Identification of a putative binding site critical for general anesthetic activation of TRPA1

Remote neurostimulation through an endogenous ion channel using a near-infrared light-activatable nanoagonist

The development of noninvasive approaches to precisely control neural activity in mammals is highly desirable. Here, we used the ion channel transient receptor potential ankyrin-repeat 1 (TRPA1) as a proof of principle, demonstrating remote near-infrared (NIR) activation of endogenous neuronal channels in mice through an engineered nanoagonist. This achievement enables specific neurostimulation in nongenetically modified mice. Initially, target-based screening identified flavins as photopharmacological agonists, allowing for the photoactivation of TRPA1 in sensory neurons upon ultraviolet A/blue light illumination. Subsequently, upconversion nanoparticles (UCNPs) were customized with an emission spectrum aligned to flavin absorption and conjugated with flavin adenine dinucleotide, creating a nanoagonist capable of NIR activation of TRPA1. Following the intrathecal injection of the nanoagonist, noninvasive NIR stimulation allows precise bidirectional control of nociception in mice through remote activation of spinal TRPA1. This study demonstrates a noninvasive NIR neurostimulation method with the potential for adaptation to various endogenous ion channels and neural processes by combining photochemical toolboxes with customized UCNPs.

Synapse-Specific Trapping of SNARE Machinery Proteins in the Anesthetized Drosophila Brain

General anesthetics disrupt brain network dynamics through multiple pathways, in part through postsynaptic potentiation of inhibitory ion channels as well as presynaptic inhibition of neuroexocytosis. Common clinical general anesthetic drugs, such as propofol and isoflurane, have been shown to interact and interfere with core components of the exocytic release machinery to cause impaired neurotransmitter release. Recent studies however suggest that these drugs do not affect all synapse subtypes equally. We investigated the role of the presynaptic release machinery in multiple neurotransmitter systems under isoflurane general anesthesia in the adult female Drosophila brain using live-cell super-resolution microscopy and optogenetic readouts of exocytosis and neural excitability. We activated neurotransmitter-specific mushroom body output neurons and imaged presynaptic function under isoflurane anesthesia. We found that isoflurane impaired synaptic release and presynaptic protein dynamics in excitatory cholinergic synapses. In contrast, isoflurane had little to no effect on inhibitory GABAergic or glutamatergic synapses. These results present a distinct inhibitory mechanism for general anesthesia, whereby neuroexocytosis is selectively impaired at excitatory synapses, while inhibitory synapses remain functional. This suggests a presynaptic inhibitory mechanism that complements the other inhibitory effects of these drugs.

Visual analysis of global research on the transient receptor potential ankyrin 1 channel: A literature review from 2002 to 2022

Background and aims

The transient receptor potential ankyrin 1 (TRPA1) channel has become a focus in pain research. However, there are no bibliometric studies that systematically analyze the existing research in this area. This study aimed to provide a systematic review of the existing literature on TRPA1 using a bibliometric analysis.

Methods

Published literature in the field of TRPA1 was collected from the Web of Science Core Collection database. Quantitative and qualitative analyses of publications, countries, institutions, authors, journals, and other entries were conducted using Excel, VOSview, and Citespace software to provide insight into global research hotspots and trends in the TRPA1 field.

Results

This study included 1189 scientific products published in 398 journals from 52 countries. The United States of America (n = 367) had the most publications, ahead of Japan (n = 212) and China (n = 199). The University of Florence (n = 55) was the most productive institution and Pierangelo Geppetti (n = 46) was the most productive author. PLoS One (n = 40) published the most articles on TRPA1. Pain, cold, inflammation, covalent modification, hyperalgesia, and oxidative stress were the most common keywords used in the studies.

Conclusion

This study provides the first bibliometric analysis of TRPA1 publications. The physiological functions of TRPA1, TRPA1, and neuropathic pain, TRPA1 as a therapeutic target, and agonists of TRPA1 are trending in TRPA1 research. Neuropathic pain, apoptosis, and sensitization could be focus areas of future research. This study provides important insight in the field of TRPA1 research.

Stilbenoid compounds inhibit NF-κB-mediated inflammatory responses in the Drosophila intestine

Introduction

Stilbenoid compounds have been described to have anti-inflammatory properties in animal models in vivo, and have been shown to inhibit Ca2+-influx through the transient receptor potential ankyrin 1 (TrpA1).

Methods

To study how stilbenoid compounds affect inflammatory signaling in vivo, we have utilized the fruit fly, Drosophila melanogaster, as a model system. To induce intestinal inflammation in the fly, we have fed flies with the intestinal irritant dextran sodium sulphate (DSS).

Results

We found that DSS induces severe changes in the bacteriome of the Drosophila intestine, and that this dysbiosis causes activation of the NF-κB transcription factor Relish. We have taken advantage of the DSS-model to study the anti-inflammatory properties of the stilbenoid compounds pinosylvin (PS) and pinosylvin monomethyl ether (PSMME). With the help of in vivo approaches, we have identified PS and PSMME to be transient receptor ankyrin 1 (TrpA1)-dependent antagonists of NF-κB-mediated intestinal immune responses in Drosophila. We have also computationally predicted the putative antagonist binding sites of these compounds at Drosophila TrpA1.

Discussion

Taken together, we show that the stilbenoids PS and PSMME have anti-inflammatory properties in vivo in the intestine and can be used to alleviate chemically induced intestinal inflammation in Drosophila.

Desflurane Post-treatment Reduces Hypoxic-ischemic Brain Injury via Reducing Transient Receptor Potential Ankyrin 1 in Neonatal Rats

Perinatal hypoxic-ischemic (HI) brain injury leads to mortality and morbidity in neonates and children. There are no effective and practical methods to attenuate this brain injury. This study determined whether desflurane, a volatile anesthetic with limited effect on the cardiovascular system, protected against HI-induced brain damage and the role of transient receptor potential ankyrin 1 (TRPA1), a mediator for simulated ischemia-induced myelin damage, in this protection. Seven-day-old male and female Sprague-Dawley rats had brain HI. They were exposed to 4.8%, 7.6% or 11.4% desflurane immediately or 4.8% desflurane at 0.5, 1 or 2 h after the HI. Brain tissue loss was evaluated 7 days later. Neurological functions and brain structures of rats with HI and 4.8% desflurane post-treatment were evaluated 4 weeks after the HI. TRPA1 expression was determined by Western blotting. HC-030031, a TRPA1 inhibitor, was used to determine the role of TRPA1 in the HI-induced brain injury. HI induced brain tissue and neuronal loss, which was attenuated by all tested concentrations of desflurane. Desflurane post-treatment also improved motor function, learning and memory in rats with brain HI. Brain HI increased the expression of TRPA1 and this increase was inhibited by desflurane. TRPA1 inhibition reduced HI-induced brain tissue loss and impairment of learning and memory. However, the combination of TRPA1 inhibition and desflurane post-treatment did not preserve brain tissues, learning and memory better than TRPA1 inhibition or desflurane post-treatment alone. Our results suggest that desflurane post-treatment induces neuroprotection against neonatal HI. This effect may be mediated by inhibiting TRPA1.

