The effect of chronic oral desipramine on capsaicin-induced allodynia and hyperalgesia: a double-blinded, placebo-controlled, crossover study

Human surrogate models of central sensitization: A critical review and practical guide

Background

As in other fields of medicine, development of new medications for management of neuropathic pain has been difficult since preclinical rodent models do not necessarily translate to the clinics. Aside from ongoing pain with burning or shock‐like qualities, neuropathic pain is often characterized by pain hypersensitivity (hyperalgesia and allodynia), most often towards mechanical stimuli, reflecting sensitization of neural transmission.

Data treatment

We therefore performed a systematic literature review (PubMed‐Medline, Cochrane, WoS, ClinicalTrials) and semi‐quantitative meta‐analysis of human pain models that aim to induce central sensitization, and generate hyperalgesia surrounding a real or simulated injury.

Results

From an initial set of 1569 reports, we identified and analysed 269 studies using more than a dozen human models of sensitization. Five of these models (intradermal or topical capsaicin, low‐ or high‐frequency electrical stimulation, thermode‐induced heat‐injury) were found to reliably induce secondary hyperalgesia to pinprick and have been implemented in multiple laboratories. The ability of these models to induce dynamic mechanical allodynia was however substantially lower. The proportion of subjects who developed hypersensitivity was rarely provided, giving rise to significant reporting bias. In four of these models pharmacological profiles allowed to verify similarity to some clinical conditions, and therefore may inform basic research for new drug development.

Conclusions

While there is no single “optimal” model of central sensitization, the range of validated and easy‐to‐use procedures in humans should be able to inform preclinical researchers on helpful potential biomarkers, thereby narrowing the translation gap between basic and clinical data.

Significance

Being able to mimic aspects of pathological pain directly in humans has a huge potential to understand pathophysiology and provide animal research with translatable biomarkers for drug development. One group of human surrogate models has proven to have excellent predictive validity: they respond to clinically active medications and do not respond to clinically inactive medications, including some that worked in animals but failed in the clinics. They should therefore inform basic research for new drug development.

Noninvasive vagus nerve stimulation alters neural response and physiological autonomic tone to noxious thermal challenge

The mechanisms by which noninvasive vagal nerve stimulation (nVNS) affect central and peripheral neural circuits that subserve pain and autonomic physiology are not clear, and thus remain an area of intense investigation. Effects of nVNS vs sham stimulation on subject responses to five noxious thermal stimuli (applied to left lower extremity), were measured in 30 healthy subjects (n = 15 sham and n = 15 nVNS), with fMRI and physiological galvanic skin response (GSR). With repeated noxious thermal stimuli a group × time analysis showed a significantly (p < .001) decreased response with nVNS in bilateral primary and secondary somatosensory cortices (SI and SII), left dorsoposterior insular cortex, bilateral paracentral lobule, bilateral medial dorsal thalamus, right anterior cingulate cortex, and right orbitofrontal cortex. A group × time × GSR analysis showed a significantly decreased response in the nVNS group (p < .0005) bilaterally in SI, lower and mid medullary brainstem, and inferior occipital cortex. Finally, nVNS treatment showed decreased activity in pronociceptive brainstem nuclei (e.g. the reticular nucleus and rostral ventromedial medulla) and key autonomic integration nuclei (e.g. the rostroventrolateral medulla, nucleus ambiguous, and dorsal motor nucleus of the vagus nerve). In aggregate, noninvasive vagal nerve stimulation reduced the physiological response to noxious thermal stimuli and impacted neural circuits important for pain processing and autonomic output.

