The differential contribution of capsaicin-sensitive afferents to behavioral and cardiovascular measures of brief and persistent nociception and to Fos expression in the formalin test

ERK2 Alone Drives Inflammatory Pain But Cooperates with ERK1 in Sensory Neuron Survival

Extracellular signal-regulated kinases 1 and 2 (ERK1/2) are highly homologous yet distinct components of signal transduction pathways known to regulate cell survival and function. Recent evidence indicates an isoform-specific role for ERK2 in pain processing and peripheral sensitization. However, the function of ERK2 in primary sensory neurons has not been directly tested. To dissect the isoform-specific function of ERK2 in sensory neurons, we used mice with Cre-loxP-mediated deletion of ERK2 in Nav1.8(+) sensory neurons that are predominantly nociceptors. We find that ERK2, unlike ERK1, is required for peripheral sensitization and cold sensation. We also demonstrate that ERK2, but not ERK1, is required to preserve epidermal innervation in a subset of peptidergic neurons. Additionally, deletion of both ERK isoforms in Nav1.8(+) sensory neurons leads to neuron loss not observed with deletion of either isoform alone, demonstrating functional redundancy in the maintenance of sensory neuron survival. Thus, ERK1 and ERK2 exhibit both functionally distinct and redundant roles in sensory neurons.

SIGNIFICANCE STATEMENT

ERK1/2 signaling affects sensory neuron function and survival. However, it was not clear whether ERK isoform-specific roles exist in these processes postnatally. Previous work from our laboratory suggested either functional redundancy of ERK isoforms or a predominant role for ERK2 in pain; however, the tools to discriminate between these possibilities were not available at the time. In the present study, we use new genetic knock-out lines to demonstrate that ERK2 in sensory neurons is necessary for development of inflammatory pain and for postnatal maintenance of peptidergic epidermal innervation. Interestingly, postnatal loss of both ERK isoforms leads to a profound loss of sensory neurons. Therefore, ERK1 and ERK2 display both functionally distinct and redundant roles in sensory neurons.

Pain behavior in the formalin test persists after ablation of the great majority of C-fiber nociceptors

Although the formalin test is a widely used model of persistent pain, the primary afferent fiber types that underlie the cellular and behavioral responses to formalin injection are largely unknown. Here we used a combined genetic and pharmacological approach to investigate the effect of ablating subsets of primary afferent nociceptors on formalin-induced nocifensive behaviors and spinal cord Fos protein expression. Intrathecal capsaicin-induced ablation of the central terminals of TRPV1+neurons greatly reduced the behavioral responses and Fos elicited by low-dose (0.5%) formalin. In contrast, genetic ablation of the MrgprD-expressing subset of non-peptidergic unmyelinated afferents, which constitute a largely non-overlapping population, altered neither the behavior nor the Fos induced by low-dose formalin. Remarkably, nocifensive behavior following high-dose (2%) formalin was unchanged in mice lacking either afferent population, or even in mice lacking both populations, which together make up the great majority of C-fiber nociceptors. Thus, at high doses, which are routinely used in the formalin test, formalin-induced "pain" behavior persists in the absence of the vast majority of C-fiber nociceptors, which points to a contribution of a large spectrum of afferents secondary to non-specific formalin-induced tissue and nerve damage.

Participation of the spinal TRPV1 receptors in formalin-evoked pain transduction: a study using a selective TRPV1 antagonist, iodo-resiniferatoxin

The involvement of spinal transient receptor potential vanilloid 1 (TRPV1) in formalin-evoked pain has remained unclear, because investigation of this kind of pain with selective antagonists has not been conducted. The purpose of this study is to investigate the participation of spinal TRPV1 in formalin-evoked pain with iodo-resiniferatoxin (I-RTX), a potent TRPV1-selective antagonist. I-RTX given intrathecally dose-dependently and significantly decreased the number of flinching responses in the formalin-evoked 1st and 2nd phase with ID50 values (drug dose producing 50% inhibition of response) of 1.0 and 3.8 μg, respectively, and concentration-dependently suppressed capsaicin-evoked calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) release from rat spinal cord slices with an IC50 value (drug concentration producing 50% inhibition of response) of 86 nm. Capsazepine, a classical non-selective TRPV1 antagonist, given intrathecally also inhibited formalin-evoked flinching in both the 1st and 2nd phase with ID50s of 420 and 200 μg, respectively, and CGRP-LI release from rat spinal cord slices with an IC50 of 7.8 μm. Ratios of in-vivo analgesic potencies of I-RTX and capsazepine well reflected their intrinsic in-vitro activity. These findings suggest that spinal TRPV1 participates in the transduction system of formalin-evoked pain.

Effects of systemic bicuculline or morphine on formalin-evoked pain-related behaviour and c-Fos expression in trigeminal nuclei after formalin injection into the lip or tongue in rats

