Role of pattern recognition receptors in chemotherapy-induced neuropathic pain

Progress in the development of effective chemotherapy is producing a growing population of patients with acute and chronic painful chemotherapy-induced peripheral neuropathy (CIPN), a serious treatment-limiting side effect for which there is currently no US Food and Drug Administration-approved treatment. CIPNs induced by diverse classes of chemotherapy drugs have remarkably similar clinical presentations, leading to the suggestion they share underlying mechanisms. Sensory neurons share with immune cells the ability to detect damage associated molecular patterns (DAMPs), molecules produced by diverse cell types in response to cellular stress and injury, including by chemotherapy drugs. DAMPs, in turn, are ligands for pattern recognition receptors (PRRs), several of which are found on sensory neurons, as well as satellite cells, and cells of the immune system. In the present experiments, we evaluated the role of two PRRs, TLR4 and RAGE, present in dorsal root ganglion (DRG), in CIPN. Antisense (AS)-oligodeoxynucleotides (ODN) against TLR4 and RAGE mRNA were administered intrathecally before ('prevention protocol') or 3 days after ('reversal protocol') the last administration of each of three chemotherapy drugs that treat cancer by different mechanisms (oxaliplatin, paclitaxel and bortezomib). TLR4 and RAGE AS-ODN prevented the development of CIPN induced by all three chemotherapy drugs. In the reversal protocol, however, while TLR4 AS-ODN completely reversed oxaliplatin- and paclitaxel-induced CIPN, in rats with bortezomib-induced CIPN it only produced a temporary attenuation. RAGE AS-ODN, in contrast, reversed CIPN induced by all three chemotherapy drugs. When a TLR4 antagonist was administered intradermally to the peripheral nociceptor terminal, it did not affect CIPN induced by any of the chemotherapy drugs. However, when administered intrathecally, to the central terminal, it attenuated hyperalgesia induced by all three chemotherapy drugs, compatible with a role of TLR4 in neurotransmission at the central terminal but not sensory transduction at the peripheral terminal. Finally, since it has been established that cultured DRG neurons can be used to study direct effects of chemotherapy on nociceptors, we also evaluated the role of TLR4 in CIPN at the cellular level, using patch-clamp electrophysiology in DRG neurons cultured from control and chemotherapy-treated rats. We found that increased excitability of small-diameter DRG neurons induced by in vivo and in vitro exposure to oxaliplatin is TLR4-dependent. Our findings suggest that in addition to the established contribution of PRR-dependent neuroimmune mechanisms, PRRs in DRG cells also have an important role in CIPN.

The Primary Cilium and its Hedgehog Signaling in Nociceptors Contribute to Inflammatory and Neuropathic Pain

The primary cilium, a 1-3 μm long hair-like structure protruding from the surface of almost all cells in the vertebrate body, is critical for neuronal development and also functions in the adult. As the migratory neural crest settles into dorsal root ganglia (DRG) sensory neurons elaborate a single primary cilium at their soma that is maintained into adult stages. While it is not known if primary cilia are expressed in nociceptors, or their potential function in the mature DRG neuron, recent studies have shown a role for Hedgehog, whose signaling demonstrates a dependence on primary cilia, in nociceptor sensitization. Here we report the expression of primary cilia in rat and mouse nociceptors, where they modulate mechanical nociceptive threshold, and contribute to inflammatory and neuropathic pain. When siRNA targeting Ift88, a primary cilium-specific intraflagellar transport (IFT) protein required for ciliary integrity, was administered by intrathecal injection, in the rat, it resulted in loss of Ift88 mRNA in DRG, and primary cilia in neuronal cell bodies, which was associated with an increase in mechanical nociceptive threshold, and abrogation of hyperalgesia induced by the pronociceptive inflammatory mediator, prostaglandin E2, and painful peripheral neuropathy induced by a neurotoxic chemotherapy drug, paclitaxel. To provide further support for the role of the primary cilium in nociceptor function we also administered siRNA for another IFT protein, Ift52. Ift52 siRNA results in loss of Ift52 in DRG and abrogates paclitaxel-induced painful peripheral neuropathy. Attenuation of Hedgehog-induced hyperalgesia by Ift88 knockdown supports a role for the primary cilium in the hyperalgesia induced by Hedgehog, and attenuation of paclitaxel chemotherapy-induced neuropathy (CIPN) by cyclopamine, which attenuates Hedgehog signaling, suggests a role of Hedgehog in CIPN. Our findings support a role of nociceptor primary cilia in the control of mechanical nociceptive threshold and in inflammatory and neuropathic pain, the latter, at least in part, Hedgehog dependent.

Isolectin B4 (IB4)-conjugated streptavidin for the selective knockdown of proteins in IB4-positive (+) nociceptors

In vivo analysis of protein function in nociceptor subpopulations using antisense oligonucleotides and short interfering RNAs is limited by their non-selective cellular uptake. To address the need for selective transfection methods, we covalently linked isolectin B4 (IB4) to streptavidin and analyzed whether it could be used to study protein function in IB4(+)-nociceptors. Rats treated intrathecally with IB4-conjugated streptavidin complexed with biotinylated antisense oligonucleotides for protein kinase C epsilon (PKCε) mRNA were found to have: a) less PKCε in dorsal root ganglia (DRG), b) reduced PKCε expression in IB4(+) but not IB4(-) DRG neurons, and c) fewer transcripts of the PKCε gene in the DRG. This knockdown in PKCε expression in IB4(+) DRG neurons is sufficient to reverse hyperalgesic priming, a rodent model of chronic pain that is dependent on PKCε in IB4(+)-nociceptors. These results establish that IB4-streptavidin can be used to study protein function in a defined subpopulation of nociceptive C-fiber afferents.

