Non-noxious A fiber afferent input enhances capsaicin-induced mechanical hyperalgesia in the rat

Postinjury stimulation triggers a transition to nociplastic pain in mice

ABSTRACT

Acute injury-induced pain can transition to chronic nociplastic pain, which predominantly affects women. To facilitate studies on the underlying mechanisms of nociplastic pain, we developed a mouse model in which postinjury thermal stimulation (intermittent 40°C water immersion for 10 minutes at 2 hours postcapsaicin) prolongs capsaicin (ie, experimental injury)-induced transient mechanical hypersensitivity outside of the injury area. Although capsaicin injection alone induced mechanical and thermal hypersensitivity that resolved in ∼7 days (slower recovery in females), the postinjury stimulation prolonged capsaicin-induced mechanical, but not thermal, hypersensitivity up to 3 weeks in both sexes. When postinjury stimulation was given at a lower intensity (30°C) or at later time points (40°C at 1-3 days postcapsaicin), chronification of mechanical hypersensitivity occurred only in females. Similar chronification could be induced by a different postinjury stimulation modality (vibration of paw) or with a different injury model (plantar incision). Notably, the 40°C postinjury stimulation did not prolong capsaicin-induced inflammation in the hind paw, indicating that the prolonged mechanical hypersensitivity in these mice arises without clear evidence of ongoing injury, reflecting nociplastic pain. Although morphine and gabapentin effectively alleviated this persistent mechanical hypersensitivity in both sexes, sexually dimorphic mechanisms mediated the hypersensitivity. Specifically, ongoing afferent activity at the previously capsaicin-injected area was critical in females, whereas activated spinal microglia were crucial in males. These results demonstrate that postinjury stimulation of the injured area can trigger the transition from transient pain to nociplastic pain more readily in females, and sex-dependent mechanisms maintain the nociplastic pain state.

In vitro and in vivo pharmacological characterization of ASP8477: A novel highly selective fatty acid amide hydrolase inhibitor

Although exogenous agonists for cannabinoid (CB) receptors are clinically effective for treating chronic pain, global activation of brain CB receptors causes frequent central nervous system (CNS) side-effects. Fatty acid amide hydrolase (FAAH) is a primary catabolic enzyme for anandamide (AEA), an endogenous CB. Recently, we discovered a novel FAAH inhibitor, 3-pyridyl 4-(phenylcarbamoyl)piperidine-1-carboxylate (ASP8477). In vitro studies demonstrated that ASP8477 inhibited human FAAH-1, FAAH-1 (P129T) and FAAH-2 activity with IC₅₀ values of 3.99, 1.65 and 57.3nM, respectively. ASP8477 at 10µM had no appreciable interactions with 65 different kinds of receptors, ion channels, transporters and enzymes, including CB₁ and CB₂ receptors and monoacylglycerol lipase. In adolescent rats, orally administered ASP8477 (0.3-10mg/kg) elevated AEA concentrations in both plasma and brain. In a capsaicin-induced secondary hyperalgesia model, a pretreatment with ASP8477 significantly improved mechanical allodynia and thermal hyperalgesia at 0.3-3mg/kg p.o. ASP8477 also significantly improved mechanical allodynia in an L5/L6 spinal nerve ligation neuropathic pain model, with an ED₅₀ value of 0.63mg/kg, and in a streptozotocin-induced diabetic neuropathy model at 3 and 10mg/kg p.o. Furthermore, ASP8477 significantly attenuated the reduction in rearing events at 1 and 3mg/kg p.o. in a monoiodoacetic acid-induced osteoarthritis model. Importantly, ASP8477 had no significant effect on motor coordination up to 30mg/kg p.o. These results indicate that ASP8477 is a potent, selective, and oral active FAAH inhibitor with activity in the CNS, with the potential to be a new analgesic agent with a wide safety margin.