This study examined differences in nociceptive responses between lip and tongue. Formalin-induced pain-related behaviour and c-Fos expression in the trigeminal caudal nucleus (Vc) with/without systemic preadministration of a gamma-aminobutyric acid (GABA) type A receptor antagonist, bicuculline (2 mg/kg, i.p., 10 min before formalin injection) or a micro-opioid receptor agonist, morphine (3 mg/kg, i.p., 10 min before formalin injection) have been studied. Formalin injection into the upper lip induced an immediate pain-related behaviour, mostly face-rubbing behaviour, for 15 min (phase 1, mean +/- SEM/5 min, 81.2 +/- 30.1), followed by a more increased activity for 15 min (phase 2, 205.4 +/- 43.6) and a decline to baseline for next 15 min (phase 3, 63.9 +/- 28.0). Formalin injection into the tongue induced similar amount of pain-related behaviour at phase 1 (67.9 +/- 16.7), followed by similar activity at phase 2 (48.6 +/- 6.2), and lesser behaviour at phase 3 (20.4 +/- 7.6). The behaviour at phase 2 decreased following preadministration of bicuculline or morphine when formalin was injected into the lip (b, 62.5 +/- 14.5; m, 95.8 +/- 10.0) but not into the tongue (b, 31.0 +/- 9.2; m, 77.4 +/- 27.0). A considerable numbers of c-Fos-immunoreactive (IR) cells were induced in the caudal and inter-medio-lateral center of superficial layers of the Vc (VcI/II; mean +/- SEM/section = 225.8 +/- 12.9) and magnocellular zone of the Vc (VcIII/IV; 67.1 +/- 4.7) 2 h after formalin injection into the lip. Much smaller numbers of c-Fos-IR cells were induced in the rostral and dorso-medial one-fourth of the VcI/II (72.6 +/- 3.7) and VcIII/IV (55.6 +/- 6.6) after formalin injection into the tongue. Following preadministration with systemic bicuculline or morphine, the formalin-induced c-Fos-IR cells were decreased more in the VcI/II when formalin was injected into the lip (VcI/II, 102.4 +/- 8.0; VcIII/IV, 32.8 +/- 1.4) than into the tongue (VcI/II, 49.5 +/- 8.1; VcIII/IV, 31.7 +/- 5.3). These results show that the lip is more sensitive to formalin-induced noxious stimulation and regulated more through GABA(A) and micro-opioid receptors than the tongue.

Chloride regulation in the pain pathway

Melzack and Wall's Gate Control Theory of Pain laid the theoretical groundwork for a role of spinal inhibition in endogenous pain control. While the Gate Control Theory was based on the notion that spinal inhibition is dynamically regulated, mechanisms underlying the regulation of inhibition have turned out to be far more complex than Melzack and Wall could have ever imagined. Recent evidence indicates that an exquisitely sensitive form of regulation involves changes in anion equilibrium potential (E(anion)), which subsequently impacts fast synaptic inhibition mediated by GABA(A), and to a lesser extent, glycine receptor activation, the prototypic ligand gated anion channels. The cation-chloride co-transporters (in particular NKCC1 and KCC2) have emerged as proteins that play a critical role in the dynamic regulation of E(anion) which in turn appears to play a critical role in hyperalgesia and allodynia following peripheral inflammation or nerve injury. This review summarizes the current state of knowledge in this area with particular attention to how such findings relate to endogenous mechanisms of hyperalgesia and allodynia and potential applications for therapeutics based on modulation of intracellular Cl(-) gradients or pharmacological interventions targeting GABA(A) receptors.

Antinociceptive effect of amygdalin isolated from Prunus armeniaca on formalin-induced pain in rats

Amygdalin is a plant glucoside isolated from the stones of rosaceous fruits, such as apricots, peaches, almond, cherries, and plums. To investigate the pain-relieving activity of amygdalin, we induced pain in rats through intraplantar injection of formalin, and evaluated the antinociceptive effect of amygdalin at doses of 0.1, 0.5, 1.0, and 10.0 mg/kg-body weight by observing nociceptive behavior such as licking, biting and shaking, the number of Fos-immunoreactive neurons in the spinal cord, and the mRNA expression of inflammatory cytokines in the plantar skin. The intramuscular injection of amygdalin significantly reduced the formalin-induced tonic pain in both early (the initial 10 min after formalin injection) and late phases (10-30 min following the initial formalin injection). During the late phase, amygdalin did reduce the formalin-induced pain in a dose-dependent manner in a dose range less than 1 mg/kg. Molecular analysis targeting c-Fos and inflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 beta (IL-1beta) also showed a significant effect of amygdalin, which matched the results of the behavioral pain analysis. These results suggest that amygdalin is effective at alleviating inflammatory pain and that it can be used as an analgesic with anti-nociceptive and anti-inflammatory activities.

Antinociceptive activity of petroleum ether fraction from the MeOH extracts of Paederia scandens in mice

The petroleum ether fraction of MeOH extract from Paederia scandens was evaluated on anti-nociceptive activity in mice using chemical and thermal models of nociception. Given orally, the petroleum ether fraction (PEF) at doses of 20, 40 and 80mg/kg produced significant inhibitions on chemical nociception induced by intraperitoneal acetic acid and subplantar formalin or capsaicin injections and on thermal nociception in the tail-flick test and in the hot plate test. More significant inhibition of nociception was observed at dose of 80mg/kg of the petroleum ether fraction. In the pentobarbital sodium-induced sleeping time test and the open-field test, the petroleum ether fraction neither significantly enhanced the pentobarbital sodium-induced sleeping time nor impaired the motor performance, indicating that the observed anti-nociception was unlikely due to sedation or motor abnormality. Moreover, the petroleum ether fraction-induced anti-nociception in both capsaicin and formalin tests was insensitive to naloxone, but was significantly antagonized by glibenclamide. These results suggested that the petroleum ether fraction produced anti-nociception possibly related to glibenclamide-sensitive K(+)-ATP channels, which merited further studies regarding the precise site and mechanism of action. The major constituents of the petroleum ether fraction (PEF) determined by GC/MS analysis, are linoleic acid, the sterols and vitamin E. Therefore it can be suggested that they exert synergetic effects and are together responsible for the antinociceptive activity of the PEF-fraction.

Extended Swimming Exercise Reduces Inflammatory and Peripheral Neuropathic Pain in Rodents

Physical exercise is often recommended to patients who have chronic pain. However, only a small number of studies report exercise-induced analgesia in the setting of inflammatory pain, and even fewer relate long-term exercise to reductions in neuropathic pain. To address these questions, we evaluated the effect of extended swimming exercise in animal models of inflammatory (intraplantar injection of dilute formalin) and neuropathic (partial peripheral nerve injury) pain. We found that 9 days of swimming exercise in 37 degrees C water for 90 min/d decreased licking and flinching responses to formalin, as compared with nonexercised control animals. In addition, 18 to 25 days of swimming decreased nerve injury-induced cold allodynia and thermal hyperalgesia in rats, and 7 days of swimming decreased nerve injury-induced thermal hyperalgesia in mice. Our data indicate that swimming exercise reduces behavioral hypersensitivity in formalin- and nerve injury-induced animal models of persistent pain.