Isolectin B4 (IB4)-conjugated streptavidin for the selective knockdown of proteins in IB4-positive (+) nociceptors

In vivo analysis of protein function in nociceptor subpopulations using antisense oligonucleotides and short interfering RNAs is limited by their non-selective cellular uptake. To address the need for selective transfection methods, we covalently linked isolectin B4 (IB4) to streptavidin and analyzed whether it could be used to study protein function in IB4(+)-nociceptors. Rats treated intrathecally with IB4-conjugated streptavidin complexed with biotinylated antisense oligonucleotides for protein kinase C epsilon (PKCε) mRNA were found to have: (a) less PKCε in dorsal root ganglia (DRG), (b) reduced PKCε expression in IB4(+) but not IB4(-) DRG neurons, and (c) fewer transcripts of the PKCε gene in the DRG. This knockdown in PKCε expression in IB4(+) DRG neurons is sufficient to reverse hyperalgesic priming, a rodent model of chronic pain that is dependent on PKCε in IB4(+)-nociceptors. These results establish that IB4-streptavidin can be used to study protein function in a defined subpopulation of nociceptive C-fiber afferents.

The Primary Cilium and its Hedgehog Signaling in Nociceptors Contribute to Inflammatory and Neuropathic Pain

The primary cilium, a 1-3 μm long hair-like structure protruding from the surface of almost all cells in the vertebrate body, is critical for neuronal development and also functions in the adult. As the migratory neural crest settles into dorsal root ganglia (DRG) sensory neurons elaborate a single primary cilium at their soma that is maintained into adult stages. While it is not known if primary cilia are expressed in nociceptors, or their potential function in the mature DRG neuron, recent studies have shown a role for Hedgehog, whose signaling demonstrates a dependence on primary cilia, in nociceptor sensitization. Here we report the expression of primary cilia in rat and mouse nociceptors, where they modulate mechanical nociceptive threshold, and contribute to inflammatory and neuropathic pain. When siRNA targeting Ift88 , a primary cilium-specific intra-flagellar transport (IFT) protein required for ciliary integrity, was administered by intrathecal injection, in the rat, it resulted in loss of Ift88 mRNA in DRG, and primary cilia in neuronal cell bodies, which was associated with an increase in mechanical nociceptive threshold, and abrogation of hyperalgesia induced by the pronociceptive inflammatory mediator, prostaglandin E 2 , and painful peripheral neuropathy induced by a neurotoxic chemotherapy drug, paclitaxel. To provide further support for the role of the primary cilium in nociceptor function we also administered siRNA for another IFT protein, Ift 52. Ift 52 siRNA results in loss of Ift 52 in DRG and abrogates paclitaxel-induced painful peripheral neuropathy. Attenuation of Hedgehog-induced hyperalgesia by Ift88 knockdown supports a role for the primary cilium in the hyperalgesia induced by Hedgehog, and attenuation of paclitaxel chemotherapy-induced neuropathy (CIPN) by cyclopamine, which attenuates Hedgehog signaling, suggests a role of Hedgehog in CIPN. Our findings support a role of nociceptor primary cilia in the control of mechanical nociceptive threshold and in inflammatory and neuropathic pain, the latter, at least in part, Hedgehog dependent.

Nociceptor Overexpression of NaV1.7 Contributes to Chronic Muscle Pain Induced by Early-Life Stress

Adult rats previously submitted to neonatal limited bedding (NLB), a model of early-life stress, display muscle mechanical hyperalgesia and nociceptor hyperexcitability, the underlying mechanism for which is unknown. Since voltage-gated sodium channel subtype 7 (NaV1.7) contributes to mechanical hyperalgesia in several preclinical pain models and is critical for nociceptor excitability, we explored its role in the muscle hyperalgesia exhibited by adult NLB rats. Western blot analyses demonstrated increased NaV1.7 protein expression in L4-L5 dorsal root ganglia (DRG) from adult NLB rats, and antisense oligodeoxynucleotide (AS ODN) targeting NaV1.7 alpha subunit mRNA attenuated the expression of NaV1.7 in DRG extracts. While this AS ODN did not affect nociceptive threshold in normal rats it significantly attenuated hyperalgesia in NLB rats. The selective NaV1.7 activator OD1 produced dose-dependent mechanical hyperalgesia that was enhanced in NLB rats, whereas the NaV1.7 blocker ProTx-II prevented OD1-induced hyperalgesia in control rats and ongoing hyperalgesia in NLB rats. AS ODN knockdown of extracellular signal-regulated kinase 1/2, which enhances NaV1.7 function, also inhibited mechanical hyperalgesia in NLB rats. Our results support the hypothesis that overexpression of NaV1.7 in muscle nociceptors play a role in chronic muscle pain induced by early-life stress, suggesting that NaV1.7 is a target for the treatment of chronic muscle pain. PERSPECTIVE: We demonstrate that early-life adversity, induced by exposure to inconsistent maternal care, produces chronic muscle hyperalgesia, which depends, at least in part, on increased expression of NaV1.7 in nociceptors.

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.