Transient receptor potential Ankyrin-1 (TRPA1) agonists suppress myelination and induce demyelination in organotypic cortical slices

Oligodendrocytes are highly specialized glial cells characterized by their production of multilayer myelin sheaths that wrap axons to speed up action potential propagation. It is due to their specific role in supporting axons that impairment of myelin structure and function leads to debilitating symptoms in a wide range of degenerative diseases, including Multiple Sclerosis and Leukodystrophies. It is known that myelin damage can be receptor-mediated and recently oligodendrocytes have been shown to express Ca2+ -permeable Transient Receptor Potential Ankyrin-1 (TRPA1) channels, whose activation can result in myelin damage in ischemia. Here, we show, using organotypic cortical slice cultures, that TRPA1 activation, by TRPA1 agonists JT010 and Carvacrol for varying lengths of time, induces myelin damage. Although TRPA1 activation does not appear to affect oligodendrocyte progenitor cell number or proliferation, it prevents myelin formation and after myelination causes internodal shrinking and significant myelin degradation. This does not occur when the TRPA1 antagonist, A967079, is also applied. Of note is that when TRPA1 agonists are applied for either 24 h, 3 days or 7 days, axon integrity appears to be preserved while mature myelinated oligodendrocytes remain but with significantly shortened internodes. These results provide further evidence that TRPA1 inhibition could be protective in demyelination diseases and a promising therapy to prevent demyelination and promote remyelination.

CYP1B1-derived epoxides modulate the TRPA1 channel in chronic pain

Pain is often debilitating, and current treatments are neither universally efficacious nor without risks. Transient receptor potential (TRP) ion channels offer alternative targets for pain relief, but little is known about the regulation or identities of endogenous TRP ligands that affect inflammation and pain. Here, transcriptomic and targeted lipidomic analysis of damaged tissue from the mouse spinal nerve ligation (SNL)-induced chronic pain model revealed a time-dependent increase in Cyp1b1 mRNA and a concurrent accumulation of 8,9-epoxyeicosatrienoic acid (EET) and 19,20-EpDPA post injury. Production of 8,9-EET and 19,20-EpDPA by human/mouse CYP1B1 was confirmed in vitro, and 8,9-EET and 19,20-EpDPA selectively and dose-dependently sensitized and activated TRPA1 in overexpressing HEK-293 cells and Trpa1-expressing/AITC-responsive cultured mouse peptidergic dorsal root ganglia (DRG) neurons. TRPA1 activation by 8,9-EET and 19,20-EpDPA was attenuated by the antagonist A967079, and mouse TRPA1 was more responsive to 8,9-EET and 19,20-EpDPA than human TRPA1. This latter effect mapped to residues Y933, G939, and S921 of TRPA1. Intra-plantar injection of 19,20-EpDPA induced acute mechanical, but not thermal hypersensitivity in mice, which was also blocked by A967079. Similarly, Cyp1b1-knockout mice displayed a reduced chronic pain phenotype following SNL injury. These data suggest that manipulation of the CYP1B1-oxylipin-TRPA1 axis might have therapeutic benefit.

TRPA1 Polymorphisms Modify the Hypotensive Responses to Propofol with No Change in Nitrite or Nitrate Levels

Anesthesia with propofol is frequently associated with hypotension. The TRPA1 gene contributes to the vasodilator effect of propofol. Hypotension is crucial for anesthesiologists because it is deleterious in the perioperative period. We tested whether the TRPA1 gene polymorphisms or haplotypes interfere with the hypotensive responses to propofol. PCR-determined genotypes and haplotype frequencies were estimated. Nitrite, nitrates, and NOx levels were measured. Propofol induced a more expressive lowering of the blood pressure (BP) without changing nitrite or nitrate levels in patients carrying CG+GG genotypes for the rs16937976 TRPA1 polymorphism and AG+AA genotypes for the rs13218757 TRPA1 polymorphism. The CGA haplotype presented the most remarkable drop in BP. Heart rate values were not impacted. The present exploratory analysis suggests that TRPA1 genotypes and haplotypes influence the hypotensive responses to propofol. The mechanisms involved are probably other than those related to NO bioavailability. With better genetic knowledge, planning anesthesia with fewer side effects may be possible.

The coumarin osthole is a non-electrophilic agonist of TRPA1

The naturally occurring coumarin osthole has antipruritic properties, and recent reports suggest that this effect is due an inhibition or desensitization of the cation channels TRPV1 and TRPV3. Osthole was also suggested to activate TRPA1, an effect that should rather be pruritic than antipruritic. Here we characterized the effects of osthole on TRPA1 by means of ratiometric calcium imaging and patch clamp electrophysiology. In HEK 293 expressing human (h) TRPA1, osthole induced a concentration-dependent increase in intracellular calcium that was inhibited by the TRPA1-inhibitor A967079. In mouse dorsal root ganglion (DRG) cells, osthole induced a strong calcium-influx that was partly mediated by TRPA1. Osthole evoked fully reversible membrane currents in whole-cell as well as cell-free inside-out recordings on hTRPA1. Osthole failed to activate the mutant hTRPA1-S873V/T874L, a previously described binding site for the non-electrophilic TRPA1-agonists menthol and carvacrol. The combined application of osthole and carvacrol diminished channel activation, suggesting a competitive binding. Finally, osthole failed to activate TRPM8 and TRPV4 but induced a modest activation of hTRPV1 expressed in HEK 293 cells. We conclude that osthole is a potent non-electrophilic agonist of TRPA1. The relevance of this property for the antipruritic effects needs to be further explored.

Structural Modeling of TRPA1 Ion Channel-Determination of the Binding Site for Antagonists

TRPA1 is a transmembrane cation channel, one of the most promising targets in the context of respiratory diseases. Its general structure has already been experimentally resolved, but the binding site of TRPA1 antagonists such as HC-030031, a model methylxanthine derivative, remains unknown. The present study aimed to determine the potential binding site of xanthine antagonists and to describe their binding mode, using a molecular modeling approach. This study represents the first attempt to bring together site-directed mutagenesis reports and the latest cryo-EM structure of an antagonist bound to TRPA1. Our research suggests that the core moiety of HC-030031 binds to a pocket formed by the TRP-like domain and the pre-S1, S4, S5 helices of one subunit. The structure, determined by cryo-EM, shows interactions of a core hypoxanthine moiety in the same area of the binding site, sharing the interaction of xanthine/hypoxanthine with Trp-711. Moreover, the predicted binding mode of HC-030031 assumes interaction with Asn-855, a residue demonstrated to be important for HC-030031 recognition in site-directed mutagenesis studies. Our model proved to be advantageous in a retrospective virtual screening benchmark; therefore, it will be useful in research on new TRPA1 antagonists among xanthine derivatives and their bioisosteres.