The control of alternative splicing by SRSF1 in myelinated afferents contributes to the development of neuropathic pain

Neuropathic pain results from neuroplasticity in nociceptive neuronal networks. Here we demonstrate that control of alternative pre-mRNA splicing, through the splice factor serine-arginine splice factor 1 (SRSF1), is integral to the processing of nociceptive information in the spinal cord. Neuropathic pain develops following a partial saphenous nerve ligation injury, at which time SRSF1 is activated in damaged myelinated primary afferent neurons, with minimal found in small diameter (IB₄ positive) dorsal root ganglia neurons. Serine arginine protein kinase 1 (SRPK1) is the principal route of SRSF1 activation. Spinal SRPK1 inhibition attenuated SRSF1 activity, abolished neuropathic pain behaviors and suppressed central sensitization. SRSF1 was principally expressed in large diameter myelinated (NF200-rich) dorsal root ganglia sensory neurons and their excitatory central terminals (vGLUT1+ve) within the dorsal horn of the lumbar spinal cord. Expression of pro-nociceptive VEGF-A_xxxa within the spinal cord was increased after nerve injury, and this was prevented by SRPK1 inhibition. Additionally, expression of anti-nociceptive VEGF-A_xxxb isoforms was elevated, and this was associated with reduced neuropathic pain behaviors. Inhibition of VEGF receptor-2 signaling in the spinal cord attenuated behavioral nociceptive responses to mechanical, heat and formalin stimuli, indicating that spinal VEGF receptor-2 activation has potent pro-nociceptive actions. Furthermore, intrathecal VEGF-A₁₆₅a resulted in mechanical and heat hyperalgesia, whereas the sister inhibitory isoform VEGF-A₁₆₅b resulted in anti-nociception. These results support a role for myelinated fiber pathways, and alternative pre-mRNA splicing of factors such as VEGF-A in the spinal processing of neuropathic pain. They also indicate that targeting pre-mRNA splicing at the spinal level could lead to a novel target for analgesic development.

Knockout of Slo2.2 enhances itch, abolishes KNa current, and increases action potential firing frequency in DRG neurons

Two mammalian genes, Kcnt1 and Kcnt2, encode pore-forming subunits of Na(+)-dependent K(+) (KNa) channels. Progress in understanding KNa channels has been hampered by the absence of specific tools and methods for rigorous KNa identification in native cells. Here, we report the genetic disruption of both Kcnt1 and Kcnt2, confirm the loss of Slo2.2 and Slo2.1 protein, respectively, in KO animals, and define tissues enriched in Slo2 expression. Noting the prevalence of Slo2.2 in dorsal root ganglion, we find that KO of Slo2.2, but not Slo2.1, results in enhanced itch and pain responses. In dissociated small diameter DRG neurons, KO of Slo2.2, but not Slo2.1, abolishes KNa current. Utilizing isolectin B4+ neurons, the absence of KNa current results in an increase in action potential (AP) firing and a decrease in AP threshold. Activation of KNa acts as a brake to initiation of the first depolarization-elicited AP with no discernible effect on afterhyperpolarizations.

Key role of the dorsal root ganglion in neuropathic tactile hypersensibility

Cutting spinal nerves just distal to the dorsal root ganglion (DRG) triggers, with rapid onset, massive spontaneous ectopic discharge in axotomized afferent A-neurons, and at the same time induces tactile allodynia in the partially denervated hindlimb. We show that secondary transection of the dorsal root (rhizotomy) of the axotomized DRG, or suppression of the ectopia with topically applied local anesthetics, eliminates or attenuates the allodynia. Dorsal rhizotomy alone does not trigger allodynia. These observations support the hypothesis that ectopic firing in DRG A-neurons induces central sensitization which leads to tactile allodynia. The question of how activity in afferent A-neurons, which are not normally nociceptive, might induce allodynia is discussed in light of the current literature.

Amelioration of neuropathic pain by novel transient receptor potential vanilloid 1 antagonist AS1928370 in rats without hyperthermic effect

Transient receptor potential vanilloid 1 (TRPV1) is activated by a variety of stimulations, such as endogenous ligands and low pH, and is believed to play a role in pain transmission. TRPV1 antagonists have been reported to be effective in several animal pain models; however, some compounds induce hyperthermia in animals and humans. We discovered the novel TRPV1 antagonist (R)-N-(1-methyl-2-oxo-1,2,3,4-tetrahydro-7-quinolyl)-2-[(2-methylpyrrolidin-1-yl)methyl]biphenyl-4-carboxamide (AS1928370) in our laboratory. AS1928370 bound to the resiniferatoxin-binding site on TRPV1 and inhibited capsaicin-mediated inward currents with an IC₅₀ value of 32.5 nM. Although AS1928370 inhibited the capsaicin-induced Ca²(+) flux in human and rat TRPV1-expressing cells, the inhibitory effect on proton-induced Ca²(+) flux was extremely small. In addition, AS1928370 showed no inhibitory effects on transient receptor potential vanilloid 4, transient receptor potential ankyrin 1, and transient receptor potential melastatin 8 in concentrations up to 10 μM. AS1928370 improved capsaicin-induced secondary hyperalgesia and mechanical allodynia in an L5/L6 spinal nerve ligation model in rats with respective ED₅₀ values of 0.17 and 0.26 mg/kg p.o. Furthermore, AS1928370 alleviated inflammatory pain in a complete Freund's adjuvant model at 10 mg/kg p.o. AS1928370 had no effect on rectal body temperature up to 10 mg/kg p.o., although a significant hypothermic effect was noted at 30 mg/kg p.o. In addition, AS1928370 showed no significant effect on motor coordination. These results suggest that blockage of the TRPV1 receptor without affecting the proton-mediated TRPV1 activation is a promising approach to treating neuropathic pain because of the potential wide safety margin against hyperthermic effects. As such, compounds such as ASP1928370 may have potential as new analgesic agents for treating neuropathic pain.