Nociceptor Interleukin 33 Receptor/ST2 Signaling in Vibration-Induced Muscle Pain in the Rat

Occupational exposure to mechanical vibration can produce the hand-arm vibration syndrome (HAVS), whose most disabling symptom is persistent muscle pain. Unfortunately, the pathophysiology of HAVS pain is still poorly understood, precluding the development of mechanism-based therapies. Since interleukin 33 (IL-33) is essential for inflammation and recovery that follows skeletal muscle injury, we explored its role in muscle pain in a model of HAVS, in adult male rats. Concomitant to mechanical hyperalgesia, an increase in IL-33 in the ipsilateral gastrocnemius muscle was observed 24 hours after vibration. A similar hyperalgesia was produced by intramuscular injection of recombinant rat IL-33 (rrIL-33, 10-300 ng). Intrathecal administration of an oligodeoxynucleotide antisense to IL-33R/ST2 mRNA decreased the expression of ST2 in DRG and attenuated both rrIL-33 and vibration-induced mechanical hyperalgesia. Together these data support the suggestion that IL-33 plays a central role in vibration-induced muscle pain by action, at least in part, on skeletal muscle nociceptors. PERSPECTIVE: Our findings provide evidence of the contribution of IL-33, acting on its canonical receptor, in nociceptors, to muscle pain induced by ergonomic vibration. This suggests that targeting IL-33/ST2 signaling may be a useful strategy for the treatment of muscle pain in HAVS.

Interleukin 6 decreases nociceptor expression of the potassium channel KV1.4 in a rat model of hand-arm vibration syndrome

Chronic muscle pain is a prominent symptom of the hand-arm vibration syndrome (HAVS), an occupational disease induced by exposure to vibrating power tools, but the underlying mechanism remains unknown. We evaluated the hypothesis that vibration induces an interleukin 6 (IL-6)-mediated downregulation of the potassium voltage-gated channel subfamily A member 4 (KV1.4) in nociceptors leading to muscle pain. Adult male rats were submitted to a protocol of mechanical vibration of the right hind limb. Twenty-four hours after vibration, muscle hyperalgesia was observed, concomitant to increased levels of IL-6 in the gastrocnemius muscle and decreased expression of KV1.4 in the dorsal root ganglia. Local injection of neutralizing antibodies against IL-6 attenuated the muscle hyperalgesia induced by vibration, whereas antisense knockdown of this channel in the dorsal root ganglia mimicked the muscle hyperalgesia observed in the model of HAVS. Finally, knockdown of the IL-6 receptor signaling subunit glycoprotein 130 (gp130) attenuated both vibration-induced muscle hyperalgesia and downregulation of KV1.4. These results support the hypothesis that IL-6 plays a central role in the induction of muscle pain in HAVS. This likely occurs through intracellular signaling downstream to the IL-6 receptor subunit gp130, which decreases the expression of KV1.4 in nociceptors.

Systemic Morphine Produces Dose-dependent Nociceptor-mediated Biphasic Changes in Nociceptive Threshold and Neuroplasticity

We investigated the dose dependence of the role of nociceptors in opioid-induced side-effects, hyperalgesia and pain chronification, in the rat. Systemic morphine produced a dose-dependent biphasic change in mechanical nociceptive threshold. At lower doses (0.003-0.03 mg/kg, s.c.) morphine induced mechanical hyperalgesia, while higher doses (1-10 mg/kg, s.c.) induced analgesia. Intrathecal (i.t.) oligodeoxynucleotide (ODN) antisense to mu-opioid receptor (MOR) mRNA, attenuated both hyperalgesia and analgesia. 5 days after systemic morphine (0.03-10 mg/kg s.c.), mechanical hyperalgesia produced by intradermal (i.d.) prostaglandin E₂ (PGE₂) was prolonged, indicating hyperalgesic priming at the peripheral terminal of the nociceptor. The hyperalgesia induced by i.t. PGE₂ (400 ng/10 µl), in groups that received 0.03 (that induced hyperalgesia) or 3 mg/kg (that induced analgesia) morphine, was also prolonged, indicating priming at the central terminal of the nociceptor. The prolongation of the hyperalgesia induced by i.d. or i.t. PGE₂, in rats previously treated with either a hyperalgesic (0.03 mg/kg, s.c.) or analgesic (3 mg/kg, s.c.) dose, was reversed by i.d. or i.t. injection of the protein translation inhibitor cordycepin (1 µg), indicative of Type I priming at both terminals. Although pretreatment with MOR antisense had no effect on priming induced by 0.03 mg/kg morphine, it completely prevented priming by 3 mg/kg morphine, in both terminals. Thus, the induction of hyperalgesia, but not priming, by low-dose morphine, is MOR-dependent. In contrast, induction of both hyperalgesia and priming by high-dose morphine is MOR-dependent. The receptor at which low-dose morphine acts to produce priming remains to be established.

Expression of a novel versican variant in dorsal root ganglia from spared nerve injury rats

The size and modular structure of versican and its gene suggest the existence of multiple splice variants. We have identified, cloned, and sequenced a previously unknown exon located within the noncoding gene sequence downstream of exon 8. This exon, which we have named exon 8β, specifies two stop-codons. mRNAs of the versican gene with exon 8β are predicted to be constitutively degraded by nonsense-mediated RNA decay. Here, we tested the hypothesis that these transcripts become expressed in a model of neuropathic pain.

