Vasomotor response to cold stimulation in human capsaicin-induced hyperalgesic area

Cooling the skin induces sympathetically driven vasoconstriction, with some vasoparalytic dilatation at the lowest temperatures. Neurogenic inflammation, on the other hand, entails vasodilatation. In this study we investigated the balance between vasoconstriction and vasodilatation in an area of experimentally induced secondary hyperalgesia (2 degrees HA), in response to low-temperature stimulations. Fourteen healthy volunteers were exposed to three 30-s long cold stimuli (20, 10, and 0 degrees C) applied, at three adjacent sites, before (baseline) and 8 min after intradermal injection of 50 microg capsaicin to the volar forearm. The cold stimuli were applied distally to the injection site within the 2 degrees HA. Blood flux (BF) and skin temperatures were measured at four different regions (proximally, and distally to the capsaicin injection and at the 0, 10, and 20 degrees C thermode sites) all within the 2 degrees HA. The vascular measurements were conducted five times. Results showed a marked increase in BF after baseline cold stimulation (P<0.001) at the 0 degrees C compared with the three other sites. In addition, vasodilatory effect (elevated BF) was found following the capsaicin injection compared with baseline for all regions (P<0.001): the non-cooled area was dilated by 450+/-5.1%; The vasoconstrictive effect for the 10 and 20 degrees C did not overcome the capsaicin vasodilatation, but did reduce it, with dilatation of 364+/-7.0% and 329+/-7.3%, respectively. For 0 degrees C, a dilatation of 407+/-6.5% was seen. It is concluded that in this experimental model, and potentially in the equivalent clinical syndromes, vasodilatation induced by the inflammation is only slightly reduced by cold stimulation such that it is still dominant, despite some cold-induced vasoconstriction.

Modulation of capsaicin-evoked visceral pain and referred hyperalgesia by protease-activated receptors 1 and 2

Protease-activated receptors (PARs) 1 and 2 are expressed in capsaicin-sensitive sensory neurons, being anti- and pro-nociceptive, respectively. Given the possible cross talk between PAR-2 and capsaicin receptors, we investigated if PAR-2 activation could facilitate capsaicin-evoked visceral pain and referred hyperalgesia in the mouse and also examined the effect of PAR-1 activation in this model. Intracolonic (i.col.) administration of capsaicin triggered visceral pain-related nociceptive behavior, followed by referred hyperalgesia. The capsaicin-evoked visceral nociception was suppressed by intraperitoneal (i.p.) TFLLR-NH2, a PAR-1-activating peptide, but not FTLLR-NH2, a control peptide, and unaffected by i.col. TFLLR-NH2. SLIGRL-NH2, a PAR-2-activating peptide, but not LRGILS-NH2, a control peptide, administered i.col., facilitated the capsaicin-evoked visceral nociception 6-18 h after administration, while i.p. SLIGRL-NH2 had no effect. The capsaicin-evoked referred hyperalgesia was augmented by i.col. SLIGRL-NH2, but not LRGILS-NH2, 6-18 h after administration, and unaffected by i.p. SLIGRL-NH2, and i.p. or i.col. TFLLR-NH2. Our data suggest that PAR-1 is antinociceptive in processing of visceral pain, whereas PAR-2 expressed in the colonic luminal surface, upon activation, produces delayed sensitization of capsaicin receptors, resulting in facilitation of visceral pain and referred hyperalgesia.

Physiological and behavioral evidence for focal nociception induced by epidural glutamate infusion in rats

STUDY DESIGN

Blinded animal study.

OBJECTIVES

To determine if an increased concentration of epidural glutamate can cause a focal nociceptive response in the lower extremities that is consistent with sciatica.

SUMMARY OF BACKGROUND DATA

It is believed that the origin of sciatic pain is related to more than physical pressure on the nerve roots. Recently, it was determined that disc material may be a significant source of free glutamate, resulting from the enzymatic degradation of matrix aggrecan proteins. We believe that this free glutamate acts as a neurotransmitter at glutamate receptors on the dorsal root ganglion (DRG) cell bodies, thereby initiating a nociceptive response.

METHODS

Rats were subject to a 72-hour epidural glutamate infusion via a mini osmotic pump. Von Frey behavioral testing was performed 24 hours before, and 24 and 72 hours after the onset of the infusion. DRG and dorsal horn tissues were analyzed for changes in receptor expression, which have been previously shown to correlate with a nociceptive state.

