, meaningful and fine-grained distributed representations of objects and conspecifics. Consistent with this connectivity, amygdala is hypothesized to contribute meaningful and fine-grained representations of intangible knowledge for integration by PRC. Behavioral, neuroimaging, neuropsychological and physiological findings further support amygdala mediation of a diversity of such representations. These representations include subjective valence, impact, economic value, noxiousness, importance, ingroup membership, social status, popularity, trustworthiness and moral features. Further, the formation of amygdala representations is little understood, and is proposed to be often implemented through embodied cognition mechanisms. The hypothesis builds on earlier work, and makes multiple novel contributions to the literature. It highlights intangible knowledge, which is an influential but insufficiently researched factor in social and other behaviors. It contributes to understanding the heavy but neglected amygdala-PRC interconnections, and the diversity of amygdala-mediated intangible knowledge representations. Amygdala is a social brain region, but it does not represent species-typical social behaviors. A novel proposal to clarify its role is postulated. The hypothesis is also suggested to illuminate amygdala's involvement in several core symptoms of autism spectrum disorder (ASD). Specifically, novel and testable explanations are proposed for the ASD symptoms of disorganized visual scanpaths, apparent social disinterest, preference for concrete cognition, aspects of the disorder's heterogeneity, and impairment in some activities of daily living. Together, the presented hypothesis demonstrates substantial explanatory potential in the neuroscience, social and clinical domains.

The acid-sensing ion channel ASIC1a mediates striatal synapse remodeling and procedural motor learning

Acid-sensing ion channel 1a (ASIC1a) is abundant in multiple brain regions, including the striatum, which serves as the input nucleus of the basal ganglia and is critically involved in procedural learning and motor memory. We investigated the functional role of ASIC1a in striatal neurons. We found that ASIC1a was critical for striatum-dependent motor coordination and procedural learning by regulating the synaptic plasticity of striatal medium spiny neurons. Global deletion of Asic1a in mice led to increased dendritic spine density but impaired spine morphology and postsynaptic architecture, which were accompanied by the decreased function of N-methyl-d-aspartate (NMDA) receptors at excitatory synapses. These structural and functional changes caused by the loss of ASIC1a were largely mediated by reduced activation (phosphorylation) of Ca2+/calmodulin-dependent protein kinase II (CaMKII) and extracellular signal-regulated protein kinases (ERKs). Consequently, Asic1a null mice exhibited poor performance on multiple motor tasks, which was rescued by striatal-specific expression of either ASIC1a or CaMKII. Together, our data reveal a previously unknown mechanism mediated by ASIC1a that promotes the excitatory synaptic function underlying striatum-related procedural learning and memory.

Mechanism of nitric oxide and acid-sensing ion channel 1a modulation of panic-like behaviour in the dorsal periaqueductal grey of the mouse

Predators induce defensive responses and fear behaviours in prey. The rat exposure test (RET) is frequently used as an animal model of panic. Nitric oxide (NO) which has been reported to be activated by the NMDA receptor, in turn mediates calcium/calmodulin-dependent protein kinase II (CaMKII) signalling pathways in defensive responses. ACCN2, the orthologous human gene of acid-sensing ion channel 1a (ASIC1a), is also associated with panic disorder; however, few studies have focused on the role of ASIC1a in the modulation of panic and calcium/CaMKII signalling by NO. In the present study, NG-nitro-L-arginine-methyl-ester (L-NAME; non-selective NOS inhibitor), S-nitroso-N-acetyl-D,L-penicillamine (SNAP; NO donor), and psalmotoxin (PcTx-1; selective ASIC1a blocker) were administered to the dorsal periaqueductal grey (dPAG) before the predator stimulus, and the roles of NO in the expression of ASIC1a, phosphorylation of CaMKIIα (p-CaMKIIα) and expression of calmodulin (CaM) were investigated. The effects of ASIC1a, p-CaMKIIα and CaM regulation were also examined. Our results showed that intra-dPAG infusion of L-NAME weakened panic-like behaviour and decreased ASIC1a, p-CaMKIIα and CaM expression levels, whereas intra-dPAG infusion of SNAP enhanced panic-like behaviour and increased ASIC1a, p-CaMKIIα and CaM levels. Intra-dPAG infusion of PcTx-1 also weakened panic-like behaviour and decreased p-CaMKIIα expression level. Taken together, these results indicate that NO and ASIC1a are involved in the modulation of RET-induced panic-like behaviour in the dPAG. NO regulates the calcium/CaMKII signalling pathways, and ASIC1a participates in this regulation.

Heteroarylguanidines as Allosteric Modulators of ASIC1a and ASIC3 Channels

Acid-sensing ion channels (ASICs) are neuronal Na⁺-selective ion channels that open in response to extracellular acidification. They are involved in pain, fear, learning, and neurodegeneration after ischemic stroke. 2-Guanidine-4-methylquinazoline (GMQ) was recently discovered as the first nonproton activator of ASIC3. GMQ is of interest as a gating modifier and pore blocker of ASICs. It has however a low potency, and exerts opposite effects on ASIC1a and ASIC3. To further explore the molecular mechanisms of GMQ action, we have used the guanidinium moiety of GMQ as a scaffold and tested the effects of different GMQ derivatives on the ASIC pH dependence and maximal current. We report that GMQ derivatives containing quinazoline and quinoline induced, as GMQ, an alkaline shift of the pH dependence of activation in ASIC3 and an acidic shift in ASIC1a. Another group of 2-guanidinopyridines shifted the pH dependence of both ASIC1a and ASIC3 to more acidic values. Several compounds induced an alkaline shift of the pH dependence of ASIC1a/2a and ASIC2a/3 heteromers. Compared to GMQ, guanidinopyridines showed a 20-fold decrease in the IC₅₀ for ASIC1a and ASIC3 current inhibition at pH 5. Strikingly, 2-guanidino-quinolines and -pyridines showed a concentration-dependent biphasic effect that resulted at higher concentrations in ASIC1a and ASIC3 inhibition (IC₅₀ > 100 μM), while causing at lower concentration a potentiation of ASIC1a, but not ASIC3 currents (EC₅₀ ≈ 10 μM). In conclusion, we describe a new family of small molecules as ASIC ligands and identify an ASIC subtype-specific potentiation by a subgroup of these compounds.