Extracellular matrix hyaluronan signals via its CD44 receptor in the increased responsiveness to mechanical stimulation

We propose that the extracellular matrix (ECM) signals CD44, a hyaluronan receptor, to increase the responsiveness to mechanical stimulation in the rat hind paw. We report that intradermal injection of hyaluronidase induces mechanical hyperalgesia, that is inhibited by co-administration of a CD44 receptor antagonist, A5G27. The intradermal injection of low (LMWH) but not high (HMWH) molecular weight hyaluronan also induces mechanical hyperalgesia, an effect that was attenuated by pretreatment with HMWH or A5G27. Pretreatment with HMWH also attenuated the hyperalgesia induced by hyaluronidase. Similarly, intradermal injection of A6, a CD44 receptor agonist, produced hyperalgesia that was inhibited by HMWH and A5G27. Inhibitors of protein kinase A (PKA) and Src, but not protein kinase C (PKC), significantly attenuated the hyperalgesia induced by both A6 and LMWH. Finally, to determine if CD44 receptor signaling is involved in a preclinical model of inflammatory pain, we evaluated the effect of A5G27 and HMWH on the mechanical hyperalgesia associated with the inflammation induced by carrageenan. Both A5G27 and HMWH attenuated carrageenan-induced mechanical hyperalgesia. Thus, while LMWH acts at its cognate receptor, CD44, to induce mechanical hyperalgesia, HMWH acts at the same receptor as an antagonist. That the local administration of HMWH or A5G27 inhibits carrageenan-induced hyperalgesia supports the suggestion that carrageenan produces changes in the ECM that contributes to inflammatory pain. These studies define a clinically relevant role for signaling by the hyaluronan receptor, CD44, in increased responsiveness to mechanical stimulation.

Role of Kv4.3 in Vibration-Induced Muscle Pain in the Rat

We hypothesized that changes in the expression of voltage-gated potassium channel (Kv) 4.3 contribute to the mechanical hyperalgesia induced by vibration injury, in a rodent model for hand-arm vibration syndrome in humans. Here we show that the exposure of the gastrocnemius muscle to vibration injury induces muscle hyperalgesia that is accompanied by a significant downregulation of Kv4.3 in affected sensory nerve fibers in dorsal root ganglia. We additionally show that the intrathecal administration of antisense oligonucleotides for Kv4.3 messenger RNA itself induces muscle hyperalgesia in the rat. Our results suggest that attenuation in the expression of Kv4.3 may contribute to neuropathic pain in people affected by hand-arm vibration syndrome.

PERSPECTIVE

Our findings establish Kv4.3 as a potential molecular target for the treatment of hand-arm vibration syndrome.

Contribution of Piezo2 to endothelium-dependent pain

Background

We evaluated the role of a mechanically-gated ion channel, Piezo2, in mechanical stimulation-induced enhancement of hyperalgesia produced by the pronociceptive vasoactive mediator endothelin-1, an innocuous mechanical stimulus-induced enhancement of hyperalgesia that is vascular endothelial cell dependent. We also evaluated its role in a preclinical model of a vascular endothelial cell dependent painful peripheral neuropathy.

Results

The local administration of oligodeoxynucleotides antisense to Piezo2 mRNA, at the site of nociceptive testing in the rat’s hind paw, but not intrathecally at the central terminal of the nociceptor, prevented innocuous stimulus-induced enhancement of hyperalgesia produced by endothelin-1 (100 ng). The mechanical hyperalgesia induced by oxaliplatin (2 mg/kg. i.v.), which was inhibited by impairing endothelial cell function, was similarly attenuated by local injection of the Piezo2 antisense. Polymerase chain reaction analysis demonstrated for the first time the presence of Piezo2 mRNA in endothelial cells.

Conclusions

These results support the hypothesis that Piezo2 is a mechano-transducer in the endothelial cell where it contributes to stimulus-dependent hyperalgesia, and a model of chemotherapy-induced painful peripheral neuropathy.

Neuronally produced versican V2 renders C-fiber nociceptors IB4 -positive

A subpopulation of nociceptors, the glial cell line-derived neurotrophic factor (GDNF)-dependent, non-peptidergic C-fibers, expresses a cell-surface glycoconjugate that can be selectively labeled with isolectin B4 (IB4 ), a homotetrameric plant lectin from Griffonia simplicifolia. We show that versican is an IB4 -binding molecule in rat dorsal root ganglion neurons. Using reverse transcriptase polymerase chain reaction (RT-PCR), in situ hybridization and immunofluorescence experiments on rat lumbar dorsal root ganglion, we provide the first demonstration that versican is produced by neurons. In addition, by probing Western blots with splice variant-specific antibodies we show that the IB4 -binding versican contains only the glycosaminoglycan alpha domain. Our data support V2 as the versican isoform that renders this subpopulation of nociceptors IB4 -positive (+). A subset of nociceptors, the GDNF-dependent non-peptidergic C-fibers can be characterized by its reactivity for isolectin B4 (IB4), a plant lectin from Griffonia simplicifolia. We have previously demonstrated that versican V2 binds IB4 in a Ca2 + -dependent manner. However, given that versican is thought to be the product of glial cells, it was questionable whether versican V2 can be accountable for the IB4-reactivity of this subset of nociceptors. The results presented here prove - for the first time - a neuronal origin of versican and suggest that versican V2 is the molecule that renders GDNF-dependent non-peptidergic C-fibers IB4-positive.

Role of nociceptor estrogen receptor GPR30 in a rat model of endometriosis pain

Endometriosis, the most common cause of chronic pelvic pain, is an estrogen-dependent disease in which classic estrogen receptors (ERα, ERβ) play an important role. Although recent evidence suggests that the novel G protein-coupled estrogen receptor (GPR30) also plays a key role in the progression of endometriosis, whether it is also involved in endometriosis pain is still unknown. Here we tested the hypothesis that GPR30 expressed by nociceptors contributes to endometriosis pain. Intramuscular injection of the GPR30 agonists raloxifene or 17β-estradiol produced a fast-onset, persistent, mechanical hyperalgesia at the site of the injection. Intrathecal antisense (AS) oligodeoxynucleotides (ODN), but not mismatch (MM) ODN, targeting mRNA for GPR30 markedly inhibited its protein expression in nociceptors and attenuated the mechanical hyperalgesia induced by local raloxifene or 17β-estradiol. Pretreatment with the GPR30 antagonist G-36 also inhibited the hyperalgesia induced by raloxifene or 17β-estradiol in naive control rats. Surgical implant of autologous uterine tissue onto the gastrocnemius muscle, which induces endometriosis-like lesions, produced local mechanical hyperalgesia. Intrathecal AS, but not MM, ODN targeting GPR30 mRNA reversibly inhibited the mechanical hyperalgesia at the site of endometriotic lesions. Finally, intralesional injection of the GPR30 antagonist G-36 also inhibited the mechanical hyperalgesia at the site of ectopic uterine tissue. We conclude that local GPR30 agonists produce persistent mechanical hyperalgesia in naive female rats, whereas local GPR30 antagonists inhibit mechanical hyperalgesia in a model of endometriosis pain. Thus, GPR30 expressed by nociceptors innervating ectopic uterine lesions might play a major role in endometriosis pain.