PERSPECTIVE

Surprisingly, few animal studies have investigated the effects of extended exercise on chronic pain. Our results support the use of exercise as a nonpharmacological approach for the management of peripheral neuropathic pain.

Intrathecal neuropeptide Y reduces behavioral and molecular markers of inflammatory or neuropathic pain

Our previous work indicates that the intrathecal administration of neuropeptide Y (NPY) acts at its cognate receptors to reduce behavioral signs of nociception in several models of inflammatory pain, including the formalin test. The present study extends these findings to a rat model of peripheral neuropathic pain, and then evaluates the hypothesis that NPY inhibits inflammation- and nerve injury-induced activation of spinal nociceptive transmission. Here we show that NPY dose-dependently reduced behavioral signs of mechanical and cold hypersensitivity in the spared nerve injury (SNI) model. Intrathecal administration of either a Y1 (BIBO3304) or a Y2 (BIIE0246) receptor antagonist dose-dependently reversed the anti-allodynic actions of NPY. To monitor the effects of NPY on the stimulus-induced activation of spinal nociresponsive neurons, we quantified protein expression of the immediate-early gene c-fos in lamina I-VI of the L4-L5 dorsal horn, with special attention to the mediolateral pattern of Fos immunohistochemical staining after SNI. Either tactile stimulation of the hindpaw ipsilateral to nerve injury, or intraplantar injection of noxious formalin, increased the number of Fos-like immunoreactive profiles. Tactile stimulation evoked a mediolateral pattern of Fos expression corresponding to the innervation territory of the uninjured (sural) nerve. We found that intrathecal NPY reduced both formalin- and SNI-induced Fos expression. NPY inhibition of SNI-induced Fos expression was localized to the sural (uninjured) innervation territory, and could be blocked by intrathecal BIBO3304 and BIIE0246. We conclude that NPY acts at spinal Y1 and Y2 receptors to reduce spinal neuron activity and behavioral signs of inflammatory or neuropathic pain.

Activation of peripheral and spinal histamine H3 receptors inhibits formalin-induced inflammation and nociception, respectively

Pharmacological activation of histamine H3 receptors is known to reduce the release of inflammatory peptides, thereby reducing pain and inflammation, but the site(s) and mechanism(s) of these effects are currently unknown. The present study addressed these questions by examining the effects of the H3 agonist immepip and the H3 antagonist thioperamide on nociceptive behaviors and swelling produced during the rat formalin test. Systemic administration of immepip (5 and 30 mg/kg, s.c.) significantly attenuated formalin-induced flinching but not licking responses during both phases. This attenuation was reversed by either systemic (15 mg/kg, i.p.) or intrathecal (20 or 50 microg) administration of thioperamide. Furthermore, immepip (30 mg/kg, s.c.) significantly inhibited formalin-induced swelling, an action which was completely reversed by systemic (15 mg/kg, i.p.), but not intrathecal (50 microg) thioperamide. Also consistent with this pattern, intrathecal immepip (50 microg) reduced flinching responses, but had no effect on formalin-induced paw swelling. The present findings suggest that activation of H3 receptors located on peripheral and spinal terminals of deep dermal fibers attenuates formalin-induced swelling and flinching, respectively. Pharmacological stimulation of H3 receptors could be an important therapeutic approach for many disorders related to deep dermal or inflammatory pain.

Roles of capsaicin-sensitive primary afferents in differential rat models of inflammatory pain: a systematic comparative study in conscious rats

To characterize the role of capsaicin-sensitive primary afferents in inflammatory pain, the effects of subcutaneous (s.c.) injection of 0.15% capsaicin on different chemical irritants-induced pathological nociception including persistent spontaneous nociception, primary thermal and mechanical hyperalgesia, and inflammatory response were systematically investigated in unanesthetized conscious rats. Four different animal models of inflammatory pain: the bee venom (BV) test, the formalin test, the carrageenan model, and the complete Freund's adjuvant (CFA) model, were employed and compared. Local pre-treatment with capsaicin produced a significant inhibition on the s.c. BV and formalin induced long-lasting persistent spontaneous nociception. However, this capsaicin-induced inhibitory effect on spontaneous nociception in the BV test was only found within the late phase (tonic nociception; 11-60 min), but not the early phase (acute nociception; 0-10 min). A complete preventing effect of capsaicin on the decreased thermal paw withdrawal latency was found in the BV, carrageenan, and CFA models. Nevertheless, pre-treatment with capsaicin only produced complete blocking effects on the decreased mechanical paw withdrawal threshold in the BV and carrageenan models, but not in the CFA model. For inflammatory response, a significant inhibition of the BV-elicited paw swelling was found following capsaicin treatment. In marked contrast, capsaicin did not produce any effects on the paw inflammation during exposure to carrageenan, CFA, and formalin. These data suggest that capsaicin-sensitive primary afferents may play differential roles in the induction and development of pathological nociception in differential inflammatory pain models. In contrast to other chemical irritants, BV-induced long-term spontaneous nociception, facilitated nociceptive behavior, and inflammation are modulated by peripheral capsaicin-sensitive afferents.

