Evidence for an antihyperalgesic effect of venlafaxine in vincristine-induced neuropathy in rat

Broad spectrum and prolonged efficacy of dimiracetam in models of neuropathic pain

Dimiracetam, a bicyclic 2-pyrrolidinone derivative originally developed as cognition enhancer, is a member of the nootropic family for which anecdotal efficacy in models of neuropathic pain has been reported. Its antineuropathic activity was evaluated in established models of neuropathic pain induced by nerve injury, chemotherapy or MIA-induced osteoarthritis. Acutely, dimiracetam was very effective in models of antiretroviral drug induced painful neuropathy, oxaliplatin-induced hyperalgesia and in the MIA-osteoarthritis. Chronic dimiracetam dosing in the MIA and ART- induced models completely reverted hyperalgesia back to the level of healthy controls. Once reached, the maximal effect was maintained despite dose diminution and increased inter-dose interval. The effect of the last dose outlasted dimiracetam half-life longer than 12 times. In synaptosomal preparations, dimiracetam counteracted the NMDA-induced release of glutamate with highest potency in the spinal cord, possibly via NMDA receptor isoforms containing pH-sensitive GluN1 and GluN2A subunits. Dimiracetam appears to be a promising and safe treatment for neuropathic pain conditions for which there are very limited therapeutic options.

Oxaliplatin-induced neuropathy in colorectal cancer: many questions with few answers

Oxaliplatin is a chemotherapeutic agent effective against advanced colorectal cancer. Unlike with other platinum-based agents, the main side effect of oxaliplatin is polyneuropathy. Oxaliplatin-induced polyneuropathy (OIPN) has a unique profile, which can be divided into acute and chronic neurotoxicity. Early identification of the neurotoxicity and alterations in dose or schedule for the medication could prevent the development of chronic symptoms, which, once established, may take many months or years to resolve or even persist throughout life with a substantial effect on quality of life. There is no doubt that the use of pharmacogenomic methods to identify genetic bases of interindividual differences in drug response has led to what is called tailoring treatment. Yet there are some challenges regarding the application of these differences. Many efforts have been made to prevent or treat OIPN. Better understanding of the mechanisms underlying the acute and chronic forms of OIPN will be a key component of future advances in the prevention and treatment of OIPN. The aim of this review is to highlight the clinical presentation, assessment, and management of OIPN, as well as the underlying pathophysiologic and pharmacogenomic background.

Modification of morphine analgesia by venlafaxine in diabetic neuropathic pain model

BACKGROUND

The purpose of this study was to investigate the influence of single or chronic (21 days) administration of the serotonin and noradrenaline reuptake inhibitor, venlafaxine, on the antinociceptive action of the opioid receptor agonist, morphine, in streptozotocin (STZ)-induced hyperalgesia.

METHODS

The studies were performed on male Wistar rats. Changes in nociceptive thresholds were determined using mechanical stimuli. Diabetes was induced by a single administration of STZ (40 mg/kg, im).

RESULTS

Venlafaxine was shown to modulate analgesic activity of morphine in STZ-induced hyperalgesia. However, whereas acute co-administration of venlafaxine increased the analgesic activity of morphine, chronic treatment with venlafaxine attenuated opioid efficacy.

CONCLUSION

Depending on the mode of administration (single or long-term), venlafaxine modulates analgesic activity of morphine. Further investigations are necessary to clarify the mechanisms of these interactions, which may be clinically relevant.

