Antidepressants and pain

Anxiolytics may promote locomotor function recovery in spinal cord injury patients

Recent findings in animal models of paraplegia suggest that specific nonbenzodiazepine anxiolytics may temporarily restore locomotor functions after spinal cord injury (SCI). Experiments using in vitro models have revealed, indeed, that selective serotonin receptor (5-HTR) ligands such as 5-HTR(1A) agonists, known as relatively safe anxiolytics, can acutely elicit episodes of rhythmic neuronal activity refered to as fictive locomotion in isolated spinal cord preparations. Along the same line, in vivo studies have recently shown that this subclass of anxiolytics can induce, shortly after systemic administration (eg, orally or subcutaneously), some locomotor-like hindlimb movements during 45-60 minutes in completely spinal cord-transected (Tx) rodents. Using 'knock-out' mice (eg, 5-HTR(7)-/-) and selective antagonists, it has been clearly established that both 5-HTR(1A) and 5-HTR(7) were critically involved in mediating the pro-locomotor effects induced by 8-OH-DPAT (typically referred to as a 5-HTR(1A) agonist) in Tx animals. Taken together, these in vitro and in vivo data strongly support the idea that 5-HTR(1A) agonists may eventually become constitutive elements of a novel first-in-class combinatorial treatment aimed at periodically inducing short episodes of treadmill stepping in SCI patients.

Usefulness of antidepressants for improving the neuropathic pain-like state and pain-induced anxiety through actions at different brain sites

Clinically, it is well known that chronic pain induces depression, anxiety, and a reduced quality of life. There have been many reports on the relationship between pain and emotion. We previously reported that chronic pain induced anxiety with changes in opioidergic function in the central nervous system. In this study, we evaluated the anxiolytic-like effects of several types of antidepressants under a chronic neuropathic pain-like state and searched for the brain site of action where antidepressants show anxiolytic or antinociceptive effects. Sciatic nerve-ligated mice exhibited thermal hyperalgesia and tactile allodynia from days 7 to 28 after nerve ligation. At 4 weeks after ligation, these mice showed a significant anxiety-related behavior in the light-dark test and the elevated plus-maze test. Under these conditions, repeated administration of antidepressants, including the tricyclic antidepressant (TCA) imipramine, the serotonin noradrenaline reuptake inhibitor (SNRI) milnacipran, and the selective serotonin reuptake inhibitor (SSRI) paroxetine, significantly prevented the anxiety-related behaviors induced by chronic neuropathic pain. These antidepressants also produced a significant reduction in thermal hyperalgesia and tactile allodynia. Moreover, the microinjection of paroxetine into the basolateral amygdala or cingulate cortex reduced anxiety-related behavior, and microinjection into the primary somatosensory cortex significantly attenuated thermal hyperalgesia. These findings suggest that serotonergic antidepressants are effective for treating anxiety associated with chronic neuropathic pain and may be useful for treating neuropathic pain with emotional dysfunction such as anxiety. Furthermore, SSRIs show anxiolytic and antinociceptive effects by acting on different brain regions.

Is there any evidence to support the use of anti-depressants in painful rheumatological conditions? Systematic review of pharmacological and clinical studies

The aim of this study was to review the evidence supporting the use of anti-depressants in painful rheumatological conditions. A systematic review of papers published between 1966 and 2007, in five European languages, on anti-depressants in rheumatological conditions was performed. Papers were scored using Jadad method and analgesic ES was calculated. We selected 78 clinical studies and 12 meta-analyses, from 140 papers. The strongest evidence of an analgesic effect of anti-depressants has been obtained for fibromyalgia. A weak analgesic effect is observed for chronic low back pain, with an efficacy level close to that of analgesics. In RA and AS, there is no analgesic effect of anti-depressants, but these drugs may help to manage fatigue and sleep disorders. There is no clear evidence of an analgesic effect inOA, but studies have poor methodological quality. Analgesic effects of anti-depressants are independent of their anti-depressant effects. Tricyclic anti-depressants (TCAs), even at low doses, have analgesic effects equivalent to those of serotonin and noradrenalin reuptake inhibitors (SNRIs), but are less well tolerated. Selective serotonin reuptake inhibitors (SSRIs) have modest analgesic effects, but higher doses are required to achieve analgesia. Anti-depressant drugs, particularly TCAs and SNRIs, have analgesic effects in chronic rheumatic painful states in which analgesics and NSAIDs are not very efficient, such as fibromyalgia and chronic low back pain. In inflammatory rheumatic diseases, anti-depressants may be useful for managing fatigue and sleep disorders. Further studies are required to compare anti-depressants with other analgesics in the management of chronic painful rheumatological conditions.