TRP channels and ASICs mediate mechanical hyperalgesia in models of inflammatory muscle pain and delayed onset muscle soreness

The roles of ion channels in sensory neurons were examined in experimental models of muscle pain in the rat. Rats were injected with 50 microl of 4% carrageenan or subjected to an eccentric exercise (ECC) of the gastrocnemius muscle (GM). The Randall-Selitto and von Frey tests were performed on the calves to evaluate mechanical hyperalgesia of the muscle. The changes in expression of four genes and proteins of ion channels in dorsal root ganglia were examined using quantitative PCR and immunohistochemistry, respectively. Effects of antagonists to transient receptor potential (TRP) channels and acid sensing ion channels (ASICs) on the mechanical hyperalgesia induced by carrageenan injection or ECC were evaluated. The mechanical hyperalgesia was observed 6-24h after carrageenan injection and 1-3 days after ECC in the Randall-Selitto test. Infiltrations of the inflammatory cells in the GM were seen in carrageenan-injected animals but not in those subjected to ECC. Expressions of genes and proteins in sensory neurons showed no changes. Intramuscular injection of antagonists to TRPV1 showed an almost complete suppressive effect on ECC-induced muscle hyperalgesia but not a carrageenan-induced one. Antagonists to TRP channels and ASICs showed suppressive effects for both carrageenan- and ECC-induced muscle hyperalgesia. The carrageenan injection and ECC models are useful models of acute inflammatory pain and delayed onset muscle soreness (DOMS), respectively, and the time course and underlying etiology might be different. TRP channels and ASICs are closely related to the development of muscle mechanical hyperalgesia, and TRPV1 is involved in ECC-induced DOMS.

Importance of hyperexcitability of DRG neurons in neuropathic pain

A number of good animal models have been developed in recent years that provide insights into the mechanisms of neuropathic pain. It now becomes evident that there are two separate peripheral components influencing neuropathic pain: one dependent on the hyperexcitability of axotomized dorsal root ganglion (DRG) neurons and the other independent of this hyperexcitability. The purpose of this review is to consider one of these components, the hyperexcitability of axotomized DRG neurons, as one of the important mechanisms underlying neuropathic pain. Several hours after nerve lesions, some axotomized DRG neurons become hyperexcitable and begin to show ongoing discharges that last many days or weeks. These ectopic discharges then enter the spinal cord and induce central sensitization, the underlying central mechanism for the generation of pain and allodynia. Although the exact causes of the development of hyperexcitability and ectopic discharges are not clear, various ion channels seem to play important roles, particularly sodium channels. In addition, important modulatory factors for ectopic discharges are purinergic and adrenergic components of the sympathetic nervous system. These findings suggest that manipulating sodium channels and/or adrenergic and purinergic receptors on axotomized DRG cells may give neuropathic pain sufferers some relief that is not available from present treatment regimens.