Acid-sensing ion channels: dual function proteins for chemo-sensing and mechano-sensing

Background

Acid-sensing ion channels (ASICs) are a group of amiloride-sensitive ligand-gated ion channels belonging to the family of degenerin/epithelial sodium channels. ASICs are predominantly expressed in both the peripheral and central nervous system and have been characterized as potent proton sensors to detect extracellular acidification in the periphery and brain.

Main body

Here we review the recent studies focusing on the physiological roles of ASICs in the nervous system. As the major acid-sensing membrane proteins in the nervous system, ASICs detect tissue acidosis occurring at tissue injury, inflammation, ischemia, stroke, and tumors as well as fatiguing muscle to activate pain-sensing nerves in the periphery and transmit pain signals to the brain. Arachidonic acid and lysophosphocholine have been identified as endogenous non-proton ligands activating ASICs in a neutral pH environment. On the other hand, ASICs are found involved in the tether model mechanotransduction, in which the extracellular matrix and cytoplasmic cytoskeletons act like a gating-spring to tether the mechanically activated ion channels and thus transmit the stimulus force to the channels. Accordingly, accumulating evidence has shown ASICs play important roles in mechanotransduction of proprioceptors, mechanoreceptors and nociceptors to monitor the homoeostatic status of muscle contraction, blood volume, and blood pressure as well as pain stimuli.

Conclusion

Together, ASICs are dual-function proteins for both chemosensation and mechanosensation involved in monitoring physiological homoeostasis and pathological signals.

Subtype-selective inhibition of acid-sensing ion channel 3 by a natural flavonoid

AIMS

Acid-sensing ion channels (ASICs) are extracellular proton-gated cation channels that have been implicated in multiple physiological and pathological processes, and peripheral ASIC3 prominently participate into the pathogenesis of chronic pain, itch, and neuroinflammation, which necessitates the need for discovery and development of novel modulators in a subtype-specific manner.

METHODS

Whole-cell patch clamp recordings and behavioral assays were used to examine the effect of several natural compounds on the ASIC-mediated currents and acid-induced nocifensive behavior, respectively.

RESULTS

We identified a natural flavonoid compound, (-)-epigallocatechin gallate (EGCG, compound 11), that acts as a potent inhibitor for the ASIC3 channel in an isoform-specific way. The compound 11 inhibited ASIC3 currents with an apparent half maximal inhibitory concentration of 13.2 μmol/L when measured at pH 5.0. However, at the concentration up to 100 μmol/L, the compound 11 had no significant impacts on the homomeric ASIC1a, 1b, and 2a channels. In contrast to most of the known ASIC inhibitors that usually bear either basic or carboxylic groups, the compound 11 belongs to the polyphenolic family. In compound 11, both the chirality and the 3-hydroxyl group of its pyrogallol part, in addition to the integrity of the gallate part, are crucial for the inhibitory efficacy. Finally, EGCG was found significantly to decrease the acid-induced nocifensive behavior in mice.

CONCLUSION

Taken together, these results thus defined a novel backbone structure for small molecule drug design targeting ASIC3 channels to treat pain-related diseases.

The Role of Ca2+ and BK Channels of Locus Coeruleus (LC) Neurons as a Brake to the CO2 Chemosensitivity Response of Rats

The cellular mechanisms by which LC neurons respond to hypercapnia are usually attributed to an "accelerator" whereby hypercapnic acidosis causes an inhibition of K⁺ channels or activation of Na⁺ and Ca⁺² channels to depolarize CO₂-sensitive neurons. Nevertheless, it is still unknown if this "accelerator" mechanism could be controlled by a brake phenomenon. Whole-cell patch clamping, fluorescence imaging microscopy and plethysmography were used to study the chemosensitive response of the LC neurons. Hypercapnic acidosis activates L-type Ca²⁺ channels and large conductance Ca-activated K⁺ (BK) channels, which function as a "brake" on the chemosensitive response of LC neurons. Our findings indicate that both Ca²⁺ and BK currents develop over the first 2 weeks of postnatal life in rat LC slices and that this brake pathway may cause the developmental decrease in the chemosensitive firing rate response of LC neurons to hypercapnic acidosis. Inhibition of this brake by paxilline (BK channel inhibitor) returns the magnitude of the chemosensitive firing rate response from LC neurons in rats older than P10 to high values similar to those in LC neurons from younger rats. Inhibition of BK channels in LC neurons by bilateral injections of paxilline into the LC results in a significant increase in the hypercapnic ventilatory response of adult rats. Our findings indicate that a BK channel-based braking system helps to determine the chemosensitive respiratory drive of LC neurons and contributes to the hypercapnic ventilatory response. Perhaps, abnormalities of this braking system could result in hypercapnia-induced respiratory disorders and panic responses.

Targeted Acid-Sensing Ion Channel Therapies for Migraine

Acid-sensing ion channels (ASICs) are a family of ion channels, consisting of four members; ASIC1 to 4. These channels are sensitive to changes in pH and are expressed throughout the central and peripheral nervous systems-including brain, spinal cord, and sensory ganglia. They have been implicated in a number of neurological conditions such as stroke and cerebral ischemia, traumatic brain injury, and epilepsy, and more recently in migraine. Their expression within areas of interest in the brain in migraine, such as the hypothalamus and PAG, their demonstrated involvement in preclinical models of meningeal afferent signaling, and their role in cortical spreading depression (the electrophysiological correlate of migraine aura), has enhanced research interest into these channels as potential therapeutic targets in migraine. Migraine is a disorder with a paucity of both acute and preventive therapies available, in which at best 50% of patients respond to available medications, and these medications often have intolerable side effects. There is therefore a great need for therapeutic development for this disabling condition. This review will summarize the understanding of the structure and CNS expression of ASICs, the mechanisms for their potential role in nociception, recent work in migraine, and areas for future research and drug development.