Pharmacological treatments of fibromyalgia: do complex conditions need complex therapies?

Fibromyalgia (FM) is a chronic pain condition, with auxiliary symptoms, such as sleep disturbances and fatigue. Although many of the mechanisms of action targeted by the drugs used to treat FM have been focused to the management of single symptoms, drugs (e.g. pregabalin, duloxetine) have now been identified that demonstrate a multidimensional effect. However, such drugs often fail to demonstrate acceptable efficacy in the majority of the patient population. Thus, the mechanisms of action of the drugs studied as treatments for FM are either identifying subgroups within the pathophysiology of the condition or suggesting that a mechanism of action that will offer universal efficacy has, as yet, to be identified.

Stage of chronicity and treatment response in patients with musculoskeletal injuries and concurrent symptoms of depression ☆

The present study examined the relation between stage of chronicity and treatment response in patients with work-related musculoskeletal conditions and concurrent depressive symptoms. Also of interest was the role of reductions in pain severity, catastrophic thinking and fear of movement/re-injury as mediators of the relation between chronicity and treatment response. A sample of 80 individuals (38 women, 42 men) with a disabling musculoskeletal pain condition and concurrent depressive symptoms participated in the research. Individuals with work absence of less than 6 months (range 12-26 weeks) were classified as early chronic (N=40), and individuals with work absence greater than 6 months (range 27-52 weeks) were classified as chronic. Both groups were matched on sex, age (+/-2 years) and severity of depressive symptoms. All participants were enrolled in a 10-week community-based disability management intervention. The early chronic group showed significantly greater reduction in depressive symptoms, and pain symptoms, than the chronic group. Regression analyses revealed that pain reduction, but not catastrophic thinking or fear of movement/re-injury, mediated the relation between chronicity and improvement in depressive symptoms. The results highlight the importance of early detection and treatment of depressive symptoms, given that treatment response decreases over time. The results also suggest that reductions in depressive symptoms might be a precondition to the effective reduction of pain symptoms in this population. Discussion addresses the factors that might contribute to treatment resistance as the period of disability extends over time.

Adverse drug interactions involving common prescription and over-the-counter analgesic agents

BACKGROUND

Eight analgesic preparations with approved indications for acute pain were among the top 200 drugs prescribed in the United States in 2006. In addition, an estimated 36 million Americans use over-the-counter (OTC) analgesics daily. Given this volume of use, it is not surprising that a number of drug interactions involving analgesic drugs have been reported.

OBJECTIVES

This article examines the pharmacologic factors that enhance the clinical relevance of potential drug interactions and reviews the literature on drug interactions involving the most commonly used analgesic preparations in the United States.

METHODS

A PubMed search was conducted for English-language articles published between January 1967 and July 2007. Among the search terms were drug interactions, acetaminophen, aspirin, ibuprofen, naproxen, celecoxib, NSAIDs, hydrocodone, oxycodone, codeine, tramadol, OTC analgesics, alcohol, ethanol, antihypertensive drugs, methotrexate, warfarin, SSRIs, paroxetine, fluoxetine, sertraline, citalopram, serotonin syndrome, MAOIs, and overdose. Controlled clinical trials, case-control studies, and case reports were included in the review.