Alterations in endocannabinoid tone following chemotherapy-induced peripheral neuropathy: effects of endocannabinoid deactivation inhibitors targeting fatty-acid amide hydrolase and monoacylglycerol lipase in comparison to reference analgesics following cisplatin treatment

Cisplatin, a platinum-derived chemotherapeutic agent, produces mechanical and coldallodynia reminiscent of chemotherapy-induced neuropathy in humans. The endocannabinoid system represents a novel target for analgesic drug development. The endocannabinoid signaling system consists of endocannabinoids (e.g. anandamide (AEA) and 2-arachidonoylglycerol (2-AG)), cannabinoid receptors (e.g. CB(1) and CB(2)) and the enzymes controlling endocannabinoid synthesis and degradation. AEA is hydrolyzed by fatty-acid amide hydrolase (FAAH) whereas 2-AG is hydrolyzed primarily by monoacylglycerol lipase (MGL). We compared effects of brain permeant (URB597) and impermeant (URB937) inhibitors of FAAH with an irreversible inhibitor of MGL (JZL184) on cisplatin-evoked behavioral hypersensitivities. Endocannabinoid modulators were compared with agents used clinically to treat neuropathy (i.e. the opioid analgesic morphine, the anticonvulsant gabapentin and the tricyclic antidepressant amitriptyline). Cisplatin produced robust mechanical and cold allodynia but did not alter responsiveness to heat. After neuropathy was fully established, groups received acute intraperitoneal (i.p.) injections of vehicle, amitriptyline (30 mg/kg), gabapentin (100 mg/kg), morphine (6 mg/kg), URB597 (0.1 or 1 mg/kg), URB937 (0.1 or 1 mg/kg) or JZL184 (1, 3 or 8 mg/kg). Pharmacological specificity was assessed by coadministering each endocannabinoid modulator with either a CB(1) (AM251 3 mg/kg), CB(2) (AM630 3 mg/kg), TRPV1 (AMG9810 3 mg/kg) or TRPA1 (HC030031 8 mg/kg) antagonist. Effects of cisplatin on endocannabinoid levels and transcription of receptors (CB(1), CB(2), TRPV1, TRPA1) and enzymes (FAAH, MGL) linked to the endocannabinoid system were also assessed. URB597, URB937, JZL184 and morphine reversed cisplatin-evoked mechanical and cold allodynia to pre-cisplatin levels. By contrast, gabapentin only partially reversed the observed allodynia while amitriptyline, administered acutely, was ineffective. CB(1) or CB(2) antagonists completely blocked the anti-allodynic effects of both FAAH (URB597, URB937) and MGL (JZL184) inhibitors to mechanical and cold stimulation. By contrast, the TRPV1 antagonist AMG9810 blocked the anti-allodynic efficacy of both FAAH inhibitors, but not the MGL inhibitor. By contrast, the TRPA1 antagonist HC30031 did not attenuate anti-allodynic efficacy of any endocannabinoid modulator. When the levels of endocannabinoids were examined, cisplatin increased both anandamide (AEA) and 2-arachidonoylglycerol (2-AG) levels in the lumbar spinal cord and decreased 2-AG levels (but not AEA) in dorsal hind paw skin. RT-PCR showed that mRNA for FAAH, but not other markers, was upregulated by cisplatin treatment in lumbar spinal cord. The present studies demonstrate that cisplatin alters endocannabinoid tone and that inhibition of endocannabinoid hydrolysis alleviates chemotherapy-induced mechanical and cold allodynia. The anti-allodynic effects of FAAH and MGL inhibitors are mediated by CB(1) and CB(2) cannabinoid receptors, whereas TRPV1, but not TRPA1, -dependent mechanisms contribute to the anti-allodynic efficacy of FAAH (but not MGL) inhibitors. Strikingly, endocannabinoid modulators potently suppressed cisplatin-evoked allodynia with a rapid onset and showed efficacy that equaled or exceeded that of major classes of anti-neuropathic pain medications used clinically. Thus, inhibition of endocannabinoid hydrolysis, via FAAH or MGL inhibitors, represents an efficacious pharmacological approach for suppressing chemotherapy-induced neuropathic pain.

Addressing both depression and pain in late life: the methodology of the ADAPT study

OBJECTIVE

To describe the methodology of the first NIH-funded clinical trial for seniors with comorbid depression and chronic low back pain.