Lactate in the brain: from metabolic end-product to signalling molecule

Lactate in the brain has long been associated with ischaemia; however, more recent evidence shows that it can be found there under physiological conditions. In the brain, lactate is formed predominantly in astrocytes from glucose or glycogen in response to neuronal activity signals. Thus, neurons and astrocytes show tight metabolic coupling. Lactate is transferred from astrocytes to neurons to match the neuronal energetic needs, and to provide signals that modulate neuronal functions, including excitability, plasticity and memory consolidation. In addition, lactate affects several homeostatic functions. Overall, lactate ensures adequate energy supply, modulates neuronal excitability levels and regulates adaptive functions in order to set the 'homeostatic tone' of the nervous system.

Role of acid-sensing ion channels in hypoxia- and hypercapnia-induced ventilatory responses

Previous reports indicate roles for acid-sensing ion channels (ASICs) in both peripheral and central chemoreception, but the contributions of ASICs to ventilatory drive in conscious, unrestrained animals remain largely unknown. We tested the hypotheses that ASICs contribute to hypoxic- and hypercapnic-ventilatory responses. Blood samples taken from conscious, unrestrained mice chronically instrumented with femoral artery catheters were used to assess arterial O₂, CO₂, and pH levels during exposure to inspired gas mixtures designed to cause isocapnic hypoxemia or hypercapnia. Whole-body plethysmography was used to monitor ventilatory parameters in conscious, unrestrained ASIC1, ASIC2, or ASIC3 knockout (^-/-) and wild-type (WT) mice at baseline, during isocapnic hypoxemia and during hypercapnia. Hypercapnia increased respiratory frequency, tidal volume, and minute ventilation in all groups of mice, but there were no differences between ASIC1^-/-, ASIC2^-/-, or ASIC3^-/- and WT. Isocapnic hypoxemia also increased respiratory frequency, tidal volume, and minute ventilation in all groups of mice. Minute ventilation in ASIC2^-/- mice during isocapnic hypoxemia was significantly lower compared to WT, but there were no differences in the responses to isocapnic hypoxemia between ASIC1^-/- or ASIC3^-/- compared to WT. Surprisingly, these findings show that loss of individual ASIC subunits does not substantially alter hypercapnic or hypoxic ventilatory responses.

Acidity and Acid-Sensing Ion Channels in the Normal and Alzheimer's Disease Brain

Alzheimer's disease prevalence has reached epidemic proportion with very few treatment options, which are associated with a multitude of side effects. A potential avenue of research for new therapies are protons, and their associated receptor: acid-sensing ion channels (ASIC). Protons are often overlooked neurotransmitters, and proton-gated currents have been identified in the brain. Furthermore, ASICs have been determined to be crucial for proper brain function. While there is more work to be done, this review is intended to highlight protons as neurotransmitters and their role along with the role of ASICs within physiological functioning of the brain. We will also cover the pathophysiological associations between ASICs and modulators of ASICs. Finally, this review will sum up how the studies of protons, ASICs and their modulators may generate new therapeutic molecules for Alzheimer's disease and other neurodegenerative diseases.

Human ASIC1a mediates stronger acid-induced responses as compared with mouse ASIC1a

Acid-sensing ion channels (ASICs) are the major proton receptor in the brain and a key mediator of acidosis-induced neuronal injuries in disease. Most of published data on ASIC function came from studies performed in mice, and relatively little is known about potential differences between human and mouse ASICs (hASIC and mASIC, respectively). This information is critical for us to better interpret the functional importance of ASICs in human disease. Here, we examined the expression of ASICs in acutely resected human cortical tissue. Compared with mouse cortex, human cortical tissue showed a similar ratio of ASIC1a:ASIC2a expression, had reduced ASIC2b level, and exhibited a higher membrane:total ratio of ASIC1a. We further investigated the mechanism for higher surface trafficking of hASIC1a in heterologous cells. A single amino acid at position 285 was critical for increased N-glycosylation and surface expression of hASIC1a. Consistent with the changes in trafficking and current, cells expressing hASIC1a or mASIC1a S285P mutant had a higher acid-activated calcium increase and exhibited worsened acidotoxicity. These data suggest that ASICs are likely to have a larger impact on acidosis-induced neuronal injuries in humans than mice, and this effect is, at least in part, a result of more efficient trafficking of hASIC1a.-Xu, Y., Jiang, Y.-Q., Li, C., He, M., Rusyniak, W. G., Annamdevula, N., Ochoa, J., Leavesley, S. J., Xu, J., Rich, T. C., Lin, M. T., Zha, X.-M. Human ASIC1a mediates stronger acid-induced responses as compared with mouse ASIC1a.

Hypoventilation Therapy Alleviates Panic by Repeated Induction of Dyspnea

BACKGROUND

Previous research has shown that hypoventilation therapy reduces panic symptoms in part by increasing basal partial pressure of carbon dioxide (PCO₂) levels. We tested an additional pathway by which hypoventilation therapy could exert its therapeutic effects: through repeated interoceptive exposure to sensations of dyspnea.

METHODS

A total of 35 patients with panic disorder were trained to perform exercises to raise their end-tidal PCO₂ levels using a portable capnometry device. Anxiety, dyspnea, end-tidal PCO₂, and respiratory rate were assessed during each exercise across 4 weeks of training. Mixed-model analysis examined whether within-exercise levels of dyspnea were predictive of reduction of panicogenic cognitions.

RESULTS

As expected, within-exercise anxiety and respiratory rate decreased over time. Unexpectedly, PCO₂ dropped significantly from the beginning to the end of exercise, with these drops becoming progressively smaller across weeks. Dyspnea increased and remained consistently above basal levels across weeks. As hypothesized, greater dyspnea was related to significantly lower panicogenic cognitions over time even after controlling for anxiety and PCO₂. Additional exploratory analyses showed that within-exercise increases in dyspnea were related to within-exercise increases in anxiety but were not related to within-exercise increases in PCO₂.

CONCLUSIONS

In support of the interoceptive exposure model, we found that greater dyspnea during hypoventilation exercises resulted in lower panicogenic cognitions even after the effect of PCO₂ was taken into account. The findings offer an additional important target in panic treatment.

