Characterization of two neuronal subclasses through constellation pharmacology

Elucidation of Medusozoan (Jellyfish) Venom Constituent Activities Using Constellation Pharmacology

Within the phylum Cnidaria, sea anemones (class Anthozoa) express a rich diversity of ion-channel peptide modulators with biomedical applications, but corollary discoveries from jellyfish (subphylum Medusozoa) are lacking. To bridge this gap, bioactivities of previously unexplored proteinaceous and small molecular weight (~15 kDa to 5 kDa) venom components were assessed in a mouse dorsal root ganglia (DRG) high-content calcium-imaging assay, known as constellation pharmacology. While the addition of crude venom led to nonspecific cell death and Fura-2 signal leakage due to pore-forming activity, purified small molecular weight fractions of venom demonstrated three main, concentration-dependent and reversible effects on defined heterogeneous cell types found in the primary cultures of mouse DRG. These three phenotypic responses are herein referred to as phenotype A, B and C: excitatory amplification (A) or inhibition (B) of KCl-induced calcium signals, and test compound-induced disturbances to baseline calcium levels (C). Most notably, certain Alatina alata venom fractions showed phenotype A effects in all DRG neurons; Physalia physalis and Chironex fleckeri fractions predominantly showed phenotype B effects in small- and medium-diameter neurons. Finally, specific Physalia physalis and Alatina alata venom components induced direct excitatory responses (phenotype C) in glial cells. These findings demonstrate a diversity of neuroactive compounds in jellyfish venom potentially targeting a constellation of ion channels and ligand-gated receptors with broad physiological implications.

The Tunicate Metabolite 2-(3,5-Diiodo-4-methoxyphenyl)ethan-1-amine Targets Ion Channels of Vertebrate Sensory Neurons

Marine tunicates produce defensive amino-acid-derived metabolites, including 2-(3,5-diiodo-4-methoxyphenyl)ethan-1-amine (DIMTA), but their mechanisms of action are rarely known. Using an assay-guided approach, we found that out of the many different sensory cells in the mouse dorsal root ganglion (DRG), DIMTA selectively affected low-threshold cold thermosensors. Whole-cell electrophysiology experiments using DRG cells, channels expressed in Xenopus oocytes, and human cell lines revealed that DIMTA blocks several potassium channels, reducing the magnitude of the afterhyperpolarization and increasing the baseline intracellular calcium concentration [Ca²⁺]_i of low-threshold cold thermosensors. When injected into mice, DIMTA increased the threshold of cold sensation by >3 °C. DIMTA may thus serve as a lead in the further design of compounds that inhibit problems in the cold-sensory system, such as cold allodynia and other neuropathic pain conditions.

Curses or Cures: A Review of the Numerous Benefits Versus the Biosecurity Concerns of Conotoxin Research

Conotoxins form a diverse group of peptide toxins found in the venom of predatory marine cone snails. Decades of conotoxin research have provided numerous measurable scientific and societal benefits. These include their use as a drug, diagnostic agent, drug leads, and research tools in neuroscience, pharmacology, biochemistry, structural biology, and molecular evolution. Human envenomations by cone snails are rare but can be fatal. Death by envenomation is likely caused by a small set of toxins that induce muscle paralysis of the diaphragm, resulting in respiratory arrest. The potency of these toxins led to concerns regarding the potential development and use of conotoxins as biological weapons. To address this, various regulatory measures have been introduced that limit the use and access of conotoxins within the research community. Some of these regulations apply to all of the ≈200,000 conotoxins predicted to exist in nature of which less than 0.05% are estimated to have any significant toxicity in humans. In this review we provide an overview of the many benefits of conotoxin research, and contrast these to the perceived biosecurity concerns of conotoxins and research thereof.

Targeting the CaVα-CaVβ interaction yields an antagonist of the N-type CaV2.2 channel with broad antinociceptive efficacy

Inhibition of voltage-gated calcium (CaV) channels is a potential therapy for many neurological diseases including chronic pain. Neuronal CaV1/CaV2 channels are composed of α, β, γ and α2δ subunits. The β subunits of CaV channels are cytoplasmic proteins that increase the surface expression of the pore-forming α subunit of CaV. We targeted the high-affinity protein-protein interface of CaVβ's pocket within the CaVα subunit. Structure-based virtual screening of 50,000 small molecule library docked to the β subunit led to the identification of 2-(3,5-dimethylisoxazol-4-yl)-N-((4-((3-phenylpropyl)amino)quinazolin-2-yl)methyl)acetamide (IPPQ). This small molecule bound to CaVβ and inhibited its coupling with N-type voltage-gated calcium (CaV2.2) channels, leading to a reduction in CaV2.2 currents in rat dorsal root ganglion sensory neurons, decreased presynaptic localization of CaV2.2 in vivo, decreased frequency of spontaneous excitatory postsynaptic potentials and miniature excitatory postsynaptic potentials, and inhibited release of the nociceptive neurotransmitter calcitonin gene-related peptide from spinal cord. IPPQ did not target opioid receptors nor did it engage inhibitory G protein-coupled receptor signaling. IPPQ was antinociceptive in naive animals and reversed allodynia and hyperalgesia in models of acute (postsurgical) and neuropathic (spinal nerve ligation, chemotherapy- and gp120-induced peripheral neuropathy, and genome-edited neuropathy) pain. IPPQ did not cause akinesia or motor impairment, a common adverse effect of CaV2.2 targeting drugs, when injected into the brain. IPPQ, a quinazoline analog, represents a novel class of CaV2.2-targeting compounds that may serve as probes to interrogate CaVα-CaVβ function and ultimately be developed as a nonopioid therapeutic for chronic pain.

Molecular characterization of frog vocal neurons using constellation pharmacology

Identification and characterization of neuronal cell classes in motor circuits are essential for understanding the neural basis of behavior. It is a challenging task, especially in a non-genetic-model organism, to identify cell-specific expression of functional macromolecules. Here, we performed constellation pharmacology, calcium imaging of dissociated neurons to pharmacologically identify functional receptors expressed by vocal neurons in adult male and female African clawed frogs, Xenopus laevis. Previously we identified a population of vocal neurons called fast trill neurons (FTNs) in the amphibian parabrachial nucleus (PB) that express N-methyl-d-aspartate (NMDA) receptors and GABA and/or glycine receptors. Using constellation pharmacology, we identified four cell classes of putative fast trill neurons (pFTNs, responsive to both NMDA and GABA/glycine applications). We discovered that some pFTNs responded to the application of substance P (SP), acetylcholine (ACh), or both. Electrophysiological recordings obtained from FTNs using an ex vivo preparation verified that SP and/or ACh depolarize FTNs. Bilateral injection of ACh, SP, or their antagonists into PBs showed that ACh receptors are not sufficient but necessary for vocal production, and SP receptors play a role in shaping the morphology of vocalizations. Additionally, we discovered that the PB of adult female X. laevis also contains all the subclasses of neurons at a similar frequency as in males, despite their sexually distinct vocalizations. These results reveal novel neuromodulators that regulate X. laevis vocal production and demonstrate the power of constellation pharmacology in identifying the neuronal subtypes marked by functional expression of cell-specific receptors in non-genetic-model organisms.NEW & NOTEWORTHY Molecular profiles of neurons are critical for understanding the neuronal functions, but their identification is challenging especially in non-genetic-model organisms. Here, we characterized the functional expression of membrane macromolecules in vocal neurons of African clawed frogs, Xenopus laevis, using a technique called constellation pharmacology. We discovered that receptors for acetylcholine and/or substance P are expressed by some classes of vocal neurons, and their activation plays a role in the production of normal vocalizations.