METHODS

Randomized controlled effectiveness trial using stepped care methodology. Participants are ≥60 years old. Phase 1 (6 weeks) is open treatment with venlafaxine xr 150 mg/day and supportive management (SM). Response is 2 weeks of PHQ-9 ≤5 and at least 30% improvement in the average numeric rating scale for pain. Nonresponders progress to phase 2 (14 weeks) in which they are randomized to high-dose venlafaxine xr (up to 300 mg/day) with problem solving therapy for depression and pain (PST-DP) or high-dose venlafaxine xr and continued SM. Primary outcomes are the univariate pain and depression response and both observed and self-reported disability. Survival analytic techniques will be used, and the clinical effect size will be estimated with the number needed to treat. We hypothesize that self-efficacy for pain management will mediate response for subjects randomized to venlafaxine xr and PST-DP.

RESULTS

Not applicable.

CONCLUSIONS

The results of this trial will inform the care of these complex patients and further understanding of comorbid pain and depression in late life.

The effect of venlafaxine on ongoing and experimentally induced pain in neuropathic pain patients: a double blind, placebo controlled study

BACKGROUND AND AIM

The aim of this randomized double blind placebo controlled study was to investigate the effectiveness and the safety of venlafaxine XR 75 and 150 mg on ongoing pain and on quantitative sensory tests in 60 patients with neuropathic pain for 8 weeks.

METHODS

Evaluation parameters consisted of ongoing pain intensity (VAS), patient satisfaction, side effects, global efficacy and tolerance. Quantitative sensory measurements taken from the affected area before and after the drug treatment included pin-prick hyperalgesia, allodynia, detection and pain thresholds to electrical and heat stimuli, temporal summation of repetitive electrical and heat stimuli.

RESULTS

A total of 55 patients completed the study. VAS scores decreased significantly compared to the baseline measurements in all groups. There was no significant difference between the groups regarding pain intensity and escape medication. The areas of allodynia and pin-prick hyperalgesia decreased significantly in venlafaxine groups compared to the placebo. There was no significant difference between the groups regarding the detection thresholds (electrical and heat). The pain threshold and the summation threshold to electrical stimuli and the summation threshold to heat stimuli increased significantly following treatment in both venlafaxine groups. In addition, the degree of the temporal summation to electrical and heat stimuli decreased significantly following treatment in both venlafaxine groups compared to the placebo.

CONCLUSION

The study showed significant effect of venlafaxine in the manifestations of hyperalgesia and temporal summation, but not on the ongoing pain intensity. Furthermore, the quantitative sensory tests provided complementing information to the clinical measures.

Chemotherapy-induced peripheral neuropathy: prevention and treatment

Chemotherapy-induced peripheral neuropathy (CIPN) is a common, dose-limiting side effect of many chemotherapeutic agents. Although many therapies have been investigated for the prevention and/or treatment of CIPN, there is no well-accepted proven therapy. In addition, there is no universally accepted, well-validated measure for the assessment of CIPN. The agents for which there are the strongest preliminary data regarding their potential efficacy in preventing CIPN are intravenous calcium and magnesium (Ca/Mg) infusions and glutathione. Agents with the strongest supporting evidence for efficacy in the treatment of CIPN include topical pain relievers, such as baclofen/amitriptyline/ketamine gel, and serotonin and norepinephrine reuptake inhibitors, such as venlafaxine and duloxetine. Other promising therapies are also reviewed in this paper. Cutaneous electrostimulation is a nonpharmacological therapy that appears, from an early pilot trial, to be potentially effective in the treatment of CIPN. Finally, there is a lack of evidence of effective treatments for the paclitaxel acute pain syndrome (P-APS), which appears to be caused by neurologic injury.