Mini-review: The nociceptive sensory functions of the polymodal receptor Transient Receptor Potential Ankyrin Type 1 (TRPA1)

Over the last 17 years since its cloning in 2003, the receptor-channel TRPA1 has received increasing attention due to its polymodal features and prominent role in pain signaling in a variety of human disease states. While evidence has been accumulating for non-neuronal TRPA1 expression, it is the presence of this channel in nociceptive nerve endings which has taken centre stage, due to its potential clinical ramifications. As a consequence, we shall focus in this review on the sensory functions of TRPA1 related to its expression in the peripheral nervous system. While substantial research has been focused on the putative role of TRPA1 in detecting irritant compounds, noxious cold and mechanical stimuli, the current overall picture is, to some extent, still cloudy. The chemosensory function of the channel is well demonstrated, as well as its involvement in the detection of oxidative and nitrosative stress; however, the other sensory features of TRPA1 have not been fully elucidated yet. The current state of the experimental evidence for these physiological roles of TRPA1 in mammals, and particularly in humans, will be discussed in this review.

Activation of Drosophila melanogaster TRPA1 Isoforms by Citronellal and Menthol

BACKGROUND

The transient receptor potential ankyrin 1 (TRPA1) cation channels function as broadly-tuned sensors of noxious chemicals in many species. Recent studies identified four functional TRPA1 isoforms in Drosophila melanogaster (dTRPA1(A) to (D)), but their responses to non-electrophilic chemicals are yet to be fully characterized.

METHODS

We determined the behavioral responses of adult flies to the mammalian TRPA1 non-electrophilic activators citronellal and menthol, and characterized the effects of these compounds on all four dTRPA1 channel isoforms using intracellular Ca²⁺ imaging and whole-cell patch-clamp recordings.

RESULTS

Wild type flies avoided citronellal and menthol in an olfactory test and this behavior was reduced in dTrpA1 mutant flies. Both compounds activate all dTRPA1 isoforms in the heterologous expression system HEK293T, with the following sensitivity series: dTRPA1(C) = dTRPA1(D) > dTRPA1(A) ≫ dTRPA1(B) for citronellal and dTRPA1(A) > dTRPA1(D) > dTRPA1(C) > dTRPA1(B) for menthol.

CONCLUSIONS

dTrpA1 was required for the normal avoidance of Drosophila melanogaster towards citronellal and menthol. All dTRPA1 isoforms are activated by both compounds, but the dTRPA1(B) is consistently the least sensitive. We discuss how these findings may guide further studies on the physiological roles and the structural bases of chemical sensitivity of TRPA1 channels.

Transient Receptor Potential Ankyrin-1 (TRPA1) Block Protects against Loss of White Matter Function during Ischaemia in the Mouse Optic Nerve

Oligodendrocytes produce myelin, which provides insulation to axons and speeds up neuronal transmission. In ischaemic conditions, myelin is damaged, resulting in mental and physical disabilities. Recent evidence suggests that oligodendrocyte damage during ischaemia can be mediated by Transient Receptor Potential Ankyrin-1 (TRPA1), whose activation raises intracellular Ca²⁺ concentrations and damages compact myelin. Here, we show that TRPA1 is constitutively active in oligodendrocytes and the optic nerve, as the specific TRPA1 antagonist, A-967079, decreases basal oligodendrocyte Ca²⁺ concentrations and increases the size of the compound action potential (CAP). Conversely, TRPA1 agonists reduce the size of the optic nerve CAP in an A-967079-sensitive manner. These results indicate that glial TRPA1 regulates neuronal excitability in the white matter under physiological as well as pathological conditions. Importantly, we find that inhibition of TRPA1 prevents loss of CAPs during oxygen and glucose deprivation (OGD) and improves the recovery. TRPA1 block was effective when applied before, during, or after OGD, indicating that the TRPA1-mediated damage is occurring during both ischaemia and recovery, but importantly, that therapeutic intervention is possible after the ischaemic insult. These results indicate that TRPA1 has an important role in the brain, and that its block may be effective in treating many white matter diseases.

Structural Pharmacology of TRP Channels

Transient receptor potential (TRP) ion channels are a super-family of ion channels that mediate transmembrane cation flux with polymodal activation, ranging from chemical to physical stimuli. Furthermore, due to their ubiquitous expression and role in human diseases, they serve as potential pharmacological targets. Advances in cryo-EM TRP channel structural biology has revealed general, as well as diverse, architectural elements and regulatory sites among TRP channel subfamilies. Here, we review the endogenous and pharmacological ligand-binding sites of TRP channels and their regulatory mechanisms.

Efficient and flexible synthesis of new photoactivatable propofol analogs

Propofol is a widely used general anesthetic, which acts by binding to and modulating several neuronal ion channels. We describe the synthesis of photoactivatable propofol analogs functionalized with an alkyne handle for bioorthogonal chemistry. Such tools are useful for detecting and isolating photolabeled proteins. We designed expedient and flexible synthetic routes to three new diazirine-based crosslinkable propofol derivatives, two of which have alkyne handles. As a proof of principle, we show that these compounds activate heterologously expressed Transient Receptor Potential Ankyrin 1 (TRPA1), a key ion channel of the pain pathway, with a similar potency as propofol in fluorescence-based functional assays. This work demonstrates that installation of the crosslinkable and clickable group on a short nonpolar spacer at the para position of propofol does not affect TRPA1 activation, supporting the utility of these chemical tools in identifying and characterizing potentially druggable binding sites in propofol-interacting proteins.

Atractylodin Produces Antinociceptive Effect through a Long-Lasting TRPA1 Channel Activation

Atractylodin (ATR) is a bioactive component found in dried rhizomes of Atractylodes lancea (AL) De Candolle. Although AL has accumulated empirical evidence for the treatment of pain, the molecular mechanism underlying the anti-pain effect of ATR remains unclear. In this study, we found that ATR increases transient receptor potential ankyrin-1 (TRPA1) single-channel activity in hTRPA1 expressing HEK293 cells. A bath application of ATR produced a long-lasting calcium response, and the response was completely diminished in the dorsal root ganglion neurons of TRPA1 knockout mice. Intraplantar injection of ATR evoked moderate and prolonged nociceptive behavior compared to the injection of allyl isothiocyanate (AITC). Systemic application of ATR inhibited AITC-induced nociceptive responses in a dose-dependent manner. Co-application of ATR and QX-314 increased the noxious heat threshold compared with AITC in vivo. Collectively, we concluded that ATR is a unique agonist of TRPA1 channels, which produces long-lasting channel activation. Our results indicated ATR-mediated anti-nociceptive effect through the desensitization of TRPA1-expressing nociceptors.

