Role of TRPM8 in dorsal root ganglion in nerve injury-induced chronic pain

Pharmacological agents targeting transient receptor potential (TRP) channels in neuropathic pain: Preclinical and clinical status

Neuropathic pain generally affects 7-10% population worldwide and an estimated ∼1 in every 20 individuals in western countries suffer and burden to society. The most limiting factor with existing therapies includes dose escalation issues, off-target side effects and poor translation of randomized trials into clinical practice. Neuropathic pain is a broad term that comprises direct injury/damage to the central and/or peripheral nervous system, leads to maladaptive changes in neuronal as well as in non-neuronal cells, which further contributes to the spontaneous pain, sensory and motor deficit along with altered sensitivity towards the noxious as well as non-noxious stimulus. Transient receptor potential (TRP) channels are polymodal, non-specific cation channels that operate as biosensors to various mechanical and chemical stimuli, including hyperosmolarity, shear stress, heat, mechanical stretch, extracellular ATP, and other products of inflammation. Modulation of these channels leads to various physiological and pathophysiological manifestations at molecular and cellular levels, leading to diseases including neuropathic pain. There are several molecules targeting TRP channels for neuropathic pain in pre-clinical studies, clinical trials and in the market. This review highlights the critical involvement of various pharmacological modulators for TRP channels targeting neuropathic pain and their possible outcomes to harness the therapeutic potential of TRP channels.

Comprehensive investigation of niosomal red palm wax gel encapsulating ginger (Zingiber officinale Roscoe): Network pharmacology, molecular docking, In vitro studies and phase 1 clinical trials

Ginger, a Zingeberaceae family member, is notable for its anti-inflammatory properties. This study explores the pharmaceutical mechanisms of ginger and red palm wax co-extract, developing novel niosomal formulations for enhanced transdermal delivery. Evaluations included physical characteristics, drug loading, in vitro release, network pharmacology, molecular docking, and biocompatibility. The niosomal ginger with red palm wax gel (NGPW) exhibited non-Newtonian fluid properties. The optimized niosome formulation (cholesterol: Tween80: Span60 = 12.5: 20: 5 w/w) showed a high yield (93.23 %), high encapsulation efficiency (54.71 %), and small size (264.33 ± 5.84 nm), prolonging in vitro anti-inflammatory activity. Human skin irritation and biocompatibility tests on 1 % NGPW showed favorable cytotoxicity and hemocompatibility results (ISO10993). Network pharmacology identified potential targets, while molecular docking highlighted high affinities between gingerol and red palm wax compounds with TRPM8 and TRPV1 proteins, suggesting pain inhibition via serotonergic synapse pathways. NGPW presents a promising transdermal pain inhibitory drug delivery strategy.

Investigating the role of nicotinic acetylcholine receptors in menthol's effects in mice

The use of menthol in tobacco products has been linked to an increased likelihood of developing nicotine dependence. The widespread use of menthol can be attributed to its unique sensory characteristics; however, emerging evidence suggests that menthol also alters sensitivity to nicotine through modulation of nicotinic acetylcholine receptors (nAChRs). Nicotinic subunits, such as β2 and α5, are of interest due to their implications in nicotine reward, reinforcement, intake regulation, and aversion. This study, therefore, examined the in vivo relevance of β2 and α5 nicotinic subunits on the pharmacological and behavioral effects of menthol. Data suggests that the α5 nicotinic subunit modulates menthol intake in mice. Overall, deletion or a reduction in function of the α5 subunit lessened aversion to menthol. α5 KO mice and mice possessing the humanized α5 SNP, a variant that confers a nicotine dependence phenotype in humans, demonstrated increased menthol intake compared to their WT counterparts and in a sex-related fashion for α5 SNP mice. We further reported that the modulatory effects of the α5 subunit do not extend to other aversive tastants like quinine, suggesting that deficits in α5* nAChR signaling may not abolish general sensitivity to the aversive effects of other noxious chemicals. Further probing into the role of α5 in other pharmacological properties of menthol revealed that the α5 subunit does not modulate the antinociceptive properties of menthol in mice and suggests that the in vivo differences observed are likely not due to the direct effects of menthol on α5-containing nAChRs in vitro.

Topical coapplication of hyaluronan with transdermal drug delivery enhancers attenuates inflammatory and neuropathic pain

ABSTRACT

We have previously shown that intradermal injection of high-molecular-weight hyaluronan (500-1200 kDa) produces localized antihyperalgesia in preclinical models of inflammatory and neuropathic pain. In the present experiments, we studied the therapeutic effect of topical hyaluronan, when combined with each of 3 transdermal drug delivery enhancers (dimethyl sulfoxide [DMSO], protamine or terpene), in preclinical models of inflammatory and neuropathic pain. Topical application of 500 to 1200 kDa hyaluronan (the molecular weight range used in our previous studies employing intradermal administration), dissolved in 75% DMSO in saline, markedly reduced prostaglandin E 2 (PGE 2 ) hyperalgesia, in male and female rats. Although topical 500- to 1200-kDa hyaluronan in DMSO vehicle dose dependently, also markedly, attenuated oxaliplatin chemotherapy-and paclitaxel chemotherapy-induced painful peripheral neuropathy (CIPN) in male rats, it lacked efficacy in female rats. However, following ovariectomy or intrathecal administration of an oligodeoxynucleotide antisense to G-protein-coupled estrogen receptor (GPR30) mRNA, CIPN in female rats was now attenuated by topical hyaluronan. Although topical coadministration of 150 to 300, 300 to 500, or 1500 to 1750 kDa hyaluronan with DMSO also attenuated CIPN, a slightly lower-molecular-weight hyaluronan (70-120 kDa) did not. The topical administration of a combination of hyaluronan with 2 other transdermal drug delivery enhancers, protamine and terpene, also attenuated CIPN hyperalgesia, an effect that was more prolonged than with DMSO vehicle. Repeated administration of topical hyaluronan prolonged the duration of antihyperalgesia. Our results support the use of topical hyaluronan, combined with chemically diverse nontoxic skin penetration enhancers, to induce marked antihyperalgesia in preclinical models of inflammatory and neuropathic pain.

Menthol Dissolved in Dimethyl Sulfoxide Protects Against Epileptiform Activity Induced by Pentylenetetrazol in Male Rats

Introduction

This research aims to investigate the protective action of menthol dissolved in dimethyl sulfoxide (DMSO) on experimental epileptiform activity induced by the intraperitoneal (IP) injection of pentylenetetrazol (PTZ) in male rats.

Methods

Thirty adult male Wistar rats weighing 200-250 g were randomly assigned to five equal groups. The control animals received normal saline (200 μL) and the rest four cohorts were considered as treatment. Menthol was dissolved in DMSO and intraperitoneally injected at the doses of 100, 200, and 400 mg/kg into the first, second, and third groups (M100, M200, and M400 V=200 μL), respectively. The fourth treatment was injected with the solvent (200 μL). The animals were anesthetized, then underwent cranial surgery and a recording electrode was implanted in the stratum radiatum of the hippocampal carbonic anhydrase 1 (CA1) region (AP=-2.76 mm, ML=-1.4 mm and DV=3 mm). The seizure activity was induced by PTZ (IP) and assessed by counting and measuring amplitudes of the spikes for 10 minutes using the eTrace program.

Results

Menthol was observed to significantly reduce the activity level of PTZ-induced epileptiform activity, as well as exert a protective and inhibitory action on proconvulsant effect of DMSO in a dose-dependent manner.

Conclusion

Menthol can potentially be used as an adjuvant to prevent seizure activity.

Highlights

Dimethyl sulfoxide (DMSO) induces proconvulsant effects, significantly increasing spike counts.Menthol 100 mg/kg also stimulates seizure activity, leading to a substantial spike count increase.Menthol 200 and 400 mg/kg exhibit inhibitory effects, decreasing seizure activity and spike counts.

Plain Language Summary

In this study, we explored the potential protective effects of menthol, dissolved in dimethyl sulfoxide (DMSO), on experimentally induced epileptiform activity in male rats. Our research involved thirty adult male Wistar rats, divided into five groups. While the control group received normal saline, the remaining four groups were treated with different doses of menthol in DMSO. The rats underwent surgery, and electrodes were implanted in the hippocampal region for recording. Using pentylenetetrazol (PTZ) to induce seizure activity, we observed that menthol, administered at varying doses, significantly reduced the level of epileptiform activity triggered by PTZ. Notably, menthol also demonstrated a protective and inhibitory effect on the proconvulsant action of DMSO, and this effect was dose-dependent. In simpler terms, our findings suggest that menthol has the potential to be used as an additional treatment to prevent seizure activity. This means that incorporating menthol, especially at specific doses, may offer a protective influence against epileptic events. This research sheds light on a promising avenue for potential therapeutic interventions, emphasizing the importance of further exploration of menthol's role in epilepsy prevention. Ultimately, our study opens the door to considering menthol as a valuable component in the development of strategies to mitigate the impact of seizures.

Molecular determinants of TRPM8 function: key clues for a cool modulation

Cold thermoreceptor neurons detect temperature drops with highly sensitive molecular machinery concentrated in their peripheral free nerve endings. The main molecular entity responsible for cold transduction in these neurons is the thermo-TRP channel TRPM8. Cold, cooling compounds such as menthol, voltage, and osmolality rises activate this polymodal ion channel. Dysregulation of TRPM8 activity underlies several physiopathological conditions, including painful cold hypersensitivity in response to axonal damage, migraine, dry-eye disease, overactive bladder, and several forms of cancer. Although TRPM8 could be an attractive target for treating these highly prevalent diseases, there is still a need for potent and specific modulators potentially suitable for future clinical trials. This goal requires a complete understanding of the molecular determinants underlying TRPM8 activation by chemical and physical agonists, inhibition by antagonists, and the modulatory mechanisms behind its function to guide future and more successful treatment strategies. This review recapitulates information obtained from different mutagenesis approaches that have allowed the identification of specific amino acids in the cavity comprised of the S1-S4 and TRP domains that determine modulation by chemical ligands. In addition, we summarize different studies revealing specific regions within the N- and C-terminus and the transmembrane domain that contribute to cold-dependent TRPM8 gating. We also highlight the latest milestone in the field: cryo-electron microscopy structures of TRPM8, which have provided a better comprehension of the 21 years of extensive research in this ion channel, shedding light on the molecular bases underlying its modulation, and promoting the future rational design of novel drugs to selectively regulate abnormal TRPM8 activity under pathophysiological conditions.