Panic-like escape response elicited in mice by exposure to CO2, but not hypoxia

Exposure to elevated concentrations of CO₂ or hypoxia has been widely used in psychiatric research as a panic provoking stimulus. However, the use of these respiratory challenges to model panic-like responses in experimental animals has been less straightforward. Little data is available, from behavioral and endocrine perspectives, to support the conclusion that a marked aversive situation, such as that experienced during panic attacks, was evoked in these animals. We here compared the behavioral responses of male CB57BL/6 mice during exposure to 20% CO₂ or 7% O₂ and its consequence on plasma levels of corticosterone. We also evaluated whether clinically-effective panicolytic drugs affect the behavioral responses expressed during CO₂ exposure. The results showed that whereas hypoxia caused a marked reduction in locomotion, inhalation of CO₂-enriched air evoked an active escape response, characterized by bouts of upward leaps directed to the border of the experimental cage, interpreted as escape attempts. Corticosterone levels were increased 30min after either of the respiratory challenges used, but it was higher in the hypoxia group. Chronic (21days), but not acute, treatment with fluoxetine or imipramine (5, 10 or 15mg/kg) or a single injection of alprazolam (0.025, 0.05 or 0.1mg/kg), but not of the anxiolytic diazepam (0.025, 0.05 or 0.1 and 1mg/kg) reduced the number of escape attempts, indicating a panicolytic-like effect. Altogether, the results suggest that whereas hypoxia increased anxiety, exposure to 20% CO₂ evoked a panic-like state. The latter condition/test protocol seems to be a simple and validated model for studying in mice pathophysiological mechanisms and the screening of novel drugs for panic disorder.

Conserved DNA Methylation Signatures in Early Maternal Separation and in Twins Discordant for CO2 Sensitivity

Respiratory and emotional responses to blood-acidifying inhalation of CO₂ are markers of some human anxiety disorders, and can be enhanced by repeatedly cross-fostering (RCF) mouse pups from their biological mother to unrelated lactating females. Yet, these dynamics remain poorly understood. We show RCF-associated intergenerational transmission of CO₂ sensitivity in normally-reared mice descending from RCF-exposed females, and describe the accompanying alterations in brain DNA methylation patterns. These epigenetic signatures were compared to DNA methylation profiles of monozygotic twins discordant for emotional reactivity to a CO₂ challenge. Altered methylation was consistently associated with repeated elements and transcriptional regulatory regions among RCF-exposed animals, their normally-reared offspring, and humans with CO₂ hypersensitivity. In both species, regions bearing differential methylation were associated with neurodevelopment, circulation, and response to pH acidification processes, and notably included the ASIC2 gene. Our data show that CO₂ hypersensitivity is associated with specific methylation clusters and genes that subserve chemoreception and anxiety. The methylation status of genes implicated in acid-sensing functions can inform etiological and therapeutic research in this field.

Anxiogenic CO2 stimulus elicits exacerbated hot flash-like responses in a rat menopause model and hot flashes in postmenopausal women

OBJECTIVE

As longitudinal studies determined that anxiety is a strong risk factor for hot flashes, we hypothesized that an anxiogenic stimulus that signals air hunger (hypercapnic, normoxic gas) would trigger an exacerbated hot flash-associated increase in tail skin temperature (TST) in a rat ovariectomy (OVEX) model of surgical menopause and hot flashes in symptomatic postmenopausal women. We also assessed TST responses in OVEX serotonin transporter (SERT) rats that models a common polymorphism that is associated with increased climacteric symptoms in postmenopausal women and increases in anxiety traits.

METHODS

OVEX and sham-OVEX rats (initial experiment) and wildtype and SERT OVEX rats (subsequent experiment) were exposed to a 5-minute infusion of 20% carbon dioxide (CO2) normoxic gas while measuring TST. Postmenopausal women were given brief 20% and 35% CO2 challenges, and hot flashes were self-reported and objectively verified.

RESULTS

Compared to controls, OVEX rats had exacerbated increases in TST, and SERT OVEX rats had prolonged TST increases following CO2. Most women reported mild/moderate hot flashes after CO2 challenges, and the hot flash severity to CO2 was positively correlated with daily hot flash frequency.

CONCLUSIONS

The studies demonstrate that this anxiogenic stimulus is capable of inducing cutaneous vasomotor responses in OVEX rats, and eliciting hot flashes in postmenopausal women. In rats, the severity of the response was mediated by loss of ovarian function and increased anxiety traits (SERT), and, in women, by daily hot flash frequency. These findings may provide insights into anxiety-related triggers and genetic risk factors for hot flashes in thermoneutral environments.

Reappraising Preclinical Models of Separation Anxiety Disorder, Panic Disorder, and CO2 Sensitivity: Implications for Methodology and Translation into New Treatments

Separation anxiety applies to multiple forms of distress responses seen in mammals during postnatal development, including separation from a caregiver. Childhood separation anxiety disorder is an important risk factor for developing panic disorder in early adulthood, and both conditions display an increased sensitivity to elevated CO₂ concentrations inhaled from the air. By interfacing epidemiological, genetic, and physiological knowledge with preclinical animal research models, it is possible to decipher the mechanisms that are central to separation anxiety and panic disorders while also suggesting possible therapies. Preclinical research models allow for environmentally controlled studies of early interferences with parental care. These models have shown that different forms of early maternal separation in mice and rats induce elevated CO₂ respiratory sensitivity, an important biomarker of separation anxiety and panic disorders. In mice, this is likely due to gene-environment interactions that affect multiple behavioural and physical phenotypes after exposure to this early adversity. Although several questions regarding the causal mechanism of separation anxiety and panic disorder remain unanswered, the identification and improved understanding of biomarkers that link these mental health conditions under the guise of preclinical research models in conjunction with human longitudinal cohort studies can help resolve these issues.

Stimuli-responsive metal–organic supercontainers as synthetic proton receptors

Functional supercontainers exhibit intriguing H⁺-dependent fluorescent switching behavior, opening exiting new opportunities for proton modulation in both chemistry and biology.