The Agonist Action of Alkylphenols on TRPA1 Relates to Their Effects on Membrane Lipid Order: Implications for TRPA1-Mediated Chemosensation

The Transient Receptor Potential Ankyrin 1 cation channel (TRPA1) is a broadly-tuned chemosensor expressed in nociceptive neurons. Multiple TRPA1 agonists are chemically unrelated non-electrophilic compounds, for which the mechanisms of channel activation remain unknown. Here, we assess the hypothesis that such chemicals activate TRPA1 by inducing mechanical perturbations in the plasma membrane. We characterized the activation of mouse TRPA1 by non-electrophilic alkylphenols (APs) of different carbon chain lengths in the para position of the aromatic ring. Having discarded oxidative stress and the action of electrophilic mediators as activation mechanisms, we determined whether APs induce mechanical perturbations in the plasma membrane using dyes whose fluorescence properties change upon alteration of the lipid environment. APs activated TRPA1, with potency increasing with their lipophilicity. APs increased the generalized polarization of Laurdan fluorescence and the anisotropy of the fluorescence of 1,6-diphenyl-1,3,5-hexatriene (DPH), also according to their lipophilicity. Thus, the potency of APs for TRPA1 activation is an increasing function of their ability to induce lipid order and membrane rigidity. These results support the hypothesis that TRPA1 senses non-electrophilic compounds by detecting the mechanical alterations they produce in the plasma membrane. This may explain how structurally unrelated non-reactive compounds induce TRPA1 activation and support the role of TRPA1 as an unspecific sensor of potentially noxious compounds.

[Effect of pre-administered flurbiprofen axetil on the EC50 of propofol during anesthesia in unstimulated patients: a randomized clinical trial]

BACKGROUND AND OBJECTIVES

Preoperative use of flurbiprofen axetil (FA) is extensively adopted to modulate the effects of analgesia. However, the relationship between FA and sedation agents remains unclear. In this study, we aimed to investigate the effects of different doses of FA on the median Effective Concentration (EC50) of propofol.

METHODS

Ninety-six patients (ASA I or II, aged 18-65 years) were randomly assigned into one of four groups in a 1:1:1:1 ratio. Group A (control group) received 10 mL of Intralipid, and groups B, C and D received 0.5 mg.kg^-1, 0.75 mg.kg^-1 and 1 mg.kg^-1 of FA, respectively, 10 minutes before induction. The depth of anesthesia was measured by the Bispectral Index (BIS). The "up-and-down" method was used to calculate the EC50 of propofol. During the equilibration period, if BIS ≤ 50 (or BIS > 50), the next patient would receive a 0.5 μg.mL^-1-lower (or-higher) propofol Target-Controlled Infusion (TCI) concentration. The hemodynamic data were recorded at baseline, 10 minutes after FA administration, after induction, after intubation, and 15 minutes after intubation.

RESULTS

The EC50 of propofol was lower in Group C (2.32 μg.mL^-1, 95% Confidence Interval [95% CI] 1.85-2.75) and D (2.39 μg.mL^-1, 95% CI 1.91-2.67) than in Group A (2.96 μg.mL^-1, 95% CI 2.55-3.33) (p = 0.023, p = 0.048, respectively). There were no significant differences in the EC50 between Group B (2.53 μg.mL^-1, 95% CI 2.33-2.71) and Group A (p ˃ 0.05). There were no significant differences in Heart Rate (HR) among groups A, B and C. The HR was significantly lower in Group D than in Group A after intubation (66 ± 6 vs. 80 ± 10 bpm, p < 0.01) and 15 minutes after intubation (61 ± 4 vs. 70 ± 8 bpm, p < 0.01). There were no significant differences among the four groups in Mean Arterial Pressure (MAP) at any time point. The MAP of the four groups was significantly lower after induction, after intubation, and 15 minutes after intubation than at baseline (p < 0.05).

CONCLUSION

High-dose FA (0.75 mg.kg^-1 or 1 mg.kg^-1) reduces the EC50 of propofol, and 1 mg.kg^-1 FA reduces the HR for adequate anesthesia in unstimulated patients. Although this result should be investigated in cases of surgical stimulation, we suggest that FA pre-administration may reduce the propofol requirement when the depth of anesthesia is measured by BIS.

A Non-covalent Ligand Reveals Biased Agonism of the TRPA1 Ion Channel

The TRPA1 ion channel is activated by electrophilic compounds through the covalent modification of intracellular cysteine residues. How non-covalent agonists activate the channel and whether covalent and non-covalent agonists elicit the same physiological responses are not understood. Here, we report the discovery of a non-covalent agonist, GNE551, and determine a cryo-EM structure of the TRPA1-GNE551 complex, revealing a distinct binding pocket and ligand-interaction mechanism. Unlike the covalent agonist allyl isothiocyanate, which elicits channel desensitization, tachyphylaxis, and transient pain, GNE551 activates TRPA1 into a distinct conducting state without desensitization and induces persistent pain. Furthermore, GNE551-evoked pain is relatively insensitive to antagonist treatment. Thus, we demonstrate the biased agonism of TRPA1, a finding that has important implications for the discovery of effective drugs tailored to different disease etiologies.

A novel, injury-free rodent model of vulnerability for assessment of acute and preventive therapies reveals temporal contributions of CGRP-receptor activation in migraine-like pain

AIM

Development and characterization of a novel injury-free preclinical model of migraine-like pain allowing mechanistic assessment of both acute and preventive treatments.

METHODS

A "two-hit" hyperalgesic priming strategy was used to induce vulnerability to a normally subthreshold challenge with umbellulone, a transient receptor potential ankyrin 1 (TRPA1) activator, in uninjured female and male C57BL/6 mice. Priming (i.e. the first hit) was induced by three consecutive daily episodes of restraint stress; repeated umbellulone was also evaluated for potential priming effects. Sixteen days after the first restraint stress, mice received inhalational umbellulone (i.e. the second hit) to elicit migraine-like pain. Medications currently used for acute or preventive migraine therapy including propranolol (a beta blocker) and sumatriptan (5HT1_B/D agonist), as well as olcegepant, an experimental calcitonin gene related peptide (CGRP) receptor antagonist and nor-Binaltorphimine (nor-BNI), an experimental long-acting kappa opioid receptor (KOR) antagonist, were investigated for their efficacy to block priming and prevent or reverse umbellulone-induced allodynia in primed animals. To assess migraine-like pain, cutaneous allodynia was determined by responses to periorbital or hindpaw probing with von Frey filaments.

RESULTS

Repeated restraint stress, but not umbellulone exposure, produced transient cutaneous allodynia that resolved within 16 d. Restraint stress produced long-lasting priming that persisted beyond 16 d, as demonstrated by reinstatement of cutaneous allodynia following inhalational umbellulone challenge. Pretreatment with propranolol or nor-BNI prior to restraint stress prevented both transient cutaneous allodynia and priming, demonstrated by a lack of umbellulone-induced cutaneous allodynia. Following establishment of restraint stress priming, olcegepant, but not propranolol or nor-BNI, prevented umbellulone-induced cutaneous allodynia. When administered 1 h after umbellulone, sumatriptan, but not olcegepant, reversed umbellulone-induced cutaneous allodynia in restraint stress-primed rats.