Effect of single-dose imipramine on chronic low-back and experimental pain. A randomized controlled trial

Antidepressants are frequently prescribed as co-analgesics in chronic pain. While their efficacy is well documented for neuropathic pain, the evidence is less clear in musculoskeletal pain conditions. The present study therefore evaluated the effect of the tricyclic antidepressant imipramine on chronic low-back pain in a randomized, double-blinded placebo-controlled design. To explore the mechanisms of action and the influence of drug metabolism, multimodal quantitative sensory tests (QST) and genotyping for cytochrome P450 2D6 (CYP2D6) were additionally performed. A single oral dose of imipramine 75 mg was compared to active placebo (tolterodine 1 mg) in 50 patients (32 females) with chronic non-specific low-back pain. Intensity of low-back pain was assessed on a 0–10 numeric rating scale at baseline and every 30 minutes after drug intake. Multimodal QST were performed at baseline and in hourly intervals for 2 hours. Pharmacogenetic influences of cytochrome P450 were addressed by CYP2D6 genotyping. No significant analgesic effect was detected neither on low-back pain nor on any of the sensory tests in the overall analyses. However, evidence for an interaction of the imipramine effect and CYP2D6 genotype was found for electrical and for pressure pain detection thresholds. Intermediate but not extensive metabolizers had a 1.20 times greater electrical pain threshold (95%-CI 1.10 to 1.31) and a 1.10 times greater pressure pain threshold (95%-CI 1.01 to 1.21) 60 minutes after imipramine than after placebo (p<0.001 and p = 0.034, respectively). The present study failed to demonstrate an immediate analgesic effect of imipramine on low-back pain. Anti-nociceptive effects as assessed by quantitative sensory tests may depend on CYP2D6 genotype, indicating that metabolizer status should be accounted for when future studies with tricyclic antidepressants are undertaken.

Adverse Effects of Antidepressants for Chronic Pain: A Systematic Review and Meta-analysis

Background

Antidepressants are widely used in the treatment of chronic pain. Applied doses are lower than those needed to unfold an antidepressive effect. While efficacy of antidepressants for chronic pain has been reported in large randomized-controlled trials (RCT), there is inconsistent data on adverse effects and tolerability. We aimed at synthesizing data from RCT to explore adverse effect profiles and tolerability of antidepressants for treatment of chronic pain.

Methods

Systematic literature research and meta-analyses were performed regarding side effects and safety of different antidepressants in the treatment of chronic pain according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. The National Center for Biotechnology Information library and MEDLINE were searched. Randomized placebo-controlled trials were included in quantitative data synthesis.

Results

Out of 1,975 screened articles, 33 papers published between 1995 and 2015 were included in our review and 23 studies were included in the meta-analyses. A higher risk for adverse effects compared to placebo was observed in all antidepressants included in our analyses, except nortriptyline. The most prevalent adverse effects were dry mouth, dizziness, nausea, headache, and constipation. Amitriptyline, mirtazapine, desipramine, venlafaxine, fluoxetine, and nortriptyline showed the highest placebo effect-adjusted risk of adverse effects. Risk for withdrawal due to adverse effects was highest in desipramine (risk ratio: 4.09, 95%-confidence interval [1.31; 12.82]) followed by milnacipran, venlafaxine, and duloxetine. The most common adverse effects under treatment with antidepressants were dry mouth, dizziness, nausea, headache, and constipation followed by palpitations, sweating, and drowsiness. However, overall tolerability was high. Each antidepressant showed distinct risk profiles of adverse effects.

Conclusion

Our synthesized data analysis confirmed overall tolerability of low-dose antidepressants for the treatment of chronic pain and revealed drug specific risk profiles. This encompassing characterization of adverse effect profiles might be useful in defining multimodal treatment regimens for chronic pain which also consider patients’ comorbidities and co-medication.

Effect of Intravenous Alfentanil on Nonpainful Thermally Induced Hyperalgesia in Healthy Volunteers

Experimental interventions that activate specific components of clinical pain are necessary for characterization of underlying mechanisms and pharmacology. Cutaneous hyperalgesia has been described that uses nonpainful heat to induce secondary hyperalgesia. This study evaluated the effect of intravenous alfentanil on experimental cutaneous hyperalgesia created using this method. Eighteen subjects participated in a randomized, double-blinded, placebo-controlled crossover study consisting of 2 sessions, 1 with alfentanil and 1 with placebo. Using a computer-controlled infusion pump, alfentanil or matching placebo was maintained at a constant plasma level of 75 ng/mL for 1 hour followed by the application of a 40°C heat stimulus to the right thenar eminence for 15 minutes. The temperature was raised by 1°C every 15 minutes until the subject reported pain or 45°C was reached. After the end point was reached, the temperature was maintained, and repeat testing was performed. The nonpainful heat created an area of secondary cutaneous hyperalgesia and significant decrease in mechanical pain threshold on heat-treated right vs untreated left during placebo administration. Alfentanil prevented the hypersensitivity when compared to placebo (P < .05) but failed to reduce the area of secondary hyperalgesia created by nonpainful heat when compared to placebo (P = .06). Neither alfentanil nor the heat lamp treatment showed any significant effect on other neurosensory measures. This study demonstrated a reliable production of cutaneous hyperalgesia using a nonpainful stimulus that is affected by the systemic delivery of alfentanil. This model for human cutaneous experimental pain may be a useful method for scientific characterization of analgesics.