RESULTS

A number of case reports and well-controlled clinical trials were identified that provided evidence of the relatively well known drug-drug interactions between prescription/OTC NSAIDs and alcohol, antihypertensive drugs, high-dose methotrexate, and lithium, as well as between frequently prescribed narcotics and other central nervous system depressants. In contrast, the ability of recent alcohol ingestion to exacerbate the hepatotoxic potential of therapeutic doses of acetaminophen is not supported by either case reports or clinical research. Use of ibuprofen according to OTC guidelines in patients taking cardioprotective doses of aspirin does not appear to interfere with aspirin's antiplatelet activity, whereas chronic prescription use of ibuprofen and other NSAIDs may interfere. Low-dose aspirin intake appears to abolish the gastroprotective effects of cyclooxygenase-2-selective inhibitors, including celecoxib. There is evidence of other less well known and potentially clinically significant drug-drug interactions, including the ability of selective serotonin reuptake inhibitors to inhibit the analgesic activity of tramadol and codeine through inhibition of their metabolic activation, to induce serotonin syndrome when used chronically in the presence of high doses of tramadol through synergistic serotonergic action, and to increase the potential for gastrointestinal bleeding associated with NSAID therapy through additive or supra-additive antiplatelet activity.

CONCLUSIONS

Considering the widespread use of analgesic agents, the overall incidence of serious drug-drug interactions involving these agents has been relatively low. The most serious interactions usually involved other interacting drugs with low therapeutic indices or chronic and/or high-dose use of an analgesic and the interacting drug.

Exacerbated mechanical allodynia in rats with depression-like behavior

Although a clinical connection between pain and depression has long been recognized, how these two conditions interact remains unclear. Here we report that both mechanical allodynia and depression-like behavior were significantly exacerbated after peripheral nerve injury in Wistar-Kyoto (WKY) rats, a genetic variation of Wistar rats with demonstrable depression-like behavior. Administration of melatonin into the anterior cingular cortex contralateral to peripheral nerve injury prevented the exacerbation of mechanical allodynia with a concurrent improvement of depression-like behavior in WKY rats. Moreover, there was a lower plasma melatonin concentration and a lower melatonin receptor expression in the anterior cingular cortex in WKY rats than in Wistar rats. These results suggest that there exists a reciprocal relationship between mechanical allodynia and depression-like behavior and the melatoninergic system in the anterior cingular cortex might play an important role in the interaction between pain and depression.

Antidepressants for non-specific low back pain

BACKGROUND

Antidepressants are commonly used in the management of low-back pain. However, their use is controversial.

OBJECTIVES

The aim of this review was to determine whether antidepressants are more effective than placebo for the treatment of non-specific low-back pain.

SEARCH STRATEGY

Randomised controlled trials were identified from MEDLINE and EMBASE (to September 2007), PsycINFO to June 2006, the Cochrane Central Register of Controlled Trials 2006, issue 2, and previous systematic reviews.

SELECTION CRITERIA

We included randomised controlled trials that compared antidepressant medication and placebo for patients with non-specific low-back pain, and used at least one clinically relevant outcome measure.

DATA COLLECTION AND ANALYSIS

Two blinded review authors independently extracted data and assessed the methodological quality of the trials. Meta-analyses were used to examine the effect of antidepressants on pain, depression and function, and the effect of antidepressant type on pain. To account for studies that could not be pooled, additional qualitative analyses were performed using the levels of evidence recommended by the Cochrane Back Review Group.

MAIN RESULTS

Ten trials that compared antidepressants with placebo were included in this review. The pooled analyses showed no difference in pain relief (six trials; standardized mean difference (SMD) -0.06 (95% confidence interval (CI) -0.28 to 0.16)) or depression (two trials; SMD 0.06 (95% CI -0.29 to 0.40)) between antidepressant and placebo treatments. The qualitative analyses found conflicting evidence on the effect of antidepressants on pain intensity in chronic low-back pain, and no clear evidence that antidepressants reduce depression in chronic low-back pain patients. Two pooled analyses showed no difference in pain relief between different types of antidepressants and placebo. Our findings were not altered by the sensitivity analyses which varied the level of methodological quality required for inclusion in the meta-analyses to allow data from additional trials to be examined. Two additional trials were identified in September 2007 and await assessment.