Antinociceptive effect of leaf essential oil from Croton sonderianus in mice

The leaf essential oil from Croton sonderianus (EOCS) was evaluated for antinociceptive activity in mice using chemical and thermal models of nociception. Given orally, the essential oil at doses of 50, 100 and 200 mg/kg produced significant inhibitions on chemical nociception induced by intraperitoneal acetic acid and subplantar formalin or capsaicin injections. However, it evidenced no efficacy against thermal nociception in hot-plate test. More prominent inhibition of acetic acid-induced writhing and capsaicin-induced hind-paw licking responses was observed at 100 and 200 mg/kg of EOCS. At similar doses, the paw licking behavior in formalin test was more potently suppressed during the late phase (20-25 min, inflammatory) than in early phase (0-5 min, neurogenic). The EOCS-induced antinociception in both capsaicin and formalin tests was insensitive to naloxone (1 mg/kg, s.c.), but was significantly antagonized by glibenclamide (2 mg/kg, i.p.). In mice, the essential oil (100 and 200 mg/kg) neither significantly enhanced the pentobarbital-sleeping time nor impaired the motor performance in rota-rod test, indicating that the observed antinociception is unlikely due to sedation or motor abnormality. These results suggest that EOCS produces antinociception possibly involving glibenclamide-sensitive KATP+ channels, which merit further studies on its efficacy in more specific models of hyperalgesia and neuropathic pain.

Effects of the high-efficacy 5-HT1A receptor agonist, F 13640 in the formalin pain model: a c-Fos study

We studied the effects of the high-efficacy 5-hydroxytryptamine1A (5-HT1A) receptor agonist, F 13640 on both formalin-induced spinal cord c-Fos protein expression and pain behaviours in the rat. Replicating earlier data, F 13640 (0.63 mg/kg, i.p.; t(-15 min)) completely inhibited the elevation and licking of the formalin-injected paw. In the same animals, and in spite of the agent as in earlier data increasing the number of c-Fos labelled nuclei when it was administered alone, F 13640 markedly reduced the number of formalin-induced c-Fos labelled nuclei. This was found in both the superficial (I-II) and deep (V-VI) dorsal horn laminae (2 h post-injection: 72+/-2% and 92+/-1% of reduction, respectively; P<0.001 in either case), spinal areas that contain neurons responsive to nociceptive stimulation. Co-operation occurred so that after the co-administration of F 13640 and formalin, c-Fos expression was inferior to that induced when either stimulation was administered alone. The data provide initial evidence for the agent's inhibitory effects on noxiously evoked c-Fos expression. The results indicate that co-operation between 5-HT1A receptor activation and nociceptive stimulation powerfully inhibits responses to severe, tonic nociception.

Sensory nerves, neurogenic inflammation and pain: missing components of alternative irritation strategies? A review and a potential strategy

The eyes and skin are highly innervated by sensory nerves; stimulation of these nerves by irritants may give rise to neurogenic inflammation, leading to sensory irritation and pain. Few in vitro models of neurogenic inflammation have been described in conjunction with alternative skin and eye irritation methods, despite the fact that the sensory innervation of these organs is well-documented. To date, alternative approaches to the Draize skin and eye irritation tests have proved largely successful at classifying severe irritants, but are generally poor at discriminating between agents with mild to moderate irritant potential. We propose that the development of in vitro models for the prediction of sensory stimulation will assist in the re-classification of the irritant potential of agents that are under-predicted by current in vitro strategies. This review describes the range of xenobiotics known to cause inflammation and pain through the stimulation of sensory nerves, as well as the endogenous mediators and receptor types that are involved. In particular, it focuses on the vanilloid receptor, its activators and its regulation, as these receptors function as integrators of responses to numerous noxious stimuli. Cell culture models and ex vivo preparations that have the potential to serve as predictors of sensory irritation are also described. In addition, as readily available sensory neuron cell line models are few in number, stem cell lines (with the capacity to differentiate into sensory neurons) are explored. Finally, a preliminary strategy to enable assessment of whether incorporation of a sensory component will enhance the predictive power of current in vitro eye and skin testing strategies is proposed.

Morphine-Induced Analgesia, Hypotension, and Bradycardia Are Enhanced in Hypertensive Rats

Several studies have emphasized an opioidergic link between the central regulation of cardiovascular function and acute noninflammatory pain. By contrast, relatively few studies have investigated the relationships between opioids, hypertension, and inflammatory pain. We used the formalin model of acute inflammatory pain to compare morphine antinociception among spontaneously hypertensive (SHR) rats, their genetic normotensive controls, Wistar-Kyoto (WKY) rats, and Sprague-Dawley (SD) rats. Measures of nociception included both behavioral and cardiovascular end-points (increased mean arterial blood pressure and heart rate). Morphine (3.0 mg/kg subcutaneously) produced greater hypotension and bradycardia in SHR than in WKY or SD rats. We next administered formalin (5%; 50 microL) and observed greater nociception during both Phase 1 and Phase 2 in SHR controls than in WKY controls. The morphine-treated groups did not differ, suggesting that morphine attenuates hypersensitivity to formalin pain in the SHR. Morphine inhibited edema but not paw hyperthermia to a greater degree in SHR, whereas Phase 1 remifentanil produced a relatively shorter delay in the onset of Phase 2 in SHR. We suggest that the presentation of essential hypertension be considered when opioid regimens are planned both during surgery (to minimize cardiovascular complications) and during the postoperative period (to optimize analgesic effects).

IMPLICATIONS

Presentation of essential hypertension should be considered when opioid regimens are planned both during surgery (to minimize cardiovascular complications) and during the postoperative period (to optimize analgesic effects).