CONCLUSION

We have developed a novel injury-free model with translational relevance that can be used to study mechanisms relevant to migraine-like pain and to evaluate novel acute or preventive treatments. Restraint stress priming induced a state of vulnerability to a subthreshold stimulus that has been referred to as "latent sensitization". The development of latent sensitization could be prevented by blockade of stress pathways with propranolol or with a kappa opioid receptor antagonist. Following establishment of latent sensitization, subthreshold stimulation with umbellulone reinstated cutaneous allodynia, likely from activation of meningeal TRPA1-expressing nociceptors. Accordingly, in restraint stress-primed animals, sumatriptan reversed umbellulone-induced cutaneous allodynia, supporting peripheral sites of action, while propranolol and nor-BNI were not effective. Surprisingly, olcegepant was effective in mice with latent sensitization when given prior to, but not after, umbellulone challenge, suggesting time-dependent contributions of calcitonin gene-related peptide receptor signaling in promoting migraine-like pain in this model. Activation of the calcitonin gene-related peptide receptor participates in initiating, but has a more limited role in maintaining, pain responses, supporting the efficacy of small molecule calcitonin gene-related peptide antagonists as preventive medications. Additionally, the effectiveness of sumatriptan in reversal of established pain thus suggests modulation of additional, non-calcitonin gene-related peptide receptor-mediated nociceptive mechanisms. Kappa opioid receptor antagonists may represent a novel preventive therapy for stress-related migraine.

Global alignment and assessment of TRP channel transmembrane domain structures to explore functional mechanisms

The recent proliferation of published TRP channel structures provides a foundation for understanding the diverse functional properties of this important family of ion channel proteins. To facilitate mechanistic investigations, we constructed a structure-based alignment of the transmembrane domains of 120 TRP channel structures. Comparison of structures determined in the absence or presence of activating stimuli reveals similar constrictions in the central ion permeation pathway near the intracellular end of the S6 helices, pointing to a conserved cytoplasmic gate and suggesting that most available structures represent non-conducting states. Comparison of the ion selectivity filters toward the extracellular end of the pore supports existing hypotheses for mechanisms of ion selectivity. Also conserved to varying extents are hot spots for interactions with hydrophobic ligands, lipids and ions, as well as discrete alterations in helix conformations. This analysis therefore provides a framework for investigating the structural basis of TRP channel gating mechanisms and pharmacology, and, despite the large number of structures included, reveals the need for additional structural data and for more functional studies to establish the mechanistic basis of TRP channel function.

Inhibition of Ligand-Gated TRPA1 by General Anesthetics

Several general anesthetics (GAs) produce pain or irritation upon administration, and this occurs predominantly through the activation of the nociceptive ion channel, transient receptor potential ankyrin type 1 (TRPA1). However, the effects of GAs on agonist-mediated TRPA1 activity are unclear. Here we show that a diverse range of noxious and non-noxious volatile anesthetics, at clinically relevant concentrations, inhibit ligand-activated TRPA1 currents. These effects are species-specific; GAs blocks rodent TRPA1 without affecting the Drosophila ortholog. Furthermore, propofol inhibits rodent but not human TRPA1. Analysis of chimeric TRPA1 proteins and mutagenesis combined reveals two amino acid residues located in the S5 domain, Ser876 and Thr877, that are critical for the inhibitory effects of isoflurane and propofol. Introduction of these residues into Drosophila TRPA1 confers anesthetic inhibition. Furthermore, several residues lining the presumptive binding pocket for noxious GAs are not required for the inhibitory effects of GAs. We conclude that anesthetics inhibit TRPA1 by interacting at a site distinct from the activation site. The inhibitory effects of GAs at TRPA1 may contribute to the diverse pharmacological action of these drugs. SIGNIFICANCE STATEMENT: We show that both noxious and non-noxious general anesthetics inhibit agonist-evoked transient receptor potential ankyrin type 1 (TRPA1) activity and identify critical amino acid residues located in the pore domain. Inhibition of TRPA1 may affect pain and vascular signaling during anesthesia.

Identification of a new class of non-electrophilic TRPA1 agonists by a structure-based virtual screening approach

Recent work has gradually been clarifying the binding site of non-electrophilic agonists on the transient receptor potential A1 (TRPA1). This study searched for non-electrophilic TRPA1 agonists by means of in silico drug discovery techniques based on three-dimensional (3-D) protein structure. First, agonist-bound pocket structures were explored using an advanced molecular dynamics simulation starting from the cryo-electron microscopic structure of TRPA1, and several pocket structures suitable for virtual screening were extracted by structure evaluation using known non-electrophilic TRPA1 agonists. Next, 49 compounds were selected as new non-electrophilic agonist candidates from a library of natural products comprising 10,555 compounds by molecular docking toward these pocket structures. Measurement of the TRPA1 agonist activity of these compounds showed notable TRPA1 activation with three compounds (decanol, 2-ethyl-1-hexanol, phenethyl butanoate). Decanol and 2-ethyl-1-hexanol, which are categorized as fatty alcohols, in particular have a novel chemical scaffold for TRPA1 activation. The results of this study are expected to be of considerable use in understanding the molecular mechanism of TRPA1 recognition by non-electrophilic agonists.

Proportional Downscaling of Glutamatergic Release Sites by the General Anesthetic Propofol at Drosophila Motor Nerve Terminals

Propofol is the most common general anesthetic used for surgery in humans, yet its complete mechanism of action remains elusive. In addition to potentiating inhibitory synapses in the brain, propofol also impairs excitatory neurotransmission. We use electrophysiological recordings from individual glutamatergic boutons in male and female larval Drosophila melanogaster motor nerve terminals to characterize this effect. We recorded from two bouton types, which have distinct presynaptic physiology and different average numbers of release sites or active zones. We show that a clinically relevant dose of propofol (3 μm) impairs neurotransmitter release similarly at both bouton types by decreasing the number of active release sites by half, without affecting release probability. In contrast, an analog of propofol has no effect on glutamate release. Coexpressing a truncated syntaxin1A protein in presynaptic boutons completely blocked this effect of propofol. Overexpressing wild-type syntaxin1A in boutons also conferred a level of resistance by increasing the number of active release sites to a physiological ceiling set by the number of active zones or T-bars, and in this way counteracting the effect of propofol. These results point to the presynaptic release machinery as a target for the general anesthetic. Proportionally equivalent effects of propofol on the number of active release sites across the different bouton types suggests that glutamatergic circuits that involve smaller boutons with fewer release sites may be more vulnerable to the presynaptic effects of the drug.

Volatile anaesthetics inhibit the thermosensitive nociceptor ion channel transient receptor potential melastatin 3 (TRPM3)

BACKGROUND

Volatile anaesthetics (VAs) are the most widely used compounds to induce reversible loss of consciousness and maintain general anaesthesia during surgical interventions. Although the mechanism of their action is not yet fully understood, it is generally believed, that VAs depress central nervous system functions mainly through modulation of ion channels in the neuronal membrane, including 2-pore-domain K+ channels, GABA and NMDA receptors. Recent research also reported their action on nociceptive and thermosensitive TRP channels expressed in the peripheral nervous system, including TRPV1, TRPA1, and TRPM8. Here, we investigated the effect of VAs on TRPM3, a less characterized member of the thermosensitive TRP channels playing a central role in noxious heat sensation.

METHODS

We investigated the effect of VAs on the activity of recombinant and native TRPM3, by monitoring changes in the intracellular Ca²⁺ concentration and measuring TRPM3-mediated transmembrane currents.