Monoaminergic Antidepressants in the Relief of Pain: Potential Therapeutic Utility of Triple Reuptake Inhibitors (TRIs)

Over 75% of depressed patients suffer from painful symptoms predicting a greater severity and a less favorable outcome of depression. Imaging, anatomical and functional studies have demonstrated the existence of common brain structures, neuronal pathways and neurotransmitters in depression and pain. In particular, the ascending serotonergic and noradrenergic pathways originating from the raphe nuclei and the locus coeruleus; respectively, send projections to the limbic system. Such pathways control many of the psychological functions that are disturbed in depression and in the perception of pain. On the other hand, the descending pathways, from monoaminergic nuclei to the spinal cord, are specifically implicated in the inhibition of nociception providing rationale for the use of serotonin (5-HT) and/or norepinephrine (NE) reuptake inhibitors (SSRIs, NRIs, SNRIs), in the relief of pain. Compelling evidence suggests that dopamine (DA) is also involved in the pathophysiology and treatment of depression. Indeed, recent insights have demonstrated a central role for DA in analgesia through an action at both the spinal and suprasinal levels including brain regions such as the periaqueductal grey (PAG), the thalamus, the basal ganglia and the limbic system. In this context, dopaminergic antidepressants (i.e., containing dopaminergic activity), such as bupropion, nomifensine and more recently triple reuptake inhibitors (TRIs), might represent new promising therapeutic tools in the treatment of painful symptoms with depression. Nevertheless, whether the addition of the dopaminergic component produces more robust effects than single- or dual-acting agents, has yet to be demonstrated. This article reviews the main pathways regulating pain transmission in relation with the monoaminergic systems. It then focuses on the current knowledge regarding the in vivo pharmacological properties and mechanism of action of monoaminergic antidepressants including SSRIs, NRIs, SNRIs and TRIs. Finally, a synthesis of the preclinical studies supporting the efficacy of these antidepressants in analgesia is also addressed in order to highlight the relative contribution of 5-HT, NE and DA to nociception.

Suppressive effect of imipramine on vincristine-induced mechanical allodynia in mice

Because chronic vincristine (VCR) treatment causes neuropathic pain, as demonstrated by mechanical allodynia, effective therapeutic strategy is required. In this study, we investigated a suppressive effect of imipramine (IMI) on VCR-induced mechanical allodynia in mice. VCR (0.1 mg/kg, intraperitoneally (i.p.)) was administered once per day for 7 d in ICR male mice. Mechanical allodynia was evaluated by withdrawal response using von Frey filaments. In VCR-treated mice, mechanical allodynia was observed on day 3, 7, and 14. On day 14, morphine (3 mg/kg, subcutaneously) slightly but significantly suppressed VCR-induced mechanical allodynia. The percent inhibition by morphine of VCR-induced mechanical allodynia was less than that of the lambda-carrageenan-induced inflammatory pain and was similar to that of nerve injury-induced neuropathic pain. Although single administration of IMI (30 mg/kg, i.p.) had no effect on VCR-induced mechanical allodynia, repeated administration of IMI (30 mg/kg, i.p.) for 7 d significantly suppressed VCR-induced mechanical allodynia. Suppressive effects by repeated IMI administration were observed in both early phase (day 0-6) and late phase (day 7-13) of VCR-induced mechanical allodynia. These results suggest that chronic VCR administration induces opioid analgesics-resistant mechanical allodynia, and repeated IMI administration may be an effective therapeutic approach for the treatment of VCR-induced mechanical allodynia.

Animal models of chemotherapy-evoked painful peripheral neuropathies

This review examines recent preclinical research on toxic peripheral neuropathy and potential therapeutic developments. Chemotherapy-induced peripheral neurotoxicity is a major clinical problem because it represents the dose-limiting side effects of a significant number of antineoplastic drugs. Patients are unable to complete full or optimal treatment schedules. The incidence of chemotherapy-induced peripheral neuropathy varies depending on the drugs and schedules used, and this can be quite high, particularly when neurophysiological methods are used to make a diagnosis. However, even when chemotherapy-induced peripheral neuropathy is not a dose-limiting side effect, its onset may severely affect the quality of life of cancer patients and cause chronic discomfort. As such, improved understanding of the pathophysiology of chemotherapy-induced neurotoxicity need for animal models is clinically relevant and will assist in the development of future neuroprotective strategies and also in the design of novel chemotherapies with improved toxicity profiles. In this review, the features of animal models of chemotherapy-induced painful neuropathy developed for 20 years, due to the administration of the most widely used drugs, such as platinum drugs, taxanes, and vinca alkaloids, will be discussed. In a second part, data available on neuroprotectants and treatment strategies, evaluated using these previous animal models in the attempt to prevent neuropathic pain, will be summarized.