Cholinergic modulation of nociceptive responses in vivo and neuropeptide release in vitro at the level of the primary sensory neuron

Muscarinic acetylcholine receptors (mAChRs) have been widely reported as pharmacological targets for the treatment of pain. However, most of these efforts have focused on CNS mAChRs and their role in modulating nociception at the level of the spinal cord. The present study examines the contribution of peripheral mAChRs in trigeminal nociceptive pathways using a combination of in vivo and in vitro approaches. In the formalin model of orofacial nociception in rats, a peri-oral co-injection of the M2 agonist arecaidine dose-dependently inhibited phase 2 nocifensive behavior up to approximately 50% at 5 nmol. This effect was blocked by co-treatment with the mAChR antagonist atropine and was not seen when arecaidine was administered under the skin of the back, a site distant from that of the formalin injection. In vitro superfusion of isolated rat buccal mucosa with the non-selective mAChR agonist muscarine or arecaidine led to a concentration-dependent inhibition of capsaicin-evoked CGRP release to 39% (EC50=255 nM) and 28% (EC50=847 nM) of control values, respectively. Both responses were blocked by the non-selective mAChR antagonist atropine or the M2 antagonist gallamine. Further, the endogenous ligand ACh produced a bi-phasic response, potentiating evoked CGRP release to 195% of control (EC50= 918nM) and inhibiting evoked CGRP release to 45% of control (EC50=255 microM), effects that were shown to be mediated by nAChRs and mAChRs, respectively. Finally, combined in situ hybridization/immunofluorescence demonstrated that m2 mRNA was present in 20% of trigeminal ganglion neurons between 30 and 60 microm in diameter and that 5-9% of these also expressed CGRP or VR1 immunoreactivity. These results show that activation of peripheral M2 receptors produces antinociception in vivo and the inhibition of nociceptor activity in vitro. While histological analyses at the level of the trigeminal neuronal cell bodies leave open the question of whether the effects of M2 agonists are direct or indirect, these data indicate that primary sensory neuronal M2 receptors may represent a viable peripheral target for the treatment of pain and inflammation.

Evidence for the involvement of vanilloid receptor in the antinociception produced by the dialdeydes unsaturated sesquiterpenes polygodial and drimanial in rats

This study investigated whether or not the neonatal treatment of rats with the sesquiterpenes polygodial or drimanial could cause persistent antinociception similar to that induced by capsaicin. Rats were injected subcutaneously 48 h after birth with capsaicin (50 mg/kg), polygodial (150 mg/kg), drimanial (150 mg/kg) or vehicle (1ml/kg). Six to eight weeks later, rats were tested in models of nociception. Treatment of rats with capsaicin, polygodial or drimanial produced significant inhibition of the first phase and, to a lesser extent, the second phase of formalin-induced nociception. A significant reduction in Complete Freund's Adjuvant and capsaicin-induced hyperalgesia was observed in the animals neonatally treated with capsaicin, polygodial or drimanial compared with vehicle-treated rats. Moreover, both sesquiterpenes caused inhibition of plasma extravasation induced by injection of capsaicin. The neonatal treatment with capsaicin, polygodial or drimanial significantly decreased [3H]-resiniferatoxin binding sites in the rat spinal cord, but only capsaicin neonatal treatment significantly reduced the expression of TRPV1 in dorsal root ganglia (DRG) when assessed by Western blot. These results extend our previous findings demonstrating that the neonatal treatment of rats with polygodial or drimanial, similar to that reported for capsaicin, produced persistent antinociception in adult animals associated with TRPV1 down-regulation in the spinal cord, but not TRPV1 expression in DRG.

Intrathecal neuropeptide Y inhibits behavioral and cardiovascular responses to noxious inflammatory stimuli in awake rats

To test the hypothesis that administration of neuropeptide Y (NPY) to the spinal cord reduces inflammatory pain, we evaluated the effects of intrathecal NPY on behavioral and cardiovascular markers of the nociception associated with intraplantar formalin injection in rats. Before the administration of formalin, NPY dose dependently increased blood pressure, an effect that could be prevented with the coadministration of the Y2 antagonist, BIIE0246. This effect lasted only 20 min, and thus was over before initiation of the formalin test. NPY dose dependently inhibited the flinching, licking, pressor, and tachycardia responses associated with formalin injection. The Y1 receptor antagonist BIBO 3304 partially reversed the antinociceptive effect of NPY at a dose that did not by itself have an effect (3 microg). We conclude that intrathecal NPY acts in part via Y1 receptors to inhibit ongoing inflammatory nociception.

Stimulation of dopamine D2 receptors in the nucleus accumbens inhibits inflammatory pain

Previous studies suggest that dopamine in the nucleus accumbens links noxious or mesolimbic stimulation with the feedback inhibition of nociception. To test the hypothesis that pharmacological agonism at dopamine receptors in the nucleus accumbens elicits antinociception, we bilaterally microinjected dopamine D1- and D2-receptor subtype selective drugs, and then evaluated behavioral responses to noxious intraplantar formalin. While the D1-selective agonist SKF 38393 was without effect at a dose of 0.5 nmol/side, the D2-selective agonist quinpirole dose-dependently (0.05-5.0 nmol/side, bilateral) inhibited the persistent phase of formalin-induced nociception. This was blocked by pre-administration of a selective D2-dopaminergic antagonist raclopride (0.3 nmol/side, bilateral). Quinpirole did not produce overt behavioral effects and did not change rotarod latency. Our results indicate that quinpirole acts at dopamine D2 receptors in the nucleus accumbens to inhibit persistent nociception at doses that circumvent confounding non-specific motor deficits, namely, sedation and motor coordination.

Electroacupuncture induces the expression of Fos in rat dorsal horn via capsaicin-insensitive afferents

Fos is expressed in rat dorsal horn neurons after electroacupuncture (E-acupuncture), but it is unclear which types of afferent fibers are involved in the expression. It is thought that the Fos expression is induced via Adelta afferents rather than C afferents, since the threshold of Adelta afferents to electrical stimulation is much lower than that of unmyelinated ones. Therefore, neonatally capsaicin treated rats lacking many C afferents were examined to clarify this. Fos expression in the dorsal horn after injection of formalin into the hindpaw was severely attenuated by neonatal capsaicin treatment. However, Fos expression after E-acupuncture to the pads of the hindpaw was unaffected by the same treatment. These results suggest that E-acupuncture induces the expression of Fos in the dorsal horn neurons via capsaicin-insensitive afferents, presumably Adelta afferents.