An integrative approach to the facile functional classification of dorsal root ganglion neuronal subclasses

Somatosensory neurons have historically been classified by a variety of approaches, including structural, anatomical, and genetic markers; electrophysiological properties; pharmacological sensitivities; and more recently, transcriptional profile differentiation. These methodologies, used separately, have yielded inconsistent classification schemes. Here, we describe phenotypic differences in response to pharmacological agents as measured by changes in cytosolic calcium concentration for the rapid classification of neurons in vitro; further analysis with genetic markers, whole-cell recordings, and single-cell transcriptomics validated these findings in a functional context. Using this general approach, which we refer to as tripartite constellation analysis (TCA), we focused on large-diameter dorsal-root ganglion (L-DRG) neurons with myelinated axons. Divergent responses to the K-channel antagonist, κM-conopeptide RIIIJ (RIIIJ), reliably identified six discrete functional cell classes. In two neuronal subclasses (L1 and L2), block with RIIIJ led to an increase in [Ca] i Simultaneous electrophysiology and calcium imaging showed that the RIIIJ-elicited increase in [Ca] i corresponded to different patterns of action potentials (APs), a train of APs in L1 neurons, and sporadic firing in L2 neurons. Genetically labeled mice established that L1 neurons are proprioceptors. The single-cell transcriptomes of L1 and L2 neurons showed that L2 neurons are Aδ-low-threshold mechanoreceptors. RIIIJ effects were replicated by application of the Kv1.1 selective antagonist, Dendrotoxin-K, in several L-DRG subclasses (L1, L2, L3, and L5), suggesting the presence of functional Kv1.1/Kv1.2 heteromeric channels. Using this approach on other neuronal subclasses should ultimately accelerate the comprehensive classification and characterization of individual somatosensory neuronal subclasses within a mixed population.

Characterization of the First Conotoxin from Conus ateralbus, a Vermivorous Cone Snail from the Cabo Verde Archipelago

Conus ateralbus is a cone snail endemic to the west side of the island of Sal, in the Cabo Verde Archipelago off West Africa. We describe the isolation and characterization of the first bioactive peptide from the venom of this species. This 30AA venom peptide is named conotoxin AtVIA (δ-conotoxin-like). An excitatory activity was manifested by the peptide on a majority of mouse lumbar dorsal root ganglion neurons. An analog of AtVIA with conservative changes on three amino acid residues at the C-terminal region was synthesized and this analog produced an identical effect on the mouse neurons. AtVIA has homology with δ-conotoxins from other worm-hunters, which include conserved sequence elements that are shared with δ-conotoxins from fish-hunting Conus. In contrast, there is no comparable sequence similarity with δ-conotoxins from the venoms of molluscivorous Conus species. A rationale for the potential presence of δ-conotoxins, that are potent in vertebrate systems in two different lineages of worm-hunting cone snails, is discussed.

Venom-Derived Peptide Modulators of Cation-Selective Channels: Friend, Foe or Frenemy

Ion channels play a key role in our body to regulate homeostasis and conduct electrical signals. With the help of advances in structural biology, as well as the discovery of numerous channel modulators derived from animal toxins, we are moving toward a better understanding of the function and mode of action of ion channels. Their ubiquitous tissue distribution and the physiological relevancies of their opening and closing suggest that cation channels are particularly attractive drug targets, and years of research has revealed a variety of natural toxins that bind to these channels and alter their function. In this review, we provide an introductory overview of the major cation ion channels: potassium channels, sodium channels and calcium channels, describe their venom-derived peptide modulators, and how these peptides provide great research and therapeutic value to both basic and translational medical research.

trans-Anethole of Fennel Oil is a Selective and Nonelectrophilic Agonist of the TRPA1 Ion Channel

Transient receptor potential (TRP) cation channels are molecular targets of various natural products. TRPA1, a member of TRP channel family, is specifically activated by natural products such as allyl isothiocyanate (mustard oil), cinnamaldehyde (cinnamon), and allicin (garlic). In this study, we demonstrated that TRPA1 is also a target of trans-anethole in fennel oil (FO) and fennel seed extract. Similar to FO, trans-anethole selectively elicited calcium influx in TRPA1-expressing mouse sensory neurons of the dorsal root and trigeminal ganglia. These FO- and anethole-induced calcium responses were blocked by a selective TRPA1 channel antagonist, HC-030031. Moreover, both FO and trans-anethole induced calcium influx and transmembrane currents in HEK293 cells stably overexpressing human TRPA1 channels, but not in regular HEK293 cells. Mutation of the amino acids S873 and T874 binding site of human TRPA1 significantly attenuated channel activation by trans-anethole, whereas pretreating with glutathione, a nucleophile, did not. Conversely, activation of TRPA1 by the electrophile allyl isothiocyanate was abolished by glutathione, but was ostensibly unaffected by mutation of the ST binding site. Finally, it was found that trans-anethole was capable of desensitizing TRPA1, and unlike allyl isothiocyanate, it failed to induce nocifensive behaviors in mice. We conclude that trans-anethole is a selective, nonelectrophilic, and seemingly less-irritating agonist of TRPA1.