Experimental studies of potential analgesics for the treatment of chemotherapy-evoked painful peripheral neuropathies

OBJECTIVE

We investigated potential analgesics for chemotherapy-evoked neuropathic pain using rats treated with paclitaxel.

DESIGN

Drugs were tested in a repeated dosing paradigm (four daily injections). Topiramate was tested with a long-term treatment paradigm (12 days). A literature search was performed to summarize prior data.

MEASURES

Mechanical stimulation of the hind paw was used to assay antiallodynic and antihyperalgesic effects acutely and 24 hours after injection.

RESULTS

Amitriptyline produced significant analgesia, but this was not apparent until after the second injection. Baclofen produced significant effects, but the response varied erratically. Mexiletine and NMED-126 (a mixed N- and T-type calcium channel blocker) produced consistent, significant analgesia when tested acutely, but the pain relief did not persist at 24 hours postinjection. Oxcarbazepine had no effect at any time. Tramadol produced consistent, near-complete analgesia when tested acutely, but the analgesia did not persist to 24 hours postinjection. Topiramate produced significant effects that were first evident after 6-8 days of dosing.

CONCLUSIONS

The present data and data from the literature review suggest that there are several potential treatments for chemotherapy-evoked neuropathic pain. Nonsteroidal anti-inflammatory drugs have little or no efficacy. Opioids have an effect, but probably only with high doses. At least some antidepressants are analgesic in these conditions. Some, but clearly not all, anticonvulsants and sodium channel blockers have efficacy. Tramadol is a particularly promising candidate. Topiramate, acetyl-L-carnitine, carbamazepine, and venlafaxine may have protective or restorative effects. Clinical trials of these candidates are needed to advance the treatment of chemotherapy-evoked pain.

Adjuvant analgesics

Adjuvant analgesics represent a diverse group of drugs that were originally developed for a primary indication other than pain. Many of these medications are currently used to enhance analgesia under specific circumstances. The proper use of adjuvant drugs is one of the keys to success in effective pain management. Since adjuvant analgesics are typically administered to patients who take multiple medications, decisions regarding administration and dosage must be made with a clear understanding of the stage of the disease and the goals of care. The article discusses major classes of adjuvant analgesics, with the focus on the mechanism of action, clinical application, and risks and benefits associated with each particular class of adjuvants.