RESULTS

Von Frey behavioral tests showed focal hyperalgesia that was maximal at the 0.02 mmol/L glutamate concentration. Significant changes in DRG glutamate receptor expression were seen for alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid, kainite, and N-methyl-D aspartate receptors. Analysis of dorsal horn glutamate receptors also showed patterns in alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid and kainate receptor expression that were consistent with a nociceptive state.

CONCLUSIONS

Epidural glutamate elicits a focal nociceptive response. Free glutamate that has been liberated from the disc material may be an important factor in the development of sciatic pain.

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.

Modulation of central hypersensitivity by nociceptive input in chronic pain after whiplash injury

OBJECTIVE

Chronic pain after whiplash injury is associated with hypersensitivity of the central nervous system to peripheral stimulation. It is unclear whether central hypersensitivity is modulated by peripheral nociceptive input. We hypothesized that changes in nociceptive input would correlate with changes in magnitude of central hypersensitivity.

DESIGN

Fifteen patients with chronic pain after whiplash injury were investigated. Changes in nociceptive input were induced by infiltration of painful and tender muscles with bupivacaine (0.25%). Such infiltrations produce either pain reduction or pain enhancement, the latter effect probably resulting from transient injection-induced trauma. We used this individual variability in correlation analyses. Changes in intensity of neck pain, as assessed by a visual analog scale (VAS), after infiltration were assumed to reflect changes in nociceptive input. Changes in pressure pain thresholds recorded at healthy tissues (nonpainful point of the neck and the second toe) were used to measure changes in central hypersensitivity. The correlations between the change in VAS score and changes in pressure pain thresholds 15 minutes after infiltration were analyzed.

RESULTS

Statistically significant negative correlations were found between change in VAS score and changes in threshold measurements performed at the neck, but not at the toe.

CONCLUSIONS

Different mechanisms underlie hyperalgesia localized at areas surrounding the site of pain and hyperalgesia generalized to distant body areas. Central hypersensitivity as a determinant of neck pain is probably a dynamic condition that is influenced by the presence and activity of a nociceptive focus.

NGF and GDNF differentially regulate TRPV1 expression that contributes to development of inflammatory thermal hyperalgesia

The transient receptor potential ion channel, TRPV1 plays an essential role in the development of inflammatory thermal hyperalgesia. We investigated the dependence of inflammatory TRPV1 induction on neurotrophic factor. Rat dorsal root ganglia (DRG) neurons were classified according to immunostaining for trk-A and IB4 and the effects of antibodies against NGF or GDNF on TRPV1 expression within the groups were then analysed by immunohistochemical means. The data were compared with the time course of trophic factor expression and the effects of their antibodies on thermal hyperalgesia against radiant heat after inflammation. Although the levels of both NGF and GDNF were increased by inflammation, NGF rapidly and transiently increased whereas GDNF increased gradually over a period of approximately one week. TRPV1 expression was increased within both trk-A positive and IB4 positive neurons after inflammation. Increased TRPV1 expression within trk-A positive neurons was prevented by anti-NGF but not by anti-GDNF, whereas TRPV1 induction within the IB4 positive group was blocked by anti-GDNF but not by anti-NGF. Both antibodies prevented the short latency of withdrawing an inflamed paw from radiant heat. These results suggest that inflammation differentially increases both NGF and GDNF, which facilitate TRPV1 expression within distinctive neurons to induce thermal hyperalgesia.

Anti-allodynic and anti-hyperalgesic effects of nociceptin receptor antagonist, JTC-801, in rats after spinal nerve injury and inflammation

The effects of nociceptin/orphanin FQ (N/OFQ) peptide receptor antagonist JTC-801 on allodynia and hyperalgesia were examined in rats in order to explore the involvement of N/OFQ system in these pathological pain states. Tactile allodynia induced by L5/L6 spinal nerve ligation was reversed by both systemic (3-30 mg/kg) and spinal (22.5 and 45 pg) JTC-801 in a dose-dependent manner. Concerning hyperalgesia induced by formalin injection into the hindpaw, JTC-801 dose-dependently suppressed the second phase, but not the first phase, of the licking behavior. Furthermore, systemic JTC-801 reduced Fos-like immunoreactivity in the dorsal horn of the spinal cord (laminae I/II). In conclusion, N/OFQ receptor antagonist JTC-801 exerted anti-allodynic and anti-hyperalgesic effects in rats, suggesting that N/OFQ system might be involved in the modulation of neuropathic pain and inflammatory hyperalgesia.