AUTHORS' CONCLUSIONS

There is no clear evidence that antidepressants are more effective than placebo in the management of patients with chronic low-back pain. These findings do not imply that severely depressed patients with back pain should not be treated with antidepressants; furthermore, there is evidence for their use in other forms of chronic pain.

Antidepressants for the treatment of chronic pain

Chronic pain represents one of the most important public health problems and, in addition to classical analgesics, antidepressants are an essential part of the therapeutic strategy. This article reviews available evidence on the efficacy and safety of antidepressants in major chronic pain conditions; namely, neuropathic pain, headaches, low back pain, fibromyalgia, irritable bowel syndrome (IBS) and cancer pain. Studies, reviews and meta-analyses published from 1991 to March 2008 were retrieved through MEDLINE, PsycINFO and the Cochrane database using numerous key words for pain and antidepressants. In summary, evidence supports the use of tricyclic antidepressants in neuropathic pain, headaches, low back pain, fibromyalgia and IBS. The efficacy of the newer serotonin and norepinephrine reuptake inhibitors is less supported by evidence, but can be recommended in neuropathic pain, migraines and fibromyalgia. To date, evidence does not support an analgesic effect of serotonin reuptake inhibitors, but beneficial effects on well-being were reported in several chronic pain conditions. These results are discussed in the light of current insights in the neurobiology of pain, the reciprocal relationship between pain and depression, and future developments in this field of research.

Patients' Representations of Antidepressants: A Clue to Nonadherence?

OBJECTIVES

Antidepressants are prescribed frequently to chronic pain patients due to their pain relief effects. This medication raises major adherence issues. Despite the adverse effects, little is known about the factors that may jeopardize adherence in chronic pain patients. We carried out a qualitative study to investigate chronic pain patients' representations of antidepressants as compared with pain-free controls.

METHODS

One hundred thirteen chronic pain patients recruited in a multidisciplinary pain clinic and 62 matched controls were questioned with standardized semistructured interviews. The interviews were submitted to content analysis.

RESULTS

Ambivalence emerged as an important aspect of "patients" and controls' views about antidepressants. Antidepressants were described as potent chemicals acting in the brain, possibly causing effects on cognition, emotions, and personality, and inducing dependence and loss of control. Positive effects were mentioned, but when respondents related their own views and experiences, the statements became less favorable. Another key point was that neither the representations of the patients and nor those of the controls comprised the analgesic properties of antidepressants.

DISCUSSION

Chronic pain patients' representations differed only little from those of controls. Antidepressants were not considered as addressing somatic problems. Thus, the prescription of antidepressants for chronic pain may be mistaken for a denial of the "reality" of pain. Although this study did not assess medication adherence, it is possible that patient representations have a bearing on adherence. Clinically, this suggests that these representations should be elicited and addressed, taking into account the patients' own models of pain.

Role of spinal serotonin 5-HT2A receptor in 2',3'-dideoxycytidine-induced neuropathic pain in the rat and the mouse

Several lines of evidence suggest that descending serotoninergic facilitatory pathways are involved in neuropathic pain. These pathways may involve 5-HT2A receptors known to play a role in spinal and peripheral sensitization. The implication of this receptor in neuropathy was investigated in a model of peripheral neuropathy induced by 2',3'-dideoxycytidine, a nucleoside analogue with reverse transcriptase inhibitory properties used in HIV/AIDS therapy. Four days after a single 100mg/kg i.v. administration in the tail vein, mitochondrial alterations in nociceptive and non-nociceptive dorsal root ganglion cells were observed at the lumbar level. These alterations were not associated with TUNEL labelling or with modification of the total number of dorsal root ganglion cells. At the same time point, 5-HT2A receptor immunolabelling was increased throughout the dorsal horn (by 49.5% in layer II and 57.8% in layer III). The number of 5-HT2A receptor immunoreactive neurons in the dorsal root ganglion was also increased by 30.7%. Four days after 2',3'-dideoxycytidine administration, rats had developed thermal allodynia as well as mechanical hyperalgesia and allodynia, which dose-dependently decreased after epidural injection of MDL 11,939, a 5-HT2A receptor antagonist. Moreover, 5-HT2A receptor knock-out mice did not develop 2',3'-dideoxycytidine-induced neuropathy whereas their control littermates displayed a neuropathy comparable to that observed in rats. Our data show that 2',3'-dideoxycytidine-induced neuropathy is associated with alterations of nociceptive and non-nociceptive peripheral cells and that the 5-HT2A receptor is involved in the peripheral sensitization of nociceptors as well as in a wide central sensitization of dorsal horn neurons.