Distinct neurochemical mechanisms are activated following administration of different P2X receptor agonists into the hindpaw of a rat

Nocifensive behaviors induced by the intradermal injection of three different P2X receptor agonists, ATP, BzATP or alpha,beta-meATP, into a hindpaw were measured in rats that were injected intrathecally with either an NMDA (MK-801) or an NK-1 (L-703,606) receptor antagonist or were pretreated systemically with the VR1 agonist resiniferatoxin (RTX). The same procedures were performed in animals injected intradermally with either capsaicin or formalin. Spinal infusion of MK-801 (10-50 nmol/10 micro l) similarly reduced the number of nociceptive events triggered by each of the P2X agonists and was also effective against capsaicin and formalin induced behaviors. Intrathecal administration of L-703,606 (50-100 nmol/10 micro l) had its greatest antinociceptive effect against capsaicin-induced behaviors followed by ATP and BzATP. L-703,606 was completely ineffective against behaviors induced by formalin or the other P2X agonist, alpha,beta-meATP. Pretreatment with RTX 2 days prior to testing significantly decreased the number of nociceptive events caused by each of the P2X agonists as well as capsaicin and formalin (capsaicin>BzATP>ATP>formalin>alpha,beta-meATP). The remaining nociceptive events in RTX animals injected with alpha,beta-meATP were significantly higher than in animals injected with either ATP or BzATP. Intradermal administration of different P2X receptor agonists induced similar levels of nocifensive behaviors and activity at spinal NMDA receptors. Capsaicin-sensitive fibers were likely activated following injection of BzATP and ATP, but not alpha,beta-meATP, and appeared to trigger the spinal release of substance P. The differences in mechanisms employed by the different P2X agonists may be a function of respective selectivity for P2X receptor subtypes.

Second phase of formalin-induced excitation of spinal dorsal horn neurons in spinalized rats is reversed by sciatic nerve block

Considerable debate persists concerning peripheral vs. central mechanisms underlying the second phase of the nociceptive response in the formalin test in the rat. To gain insight into the neurophysiological basis of this pain, we investigated the effects of block of afferent nerve conduction during the second phase of formalin-evoked excitation of single nociceptive neurons recorded extracellularly from rat spinal dorsal horn segments (L(3-4)) in pentobarbital-anaesthetized, male Sprague-Dawley rats. Rats were spinally transected (T(9)) to examine exclusively peripheral and spinal nociceptive processing. In six control rats, hind paw intraplantar formalin injection (50 microL, 2.5%) induced the typical biphasic increase in the discharge rate of the six wide dynamic range neurons tested. This response consisted of a relatively brief immediate phase (approximately 5 min), followed by decreased firing. An ensuing second phase of elevated discharge began approximately 35 min after injection and persisted to at least 80 min. In this control group, 0.9% saline was applied to the exposed ipsilateral sciatic nerve after onset of the second phase (40 min after formalin injection). In a group of six test rats, application of 2% lidocaine instead of saline reversed the second phase of excitation in all six wide dynamic range neurons examined. When the firing rate was normalized to that at 40 min (100%), the time of saline or lidocaine administration, the rate at 50 min was 120 +/- 7.5% in the saline-treated group and 31 +/- 7.4% in the lidocaine-treated group; following lidocaine treatment firing rate remained markedly less than that before administration throughout the remainder of the recording. It is concluded that: (i) spinal mechanisms alone are not sufficient for induction and maintenance of second phase increased discharge of spinal nociceptive dorsal horn neurons; (ii) descending influences via supraspinal inputs are not causal in the development and maintenance of second phase increased discharge and (iii) tonic input from afferent neurons during the second phase plays a primary and essential role in generating and sustaining the second phase of elevated discharge of dorsal horn neurons and, thus, presumably the second phase of nociceptive scores in the formalin test. The data in this study reveal how much of an altered synaptically elicited response in the spinal dorsal horn can be attributed to postsynaptic plastic changes vs. how much can be simply due to increased synaptic input. The present results are important not only in the context of the formalin test but also in the context of other models related to inflammatory pain and neuropathic pain.

Effects of nefopam on the spinal nociceptive processes: a c-Fos protein study in the rat

We have evaluated the effects of nefopam on the spinal c-Fos protein expression in the model of acute (noxious heat) and persistent (intraplantar injection of formalin) nociception in the rat. One and two hours after i.pl. formalin injection, c-Fos immunoreactive (c-Fos-IR) nuclei were preferentially located in the superficial (I-II) and deep (V-VI) laminae of the spinal dorsal horn of segments L4-L5, i.e. spinal areas containing numerous neurons responding exclusively, or not, to peripheral nociceptive stimuli. The doses of 15 and 30 mg/kg (s.c.) of nefopam had significant reducing effects on the formalin-evoked spinal c-Fos protein expression (36+/-14% and 47+/-9% reduction of the total number of c-Fos-IR nuclei per section, respectively, P<0.05 for both). These reducing effects of nefopam were not detectable 2 h after formalin. These results provide evidence that the significant effects of nefopam are time-limited in the formalin model of persistent nociception. One hour after noxious heat stimulation (52 degrees C for 15 s), c-Fos-IR nuclei were principally located in the superficial laminae I-II of the spinal dorsal horn (about 90% of the total number of c-Fos-IR nuclei per section). Nefopam (15 mg/kg s.c.) significantly reduced the noxious heat-evoked spinal c-Fos protein expression (33+/-3% reduction of the total number of c-Fos-IR nuclei, P<0.0001). The present results provide first evidence for the reducing effects of nefopam on the noxiously evoked spinal c-Fos protein expression, principally in acute nociceptive processes. These results suggest that nefopam may produce antinociceptive effects mainly in acute pain states.