Widespread Sensory Hypersensitivity Is a Feature of Chronic Whiplash-Associated Disorder but not Chronic Idiopathic Neck Pain

OBJECTIVES

To investigate sensory changes present in patients with chronic whiplash-associated disorders and chronic idiopathic neck pain using a variety of quantitative sensory tests to better understand the pain processing mechanisms underlying persistent symptoms.

METHODS

A case control study was used with 29 subjects with chronic whiplash-associated disorders, 20 subjects with chronic idiopathic neck pain, and 20 pain-free volunteers. Pressure pain thresholds were measured over the articular pillars of C2-C3, C5-C6, the median, radial, and ulnar nerve trunks in the arm and over a remote site, the muscle belly of tibialis anterior. Heat pain thresholds, cold pain thresholds, and von Frey hair sensibility were measured over the cervical spine, tibialis anterior, and deltoid insertion. Anxiety was measured with the Short-Form of the Spielberger State Anxiety Inventory.

RESULTS

Pressure pain thresholds were decreased over cervical spine sites in both subject groups when compared with controls (P < 0.05). In the chronic whiplash-associated disorders group, pressure pain thresholds were also decreased over the tibialis anterior, median, and radial nerve trunks (P < 0.001). Heat pain thresholds were decreased and cold pain thresholds increased at all sites (P < 0.03). No differences in heat pain thresholds or cold pain thresholds were evident in the idiopathic neck pain group at any site compared with the control group (P > 0.27). No abnormalities in von Frey hair sensibility were evident in either neck pain group (P > 0.28).

DISCUSSION

Both chronic whiplash-associated disorders and idiopathic neck pain groups were characterized by mechanical hyperalgesia over the cervical spine. Whiplash subjects showed additional widespread hypersensitivity to mechanical pressure and thermal stimuli, which was independent of state anxiety and may represent changes in central pain processing mechanisms. This may have implications for future treatment approaches.

Association of striatal dopamine D2/D3 receptor binding potential with pain but not tactile sensitivity or placebo analgesia

Striatal dopamine D2/D3 receptors have been suggested to play a role in pain sensitivity and placebo effect. We studied whether the association of dopamine D2/D3 receptor binding potential (BP) with sensory thresholds is specific to the modality of pain, and whether striatal dopamine D2/D3 receptor BP predicts the magnitude of placebo analgesia. Pain and tactile thresholds, and placebo analgesia were assessed in eight healthy human male subjects who had previously participated in a dopamine D2/D3 receptor positron emission tomography study with [11C]raclopride. The results show that the cutaneous heat pain threshold was inversely correlated with dopamine D2/D3 receptor BP in the right putamen, but responses to tactile stimulation did not correlate with striatal dopamine D2/D3 receptor BP. Placebo-induced elevation of the heat pain threshold did not correlate with striatal dopamine D2/D3 receptor BP. These results suggest that the influence of striatal dopamine D2/D3 receptors on sensory thresholds is selective for the modality of pain. Moreover, striatal dopamine D2/D3 receptor BP appears not to predict individual's analgesic response to placebo.

Effects of α1- and α2-adrenoreceptor antagonists on cold allodynia in a rat tail model of neuropathic pain

Systemic administrations (0.1, 0.5, and 2 mg/kg) of alpha1-adrenoreceptor (AR) antagonist prazosin dose-dependently attenuated cold allodynia in a rat tail model of neuropathic pain, whereas alpha2-AR antagonist yohimbine exacerbated it. These results suggest that the functions of alpha1- and alpha2-AR in this model are excitatory and inhibitory, respectively, consistent with their general properties. It is also proposed that cold allodynia can be reversed by alpha1-AR antagonist and exacerbated by alpha2-AR antagonist.

