Alleviation of neuropathic pain by trazodone in rats

Abstract Neuropathic pain is generally characterised by an abnormal sensation (dysesthesia), an increased response to painful stimuli (hyperalgesia), and pain in response to a stimulus that does not normally provoke pain (allodynia). The present study was designed to investigate the effect of trazodone (5mg/kg and 10mg/kg) on peripheral neuropathic pain induced by partial sciatic nerve ligation in rats. Mechanical hyperalgesia, cold allodynia and thermal hyperalgesia were assessed by performing the pinprick, acetone, and hot plate tests, respectively. Biochemically, lipid peroxidation level and total calcium levels were measured. However, trazodone administration (5 and 10 mg/ kg i.p.) for 21days significantly diminished partial sciatic nerve ligation-induced neuropathic pain along with areduction in oxidative stress and calcium levels. The results of the present study suggest that trazodone is effective in attenuating partial sciatic nerve ligation-inducedpainful neuropathic states, which may be attributed to decreased oxidative stress and calcium levels.

The pelvic floor muscle hyperalgesia (PFMH) scoring system: a new classification tool to assess women with chronic pelvic pain: multicentre pilot study of validity and reliability.

OBJECTIVE: The contribution of pelvic floor muscle tenderness to chronic pelvic pain (CPP) is well established in the literature. However pelvic floor muscle hyperalgesia (PFMH) is often missed during vaginal examination of women with CPP. To our knowledge criteria for diagnosing PFMH has not been established or validated so far. The aim of this study is to assess the validity and reliability of the PFMH scoring system. STUDY DESIGN: Women with and without PFMH were recruited prospectively. Digital pelvic examination was performed to detect any pain. All women were asked to report of any discomfort or pain evoked by digital palpation of the PFMs and to rate the severity of pain/discomfort as none (grade 0), mild (grade I) moderate (grade II) or severe (grade III). All women were also asked to describe the severity of the pain/discomfort using a visual analogue scale (VAS). Following examination a PFMH score was given according to each patient's reactions. Intra-observer and inter-observer reliability was assessed. Construct and content validity was also determined. RESULTS: 111 (44 symptomatic and 67 controls) were recruited. Intraobserver reliability had ICCs between 0.426 and 0.804. Interobserver reliability had ICCs between 0.724 and 0.917. There was a good correlation between PFMH scores and VAS scores (rho 0.994, p<0.01). Total scores between symptomatic and controls were significantly different (p<0.01 Mann-Whitney U test). CONCLUSION: The PFMH scoring system is a simple, reliable, valid and easy screening tool for in the assessment of women with CPP.

Development of a bedside pain assessment kit for the classification of patients with osteoarthritis.

There are no standardized bedside assessments for subtyping patients with osteoarthritis (OA) based on pain mechanisms. Thus, we developed a bedside sensory testing kit (BSTK) to classify OA patients based on sensory profiles potentially indicative of pain mechanism. After usability and informal reliability testing (n = 22), the kit was tested in a formal reliability study (n = 20). Patients completed questionnaires and sensory testing: pressure algometry to detect hyperalgesia; repeat algometry after heterotopic noxious conditioning stimulation to measure diffuse noxious inhibitory control (DNIC); light touch using Von Frey filaments; and cold allodynia using a brass rod. The procedure was brief and well tolerated. Algometry and filament testing were highly reliable [intra-class correlation coefficients (ICCs) 0.71-0.91]; DNIC was acceptably reliable (ICCs 0.53-0.91); brass rod reliability was inconclusive. Patients were classified empirically into four groups: "All abnormal findings" (primary and secondary hyperalgesia and dysfunctional DNIC); "all normal findings"; and two intermediate groups. The "all abnormal findings" group had more neuropathic pain symptoms, and lower WOMAC total, stiffness, and activity scores than the "all normal findings" group. Simple BSTK procedures, consolidated in a kit, reliably classified OA patients into subgroups based on sensory profile, suggesting that OA patients differ in underlying pain mechanisms. Further research is needed to confirm these subgroups and determine their validity in predicting response to treatment.

Healthy volunteers can be phenotyped using cutaneous sensitization pain models.