RESULTS

All the investigated VAs (chloroform, halothane, isoflurane, sevoflurane) inhibited both the agonist-induced (pregnenolone sulfate, CIM0216) and heat-activated Ca²⁺ signals and transmembrane currents in a concentration dependent way in HEK293T cells overexpressing recombinant TRPM3. Among the tested VAs, halothane was the most potent blocker (IC₅₀ = 0.52 ± 0.05 mM). We also investigated the effect of VAs on native TRPM3 channels expressed in sensory neurons of the dorsal root ganglia. While VAs activated certain sensory neurons independently of TRPM3, they strongly and reversibly inhibited the agonist-induced TRPM3 activity.

CONCLUSIONS

These data provide a better insight into the molecular mechanism beyond the analgesic effect of VAs and propose novel strategies to attenuate TRPM3 dependent nociception.

TRPA1 modulation by piperidine carboxamides suggests an evolutionarily conserved binding site and gating mechanism

The transient receptor potential ankyrin 1 (TRPA1) channel functions as an irritant sensor and is a therapeutic target for treating pain, itch, and respiratory diseases. As a ligand-gated channel, TRPA1 can be activated by electrophilic compounds such as allyl isothiocyanate (AITC) through covalent modification or activated by noncovalent agonists through ligand binding. However, how covalent modification leads to channel opening and, importantly, how noncovalent binding activates TRPA1 are not well-understood. Here we report a class of piperidine carboxamides (PIPCs) as potent, noncovalent agonists of human TRPA1. Based on their species-specific effects on human and rat channels, we identified residues critical for channel activation; we then generated binding modes for TRPA1-PIPC interactions using structural modeling, molecular docking, and mutational analysis. We show that PIPCs bind to a hydrophobic site located at the interface of the pore helix 1 (PH1) and S5 and S6 transmembrane segments. Interestingly, this binding site overlaps with that of known allosteric modulators, such as A-967079 and propofol. Similar binding sites, involving π-helix rearrangements on S6, have been recently reported for other TRP channels, suggesting an evolutionarily conserved mechanism. Finally, we show that for PIPC analogs, predictions from computational modeling are consistent with experimental structure-activity studies, thereby suggesting strategies for rational drug design.

Mammalian Transient Receptor Potential TRPA1 Channels: From Structure to Disease

The transient receptor potential ankyrin (TRPA) channels are Ca²⁺-permeable nonselective cation channels remarkably conserved through the animal kingdom. Mammals have only one member, TRPA1, which is widely expressed in sensory neurons and in non-neuronal cells (such as epithelial cells and hair cells). TRPA1 owes its name to the presence of 14 ankyrin repeats located in the NH₂ terminus of the channel, an unusual structural feature that may be relevant to its interactions with intracellular components. TRPA1 is primarily involved in the detection of an extremely wide variety of exogenous stimuli that may produce cellular damage. This includes a plethora of electrophilic compounds that interact with nucleophilic amino acid residues in the channel and many other chemically unrelated compounds whose only common feature seems to be their ability to partition in the plasma membrane. TRPA1 has been reported to be activated by cold, heat, and mechanical stimuli, and its function is modulated by multiple factors, including Ca²⁺, trace metals, pH, and reactive oxygen, nitrogen, and carbonyl species. TRPA1 is involved in acute and chronic pain as well as inflammation, plays key roles in the pathophysiology of nearly all organ systems, and is an attractive target for the treatment of related diseases. Here we review the current knowledge about the mammalian TRPA1 channel, linking its unique structure, widely tuned sensory properties, and complex regulation to its roles in multiple pathophysiological conditions.

Propofol Induces Cardioprotection Against Ischemia-Reperfusion Injury via Suppression of Transient Receptor Potential Vanilloid 4 Channel

Ca²⁺ entry via the transient receptor potential vanilloid 4 (TRPV4) channel contributes to Ca²⁺ overload and triggers many pathophysiological conditions, including myocardial ischemia/reperfusion (I/R) injury. Propofol, a widely used intravenous anesthetic, attenuates myocardial I/R injury. However, the mechanism of propofol remains to be examined. The present study aims to test the hypothesis that propofol attenuates myocardial I/R injury through the suppression of TRPV4. We used a murine ex vivo model of myocardial I/R and in vitro cultured myocytes subjected to hypoxia/reoxygenation (H/R). Propofol or TRPV4 antagonist, HC-067047, attenuates myocardial I/R injury in isolated hearts. In addition, propofol, HC-067047, or TRPV4-siRNA attenuates H/R-induced intracellular Ca²⁺ concentration ([Ca²⁺]_i) increase and cell viability reduction. On the contrary, TRPV4 agonist GSK1016790A exacerbates both ex vivo and in vitro myocardial injury. Pretreatment with propofol reverses the myocardial injury and intracellular Ca²⁺ overload induced by GSK1016790A at least in vitro. However, neither the combination of propofol and HC-067047 nor applying propofol to cells transfected with TRPV4-siRNA creates additional protective effects. In addition, propofol dose-dependently inhibits TRPV4-mediated Ca²⁺ entry induced by GSK1016790A and 4α-PDD. Propofol attenuates myocardial I/R injury partially through the suppression of TRPV4 channel and the subsequent inhibition of intracellular Ca²⁺ overload.

Drosophila menthol sensitivity and the Precambrian origins of transient receptor potential-dependent chemosensation

Transient receptor potential (TRP) cation channels are highly conserved, polymodal sensors which respond to a wide variety of stimuli. Perhaps most notably, TRP channels serve critical functions in nociception and pain. A growing body of evidence suggests that transient receptor potential melastatin (TRPM) and transient receptor potential ankyrin (TRPA) thermal and electrophile sensitivities predate the protostome-deuterostome split (greater than 550 Ma). However, TRPM and TRPA channels are also thought to detect modified terpenes (e.g. menthol). Although terpenoids like menthol are thought to be aversive and/or harmful to insects, mechanistic sensitivity studies have been largely restricted to chordates. Furthermore, it is unknown if TRP-menthol sensing is as ancient as thermal and/or electrophile sensitivity. Combining genetic, optical, electrophysiological, behavioural and phylogenetic approaches, we tested the hypothesis that insect TRP channels play a conserved role in menthol sensing. We found that topical application of menthol to Drosophila melanogaster larvae elicits a Trpm- and TrpA1-dependent nocifensive rolling behaviour, which requires activation of Class IV nociceptor neurons. Further, in characterizing the evolution of TRP channels, we put forth the hypotheses that three previously undescribed TRPM channel clades (basal, αTRPM and βTRPM), as well as TRPs with residues critical for menthol sensing, were present in ancestral bilaterians. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.

Is TRPA1 Burning Down TRPV1 as Druggable Target for the Treatment of Chronic Pain?