NOP receptor mediates anti-analgesia induced by agonist-antagonist opioids

Clinical studies have shown that agonist-antagonist opioid analgesics that produce their analgesic effect via action on the kappa-opioid receptor, produce a delayed-onset anti-analgesia in men but not women, an effect blocked by co-administration of a low dose of naloxone. We now report the same time-dependent anti-analgesia and its underlying mechanism in an animal model. Using the Randall-Selitto paw-withdrawal assay in male rats, we found that nalbuphine, pentazocine, and butorphanol each produced analgesia during the first hour followed by anti-analgesia starting at ∼90min after administration in males but not females, closely mimicking its clinical effects. As observed in humans, co-administration of nalbuphine with naloxone in a dose ratio of 12.5:1 blocked anti-analgesia but not analgesia. Administration of the highly selective kappa-opioid receptor agonist U69593 produced analgesia without subsequent anti-analgesia, and confirmed by the failure of the selective kappa antagonist nor-binaltorphimine to block nalbuphine-induced anti-analgesia, indicating that anti-analgesia is not mediated by kappa-opioid receptors. We therefore tested the role of other receptors in nalbuphine anti-analgesia. Nociceptin/orphanin FQ (NOP) and sigma-1 and sigma-2 receptors were chosen on the basis of their known anti-analgesic effects and receptor binding studies. The selective NOP receptor antagonists, JTC801, and J-113397, but not the sigma receptor antagonist, BD 1047, antagonized nalbuphine anti-analgesia. Furthermore, the NOP receptor agonist NNC 63-0532 produced anti-analgesia with the same delay in onset observed with the three agonist-antagonists, but without producing preceding analgesia and this anti-analgesia was also blocked by naloxone. These results strongly support the suggestion that clinically used agonist-antagonists act at the NOP receptor to produce anti-analgesia.

Second messengers mediating the expression of neuroplasticity in a model of chronic pain in the rat

Hyperalgesic priming is a model of the transition from acute to chronic pain, in which previous activation of cell surface receptors or direct activation of protein kinase C epsilon markedly prolongs mechanical hyperalgesia induced by pronociceptive cytokines. We recently demonstrated a role of peripheral protein translation, alpha-calmodulin-dependent protein kinase II (αCaMKII) activation, and the ryanodine receptor in the induction of hyperalgesic priming. In the present study, we tested if they also mediate the prolonged phase of prostaglandin E2-induced hyperalgesia. We found that inhibition of αCaMKII and local protein translation eliminates the prolonged phase of prostaglandin E2 hyperalgesia. Although priming induced by receptor agonists or direct activation of protein kinase C epsilon occurs in male but not female rats, activation of αCaMKII and the ryanodine receptor also produces priming in females. As in males, the prolonged phase of prostaglandin E2-induced hyperalgesia in female rats is also protein kinase C epsilon-, αCaMKII-, and protein translation-dependent. In addition, in both male and female primed rats, the prolonged prostaglandin E2-induced hyperalgesia was significantly attenuated by inhibition of MEK/ERK. On the basis of these data, we suggest that the mechanisms previously shown to be involved in the induction of the neuroplastic state of hyperalgesic priming also mediate the prolongation of hyperalgesia.

PERSPECTIVES

The data provided by this study suggest that direct intervention on specific targets may help to alleviate the expression of chronic hyperalgesic conditions.

Ectopic endometrium-derived leptin produces estrogen-dependent chronic pain in a rat model of endometriosis

Endometriosis pain is a very common and extremely disabling condition whose mechanism is still poorly understood. While increased levels of leptin have been reported in patients with endometriosis, their contribution to endometriosis pain has not been explored. Using a rodent model of endometriosis we provide evidence for an estrogen-dependent contribution of leptin in endometriosis-induced pain. Rats implanted with autologous uterine tissue onto the gastrocnemius muscle developed endometriosis-like lesions and local chronic pain. Compared to eutopic uterine tissue, leptin mRNA and protein were up-regulated in the endometriosis-like lesions. Intramuscular injection of recombinant leptin in naive rats produced dose-dependent local mechanical hyperalgesia and nociceptor sensitization to mechanical stimulation. Ovariectomy attenuated the mechanical hyperalgesia induced by recombinant leptin, in rats treated with vehicle compared to those treated with 17β-estradiol replacement, at 1 and 24 h after leptin injection. Finally, intralesional injections of a pegylated leptin receptor (Ob-R) antagonist or of an inhibitor of Janus kinase2, which transduces the Ob-R signal, markedly attenuated pain in the endometriosis model. Taken together these data support the hypothesis that leptin, generated in ectopic endometrial lesions produces mechanical hyperalgesia by acting on nociceptors innervating the lesion. This sensitivity to leptin is dependent on estrogen levels. Thus, interventions targeting leptin signaling, especially in combination with interventions that lower estrogen levels, might be useful for the treatment of endometriosis pain.