Progress in the Structural Basis of thermoTRP Channel Polymodal Gating

The thermosensory transient receptor potential (thermoTRP) family of ion channels is constituted by several nonselective cation channels that are activated by physical and chemical stimuli functioning as paradigmatic polymodal receptors. Gating of these ion channels is achieved through changes in temperature, osmolarity, voltage, pH, pressure, and by natural or synthetic chemical compounds that directly bind to these proteins to regulate their activity. Given that thermoTRP channels integrate diverse physical and chemical stimuli, a thorough understanding of the molecular mechanisms underlying polymodal gating has been pursued, including the interplay between stimuli and differences between family members. Despite its complexity, recent advances in cryo-electron microscopy techniques are facilitating this endeavor by providing high-resolution structures of these channels in different conformational states induced by ligand binding or temperature that, along with structure-function and molecular dynamics, are starting to shed light on the underlying allosteric gating mechanisms. Because dysfunctional thermoTRP channels play a pivotal role in human diseases such as chronic pain, unveiling the intricacies of allosteric channel gating should facilitate the development of novel drug-based resolving therapies for these disorders.

The distinctive role of menthol in pain and analgesia: Mechanisms, practices, and advances

Menthol is an important flavoring additive that triggers a cooling sensation. Under physiological condition, low to moderate concentrations of menthol activate transient receptor potential cation channel subfamily M member 8 (TRPM8) in the primary nociceptors, such as dorsal root ganglion (DRG) and trigeminal ganglion, generating a cooling sensation, whereas menthol at higher concentration could induce cold allodynia, and cold hyperalgesia mediated by TRPM8 sensitization. In addition, the paradoxical irritating properties of high concentrations of menthol is associated with its activation of transient receptor potential cation channel subfamily A member 1 (TRPA1). Under pathological situation, menthol activates TRPM8 to attenuate mechanical allodynia and thermal hyperalgesia following nerve injury or chemical stimuli. Recent reports have recapitulated the requirement of central group II/III metabotropic glutamate receptors (mGluR) with endogenous κ-opioid signaling pathways for menthol analgesia. Additionally, blockage of sodium channels and calcium influx is a determinant step after menthol exposure, suggesting the possibility of menthol for pain management. In this review, we will also discuss and summarize the advances in menthol-related drugs for pathological pain treatment in clinical trials, especially in neuropathic pain, musculoskeletal pain, cancer pain and postoperative pain, with the aim to find the promising therapeutic candidates for the resolution of pain to better manage patients with pain in clinics.

Second messengers mediating high-molecular-weight hyaluronan-induced antihyperalgesia in rats with chemotherapy-induced peripheral neuropathy

ABSTRACT

High-molecular-weight hyaluronan (HMWH) is an agonist at cluster of differentiation (CD)44, the cognate hyaluronan receptor, on nociceptors, where it acts to induce antihyperalgesia in preclinical models of inflammatory and neuropathic pain. In the present experiments, we studied the CD44 second messengers that mediate HMWH-induced attenuation of pain associated with oxaliplatin and paclitaxel chemotherapy-induced peripheral neuropathy (CIPN). While HMWH attenuated CIPN only in male rats, after ovariectomy or intrathecal administration of an oligodeoxynucleotide (ODN) antisense to G protein-coupled estrogen receptor (GPR30) mRNA, female rats were also sensitive to HMWH. Intrathecal administration of an ODN antisense to CD44 mRNA markedly attenuated HMWH-induced antihyperalgesia in male rats with CIPN induced by oxaliplatin or paclitaxel. Intradermal administration of inhibitors of CD44 second messengers, RhoA (member of the Rho family of GTPases), phospholipase C, and phosphatidylinositol (PI) 3-kinase gamma (PI3Kγ), attenuated HMWH-induced antihyperalgesia as does intrathecal administration of an ODN antisense to PI3Kγ. Our results demonstrated that HMWH induced antihyperalgesia in CIPN, mediated by its action at CD44 and downstream signaling by RhoA, phospholipase C, and PI3Kγ.

Activation of peripheral TRPM8 mitigates ischemic stroke by topically applied menthol

Background

No reports exist as to neuroprotective effects associated with topical activation of transient receptor potential melastatin 8 (TRPM8), a noted cold receptor. In the present study, we identified whether activating peripheral TRPM8 can be an adjuvant therapy for ischemic stroke.

Methods

Menthol, an agonist of TRPM8, was applied orally or topically to all paws or back of the mouse after middle cerebral artery occlusion (MCAO). We used Trpm8 gene knockout (Trpm8^−/−) mice or TRPM8 antagonist and lidocaine to validate the roles of TRPM8 and peripheral nerve conduction in menthol against ischemic stroke.

Results

Application of menthol 16% to paw derma attenuated infarct volumes and ameliorated sensorimotor deficits in stroke mice induced by MCAO. The benefits of topically applied menthol were associated with reductions in oxidative stress, neuroinflammation and infiltration of monocytes and macrophages in ischemic brains. Antagonizing TRPM8 or Trpm8 knockout dulls the neuroprotective effects of topically application of menthol against MCAO. Immunohistochemistry analyses revealed significantly higher TRPM8 expression in skin tissue samples obtained from the paws compared with skin from the backs, which was reflected by significantly smaller infarct lesion volumes and better sensorimotor function in mice treated with menthol on the paws compared with the back. Blocking conduction of peripheral nerve in the four paws reversed the neuroprotective effects of topical menthol administrated to paws. On the other hand, oral menthol dosing did not assist with recovery from MCAO in our study.

Conclusion

Our results suggested that activation of peripheral TRPM8 expressed in the derma tissue of limbs with sufficient concentration of menthol is beneficial to stroke recovery. Topical application of menthol on hands and feet could be a novel and simple-to-use therapeutic strategy for stroke patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02553-4.

Peripheral Ion Channel Gene Screening in Painful- and Painless-Diabetic Neuropathy

Neuropathic pain is common in diabetic peripheral neuropathy (DN), probably caused by pathogenic ion channel gene variants. Therefore, we performed molecular inversion probes-next generation sequencing of 5 transient receptor potential cation channels, 8 potassium channels and 2 calcium-activated chloride channel genes in 222 painful- and 304 painless-DN patients. Twelve painful-DN (5.4%) patients showed potentially pathogenic variants (five nonsense/frameshift, seven missense, one out-of-frame deletion) in ANO3 (n = 3), HCN1 (n = 1), KCNK18 (n = 2), TRPA1 (n = 3), TRPM8 (n = 3) and TRPV4 (n = 1) and fourteen painless-DN patients (4.6%-three nonsense/frameshift, nine missense, one out-of-frame deletion) in ANO1 (n = 1), KCNK18 (n = 3), KCNQ3 (n = 1), TRPA1 (n = 2), TRPM8 (n = 1), TRPV1 (n = 3) and TRPV4 (n = 3). Missense variants were present in both conditions, presumably with loss- or gain-of-functions. KCNK18 nonsense/frameshift variants were found in painless/painful-DN, making a causal role in pain less likely. Surprisingly, premature stop-codons with likely nonsense-mediated RNA-decay were more frequent in painful-DN. Although limited in number, painful-DN patients with ion channel gene variants reported higher maximal pain during the night and day. Moreover, painful-DN patients with TRP variants had abnormal thermal thresholds and more severe pain during the night and day. Our results suggest a role of ion channel gene variants in neuropathic pain, but functional validation is required.

β-Lactam TRPM8 Antagonist RGM8-51 Displays Antinociceptive Activity in Different Animal Models

Transient receptor potential melastatin subtype 8 (TRPM8) is a cation channel extensively expressed in sensory neurons and implicated in different painful states. However, the effectiveness of TRPM8 modulators for pain relief is still a matter of discussion, since structurally diverse modulators lead to different results, depending on the animal pain model. In this work, we described the antinociceptive activity of a β–lactam derivative, RGM8-51, showing good TRPM8 antagonist activity, and selectivity against related thermoTRP channels and other pain-mediating receptors. In primary cultures of rat dorsal root ganglion (DRG) neurons, RGM8-51 potently reduced menthol-evoked neuronal firing without affecting the major ion conductances responsible for action potential generation. This compound has in vivo antinociceptive activity in response to cold, in a mouse model of oxaliplatin-induced peripheral neuropathy. In addition, it reduces cold, mechanical and heat hypersensitivity in a rat model of neuropathic pain arising after chronic constriction of the sciatic nerve. Furthermore, RGM8-51 exhibits mechanical hypersensitivity-relieving activity, in a mouse model of NTG-induced hyperesthesia. Taken together, these preclinical results substantiate that this TRPM8 antagonist is a promising pharmacological tool to study TRPM8-related diseases.