Conotoxin κM-RIIIJ, a tool targeting asymmetric heteromeric Kv1 channels

The vast complexity of native heteromeric K⁺ channels is largely unexplored. Defining the composition and subunit arrangement of individual subunits in native heteromeric K⁺ channels and establishing their physiological roles is experimentally challenging. Here we systematically explored this "zone of ignorance" in molecular neuroscience. Venom components, such as peptide toxins, appear to have evolved to modulate physiologically relevant targets by discriminating among closely related native ion channel complexes. We provide proof-of-principle for this assertion by demonstrating that κM-conotoxin RIIIJ (κM-RIIIJ) from Conus radiatus precisely targets "asymmetric" K_v channels composed of three K_v1.2 subunits and one K_v1.1 or K_v1.6 subunit with 100-fold higher apparent affinity compared with homomeric K_v1.2 channels. Our study shows that dorsal root ganglion (DRG) neurons contain at least two major functional K_v1.2 channel complexes: a heteromer, for which κM-RIIIJ has high affinity, and a putative K_v1.2 homomer, toward which κM-RIIIJ is less potent. This conclusion was reached by (i) covalent linkage of members of the mammalian Shaker-related K_v1 family to K_v1.2 and systematic assessment of the potency of κM-RIIIJ block of heteromeric K⁺ channel-mediated currents in heterologous expression systems; (ii) molecular dynamics simulations of asymmetric K_v1 channels providing insights into the molecular basis of κM-RIIIJ selectivity and potency toward its targets; and (iii) evaluation of calcium responses of a defined population of DRG neurons to κM-RIIIJ. Our study demonstrates that bioactive molecules present in venoms provide essential pharmacological tools that systematically target specific heteromeric K_v channel complexes that operate in native tissues.

Volatile Terpenes and Brain Function: Investigation of the Cognitive and Mood Effects of Mentha × Piperita L. Essential Oil with In Vitro Properties Relevant to Central Nervous System Function

Background: Extracts of several members of the monoterpene-rich Lamiaceae sub-family Nepetoideae, including those from the Salvia (sage), Melissa (Lemon balm) and Rosmarinus (rosemary) genera, evince cognitive and mood effects in humans that are potentially related to their effects on cholinergic and GABAergic neurotransmission. To date, despite promising in vitro properties, the cognitive and mood effects of the closely related Mentha spicata (spearmint) and Mentha piperita (peppermint) remain unexplored. This study therefore assessed the human cognitive/mood effects of the M. spicata/piperita essential oil with the most promising, brain-relevant in vitro properties according to pre-trial in vitro screening. Design: Organic spearmint and peppermint (Mentha spicata/piperita) essential oils were pre-screened for neurotransmitter receptor binding and acetylcholinesterase (AChE) inhibition. In a double-blind, placebo-controlled, balanced cross-over study, 24 participants (mean age 25.2 years) consumed single doses of encapsulated placebo and 50 µL and 100 µL of the most promising essential oil (peppermint with nicotinic/GABAA receptor binding and AChE inhibitory properties, that increased calcium influx in a CAD cell neuronal model). Psychological functioning was assessed with mood scales and a range of standardised, cognitively demanding tasks pre-dose and at 1, 3 and 6 h post-dose. Results: The highest (100 µL) dose of essential oil improved performance on the cognitively demanding Rapid Visual Information Processing task (RVIP) at 1 h and 3 h post-dose and both doses attenuated fatigue and improved performance of the Serial 3 s subtraction task at 3 h post-dose. Conclusion: Peppermint (Mentha piperita) essential oil with high levels of menthol/menthone and characteristic in vitro cholinergic inhibitory, calcium regulatory and GABA_A/nicotinic receptor binding properties, beneficially modulated performance on demanding cognitive tasks and attenuated the increase in mental fatigue associated with extended cognitive task performance in healthy adults. Future investigations should consider investigating higher doses.

Sodium channels and pain: from toxins to therapies

Voltage-gated sodium channels (NaV channels) are essential for the initiation and propagation of action potentials that critically influence our ability to respond to a diverse range of stimuli. Physiological and pharmacological studies have linked abnormal function of NaV channels to many human disorders, including chronic neuropathic pain. These findings, along with the description of the functional properties and expression pattern of NaV channel subtypes, are helping to uncover subtype specific roles in acute and chronic pain and revealing potential opportunities to target these with selective inhibitors. High-throughput screens and automated electrophysiology platforms have identified natural toxins as a promising group of molecules for the development of target-specific analgesics. In this review, the role of toxins in defining the contribution of NaV channels in acute and chronic pain states and their potential to be used as analgesic therapies are discussed.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Pain therapeutics from cone snail venoms: From Ziconotide to novel non-opioid pathways

There have been numerous attempts to develop non-opioid drugs for severe pain, but the vast majority of these efforts have failed. A notable exception is Ziconotide (Prialt®), approved by the FDA in 2004. In this review, we summarize the present status of Ziconotide as a therapeutic drug and introduce a wider framework: the potential of venom peptides from cone snails as a resource providing a continuous pipeline for the discovery of non-opioid pain therapeutics. An auxiliary theme that we hope to develop is that these venoms, already a validated starting point for non-opioid drug leads, should also provide an opportunity for identifying novel molecular targets for future pain drugs. This review comprises several sections: the first focuses on Ziconotide as a therapeutic (including a historical retrospective and a clinical perspective); followed by sections on other promising Conus venom peptides that are either in clinical or pre-clinical development. We conclude with a discussion on why the outlook for discovery appears exceptionally promising. The combination of new technologies in diverse fields, including the development of novel high-content assays and revolutionary advancements in transcriptomics and proteomics, puts us at the cusp of providing a continuous pipeline of non-opioid drug innovations for pain. SIGNIFICANCE: The current opioid epidemic is the deadliest drug crisis in American history. Thus, this review on the discovery of non-opioid pain therapeutics and pathways from cone snail venoms is significant and timely.

Morphological and functional changes in TRPM8-expressing corneal cold thermoreceptor neurons during aging and their impact on tearing in mice

Morphological and functional alterations of peripheral somatosensory neurons during the aging process lead to a decline of somatosensory perception. Here, we analyze the changes occurring with aging in trigeminal ganglion (TG), TRPM8-expressing cold thermoreceptor neurons innervating the mouse cornea, which participate in the regulation of basal tearing and blinking and have been implicated in the pathogenesis of dry eye disease (DED). TG cell bodies and axonal branches were examined in a mouse line (TRPM8^BAC -EYFP) expressing a fluorescent reporter. In 3 months old animals, about 50% of TG cold thermoreceptor neurons were intensely fluorescent, likely providing strongly fluorescent axons and complex corneal nerve terminals with ongoing activity at 34°C and low-threshold, robust responses to cooling. The remaining TRPM8⁺ corneal axons were weakly fluorescent with nonbeaded axons, sparsely ramified nerve terminals, and exhibited a low-firing rate at 34°C, responding moderately to cooling pulses as do weakly fluorescent TG neurons. In aged (24 months) mice, the number of weakly fluorescent TG neurons was strikingly high while the morphology of TRPM8⁺ corneal axons changed drastically; 89% were weakly fluorescent, unbranched, and often ending in the basal epithelium. Functionally, 72.5% of aged cold terminals responded as those of young animals, but 27.5% exhibited very low-background activity and abnormal responsiveness to cooling pulses. These morpho-functional changes develop in parallel with an enhancement of tear's basal flow and osmolarity, suggesting that the aberrant sensory inflow to the brain from impaired peripheral cold thermoreceptors contributes to age-induced abnormal tearing and to the high incidence of DED in elderly people.