Treatment of fibromyalgia and its symptoms

The main symptoms of fibromyalgia syndrome (FM) are pain, stiffness, subjective weakness and muscle fatigue. Pain in FM usually fluctuates, as well as being 'deep' and is always associated with local or generalized tenderness (hyperalgesia and allodynia). The pathogenesis of such peripheral and/or CNS changes in FM is unclear, but peripheral tissue changes, specifically in muscles, have been implicated. Indirect evidence from interventions that attenuate tonic peripheral impulse input in patients with FM suggest that overall FM pain is dependent on nociception. More importantly, FM-associated widespread mechanical hyperalgesia and allodynia can also be improved or abolished by removal of peripheral pain impulse input. In addition, FM patients show evidence of abnormal stress reactivity, including blunting of the hypothalamic-pituitary-adrenal axis and increased autonomic nervous system responsiveness. Thus, therapeutic interventions in FM should target not only pain reductions, but also improvements of peripheral/central sensitization and neuroendocrine/autonomic abnormalities. Despite the complexity of FM, there are pharmacologic and non-pharmacologic interventions that are available that have clinical benefit. Present evidence indicates efficacy of antidepressants, cardiovascular exercise and cognitive behavioral therapy. Based on this evidence, a stepwise program emphasizing education, medications, exercise and cognitive therapy can be recommended.

Role of serotonin 5-HT1A and opioid receptors in the antiallodynic effect of tramadol in the chronic constriction injury model of neuropathic pain in rats

RATIONALE

Tramadol (1RS, 2RS)-2-[(dimethylamino)-methyl]-1-(3-methoxyphenyl)-cyclohexanol) is an atypical centrally acting analgesic agent with weak opioid receptor affinity that, like some antidepressants, enhances the extraneuronal concentrations of the monoamine neurotransmitters, noradrenaline and serotonin, by interfering with their re-uptake and release mechanisms.

OBJECTIVES

The present study was undertaken to evaluate the potential role of 5-HT(1A) receptors and opioids receptors in the analgesic effect of tramadol in neuropathic pain. With this aim, the effect of either a selective 5-HT(1A) receptor antagonist (WAY-100635, N-2-[4-(2-methoxyphenyl-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexane carboxamide) or a selective 5-HT(1A) receptor agonist (8-OH-DPAT, 8-hydroxy-2-(di-n-propylamine) tetralin hydrobromide) or an opioid receptor antagonist (naloxone; naloxone hydrochloride dihydrate) was investigated in combination with tramadol by means of the cold-plate test in the chronic constriction injury model in rats.

RESULTS

The results showed that WAY-100635 (0.8 mg/kg) significantly enhanced the antiallodynic effect of non-effective doses of tramadol (5-10 mg/kg). In contrast, 8-OH-DPAT (0.5 mg/kg) counteracted the antiallodynic effect of an effective dose of tramadol (22 mg/kg). Naloxone (0.5 mg/kg) partially counteracted the antiallodynic effect of tramadol (22 mg/kg).

CONCLUSIONS

These findings suggest the involvement of opioid and 5-HT(1A) receptors in the antinociceptive effect of tramadol and support the idea that the combination of tramadol with compounds having 5-HT(1A) antagonist properties could be a new strategy to improve tramadol-induced analgesia in neuropathic pain.