In Vitro and In Vivo Pharmacological Characterization of a Novel TRPM8 Inhibitor Chemotype Identified by Small-Scale Preclinical Screening

Transient receptor potential melastatin type 8 (TRPM8) is a target for the treatment of different physio-pathological processes. While TRPM8 antagonists are reported as potential drugs for pain, cancer, and inflammation, to date only a limited number of chemotypes have been investigated and thus a limited number of compounds have reached clinical trials. Hence there is high value in searching for new TRPM8 antagonistic to broaden clues to structure-activity relationships, improve pharmacological properties and explore underlying molecular mechanisms. To address this, the EDASA Scientific in-house molecular library has been screened in silico, leading to identifying twenty-one potentially antagonist compounds of TRPM8. Calcium fluorometric assays were used to validate the in-silico hypothesis and assess compound selectivity. Four compounds were identified as selective TRPM8 antagonists, of which two were dual-acting TRPM8/TRPV1 modulators. The most potent TRPM8 antagonists (BB 0322703 and BB 0322720) underwent molecular modelling studies to highlight key structural features responsible for drug-protein interaction. The two compounds were also investigated by patch-clamp assays, confirming low micromolar potencies. The most potent compound (BB 0322703, IC50 1.25 ± 0.26 μM) was then profiled in vivo in a cold allodinya model, showing pharmacological efficacy at 30 μM dose. The new chemotypes identified showed remarkable pharmacological properties paving the way to further investigations for drug discovery and pharmacological purposes.

P2Y1 purinergic receptor inhibition attenuated remifentanil-induced postoperative hyperalgesia via decreasing NMDA receptor phosphorylation in dorsal root ganglion

BACKGROUND

Remifentanil-induced postoperative hyperalgesia is an intractable side effect of the clinical use of remifentanil, the mechanism of which remains obscure, especially in the peripheral nervous system. N-methyl-D-aspartate receptor (NMDAR) phosphorylation in dorsal root ganglion (DRG) plays a pronociceptive role in neuropathic pain. The contribution of the P2Y1 purinergic receptor (P2Y1R) in DRG to pain hypersensitivity derived from various origins and P2Y1R upregulation-induced NMDAR activation in neurons have also been uncovered. This study aimed to investigate whether P2Y1R participates in nociceptive processing in the DRG and spinal cord in remifentanil-induced postoperative hyperalgesia.

METHODS

Rats with remifentanil-induced postoperative hyperalgesia were intrathecally injected with NMDAR antagonist MK801 or P2Y1R antagonist MRS2179 at 10 min prior to remifentanil infusion. Mechanical allodynia, heat hyperalgesia, and cold hyperalgesia were measured at -24 h, 2 h, 6 h, 24 h, and 48 h following remifentanil infusion. The P2Y1R expression and NMDAR expression and phosphorylation in DRG ipsilateral to the incision were detected by Western blot and immunofluorescence.

RESULTS

Incision and remifentanil induced mechanical allodynia, heat hyperalgesia, and cold hyperalgesia accompanied by upregulated P2Y1R expression, increased NMDAR subunit NR1 expression and phosphorylation at Ser896, and NR2B expression and phosphorylation at Tyr1472 in DRG. Inhibition of NMDAR phosphorylation by MK801 effectively attenuated remifentanil-induced postoperative hyperalgesia. Furthermore, P2Y1R blockade by MRS2179 not only lessened remifentanil-evoked postoperative hypersensitivity to mechanical, heat, and cold stimuli, but also suppressed the increases in NR1 and NR2B expression and phosphorylation in DRG induced by incision and remifentanil.

CONCLUSION

The process by which P2Y1R mediates NMDAR expression and phosphorylation represents a mechanism of remifentanil-induced postoperative hyperalgesia in the DRG and/or spinal cord.

PI3Kγ/AKT Signaling in High Molecular Weight Hyaluronan (HMWH)-Induced Anti-Hyperalgesia and Reversal of Nociceptor Sensitization

High molecular weight hyaluronan (HMWH), a well-established treatment for osteoarthritis pain, is anti-hyperalgesic in preclinical models of inflammatory and neuropathic pain. HMWH-induced anti-hyperalgesia is mediated by its action at cluster of differentiation 44 (CD44), the cognate hyaluronan receptor, which can signal via phosphoinositide 3-kinase (PI3K), a large family of kinases involved in diverse cell functions. We demonstrate that intrathecal administration of an oligodeoxynucleotide (ODN) antisense to mRNA for PI3Kγ (a Class I PI3K isoform) expressed in dorsal root ganglia (DRGs), and intradermal administration of a PI3Kγ-selective inhibitor (AS605240), markedly attenuates HMWH-induced anti-prostaglandin E₂ (PGE₂) hyperalgesia, in male and female rats. Intradermal administration of inhibitors of mammalian target of rapamycin (mTOR; rapamycin) and protein kinase B (AKT; AKT Inhibitor IV), signaling molecules downstream of PI3Kγ, also attenuates HMWH-induced anti-hyperalgesia. In vitro patch-clamp electrophysiology experiments on cultured nociceptors from male rats demonstrate that some HMWH-induced changes in generation of action potentials (APs) in nociceptors sensitized by PGE₂ are PI3Kγ dependent (reduction in AP firing rate, increase in latency to first AP and increase in slope of current ramp required to induce AP) and some are PI3Kγ independent [reduction in recovery rate of AP afterhyperpolarization (AHP)]. Our demonstration of a role of PI3Kγ in HMWH-induced anti-hyperalgesia and reversal of nociceptor sensitization opens a novel line of research into molecular targets for the treatment of diverse pain syndromes.SIGNIFICANCE STATEMENT We have previously demonstrated that high molecular weight hyaluronan (HMWH) attenuates inflammatory hyperalgesia, an effect mediated by its action at cluster of differentiation 44 (CD44), the cognate hyaluronan receptor, and activation of its downstream signaling pathway, in nociceptors. In the present study, we demonstrate that phosphoinositide 3-kinase (PI3K)γ and downstream signaling pathway, protein kinase B (AKT) and mammalian target of rapamycin (mTOR), are crucial for HMWH to induce anti-hyperalgesia.

Sexually Dimorphic Role of Toll-like Receptor 4 (TLR4) in High Molecular Weight Hyaluronan (HMWH)-induced Anti-hyperalgesia

High molecular weight hyaluronan (HMWH), a prominent component of the extracellular matrix binds to and signals via multiple receptors, including cluster of differentiation 44 (CD44) and toll-like receptor 4 (TLR4). We tested the hypothesis that, in the setting of inflammation, HMWH acts at TLR4 to attenuate hyperalgesia. We found that the attenuation of prostaglandin E2 (PGE2)-induced hyperalgesia by HMWH was attenuated by a TLR4 antagonist (NBP2-26245), but only in male and ovariectomized female rats. In this study we sought to evaluated the role of the TLR4 signaling pathway in anti-hyperalgesia induced by HMWH in male rats. Decreasing expression of TLR4 in nociceptors, by intrathecal administration of an oligodeoxynucleotide (ODN) antisense to TLR4 mRNA, also attenuated HMWH-induced anti-hyperalgesia, in male and ovariectomized female rats. Estrogen replacement in ovariectomized females reconstituted the gonad-intact phenotype. The administration of an inhibitor of myeloid differentiation factor 88 (MyD88), a TLR4 second messenger, attenuated HMWH-induced anti-hyperalgesia, while an inhibitor of the MyD88-independent TLR4 signaling pathway did not. Since it has previously been shown that HMWH-induced anti-hyperalgesia is also mediated, in part by CD44 we evaluated the effect of the combination of ODN antisense to TLR4 and CD44 mRNA. This treatment completely reversed HMWH-induced anti-hyperalgesia in male rats. Our results demonstrate a sex hormone-dependent, sexually dimorphic involvement of TLR4 in HMWH-induced anti-hyperalgesia, that is MyD88 dependent. PERSPECTIVE: The role of TLR4 in anti-hyperalgesia induced by HMWH is a sexually dimorphic, TLR4 dependent inhibition of inflammatory hyperalgesia that provides a novel molecular target for the treatment of inflammatory pain.

Long-Term Sequelae of Frostbite-A Scoping Review

Frostbite is tissue damage caused by freezing temperatures and constitutes an important cause of morbidity in cold climate zones and high altitude. The direct effects of sub-zero temperatures lead to tissue freezing, electrolyte shifts and pH alterations, microvascular damage, and eventually to cell death. Upon rewarming, inflammatory reperfusion injury and thrombosis may lead to further tissue damage. Several studies and various case reports show that many patients suffer from long-term sequelae such as vasomotor disturbances (associated with susceptibility to refreezing), and neuropathic and nociceptive pain, as well as damage to skeletal structures. There are still many uncertainties regarding the pathophysiology of these sequelae. It has been shown that the transient receptor potential channel (TRP) family plays a role in cold allodynia. Botulinum Toxin type A (BTX-A) injections have been reported to be beneficial in vasomotor and neuropathic disturbances secondary to frostbite. Epidural sympathetic block has been used for short-term treatment of frostbite induced chronic pain. Furthermore, amitriptyline, gabapentinoids, and duloxetine may have some benefits. Frostbite arthritis clinically resembles regular osteoarthritis. In children there is a risk of epiphyseal cartilage damage leading to bone deformities. Despite some promising therapeutic concepts, the scarcity of data on frostbite long-term sequelae in the literature indicates the need of more in-depth studies of this pathology in all its aspects.

P2Y1 Purinergic Receptor Contributes to Remifentanil-Induced Cold Hyperalgesia via Transient Receptor Potential Melastatin 8-Dependent Regulation of N-methyl-d-aspartate Receptor Phosphorylation in Dorsal Root Ganglion

BACKGROUND

Remifentanil can induce postinfusion cold hyperalgesia. N-methyl-d-aspartate receptor (NMDAR) activation and upregulation of transient receptor potential melastatin 8 (TRPM8) membrane trafficking in dorsal root ganglion (DRG) are critical to cold hyperalgesia derived from neuropathic pain, and TRPM8 activation causes NMDAR-dependent cold response. Contribution of P2Y1 purinergic receptor (P2Y1R) activation in DRG to cold pain hypersensitivity and NMDAR activation induced by P2Y1R upregulation in neurons are also unraveled. This study explores whether P2Y1R contributes to remifentanil-induced cold hyperalgesia via TRPM8-dependent regulation of NMDAR phosphorylation in DRG.