A literature review on the pharmacological sensitivity of human evoked hyperalgesia pain models

AIMS

Human evoked pain models can be used to determine the efficacy of new and existing analgesics and to aid in the identification of new targets. Aspects of neuropathic pain can be simulated by inducing hyperalgesia resulting from provoked sensitization. The present literature review aimed to provide insight into the sensitivity of different hyperalgesia and allodynia models of pharmacological treatment.

METHODS

A literature search was performed to identify randomized, double-blind, placebo-controlled studies that included human hyperalgesia pain models and investigated the pharmacodynamic effects of different classes of drugs.

RESULTS

Three hyperalgesia models [ultraviolet B (UVB) irradiation, capsaicin and thermode burn] have been used extensively. Assessment of hyperalgesia/allodynia and pharmacological effect are measured using challenge tests, which generally comprise thermal (heat/cold) or mechanical stimulation (pin-prick, stroking or impact). The UVB model was sensitive to the antihyperalgesic effects of nonsteroidal anti-inflammatory drugs (NSAIDs) and opioids. The capsaicin model was partially sensitive to opioids. The burn model did not detect any antihyperalgesic effects when NSAIDs or local anaesthetics were administered but responded to the effects of N-methyl D-aspartate (NMDA) receptor antagonists by moderately reducing mechanical hyperalgesia.

CONCLUSIONS

Based on pharmacological sensitivity, the UVB model adequately reflects inflammatory pain and was sensitive to NSAIDs and opioids. Findings from the capsaicin and burn models raised questions about the translatability of these models to the treatment of neuropathic pain. There is a need for a reproducible and predictive model of neuropathic pain, either in healthy subjects or in patients.

A Randomized, Double-Blind, Placebo-Controlled, Crossover Study of the T-Type Calcium Channel Blocker ABT-639 in an Intradermal Capsaicin Experimental Pain Model in Healthy Adults

OBJECTIVE

This randomized, double-blind, placebo-controlled, crossover trial evaluated the pharmacodynamic effects of a single 100-mg dose of ABT-639, a peripherally active, selective T-type Ca_v3.2 channel blocker, with the intradermal capsaicin pain model using pregabalin 300 mg as a positive control.

SUBJECTS

Healthy adult males (aged 21 to 55 years) were randomly assigned to receive single oral doses of ABT-639, pregabalin, and placebo.

METHODS

Serial measurements for area (cm²) of hyperalgesia, allodynia, and flare response were performed over a 20-minute period after each capsaicin injection at 1 and 4 hours post-dose. Capsaicin injections were administered in different arms as determined by random assignment. Serial measurements for spontaneous pain and elicited pain were performed over a 60-minute period at 1 and 4 hours post-dose using a 100-mm visual analog scale. Standard safety evaluations were performed.

RESULTS

Nineteen participants were randomized and included in the analysis. No significant differences were observed between ABT-639 and placebo in spontaneous pain, elicited pain, and areas of allodynia, hyperalgesia, and flare after intradermal capsaicin injection at 1 and 4 hours post-dose. In contrast, pregabalin demonstrated significant reductions in spontaneous pain at 1 and 4 hours post-dose, and elicited pain and areas of allodynia and hyperalgesia at 4 hours post-dose compared with placebo. ABT-639 demonstrated acceptable safety and tolerability; somnolence and euphoric mood were the most commonly reported adverse events.

CONCLUSIONS

These data indicate that a single 100-mg dose of ABT-639 had no effect on experimental pain induced by intradermal capsaicin injection.