Over the last decades, a great array of molecular mediators have been identified as potential targets for the treatment of chronic pain. Among these mediators, transient receptor potential (TRP) channel superfamily members have been thoroughly studied. Namely, the nonselective cationic channel, transient receptor potential ankyrin subtype 1 (TRPA1), has been described as a chemical nocisensor involved in noxious cold and mechanical sensation and as rivalling TRPV1, which traditionally has been considered as the most important TRP channel involved in nociceptive transduction. However, few TRPA1-related drugs have succeeded in clinical trials. In the present review, we attempt to discuss the latest data on the topic and future directions for pharmacological intervention.

Sevoflurane increases locomotion activity in mice

Clinical observations show emergence of agitation and hyperactivity during the anesthesia induction and/or recovery period post-anesthesia. However, an animal model to illustrate this clinical phenomenon has not yet been established. We therefore set out to investigate whether sevoflurane, a commonly used anesthetic, could alter locomotion in mice during the anesthesia induction and recovery period post-anesthesia. The activity of the mice was recorded 5 minutes before, during (for 30 minutes), and 40 minutes after the administration of the anesthetic sevoflurane [1-, 1.5- and 2-fold minimum alveolar concentration] at 37⁰ C. The total walking distance and velocity of movement were measured and quantified as the indexes of locomotion. We found that the anesthetic sevoflurane increased the locomotion of the mice during the induction period of the anesthesia. During the recovery phase after anesthesia, the mice exhibited increased locomotion for a short period of time (about 5 minutes) and then displayed a sharp decrease in mobility for up to 60 minutes following the end of anesthesia administration. The anesthetic sevoflurane did not significantly alter the food intake and body weight of the mice. Furthermore, we found that Alzheimer’s disease transgenic mice exhibited a greater degree of sevoflurane-induced hyperactivity than the wild-type mice did. Our results showed that inhalation of the anesthetic sevoflurane induced an acute hyperactivity in mice, particularly among Alzheimer’s disease transgenic mice. These findings from the pilot studies have established an animal model to promote further studies into postoperative emergence agitation, hyperactivity and the underlying mechanisms into these conditions.

Alkylphenol inverse agonists of HCN1 gating: H-bond propensity, ring saturation and adduct geometry differentially determine efficacy and potency

BACKGROUND AND PURPOSE

In models of neuropathic pain, inhibition of HCN1 is anti-hyperalgesic. 2,6-di-iso-propyl phenol (propofol) and its non-anesthetic congener, 2,6-di-tert-butyl phenol, inhibit HCN1 channels by stabilizing closed state(s).

EXPERIMENTAL APPROACH

Using in vitro electrophysiology and kinetic modeling, we systematically explore the contribution of ligand architecture to alkylphenol-channel coupling.

KEY RESULTS

When corrected for changes in hydrophobicity (and propensity for intra-membrane partitioning), the decrease in potency upon 1-position substitution (NCO∼OH >> SH >>> F) mirrors the ligands' H-bond acceptor (NCO > OH > SH >>> F) but not donor profile (OH > SH >>> NCO∼F). H-bond elimination (OH to F) corresponds to a ΔΔG of ∼4.5 kCal mol^-1 loss of potency with little or no disruption of efficacy. Substitution of compact alkyl groups (iso-propyl, tert-butyl) with shorter (ethyl, methyl) or more extended (sec-butyl) adducts disrupts both potency and efficacy. Ring saturation (with the obligate loss of both planarity and π electrons) primarily disrupts efficacy.

CONCLUSIONS AND IMPLICATIONS

A hydrophobicity-independent decrement in potency at higher volumes suggests the alkylbenzene site has a volume of ≥800 ų. Within this, a relatively static (with respect to ligand) H-bond donor contributes to initial binding with little involvement in generation of coupling energy. The influence of π electrons/ring planarity and alkyl adducts on efficacy reveals these aspects of the ligand present towards a face of the channel that undergoes structural changes during opening. The site's characteristics suggest it is "druggable"; introduction of other adducts on the ring may generate higher potency inverse agonists.

TRPA1: a molecular view

The transient receptor potential ankyrin 1 (TRPA1) ion channel is expressed in pain-sensing neurons and other tissues and has become a major target in the development of novel pharmaceuticals. A remarkable feature of the channel is its long list of activators, many of which we are exposed to in daily life. Many of these agonists induce pain and inflammation, making TRPA1 a major target for anti-inflammatory and analgesic therapies. Studies in human patients and in experimental animals have confirmed an important role for TRPA1 in a number of pain conditions. Over the recent years, much progress has been made in elucidating the molecular structure of TRPA1 and in discovering binding sites and modulatory sites of the channel. Because the list of published mutations and important molecular sites is steadily growing and because it has become difficult to see the forest for the trees, this review aims at summarizing the current knowledge about TRPA1, with a special focus on the molecular structure and the known binding or gating sites of the channel.

TRPA1 ankyrin repeat six interacts with a small molecule inhibitor chemotype

TRPA1, a member of the transient receptor potential channel (TRP) family, is genetically linked to pain in humans, and small molecule inhibitors are efficacious in preclinical animal models of inflammatory pain. These findings have driven significant interest in development of selective TRPA1 inhibitors as potential analgesics. The majority of TRPA1 inhibitors characterized to date have been reported to interact with the S5 transmembrane helices forming part of the pore region of the channel. However, the development of many of these inhibitors as clinical drug candidates has been prevented by high lipophilicity, low solubility, and poor pharmacokinetic profiles. Identification of alternate compound interacting sites on TRPA1 provides the opportunity to develop structurally distinct modulators with novel structure-activity relationships and more desirable physiochemical properties. In this paper, we have identified a previously undescribed potent and selective small molecule thiadiazole structural class of TRPA1 inhibitor. Using species ortholog chimeric and mutagenesis strategies, we narrowed down the site of interaction to ankyrinR #6 within the distal N-terminal region of TRPA1. To identify the individual amino acid residues involved, we generated a computational model of the ankyrinR domain. This model was used predictively to identify three critical amino acids in human TRPA1, G238, N249, and K270, which were confirmed by mutagenesis to account for compound activity. These findings establish a small molecule interaction region on TRPA1, expanding potential avenues for developing TRPA1 inhibitor analgesics and for probing the mechanism of channel gating.

Novel amide derivatives of 1,3-dimethyl-2,6-dioxopurin-7-yl-alkylcarboxylic acids as multifunctional TRPA1 antagonists and PDE4/7 inhibitors: A new approach for the treatment of pain

A series of novel amide derivatives of 1,3-dimethyl-2,6-dioxopurin-7-yl-alkylcarboxylic acids designed using a structure-based computational approach was synthesized and assayed to evaluate their ability to block human TRPA1 channel and inhibit PDE4B/7A activity. We identified compounds 16 and 27 which showed higher potency against TRPA1 compared to HC-030031. In turn, compound 36 was the most promising multifunctional TRPA1 antagonist and PDE4B/7A dual inhibitor with IC₅₀ values in the range of that of the reference rolipram and BRL-50481, respectively. Compound 36 as a combined TRPA1/PDE4B/PDE7A ligand was characterized by a distinct binding mode in comparison to 16 and 27, in the given protein targets. The inhibition of both cAMP-specific PDE isoenzymes resulted in a strong anti-TNF-α effect of 36in vivo. Moreover, the potent anti-inflammatory and analgesic efficacy of 36 was observed in animal models of pain and inflammation (formalin test in mice and carrageenan-induced paw edema in rats). This compound also displayed significant antiallodynic properties in the early phase of chemotherapy-induced peripheral neuropathy in mice. In turn, the pure TRPA1 antagonists 16 and 27 revealed a statistically significant antiallodynic effect in the formalin test and in the von Frey test performed in both phases of oxaliplatin-induced allodynia. Antiallodynic activity of the test compounds 16, 27 and 36 was observed at a dose range comparable to that of the reference drug - pregabalin. In conclusion, the proposed approach of pain treatment based on the concomitant blocking of TRPA1 channel and PDE4B/7A inhibitory activity appears to be interesting research direction for the future search for novel analgesics.