Mice lacking central serotonergic neurons show enhanced inflammatory pain and an impaired analgesic response to antidepressant drugs

A large body of literature has implicated serotonin [5-hydroxytryptamine (5-HT)] in descending modulation of nociceptive transmission. Here, we have studied the pain behavior of Lmx1b conditional knock-out mice (Lmx1b(f/f/p)), which lack 5-HT neurons in the CNS. Lmx1b(f/f/p) mutant mice showed normal thermal and visceral pain responses but were less sensitive to mechanical stimuli and exhibited enhanced inflammatory pain compared with their littermate control mice. Importantly, the analgesic effect of several antidepressant drugs, including selective serotonin reuptake inhibitors (SSRIs), serotonin-norepinephrine reuptake inhibitors (SNRIs), and tricyclic antidepressants, was either abolished or greatly attenuated in Lmx1b(f/f/p) mice. Moreover, in the acute versus persistent pain settings, the analgesic actions of the SNRI duloxetine and the SSRI fluoxetine were differentially affected. Together, our results provide in vivo genetic evidence demonstrating that although the predominant role of the central 5-HT system in inflammatory pain is inhibitory, its role in acute mechanical pain is facilitatory. The findings that the analgesic effects of various antidepressant drugs are differentially dependent on the central 5-HT system should help us to understand the mechanism of the analgesic action of different classes of antidepressants in the management of persistent pain.

Mechanical and cold hypersensitivity in nerve-injured C57BL/6J mice is not associated with fear-avoidance- and depression-related behaviour

BACKGROUND

Neuropathic pain is associated with significant co-morbidity, including anxiety and depression, which impact considerably on the overall patient experience. However, pain co-morbidity symptoms are rarely assessed in animal models of neuropathic pain. To improve the clinical validity of a widely used rodent model of traumatic peripheral neuropathy, we have investigated fear-avoidance- and depression-related behaviours in nerve-injured and sham-operated mice over a 4 week period.

METHODS

Male C57BL/6J mice were subjected to partial sciatic nerve ligation (PSNL) or sham surgery and were assessed on days 7, 14, and 28 after operation. Withdrawal thresholds to punctate mechanical and cooling stimuli were measured. Mice were tested on the novel open-field and elevated plus-maze tests for fear-avoidance behaviour, and on the tail suspension test for depression-related behaviour.

RESULTS

Hypersensitivity to punctate mechanical and cool stimuli was evident up to day 28 after PSNL. However, there was no change in fear-avoidance- or depression-related behaviours regardless of interval after-surgery.

CONCLUSION

These data demonstrate that pain behaviour in nerve-injured C57BL/6J mice was not associated with alterations in emotion-related behaviours.

In Vivo Effect of Venlafaxine on Locus Coeruleus Neurons: Role of Opioid, α2-Adrenergic, and 5-Hydroxytryptamine1A Receptors

The locus coeruleus (LC) is involved in several neural pathways responsible for some somatic and emotional processes, such as pain and depression; its activity is regulated by several receptors, such as opioid, alpha(2)-adrenergic, and 5-hydroxytryptamine (5-HT)(1A) receptors. The present study investigates the in vivo effects of venlafaxine, an antidepressant with analgesic properties, on locus coeruleus neurons, and its modulation by opioid, alpha(2)-adrenergic, and 5-HT(1A) receptors. The results show that acute administration of venlafaxine produced a dose-dependent, complete inhibition of LC activity. This inhibitory effect was not reversed by the opioid receptor antagonist naloxone, but subsequent administration of idazoxan, an alpha(2)-adrenoceptor antagonist, did reverse it. The preadministration of the 5-HT(1A) receptor agonist 8-hydroxy-2-dipropylaminotetralin (8-OH-DPAT) (1 and 40 microg/kg) significantly enhanced the venlafaxine inhibitory effect, decreasing the ED(50) by 56 and 44%, respectively. A 14-day treatment with venlafaxine (40 mg/kg/day) induced a suppression of the firing activity of LC neurons. In these treated animals, venlafaxine produced an inhibitory effect similar to that in nontreated animals. This inhibitory effect was not reversed by naloxone, but it was reversed by idazoxan. In addition, the preadministration of 8-OH-DPAT (40 microg/kg) significantly enhanced the venlafaxine effect, decreasing the ED(50) by 60%. These results suggest that the effect of venlafaxine on LC neurons is modulated by alpha(2)-adrenergic and 5-HT(1A) receptors, and not by opioid receptors. These data could contribute to the further understanding of the antidepressant and analgesic mechanism of action of venlafaxine.