Adjuvant analgesics

Chronic pain, whether arising from viscera, bone, or any other tissue or structure, is, more often than commonly thought, the result of a mixture of pain mechanisms, and therefore there is no simple formula available to manage chronic complex pain states. Box 1 summarizes a pharmacological algorithm for difficult-to-treat chronic pain, which merely introduces the medication aspect of the treatment. In effect, any comprehensive algorithm should call for an interdisciplinary approach that would include rehabilitation, as well as psychosocial, and when indicated, interventional techniques. Box 1 Analgesic algorithm for difficult-to-treat pain syndromes. Pharmacological Interventions. Moderate to severe pain/functional impairment; pain with a score of >4 on the brief pain inventory. 1. Gabapentinoid (gabapentin, pregabalin)+/-Opioid/opioid rotation or 2. Antidepressant (TCA, duloxetine, venlafaxine)+/-Opioid/opioid rotation or 3. Gabapentinoid+antidepressant+Opioid/opioid rotation; in addition, may consider trials of one or more of the following adjuvants when clinically appropriate: Topical therapies for cutaneous allodynia/hyperalgesia. Anti-inflammatory drugs (corticosteroids for acute inflammatory neuropathic pain)IV bisphosphonates for cancer bone pain or CRPS/RSDNon-gabapentinoid AEDs such as carbamazepine or oxcarbazepine or lamotrigine+/-baclofen for intermittent lancinating pain due to cranial neuralgiasNMDA antagonists Mexiletine On a compassionate basis, according to the patient's clinical condition and pain mechanism, the physician may want to consider an empirical trial of one or more of the emergent topical, oral or parenteral/intrathecal therapies as discussed in the text. If SMP, consider topical clonidine and sympatholytic interventions; if clinically feasible, trials of topical therapies, eg, lidocaine 5% patch, may be considered for a variety of pain states and features.The major rationale for introducing adjuvants is to better balance efficacy and adverse effects. The following scenarios should prompt the use of adjuvants in clinical practice: The toxic limit of a primary analgesic has been reached. The therapeutic benefit of a primary analgesic has plateaued, eg, treatment has reached its true efficacy limit or pharmachodynamic tolerance has developed. The primary analgesic is contraindicated, eg, substance abuse, aberrant behavior, organ failure, allergy, and so forth. Subjective and qualitative symptoms demand broader coverage. Patients often convey that different medications will impart distinct analgesic benefits. Presence of disabling nonpainful complaints and need to manage symptoms such as insomnia, depression, anxiety, and fatigue that all cause worsening of the patient's quality of life and function. Physicians have also been drawn to the adjuvants secondary to new realities of clinical practice. Moreover, aversion to addiction and diversion remains a potent force that shapes prescribing profiles.

Antidepressants enhance the antinociceptive effects of carbamazepine in the acetic acid-induced writhing test in mice

Some antidepressants, as well as antiepileptics, are effective for treating pain of varying etiology. The present study was designed to characterize the antinociceptive effects of imipramine, a tricyclic antidepressant, fluvoxamine, a selective serotonin reuptake inhibitor, milnacipran, a serotonin noradrenaline reuptake inhibitor, and carbamazepine, an antiepileptic drug, using the acetic acid-induced writhing test in mice. Imipramine (1.25-10 mg/kg, i.p.), fluvoxamine (5-40 mg/kg, i.p.) and milnacipran (2.5-20 mg/kg, i.p.) all dose-dependently and significantly reduced the number of writhes induced by the injection of acetic acid (0.8% (v/v)), although the maximal effect of milnacipran was weaker than those of imipramine and fluvoxamine. Similarly, carbamazepine (5-20 mg/kg, i.p.) also showed a dose-dependent and significant antinociceptive effect. In combination studies, the co-administration of a sub-effective dose of carbamazepine (5 mg/kg, i.p.) with imipramine (1.25 and 2.5 mg/kg, i.p.), fluvoxamine (10 mg/kg, i.p.) or milnacipran (1.25 and 2.5 mg/kg, i.p.) significantly reduced the number of writhes. Additionally, the hole-board test revealed that the medications with significant antinociceptive effects barely produced changes in motor activity that could possibly affect writhing behavior. Thus, the present study demonstrated that the antinociceptive effect of carbamazepine is enhanced by combination with imipramine, fluvoxamine and milnacipran. Therefore, the combined therapy using antidepressants and carbamazepine may be useful clinically for the control of pain.

Venlafaxine compromises the antinociceptive actions of gabapentin in rat models of neuropathic and persistent pain

RATIONALE

Neuropathic pain is associated with a number of disease states of diverse aetiology that can share common pathophysiological mechanisms. Antiepileptic drugs modulate ion channel function and antidepressants increase extracellular monoamine levels, and both drug classes variously attenuate signs and symptoms of neuropathic pain. Thus, coadministration of the antiepileptic gabapentin and the antidepressant venlafaxine may provide superior pain relief to administration of either drug alone.

OBJECTIVES

To systematically establish the pain relieving efficacies of venlafaxine and gabapentin alone and in combination.

MATERIALS AND METHODS

Gabapentin (50 and 100 mg/kg, s.c.) and venlafaxine (10, 25, 50 mg/kg, s.c.) were tested alone or in combination in the rat spared nerve injury (SNI) model of neuropathic pain and the rat formalin test of persistent pain. Diuresis was measured in a separate experiment after administration of venlafaxine.