Location and Character of Volatile General Anesthetics Binding Sites in the Transmembrane Domain of TRPV1

It has been proposed that general anesthesia results from direct multisite interactions with multiple and diverse ion channels in the brain. An understanding of the mechanisms by which general anesthetics modulate ion channels is essential to clarify their underlying behavior and their role in reversible immobilization and amnesia. Despite the fact that volatile general anesthetics are drugs that primarily induce insensitivity to pain, they have been reported to sensitize and active the vanilloid-1 receptor, TRPV1, which is known to mediate the response of the nervous system to certain harmful stimuli and which plays a crucial role in the pain pathway. Currently, the mechanism of action of anesthetics is unknown and the precise molecular sites of interaction have not been identified. Here, using ∼2.5 μs of classical molecular dynamics simulations and metadynamics, we explore these enigmas. Binding sites are identified and the strength of the association is further characterized using alchemical free-energy calculations. Anesthetic binding/unbinding proceeds primarily through a membrane-embedded pathway, and subsequently, a complex scenario is established involving multiple binding sites featuring single or multiple occupancy states of two small volatile drugs. One of the five anesthetic binding sites reported was previously identified experimentally, and another one, importantly, is identical to that of capsaicin, one of the chemical stimuli that activate TRPV1. However, in contrast to capsaicin, isoflurane and chloroform binding free-energies render modest to no association compared to capsaicin, suggesting a different activation mechanism. Uncovering chloroform and isoflurane modulatory sites will further our understanding of the TRPV1 molecular machinery and open the possibility of developing site-specific drugs.

Propofol inhibits prokaryotic voltage-gated Na+ channels by promoting activation-coupled inactivation

Propofol is widely used in the clinic for the induction and maintenance of general anesthesia. As with most general anesthetics, however, our understanding of its mechanism of action remains incomplete. Local and general anesthetics largely inhibit voltage-gated Na⁺ channels (Navs) by inducing an apparent stabilization of the inactivated state, associated in some instances with pore block. To determine the biophysical and molecular basis of propofol action in Navs, we investigated NaChBac and NavMs, two prokaryotic Navs with distinct voltage dependencies and gating kinetics, by whole-cell patch clamp electrophysiology in the absence and presence of propofol at clinically relevant concentrations (2-10 µM). In both Navs, propofol induced a hyperpolarizing shift of the pre-pulse inactivation curve without any significant effects on recovery from inactivation at strongly hyperpolarized voltages, demonstrating that propofol does not stabilize the inactivated state. Moreover, there was no evidence of fast or slow pore block by propofol in a non-inactivating NaChBac mutant (T220A). Propofol also induced hyperpolarizing shifts of the conductance-voltage relationships with negligible effects on the time constants of deactivation at hyperpolarized voltages, indicating that propofol does not stabilize the open state. Instead, propofol decreases the time constants of macroscopic activation and inactivation. Adopting a kinetic scheme of Nav gating that assumes preferential closed-state recovery from inactivation, a 1.7-fold acceleration of the rate constant of activation and a 1.4-fold acceleration of the rate constant of inactivation were sufficient to reproduce experimental observations with computer simulations. In addition, molecular dynamics simulations and molecular docking suggest that propofol binding involves interactions with gating machinery in the S4-S5 linker and external pore regions. Our findings show that propofol is primarily a positive gating modulator of prokaryotic Navs, which ultimately inhibits the channels by promoting activation-coupled inactivation.

Recent progress on the molecular pharmacology of propofol

The precise mechanism by which propofol enhances GABAergic transmission remains unclear, but much progress has been made regarding the underlying structural and dynamic mechanisms. Furthermore, it is now clear that propofol has additional molecular targets, many of which are functionally influenced at concentrations achieved clinically. Focusing primarily on molecular targets, this brief review attempts to summarize some of this recent progress while pointing out knowledge gaps and controversies. It is not intended to be comprehensive but rather to stimulate further thought, discussion, and study on the mechanisms by which propofol produces its pleiotropic effects.

Thermally activated TRP channels: molecular sensors for temperature detection

Temperature sensing is one of the oldest capabilities of living organisms, and is essential for sustaining life, because failure to avoid extreme noxious temperatures can result in tissue damage or death. A subset of members of the transient receptor potential (TRP) ion channel family is finely tuned to detect temperatures ranging from extreme cold to noxious heat, giving rise to thermoTRP channels. Structural and functional experiments have shown that thermoTRP channels are allosteric proteins, containing different domains that sense changes in temperature, among other stimuli, triggering pore opening. Although temperature-dependence is well characterized in thermoTRP channels, the molecular nature of temperature-sensing elements remains unknown. Importantly, thermoTRP channels are involved in pain sensation, related to pathological conditions. Here, we provide an overview of thermoTRP channel activation. We also discuss the structural and functional evidence supporting the existence of an intrinsic temperature sensor in this class of channels, and we explore the basic thermodynamic principles for channel activation. Finally, we give a view of their role in painful pathophysiological conditions.

Sites Contributing to TRPA1 Activation by the Anesthetic Propofol Identified by Photoaffinity Labeling

In addition to inducing anesthesia, propofol activates a key component of the pain pathway, the transient receptor potential ankyrin 1 ion channel (TRPA1). Recent mutagenesis studies suggested a potential activation site within the transmembrane domain, near the A-967079 cavity. However, mutagenesis cannot distinguish between protein-based and ligand-based mechanisms, nor can this site explain the complex modulation by propofol. Thus more direct approaches are required to reveal potentially druggable binding sites. Here we apply photoaffinity labeling using a propofol derivative, meta-azipropofol, for direct identification of binding sites in mouse TRPA1. We confirm that meta-azipropofol activates TRPA1 like the parent anesthetic, and identify two photolabeled residues (V954 and E969) in the S6 helix. In combination with docking to closed and open state models of TRPA1, photoaffinity labeling suggested that the A-967079 cavity is a positive modulatory site for propofol. Further, the photoaffinity labeling of E969 indicated pore block as a likely mechanism for propofol inhibition at high concentrations. The direct identification of drug-binding sites clarifies the molecular mechanisms of important TRPA1 agonists, and will facilitate drug design efforts to modulate TRPA1.