Involvement of primary sensory afferents, postganglionic sympathetic nerves and mast cells in the formalin-evoked peripheral release of adenosine

Injection of formalin into the rat hind paw produces a dose-dependent local peripheral release of adenosine. Low doses of formalin (0.5-2.5%) evoke release during the first 10 min following injection, while a high dose of formalin (5%) evokes release lasting for 60 min. The current study was designed to determine the possible origin of release produced by two doses of formalin (1.5% and 5%). Microdialysis probes were implanted into the subcutaneous tissue under the glabrous skin of the hind paw of anaesthetized rats, and adenosine was determined by high performance liquid chromatography. Pretreatment with capsaicin, a neurotoxin selective for unmyelinated small diameter primary afferent nerves, markedly reduced the adenosine released by 1.5% formalin and the early phase of release by 5% formalin. Acute injection of 1% capsaicin to the hind paw of untreated rats also induced adenosine release. Pretreatment with 6-hydroxydopamine, a neurotoxin selective for sympathetic postganglionic nerve terminals, had no effect on release evoked by 1.5% formalin, but significantly reduced adenosine release during the late phase of release induced by 5% formalin. Pretreatment with compound 48/80, which degranulates mast cells, had no effect on adenosine release evoked by either concentration of formalin. We conclude that the origin of the adenosine released peripherally by formalin depends on the formalin concentration. At the lower concentration (1.5%), release is predominantly from unmyelinated sensory afferent nerve terminals, while at the higher concentration (5%), unmyelinated afferent nerve terminals are involved in the early phase, while sympathetic postganglionic nerve terminals are involved in the later phase. Mast cells do not contribute to release of adenosine evoked by either concentration of formalin.

An automated flinch detecting system for use in the formalin nociceptive bioassay

The biphasic display of paw-flinch behavior in the rat after injection of formalin into the dorsum of the hind paw is used for the screening of anti-hyperalgesic agents. Described and characterized here is a less labor-intensive system for counting flinch activity by detecting movement of a small metal band placed on the formalin-injected paw. A signal is generated as the band breaks the electromagnetic field of a loop antenna located under the rat and processed through an algorithm that determines flinch activity using 1) amplitude, 2) zero-voltage crossing, and 3) signal duration. Flinches are summed and stored over a selected collection interval throughout the assay for later analysis. Studies have validated the measures with respect to 1) system stability over time; 2) system-to-“practiced observer” correlation on flinch detection, r 2 = 0.94; 3) system variables including time of day, sex, age, and body weight; and 4) 50% effective dose values similar to those previously reported for intrathecal morphine and the NMDA antagonist MK-801.

Pivotal role of capsaicin-sensitive primary afferents in development of both heat and mechanical hyperalgesia induced by intraplantar bee venom injection

To investigate the roles of primary afferent fibers in development of the bee venom (BV)-induced persistent spontaneous nociception (PSN) and hyperalgesia (HA), the sciatic nerve or both the sciatic and saphenous nerves of rats were topically treated with capsaicin respectively under pentobarbital anesthesia to destroy the capsaicin-sensitive primary afferent (CSPA) fibers. Effect of the sciatic nerve capsaicin on the formalin-induced PSN was also evaluated. Destruction of the CSPA fibers of the sciatic nerve or both the sciatic and saphenous nerves only produced 34 or 69% inhibition of the mean total number of 1 h BV-induced paw flinches. However, the total number of 1 h formalin-induced paw flinches was inhibited by 90% (85% for phase 1 and 91% for phase 2). In naïve rats, destruction of the CSPA fibers of the sciatic nerve caused 237 and 60% increase in paw withdrawal thermal latency (PWTL) to radiant heat in the injection site (paw pad) and at the heel of the treated hind paw compared to the baseline values. However, it was without significant influence upon the PWTL in the non-treated side or the paw withdrawal mechanical threshold (PWMT) to von Frey filament stimuli in both hind paws. In the BV-treated rats, the CSPA fiber destruction of the sciatic nerve completely blocked development of the heat and mechanical HA in the BV injection site. However, the reduction in either PWTL (drop to baseline level) or PWMT (drop by 56% from the baseline level) at the heel of the BV-treated side was not affected by this treatment. However, destruction of the CSPA fibers of both the sciatic and saphenous nerves was able to block development of both heat and mechanical HA in the whole BV-treated hind paw and heat hyperalgesia in the non-injected hind paw. Taken together, we conclude that: (1) the CSPA (C- and A delta-) fibers play a pivotal role in mediation of either the heat or the mechanical hyperalgesia induced by s.c. BV; (2) the CSPA fibers may play a crucial role in mediation of the formalin-induced PSN, but play a partial role in the BV-induced nociceptive process; (3) in addition to the sciatic nerve, the saphenous nerve is also involved in mediation of the BV-induced PSN as well as heat and mechanical hyperalgesia, while it is not likely to be involved in the formalin-induced nociception.

Hypoalgesia and hyperalgesia with inherited hypertension in the rat

Many studies indicate that blood pressure control systems can attenuate pain (hypoalgesia) of short duration; however, we recently found exaggerated nociceptive responses (hyperalgesia) of persistent duration in the spontaneously hypertensive rat (SHR). Here, we used SHR, Dahl Salt-Sensitive (SS), and normotensive control rats to evaluate the contribution of sustained elevations in arterial pressure to nociceptive responses. Compared with Sprague-Dawley and/or Wistar-Kyoto controls, SHR were 1) hypoalgesic in the hot plate test and 2) hyperalgesic in longer latency tail and paw-withdrawal tests and in two models of inflammatory nociception. These differences were not observed between SS and salt-resistant controls fed a high-salt diet. Inflammatory hyperalgesia in SHR was correlated with neither paw edema nor the number of Fos-positive spinal cord neurons. Our results indicate that "pain" phenotype of the SHR is not restricted to hypoalgesia. This phenotype is related to genetic factors or to the autonomic systems that control blood pressure and not to sustained elevations in blood pressure, differences in spinal neuron activity, or inflammatory edema.