Cellular Localization of Acid-Sensing Ion Channel 1 in Rat Nucleus Tractus Solitarii

By determining its cellular localization in the nucleus tractus solitarii (NTS), we sought anatomical support for a putative physiological role for acid-sensing ion channel Type 1 (ASIC1) in chemosensitivity. Further, we sought to determine the effect of a lesion that produces gliosis in the area. In rats, we studied ASIC1 expression in control tissue with that in tissue with gliosis, which is associated with acidosis, after saporin lesions. We hypothesized that saporin would increase ASIC1 expression in areas of gliosis. Using fluorescent immunohistochemistry and confocal microscopy, we found that cells and processes containing ASIC1-immunoreactivity (IR) were present in the NTS, the dorsal motor nucleus of vagus, and the area postrema. In control tissue, ASIC1-IR predominantly colocalized with IR for the astrocyte marker, glial fibrillary acidic protein (GFAP), or the microglial marker, integrin αM (OX42). The subpostremal NTS was the only NTS region where neurons, identified by protein gene product 9.5 (PGP9.5), contained ASIC1-IR. ASIC1-IR increased significantly (157 ± 8.6% of control, p < 0.001) in the NTS seven days after microinjection of saporin. As we reported previously, GFAP-IR was decreased in the center of the saporin injection site, but GFAP-IR was increased in the surrounding areas where OX42-IR, indicative of activated microglia, was also increased. The over-expressed ASIC1-IR colocalized with GFAP-IR and OX42-IR in those reactive astrocytes and microglia. Our results support the hypothesis that ASIC1 would be increased in activated microglia and in reactive astrocytes after injection of saporin into the NTS.

Effects of 35% carbon dioxide (CO2) inhalation in patients with post-traumatic stress disorder (PTSD): A double-blind, randomized, placebo-controlled, cross-over trial

BACKGROUND

In patients with post-traumatic stress disorder (PTSD) two open pilot studies about the effects of 35% carbon dioxide (CO₂) exist. One shows an augmented panicogenic and anxiogenic response (Muhtz et al., 2011), the other does not (Talesnik et al. 2007). We further characterized the CO₂ reactivity in PTSD using for the first time placebo-controlled and double-blind conditions.

METHODS

In 20 patients with PTSD we assessed panic, anxiety, dissociative and PTSD symptoms after a single vital capacity inhalation of 35% CO₂ compared to a placebo gas condition in a within-participant cross-over, placebo-controlled, double-blind and randomized design.

RESULTS

Inhalation of 35% CO₂ versus placebo provoked significantly increased panic, anxiety, dissociative and PTSD symptoms. The reaction to placebo gas was minimal. Order of inhalation, patients' sex or age did not influence the results. The panic and anxiety response under CO₂ was considerably higher in the PTSD patients than in healthy controls from our previous open study.

CONCLUSIONS

The results corroborate that our preceding findings of an increased CO₂ reactivity in patients with PTSD are not false positive due to the open design or the lack of placebo control. Replication in a larger number of PTSD patients and matched control subjects is needed. The potential role of childhood traumatisation, psychiatric comorbidity, psychotropic medication and trait dissociation in prior contradictory reports should be clarified.

Naked mole-rat acid-sensing ion channel 3 forms nonfunctional homomers, but functional heteromers

Acid-sensing ion channels (ASICs) form both homotrimeric and heterotrimeric ion channels that are activated by extracellular protons and are involved in a wide range of physiological and pathophysiological processes, including pain and anxiety. ASIC proteins can form both homotrimeric and heterotrimeric ion channels. The ASIC3 subunit has been shown to be of particular importance in the peripheral nervous system with pharmacological and genetic manipulations demonstrating a role in pain. Naked mole-rats, despite having functional ASICs, are insensitive to acid as a noxious stimulus and show diminished avoidance of acidic fumes, ammonia, and carbon dioxide. Here we cloned naked mole-rat ASIC3 (nmrASIC3) and used a cell-surface biotinylation assay to demonstrate that it traffics to the plasma membrane, but using whole-cell patch clamp electrophysiology we observed that nmrASIC3 is insensitive to both protons and the non-proton ASIC3 agonist 2-guanidine-4-methylquinazoline. However, in line with previous reports of ASIC3 mRNA expression in dorsal root ganglia neurons, we found that the ASIC3 antagonist APETx2 reversibly inhibits ASIC-like currents in naked mole-rat dorsal root ganglia neurons. We further show that like the proton-insensitive ASIC2b and ASIC4, nmrASIC3 forms functional, proton-sensitive heteromers with other ASIC subunits. An amino acid alignment of ASIC3s between 9 relevant rodent species and human identified unique sequence differences that might underlie the proton insensitivity of nmrASIC3. However, introducing nmrASIC3 differences into rat ASIC3 (rASIC3) produced only minor differences in channel function, and replacing the nmrASIC3 sequence with that of rASIC3 did not produce a proton-sensitive ion channel. Our observation that nmrASIC3 forms nonfunctional homomers may reflect a further adaptation of the naked mole-rat to living in an environment with high-carbon dioxide levels.

Neonatal maternal separation opposes the facilitatory effect of castration on the respiratory response to hypercapnia of the adult male rat: Evidence for the involvement of the medial amygdala

Respiratory manifestations of panic disorder (PD) include a greater respiratory instability and enhanced responsiveness to CO₂ compared to normal individuals. Although the prevalence of PD is approximately three times greater in women compared to men, the origins of this sexual dimorphism remain poorly understood. Similar to PD patients, adult female rats previously subjected to neonatal maternal separation (NMS) show an increase in their ventilatory response to CO₂ . Because this effect of NMS is not observed in males, we hypothesised that testosterone prevents NMS-induced hyper-responsiveness to CO₂ . Pups subjected to NMS were placed in an incubator for 3 h d^-1 from postnatal days 3-12. Control pups remained undisturbed. At adulthood (8-10 weeks of age), rats were then subjected either to sham surgery or castration. Fourteen days later, breathing was measured at rest (room air) and during acute exposure to hypercapnia (5 and 10% CO₂ for 10 minutes each) using plethysmography. To gain insight into the mechanisms involved, c-fos expression was used as an indicator of neuronal activation. Brains were collected following air or CO₂ exposure for quantification of c-fos positive cells by immunohistochemistry in selected regions, including the paraventricular nucleus of the hypothalamus, the dorsomedial hypothalamus and the amygdalar complex. Castration produced a 100% increase of hyperventilatory response to 10% CO₂ in control rats. Unexpectedly, castration had no effect on the hyperventilatory response of NMS rats. The intensity of the hypercapnic response was inversely correlated with c-fos expression in the medial amygdala. We conclude that testosterone prevents the hyper-responsiveness to CO₂ , whereas NMS attenuates sensitivity to hormone withdrawal. We propose that an inhibitory influence from the medial amygdala contributes to this effect.