Electrophysiological correlates of hyperalgesic priming in vitro and in vivo

We have modeled the transition from acute to chronic pain in the rat. In this model (termed hyperalgesic priming) a chronic state develops after a prior inflammatory process or exposure to an inflammatory mediator, in which response to subsequent exposure to prostaglandin E2 (PGE2) is characterized by a protein kinase Cε-dependent marked prolongation of mechanical hyperalgesia. To assess the effect of priming on the function of the nociceptor, we have performed in vitro patch clamp and in vivo single-fiber electrophysiology studies using tumor necrosis factor α to induce priming. In vitro, the only change observed in nociceptors cultured from primed animals was a marked hyperpolarization in resting membrane potential (RMP); prolonged sensitization, measured at 60 minutes, could not be tested in vitro. However, complimentary with behavioral findings, in vivo baseline mechanical nociceptive threshold was significantly elevated compared to controls. Thirty minutes after injection of PGE2 into the peripheral receptive field, both primed and control nociceptors showed enhanced response to mechanical stimulation. However, 60 minutes after PGE2 administration, the response to mechanical stimulation was further increased in primed but not in control nociceptors. Thus, at the level of the primary afferent nociceptor, it is possible to demonstrate both altered function at baseline and prolonged PGE2-induced sensitization. Intrathecal antisense (AS) to Kv7.2, which contributes to RMP in sensory neurons, reversibly prevented the expression of priming in both behavioral and single-fiber electrophysiology experiments, implicating these channels in the expression of hyperalgesic priming.

Peripheral administration of translation inhibitors reverses increased hyperalgesia in a model of chronic pain in the rat

Chronic pain is extremely difficult to manage, in part due to lack of progress in reversing the underlying pathophysiology. Since translation of messenger ribonucleic acids (mRNAs) in the peripheral terminal of the nociceptor plays a role in the transition from acute to chronic pain, we tested the hypothesis that transient inhibition of translation in the peripheral terminal of the nociceptor could reverse hyperalgesic priming, a model of transition from acute to chronic pain. We report that injection of translation inhibitors rapamycin and cordycepin, which inhibit translation by different mechanisms, at the peripheral terminal of the primed nociceptor produces reversal of priming in the rat that outlasted the duration of action of these drugs to prevent the development of priming. These data support the suggestion that interruption of translation in the nociceptor can reverse a preclinical model of at least 1 form of chronic pain.

PERSPECTIVE

This study provides evidence that ongoing protein translation in the sensory neuron terminals is involved in pain chronification, and local treatment that transiently interrupts this translation may be a useful therapy to chronic pain.

IB4(+) nociceptors mediate persistent muscle pain induced by GDNF

Skeletal muscle is a well-known source of glial cell line-derived neurotrophic factor (GDNF), which can produce mechanical hyperalgesia. Since some neuromuscular diseases are associated with both increased release of GDNF and intense muscle pain, we explored the role of GDNF as an endogenous mediator in muscle pain. Intramuscularly injected GDNF induced a dose-dependent (0.1-10 ng/20 μl) persistent (up to 3 wk) mechanical hyperalgesia in the rat. Once hyperalgesia subsided, injection of prostaglandin E(2) at the site induced a prolonged mechanical hyperalgesia (>72 h) compared with naïve rats (<4 h; hyperalgesic priming). Selective neurotoxic destruction of IB4(+) nociceptors attenuated both GDNF hyperalgesia and hyperalgesic priming. Ergonomic muscular injury induced by eccentric exercise or mechanical vibration increased muscle GDNF levels at 24 h, a time point where rats also exhibited marked muscle hyperalgesia. Intrathecal antisense oligodeoxynucleotides to mRNA encoding GFRα1, the canonical binding receptor for GDNF, reversibly inhibited eccentric exercise- and mechanical vibration-induced muscle hyperalgesia. Finally, electrophysiological recordings from nociceptors innervating the gastrocnemius muscle in anesthetized rats, revealed significant increase in response to sustained mechanical stimulation after local GDNF injection. In conclusion, these data indicate that GDNF plays a role as an endogenous mediator in acute and induction of chronic muscle pain, an effect likely to be produced by GDNF action at GFRα1 receptors located in IB4(+) nociceptors.

Transient decrease in nociceptor GRK2 expression produces long-term enhancement in inflammatory pain

In heterozygous mice, attenuation of G-protein-coupled receptor kinase 2 (GRK2) level in nociceptors is associated with enhanced and prolonged inflammatory hyperalgesia. To further elucidate the role of GRK2 in nociceptor function we reversibly decreased GRK2 expression using intrathecal antisense oligodeoxynucleotide (AS-ODN). GRK2 AS-ODN administration led to an enhanced and prolonged hyperalgesia induced by prostaglandin E(2), epinephrine and carrageenan. Moreover, this effect persisted unattenuated 2weeks after the last dose of antisense, well after GRK2 protein recovered, suggesting that transient attenuation of GRK2 produced neuroplastic changes in nociceptor function. Unlike hyperalgesic priming induced by transient activation of protein kinase C epsilon (PKCε), (Aley et al., 2000; Parada et al., 2003b), the enhanced and prolonged hyperalgesia following attenuation of GRK2 is PKCε- and cytoplasmic polyadenylation element binding protein (CPEB)-independent and is protein kinase A (PKA)- and Src tyrosine kinase (Src)-dependent. Finally, rats treated with GRK2 AS-ODN exhibited enhanced and prolonged hyperalgesia induced by direct activation of second messengers, adenyl cyclase, Epac or PKA, suggesting changes downstream of G-protein-coupled receptors. Because inflammation can produce a decrease in GRK2, such a mechanism could help explain a predilection to develop chronic pain, after resolution of acute inflammation.