Quantitative Analysis of Mouse Dural Afferent Neurons Expressing TRPM8, VGLUT3, and NF200

OBJECTIVE

To quantify the abundance of dural afferent neurons expressing transient receptor potential channel melastatin 8 (TRPM8), vesicular glutamate transporter 3 (VGLUT3), and neurofilament 200 (NF200) in adult mice.

BACKGROUND

With the increasing use of mice as a model system to study headache mechanisms, it is important to understand the composition of dural afferent neurons in mice. In a previous study, we have measured the abundance of mouse dural afferent neurons that express neuropeptide calcitonin gene-related peptide as well as two TRP channels TRPV1 and TRPA1, respectively. Here, we conducted quantitative analysis of three other dural afferent subpopulations in adult mice.

METHODS

We used the fluorescent tracer Fluoro-Gold to retrogradely label dural afferent neurons in adult mice expressing enhanced green fluorescent protein in discrete subpopulations of trigeminal ganglion (TG) neurons. Mechanoreceptors with myelinated fibers were identified by NF200 immunoreactivity. We also conducted Ca²⁺ -imaging experiments to test the overlap between TRPM8 and VGLUT3 expression in mouse primary afferent neurons (PANs).

RESULTS

The abundance of TRPM8-expressing neurons in dural afferent neurons was significantly lower than that in total TG neurons. The percentages of dural afferent neurons expressing VGLUT3 and NF200 were comparable to those of total TG neurons, respectively. TRPM8 agonist menthol evoked Ca²⁺ influx in less than 7% VGLUT3-expressing PANs in adult mice.

CONCLUSIONS

TG neurons expressing TRPM8, VGLUT3, and NF200 all innervate adult mouse dura. TRPM8 and VGLUT3 are expressed in distinct subpopulations of PANs in adult mice. These results provide an anatomical basis to investigate headache mechanisms in mouse models.

TRPA1 expression levels and excitability brake by KV channels influence cold sensitivity of TRPA1-expressing neurons

The molecular sensor of innocuous (painless) cold sensation is well-established to be transient receptor potential cation channel, subfamily M, member 8 (TRPM8). However, the role of transient receptor potential cation channel, subfamily A, member 1 (TRPA1) in noxious (painful) cold sensation has been controversial. We find that TRPA1 channels contribute to the noxious cold sensitivity of mouse somatosensory neurons, independent of TRPM8 channels, and that TRPA1-expressing neurons are largely non-overlapping with TRPM8-expressing neurons in mouse dorsal-root ganglia (DRG). However, relatively few TRPA1-expressing neurons (e.g., responsive to allyl isothiocyanate or AITC, a selective TRPA1 agonist) respond overtly to cold temperature in vitro, unlike TRPM8-expressing neurons, which almost all respond to cold. Using somatosensory neurons from TRPM8-/- mice and subtype-selective blockers of TRPM8 and TRPA1 channels, we demonstrate that responses to cold temperatures from TRPA1-expressing neurons are mediated by TRPA1 channels. We also identify two factors that affect the cold-sensitivity of TRPA1-expressing neurons: (1) cold-sensitive AITC-sensitive neurons express relatively more TRPA1 transcripts than cold-insensitive AITC-sensitive neurons and (2) voltage-gated potassium (K_V) channels attenuate the cold-sensitivity of some TRPA1-expressing neurons. The combination of these two factors, combined with the relatively weak agonist-like activity of cold temperature on TRPA1 channels, partially explains why few TRPA1-expressing neurons respond to cold. Blocking K_V channels also reveals another subclass of noxious cold-sensitive DRG neurons that do not express TRPM8 or TRPA1 channels. Altogether, the results of this study provide novel insights into the cold-sensitivity of different subclasses of somatosensory neurons.

Veratridine produces distinct calcium response profiles in mouse Dorsal Root Ganglia neurons

Nociceptors are a subpopulation of dorsal root ganglia (DRG) neurons that detect noxious stimuli and signal pain. Veratridine (VTD) is a voltage-gated sodium channel (VGSC) modifier that is used as an "agonist" in functional screens for VGSC blockers. However, there is very little information on VTD response profiles in DRG neurons and how they relate to neuronal subtypes. Here we characterised VTD-induced calcium responses in cultured mouse DRG neurons. Our data shows that the heterogeneity of VTD responses reflects distinct subpopulations of sensory neurons. About 70% of DRG neurons respond to 30-100 μM VTD. We classified VTD responses into four profiles based upon their response shape. VTD response profiles differed in their frequency of occurrence and correlated with neuronal size. Furthermore, VTD response profiles correlated with responses to the algesic markers capsaicin, AITC and α, β-methylene ATP. Since VTD response profiles integrate the action of several classes of ion channels and exchangers, they could act as functional "reporters" for the constellation of ion channels/exchangers expressed in each sensory neuron. Therefore our findings are relevant to studies and screens using VTD to activate DRG neurons.

TRPM8 and Migraine

Migraine is among the most common diseases on earth and one of the most disabling, the latter due in large part to poor treatment efficacy. Development of new therapeutics is dependent on the identification of mechanisms contributing to migraine and discovery of targets for new drugs. Numerous genome-wide association studies (GWAS) have implicated the transient receptor-potential M8 (TRPM8) channel in migraine. This channel is predominantly expressed on peripheral sensory neurons and is known as the sensor for cold temperature in cutaneous tissue but is also expressed on deep visceral afferents where cold is not likely a stimulus. Consequently, a number of alternative endogenous agonists have been proposed. Apart from its role in cold sensation, TRPM8 also contributes to cold allodynia after nerve injury or inflammation, and it is necessary for cooling/menthol-based analgesia. How it might contribute to migraine is less clear. The purpose of this review is to discuss the anatomical and physiological mechanisms by which meningeal TRPM8 may play a role in migraine as well as the potential of TRPM8 as a therapeutic target. TRPM8 is expressed on sensory afferents innervating the meninges, and these neurons are subject to developmental changes that may influence their contribution to migraine. As in viscera, meningeal TRPM8 channels are unlikely to be activated by temperature fluctuations and their endogenous ligands remain unknown. Preclinical migraine studies show that activation of meningeal TRPM8 by exogenous agonists can both cause and alleviate headache behaviors, depending on whether other meningeal afferents concurrently receive noxious stimuli. This is reminiscent of the fact that cold can trigger migraine in humans but menthol can also alleviate headache. We propose that both TRPM8 agonists and antagonists may be potential therapeutics, depending on how migraine is triggered in individual patients. In this regard, TRPM8 may be a novel target for personalized medicine in migraine treatment.