The nonproton ligand of acid-sensing ion channel 3 activates mollusk-specific FaNaC channels via a mechanism independent of the native FMRFamide peptide

The degenerin/epithelial sodium channel (DEG/ENaC) superfamily of ion channels contains subfamilies with diverse functions that are fundamental to many physiological and pathological processes, ranging from synaptic transmission to epileptogenesis. The absence in mammals of some DEG/ENaCs subfamily orthologues such as FMRFamide peptide-activated sodium channels (FaNaCs), which have been identified only in mollusks, indicates that the various subfamilies diverged early in evolution. We recently reported that the nonproton agonist 2-guanidine-4-methylquinazoline (GMQ) activates acid-sensing ion channels (ASICs), a DEG/ENaC subfamily mainly in mammals, in the absence of acidosis. Here, we show that GMQ also could directly activate the mollusk-specific FaNaCs. Differences in ion selectivity and unitary conductance and effects of substitutions at key residues revealed that GMQ and FMRFamide activate FaNaCs via distinct mechanisms. The presence of two activation mechanisms in the FaNaC subfamily diverging early in the evolution of DEG/ENaCs suggested that dual gating is an ancient feature in this superfamily. Notably, the GMQ-gating mode is still preserved in the mammalian ASIC subfamily, whereas FMRFamide-mediated channel gating was lost during evolution. This implied that GMQ activation may be essential for the functions of mammalian DEG/ENaCs. Our findings provide new insights into the evolution of DEG/ENaCs and may facilitate the discovery and characterization of their endogenous agonists.

Amiloride-sensitive cation channel 2 genotype affects the response to a carbon dioxide panic challenge

Until recently, genetic research into panic disorder (PD) has had only limited success. Inspired by rodent research, demonstrating that the acid-sensing ion channel 1a (ASIC1a) is critically involved in the behavioral fear response to carbon dioxide (CO₂) exposure, variants in the human homologue gene amiloride-sensitive cation channel 2 (ACCN2) were shown to be associated with PD. However, the relationship between changes in brain pH and ACCN2, as done in rodents by CO₂ exposure, has not been investigated yet in humans. Here, we examined this link between the ACCN2 gene and the response to CO₂ exposure in two studies: in healthy volunteers as well as PD patients and using both behavioral and physiological outcome measures. More specifically, 107 healthy volunteers and 183 PD patients underwent a 35% CO₂ inhalation. Negative affect was assessed using visual analogue scales and the panic symptom list (PSL), and, in healthy volunteers, cardiovascular measurements. The single nucleotide polymorphism rs10875995 was significantly associated with a higher emotional response in PD patients and with an increase in systolic as well as diastolic blood pressure in healthy subjects. In all measurements, subjects homozygous for the T-allele showed a heightened reactivity to CO₂. Furthermore, a trend towards an rs685012 genotype effect on the emotional response was found in PD patients. We provide the first evidence that genetic variants in the ACCN2 are associated with differential sensitivity to CO₂ in PD patients as well as healthy volunteers, further supporting ACCN2 as a promising candidate for future research to improve current treatment options.

Molecular Basis for Allosteric Inhibition of Acid-Sensing Ion Channel 1a by Ibuprofen

A growing body of evidence links certain aspects of nonsteroidal anti-inflammatory drug (NSAID) pharmacology with acid-sensing ion channels (ASICs), a small family of excitatory neurotransmitter receptors implicated in pain and neuroinflammation. The molecular basis of NSAID inhibition of ASICs has remained unknown, hindering the exploration of this line of therapy. Here, we characterized the mechanism of inhibition, explored the molecular determinants of sensitivity, and sought to establish informative structure-activity relationships, using electrophysiology, site-directed mutagenesis, and voltage-clamp fluorometry. Our results show that ibuprofen is an allosteric inhibitor of ASIC1a, which binds to a crucial site in the agonist transduction pathway and causes conformational changes that oppose channel activation. Ibuprofen inhibits several ASIC subtypes, but certain ibuprofen derivatives show some selectivity for ASIC1a over ASIC2a and vice versa. These results thus define the NSAID/ASIC interaction and pave the way for small-molecule drug design targeting pain and inflammation.

Review of CO₂ as a Euthanasia Agent for Laboratory Rats and Mice

Selecting an appropriate, effective euthanasia agent is controversial. Several recent publications provide clarity on the use of CO2 in laboratory rats and mice. This review examines previous studies on CO2 euthanasia and presents the current body of knowledge on the subject. Potential areas for further investigation and recommendations are provided.

ASIC1A in neurons is critical for fear-related behaviors

Acid-sensing ion channels (ASICs) have been implicated in fear-, addiction- and depression-related behaviors in mice. While these effects have been attributed to ASIC1A in neurons, it has been reported that ASICs may also function in nonneuronal cells. To determine if ASIC1A in neurons is indeed required, we generated neuron-specific knockout (KO) mice with floxed Asic1a alleles disrupted by Cre recombinase driven by the neuron-specific synapsin I promoter (SynAsic1a KO mice). We confirmed that Cre expression occurred in neurons, but not all neurons, and not in nonneuronal cells including astrocytes. Consequent loss of ASIC1A in some but not all neurons was verified by western blotting, immunohistochemistry and electrophysiology. We found ASIC1A was disrupted in fear circuit neurons, and SynAsic1a KO mice exhibited prominent deficits in multiple fear-related behaviors including Pavlovian fear conditioning to cue and context, predator odor-evoked freezing and freezing responses to carbon dioxide inhalation. In contrast, in the nucleus accumbens ASIC1A expression was relatively normal in SynAsic1a KO mice, and consistent with this observation, cocaine conditioned place preference (CPP) was normal. Interestingly, depression-related behavior in the forced swim test, which has been previously linked to ASIC1A in the amygdala, was also normal. Together, these data suggest neurons are an important site of ASIC1A action in fear-related behaviors, whereas other behaviors likely depend on ASIC1A in other neurons or cell types not targeted in SynAsic1a KO mice. These findings highlight the need for further work to discern the roles of ASICs in specific cell types and brain sites.