Mitochondrial dependence of nerve growth factor-induced mechanical hyperalgesia

Mitochondria are present at high concentration at the site of sensory transduction in the peripheral terminals of nociceptors. Because nerve growth factor (NGF), which induces nociceptor sensitization by acting on the high-affinity tropomyosin receptor kinase A (TrkA) receptor, also produces local recruitment of mitochondria in DRG neurons, we evaluated the role of mitochondria in NGF-induced mechanical hyperalgesia. Inhibition of 3 major mitochondrial functions-oxidation of nutrients, adenosine triphosphate (ATP) production, and generation of reactive oxygen species--markedly attenuated NGF-induced mechanical hyperalgesia in the rat. Disruption of microtubules, which are required for the trafficking and subcellular localization of mitochondria, also attenuated NGF-induced hyperalgesia. Our results suggest a contribution of mitochondrial localization and function to NGF-dependent pain syndromes.

Nociceptor subpopulations involved in hyperalgesic priming

We have previously developed a model in the rat for the transition from acute to chronic pain, hyperalgesic priming, in which a long-lasting neuroplastic change in signaling pathways mediates a prolongation of proinflammatory cytokine-induced nociceptor sensitization and mechanical hyperalgesia, induced at the site of a previous inflammatory insult. Induction of priming is mediated by activation of protein kinase C(epsilon) (PKC(epsilon)) in the peripheral terminal of the primary afferent nociceptor. Given that hyperalgesic mediator-induced PKC(epsilon) translocation occurs in isolectin B4 (IB4)(+)-nonpeptidergic but not in receptor tyrosine kinase (TrkA)(+)-peptidergic nociceptors, we tested the hypothesis that hyperalgesic priming was restricted to the IB4(+) subpopulation of nociceptors. After recovery from nerve growth factor (NGF)- and GDNF-induced hyperalgesia, a proinflammatory cytokine, prostaglandin E(2) (PGE(2)) induced, PKC(epsilon)-dependent, markedly prolonged hyperalgesia, two features that define the development of the primed state. Thus, hyperalgesic priming occurs in both the IB4(+)-nonpeptidergic and TrkA(+)-peptidergic subpopulations of nociceptive afferents. Of note, however, while attenuation of PKC(epsilon) prevented NGF-induced priming, the hyperalgesia induced by NGF is PKC(epsilon) independent. We propose that separate intracellular pools of PKC(epsilon), in the peripheral terminals of nociceptors, mediate nociceptor sensitization and the induction of hyperalgesic priming.

Dependence of monocyte chemoattractant protein 1 induced hyperalgesia on the isolectin B4-binding protein versican

The type 1 chemokine monocyte chemoattractant protein (MCP-1) has been implicated in the generation of inflammatory and neuropathic pain, but the underlying mechanism remains poorly understood. Here we show that mechanical hyperalgesia induced by intradermal injection of MCP-1 in the rat is blocked by the intrathecal administration of isolectin B4 (IB4)-saporin, a selective neurotoxin for IB4(+)/Ret(+)-nociceptors. MCP-1-induced hyperalgesia is also attenuated by intrathecal antisense oligodeoxynucleotides targeting mRNA for versican, a molecule that binds MCP-1 and that also renders the Ret-expressing nociceptors IB4-positive (+). Finally, peripheral administration of ADAMTS-4 or chondroitinase ABC, two enzymes that disrupt versican integrity by the degradation of the versican core-protein or its chondroitin sulfate glycosaminoglycan side chains, respectively, also attenuated MCP-1 hyperalgesia at the site of nociceptive testing. We suggest that versican's glycosaminoglycan side chains present MCP-1 to a CCR2 expressing cell type in the skin that, in turn, selectively activates IB4(+)/Ret(+) nociceptors, thereby contributing to enhanced mechanical sensitivity under inflammatory conditions.

Neurotoxic catecholamine metabolite in nociceptors contributes to painful peripheral neuropathy

The neurotoxic effects of catecholamine metabolites have been implicated in neurodegenerative diseases. As some sensory neurons express tyrosine hydroxylase and monoamine oxidase (MAO), we investigated the potential contribution of catecholamine metabolites to neuropathic pain in a model of alcoholic neuropathy. The presence of catecholamines in sensory neurons is supported by capsaicin-stimulated epinephrine release, an effect enhanced in ethanol-fed rats. mRNA for enzymes in dorsal root ganglia involved in catecholamine uptake and metabolism, dopamine beta-hydroxylase and MAO-A, were decreased by neonatal administration of capsaicin. Ethanol-induced hyperalgesia was attenuated by systemic and local peripheral administration of inhibitors of MAO-A, reduction of norepinephrine transporter (NET) in sensory neurons and a NET inhibitor. Finally, intradermal injection of 3,4-dihydroxyphenylglycolaldehyde (DOPEGAL), a neurotoxic MAO-A catecholamine metabolite, produced robust mechanical hyperalgesia. These observations suggest that catecholamines in nociceptors are metabolized to neurotoxic products by MAO-A, which can cause neuronal dysfunction underlying neuropathic pain.