Prolonged maintenance of capsaicin-induced hyperalgesia by brief daily vibration stimuli

This study tests the hypothesis that central sensitization initiated by nociceptive input can be maintained by repeated brief innocuous peripheral inputs. Capsaicin was injected intradermally into the hind paw of adult rats. Three different types of daily cutaneous mechanical stimulations (vibration, soft brush, or pressure) were applied to the capsaicin-injected paw for a period of 2 weeks. Daily stimulation consisted of a 10-s stimulation repeated every 30s for 30 min. Foot withdrawal thresholds to von Frey stimuli applied to the paw were measured once a day for 4 weeks. The capsaicin-only group (control rats without daily stimulation) showed hyperalgesia lasting for 3 days. In contrast, hyperalgesia persisted for 2 weeks in the group that received vibration stimulation. Neither the soft brush nor the pressure group showed a significant difference in mechanical threshold from the control group (capsaicin only). The vibration-induced prolonged hyperalgesia was significantly reduced by systemic injection of ifenprodil, an NMDA-receptor antagonist, but it was not influenced by either an AMPA-receptor blocker or a reactive oxygen species (ROS) scavenger. Furthermore, a dorsal column lesion did not interfere with the prolongation of hyperalgesia. Data suggest that vibration-induced prolongation of hyperalgesia is mediated by spinal NMDA-receptors, and a similar mechanism may underlie some forms of chronic pain with no obvious causes, such as complex regional pain syndrome type 1 (CRPS-1).

Comparison of antinociceptive actions of standard analgesics in attenuating capsaicin and nerve-injury-induced mechanical hypersensitivity

Intradermal capsaicin injection produces immediate spontaneous pain behaviors, and a secondary mechanical hypersensitivity (SMH) that is employed in the clinic as a model potentially predictive of human neuropathic pain. Presently, we have characterized capsaicin-induced SMH in rats, and compared pharmacological actions of standard analgesics in this and two nerve injury models, the L5/L6 spinal nerve ligation (SNL) and sciatic nerve chronic constriction injury (CCI) models. Intraplantar capsaicin produced dose-related SMH (enhanced paw withdrawal response to von Frey monofilament stimulation at an area away from injection site) that lasted for over 4 h. While pretreatment with a potent selective transient receptor potential vanilloid receptor-1 (TRPV1) antagonist A-425619 (1-isoquinolin-5-yl-3-(4-trifluoromethyl-benzyl)-urea) prevented development of acute nocifensive (flinching) behavior immediately following capsaicin injection (ED(50)=4.9 mg/kg), the compound failed to attenuate the SMH when administered 2 h following capsaicin (10 microg/10 microl). Additional standard analgesics were also tested 3 h following intraplantar capsaicin in the SMH model. Comparison of their potencies in attenuating mechanical hypersensitivity in capsaicin, SNL and CCI models revealed similar ED(50)s for morphine (2.3 mg/kg, 1.6 mg/kg and 3.2 mg/kg, respectively), gabapentin (33.1 mg/kg, 33.9 mg/kg and 26.3 mg/kg, respectively) and lamotrigine (9.1 mg/kg, 8.9 mg/kg and 15.5 mg/kg, respectively). Duloxetine produced 50-65% effect at the highest tested dose (50 mg/kg), whereas the highest tested doses of morphine (10 mg/kg), gabapentin (85.5 mg/kg) and lamotrigine (30 mg/kg) all produced >70% efficacy in capsaicin SMH, SNL and CCI models. In contrast, celecoxib and ibuprofen showed weak effects in all three models. All standard analgesics generally had weak efficacy in attenuating capsaicin-induced immediate acute flinching behavior when administered before capsaicin. These results provide further support to the suggestions that distinct pharmacological mechanisms underlie capsaicin-induced acute nocifensive and SMH behaviors, and certain neuronal mechanisms underlying neuropathic pain states are also contributory to capsaicin-induced SMH.

Areas of capsaicin-induced secondary hyperalgesia and allodynia are reduced by a single chiropractic adjustment: a preliminary study

INTRODUCTION

The aim of the study was to investigate the hypoalgesic effects of a single spinal manipulation treatment on acute inflammatory reactions and pain induced by cutaneous application of capsaicin.

METHODS

Twenty healthy subjects participated in the experiment, which consisted of 2 sessions. In both sessions, following control measurements, topical capsaicin was applied to the right or left forearm to induce cutaneous inflammatory reactions. The cream was removed after 20 minutes. Then subjects received either spinal manipulation treatment (SMT) or "nonspinal manipulation treatment" (N-SMT), respectively. In control as well as pretreatment and posttreatment intervals, the following tests were performed: measurement of the areas of mechanical hyperalgesia and stroking allodynia, assessment of spontaneous pain, and measurement of blood flow.