[Pathophysiology of radicular pain]

Pathophysiological mechanisms of low back pain and radicular pain produced by lumbar disorders are still controversial. We have made experimental animal models and have elucidated the mechanisms of hyperalgesia, which is a pain related behavior, in these animals. We have not demonstrated that only mechanical compression of the nerve roots in the lumbar spine results in hyperalgesia in rat. We found that application of the nucleus pulposus to the nerve roots produces time-dependent reversible hyperalgesia in the affected hindpaw of the rat, and that the hyperalgesia is related to bioactive substances in the arachidonic acid cascade. We have also reported that inflammatory granulation tissue around the nerve root is related to hyperalgesia rather than the nucleus pulposus itself, and that mechanical compression to the nerve root after application of the nucleus pulposus produces different type of hyperalgesia. Collectively, not only mechanical compression of the cauda equina and nerve root but also various inflammatory substances are related to pathophysiological mechanisms of radicular pain in patients with lumbar disorders such as disc herniation. Elucidating pathophysiological mechanisms of pain-related behaviors such as hyperalgesia is important to establish a new strategy for the treatment of lumbar painful radiculopathy.

Modulation of peripheral inflammation in sensory ganglia by nuclear factor (kappa)B decoy oligodeoxynucleotide: involvement of SRC kinase pathway

Nuclear factor kappa B (NF(kappa)B) transcription factor plays a key role in the expression of many genes involved in the inflammatory process. We used the Freund's Complete Adjuvant (FCA)-induced model of peripheral inflammation to investigate the anti-inflammatory effects of double stranded oligodeoxynucleotides (ODN) with consensus NF(kappa)B sequence as transcription factor decoys to inhibit NF(kappa)kappaB activation in the dorsal root ganglia (DRG). Local administration of the wild-type-, but not mutant-ODN decoy, dose-dependently inhibited edema formation and paw withdrawal latency as a measure of hyperalgesic response induced by FCA in rat paw. Biochemical assays performed in ipsilateral L4/L5 dorsal root ganglia obtained following FCA/wild-type ODN treatment showed: (1) an inhibition of the activity of c-Src kinase, a member of the non-receptor tyrosine kinase super family, (2) a decreased level of p65 NF(kappa)B subunit, and (3) an inhibition of cyclooxygenase-2 (COX-2) protein expression, a major pro-inflammatory enzyme transcriptionally controlled by NF(kappa)B. The present results indicate that the wild-type ODN decoy may act as a competitor for NF(kappa)B binding to its cognate recognition sequence as well as a modulator of c-Src activity in the DRG. The NF(kappa)B/c-Src interaction may represent a novel pathway for further exploring the molecular mechanism of inflammatory pain.

Study of the involvement of K+ channels in the peripheral antinociception of the kappa-opioid receptor agonist bremazocine

The involvement of the nitric oxide (NO)/cyclic GMP pathway in the molecular mechanisms of antinociceptive drugs like morphine has been previously shown by our group. Additionally, it is known that the desensitisation of nociceptors by K(+) channel opening should be the final target for several analgesic drugs including nitric oxide donors and exogenous micro-opioid receptor agonists. In our previous study, we demonstrated that bremazocine, a kappa-opioid receptor agonist, induces peripheral antinociception by activating nitric oxide/cyclic GMP pathway. In the current study, we assessed whether bremazocine is capable to activate K(+) channels eliciting antinociception. Bremazocine (20, 40 and 50 microg) dose-dependently reversed the hyperalgesia induced in the rat paw by local injection of carrageenan (250 microg) or prostaglandin E(2) (2 microg), measured by the paw pressure test. Using the selective kappa-opioid receptor antagonist nor-binaltorphimine (Nor-BNI, 200 microg/paw), it was confirmed that bremazocine (50 microg/paw) acts specifically on the kappa-opioid receptors present at peripheral sites. Prior treatment with the ATP-sensitive K(+) channel blockers glibenclamide (40, 80 and 160 microg) and tolbutamide (40, 80 and 160 microg) did not antagonise the antinociceptive effect of bremazocine (50 microg). The same results were obtained when we used prostaglandin E(2) (2 microg) as the hyperalgesic stimulus. The supposed participation of other types of K(+) channels was tested using the Ca(2+)-activated K(+) channel blockers dequalinium (12.5, 25 and 50 microg) and charybdotoxin (0.5, 1 and 2 microg) and different types of the non-selective K(+) channel blockers tetraethylammonium (25, 50 and 100 microg) and 4-aminopyridine (10, 25 and 50 microg). None of the K(+) channel blockers reversed the antinociceptive effect of bremazocine. On the basis of these results, we suggest that K(+) channels are not involved in the peripheral antinociceptive effect of bremazocine, although this opioid receptor agonist induces nitric oxide/cGMP pathway activation.