BACKGROUND: Human experimental pain models leading to development of secondary hyperalgesia are used to estimate efficacy of analgesics and antihyperalgesics. The ability to develop an area of secondary hyperalgesia varies substantially between subjects, but little is known about the agreement following repeated measurements. The aim of this study was to determine if the areas of secondary hyperalgesia were consistently robust to be useful for phenotyping subjects, based on their pattern of sensitization by the heat pain models. METHODS: We performed post-hoc analyses of 10 completed healthy volunteer studies (n = 342 [409 repeated measurements]). Three different models were used to induce secondary hyperalgesia to monofilament stimulation: the heat/capsaicin sensitization (H/C), the brief thermal sensitization (BTS), and the burn injury (BI) models. Three studies included both the H/C and BTS models. RESULTS: Within-subject compared to between-subject variability was low, and there was substantial strength of agreement between repeated induction-sessions in most studies. The intraclass correlation coefficient (ICC) improved little with repeated testing beyond two sessions. There was good agreement in categorizing subjects into 'small area' (1(st) quartile [<25%]) and 'large area' (4(th) quartile [>75%]) responders: 56-76% of subjects consistently fell into same 'small-area' or 'large-area' category on two consecutive study days. There was moderate to substantial agreement between the areas of secondary hyperalgesia induced on the same day using the H/C (forearm) and BTS (thigh) models. CONCLUSION: Secondary hyperalgesia induced by experimental heat pain models seem a consistent measure of sensitization in pharmacodynamic and physiological research. The analysis indicates that healthy volunteers can be phenotyped based on their pattern of sensitization by the heat [and heat plus capsaicin] pain models.

Pharmacological and behavioral characterization of the saphenous chronic constriction injury model of neuropathic pain in rats.

The aim of the present study was to develop a new experimental pain model by adapting the chronic constriction injury (CCI) model of the sciatic nerve to the exclusively sensory saphenous nerve in rats. Animals were divided into naïve, sham, and two experimental groups, in which two or four 4-0 chromic gut ligatures were loosely ligated around the saphenous nerve. Then, behavioral signs of neuropathic pain were observed for 8 weeks. In rats with four ligatures, prominent mechanical allodynia and thermal hyperalgesia developed; these behavioral signs were not prominent in rats with two ligatures. Pharmacological analysis was made in rats with four loose ligations; morphine and WIN 55,212-2, a cannabinoid agonist, reversed all of the modalities tested, whereas gabapentin only suppressed mechanical allodynia and amitriptyline only reduced mechanical hyperalgesia. Our data establish a rat model of saphenous CCI with significant allodynia and hyperalgesia, which is sensitive to a number of analgesic compounds.

Spinal 5-HT receptors that contribute to the pain-relieving effects of spinal cord stimulation in a rat model of neuropathy.

Spinal cord stimulation (SCS) is extensively employed in the management of neuropathic pain, but the underlying mechanisms are only partially understood. Recently, we demonstrated that the pain-relieving effect of SCS appears to involve the spinal serotonin system, and the present study aimed at identifying the types of the spinal serotonin receptors involved. Experiments were performed on rats with neuropathy produced by partial ligation of the sciatic nerve. Tactile sensitivity was assessed using von Frey filaments, and cold and heat sensitivity with cold spray and radiant heat, respectively. Selective 5-HT receptor antagonists, methiothepin (5-HT(1,6,7)), ketanserin tartrate (5-HT(2A)), TICM (5-HT(3)), SDZ-205,557 (5-HT(4)), as well as receptor agonists, α-m-5-HT (5-HT(2)), m-CPBG (5-HT(3)) in per se ineffective doses, or vehicle, were administrated intrathecally 5 minutes prior to the application of SCS. Ketanserin and SDZ-205,557 significantly attenuated the suppressive effect of SCS on tactile hypersensitivity, while methiothepin and TICM were ineffective. The suppressive effect on cold hypersensitivity of SCS was counteracted by ketanserin only. None of the 5-HT receptor antagonists attenuated the suppressive effect on heat hyperalgesia of SCS. Subeffective doses of α-m-5-HT and m-CPBG enhanced the suppressive effect of SCS on tactile hypersensitivity. The enhancing effect of m-CPBG was abolished by a γ-aminobutyric acid (GABA)(A) or GABA(B) antagonist intrathecally. These results suggest that the activation of 5-HT(2A), 5-HT(3), and 5-HT(4) receptors plays an important role in SCS-induced relief of neuropathic pain. The activation of 5-HT(3) receptors appears to operate via spinal GABAergic interneurons.