RESULTS

The results confirmed that topical capsaicin induced inflammatory reactions based on occurrence of hyperalgesia and allodynia, augmented pain perception, and increased blood flow following capsaicin application compared with the control session. When compared with N-SMT, spontaneous pain was rated significantly lower post-SMT (P <.014). In addition, areas of both secondary hyperalgesia and allodynia decreased after SMT (hyperalgesia: P <.007; allodynia: P <.003). However, there was no significant treatment effect for local blood flow.

CONCLUSION

These results suggest hypoalgesic effects following a single SMT. As local vascular parameter was not affected by the single SMT, the hypoalgesic effects appear to be due to central mechanisms.

Cannabinoid receptor-independent actions of the aminoalkylindole WIN 55,212-2 on trigeminal sensory neurons

The prototypical aminoalkylindole cannabinoid WIN 55,212-2 (WIN-2) has been shown to produce antihyperalgesia through a peripheral mechanism of action. However, it is not known whether WIN-2 exerts this action directly via cannabinoid receptors located on primary afferents or if other, perhaps indirect or noncannabinoid, mechanisms are involved. To address this question, we have examined the specific actions of WIN-2 on trigeminal ganglion (TG) neurons in vitro by quantifying its ability to modulate the evoked secretion of the proinflammatory neuropeptide CGRP as well as the inflammatory mediator-induced generation of cAMP. WIN-2 evoked CGRP release from TG neurons in vitro (EC(50)=26 microm) in a concentration- and calcium-dependent manner, which was mimicked by the cannabinoid receptor-inactive enantiomer WIN 55,212-3 (WIN-3). Moreover, WIN-2-evoked CGRP release was attenuated by the nonselective cation channel blocker ruthenium red but not by the vanilloid receptor type 1 (TRPV1) antagonist capsazepine, suggesting that, unlike certain endogenous and synthetic cannabinoids, WIN-2 is not a TRPV1 agonist but rather acts at an as yet unidentified cation channel. The inhibitory effects of WIN-2 on TG neurons were also examined. WIN-2 neither inhibited capsaicin-evoked CGRP release nor did it inhibit forskolin-, isoproteranol- or prostaglandin E(2)-stimulated cAMP accumulation. On the other hand, WIN-2 significantly inhibited (EC(50)=1.7 microm) 50 mm K(+)-evoked CGRP release by approximately 70%. WIN-2 inhibition of 50 mm K(+)-evoked CGRP release was not reversed by antagonists of cannabinoid type 1 (CB1) receptor, but was mimicked in magnitude and potency (EC(50)=2.7 microm) by its cannabinoid-inactive enantiomer WIN-3. These findings indicate that WIN-2 exerts both excitatory and inhibitory effects on TG neurons, neither of which appear to be mediated by CB1, CB2 or TRPV1 receptors, but by a novel calcium-dependent mechanism. The ramifications of these results are discussed in relation to our current understanding of cannabinoid/vanilloid interactions with primary sensory neurons.

Modulation of trigeminal sensory neuron activity by the dual cannabinoid-vanilloid agonists anandamide, N-arachidonoyl-dopamine and arachidonyl-2-chloroethylamide

1. Peripheral cannabinoids have been shown to suppress nociceptive neurotransmission in a number of behavioral and neurophysiological studies. It is not known, however, whether cannabinoids exert this action through direct interactions with nociceptors in the periphery and/or if other processes are involved. To gain a better understanding of the direct actions of cannabinoid-vanilloid agonists on sensory neurons, we examined the effects of these compounds on trigeminal ganglion (TG) neurons in vitro. 2. AEA (EC(50)=11.0 microM), NADA (EC(50)=857 nM) and arachidonyl-2-chloroethylamide ACEA (EC(50)=14.0 microM) each evoked calcitonin gene-related peptide (CGRP) release from TG neurons. The TRPV1 antagonists iodo-resiniferatoxin (I-RTX) and capsazepine (CPZ) each obtunded AEA-, NADA-, ACEA- and capsaicin (CAP)-evoked CGRP release with individually equivalent IC(50)'s for each of the compounds (I-RTX IC(50) range=2.6-4.0 nM; CPZ IC(50) range=523-1140 microM). 3. The pro-inflammatory mediator prostaglandin E(2) significantly increased the maximal effect of AEA-evoked CGRP release without altering the EC(50). AEA, ACEA and CAP stimulated cAMP accumulation in TG neurons in a calcium- and TRPV1-dependent fashion. Moreover, the protein kinase inhibitor staurosporine significantly inhibited AEA- and CAP-evoked CGRP release. 4. The pungency of AEA, NADA, ACEA and CAP in the rat eye-wipe assay was also assessed. Interestingly, when applied intraocularly, NADA or CAP each produced nocifensive responses, while AEA or ACEA did not. 5. Finally, the potential inhibitory effects of these cannabinoids on TG nociceptors were evaluated. Neither AEA nor ACEA decreased CAP-evoked CGRP release. Furthermore, neither of the cannabinoid receptor type 1 antagonists SR141716A nor AM251 had any impact on either basal or CAP-evoked CGRP release. AEA also did not inhibit 50 mM K(+)-evoked CGRP release and did not influence bradykinin-stimulated inositol phosphate accumulation. 6. We conclude that the major action of AEA, NADA and ACEA on TG neurons is excitatory, while, of these, only NADA is pungent. These findings are discussed in relation to our current understanding of interactions between the cannabinoid and vanilloid systems and nociceptive processing in the periphery.