Effects of unilateral suckling on nursing behavior and c-fos activity in the caudal periaqueductal gray in rats

In rats, suckling elicits kyphosis-the bilaterally symmetrical, upright, humpbacked nursing posture-and maximal expression of the immediate early gene c-fos in a region of the caudal periaqueductal gray (cPAG) that mediates the sensorimotor integration of kyphosis. We determined the effects of prepartum unilateral nipple removal on nursing behavior and c-fos expression during a 60-min mother-litter interaction on Day 7 postpartum. Compared with dams suckled by 6 pups bilaterally, dams suckled unilaterally displayed essentially normal maternal behaviors, including kyphosis. Unilaterally suckled dams, however, showed an increase in the abnormal prone nursing posture, a decrease in proportion of kyphotic nursing of total time over pups, and a 20% higher contralateral/ipsilateral ratio of cPAG neurons expressing c-fos. These results are consistent with an incompletely lateralized neural pathway conveying suckling stimulation to the cPAG and provide a mechanism whereby kyphosis is elicited by unilateral suckling when pups initiate nursing from their supine dam.

Evidence for the involvement of spinal endogenous ATP and P2X receptors in nociceptive responses caused by formalin and capsaicin in mice

1. The aim of the present study is to characterize the role of spinal endogenous ATP and P2X receptors in the generation of neurogenic and inflammatory pain. We examined the effects of intrathecal treatment with P2X receptor antagonists on the formalin- and capsaicin-induced nociceptive behaviours in mice. 2. Intrathecal pretreatment with the general P2 receptor antagonist, pyridoxal-phosphate-6-azophenyl-2', 4'-disulphonic acid (PPADS), significantly suppressed both the first and second phases of the formalin-induced nociceptive behaviour. The second phase of the nociceptive response was also suppressed by intrathecal treatment with PPADS after the first phase. Furthermore, pretreatment with the selective antagonist for the P2X1, P2X3 and P2X2+3 receptors, 2',3'-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate (TNP-ATP), significantly reduced the first phase, but not the second phase. The second phase was also not suppressed by intrathecal TNP-ATP after the first phase. 3. Capsaicin-induced nociceptive behaviour that has been shown to be a model for neurogenic pain, was also significantly suppressed by intrathecal pretreatment with PPADS or TNP-ATP. 4. Nociceptive behaviour in the first phase of the formalin test and in the capsaicin test were significantly inhibited by intrathecal pretreatment with alpha, beta-methylene ATP (alpha,betameATP: 5 microg mouse-1) 15 min prior to injection of formalin or capsaicin. This treatment has been previously shown to desensitize spinal P2X3 receptor subtypes in vivo. 5. These findings suggest that spinal endogenous ATP may play a role in (1) the formalin- and capsaicin-induced neurogenic pain via the PPADS- and TNP-ATP-sensitive P2X receptors which are also desensitized by alpha,betameATP (perhaps the P2X3 receptor subtype) and (2) formalin-induced inflammatory pain via PPADS-sensitive, TNP-ATP- and alpha,betameATP-insensitive P2X (and/or P2Y) receptors.

Opioidergic and adrenergic modulation of formalin-evoked spinal c-fos mRNA expression and nocifensive behavior in the rat

Fos protein expression has been used to reflect neuronal activation in pain processing pathways although analgesics may uncouple behavioral and Fos responses. We determine whether formalin-induced spinal c-fos mRNA expression (Northern blotting) correlates with nocifensive behavior following pretreatment with morphine, the alpha2-adrenoceptor agonist dexmedetomidine, or their respective antagonists naloxone and atipamezole. Both opiate and alpha2-adrenoceptor agonists reduced formalin-induced c-fos gene transcription and nocifensive behavior via their cognate receptors. Unexpectedly, blockade of either the opiate or alpha2-adrenergic receptors, alone, caused an increase in formalin-evoked c-fos mRNA; while blocking the opiate receptor had no effect on formalin-induced behavior, alpha2-adrenoceptor block had an analgesic effect, indicating discordance between c-fos message transcription and nocifensive behavior. We concluded that the formalin-induced spinal c-fos signal was a poor predictor of the behavioral response to pharmacological manipulation of pain processing pathways.

The effect of genotype on sensitivity to inflammatory nociception: characterization of resistant (A/J) and sensitive (C57BL/6J) inbred mouse strains

The important role of genetic factors in the mediation of sensitivity to pain and pain inhibition is being increasingly appreciated. In an attempt to systematically study the genotypic influences on inflammatory nociception, we conducted a survey of the nociceptive responsivity of three common outbred mouse strains and 11 inbred mouse strains on the formalin test. The formalin test is known to display a biphasic temporal pattern of behavioral and electrophysiological activity, defined by an acute/early phase and a tonic/late phase. Nociceptive sensitivity (licking/biting of the affected area) to a subcutaneous injection of 5% formalin (25 microl volume) into the plantar surface of the right hindpaw displayed moderate heritability in both phases (0.38 and 0.46, respectively). One strain, A/J, was identified as extremely resistant to formalin nociception, displaying total licking in the acute and tonic phases that was 60% and 87% lower, respectively, than the grand mean of all strains. A subsequent series of experiments were performed to characterize the difference between A/J and C57BL/6J mice. The findings establish this inbred strain comparison as a useful genetic model of nociceptive sensitivity.