METHODS

Rats with remifentanil-induced cold hyperalgesia were injected with TRPM8 antagonist or P2Y1R antagonist at 10 minutes before remifentanil infusion. Cold hyperalgesia (paw lift number and withdrawal duration on cold plate) was measured at -24, 2, 6, 24, and 48 hours following remifentanil infusion. After the last behavioral test, P2Y1R expression, TRPM8 expression and membrane trafficking, and NMDAR subunit (NR1 and NR2B) expression and phosphorylation in DRG were detected by western blot, and colocalization of P2Y1R with TRPM8 was determined by double-labeling immunofluorescence. Two-way repeated measures analysis of variance (ANOVA) or 2 × 2 factorial design ANOVA with repeated measures was used to analyze behavioral data of cold hyperalgesia. One-way ANOVA followed by Bonferroni post hoc comparisons was used to analyze the data in western blot and immunofluorescence.

RESULTS

Remifentanil infusion (1 μg·kg-1·min-1 for 60 minutes) induced cold hyperalgesia (hyperalgesia versus control, paw lift number and withdrawal duration on cold plate at 2-48 hours, P < .0001) with upregulated NR1 (hyperalgesia versus naive, 48 hours, mean ± standard deviation [SD], 114.00% ± 12.48% vs 41.75% ± 5.20%, P < .005) and NR2B subunits expression (104.13% ± 8.37% vs 24.63% ± 4.87%, P < .005), NR1 phosphorylation at Ser896 (91.88% ± 7.08% vs 52.00% ± 7.31%, P < .005) and NR2B phosphorylation at Tyr1472 (115.75% ± 8.68% vs 59.75% ± 7.78%, P < .005), TRPM8 expression (115.38% ± 9.27% vs 40.50% ± 4.07%, P < .005) and membrane trafficking (112.88% ± 5.62% vs 48.88% ± 6.49%, P < .005), and P2Y1R expression (128.25% ± 14.86% vs 45.13% ± 7.97%, P < .005) in DRG. Both TRPM8 and P2Y1R antagonists attenuated remifentanil-induced cold hyperalgesia and downregulated increased NR1 and NR2B expression and phosphorylation induced by remifentanil (remifentanil + RQ-00203078 versus remifentanil + saline, NR1 phosphorylation, 69.38% ± 3.66% vs 92.13% ± 4.85%; NR2B phosphorylation, 72.25% ± 6.43% vs 111.75% ± 11.00%, P < .0001). NMDAR activation abolished inhibition of TRPM8 and P2Y1R antagonists on remifentanil-induced cold hyperalgesia. P2Y1R antagonist inhibited remifentanil-evoked elevations in TRPM8 expression and membrane trafficking and P2Y1R-TRPM8 coexpression (remifentanil + 2'-deoxy-N6-methyl adenosine 3',5'-diphosphate [MRS2179] versus remifentanil + saline, coexpression, 8.33% ± 1.33% vs 22.19% ± 2.15%, P < .0001).

CONCLUSIONS

Attenuation of remifentanil-induced cold hyperalgesia by P2Y1R inhibition is attributed to downregulations in NMDAR expression and phosphorylation via diminishing TRPM8 expression and membrane trafficking in DRG.

The cool things to know about TRPM8!

Transient receptor potential melastatin 8 (TRPM8) channels play a central role in the detection of environmental cold temperatures in the somatosensory system. TRPM8 is found in a subset of unmyelinated (C-type) afferents located in the dorsal root (DRG) and trigeminal ganglion (TG). Cold hypersensitivity is a common symptom of neuropathic pain conditions caused by cancer therapy, spinal cord injury, viral infection, multiple sclerosis, diabetes, or withdrawal symptoms associated with chronic morphine treatment. Therefore, TRPM8 has received great attention as a therapeutic target. However, as the activity of TRPM8 is unique in sensing cool temperature as well as warming, it is critical to understand the signaling transduction pathways that control modality-specific activity of TRPM8 in healthy versus pathological settings. This review summarizes recent advances in our understanding of the mechanisms involved in the regulation of the TRPM8 activity.

Peripheral Neuropathic Pain: From Experimental Models to Potential Therapeutic Targets in Dorsal Root Ganglion Neurons

Neuropathic pain exerts a global burden caused by the lesions in the somatosensory nerve system, including the central and peripheral nervous systems. The mechanisms of nerve injury-induced neuropathic pain involve multiple mechanisms, various signaling pathways, and molecules. Currently, poor efficacy is the major limitation of medications for treating neuropathic pain. Thus, understanding the detailed molecular mechanisms should shed light on the development of new therapeutic strategies for neuropathic pain. Several well-established in vivo pain models were used to investigate the detail mechanisms of peripheral neuropathic pain. Molecular mediators of pain are regulated differentially in various forms of neuropathic pain models; these regulators include purinergic receptors, transient receptor potential receptor channels, and voltage-gated sodium and calcium channels. Meanwhile, post-translational modification and transcriptional regulation are also altered in these pain models and have been reported to mediate several pain related molecules. In this review, we focus on molecular mechanisms and mediators of neuropathic pain with their corresponding transcriptional regulation and post-translational modification underlying peripheral sensitization in the dorsal root ganglia. Taken together, these molecular mediators and their modification and regulations provide excellent targets for neuropathic pain treatment.

Regeneration of mechanical sciatic nerve injury is affected by cold and heat exposure: involvements of the TRPM2 and TRPM8 channels

BACKGROUND

Mechanical sciatic nerve (MSN) injury has a high rate within trauma cases. Heat and cold exposure in the treatments of MSN injuries have been clinically used in human. The MSN injury results in apoptosis, overload Ca²⁺ influx, and reactive oxygen species (ROS) generation in the sciatic nerve. TRPM2 and TRPM8 cation channels are activated by ROS. TRPM2 is activated by warmth (36-38°C) and heat (45-47°C), although TRPM8 is activated by cold (0-25°C). Heat or cold exposure may aid recovery MSN injury through modulation of TRPM2 and TRPM8 in sciatic nerve.

OBJECTIVE

The protective roles of cold and heat treatments via modulation of TRPM2 and TRPM8 were evaluated on MSN injury-induced neurotoxicity in in vitro models of mouse and the SH-SY5Y cell line.

METHOD

The mice sciatic nerves and SH-SY5Y cells were divided into control (37°C), cold (10°C), and moderate heat (40°C) groups.

RESULTS

Our data identified a decrease in injury diameter in the neurons following heat exposure, but not cold exposure. In addition, the results of laser confocal microscopy analyses were indicative of a protective role of TRPM8 antagonist (ACA) against cold-induced increases in Ca²⁺ influx in the sciatic nerve and TRPM8 expressing SH-SY5Y cells. The results of the automatic plate reader and laser confocal microscope assays indicated a protective role of heat treatment against MSN injury-induced increases in apoptosis, mitochondrial ROS, cytosolic ROS, caspase -3, and -9 in the neurons.

CONCLUSIONS

The heat treatment via possible modulation of TRPM2 channel and heat shock proteins induced protective actions against injury-mediated increases of oxidative stress, excitotoxicity, and apoptosis in the sciatic nerve and SH-SY5Y cells.

Involvement of TACAN, a Mechanotransducing Ion Channel, in Inflammatory But Not Neuropathic Hyperalgesia in the Rat

TACAN (Tmem120A), a mechanotransducing ion channel highly expressed in a subset of nociceptors, has recently been shown to contribute to detection of noxious mechanical stimulation. In the present study we evaluated its role in sensitization to mechanical stimuli associated with preclinical models of inflammatory and chemotherapy-induced neuropathic pain (CIPN). Intrathecal administration of an oligodeoxynucleotide antisense (AS-ODN) to TACAN mRNA attenuated TACAN protein expression in rat dorsal root ganglia (DRG). While TACAN AS-ODN produced only a modest increase in mechanical nociceptive threshold, it markedly reduced mechanical hyperalgesia produced by intradermal administration of prostaglandin E₂, tumor necrosis factor alpha, and low molecular weight hyaluronan, and systemic administration of lipopolysaccharide, compatible with a prominent role of TACAN in mechanical hyperalgesia produced by inflammation. In contrast, TACAN AS-ODN had no effect on mechanical hyperalgesia associated with CIPN produced by oxaliplatin or paclitaxel. Our results provide evidence that TACAN plays a role in mechanical hyperalgesia induced by pronociceptive inflammatory mediators, but not CIPN, compatible with multiple mechanisms mediating mechanical nociception, and sensitization to mechanical stimuli in preclinical models of inflammatory versus CIPN. PERSPECTIVE: We evaluated the role of TACAN, a mechanotransducing ion channel in nociceptors, in preclinical models of inflammatory and CIPN. Attenuation of TACAN expression reduced hyperalgesia produced by inflammatory mediators but had not chemotherapeutic agents. Our findings support the presence of multiple mechanotransducers in nociceptors.