The role of uninjured nerve in spinal nerve ligated rats points to an improved animal model of neuropathic pain

L5 and L6 spinal nerve ligation (SNL) in rats leads to behavioral signs of neuropathic pain including mechanical allodynia. The purposes of this study were to investigate the role of the intact L4 spinal nerve in the development of mechanical allodynia following L5 and L6 SNL and, as a result, to develop a modified model of neuropathic pain. As a first set of experiments, in addition to tight ligation of the left L5 and L6 spinal nerves, the intact L4 spinal nerve was manipulated either (1) by gentle repeated stretching of the L4 spinal nerve immediately after L5 and L6 SNL or (2) by intermittent mechanical stimulation to the ipsilateral paw during the first week after SNL. Tactile sensitivity was measured by determining the foot withdrawal threshold before and after SNL. Mild irritation of L4 spinal nerve and application of mechanical stimuli to the ipsilateral paw significantly increased the development of mechanical allodynia after SNL. In a second set of experiments, SNL was produced by tightly ligating only the left L5 spinal nerve with or without a loop of 5-0 chromic gut placed loosely around the L4 spinal nerve. This additional L4 loop significantly increased long-lasting tactile sensitivity compared to L5 SNL alone. These results suggest that afferent activity of the intact L4 spinal nerve aids in the development of mechanical allodynia in the SNL model of neuropathic pain. The addition of a chromic gut loop around the intact L4 spinal nerve can augment the development of mechanical allodynia following SNL of L5. We propose this latter as a useful and practical animal model of neuropathic pain.

The neuronal distribution of cannabinoid receptor type 1 in the trigeminal ganglion of the rat

Cannabinoid compounds have been shown to produce antinociception and antihyperalgesia by acting upon cannabinoid receptors located in both the CNS and the periphery. A potential mechanism by which cannabinoids could inhibit nociception in the periphery is the activation of cannabinoid receptors located on one or more classes of primary nociceptive neurons. To address this hypothesis, we evaluated the neuronal distribution of cannabinoid receptor type 1 (CB1) in the trigeminal ganglion (TG) of the adult rat through combined in situ hybridization (ISH) and immunohistochemistry (IHC). CB1 receptor mRNA was localized mainly to medium and large diameter neurons of the maxillary and mandibular branches of the TG. Consistent with this distribution, in a de facto nociceptive sensory neuron population that exhibited vanilloid receptor type 1 immunoreactivity, colocalization with CB1 mRNA was also sparse (<5%). Furthermore, very few neurons (approximately 5%) in the peptidergic (defined as calcitonin gene-related peptide- or substance P-immunoreactive) or the isolectin B4-binding sensory neuron populations contained CB1 mRNA. In contrast, and consistent with the neuron-size distribution for CB1, nearly 75% of CB1-positive neurons exhibited N52-immunoreactivity, a marker of myelinated axons. These results indicate that in the rat TG, CB1 receptors are expressed predominantly in neurons that are not thought to subserve nociceptive neurotransmission in the noninjured animal. Taken together with the absence of an above background in situ signal for CB2 mRNA in TG neurons, these findings suggest that the peripherally mediated antinociceptive effects of cannabinoids may involve either as yet unidentified receptors or interaction with afferent neuron populations that normally subserve non-nociceptive functions.