Skin Matters: A Review of Topical Treatments for Chronic Pain. Part One: Skin Physiology and Delivery Systems

Chronic pain is a complex disorder with multiple etiologies for which the pathologic mechanisms are still largely unknown, making effective treatment a difficult clinical task. Achieving pain relief along with improved function and quality of life is the primary goal of pain clinicians; however, most patients and healthcare professionals consider 30% pain improvement to be clinically significant—a success level that would be unacceptable in other areas of medicine. Furthermore, patients with chronic pain frequently have multiple comorbidities, including depression and sleep apnea, and most have seen several physicians prior to being seen by a pain specialist, have more than three specific pain generators, and are taking multiple medications. The addition of further oral medications to control pain increases the risk of drug–drug interactions and side effects. However, topical analgesics have the advantage of local application with limited systemic levels of drug. Topical therapies benefit from reduced side effects, lower risk of drug–drug interactions, better patient acceptability/compliance, and improved tolerability. This two-part paper is a review of topical analgesics and their potential role in the treatment of chronic pain.

A randomized, double-blind, crossover study to evaluate the depth response relationship of intradermal capsaicin-induced pain and hyperalgesia in healthy adult volunteers

OBJECTIVE

The purpose of this study was to evaluate pain and hyperalgesia in response to different depths of intradermal (ID) capsaicin injections in healthy volunteers.

DESIGN

Double-blind, cross-over study.

SETTING

Clinical Research Laboratory.

SUBJECTS

Fifteen healthy male subjects received ID capsaicin injections into the volar aspect of each forearm at depths of 1 mm, 3 mm, 5 mm, and 7 mm. After injection, spontaneous pain, elicited pain, flare response, heat thresholds, and area of hyperalgesia were measured at various time points.

OUTCOMES MEASURE

Spontaneous pain, elicited pain (pinprick, stroking, and hot pain), hyperalgesia area, and allodynia area.

RESULTS

No significant difference was found between any depths in spontaneous pain, elicited pain (pinprick, stroking, hot pain), hyperalgesia area, or allodynia area. A significant difference was found in the change in heat threshold between 5 mm and 1 mm, 7 mm and 1 mm, 5 mm and 3 mm, 7 mm and 3 mm depths. A significant difference was found in flare area between 5 mm and 3 mm depths. A significant difference was found in systolic blood pressure area under the curve (AUC) between 7 mm and 1 mm depths, and for both systolic and diastolic pressures for 5 mm and 1 mm depths, and 5 mm and 3 mm depths. A significant difference was found in pulse AUC between 5 mm and 1 mm depths and 5 mm and 3 mm depths.

CONCLUSIONS

Injection of capsaicin at different depths in the skin had different effects on heart rate and blood pressure but no effect on pain. These results may have implications on the pharmacology and analgesic predictive value of the model of ID capsaicin.

Evidence for acute central sensitization to prolonged experimental pain in posttraumatic stress disorder

BACKGROUND

Post-traumatic stress disorder (PTSD) and pain have a well-documented high comorbidity; however, the underlying mechanisms of this comorbidity are currently poorly understood. The aim of this psychophysical study was to investigate the behavioral response to a prolonged suprathreshold pain stimulus in subjects with combat-related PTSD and combat controls (CC) for clinical evidence of central sensitization.

METHODS

Ten male subjects with current PTSD related to combat and 11 CC male subjects underwent baseline quantitative sensory testing (QST), temporal pain summation, and psychological profiling followed by an intramuscular injection of capsaicin into the quadriceps muscle.

RESULTS

There was no significant between-group difference for the initial maximal pain response or an initial pain reduction for the first 15 minutes postinjection on QST or pain ratings. However, we observed significantly higher scores in the PTSD group for the second 15 minutes postinjection on both pain intensity and pain unpleasantness ratings. Assessment of temporal summation to repetitive pressure stimuli showed significantly higher subjective pain in the PTSD group.

CONCLUSION

These findings are consistent with a significantly higher degree of acute central sensitization in individuals with PTSD. Increased acute central sensitization may underlie increased vulnerability for developing pain-related conditions following combat trauma.