Abrupt Onset and Termination of Cutaneous Allodynia (Central Sensitization) During Attacks of SUNCT

The presence of central sensitization and cutaneous allodynia has not been readily studied in other primary headache syndromes outside of migraine. If central sensitization does occur, is the temporal profile any different in the short-lasting more aggressive syndromes such as SUNCT than in migraine? A patient with SUNCT was examined during and in between attacks looking for the presence and duration of cutaneous allodynia.

Benzodiazepine system is involved in hyperalgesia in rats induced by the exposure to extremely low frequency magnetic fields

Many reports demonstrate that extremely low frequency magnetic fields (ELF MFs, 60 Hz) may be involved in hyperalgesia. In a previous investigation, we suggested that MFs may produce hyperalgesia and such a response may be regulated by the benzodiazepine system. In order to further confirm this effect of MFs, we used diazepam and/or flumazenil with MFs exposure. When testing the pain threshold of rats using hot plate tests, MFs or diazepam (0.5 microg, i.c.v.; a benzodiazepine receptor agonist) induced hyperalgesic effects with the reduction of latency. These effects were blocked by a pretreatment of flumazenil (1.5 mg/kg, i.p.; a benzodiazepine receptor antagonist). When the rats were exposed simultaneously to MFs and diazepam, the latency tended to decrease without statistical significance. The induction of hyperalgesia by co-exposure to MFs and diazepam was also blocked by flumazenil. However, the pretreatment of GABA receptor antagonists such as bicuculline (0.1 microg, i.c.v.; a GABA(A) antagonist) or phaclofen (10 microg, i.c.v.; a GABA(B) antagonist) did not antagonize the hyperalgesic effect of MFs. These results suggest that the benzodiazepine system may be involved in MFs-induced hyperalgesia.

A cascade of cytokines mediates mechanical inflammatory hypernociception in mice

The hypernociceptive effects of cytokines [TNF-alpha, keratinocyte-derived chemokine (KC), and IL-1beta] and their participation in carrageenan (Cg)-induced inflammatory hypernociception in mice were investigated. Nociceptor sensitization (hypernociception) was quantified with an electronic version of the von Frey filament test in WT and TNF receptor type 1 knockout mice (TNF-R1-/-). TNF-alpha-induced hypernociception was abolished in TNF-R1-/- mice, partially inhibited by pretreatment with IL-1 receptor antagonist (IL-1ra) or indomethacin and unaffected by Ab against KC (AbKC) or guanethidine. IL-1ra and indomethacin pretreatment strongly inhibited the hypernociception induced by IL-1beta, which was not altered by AbKC or guanethidine or by knocking out TNF-R1. KC-induced hypernociception was abolished by AbKC, inhibited by pretreatment with indomethacin plus guanethidine, and partially inhibited by IL-1ra, indomethacin, or guanethidine. In contrast, KC-induced hypernociception was not altered by knocking out TNF-R1. Cg-induced hypernociception was abolished by administration of indomethacin plus guanethidine, diminished in TNF-R1-/- mice, and partially inhibited in WT mice pretreated with AbKC, IL-1ra, indomethacin, or guanethidine. TNF-alpha, KC, and IL-1beta concentrations were elevated in the skin of Cg-injected paws. The TNF-alpha and KC concentrations rose concomitantly and peaked before that of IL-1beta. In mice, the cytokine cascade begins with the release of TNF-alpha (acting on TNF-R1 receptor) and KC, which stimulate the release of IL-1beta. As in rats, the final mediators of this cascade were prostaglandins released by IL-1beta and sympathetic amines released by KC. These results extend to mice the concept that the release of primary mediators responsible for hypernociception is preceded by a cascade of cytokines.

Stepwise motor and all-or-none sensory recovery is associated with nonlinear sparing after incremental spinal cord injury in rats