GDNF hyperalgesia is mediated by PLCgamma, MAPK/ERK, PI3K, CDK5 and Src family kinase signaling and dependent on the IB4-binding protein versican

The function of the isolectin B4 (IB4+)-binding and GDNF-dependent Ret (Ret+)-expressing non-peptidergic subpopulation of nociceptors remain poorly understood. We demonstrate that acute administration of GDNF sensitizes nociceptors and produces mechanical hyperalgesia in the rat. Intrathecal IB4-saporin, a selective toxin for IB4+/Ret+-nociceptors, attenuates GDNF but not NGF hyperalgesia. Conversely, intrathecal antisense to Trk A attenuated NGF but not GDNF hyperalgesia. Intrathecal administration of antisense oligodeoxynucleotides targeting mRNA for versican, the molecule that renders the Ret-expressing nociceptors IB4-positive (+), also attenuated GDNF but not NGF hyperalgesia, as did ADAMTS-4, a matrix metalloprotease known to degrade versican. Finally, inhibitors for all five signaling pathways known to be activated by GDNF at GFRa1/Ret: PLCc, CDK5, PI3K,MAPK/ERK and Src family kinases, attenuated GDNF hyperalgesia. Our results demonstrate a role of the non-peptidergic nociceptors in pain produced by the neurotrophin GDNF and suggest that the IB4-binding protein versican functions in the expression of this phenotype.

Proinflammatory cytokines mediating burn-injury pain

Thermal burns induce pain at the site of injury, mechanical hyperalgesia, associated with a complex time-dependent inflammatory response. To determine the contribution of inflammatory mediators to burn injury-induced mechanical hyperalgesia, we measured dynamic changes in the levels of three potent hyperalgesic cytokines, interleukin IL-1 beta, IL-6, and tumor necrosis factor-alpha (TNFalpha), in skin of the rat, following a partial-thickness burn injury. Only IL-6 demonstrated a sustained increase ipsilateral but not contralateral to the burn, correlating with the prolonged ipsilateral mechanical hyperalgesia. Spinal intrathecal injection of oligodeoxynucleotides antisense for gp130, a receptor subunit shared by members of the IL-6 family of cytokines, attenuated both burn- and intradermal IL-6-induced hyperalgesia, as did intradermal injection of anti-IL-6 function blocking antibodies. These studies suggest that IL-6 is an important mediator of burn-injury pain.

PLC-β3 signals upstream of PKCε in acute and chronic inflammatory hyperalgesia

While protein kinase C epsilon has been shown to contribute to acute and chronic mechanical hyperalgesia, its upstream signaling pathway has received little attention. Since phospholipase C can signal to PKC epsilon and has been implicated in nociceptor sensitization, we tested if it is upstream of PKC epsilon in mechanisms underlying primary mechanical hyperalgesia. In the rat, the PKC epsilon-dependent mechanical hyperalgesia and hyperalgesic priming (i.e., a form of chronic latent enhanced hyperalgesia) induced by carrageenan were attenuated by a non-selective PLC inhibitor U-73122. A lipid mediator of PLC signaling, l-alpha-lysophosphatidylcholine produced dose-dependent mechanical hyperalgesia and hyperalgesic priming, which was attenuated by EAVSLKPT, a selective PKC epsilon inhibitor. However, U-73122 did not attenuate hyperalgesia induced by psi epsilon RACK, a selective PKC epsilon activator. Antisense to PLC-beta 3 isoform, which was found in small-diameter dorsal root ganglion neurons, also attenuated carrageenan-induced acute and chronic-latent hyperalgesia. These studies support the suggestion that PLC-beta 3 is an important upstream signaling molecule for PKC epsilon-mediated acute and chronic inflammatory pain.

Identification of versican as an isolectin B4-binding glycoprotein from mammalian spinal cord tissue

Nociceptors are specialized nerve fibers that transmit noxious pain stimuli to the dorsal horn of the spinal cord. A subset of nociceptors, the nonpeptidergic C-fibers, is characterized by its reactivity for the plant isolectin B4 (IB4) from Griffonia simplicifolia. The molecular nature of the IB4-reactive glycoconjugate, although used as a neuroanatomical marker for more than a decade, has remained unknown. We here present data which strongly suggest that a splice variant of the extracellular matrix proteoglycan versican is the IB4-reactive glycoconjugate associated with these nociceptors. We isolated (by subcellular fractionation and IB4 affinity chromatography) a glycoconjugate from porcine spinal cord tissue that migrated in SDS/PAGE as a single distinct protein band at an apparent molecular mass of > 250 kDa. By using MALDI-TOF/TOF MS, we identified this glycoconjugate unambiguously as a V2-like variant of versican. Moreover, we demonstrate that the IB4-reactive glycoconjugate and the versican variant can be co-released from spinal cord membranes by hyaluronidase, and that the IB4-reactive glycoconjugate and the versican variant can be co-precipitated by an anti-versican immunoglobulin and perfectly co-migrate in SDS/PAGE. Our findings shed new light on the role of the extracellular matrix, which is thought to be involved in plastic changes underlying pain-related phenomena such as hyperalgesia and allodynia.

Identification and characterization of a Ca2+ -sensitive interaction of the vanilloid receptor TRPV1 with tubulin

The vanilloid receptor TRPV1 plays a well-established functional role in the detection of a range of chemical and thermal noxious stimuli, such as those associated with tissue inflammation and the resulting pain. TRPV1 activation results in membrane depolarization, but may also trigger intracellular Ca2+ -signalling events. In a proteomic screen for proteins associated with the C-terminal sequence of TRPV1, we identified beta-tubulin as a specific TRPV1-interacting protein. We demonstrate that the TRPV1 C-terminal tail is capable of binding tubulin dimers, as well as of binding polymerized microtubules. The interaction is Ca2+ -sensitive, and affects microtubule properties, such as microtubule sensitivity towards low temperatures and nocodazole. Our data thus provide compelling evidence for the interaction of TRPV1 with the cytoskeleton. The Ca2+ -sensitivity of this interaction suggests that the microtubule cytoskeleton at the cell membrane may be a downstream effector of TRPV1 activation.