Transient acidosis while retrieving a fear-related memory enhances its lability

Attenuating the strength of fearful memories could benefit people disabled by memories of past trauma. Pavlovian conditioning experiments indicate that a retrieval cue can return a conditioned aversive memory to a labile state. However, means to enhance retrieval and render a memory more labile are unknown. We hypothesized that augmenting synaptic signaling during retrieval would increase memory lability. To enhance synaptic transmission, mice inhaled CO2 to induce an acidosis and activate acid sensing ion channels. Transient acidification increased the retrieval-induced lability of an aversive memory. The labile memory could then be weakened by an extinction protocol or strengthened by reconditioning. Coupling CO2 inhalation to retrieval increased activation of amygdala neurons bearing the memory trace and increased the synaptic exchange from Ca2+-impermeable to Ca2+-permeable AMPA receptors. The results suggest that transient acidosis during retrieval renders the memory of an aversive event more labile and suggest a strategy to modify debilitating memories.

A selectivity filter at the intracellular end of the acid-sensing ion channel pore

Increased extracellular proton concentrations during neurotransmission are converted to excitatory sodium influx by acid-sensing ion channels (ASICs). 10-fold sodium/potassium selectivity in ASICs has long been attributed to a central constriction in the channel pore, but experimental verification is lacking due to the sensitivity of this structure to conventional manipulations. Here, we explored the basis for ion selectivity by incorporating unnatural amino acids into the channel, engineering channel stoichiometry and performing free energy simulations. We observed no preference for sodium at the “GAS belt” in the central constriction. Instead, we identified a band of glutamate and aspartate side chains at the lower end of the pore that enables preferential sodium conduction.

DOI:http://dx.doi.org/10.7554/eLife.24630.001

The Choice of Euthanasia Method Affects Metabolic Serum Biomarkers

The impact of euthanasia methods on endocrine and metabolic parameters in rodent tissues and biological fluids is highly relevant for the accuracy and reliability of the data collected. However, few studies concerning this issue are found in the literature. We compared the effects of three euthanasia methods currently used in animal experimentation (i.e. decapitation, CO₂ inhalation and pentobarbital injection) on the serum levels of corticosterone, insulin, glucose, triglycerides, cholesterol and a range of free fatty acids in rats. The corticosterone and insulin levels were not significantly affected by the euthanasia protocol used. However, euthanasia by an overdose of pentobarbital (120 mg/kg intraperitoneal injection) increased the serum levels of glucose, and decreased cholesterol, stearic and arachidonic acids levels compared with euthanasia by CO₂ inhalation and decapitation. CO₂ inhalation appears to increase the serum levels of triglycerides, while euthanasia by decapitation induced no individual discrepant biomarker level. We conclude that choice of the euthanasia methods is critical for the reliability of serum biomarkers and indicate the importance of selecting adequate euthanasia methods for metabolic analysis in rodents. Decapitation without anaesthesia may be the most adequate method of euthanasia when taking both animal welfare and data quality in consideration.

Dual-hormone stress reactivity predicts downstream war-zone stress-evoked PTSD

BACKGROUND

The crucial role of the hypothalamic-pituitary-adrenal axis (HPA) in stress-related homeostasis suggests dysregulated HPA involvement in the pathogenesis of post-traumatic stress disorder (PTSD), yet most studies examining linkages between HPA axis measures and PTSD have yielded null findings. One untested explanation for this inconsistency is a failure to account for simultaneous adrenal and gonadal influence. Here we tested the singular and interactive effects of cortisol (C_R) and testosterone (T_R) reactivity as moderators of war-zone stress evoked PTSD emergence in the war-zone.

METHODS

U.S. soldiers (N=120) scheduled for deployment to Iraq completed pre-deployment measures of C_R and T_R stress reactivity to a CO₂ inhalation challenge. Once deployed, monthly assessments of exposure to traumatic war-zone stressors and PTSD symptoms were collected via a web-based assessment system.

RESULTS

Cortisol hypo-reactivity potentiated the pathogenic impact of war-zone stressors only in soldiers for whom the CO₂ challenge did not elevate testosterone, suggesting that the dual hormone stress reactivity profile of blunted cortisol and testosterone may confer increased risk for PTSD emergence by potentiating the pathogenic effects of war-zone stressors.

CONCLUSIONS

Findings underscore the utility of assessing both HPA and HPG stress reactivity when assessing PTSD vulnerability and may help inform efforts for enhanced soldier screening and inoculation to war-zone stressors.

Pharmacotherapy in Generalized Anxiety Disorder: Novel Experimental Medicine Models and Emerging Drug Targets

Many pharmacological and psychological approaches have been found efficacious in patients with generalized anxiety disorder (GAD), but many treatment-seeking patients will not respond and others will relapse despite continuing with interventions that initially had beneficial effects. Other patients will respond but then stop treatment early because of untoward effects such as sexual dysfunction, drowsiness, and weight gain. There is much scope for the development of novel approaches that could have greater overall effectiveness or acceptability than currently available interventions or that have particular effectiveness in specific clinical subgroups. 'Experimental medicine' studies in healthy volunteers model disease states and represent a proof-of-concept approach for the development of novel therapeutic interventions: they determine whether to proceed to pivotal efficacy studies and so can reduce delays in translating innovations into clinical practice. Investigations in healthy volunteers challenged with the inhalation of air 'enriched' with 7.5% carbon dioxide (CO₂) indicate this technique provides a validated and robust experimental medicine model, mirroring the subjective, autonomic, and cognitive features of GAD. The anxiety response during CO₂ challenge probably involves both central noradrenergic neurotransmission and effects on acid-base sensitive receptors and so may stimulate development of novel agents targeted at central chemosensors. Increasing awareness of the potential role of altered cytokine balance in anxiety and the interplay of cytokines with monoaminergic mechanisms may also encourage the investigation of novel agents with modulating effects on immunological profiles. Although seemingly disparate, these two approaches to treatment development may pivot on a shared mechanism in exerting anxiolytic-like effects through pharmacological effects on acid-sensing ion channels.