Involvement of Neural Transient Receptor Potential Channels in Peripheral Inflammation

Transient receptor potential (TRP) channels are a superfamily of non-selective cation channels that act as polymodal sensors in many tissues throughout mammalian organisms. In the context of ion channels, they are unique for their broad diversity of activation mechanisms and their cation selectivity. TRP channels are involved in a diverse range of physiological processes including chemical sensing, nociception, and mediating cytokine release. They also play an important role in the regulation of inflammation through sensory function and the release of neuropeptides. In this review, we discuss the functional contribution of a subset of TRP channels (TRPV1, TRPV4, TRPM3, TRPM8, and TRPA1) that are involved in the body’s immune responses, particularly in relation to inflammation. We focus on these five TRP channels because, in addition to being expressed in many somatic cell types, these channels are also expressed on peripheral ganglia and nerves that innervate visceral organs and tissues throughout the body. Activation of these neural TRP channels enables crosstalk between neurons, immune cells, and epithelial cells to regulate a wide range of inflammatory actions. TRP channels act either through direct effects on cation levels or through indirect modulation of intracellular pathways to trigger pro- or anti-inflammatory mechanisms, depending on the inflammatory disease context. The expression of TRP channels on both neural and immune cells has made them an attractive drug target in diseases involving inflammation. Future work in this domain will likely yield important new pathways and therapies for the treatment of a broad range of disorders including colitis, dermatitis, sepsis, asthma, and pain.

Menthol facilitates excitatory and inhibitory synaptic transmission in rat medullary dorsal horn neurons

Menthol, which acts as an agonist for transient receptor potential melastatin 8 (TRPM8), has complex effects on nociceptive transmission, including pain relief and hyperalgesia. Here, we addressed the effects of menthol on spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs, respectively) in medullary dorsal horn neurons, using a whole-cell patch-clamp technique. Menthol significantly increased sEPSC frequency, in a concentration-dependent manner, without affecting current amplitudes. The menthol-induced increase in sEPSC frequency could be completely blocked by AMTB, a TRPM8 antagonist, but was not blocked by HC-030031, a transient receptor potential ankyrin 1 (TRPA1) antagonist. Menthol still increased sEPSC frequency in the presence of Cd²⁺, a general voltage-gated Ca²⁺ channel blocker, suggesting that voltage-gated Ca²⁺ channels are not involved in the menthol-induced increase in sEPSC frequency. However, menthol failed to increase sEPSC frequency in the absence of extracellular Ca²⁺, suggesting that TRPM8 on primary afferent terminals is Ca²⁺ permeable. On the other hand, menthol also increased sIPSC frequency, without affecting current amplitudes. The menthol-induced increase in sIPSC frequency could be completely blocked by either AMTB or CNQX, an AMPA/KA receptor antagonist, suggesting that the indirect increase in excitability of inhibitory interneurons may lead to the facilitation of spontaneous GABA and/or glycine release. The present results suggested that menthol exerts analgesic effects, via the enhancement of inhibitory synaptic transmission, through central feed-forward neural circuits within the medullary dorsal horn region.

Exploration of TRPM8 Binding Sites by β-Carboline-Based Antagonists and Their In Vitro Characterization and In Vivo Analgesic Activities

Transient receptor potential melastatin 8 (TRPM8) ion channel represents a valuable pharmacological option for several therapeutic areas. Here, a series of conformationally restricted derivatives of the previously described TRPM8 antagonist N,N′-dibenzyl tryptophan 4 were prepared and characterized in vitro by Ca²⁺-imaging and patch-clamp electrophysiology assays. Molecular modeling studies led to identification of a broad and well-defined interaction network of these derivatives inside the TRPM8 binding site, underlying their antagonist activity. The (5R,11aS)-5-(4-chlorophenyl)-2-(4-fluorobenzyl)-5,6,11,11a-tetrahydro-1H-imidazo[1′,5′:1,6]pyrido[3,4-b]indole-1,3(2H)-dione (31a) emerged as a potent (IC₅₀ = 4.10 ± 1.2 nM), selective, and metabolically stable TRPM8 antagonist. In vivo, 31a showed significant target coverage in an icilin-induced WDS (at 11.5 mg/kg ip), an oxaliplatin-induced cold allodynia (at 10–30 μg sc), and CCI-induced thermal hyperalgesia (at 11.5 mg/kg ip) mice models. These results confirm the tryptophan moiety as a solid pharmacophore template for the design of highly potent modulators of TRPM8-mediated activities.

Mechanisms Mediating High-Molecular-Weight Hyaluronan-Induced Antihyperalgesia

We evaluated the mechanism by which high-molecular-weight hyaluronan (HMWH) attenuates nociceptor sensitization, in the setting of inflammation. HMWH attenuated mechanical hyperalgesia induced by the inflammatory mediator prostaglandin E2 (PGE₂) in male and female rats. Intrathecal administration of an oligodeoxynucleotide antisense (AS-ODN) to mRNA for cluster of differentiation 44 (CD44), the cognate hyaluronan receptor, and intradermal administration of A5G27, a CD44 receptor antagonist, both attenuated antihyperalgesia induced by HMWH. In male rats, HMWH also signals via Toll-like receptor 4 (TLR4), and AS-ODN for TLR4 mRNA administered intrathecally, attenuated HMWH-induced antihyperalgesia. Since HMWH signaling is dependent on CD44 clustering in lipid rafts, we pretreated animals with methyl-β-cyclodextrin (MβCD), which disrupts lipid rafts. MβCD markedly attenuated HMWH-induced antihyperalgesia. Inhibitors for components of intracellular signaling pathways activated by CD44, including phospholipase C and phosphoinositide 3-kinase (PI3K), also attenuated HMWH-induced antihyperalgesia. Furthermore, in vitro application of HMWH attenuated PGE₂-induced sensitization of tetrodotoxin-resistant sodium current, in small-diameter dorsal root ganglion neurons, an effect that was attenuated by a PI3K inhibitor. Our results indicate a central role of CD44 signaling in HMWH-induced antihyperalgesia and suggest novel therapeutic targets, downstream of CD44, for the treatment of pain generated by nociceptor sensitization.SIGNIFICANCE STATEMENT High-molecular-weight-hyaluronan (HMWH) is used to treat osteoarthritis and other pain syndromes. In this study we demonstrate that attenuation of inflammatory hyperalgesia by HMWH is mediated by its action at cluster of differentiation 44 (CD44) and activation of its downstream signaling pathways, including RhoGTPases (RhoA and Rac1), phospholipases (phospholipases Cε and Cγ1), and phosphoinositide 3-kinase, in nociceptors. These findings contribute to our understanding of the antihyperalgesic effect of HMWH and support the hypothesis that CD44 and its downstream signaling pathways represent novel therapeutic targets for the treatment of inflammatory pain.

TRPM8 channels: A review of distribution and clinical role

Ion channels are important therapeutic targets due to their plethoric involvement in physiological and pathological consequences. The transient receptor potential cation channel subfamily M member 8 (TRPM8) is a nonselective cation channel that controls Ca²⁺ homeostasis. It has been proposed to be the predominant thermoreceptor for cellular and behavioral responses to cold stimuli in the transient receptor potential (TRP) channel subfamilies and exploited so far to reach the clinical-stage of drug development. TRPM8 channels can be found in multiple organs and tissues, regulating several important processes such as cell proliferation, migration and apoptosis, inflammatory reactions, immunomodulatory effects, pain, and vascular muscle tension. The related disorders have been expanded to new fields ranging from cancer and migraine to dry eye disease, pruritus, irritable bowel syndrome (IBS), and chronic cough. This review is aimed to summarize the distribution of TRPM8 and disorders related to it from a clinical perspective, so as to broaden the scope of knowledge of researchers to conduct more studies on this subject.

Chronic morphine regulates TRPM8 channels via MOR-PKCβ signaling

Postoperative shivering and cold hypersensitivity are major side effects of acute and chronic opioid treatments respectively. TRPM8 is a cold and menthol-sensitive channel found in a subset of dorsal root ganglion (DRG) nociceptors. Deletion or inhibition of the TRPM8 channel was found to prevent the cold hyperalgesia induced by chronic administration of morphine. Here, we examined the mechanisms by which morphine was able to promote cold hypersensitivity in DRG neurons and transfected HEK cells. Mice daily injected with morphine for 5 days developed cold hyperalgesia. Treatment with morphine did not alter the expressions of cold sensitive TREK-1, TRAAK and TRPM8 in DRGs. However, TRPM8-expressing DRG neurons isolated from morphine-treated mice exhibited hyperexcitability. Sustained morphine treatment in vitro sensitized TRPM8 responsiveness to cold or menthol and reduced activation-evoked desensitization of the channel. Blocking phospholipase C (PLC) as well as protein kinase C beta (PKCβ), but not protein kinase A (PKA) or Rho-associated protein kinase (ROCK), restored channel desensitization. Identification of two PKC phosphorylation consensus sites, S1040 and S1041, in the TRPM8 and their site-directed mutation were able to prevent the MOR-induced reduction in TRPM8 desensitization. Our results show that activation of MOR by morphine 1) promotes hyperexcitability of TRPM8-expressing neurons and 2) induces a PKCβ-mediated reduction of TRPM8 desensitization. This MOR-PKCβ dependent modulation of TRPM8 may underlie the onset of cold hyperalgesia caused by repeated administration of morphine. Our findings point to TRPM8 channel and PKCβ as important targets for opioid-induced cold hypersensitivity.