Evaluation of anti-hyperalgesic and analgesic effects of two benzodiazepines in human experimental pain: a randomized placebo-controlled study

Background and Aims

Compounds that act on GABA-receptors produce anti-hyperalgesia in animal models, but little is known on their effects in humans. The aim of this study was to explore the potential usefulness of GABA-agonism for the control of pain in humans. Two agonists at the benzodiazepine-binding site of GABA_A-receptors (clobazam and clonazepam) were studied using multiple experimental pain tests. Positive results would support further investigation of GABA agonism for the control of clinical pain.

Methods

In a randomized double-blind crossover design, 16 healthy male volunteers received clobazam 20 mg, clonazepam 1 mg and tolterodine 1 mg (active placebo). The area of static hyperalgesia after intradermal capsaicin injection was the primary endpoint. Secondary endpoints were: area of dynamic hyperalgesia, response to von Frey hair stimulation, pressure pain thresholds, conditioned pain modulation, cutaneous and intramuscular electrical pain thresholds (1, 5 and 20 repeated stimulation), and pain during cuff algometry.

Results

For the primary endpoint, an increase in the area of static hyperalgesia was observed after administration of placebo (p<0.001), but not after clobazam and clonazepam. Results suggestive for an anti-hyperalgesic effect of the benzodiazepines were obtained with all three intramuscular pain models and with cuff algometry. No effect could be detected with the other pain models employed.

Conclusions

Collectively, the results are suggestive for a possible anti-hyperalgesic effect of drugs acting at the GABA_A-receptors in humans, particularly in models of secondary hyperalgesia and deep pain. The findings are not conclusive, but support further clinical research on pain modulation by GABAergic drugs. Because of the partial results, future research should focus on compounds acting selectively on subunits of the GABA complex, which may allow the achievement of higher receptor occupancy than unselective drugs. Our data also provide information on the most suitable experimental models for future investigation of GABAergic compounds.

Trial Registration

ClinicalTrials.gov NCT01011036

Effects of monoamine reuptake inhibitors in assays of acute pain-stimulated and pain-depressed behavior in rats

Pain is associated with stimulation of some behaviors (eg, withdrawal reflexes) but depression of many other behaviors (eg, feeding, locomotion, positively reinforced operant behavior). Drugs that block reuptake of serotonin, norepinephrine, and/or dopamine are widely used to treat depression, and they have also emerged as useful drugs for treatment of pain. This study compared effects of selective and mixed-action inhibitors of serotonin, norepinephrine, and/or dopamine reuptake in assays of acute pain-stimulated and pain-depressed behavior. Intraperitoneal injection of dilute acid served as a noxious stimulus to stimulate a writhing response or depress intracranial self-stimulation (ICSS) in Sprague Dawley rats. Selective reuptake inhibitors of serotonin (citalopram, clomipramine) and norepinephrine (nisoxetine, nortriptyline) and a mixed-action reuptake inhibitor of serotonin and norepinephrine (milnacipran) blocked acid-stimulated writhing but failed to block acid-induced depression of ICSS. Selective dopamine reuptake inhibitors (RTI-113 [3ß-(4-chlorophenyl)tropane-2ß-carboxylic acid phenyl ester hydrochloride], bupropion) and a triple reuptake inhibitor of dopamine, serotonin, and norepinephrine (RTI-112 [3ß-(3-methyl-4-chlorophenyl)tropane-2ß-carboxylic acid methyl ester hydrochloride]) blocked both acid-stimulated writhing and acid-induced depression of ICSS, although these drugs also produced an abuse-related facilitation of ICSS in the absence of the noxious stimulus. These results support further consideration of dopamine reuptake inhibitors as candidate analgesics, although abuse liability remains a concern.

PERSPECTIVE

Monoamine reuptake inhibitors are used to treat depression and some forms of pain. This study examined effects of monoamine reuptake inhibitors in a preclinical assay of pain-related behavioral depression. The results support further consideration of dopamine reuptake inhibitors as candidate analgesics under selected circumstances, although abuse liability remains a concern.