[Reproducible efficacy of tricyclic antidepressant on chronic pain in an elderly patient with osteoporosis]

An 81-year-old woman was admitted due to exacerbation of chronic back pain from a vertebral osteoporosis fracture. The lumbar MRI examination revealed compression fracture of Th12 and L1 bones. Initial treatment with roxoprofen, calcitonin, bupurenorfin, and morphine did not achieve pain reduction in the patient. Because her geriatric depression scale score was low, we next tried to treat the pain using an antidepressant. Although the pain was improved by amitriptyline, the side effects of dry mouth and urinary incontinence were occurred. Milnacipran, a serotonin and norepinephrine reuptake inhibitor (SNRI), was then tried for the treatment of the chronic pain instead of amitriptyline, but the pain was increased. Then, she was given amitriptyline again for treatment of the chronic back pain instead of SNRI. The second-time amitriptyline treatment was effective to reduce the pain, with minimal side-effects. Because chronic pain due to osteoporosis is often difficult to treat in elderly patients, the classic antidepressant, amitriptyline, may help pain control by narcotics and anti-inflammatory agents in some elderly patients.

Management of painful neuropathies

Inadequately assessed and poorly managed pain contributes both to high general medical cost and to increasing numbers of societal problems. A large determining factor of inadequate pain care is the lack of understanding about the complex nature of pain and of a rational approach to its assessment and treatment. The less than adequate assessment of neuropathic pain and its relatively poor response to typical analgesic medications are major determinants to the undertreatment of pain. Other than the important management concept of treating the underlying cause, current approaches to the treatment of pain in general, and neuropathic pain in particular, have shifted away from treatment of individual syndromes toward the identification and management of common symptoms and the mechanisms upon which such symptoms are presumed to be based. This article summarizes the features of neuropathic pain that commonly appear in most peripheral neuropathies, regardless of the mechanism of injury, and provides an approach for the selection of treatment.

Tramadol has a Better Potency Ratio Relative to Morphine in Neuropathic than in Nociceptive Pain Models

BACKGROUND AND OBJECTIVE

Treatment of neuropathic pain remains a challenge and the role of various analgesics in this setting is still debated. The effects of tramadol, an atypically acting analgesic with a combined opioid and monoaminergic mechanism of action, and morphine, a prototypical opioid, were tested in rat models of neuropathic and nociceptive pain.

METHODS

Cold allodynia and mechanical hypersensitivity, symptoms of neuropathic pain, were studied in rat models of mononeuropathic pain. Cold allodynia was analyzed in the chronic constriction injury (CCI) model and mechanical hypersensitivity was analyzed in the spinal nerve ligation (SNL) model. Heat-induced rat tail-flick latencies were determined as measure for nociceptive pain.

RESULTS

Cold allodynia and mechanical hypersensitivity were strongly attenuated with similar absolute potency after intravenous administration of tramadol and morphine. The doses of drug that were calculated to result in 50% pain inhibition (ED(50)) for tramadol and morphine were 2.1 and 0.9 mg/kg, respectively, in CCI rats and 4.3 and 3.7 mg/kg, respectively, in SNL rats. In the tail-flick assay of acute nociception, the potency of the two drugs differed markedly, as seen by ED(50) values of 5.5 and 0.7 mg/kg intravenously for tramadol and morphine, respectively. Accordingly, the analgesic potency ratio (ED(50) tramadol/ED(50) morphine) of both compounds differed in neuropathic (potency ratio 2.3 in CCI and 1.2 in SNL) and nociceptive pain models (potency ratio 7.8), suggesting a relative increase in potency of tramadol in neuropathic pain compared with nociceptive pain.

CONCLUSION

The results of this study are consistent with clinical data supporting the efficacy of opioids in neuropathic pain conditions, and furthermore suggest an additional contribution of the monoaminergic mechanism of tramadol in the treatment of neuropathic pain states.