Role of Nociceptor Toll-like Receptor 4 (TLR4) in Opioid-Induced Hyperalgesia and Hyperalgesic Priming

In addition to analgesia, opioids produce opioid-induced hyperalgesia (OIH) and neuroplasticity characterized by prolongation of inflammatory-mediator-induced hyperalgesia (hyperalgesic priming). We evaluated the hypothesis that hyperalgesia and priming induced by opioids are mediated by similar nociceptor mechanisms. In male rats, we first evaluated the role of nociceptor Toll-like receptor 4 (TLR4) in OIH and priming induced by systemic low-dose morphine (LDM, 0.03 mg/kg). Intrathecal oligodeoxynucleotide antisense to TLR4 mRNA (TLR4 AS-ODN) prevented OIH and prolongation of prostaglandin E₂ hyperalgesia (priming) induced by LDM. In contrast, high-dose morphine (HDM, 3 mg/kg) increased nociceptive threshold (analgesia) and induced priming, neither of which was attenuated by TLR4 AS-ODN. Protein kinase C ε (PKCε) AS-ODN also prevented LDM-induced hyperalgesia and priming, whereas analgesia and priming induced by HDM were unaffected. Treatment with isolectin B4 (IB4)-saporin or SSP-saporin (which deplete IB4⁺ and peptidergic nociceptors, respectively), or their combination, prevented systemic LDM-induced hyperalgesia, but not priming. HDM-induced priming, but not analgesia, was markedly attenuated in both saporin-treated groups. In conclusion, whereas OIH and priming induced by LDM share receptor and second messenger mechanisms in common, action at TLR4 and signaling via PKCε, HDM-induced analgesia, and priming are neither TLR4 nor PKCε dependent. OIH produced by LDM is mediated by both IB4⁺ and peptidergic nociceptors, whereas priming is not dependent on the same population. In contrast, priming induced by HDM is mediated by both IB4⁺ and peptidergic nociceptors. Implications for the use of low-dose opioids combined with nonopioid analgesics and in the treatment of opioid use disorder are discussed.SIGNIFICANCE STATEMENT Opioid-induced hyperalgesia (OIH) and priming are common side effects of opioid agonists such as morphine, which acts at μ-opioid receptors. We demonstrate that OIH and priming induced by systemic low-dose morphine (LDM) share action at Toll-like receptor 4 (TLR4) and signaling via protein kinase C ε (PKCε) in common, whereas systemic high-dose morphine (HDM)-induced analgesia and priming are neither TLR4 nor PKCε dependent. OIH produced by systemic LDM is mediated by isolectin B4-positive (IB4⁺) and peptidergic nociceptors, whereas priming is dependent on a different class of nociceptors. Priming induced by systemic HDM is, however, mediated by both IB4⁺ and peptidergic nociceptors. Our findings may provide useful information for the use of low-dose opioids combined with nonopioid analgesics to treat pain and opioid use disorders.

Role of GPCR (mu-opioid)-receptor tyrosine kinase (epidermal growth factor) crosstalk in opioid-induced hyperalgesic priming (type II)

Repeated stimulation of mu-opioid receptors (MORs), by an MOR-selective agonist DAMGO induces type II priming, a form of nociceptor neuroplasticity, which has 2 components: opioid-induced hyperalgesia (OIH) and prolongation of prostaglandin-E2 (PGE2)-induced hyperalgesia. We report that intrathecal antisense knockdown of the MOR in nociceptors, prevented the induction of both components of type II priming. Type II priming was also eliminated by SSP-saporin, which destroys the peptidergic class of nociceptors. Because the epidermal growth factor receptor (EGFR) participates in MOR signaling, we tested its role in type II priming. The EGFR inhibitor, tyrphostin AG 1478, prevented the induction of prolonged PGE2-induced hyperalgesia, but not OIH, when tested out to 30 days after DAMGO. However, even when repeatedly injected, an EGFR agonist did not induce hyperalgesia or priming. A phosphopeptide, which blocks the interaction of Src, focal adhesion kinase (FAK), and EGFR, also prevented DAMGO-induced prolongation of PGE2 hyperalgesia, but only partially attenuated the induction of OIH. Inhibitors of Src and mitogen-activated protein kinase (MAPK) also only attenuated OIH. Inhibitors of matrix metalloproteinase, which cleaves EGF from membrane protein, markedly attenuated the expression, but did not prevent the induction, of prolongation of PGE2 hyperalgesia. Thus, although the induction of prolongation of PGE2-induced hyperalgesia at the peripheral terminal of peptidergic nociceptor is dependent on Src, FAK, EGFR, and MAPK signaling, Src, FAK, and MAPK signaling is only partially involved in the induction of OIH.

Intrathecal TRPM8 blocking attenuates cold hyperalgesia via PKC and NF-κB signaling in the dorsal root ganglion of rats with neuropathic pain

Background: TRPM8 channel plays central roles in the sensitization of nociceptive transduction and is thought as one of the potential targets for the treatment of neuropathic pain. However, the specific molecular mechanisms are still less clear.

Methods: Sciatic chronic constriction injury (CCI) rats were intrathecally administered with AMTB (TRPM8-selective antagonist) or PDTC (nuclear factor-kappa B (NF-κB) inhibitor). Cold-, thermal- and mechanical-pain thresholds were examined in CCI and sham-operated rats before and after intrathecal administration of AMTB or PDTC. Protein expression levels of TRPM8 and NF-κB p65, p-PKC/PKC value and p-PKA/PKA value in the CCI ipsilateral L4-6 dorsal root ganglions (DRGs) were analyzed. In addition, the co-expression of TRPM8 and NF-κB was evaluated in DRG.

Results: Intrathecal injection of AMTB decreased the cold hypersensitivity and aggravated the thermal-hyperalgesia in the next 2 weeks after CCI surgery. The protein expression of TRPM8 and NF-κB p65 in the ipsilateral DRGs significantly increased after CCI surgery, which can be reversed by intrathecal administration of AMTB. The PKC, PKA, p-PKC/PKC and p-PKA/PKA values showed significantly increase after CCI surgery, while intrathecal AMTB administration offset the expression increase of PKC, p-PKC and p-PKC/PKC but PKA or p-PKA/PKA in the DRG. NF-κB inhibitor not only efficiently increased the cold-, thermal-pain threshold of CCI rats, but also enhanced AMTB’s anti-cold pain effect although exerted no anti-thermal hyperalgesia effect compared with TRPM8 blockade group. Immunofluorescence results showed co-expression of TRPM8 and NF-κB in DRG neurons.

Conclusion: TRPM8 channels in DRGs participate in the pathogenesis of cold and thermal hyperalgesia (not mechanical allodynia) in rats with neuropathic pain, which could be regulated by PKC (not PKA) and NF-κB signaling. TRPM8 channel, PKC and NF-κB are potential targets for cold hyperalgesia treatment in neuropathic pain patients.

Mu-opioid Receptor (MOR) Biased Agonists Induce Biphasic Dose-dependent Hyperalgesia and Analgesia, and Hyperalgesic Priming in the Rat

Stimulation of the mu-opioid receptor (MOR) on nociceptors with fentanyl can produce hyperalgesia (opioid-induced hyperalgesia, OIH) and hyperalgesic priming, a model of transition to chronic pain. We investigated if local and systemic administration of biased MOR agonists (PZM21 and TRV130 [oliceridine]), which preferentially activate G-protein over β-arrestin translocation, and have been reported to minimize some opioid side effects, also produces OIH and priming. Injected intradermally (100 ng), both biased agonists induced mechanical hyperalgesia and, when injected at the same site, 5 days later, prostaglandin E₂ (PGE₂) produced prolonged hyperalgesia (priming). OIH and priming were both prevented by intrathecal treatment with an oligodeoxynucleotide (ODN) antisense (AS) for MOR mRNA. Agents that reverse Type I (the protein translation inhibitor cordycepin) and Type II (combination of Src and mitogen-activated protein kinase [MAPK] inhibitors) priming, or their combination, did not reverse priming induced by local administration of PZM21 or TRV130. While systemic PZM21 at higher doses (1 and 10 mg/kg) induced analgesia, lower doses (0.001, 0.01, 0.1, and 0.3 mg/kg) induced hyperalgesia; all doses induced priming. Hyperalgesia, analgesia and priming induced by systemic administration of PZM21 were also prevented by MOR AS-ODN. And, priming induced by systemic PZM21 was also not reversed by intradermal cordycepin or the combination of Src and MAPK inhibitors. Thus, maintenance of priming induced by biased MOR agonists, in the peripheral terminal of nociceptors, has a novel mechanism.

Identification of key genes and pathways involved in response to pain in goat and sheep by transcriptome sequencing

Purpose

This aim of this study was to investigate the key genes and pathways involved in the response to pain in goat and sheep by transcriptome sequencing.

Methods

Chronic pain was induced with the injection of the complete Freund’s adjuvant (CFA) in sheep and goats. The animals were divided into four groups: CFA-treated sheep, control sheep, CFA-treated goat, and control goat groups (n = 3 in each group). The dorsal root ganglions of these animals were isolated and used for the construction of a cDNA library and transcriptome sequencing. Differentially expressed genes (DEGs) were identified in CFA-induced sheep and goats and gene ontology (GO) enrichment analysis was performed.

Results

In total, 1748 and 2441 DEGs were identified in CFA-treated goat and sheep, respectively. The DEGs identified in CFA-treated goats, such as C-C motif chemokine ligand 27 (CCL27), glutamate receptor 2 (GRIA2), and sodium voltage-gated channel alpha subunit 3 (SCN3A), were mainly enriched in GO functions associated with N-methyl-d-aspartate (NMDA) receptor, inflammatory response, and immune response. The DEGs identified in CFA-treated sheep, such as gamma-aminobutyric acid (GABA)-related DEGs (gamma-aminobutyric acid type A receptor gamma 3 subunit [GABRG3], GABRB2, and GABRB1), SCN9A, and transient receptor potential cation channel subfamily V member 1 (TRPV1), were mainly enriched in GO functions related to neuroactive ligand-receptor interaction, NMDA receptor, and defense response.

Conclusions

Our data indicate that NMDA receptor, inflammatory response, and immune response as well as key DEGs such as CCL27, GRIA2, and SCN3A may regulate the process of pain response during chronic pain in goats. Neuroactive ligand-receptor interaction and NMDA receptor as well as GABA-related DEGs, SCN9A, and TRPV1 may modulate the process of response to pain in sheep. These DEGs may serve as drug targets for preventing chronic pain.

Electronic supplementary material

The online version of this article (10.1186/s40659-018-0174-7) contains supplementary material, which is available to authorized users.