NGX426, an oral AMPA-kainate antagonist, is effective in human capsaicin-induced pain and hyperalgesia

BACKGROUND

Non-N-methyl-D-aspartate receptor subtypes modulate neurotransmitter release and mediate fast excitatory postsynaptic potentials. This study evaluated the effects of an oral prodrug to tezampanel, a selective α-amino-3-hydroxy-5-methly-4-isoxazole-proprionic acid/kainate receptor antagonist, on intradermal capsaicin-induced pain and hyperalgesia.

METHODS

This was a randomized, double blind, crossover, placebo-controlled study. Eighteen subjects received 150 or 90 mg NGX426, or placebo, separated by a washout of 6 ± 2 days. In each treatment period, two intradermal injections of capsaicin were given in the volar region of alternate forearms at 30- and 120-minute drug/placebo administration. Spontaneous pain, elicited pain, and area of hyperalgesia were determined at certain time points after each injection. Subjects were asked to rate the painfulness of a 1-minute long 45°C heat stimulus (brief thermal stimulation [BTS]) applied to the anterior thigh at 4 hours and 30 minutes following drug administration, then every 30 minutes through 6 hours following drug administration.

RESULTS

The 150-mg dose produced a statistically definitive reduction in spontaneous pain for all time points relative to placebo. The 90-mg dose produced a statistically significant reduction for the early time point and the entire time interval. Both doses significantly reduced elicited pain at all time points. For the BTS, the 150-mg group reached statistical significance compared with placebo at the 270-minute time point only.

CONCLUSIONS

This study demonstrated that NGX426 reduces capsaicin-induced pain and hyperalgesia in human volunteers with low incidence of side effects that suggests that this class of drug may be effective in the treatment of clinical pain.

Effect of transdermal opioids in experimentally induced superficial, deep and hyperalgesic pain

BACKGROUND AND PURPOSE

Chronic pain and hyperalgesia can be difficult to treat with classical opioids acting predominately at the µ-opioid receptor. Buprenorphine and its active metabolite are believed to act through µ-, κ- and δ-receptors and may therefore possess different analgesic and anti-hyperalgesic effects compared with pure µ-receptor agonists, for example, fentanyl. Here, we have compared the analgesic and anti-hyperalgesic effects of buprenorphine and fentanyl.

EXPERIMENTAL APPROACH

Twenty-two healthy volunteers were randomized to treatment with transdermal buprenorphine (20 µg·h(-1), 144 h), fentanyl (25 µg·h(-1), 72 h) or placebo patches in a double-blind, cross-over experimental pain study. The experimental pain tests (phasic pain, sensitization) involved pressure at the tibial bone, cutaneous electrical and thermal stimulation, intramuscular nerve growth factor, UVB light burn injury model and intradermal capsaicin-induced hyperalgesia. Pain testing was carried out at baseline, 24, 48, 72 and 144 h after application of the drugs.

KEY RESULTS

Compared with placebo, buprenorphine, but not fentanyl, significantly attenuated pressure at the tibial bone as well as pressure pain in the primary hyperalgesic area induced by UVB light The two drugs were equipotent and better than placebo against cutaneous thermal pain stimulation), but failed to show significant analgesic effect to cutaneous electrical stimulation, nerve growth factor-induced muscle soreness and to capsaicin-induced hyperalgesia.

CONCLUSIONS AND IMPLICATIONS

Buprenorphine, but not fentanyl, showed analgesic effects against experimentally induced, bone-associated pain and primary hyperalgesia compared with placebo. These tissue- and modality-differentiated properties may reflect the variable effects of opioid drugs observed in individual patients.

Long-term potentiation in spinal nociceptive pathways as a novel target for pain therapy

Long-term potentiation (LTP) in nociceptive spinal pathways shares several features with hyperalgesia and has been proposed to be a cellular mechanism of pain amplification in acute and chronic pain states. Spinal LTP is typically induced by noxious input and has therefore been hypothesized to contribute to acute postoperative pain and to forms of chronic pain that develop from an initial painful event, peripheral inflammation or neuropathy. Under this assumption, preventing LTP induction may help to prevent the development of exaggerated postoperative pain and reversing established LTP may help to treat patients who have an LTP component to their chronic pain. Spinal LTP is also induced by abrupt opioid withdrawal, making it a possible mechanism of some forms of opioid-induced hyperalgesia. Here, we give an overview of targets for preventing LTP induction and modifying established LTP as identified in animal studies. We discuss which of the various symptoms of human experimental and clinical pain may be manifestations of spinal LTP, review the pharmacology of these possible human LTP manifestations and compare it to the pharmacology of spinal LTP in rodents.