Spinal cord injury (SCI) causes motor and sensory deficits that impair functional performance. While more functional recovery occurs with greater white matter sparing (WMS), it is unclear which locomotor features are more vulnerable to SCI than others, if recovery of certain features depends on specific amounts of WMS, and whether motor recovery patterns differ from sensory recovery. Locomotor and sensory recovery after graded contusive SCI with cord displacements of 0.3, 0.5, 0.7, 0.9, 1.1, 1.25, and 1.3 mm was examined for 6 weeks in 80 female Sprague-Dawley rats. Seven SCI gradations resulted in three locomotor performance levels measured with BBB (P < 0.01): High: laminectomy (LAM) controls and 0.3 (19.87 +/- 0.35 SEM); Intermediate: 0.5-0.9 (13.71 +/- 0.32); and Low: 1.1-1.3 (9.23 +/- 0.36). Normal paw position was most susceptible to SCI requiring 90% WMS, while consistent plantar stepping was least susceptible depending on 10% WMS. A threshold at the 0.9 severity for coordination, toe clearance, and nearly normal trunk stability and tail usage required 25% WMS. Analysis of interlimb coordination using new phase dispersion (PD) techniques delineated three recovery patterns: synchronous (0.3), modified concordance (0.5, 0.7), and disengaged (0.9, 1.1). Lesion severity correlated to WMS (r(2) = 0.96) and to BBB (r(2) = 0.87) by nonlinear polynomial regressions. Mechanical allodynia developed only after injuries resulting in < or =10% WMS. Nonlinear motor and sensory recovery patterns suggest that small reparative changes may substantially improve function in individuals with SCI. A hierarchical locomotor recovery based on simple segmental versus complex supraspinal motor control is proposed.

Stability of neuropathic pain symptoms in partial sciatic nerve ligation in rats is affected by suture material

Many factors affect the development of neuropathic pain behavior in animal models. In this letter, we describe the differences in the development of neuropathic pain behavior observed when the partial sciatic nerve ligation (PNL) is performed with either a synthetic silk or chromic catgut ligation. To characterize nociceptive changes over time after surgery, neutral plate, hot plate, Von Frey, pinprick, acetone spray and cold plate testing was performed. The results indicated that a chromic catgut ligature caused cold allodynia, chemical hyperreactivity, mechanical hyperalgesia and hypersensitivity that remained present for the entire 56 days post-surgical observation period. With the synthetic silk ligature, comparable functional deficits were present in the initial phase after surgery, but several of these deficits diminished over time 21-28 days post-surgery. In conclusion, performing the PNL using chromic catgut suture thread gives rise to more robust sensory deficits than when synthetic silk is used. Therefore, the material that is used for the ligature in the partial sciatic ligation model has an effect on the outcome of the observed sensory abnormalities.

The monosodium iodoacetate model of osteoarthritis: a model of chronic nociceptive pain in rats?

Osteoarthritis (OA) is a widespread condition affecting the elderly population. One of the most prominent features but least studied symptoms is chronic pain associated with OA. The study objective was to determine pain endpoints in rats with monosodium iodoacetate (MIA) induced OA, and to investigate the efficacy of common nociceptive agents. Sprague-Dawley rats received an intraarticular injection of either 25 microl 80 mg/ml MIA or 25 microl 0.9% sterile saline into the right knee joint. Changes in von Frey thresholds and latencies to stroking with a cotton bud (punctate and dynamic allodynia, respectively) were measured pre- and for up to 10 weeks post-intraarticular injection. Changes in hind paw weight distribution were also determined. Both punctate allodynia and a weight bearing deficit were observed in MIA-treated rats for up to 10 weeks. Interestingly, dynamic allodynia was not detected at any time point tested. Morphine (0.3-3 mg/kg, s.c.) and tramadol (3-100 mg/kg, p.o.) dose-dependently inhibited punctate allodynia and partially reversed weight bearing deficit. In conclusion, the MIA model of OA is reproducible and mimics OA pain in humans. Analgesic drug studies indicate this model may be useful for investigating chronic nociceptive pain.

Muscular mechanical hyperalgesia revealed by behavioural pain test and c-Fos expression in the spinal dorsal horn after eccentric contraction in rats

Delayed onset muscle soreness (DOMS) is quite common, but the mechanism for this phenomenon is still not understood; even the existence of muscle tenderness (mechanical hyperalgesia) has not been demonstrated in experimental models. We developed an animal model of DOMS by inducing eccentric contraction (lengthening contraction, ECC) to the extensor digitorum longus muscle (EDL), and investigated the existence of mechanical hyperalgesia in the EDL by means of behavioural pain tests (Randall-Selitto test and von Frey hair test, applied to/through the skin on the EDL muscle) and c-Fos expression in the spinal dorsal horn. We found that the mechanical withdrawal threshold measured with the Randall-Selitto apparatus decreased significantly between 1 and 3 days after ECC, while that measured by von Frey hairs did not. The group that underwent stretching of the muscle only (SHAM group) showed no change in mechanical pain threshold in either test. These results demonstrated that the pain threshold of deep tissues (possibly of the muscle) decreased after ECC. c-Fos immunoreactivity in the dorsal horn (examined 2 days after ECC/SHAM exercise) was not changed by either ECC or compression (1568 mN) to the EDL muscle by itself, but it was significantly increased by applying compression to the EDL muscle 2 days after ECC. This increase was observed in the superficial dorsal horn of the L4 segment of the ipsilateral side, and was clearly suppressed by morphine treatment (10 mg kg(-1), i.p.). These results demonstrated the existence of mechanical hyperalgesia in the muscle subjected to ECC. This model could be used for future study of the neural mechanism of muscle soreness.