Upregulation of proinflammatory cytokines and nerve growth factor by intraplantar injection of capsaicin in rats

Capsaicin-sensitive primary afferents (CSPA) are known to be involved in nociception and neurogenic inflammation. Extensive research has been devoted to the sensory role of these fibres but less attention has been paid to their local effector function. This study aimed at gaining more insight into the molecular mechanisms underlying the neurogenic inflammation induced by this special group of afferent fibres. Different groups of rats (n = 5 in each group), either naive or subjected to selective ablation of their CSPA, received individual intraplantar injections of saline, capsaicin, its vehicle or capsaicin preceded by its antagonist, capsazepine. Acute tests for nociception were used to assess the variations of the nociceptive thresholds. Variations of the levels of proinflammatory cytokines and nerve growth factor (NGF) were measured by enzyme-linked immunosorbent assay (ELISA). Intraplantar injection of capsaicin (10 microg in 50 microl) produced a sustained thermal and mechanical hyperalgesia that peaked at 3-6 h and disappeared 24 h following the injection. Similar capsaicin injection in further groups of rats produced an early upregulation of the proinflammatory cytokines and NGF, which peaked at 30-60 min and returned to control levels within 2-5 h. Similar effects were observed following the application of either capsaicin or intense electrical stimulation on the cut end of the distal portion of the sciatic nerve. The effects of capsaicin were abolished in rats subjected to selective ablation of their CSPA. These results demonstrate that CSPA can simultaneously challenge the immune system through the release of proinflammatory mediators and the central nervous system through nociceptive signalling and can therefore serve as a common afferent pathway to both immune and nervous systems.

Progressive tactile hypersensitivity after a peripheral nerve crush: non-noxious mechanical stimulus-induced neuropathic pain

Neuropathic pain syndromes are characterized by spontaneous pain and by stimulus-evoked allodynia and hyperalgesia. Stimulus-induced pain, i.e. the capacity of external stimuli to alter sensory processing so as to generate a pain hypersensitivity that outlasts the initiating stimulus, is usually present only after intense activation of nociceptors. In abnormal pain states, however, such as after capsaicin injection or inflammation, a stimulus-induced incremental pain can be generated by repetitive light touch, termed progressive tactile hypersensitivity (PTH). In the present study, we have examined whether PTH also occurs in two experimental models of neuropathic pain: a crush injury of the sciatic nerve and the spared nerve injury (SNI) model. When applied during the first weeks after injury to the territory of the injured crushed nerve, repeated low-intensity mechanical stimulation did not change the mechanical withdrawal threshold response. However, 10 weeks and after, the same repeated stimulation induced a progressive tactile hypersensitivity that persisted after discontinuation of the tactile stimulation. Following SNI, repeated stimulation of the hypersensitive skin territory, corresponding to the intact spared sural nerve, never induced PTH. Tactile stimulation of regenerating afferents but not spared non-injured afferents, can induce, therefore, PTH and such a stimulus-induced alteration in pain processing may contribute to clinical neuropathic pain.

Functional downregulation of P2X3 receptor subunit in rat sensory neurons reveals a significant role in chronic neuropathic and inflammatory pain

The excitation of nociceptive sensory neurons by ATP released in injured tissue is believed to be mediated partly by P2X3 receptors. Although an analysis of P2X3 knock-out mice has revealed some deficits in nociceptive signaling, detailed analysis of the role of these receptors is hampered by the lack of potent specific pharmacological tools. Here we have used antisense oligonucleotides (ASOs) to downregulate P2X3 receptors to examine their role in models of chronic pain in the rat. ASOs and control missense oligonucleotides (180 microg/d) were administered intrathecally to naive rats for up to 7 d via a lumbar indwelling cannula attached to an osmotic minipump. Functional downregulation of the receptors was confirmed by alphabeta-methylene ATP injection into the hindpaw, which evoked significantly less mechanical hyperalgesia as early as 2 d after treatment with ASOs relative to controls. At this time point, P2X3 protein levels were significantly downregulated in lumbar L4 and L5 dorsal root ganglia. After 7 d of ASO treatment, P2X3 protein levels were reduced in the primary afferent terminals in the lumbar dorsal horn of the spinal cord. In models of neuropathic (partial sciatic ligation) and inflammatory (complete Freund's adjuvant) pain, inhibition of the development of mechanical hyperalgesia as well as significant reversal of established hyperalgesia were observed within 2 d of ASO treatment. The time course of the reversal of hyperalgesia is consistent with downregulation of P2X3 receptor protein and function. This study demonstrates the utility of ASO approaches for validating gene targets in in vivo pain models and provides evidence for a role of P2X3 receptors in the pathophysiology of chronic pain.