Assessing analgesic actions of opioids by experimental pain models in healthy volunteers - an updated review

AIM

Experimental pain models may help to evaluate the mechanisms of action of analgesics and target the clinical indications for their use. This review addresses how the efficacy of opioids can be assessed in human volunteers using experimental pain models. The drawback with the different study designs is also discussed.

METHOD

A literature search was completed for randomized controlled studies which included human experimental pain models, healthy volunteers and opioids.

RESULTS

Opioids with a strong affinity for the micro-opioid receptor decreased the sensation in a variety of experimental pain modalities, but strong tonic pain was attenuated more than short lasting pain and non-painful sensations. The effects of opioids with weaker affinity for the micro-opioid receptor were detected by a more narrow range of pain models, and the assessment methods needed to be more sensitive.

CONCLUSION

The way the pain is induced, assessed and summarized is very important for the sensitivity of the pain models. This review gives an overview of how different opioids perform in experimental pain models. Generally experimental pain models need to be designed with careful consideration of pharmacological mechanisms and pharmacokinetics of analgesics. This knowledge can aid the decisions needed to be taken when designing experimental pain studies for compounds entering phase 1 clinical trials.

Efficacy of Duloxetine, a Potent and Balanced Serotonergic and Noradrenergic Reuptake Inhibitor, in Inflammatory and Acute Pain Models in Rodents

Duloxetine, a selective but balanced serotonergic and noradrenergic reuptake inhibitor, was evaluated in the acute nociceptive pain models of tail flick and hot plate in mice and in the persistent and/or inflammatory pain models of acetic acid-induced writhing in mice, carrageenan-induced thermal hyperalgesia and mechanical allodynia in rats, and capsaicin-induced mechanical allodynia in rats. In acute pain models, duloxetine had no significant effect on response latency in the mouse tail-flick test but produced modest increases in response latencies in the mouse hot plate test. Morphine produced dose-related analgesic effects in both the mouse tail-flick and hot plate tests. In models of inflammatory and/or persistent pain, duloxetine, morphine, and ibuprofen produced dose-related decreases in acetic acid-induced writhing in mice. Duloxetine, ibuprofen, and gabapentin also produced dose-dependent reversals of both thermal hyperalgesia and mechanical allodynia produced by carrageenan in rats. In addition, both duloxetine and morphine produced a significant reduction of capsaicin-induced mechanical allodynia in rats. Duloxetine and gabapentin were without substantial effect on the Rotorod test in mice, whereas morphine and ibuprofen produced a significant impairment. Our data indicate that duloxetine may be efficacious in the treatment of persistent and/or inflammatory pain states at doses that have modest or no effect on acute nociception or motor performance.

Experimental Human Pain Models: A Review of Standardised Methods for Preclinical Testing of Analgesics

Treatment of pain is one of the major challenges in clinical medicine. However, it is often difficult to evaluate the effect of a treatment, as the many symptoms of the underlying diseases often confound this assessment. Furthermore, as the pain mechanisms in many diseases are poorly understood, the limited successful trial and error approach is most often used in the selection of analgesics. Hence, there is a need for new methods in the characterization and treatment of pain. Human experimental pain models offer the possibility to explore the pain system under controlled settings. The models can also be used to screen the analgesic profiles of drugs targeted to treat pain. This review gives a brief introduction to the methods used to evoke and assess pain in the skin, muscle and viscera. New methods using multimodal stimulation and activation of central pain mechanisms can to a higher degree mimic the clinical situation, and such methods are recommended in the future screening of analgesics. Examples of the use of experimental pain models in the testing of analgesics are given. With these models the therapeutic spectrum may be defined from a differentiated knowledge on the effect of drugs on the pain system. Such information may be used in the future guidelines for trials and clinical use of analgesics.