The hitchhiker's guide to the voltage-gated sodium channel galaxy

Eukaryotic voltage-gated sodium (Na_v) channels contribute to the rising phase of action potentials and served as an early muse for biophysicists laying the foundation for our current understanding of electrical signaling. Given their central role in electrical excitability, it is not surprising that (a) inherited mutations in genes encoding for Na_v channels and their accessory subunits have been linked to excitability disorders in brain, muscle, and heart; and (b) Na_v channels are targeted by various drugs and naturally occurring toxins. Although the overall architecture and behavior of these channels are likely to be similar to the more well-studied voltage-gated potassium channels, eukaryotic Na_v channels lack structural and functional symmetry, a notable difference that has implications for gating and selectivity. Activation of voltage-sensing modules of the first three domains in Na_v channels is sufficient to open the channel pore, whereas movement of the domain IV voltage sensor is correlated with inactivation. Also, structure–function studies of eukaryotic Na_v channels show that a set of amino acids in the selectivity filter, referred to as DEKA locus, is essential for Na⁺ selectivity. Structures of prokaryotic Na_v channels have also shed new light on mechanisms of drug block. These structures exhibit lateral fenestrations that are large enough to allow drugs or lipophilic molecules to gain access into the inner vestibule, suggesting that this might be the passage for drug entry into a closed channel. In this Review, we will synthesize our current understanding of Na_v channel gating mechanisms, ion selectivity and permeation, and modulation by therapeutics and toxins in light of the new structures of the prokaryotic Na_v channels that, for the time being, serve as structural models of their eukaryotic counterparts.

Metabolic model for diversity-generating biosynthesis

A conventional metabolic pathway leads to a specific product. In stark contrast, there are diversity-generating metabolic pathways that naturally produce different chemicals, sometimes of great diversity. We demonstrate that for one such pathway, tru, each ensuing metabolic step is slower, in parallel with the increasing potential chemical divergence generated as the pathway proceeds. Intermediates are long lived and accumulate progressively, in contrast with conventional metabolic pathways, in which the first step is rate-limiting and metabolic intermediates are short-lived. Understanding these fundamental differences enables several different practical applications, such as combinatorial biosynthesis, some of which we demonstrate here. We propose that these principles may provide a unifying framework underlying diversity-generating metabolism in many different biosynthetic pathways.

Classifying neuronal subclasses of the cerebellum through constellation pharmacology

A pressing need in neurobiology is the comprehensive identification and characterization of neuronal subclasses within the mammalian nervous system. To this end, we used constellation pharmacology as a method to interrogate the neuronal and glial subclasses of the mouse cerebellum individually and simultaneously. We then evaluated the data obtained from constellation-pharmacology experiments by cluster analysis to classify cells into neuronal and glial subclasses, based on their functional expression of glutamate, acetylcholine, and GABA receptors, among other ion channels. Conantokin peptides were used to identify N-methyl-d-aspartate (NMDA) receptor subtypes, which revealed that neurons of the young mouse cerebellum expressed NR2A and NR2B NMDA receptor subunits. Additional pharmacological tools disclosed differential expression of α-amino-3-hydroxy-5-methyl-4-isoxazloepropionic, nicotinic acetylcholine, and muscarinic acetylcholine receptors in different neuronal and glial subclasses. Certain cell subclasses correlated with known attributes of granule cells, and we combined constellation pharmacology with genetically labeled neurons to identify and characterize Purkinje cells. This study illustrates the utility of applying constellation pharmacology to classify neuronal and glial subclasses in specific anatomical regions of the brain.

Constellation pharmacology: a new paradigm for drug discovery

Constellation pharmacology is a cell-based high-content phenotypic-screening platform that utilizes subtype-selective pharmacological agents to elucidate the cell-specific combinations (constellations) of key signaling proteins that define specific cell types. Heterogeneous populations of native cells, in which the different individual cell types have been identified and characterized, are the foundation for this screening platform. Constellation pharmacology is useful for screening small molecules or for deconvoluting complex mixtures of biologically active natural products. This platform has been used to purify natural products and discover their molecular mechanisms. In the ongoing development of constellation pharmacology, there is a positive feedback loop between the pharmacological characterization of cell types and screening for new drug candidates. As constellation pharmacology is used to discover compounds with novel targeting-selectivity profiles, those new compounds then further help to elucidate the constellations of specific cell types, thereby increasing the content of this high-content platform.

Structure and function of μ-conotoxins, peptide-based sodium channel blockers with analgesic activity

μ-Conotoxins block voltage-gated sodium channels (VGSCs) and compete with tetrodotoxin for binding to the sodium conductance pore. Early efforts identified µ-conotoxins that preferentially blocked the skeletal muscle subtype (NaV1.4). However, the last decade witnessed a significant increase in the number of µ-conotoxins and the range of VGSC subtypes inhibited (NaV1.2, NaV1.3 or NaV1.7). Twenty µ-conotoxin sequences have been identified to date and structure-activity relationship studies of several of these identified key residues responsible for interactions with VGSC subtypes. Efforts to engineer-in subtype specificity are driven by in vivo analgesic and neuromuscular blocking activities. This review summarizes structural and pharmacological studies of µ-conotoxins, which show promise for development of selective blockers of NaV1.2, and perhaps also NaV1.1,1.3 or 1.7.

Mouse DRG Cell Line with Properties of Nociceptors

In vitro cell lines from DRG neurons aid drug discovery because they can be used for early stage, high-throughput screens for drugs targeting pain pathways, with minimal dependence on animals. We have established a conditionally immortal DRG cell line from the Immortomouse. Using immunocytochemistry, RT-PCR and calcium microfluorimetry, we demonstrate that the cell line MED17.11 expresses markers of cells committed to the sensory neuron lineage. Within a few hours under differentiating conditions, MED17.11 cells extend processes and following seven days of differentiation, express markers of more mature DRG neurons, such as NaV1.7 and Piezo2. However, at least at this time-point, the nociceptive marker NaV1.8 is not expressed, but the cells respond to compounds known to excite nociceptors, including the TRPV1 agonist capsaicin, the purinergic receptor agonist ATP and the voltage gated sodium channel agonist, veratridine. Robust calcium transients are observed in the presence of the inflammatory mediators bradykinin, histamine and norepinephrine. MED17.11 cells have the potential to replace or reduce the use of primary DRG culture in sensory, pain and developmental research by providing a simple model to study acute nociception, neurite outgrowth and the developmental specification of DRG neurons.