Factor analysis of responses to thermal, electrical, and mechanical painful stimuli supports the importance of multi-modal pain assessment.

During the last decade, a multi-modal approach has been established in human experimental pain research for assessing pain thresholds and responses to various experimental pain modalities. Studies have concluded that differences in responses to pain stimuli are mainly related to variation between individuals rather than variation in response to different stimulus modalities. In a factor analysis of 272 consecutive volunteers (137 men and 135 women) who underwent tests with different experimental pain modalities, it was determined whether responses to different pain modalities represent distinct individual uncorrelated dimensions of pain perception. Volunteers underwent single painful electrical stimulation, repeated painful electrical stimulation (temporal summation), test for reflex receptive field, pressure pain stimulation, heat pain stimulation, cold pain stimulation, and a cold pressor test (ice water test). Five distinct factors were found representing responses to 5 distinct experimental pain modalities: pressure, heat, cold, electrical stimulation, and reflex-receptive fields. Each of the factors explained approximately 8% to 35% of the observed variance, and the 5 factors cumulatively explained 94% of the variance. The correlation between the 5 factors was near null (median ρ=0.00, range -0.03 to 0.05), with 95% confidence intervals for pairwise correlations between 2 factors excluding any relevant correlation. Results were almost similar for analyses stratified according to gender and age. Responses to different experimental pain modalities represent different specific dimensions and should be assessed in combination in future pharmacological and clinical studies to represent the complexity of nociception and pain experience.

Distribution of signs and symptoms of complex regional pain syndrome type I in patients meeting the diagnostic criteria of the International Association for the Study of Pain.

The aim of the present study was to describe the occurrence of signs and symptoms in CRPS I patients meeting the IASP (Orlando) criteria, assess the occurrence of signs and symptoms in relation to disease duration and compare these to historical data based on a different diagnostic criteria set. Six hundred and ninety-two ambulatory patients meeting the IASP criteria for CRPS I referred to the outpatient clinics of five participating centers were included in this cross-sectional study. Characteristics were recorded in a standardized fashion and categorized according to the factor structure proposed by Bruehl/Harden. Subgroups were classified according to the duration of complaints and compared to historical data as described by Veldman et al. The Chi-square test corrected for multiple comparisons was used for statistical analysis. The prevalence of sensory signs was higher in patients with longer disease duration, especially for the allodynia's and hyperalgesia (all p<0.001). Signs in vasomotor (color difference; p=0.0007) and sudomotor (edema; p<0.0001) subgroups were less frequently present in patients with longer disease duration (i.e. >6 months). Prevalences of signs in the motor subgroup were all higher (p<0.0001) in patients with longer disease duration, except for limited range of motion. Occurrence of signs was significantly lower (<0.001) than those reported by Veldman et al., except for hyperesthesia and dystonia. Occurrence rates may vary at different time points after onset of CRPS, which may be of influence for diagnosing patients with novel derived diagnostic criteria. We argue for a mechanism based description of CRPS I based on one set of uniform generally accepted diagnostic criteria in future studies.

Effect of capsaicin treatment on nociceptors in rat glabrous skin one day after plantar incision.

Dilute capsaicin produces a differential effect on incision-related pain behaviors depending upon the test; it reduces heat hyperalgesia and guarding pain but not mechanical hyperalgesia. This suggests that common mechanisms for heat hyperalgesia and guarding pain occur, and distinct mechanisms exist for mechanical hyperalgesia. The purpose of the present study was to evaluate the effect of capsaicin treatment on the activity of cutaneous nociceptors sensitized by incision to understand the mechanisms for the selective action of dilute capsaicin on incisional pain. We compared the effect of 0.05% capsaicin vs. vehicle treatment on pain behaviors after incision and on the activity of nociceptors from these same rats using the in vitro glabrous skin-nerve preparation. Immunohistochemical expression of protein gene product 9.5 (PGP9.5), neurofilament 200, calcitonin gene related peptide (CGRP) and isolectin B4 (IB4) in skin was also evaluated 1 week after 0.05% capsaicin infiltration. Infiltration of 0.05% capsaicin decreased CGRP and IB4/PGP9.5-immunoreactivity of nociceptors in skin. The same dose of capsaicin that inhibited heat hyperalgesia and guarding behavior interfered with chemo- and heat sensitivity of C-fibers. Neither mechanical hyperalgesia nor mechanosensitivity of nociceptors was affected by capsaicin, suggesting that the concentration of capsaicin used in this study did not cause fiber degeneration. These results demonstrate that nociceptors desensitized by capsaicin contribute to heat hyperalgesia and guarding pain after plantar incision. These putative TRPV1-expressing C-fibers are sensitized to heat and acid after incision, and the transduction of heat and chemical stimuli after plantar incision is impaired by dilute capsaicin.