Targeting nociceptive transient receptor potential channels to treat chronic pain: current state of the field

Control of chronic pain is frequently inadequate and/or associated with intolerable adverse effects, prompting a frantic search for new therapeutics and new therapeutic targets. Nearly two decades of preclinical and clinical research supports the involvement of transient receptor potential (TRP) channels in temperature perception, nociception and sensitization. Although there has been considerable excitement around the therapeutic potential of this channel family since the cloning and identification of TRPV1 cation channels as the capsaicin receptor more than 20 years ago, only modulators of a few channels have been tested clinically. TRPV1 channel antagonists have suffered from side effects related to the channel's role in temperature sensation; however, high dose formulations of capsaicin have reached the market and shown therapeutic utility. A number of potent, small molecule antagonists of TRPA1 channels have recently advanced into clinical trials for the treatment of inflammatory and neuropathic pain, and TRPM8 antagonists are following closely behind for cold allodynia. TRPV3, TRPV4, TRPM2 and TRPM3 channels have also been of significant interest. This review discusses the preclinical promise and status of novel analgesic agents that target TRP channels and the challenges that these compounds may face in development and clinical practice.

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.

Antinociceptive effect of two novel transient receptor potential melastatin 8 antagonists in acute and chronic pain models in rat

BACKGROUND AND PURPOSE

Transient receptor potential (TRP) channels are a superfamily of non-selective cation permeable channels involved in peripheral sensory signalling. Animal studies have shown that several TRPs are important players in pain modulation. Among them, the TRP melastatin 8 (TRPM8) has elicited more interest for its controversial role in nociception. This channel, expressed by a subpopulation of sensory neurons in dorsal root ganglia (DRG) and trigeminal ganglia (TG), is activated by cold temperatures and cooling agents. In experimental neuropathic pain models, an up-regulation of this receptor in DRG and TG has been observed, suggesting a key role for TRPM8 in the development and maintenance of pain. Consistent with this hypothesis, TRPM8 knockout mice are less responsive to pain stimuli.

EXPERIMENTAL APPROACH

In this study, the therapeutic potential and efficacy of two novel TRPM8 antagonists, DFL23693 and DFL23448, were tested.

KEY RESULTS

Two potent and selective TRPM8 antagonists with distinct pharmacokinetic profiles, DFL23693 and DFL23448, have been fully characterized in vitro. In vivo studies in well-established models, namely, the wet-dog shaking test and changes in body temperature, confirmed their ability to block the TRPM8 channel. Finally, TRPM8 blockage resulted in a significant antinociceptive effect in formalin-induced orofacial pain and in chronic constriction injury-induced neuropathic pain, confirming an important role for this channel in pain perception.

CONCLUSION AND IMPLICATIONS

Our findings, in agreement with previous literature, encourage further studies for a better comprehension of the therapeutic potential of TRPM8 blockers as novel agents for pain management.

Hyperalgesic priming (type II) induced by repeated opioid exposure: maintenance mechanisms

We previously developed a model of opioid-induced neuroplasticity in the peripheral terminal of the nociceptor that could contribute to opioid-induced hyperalgesia, type II hyperalgesic priming. Repeated administration of mu-opioid receptor (MOR) agonists, such as DAMGO, at the peripheral terminal of the nociceptor, induces long-lasting plasticity expressed, prototypically as opioid-induced hyperalgesia and prolongation of prostaglandin E2-induced hyperalgesia. In this study, we evaluated the mechanisms involved in the maintenance of type II priming. Opioid receptor antagonist, naloxone, induced hyperalgesia in DAMGO-primed paws. When repeatedly injected, naloxone-induced hyperalgesia, and hyperalgesic priming, supporting the suggestion that maintenance of priming involves changes in MOR signaling. However, the knockdown of MOR with oligodeoxynucleotide antisense did not reverse priming. Mitogen-activated protein kinase and focal adhesion kinase, which are involved in the Src signaling pathway, previously implicated in type II priming, also inhibited the expression, but not maintenance of priming. However, when Src and mitogen-activated protein kinase inhibitors were coadministered, type II priming was reversed, in male rats. A second model of priming, latent sensitization, induced by complete Freund's adjuvant was also reversed, in males. In females, the inhibitor combination was only able to inhibit the expression and maintenance of DAMGO-induced priming when knockdown of G-protein-coupled estrogen receptor 30 (GPR30) in the nociceptor was performed. These findings demonstrate that the maintenance of DAMGO-induced type II priming, and latent sensitization is mediated by an interaction between, Src and MAP kinases, which in females is GPR30 dependent.

Fentanyl Induces Rapid Onset Hyperalgesic Priming: Type I at Peripheral and Type II at Central Nociceptor Terminals

Systemic fentanyl induces hyperalgesic priming, long-lasting neuroplasticity in nociceptor function characterized by prolongation of inflammatory mediator hyperalgesia. To evaluate priming at both nociceptor terminals, we studied, in male Sprague Dawley rats, the effect of local administration of agents that reverse type I (protein translation) or type II [combination of Src and mitogen-activated protein kinase (MAPK)] priming. At the central terminal, priming induced by systemic, intradermal, or intrathecal fentanyl was reversed by the combination of Src and MAPK inhibitors, but at the peripheral terminal, it was reversed by the protein translation inhibitor. Mu-opioid receptor (MOR) antisense prevented fentanyl hyperalgesia and priming. To determine whether type I and II priming occur in the same population of neurons, we used isolectin B4-saporin or [Sar⁹, Met(O₂)¹¹]-substance P-saporin to deplete nonpeptidergic or peptidergic nociceptors, respectively. Following intrathecal fentanyl, central terminal priming was prevented by both saporins, whereas that in peripheral terminal was not attenuated even by their combination. However, after intradermal fentanyl, priming in the peripheral terminal requires both peptidergic and nonpeptidergic nociceptors, whereas that in the central terminal is dependent only on peptidergic nociceptors. Pretreatment with dantrolene at either terminal prevented fentanyl-induced priming in both terminals, suggesting communication between central and peripheral terminals mediated by intracellular Ca²⁺ signaling. In vitro application of fentanyl increased cytoplasmic Ca²⁺ concentration in dorsal root ganglion neurons, which was prevented by pretreatment with dantrolene and naloxone. Therefore, acting at MOR in the nociceptor, fentanyl induces hyperalgesia and priming rapidly at both the central (type II) and peripheral (type I) terminal and this is mediated by Ca²⁺ signaling.SIGNIFICANCE STATEMENT Fentanyl, acting at the μ-opioid receptor (MOR), induces hyperalgesia and hyperalgesic priming at both the central and peripheral terminal of nociceptors and this is mediated by endoplasmic reticulum Ca²⁺ signaling. Priming in the central terminal is type II, whereas that in the peripheral terminal is type I. Our findings may provide useful information for the design of drugs with improved therapeutic profiles, selectively disrupting individual MOR signaling pathways, to maintain an adequate long-lasting control of pain.

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.

Neuropathic pain and dry eye

Dry eye is a common, multifactorial disease currently diagnosed by a combination of symptoms and signs. Its epidemiology and clinical presentation have many similarities with neuropathic pain outside the eye. This review highlights the similarities between dry eye and neuropathic pain, focusing on clinical features, somatosensory function, and underlying pathophysiology. Implications of these similarities on the diagnosis and treatment of dry eye are discussed.

CD44 Signaling Mediates High Molecular Weight Hyaluronan-Induced Antihyperalgesia

We studied, in male Sprague Dawley rats, the role of the cognate hyaluronan receptor, CD44 signaling in the antihyperalgesia induced by high molecular weight hyaluronan (HMWH). Low molecular weight hyaluronan (LMWH) acts at both peptidergic and nonpeptidergic nociceptors to induce mechanical hyperalgesia that is prevented by intrathecal oligodeoxynucleotide antisense to CD44 mRNA, which also prevents hyperalgesia induced by a CD44 receptor agonist, A6. Ongoing LMWH and A6 hyperalgesia are reversed by HMWH. HMWH also reverses the hyperalgesia induced by diverse pronociceptive mediators, prostaglandin E₂, epinephrine, TNFα, and interleukin-6, and the neuropathic pain induced by the cancer chemotherapy paclitaxel. Although CD44 antisense has no effect on the hyperalgesia induced by inflammatory mediators or paclitaxel, it eliminates the antihyperalgesic effect of HMWH. HMWH also reverses the hyperalgesia induced by activation of intracellular second messengers, PKA and PKC_ε, indicating that HMWH-induced antihyperalgesia, although dependent on CD44, is mediated by an intracellular signaling pathway rather than as a competitive receptor antagonist. Sensitization of cultured small-diameter DRG neurons by prostaglandin E₂ is also prevented and reversed by HMWH. These results demonstrate the central role of CD44 signaling in HMWH-induced antihyperalgesia, and establish it as a therapeutic target against inflammatory and neuropathic pain.SIGNIFICANCE STATEMENT We demonstrate that hyaluronan (HA) with different molecular weights produces opposing nociceptive effects. While low molecular weight HA increases sensitivity to mechanical stimulation, high molecular weight HA reduces sensitization, attenuating inflammatory and neuropathic hyperalgesia. Both pronociceptive and antinociceptive effects of HA are mediated by activation of signaling pathways downstream CD44, the cognate HA receptor, in nociceptors. These results contribute to our understanding of the role of the extracellular matrix in pain, and indicate CD44 as a potential therapeutic target to alleviate inflammatory and neuropathic pain.