Spinal blockade of TNF blocks spinal nerve ligation-induced increases in spinal P-p38

Spinal nerve ligation (SNL) results in a profound long lasting allodynia and increases in phosphorylated p38 in dorsal root ganglia (DRG) neurons and spinal cord microglia. We have previously shown that systemic etanercept, a tumor necrosis factor (TNF) antagonist, reduced allodynia by 42% and blocked SNL-induced increases in P-p38 levels in the L5 and L6 DRG, but not in the ipsilateral lumbar spinal cord. The present experiments demonstrated that intrathecal etanercept (100 microg) prevents SNL-induced increased levels of spinal P-p38. Pretreatment, but not posttreatment, with intrathecal etanercept (100 microg), given every third day, reduced mechanical allodynia by 50%. This therapeutic benefit was maintained for at least 7 days after cessation of treatment. Combined systemic and intrathecal administration of etanercept was no more effective than intrathecal treatment alone. These data imply that TNF provides the trigger for phosphorylation of p38 in both DRG neurons and spinal microglia.

Amitriptyline produces multiple influences on the peripheral enhancement of nociception by P2X receptors

Peripherally administered amitriptyline exhibits potential to be a locally active analgesic, while ATP augments peripheral nociception by interacting with P2X(3) receptors on sensory afferents. The present study examined the effects of amitriptyline on flinching and biting/licking behaviours and thermal hyperalgesia produced by alphabeta-methylene-ATP (alphabeta-MeATP), a ligand for P2X(3) receptors, following intraplantar administration into the hindpaw of rats. Coadministration of low doses of amitriptyline (up to 100 nmol) with alphabeta-MeATP augmented thermal hyperalgesia and flinching behaviours. The most active dose of amitriptyline (100 nmol) had no intrinsic effect. Augmentation of alphabeta-MeATP actions appears to be due to increased tissue levels of biogenic amines resulting from inhibition of uptake, as phentolamine (alpha(1)/alpha(2)-adrenergic receptor antagonist) and methysergide (5-hydroxytryptamine or 5-HT(1)/5-HT(2) receptor antagonist) inhibit the augmented flinching produced by alphabeta-MeATP/amitriptyline. When noradrenaline and 5-HT were coadministered with alphabeta-MeATP (both increase the effect of alphabeta-MeATP), amitriptyline had no effect on flinching produced by alphabeta-MeATP/noradrenaline but inhibited flinching produced by alphabeta-MeATP/5-HT. In the presence of low concentrations of formalin (0.5%, 1%; which also increase the effect alphabeta-MeATP), amitriptyline inhibited augmented behaviours. Higher doses of amitriptyline (300-1000 nmol) increased thermal thresholds, suppressed thermal hyperalgesia produced by alphabeta-MeATP, and inhibited flinching produced by alphabeta-MeATP. Collectively, these results indicate that amitriptyline produces complex influences on peripheral pain signaling by P2X receptors. Lower doses augment nociception by alphabeta-MeATP (probably by inhibiting noradrenaline and 5-HT uptake) but inhibit alphabeta-MeATP responses in the presence of inflammatory mediators (perhaps reflecting receptor blocking properties); higher doses uniformly inhibit nociception by alphabeta-MeATP (perhaps reflecting local anesthetic properties).