Limited BDNF contributes to the failure of injury to skin afferents to produce a neuropathic pain condition.

Although a large body of evidence has shown that peripheral nerve injury usually induces neuropathic pain, there are also clinical studies demonstrating that injury of the sural nerve, which almost only innervates skin, fails to do so. The underlying mechanism, however, is largely unknown. In the present work, we found that the transection of either the gastrocnemius-soleus (GS) nerve innervating skeletal muscle or tibial nerve supplying both muscle and skin, but not of the sural nerve produced a lasting mechanical allodynia and thermal hyperalgesia in adult rats. High-frequency stimulation (HFS) or injury of either the tibial nerve or the GS nerve induced late-phase long-term potentiation (L-LTP) of C-fiber-evoked field potentials in spinal dorsal horn, while HFS or injury of the sural nerve only induced early-phase LTP (E-LTP). Furthermore, HFS of the tibial nerve induced L-LTP of C-fiber responses evoked by the stimulation of the sural nerve and the heterotopic L-LTP was completely prevented by spinal application of TrkB-Fc (a BDNF scavenger). Spinal application of low dose BDNF (10pg/ml) enabled HFS of the sural nerve to produce homotopic L-LTP. Finally, we found that injury of the GS nerve but not that of the sural nerve up-regulated BDNF in DRG neurons, and that the up-regulation of BDNF occurred not only in injured neurons but also in many uninjured ones. Therefore, the sural nerve injury failing to produce neuropathic pain may be due to the nerve containing insufficient BDNF under both physiological and pathological conditions.

Acetazolamide and midazolam act synergistically to inhibit neuropathic pain.

Treatment of neuropathic pain is a major clinical challenge that has been met with minimal success. After peripheral nerve injury, a decrease in the expression of the K-Cl cotransporter KCC2, a major neuronal Cl(-) extruder, leads to pathologic alterations in GABA(A) and glycine receptor function in the spinal cord. The down-regulation of KCC2 is expected to cause a reduction in Cl(-) extrusion capacity in dorsal horn neurons, which, together with the depolarizing efflux of HCO(3)(-) anions via GABA(A) channels, would result in a decrease in the efficacy of GABA(A)-mediated inhibition. Carbonic anhydrases (CA) facilitate intracellular HCO(3)(-) generation and hence, we hypothesized that inhibition of CAs would enhance the efficacy of GABAergic inhibition in the context of neuropathic pain. Despite the decrease in KCC2 expression, spinal administration of benzodiazepines has been shown to be anti-allodynic in neuropathic conditions. Thus, we also hypothesized that spinal inhibition of CAs might enhance the anti-allodynic effects of spinally administered benzodiazepines. Here, we show that inhibition of spinal CA activity with acetazolamide (ACT) reduces neuropathic allodynia. Moreover, we demonstrate that spinal co-administration of ACT and midazolam (MZL) act synergistically to reduce neuropathic allodynia after peripheral nerve injury. These findings indicate that the combined use of CA inhibitors and benzodiazepines may be effective in the clinical management of neuropathic pain in humans.

An anti-nociceptive role for ceftriaxone in chronic neuropathic pain in rats.