Insights into the origins of fish hunting in venomous cone snails from studies of Conus tessulatus

Prey shifts in carnivorous predators are events that can initiate the accelerated generation of new biodiversity. However, it is seldom possible to reconstruct how the change in prey preference occurred. Here we describe an evolutionary "smoking gun" that illuminates the transition from worm hunting to fish hunting among marine cone snails, resulting in the adaptive radiation of fish-hunting lineages comprising ∼100 piscivorous Conus species. This smoking gun is δ-conotoxin TsVIA, a peptide from the venom of Conus tessulatus that delays inactivation of vertebrate voltage-gated sodium channels. C. tessulatus is a species in a worm-hunting clade, which is phylogenetically closely related to the fish-hunting cone snail specialists. The discovery of a δ-conotoxin that potently acts on vertebrate sodium channels in the venom of a worm-hunting cone snail suggests that a closely related ancestral toxin enabled the transition from worm hunting to fish hunting, as δ-conotoxins are highly conserved among fish hunters and critical to their mechanism of prey capture; this peptide, δ-conotoxin TsVIA, has striking sequence similarity to these δ-conotoxins from piscivorous cone snail venoms. Calcium-imaging studies on dissociated dorsal root ganglion (DRG) neurons revealed the peptide's putative molecular target (voltage-gated sodium channels) and mechanism of action (inhibition of channel inactivation). The results were confirmed by electrophysiology. This work demonstrates how elucidating the specific interactions between toxins and receptors from phylogenetically well-defined lineages can uncover molecular mechanisms that underlie significant evolutionary transitions.

Discovery by proteogenomics and characterization of an RF-amide neuropeptide from cone snail venom

In this study, a proteogenomic annotation strategy was used to identify a novel bioactive peptide from the venom of the predatory marine snail Conus victoriae. The peptide, conorfamide-Vc1 (CNF-Vc1), defines a new gene family. The encoded mature peptide was unusual for conotoxins in that it was cysteine-free and, despite low overall sequence similarity, contained two short motifs common to known neuropeptides/hormones. One of these was the C-terminal RF-amide motif, commonly observed in neuropeptides from a range of organisms, including humans. The mature venom peptide was synthesized and characterized structurally and functionally. The peptide was bioactive upon injection into mice, and calcium imaging of mouse dorsal root ganglion (DRG) cells revealed that the peptide elicits an increase in intracellular calcium levels in a subset of DRG neurons. Unusually for most Conus venom peptides, it also elicited an increase in intracellular calcium levels in a subset of non-neuronal cells.

BIOLOGICAL SIGNIFICANCE

Our findings illustrate the utility of proteogenomics for the discovery of novel, functionally relevant genes and their products. CNF-Vc1 should be useful for understanding the physiological role of RF-amide peptides in the molluscan and mammalian nervous systems.

Pharmacological fractionation of tetrodotoxin-sensitive sodium currents in rat dorsal root ganglion neurons by μ-conotoxins

BACKGROUND AND PURPOSE

Adult rat dorsal root ganglion (DRG) neurons normally express transcripts for five isoforms of the α-subunit of voltage-gated sodium channels: NaV 1.1, 1.6, 1.7, 1.8 and 1.9. Tetrodotoxin (TTX) readily blocks all but NaV 1.8 and 1.9, and pharmacological agents that discriminate among the TTX-sensitive NaV 1-isoforms are scarce. Recently, we used the activity profile of a panel of μ-conotoxins in blocking cloned rodent NaV 1-isoforms expressed in Xenopus laevis oocytes to conclude that action potentials of A- and C-fibres in rat sciatic nerve were, respectively, mediated primarily by NaV 1.6 and NaV 1.7.

EXPERIMENTAL APPROACH

We used three μ-conotoxins, μ-TIIIA, μ-PIIIA and μ-SmIIIA, applied individually and in combinations, to pharmacologically differentiate the TTX-sensitive INa of voltage-clamped neurons acutely dissociated from adult rat DRG. We examined only small and large neurons whose respective INa were >50% and >80% TTX-sensitive.

KEY RESULTS

In both small and large neurons, the ability of the toxins to block TTX-sensitive INa was μ-TIIIA < μ-PIIIA < μ-SmIIIA, with the latter blocking ≳90%. Comparison of the toxin-susceptibility profiles of the neuronal INa with recently acquired profiles of rat NaV 1-isoforms, co-expressed with various NaV β-subunits in X. laevis oocytes, were consistent: NaV 1.1, 1.6 and 1.7 could account for all of the TTX-sensitive INa , with NaV 1.1 < NaV 1.6 < NaV 1.7 for small neurons and NaV 1.7 < NaV 1.1 < NaV 1.6 for large neurons.

CONCLUSIONS AND IMPLICATIONS

Combinations of μ-conotoxins can be used to determine the probable NaV 1-isoforms underlying the INa in DRG neurons. Preliminary experiments with sympathetic neurons suggest that this approach is extendable to other neurons.

A family of excitatory peptide toxins from venomous crassispirine snails: using Constellation Pharmacology to assess bioactivity

The toxinology of the crassispirine snails, a major group of venomous marine gastropods within the superfamily Conoidea, is largely unknown. Here we define the first venom peptide superfamily, the P-like crassipeptides, and show that the organization of their gene sequences is similar to conotoxin precursors. We provide evidence that one peptide family within the P-like crassipeptide superfamily includes potassium-channel (K-channel) blockers, the κP-crassipeptides. Three of these peptides were chemically synthesized (cce9a, cce9b and iqi9a). Using conventional electrophysiology, cce9b was shown to be an antagonist of both a human Kv1.1 channel isoform (Shaker subfamily of voltage-gated K channels) and a Drosophila K-channel isoform. We assessed the bioactivity of these peptides in native mammalian dorsal root ganglion neurons in culture. We demonstrate that two of these crassipeptides, cce9a and cce9b, elicited an excitatory phenotype in a subset of small-diameter capsaicin-sensitive mouse DRG neurons that were also affected by κJ-conotoxin PlXIVA (pl14a), a blocker of Kv1.6 channels. Given the vast complexity of heteromeric K-channel isoforms, this study demonstrates that the crassispirine venoms are a potentially rich source for discovering novel peptides that can help to identify and characterize the diversity of K-channel subtypes expressed in native neurons and other cell types.