Unilateral spinal nerve ligation leads to an asymmetrical distribution of mast cells in the thalamus of female but not male mice

Mast cells are restricted to the leptomeninges and thalamus of healthy mice. These populations are increased by stress and highly sensitive to reproductive hormones. To examine the influence of nociception, a form of stress, on thalamic mast cells, we ligated the left fifth lumbar spinal nerve of male and female mice to induce hyperalgesia. Two, 7 and 14 days later, mice were killed and thalami examined histologically using toluidine blue stain. The total number of thalamic mast cells was not influenced by ligation of the spinal nerve compared to sham-operation in either female or male mice. However, in females, the percent of thalamic mast cells located on the side of the thalamus contralateral to the ligation was greater on days 2 and 7, coincident with mechanical hyperalgesia. At these times, areas in which mast cells were most dense contralateral to nerve-injury included the posterior (Po) and lateral geniculate (LG) nuclei compared to their symmetrical distribution in sham-operated mice. These data suggest that local nociceptive signals to each side of the thalamus rather than stress hormones influence the location of mast cells during the development of allodynia and hyperalgesia. In addition, both hyperalgesia and mast cell distribution induced by nerve-ligation differ in females compared to males, reflecting a novel neuroimmune response to pain within the CNS.

Calcitonin gene-related peptide receptor activation produces PKA- and PKC-dependent mechanical hyperalgesia and central sensitization

Calcitonin gene-related peptide (CGRP), acting through CGRP receptors, produces behavioral signs of mechanical hyperalgesia in rats and sensitization of wide dynamic range (WDR) neurons in the spinal cord dorsal horn. Although involvement of CGRP receptors in central sensitization has been confirmed, the second-messenger systems activated by CGRP receptor stimulation and involved in pain transmission are not clear. This study tested whether the hyperalgesia and sensitizing effects of CGRP receptor activation on WDR neurons are mediated by protein kinase A or C (PKA or PKC) signaling. Intrathecal injection of CGRP in rats produced mechanical hyperalgesia, as shown by paw withdrawal threshold tests. CGRP-induced hyperalgesia was attenuated significantly by the CGRP1 receptor antagonist, CGRP8-37. The effect was also attenuated significantly by a PKA inhibitor (H89) or a PKC inhibitor (chelerythrine chloride). Electrophysiological experiments demonstrated that superfusion of the spinal cord with CGRP-induced sensitization of spinal dorsal horn neurons. The CGRP effect could be blocked by CGRP8-37. Either a PKA or PKC inhibitor (H89 or chelerythrine) also attenuated this effect of CGRP. These results are consistent with the hypothesis that CGRP produces hyperalgesia by a direct action on CGRP1 receptors in the spinal cord dorsal horn and suggest that the effects of CGRP are mediated by both PKA and PKC second-messenger pathways.

Neurotrophin-3 suppresses thermal hyperalgesia associated with neuropathic pain and attenuates transient receptor potential vanilloid receptor-1 expression in adult sensory neurons

Neurotrophin-3 (NT-3) negatively modulates nerve growth factor (NGF) receptor expression and associated nociceptive phenotype in intact neurons, suggesting a beneficial role in treating aspects of neuropathic pain mediated by NGF. We report that NT-3 is effective at suppressing thermal hyperalgesia associated with chronic constriction injury (CCI); however, NT-3 does not alter the mechanical hypersensitivity that also develops with CCI. Thermal hyperalgesia is critically linked to expression and activation of the capsaicin receptor, transient receptor potential vanilloid receptor-1 (TRPV1). Thus, its modulation by NT-3 after CCI was examined. CCI results in elevated TRPV1 expression at both the mRNA and protein levels in predominantly small-to-medium neurons, with the percentage of neurons expressing TRPV1 remaining unchanged at approximately 56%. Attenuation of thermal hyperalgesia mediated by NT-3 correlates with decreased TRPV1 expression such that only approximately 26% of neurons ipsilateral to CCI expressed detectable TRPV1 mRNA. NT-3 effected a decrease in expression of the activated component of the signaling pathway linked to regulation of TRPV1 expression, phospho-p38 MAPK (Ji et al., 2002), in neurons ipsilateral to CCI. Exogenous NT-3 could both prevent the onset of thermal hyperalgesia and reverse established thermal hyperalgesia and elevated TRPV1 expression 1 week after CCI. Continuous infusion is required for suppression of both thermal hyperalgesia and TRPV1 expression, because removal of NT-3 resulted in a prompt reestablishment of the hyperalgesic state and corresponding CCI-associated TRPV1 phenotype. In conclusion, although NGF drives inflammation-associated thermal hyperalgesia via its regulation of TRPV1 expression, NT-3 is now identified as a potent negative modulator of this state.