Glial glutamate transporter-1 (GLT-1) plays an essential role in the maintenance of glutamate homeostasis and is involved in the development and maintenance of pathological pain. The present study was undertaken (1) to observe the anti-nociceptive effects of ceftriaxone (Cef) in a chronic neuropathic pain model induced by chronic constrictive nerve injury (CCI) of the sciatic nerve and (2) to identify the role of spinal GLT-1 in the process. CCI induced significant thermal hyperalgesia and mechanical allodynia, which began from postoperative day 3 and lasted to day 21. This long-term hyperalgesia was accompanied by significant down-regulation of GLT-1 expression in the L4-L6 segments of the spinal dorsal horn, as revealed by immunohistochemistry and Western blot. Intraperitoneal preventive and therapeutic administration of Cef effectively prevented or reversed, respectively, the development of thermal hyperalgesia, mechanical allodynia, and GLT-1 down-regulation in the spinal dorsal horn. To further determine whether the above anti-nociceptive effects of Cef are a result of the up-regulation of spinal GLT-1 expression and its function, we further observed the effects of intrathecal administration of Cef in the same model. It was found that intrathecal administration of Cef led to the specific up-regulation of GLT-1 expression and glutamate uptake ((3)H-glutamate) in the spinal dorsal horn, and similar anti-nociceptive effects to those of intraperitoneal administration of Cef. The above effects of intrathecal Cef administration were all significantly inhibited by intrathecal administration of GLT-1 antisense oligodeoxynucleotides (As-ODNs). These results indicate that Cef plays an anti-nociceptive role by up-regulating spinal GLT-1 expression and its function.

Nociceptive sensitization by complement C5a and C3a in mouse.

Activation of the complement system by injury increases inflammation by producing complement fragments C5a and C3a which are able to recruit and activate immune cells. Complement activation may contribute to pain after inflammation and injury. In this study, we examined whether C5a and C3a elicit nociception when injected into mouse hind paws in vivo, and whether C5a and C3a activate and/or sensitize mechanosensitive nociceptors when applied on peripheral terminals in vitro. We also examined the dorsal root ganglia (DRG) for C5a receptor (C5aR) mRNA and effects of C5a and C3a on intracellular Ca(2+) concentration ([Ca(2+)](i)) using Ca(2+) imaging. Heat hyperalgesia was elicited by intraplantar injection of C5a, and mechanical hyperalgesia by C5a and C3a. After exposure to either C5a or C3a, C-nociceptors were sensitized to heat as evidenced by an increased proportion of heat responsive fibers, lowered response threshold to heat and increased action potentials during and after heat stimulation. A-nociceptors were activated by complement. However, no change was observed in mechanical responses of A- and C-nociceptors after C5a and C3a application. The presence of C5aR mRNA was detected in DRG. C5a and C3a application elevated [Ca(2+)](i) and facilitated capsaicin-induced [Ca(2+)](i) responses in DRG neurons. The results suggest a potential role for complement fragments C5a and C3a in nociception by activating and sensitizing cutaneous nociceptors.

In vivo silencing of the Ca(V)3.2 T-type calcium channels in sensory neurons alleviates hyperalgesia in rats with streptozocin-induced diabetic neuropathy.

Earlier, we showed that streptozocin (STZ)-induced type 1 diabetes in rats leads to the development of painful peripheral diabetic neuropathy (PDN) manifested as thermal hyperalgesia and mechanical allodynia accompanied by significant enhancement of T-type calcium currents (T-currents) and cellular excitability in medium-sized dorsal root ganglion (DRG) neurons. Here, we studied the in vivo and in vitro effects of gene-silencing therapy specific for the Ca(V)3.2 isoform of T-channels, on thermal and mechanical hypersensitivities, and T-current expression in small- and medium-sized DRG neurons of STZ-treated rats. We found that silencing of the T-channel Ca(V)3.2 isoform using antisense oligonucleotides, had a profound and selective anti-hyperalgesic effect in diabetic rats and is accompanied by significant down-regulation of T-currents in DRG neurons. Anti-hyperalgesic effects of Ca(V)3.2 antisense oligonucleotides in diabetic rats were similar in models of rapid and slow onset of hyperglycemia following intravenous and intraperitoneal injections of STZ, respectively. Furthermore, treatments of diabetic rats with daily insulin injections reversed T-current alterations in DRG neurons in parallel with reversal of thermal and mechanical hypersensitivities in vivo. This confirms that Ca(V)3.2 T-channels, important signal amplifiers in peripheral sensory neurons, may contribute to the cellular hyperexcitability that ultimately leads to the development of painful PDN.