Griseorhodins D-F, neuroactive intermediates and end products of post-PKS tailoring modification in Griseorhodin biosynthesis

The griseorhodins belong to a family of extensively modified aromatic polyketides that exhibit activities such as inhibition of HIV reverse transcriptase and human telomerase. The vast structural diversity of this group of polyketides is largely introduced by enzymatic oxidations, which can significantly influence the bioactivity profile. Four new compounds, griseorhodins D-F, were isolated from a griseorhodin producer, Streptomyces sp. CN48+, based upon their enhancement of calcium uptake in a mouse dorsal root ganglion primary cell culture assay. Two of these compounds, griseorhodins D1 and D2, were shown to be identical to the major, previously uncharacterized products of a grhM mutant in an earlier griseorhodin biosynthesis study. Their structures enabled the establishment of a more complete hypothesis for the biosynthesis of griseorhodins and related compounds. The other two compounds, griseorhodins E and F, represent new products of post-polyketide synthase tailoring in griseorhodin biosynthesis and showed significant binding activity in a human dopamine active transporter assay.

Defining modulatory inputs into CNS neuronal subclasses by functional pharmacological profiling

Previously we defined neuronal subclasses within the mouse peripheral nervous system using an experimental strategy called "constellation pharmacology." Here we demonstrate the broad applicability of constellation pharmacology by extending it to the CNS and specifically to the ventral respiratory column (VRC) of mouse brainstem, a region containing the neuronal network controlling respiratory rhythm. Analysis of dissociated cells from this locus revealed three major cell classes, each encompassing multiple subclasses. We broadly analyzed the combinations (constellations) of receptors and ion channels expressed within VRC cell classes and subclasses. These were strikingly different from the constellations of receptors and ion channels found in subclasses of peripheral neurons from mouse dorsal root ganglia. Within the VRC cell population, a subset of dissociated neurons responded to substance P, putatively corresponding to inspiratory pre-Bötzinger complex (preBötC) neurons. Using constellation pharmacology, we found that these substance P-responsive neurons also responded to histamine, and about half responded to bradykinin. Electrophysiological studies conducted in brainstem slices confirmed that preBötC neurons responsive to substance P exhibited similar responsiveness to bradykinin and histamine. The results demonstrate the predictive utility of constellation pharmacology for defining modulatory inputs into specific neuronal subclasses within central neuronal networks.

Investigating neuronal cell types through comparative cellular physiology

Comprehensive understanding of nervous systems would require the unambiguous identification of all neuronal cell types, which are distinguished by gene-expression differences, particularly in plasma-membrane receptors and ion channels. Toward this ultimate goal, a recent paper initiated an approach to identify and study divergent neuronal cell types through comparative cellular physiology.

Using constellation pharmacology to define comprehensively a somatosensory neuronal subclass

Change is intrinsic to nervous systems; change is required for learning and conditioning and occurs with disease progression, normal development, and aging. To better understand mammalian nervous systems and effectively treat nervous-system disorders, it is essential to track changes in relevant individual neurons. A critical challenge is to identify and characterize the specific cell types involved and the molecular-level changes that occur in each. Using an experimental strategy called constellation pharmacology, we demonstrate that we can define a specific somatosensory neuronal subclass, cold thermosensors, across different species and track changes in these neurons as a function of development. Cold thermosensors are uniformly responsive to menthol and innocuous cool temperature (17 °C), indicating that they express TRPM8 channels. A subset of cold thermosensors expressed α7 nicotinic acetylcholine receptors (nAChRs) but not other nAChR subtypes. Differences in temperature threshold of cold thermosensors correlated with functional expression of voltage-gated K channels Kv1.1/1.2: Relatively higher expression of KV1.1/1.2 channels resulted in a higher threshold response to cold temperature. Other signaling components varied during development and between species. In cold thermosensors of neonatal mice and rats, ATP receptors were functionally expressed, but the expression disappeared with development. This developmental change occurred earlier in low-threshold than high-threshold cold thermosensors. Most rat cold thermosensors expressed TRPA1 channels, whereas mouse cold thermosensors did not. The broad implications of this study are that it is now feasible to track changes in receptor and ion-channel expression in individual neuronal subclasses as a function of development, learning, disease, or aging.

Synthesis and bioactivity of nobilamide B

An alternative and facile solution/solid-phase approach is reported for the total synthesis of neuroactive peptide, nobilamide B. Z-Dhb was formed in solution via EDC/CuCl induced elimination. The solid-phase synthesis employed HBTU/Oxyma Pure™ coupling using Barlos resin. Synthetic nobilamide B was also found to be neuroactive in primary cultures of dorsal root ganglion (DRG) neurons.

Animal toxins influence voltage-gated sodium channel function

Voltage-gated sodium (Nav) channels are essential contributors to neuronal excitability, making them the most commonly targeted ion channel family by toxins found in animal venoms. These molecules can be used to probe the functional aspects of Nav channels on a molecular level and to explore their physiological role in normal and diseased tissues. This chapter summarizes our existing knowledge of the mechanisms by which animal toxins influence Nav channels as well as their potential application in designing therapeutic drugs.

Amplified cold transduction in native nociceptors by M-channel inhibition

Topically applied camphor elicits a sensation of cool, but nothing is known about how it affects cold temperature sensing. We found that camphor sensitizes a subpopulation of menthol-sensitive native cutaneous nociceptors in the mouse to cold, but desensitizes and partially blocks heterologously expressed TRPM8 (transient receptor potential cation channel subfamily M member 8). In contrast, camphor reduces potassium outward currents in cultured sensory neurons and, in cold nociceptors, the cold-sensitizing effects of camphor and menthol are additive. Using a membrane potential dye-based screening assay and heterologously expressed potassium channels, we found that the effects of camphor are mediated by inhibition of Kv7.2/3 channels subtypes that generate the M-current in neurons. In line with this finding, the specific M-current blocker XE991 reproduced the cold-sensitizing effect of camphor in nociceptors. However, the M-channel blocking effects of XE991 and camphor are not sufficient to initiate cold transduction but require a cold-activated inward current generated by TRPM8. The cold-sensitizing effects of XE991 and camphor are largest in high-threshold cold nociceptors. Low-threshold corneal cold thermoreceptors that express high levels of TRPM8 and lack potassium channels are not affected by camphor. We also found that menthol--like camphor--potently inhibits Kv7.2/3 channels. The apparent functional synergism arising from TRPM8 activation and M-current block can improve the effectiveness of topical coolants and cooling lotions, and may also enhance TRPM8-mediated analgesia.