Biological markers for anxiety disorders, OCD and PTSD: A consensus statement. Part II: Neurochemistry, neurophysiology and neurocognition

OBJECTIVE

Biomarkers are defined as anatomical, biochemical or physiological traits that are specific to certain disorders or syndromes. The objective of this paper is to summarise the current knowledge of biomarkers for anxiety disorders, obsessive-compulsive disorder (OCD) and posttraumatic stress disorder (PTSD).

METHODS

Findings in biomarker research were reviewed by a task force of international experts in the field, consisting of members of the World Federation of Societies for Biological Psychiatry Task Force on Biological Markers and of the European College of Neuropsychopharmacology Anxiety Disorders Research Network.

RESULTS

The present article (Part II) summarises findings on potential biomarkers in neurochemistry (neurotransmitters such as serotonin, norepinephrine, dopamine or GABA, neuropeptides such as cholecystokinin, neurokinins, atrial natriuretic peptide, or oxytocin, the HPA axis, neurotrophic factors such as NGF and BDNF, immunology and CO2 hypersensitivity), neurophysiology (EEG, heart rate variability) and neurocognition. The accompanying paper (Part I) focuses on neuroimaging and genetics.

CONCLUSIONS

Although at present, none of the putative biomarkers is sufficient and specific as a diagnostic tool, an abundance of high quality research has accumulated that should improve our understanding of the neurobiological causes of anxiety disorders, OCD and PTSD.

Functional neuroanatomy in panic disorder: Status quo of the research

AIM

To provide an overview of the current research in the functional neuroanatomy of panic disorder.

METHODS

Panic disorder (PD) is a frequent psychiatric disease. Gorman et al (1989; 2000) proposed a comprehensive neuroanatomical model of PD, which suggested that fear- and anxiety-related responses are mediated by a so-called "fear network" which is centered in the amygdala and includes the hippocampus, thalamus, hypothalamus, periaqueductal gray region, locus coeruleus and other brainstem sites. We performed a systematic search by the electronic database PubMed. Thereby, the main focus was laid on recent neurofunctional, neurostructural, and neurochemical studies (from the period between January 2012 and April 2016). Within this frame, special attention was given to the emerging field of imaging genetics.

RESULTS

We noted that many neuroimaging studies have reinforced the role of the "fear network" regions in the pathophysiology of panic disorder. However, recent functional studies suggest abnormal activation mainly in an extended fear network comprising brainstem, anterior and midcingulate cortex (ACC and MCC), insula, and lateral as well as medial parts of the prefrontal cortex. Interestingly, differences in the amygdala activation were not as consistently reported as one would predict from the hypothesis of Gorman et al (2000). Indeed, amygdala hyperactivation seems to strongly depend on stimuli and experimental paradigms, sample heterogeneity and size, as well as on limitations of neuroimaging techniques. Advanced neurochemical studies have substantiated the major role of serotonergic, noradrenergic and glutamatergic neurotransmission in the pathophysiology of PD. However, alterations of GABAergic function in PD are still a matter of debate and also their specificity remains questionable. A promising new research approach is "imaging genetics". Imaging genetic studies are designed to evaluate the impact of genetic variations (polymorphisms) on cerebral function in regions critical for PD. Most recently, imaging genetic studies have not only confirmed the importance of serotonergic and noradrenergic transmission in the etiology of PD but also indicated the significance of neuropeptide S receptor, CRH receptor, human TransMEMbrane protein (TMEM123D), and amiloride-sensitive cation channel 2 (ACCN2) genes.

CONCLUSION

In light of these findings it is conceivable that in the near future this research will lead to the development of clinically useful tools like predictive biomarkers or novel treatment options.

Acid-Sensing Ion Channel Pharmacology, Past, Present, and Future …

pH is one of the most strictly controlled parameters in mammalian physiology. An extracellular pH of ~7.4 is crucial for normal physiological processes, and perturbations to this have profound effects on cell function. Acidic microenvironments occur in many physiological and pathological conditions, including inflammation, bone remodeling, ischemia, trauma, and intense synaptic activity. Cells exposed to these conditions respond in different ways, from tumor cells that thrive to neurons that are either suppressed or hyperactivated, often fatally. Acid-sensing ion channels (ASICs) are primary pH sensors in mammals and are expressed widely in neuronal and nonneuronal cells. There are six main subtypes of ASICs in rodents that can form homo- or heteromeric channels resulting in many potential combinations. ASICs are present and activated under all of the conditions mentioned earlier, suggesting that they play an important role in how cells respond to acidosis. Compared to many other ion channel families, ASICs were relatively recently discovered-1997-and there is a substantial lack of potent, subtype-selective ligands that can be used to elucidate their structural and functional properties. In this chapter I cover the history of ASIC channel pharmacology, which began before the proteins were even identified, and describe the current arsenal of tools available, their limitations, and take a glance into the future to predict from where new tools are likely to emerge.

Peripheral sensory neuron injury contributes to neuropathic pain in experimental autoimmune encephalomyelitis

Multiple sclerosis (MS)-induced neuropathic pain deteriorates quality of life in patients but is often refractory to treatment. In experimental autoimmune encephalomyelitis (EAE), a rodent model of MS, animals develop neuropathy and inflammation-induced tissue acidosis, which suggests the involvement of acid-sensing ion channels (ASICs). Also, peripheral neuropathy is reported in MS patients. However, the involvement of the peripheral nervous system (PNS) in MS neuropathic pain remains elusive. This study investigated the contribution of ASICs and peripheral neuropathy in MS-induced neuropathic pain. Elicited pain levels were as high in Asic1a^-/-, Asic2^-/- and Asic3^-/- mice as wild-type mice even though only Asic1a^-/- mice showed reduced EAE disease severity, which indicates that pain in EAE was independent of disease severity. We thus adopted an EAE model without pertussis toxin (EAEnp) to restrain activated immunity in the periphery and evaluate the PNS contribution to pain. Both EAE and EAEnp mice showed similar pain behaviors and peripheral neuropathy in nerve fibers and DRG neurons. Moreover, pregabalin significantly reduced neuropathic pain in both EAE and EAEnp mice. Our findings highlight the essential role of the PNS in neuropathic pain in EAE and pave the way for future development of analgesics without side effects in the CNS.