Downregulations of TRPM8 expression and membrane trafficking in dorsal root ganglion mediate the attenuation of cold hyperalgesia in CCI rats induced by GFRα3 knockdown

BACKGROUND

Cold hyperalgesia is an intractable sensory abnormality commonly seen in peripheral neuropathies. Although glial cell line-derived neurotrophic factor family receptor alpha3 (GFRα3) is required for the formation of pathological cold pain has been revealed, potential transduction mechanism is poorly elucidated. We have previously demonstrated the contribution of enhanced activity of transient receptor potential melastatin 8 (TRPM8) to cold hyperalgesia in neuropathic pain using a rat model of chronic constriction injury (CCI) to the sciatic nerve. Recently, the enhancement of TRPM8 activity is attributed to the increased TRPM8 plasma membrane trafficking. In addition, TRPM8 can be sensitized by the activation of GFRα3, leading to increased cold responses in vivo. The aim of this study was to investigate whether GFRα3 could influence cold hyperalgesia of CCI rats via modulating TRPM8 expression and plasma membrane trafficking in dorsal root ganglion (DRG).

METHODS

Mechanical allodynia, cold and heat hyperalgesia were measured on 1day before CCI and the 1st, 4th, 7th, 10th and 14th day after CCI. TRPM8 total expression and membrane trafficking as well as GFRα3 expression in DRG were detected by immunofluorescence and western blot. Furthermore, GFRα3 small interfering RNA (siRNA) was intrathecally administrated to reduce GFRα3 expression in DRG, and the effects of GFRα3 knockdown on CCI-induced behavioral sensitization as well as TRPM8 total expression and membrane trafficking in both mRNA and protein levels were investigated, and the change in coexpression of TRPM8 with GFRα3 was also evaluated. Then, the effect of GFRα3 activation with artemin on pain behavior of CCI rats pretreated with the selective TRPM8 antagonist RQ-00203078 was observed.

RESULTS

Here we found that TRPM8 total expression and plasma membrane trafficking as well as GFRα3 expression in DRG were initially increased on the 4th day after CCI, and maintained at the peak level from the 10th to the 14th day, which entirely conformed with the induction and maintenance of behavioral-reflex facilitation following CCI. The coexpression of TRPM8 with GFRα3, which was mainly located in peptidergic C-fibers DRG neurons, was also increased after CCI. Downregulation of GFRα3 protein in DRG attenuated CCI-induced cold hyperalgesia without affecting mechanical allodynia and heat hyperalgesia, and reduced the upregulations of TRPM8 total expression and plasma membrane trafficking as well as coexpression of TRPM8 with GFRα3 induced by CCI. Additionally, the inhibition of TRPM8 abolished the influence of GFRα3 activation on cold hyperalgesia after CCI.

CONCLUSION

Our results demonstrate that GFRα3 knockdown specially inhibits cold hyperalgesia following CCI via decreasing the expression level and plasma membrane trafficking of TRPM8 in DRG. GFRα3 and its downstream mediator, TRPM8, represent a new analgesia axis which can be further exploited in sensitized cold reflex under the condition of chronic pain.

Nociceptor plasticity: A closer look

Nociceptors are receptors specifically involved in detecting a tissue damage and transducing it in an electrical signal. Nociceptor activation provoked by any kind of acute lesion is related to the release of several mediators of inflammation, within the framework of a process defined as "peripheral sensitization." This results in an exaggerated response to the painful stimulus, clinically defined as "primary hyperalgesia." The concept of "neuroplasticity" may explain the adaptive mechanisms carried out by the Nervous System in relation to a "harmful" damage; also, neuroplasticity mechanisms are also fundamental for rehabilitative intervention protocols. Here we review several studies that addressed the role of different receptors and ionic channels discovered on nociceptor surface and their role in pain perception. The changes in expression, distribution, and functioning of receptors and ionic channels are thought to be a part of the neuroplasticity property, through which the Nervous System constantly adapts to external stimuli. Moreover, some of the reviewed mediators are also been associated to "central sensitization," a process that results in pain chronicization when the painful stimulation is particularly prolonged or intense, and lastly leads to the memorization of the uncomfortable painful perception.

Development of TRPM8 Antagonists to Treat Chronic Pain and Migraine

A review. Development of pharmaceutical antagonists of transient receptor potential melastatin 8 (TRPM8) have been pursued for the treatment of chronic pain and migraine. This review focuses on the current state of this progress.

Role of the Excitability Brake Potassium Current IKD in Cold Allodynia Induced by Chronic Peripheral Nerve Injury

Cold allodynia is a common symptom of neuropathic and inflammatory pain following peripheral nerve injury. The mechanisms underlying this disabling sensory alteration are not entirely understood. In primary somatosensory neurons, cold sensitivity is mainly determined by a functional counterbalance between cold-activated TRPM8 channels and Shaker-like Kv1.1-1.2 channels underlying the excitability brake current I_KD Here we studied the role of I_KD in damage-triggered painful hypersensitivity to innocuous cold. We found that cold allodynia induced by chronic constriction injury (CCI) of the sciatic nerve in mice, was related to both an increase in the proportion of cold-sensitive neurons (CSNs) in DRGs contributing to the sciatic nerve, and a decrease in their cold temperature threshold. I_KD density was reduced in high-threshold CSNs from CCI mice compared with sham animals, with no differences in cold-induced TRPM8-dependent current density. The electrophysiological properties and neurochemical profile of CSNs revealed an increase of nociceptive-like phenotype among neurons from CCI animals compared with sham mice. These results were validated using a mathematical model of CSNs, including I_KD and TRPM8, showing that a reduction in I_KD current density shifts the thermal threshold to higher temperatures and that the reduction of this current induces cold sensitivity in former cold-insensitive neurons expressing low levels of TRPM8-like current. Together, our results suggest that cold allodynia is largely due to a functional downregulation of I_KD in both high-threshold CSNs and in a subpopulation of polymodal nociceptors expressing TRPM8, providing a general molecular and neural mechanism for this sensory alteration.SIGNIFICANCE STATEMENT This paper unveils the critical role of the brake potassium current I_KD in damage-triggered cold allodynia. Using a well-known form of nerve injury and combining behavioral analysis, calcium imaging, patch clamping, and pharmacological tools, validated by mathematical modeling, we determined that the functional expression of I_KD is reduced in sensory neurons in response to peripheral nerve damage. This downregulation not only enhances cold sensitivity of high-threshold cold thermoreceptors signaling cold discomfort, but it also transforms a subpopulation of polymodal nociceptors signaling pain into neurons activated by mild temperature drops. Our results suggest that cold allodynia is linked to a reduction of I_KD in both high-threshold cold thermoreceptors and nociceptors expressing TRPM8, providing a general model for this form of cold-induced pain.

Marked sexual dimorphism in 5-HT1 receptors mediating pronociceptive effects of sumatriptan

Amongst the side effects of triptans, a substantial percentage of patients experience injection site pain and tenderness, the underlying mechanism of which is unknown. We found that the dose range from 10fg to 1000ng (intradermal) of sumatriptan induced a complex dose-dependent mechanical hyperalgesia in male rats, with distinct peaks, at 1pg and 10ng, but no hyperalgesia at 1ng. In contrast, in females, there was 1 broad peak. The highest dose (1000ng) did not produce hyperalgesia in either sex. We evaluated the receptors mediating sumatriptan hyperalgesia (1pg, 1 and 10ng). In males, the injection of an antagonist for the serotonin (5-HT) receptor subtype 1B (5-HT_1B), but not 5-HT_1D, markedly inhibited sumatriptan (1pg)-induced hyperalgesia, at 10ng a 5-HT_1D receptor antagonist completely eliminated hyperalgesia. In contrast, in females, the 5-HT_1D, but not 5-HT_1B, receptor antagonist completely blocked sumatriptan (1pg and 10ng) hyperalgesia and both 5-HT_1B and 5-HT_1D receptor antagonists attenuated hyperalgesia (1ng) in females, which is GPR30 estrogen receptor dependent. While selective 5-HT_1D or 5-HT_1B, agonists produce robust hyperalgesia in female and male rats, respectively, when co-injected the hyperalgesia induced in both sexes was attenuated. Mechanical hyperalgesia induced by sumatriptan (1pg and 10ng) is dependent on the G-protein α_i subunit and protein kinase A (PKA), in IB4-positive and negative nociceptors. Understanding the mechanisms responsible for the complex dose dependence for triptan hyperalgesia may provide useful information for the design of anti-migraine drugs with improved therapeutic profiles.

Adenosine-A1 receptor agonist induced hyperalgesic priming type II

We have recently shown that repeated exposure of the peripheral terminal of the primary afferent nociceptor to the mu-opioid receptor (MOR) agonist DAMGO ([D-Ala, N-Me-Phe, Gly-ol]-enkephalin acetate salt) induces a model of transition to chronic pain that we have termed type II hyperalgesic priming. Similar to type I hyperalgesic priming, there is a markedly prolonged response to subsequent administration of proalgesic cytokines, prototypically prostaglandin E2 (PGE2). However, type II hyperalgesic priming differs from type I in being rapidly induced, protein kinase A (PKA), rather than PKCε dependent, not reversed by a protein translation inhibitor, occurring in female as well as in male rats, and isolectin B4-negative neuron dependent. We report that, as with the repeated injection of a MOR agonist, the repeated administration of an agonist at the A1-adenosine receptor, also a Gi-protein coupled receptor, N-cyclopentyladenosine (CPA), also produces priming similar to DAMGO-induced type II hyperalgesic priming. In this study, we demonstrate that priming induced by repeated exposure to this A1-adenosine receptor agonist shares the same mechanisms, as MOR-agonist induced priming. However, the prolongation of PGE2 hyperalgesia induced by repeated administration of CPA depends on G-protein αi subunit activation, differently from DAMGO-induced type II priming, in which it depends on the β/γ subunit. These data implicate a novel form of Gi-protein signaling pathway in the type II hyperalgesic priming induced by repeated administration of an agonist at A1-adenosine receptor to the peripheral terminal of the nociceptor.