Comparative functional expression of nAChR subtypes in rodent DRG neurons

We investigated the functional expression of nicotinic acetylcholine receptors (nAChRs) in heterogeneous populations of dissociated rat and mouse lumbar dorsal root ganglion (DRG) neurons by calcium imaging. By this experimental approach, it is possible to investigate the functional expression of multiple receptor and ion-channel subtypes across more than 100 neuronal and glial cells simultaneously. Based on nAChR expression, DRG neurons could be divided into four subclasses: (1) neurons that express predominantly α3β4 and α6β4 nAChRs; (2) neurons that express predominantly α7 nAChRs; (3) neurons that express a combination of α3β4/α6β4 and α7 nAChRs; and (4) neurons that do not express nAChRs. In this comparative study, the same four neuronal subclasses were observed in mouse and rat DRG. However, the expression frequency differed between species: substantially more rat DRG neurons were in the first three subclasses than mouse DRG neurons, at all developmental time points tested in our study. Approximately 70-80% of rat DRG neurons expressed functional nAChRs, in contrast to only ~15-30% of mouse DRG neurons. Our study also demonstrated functional coupling between nAChRs, voltage-gated calcium channels, and mitochondrial Ca(2) (+) transport in discrete subsets of DRG neurons. In contrast to the expression of nAChRs in DRG neurons, we demonstrated that a subset of non-neuronal DRG cells expressed muscarinic acetylcholine receptors and not nAChRs. The general approach to comparative cellular neurobiology outlined in this paper has the potential to better integrate molecular and systems neuroscience by uncovering the spectrum of neuronal subclasses present in a given cell population and the functionally integrated signaling components expressed in each subclass.

Biodiversity of cone snails and other venomous marine gastropods: evolutionary success through neuropharmacology

Venomous marine snails (superfamily Conoidea) are a remarkably biodiverse marine invertebrate lineage (featuring more than 10,000 species). Conoideans use complex venoms (up to 100 different components for each species) to capture prey and for other biotic interactions. Molecular phylogeny and venom peptide characterization provide an unusual multidisciplinary view of conoidean biodiversity at several taxonomic levels. Venom peptides diverge between species at an unprecedented rate through hypermutation within gene families. Clade divergence within a genus occurs without recruiting new gene families when a saltatory event, such as colonization of new prey types (e.g., fish), leads to a new radiation. Divergence between genera in the same family involves substantial divergence in gene families. In the superfamily Conoidea, the family groups recruited distinct sets of different venom gene superfamilies. The associated morphological, behavioral, and prey-preference changes that accompany these molecular changes are unknown for most conoidean lineages, except for one genus, Conus, for which many associated phenotypic changes have been documented.

Neuroactive diol and acyloin metabolites from cone snail-associated bacteria

The bacterium Gordonia sp. 647W.R.1a.05 was cultivated from the venom duct of the cone snail, Conus circumcisus. The Gordonia sp. organic extract modulated the action potential of mouse dorsal root ganglion neurons. Assay-guided fractionation led to the identification of the new compound circumcin A (1) and 11 known analogs (2-12). Two of these compounds, kurasoin B (7) and soraphinol A (8), were active in a human norepinephrine transporter assay with Ki values of 2575 and 867 nM, respectively. No neuroactivity had previously been reported for compounds in this structural class. Gordonia species have been reproducibly isolated from four different cone snail species, indicating a consistent association between these organisms.

A bacterial source for mollusk pyrone polyketides

In the oceans, secondary metabolites often protect otherwise poorly defended invertebrates, such as shell-less mollusks, from predation. The origins of these metabolites are largely unknown, but many of them are thought to be made by symbiotic bacteria. In contrast, mollusks with thick shells and toxic venoms are thought to lack these secondary metabolites because of reduced defensive needs. Here, we show that heavily defended cone snails also occasionally contain abundant secondary metabolites, γ-pyrones known as nocapyrones, which are synthesized by symbiotic bacteria. The bacteria, Nocardiopsis alba CR167, are related to widespread actinomycetes that we propose to be casual symbionts of invertebrates on land and in the sea. The natural roles of nocapyrones are unknown, but they are active in neurological assays, revealing that mollusks with external shells are an overlooked source of secondary metabolite diversity.

TRPM8 is the principal mediator of menthol-induced analgesia of acute and inflammatory pain

Menthol, the cooling natural product of peppermint, is widely used in medicinal preparations for the relief of acute and inflammatory pain in sports injuries, arthritis, and other painful conditions. Menthol induces the sensation of cooling by activating TRPM8, an ion channel in cold-sensitive peripheral sensory neurons. Recent studies identified additional targets of menthol, including the irritant receptor, TRPA1, voltage-gated ion channels and neurotransmitter receptors. It remains unclear which of these targets contribute to menthol-induced analgesia, or to the irritating side effects associated with menthol therapy. Here, we use genetic and pharmacological approaches in mice to probe the role of TRPM8 in analgesia induced by L-menthol, the predominant analgesic menthol isomer in medicinal preparations. L-menthol effectively diminished pain behavior elicited by chemical stimuli (capsaicin, acrolein, acetic acid), noxious heat, and inflammation (complete Freund's adjuvant). Genetic deletion of TRPM8 completely abolished analgesia by L-menthol in all these models, although other analgesics (acetaminophen) remained effective. Loss of L-menthol-induced analgesia was recapitulated in mice treated with a selective TRPM8 inhibitor, AMG2850. Selective activation of TRPM8 with WS-12, a menthol derivative that we characterized as a specific TRPM8 agonist in cultured sensory neurons and in vivo, also induced TRPM8-dependent analgesia of acute and inflammatory pain. L-menthol- and WS-12-induced analgesia was blocked by naloxone, suggesting activation of endogenous opioid-dependent analgesic pathways. Our data show that TRPM8 is the principal mediator of menthol-induced analgesia of acute and inflammatory pain. In contrast to menthol, selective TRPM8 agonists may produce analgesia more effectively, with diminished side effects.

An animal model of oxaliplatin-induced cold allodynia reveals a crucial role for Nav1.6 in peripheral pain pathways

Cold allodynia, pain in response to cooling, occurs during or within hours of oxaliplatin infusion and is thought to arise from a direct effect of oxaliplatin on peripheral sensory neurons. To characterize the pathophysiological mechanisms underlying acute oxaliplatin-induced cold allodynia, we established a new intraplantar oxaliplatin mouse model that rapidly developed long-lasting cold allodynia mediated entirely through tetrodotoxin-sensitive Nav pathways. Using selective inhibitors and knockout animals, we found that Nav1.6 was the key isoform involved, while thermosensitive transient receptor potential channels were not involved. Consistent with a crucial role for delayed-rectifier potassium channels in excitability in response to cold, intraplantar administration of the K(+)-channel blocker 4-aminopyridine mimicked oxaliplatin-induced cold allodynia and was also inhibited by Nav1.6 blockers. Intraplantar injection of the Nav1.6 activator Cn2 elicited spontaneous pain, mechanical allodynia, and enhanced 4-aminopyridine-induced cold allodynia. These findings provide behavioural evidence for a crucial role of Nav1.6 in multiple peripheral pain pathways including cold allodynia.