The c-kit signaling pathway is involved in the development of persistent pain.

Protein kinase signal transduction pathways play critical roles in regulating nociception. Here we show that c-kit, a tyrosine kinase receptor, is expressed in lamina I and II layer of the dorsal horn. Moreover, the superficial c-kit(+) fibers originate from the dorsal root ganglion, and c-kit in lamina II inner layer comes from intrinsic expression of the spinal cord. Kit(W-v) mice, which contain a hypomorphic mutation, exhibited normal acute pain in most pain behavior tests. In the formalin test, the first phase was not affected, whereas the second phase pain response of Kit(W-v) mice was significantly reduced relative to wild-type littermates. Kit(W-v) mice also showed abnormal neuropathic pain, notably in the contralateral side of nerve injury. The expression and release of CGRP and substance P were not altered by the c-kit mutation. Together, these results implicate c-kit-mediated signal transduction in the development of persistent pain.

The effects of thalidomide and minocycline on taxol-induced hyperalgesia in rats.

Chemotherapy-induced pain is the most common treatment-limiting complication encountered by cancer patients receiving taxane-, vinca alkaloid- or platin-based chemotherapy. Several lines of evidence indicate that activation of pro-inflammatory cascades involving the release of cytokines including tumor necrosis factor-alpha (TNF-alpha), interleukin-1 beta (IL-1beta) and interleukin-6 (IL-6) as well as various growth factors are key events in the pathogenesis of many types of nerve-injury related pain. Similar mechanisms might also be involved in the etiology of chemotherapy-induced pain. Thalidomide and minocycline have profound immunomodulatory actions in addition to their originally intended pharmacological actions. These compounds were evaluated here for effects in preventing the development of taxol-induced mechanical and thermal hyperalgesia in rats. Thalidomide (50.0 mg/kg) reduced taxol-induced mechanical allodynia and hyperalgesia whereas minocycline (20.0 mg/kg) reduced taxol-induced mechanical hyperalgesia and allodynia as well as taxol-induced thermal hyperalgesia. These results suggest that immunomodulatory agents may provide a treatment option for the protection or reversal of chemotherapy-related pain.

Transient receptor potential ankyrin-1 participates in visceral hyperalgesia following experimental colitis.

Transient receptor potential ankyrin-1 (TRPA1) is an important receptor that contributes to inflammatory pain. However, previous studies were mainly concerned with its function in somatic hyperalgesia while few referred to visceral, especially colonic inflammatory hyperalgesia. The present study was aimed to investigate the role of TRPA1 in visceral hyperalgesia after trinitrobenzene sulfonic acid (TNBS)-induced colitis. Results indicate that TNBS induced a significant increase in visceral sensitivity to colonic distension and chemical irritation accompanied by up-regulation of TRPA1 in colonic afferent dorsal root ganglia (DRG). Intrathecal administration of TRPA1 antisense (AS) oligodeoxynucleotide (ODN) reduced the TRPA1 expression in DRG as well as suppressed the colitis-induced hyperalgesia to nociceptive colonic distension and intracolonic allyl isothiocyanate (AITC). Meanwhile the TRPA1 antisense ODN had no effect on transient receptor potential vanilloid-1 (TRPV1) expression, which was proposed to highly co-express with TRPA1, and no effect on the response to TRPV1 agonist, capsaicin. These data suggest an apparent role of TRPA1 in visceral hyperalgesia following colitis that might provide a novel therapeutic target for the relief of pain.

Quantitative sensory testing with Von Frey monofilaments in patients with allodynia: what are we quantifying?

The International Association for the Study of Pain defines allodynia as pain due to a stimulus that does not normally provoke pain and hyperalgesia as an increased response to a stimulus, which is normally painful. However, does "normally painful" mean "any stimulation of nociceptors" or "the subjective pain response?" We argue that "normally painful" should not mean "any stimulation of nociceptors," as Von Frey monofilaments may evoke a pricking sensation--which implies the involvement of nociceptors--without necessarily leading to a subjective pain perception. In this paper, we propose that the diagnosis of either allodynia or hyperalgesia should be based on the patient's report, that is, painful versus not painful, rather than on the (sub) type of afferent fiber involved.