RESULTS

Hindpaw mechanical allodynia was dose-dependently reversed by gabapentin (50 and 100 mg/kg, s.c.), whereas venlafaxine was ineffective (10 and 50 mg/kg, s.c.). Both gabapentin and venlafaxine also attenuated hindpaw mechanical hyperalgesia. Surprisingly, coadministration of venlafaxine (50 mg/kg) significantly lowered the antiallodynic effect of both doses of gabapentin by up to 60% in spared-nerve-injury rats and a negative antinociceptive interaction between gabapentin and venlafaxine was also observed in the rat formalin test. We demonstrated that venlafaxine administration was associated with a dose-dependent increase in urine output over the time course of the nociceptive experiments.

CONCLUSION

Venlafaxine compromises the antiallodynic effects of coadministered gabapentin most probably as consequence-increased diuresis.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Emerging therapies for neuropathic pain

Neuropathic pain develops as a result of damage to either the peripheral or central nervous system. It is characterised by spontaneous burning pain and/or ongoing pain with accompanying hyperalgesia and allodynia. Neuropathic pain is difficult to treat as it is often refractory to conventional analgesic treatments, with most patients obtaining only partial relief. At present, there are four major medication categories that are considered first-line treatment for neuropathic pain: antidepressants, anticonvulsants, local anaesthetic/topical agents and opioids. The efficacy of these treatments in neuropathic pain, excepting opioids, has been discovered serendipitously. However, responder rates and overall efficacy is poor with these agents and tolerability or side effects are often limiting. This update will review existing treatment options for neuropathic pain, and highlight more recent advances in the development of novel analgesics to treat this chronic disorder.

Serotonin and noradrenaline reuptake inhibitors in animal models of pain

Animal models of chronic pain serve as an experimental basis for testing new therapeutic interventions and for mechanistic investigations. In an animal model of chronic pain, based on the injection of formalin into the paw of a rodent, inhibitors of noradrenaline reuptake such as nisoxetine, nortriptyline and maprotiline and dual inhibitors of the noradrenaline and serotonin reuptake such as imipramine and milnacipran produce potent anti-nociceptive effects, whereas selective serotonin reuptake inhibitors, such as fluvoxamine, are much less potent. In another model, neuropathic pain resulting from the chronic constriction injury of the sciatic nerve was prevented by the dual uptake inhibitor, venlafaxine. The experimental model involving ligation of the 5th spinal nerve induces behavioural signs in rats and mice that are similar to the symptoms of human neuropathic pain. In this model amitriptyline, a non-selective serotonin and noradrenaline reuptake blocker, the preferential noradrenaline reuptake inhibitor, desipramine and the selective serotonin and noradrenaline reuptake inhibitors, milnacipran and duloxetine, produce a decrease in pain sensitivity whereas the selective serotonin reuptake inhibitor, fluoxetine, is ineffective. Antidepressants acting on the noradrenergic or both the noradrenergic and serotonergic systems thus appear to be more effective than those working on the serotonin system alone.

Treatment of Pain Syndromes with Venlafaxine

Major depressive disorder (MDD) and anxiety disorders such as generalized anxiety disorder (GAD) are often accompanied by chronic painful symptoms. Examples of such symptoms are backache, headache, gastrointestinal pain, and joint pain. In addition, pain generally not associated with major depression or an anxiety disorder, such as peripheral neuropathic pain (e.g., diabetic neuropathy and postherpetic neuralgia), cancer pain, and fibromyalgia, can be challenging for primary care providers to treat. Antidepressants that block reuptake of both serotonin and norepinephrine, such as the tricyclic antidepressants (e.g., amitriptyline), have been used to treat pain syndromes in patients with or without comorbid MDD or GAD. Venlafaxine, a serotonin and norepinephrine reuptake inhibitor, has been safe and effective in animal models, healthy human volunteers, and patients for treatment of various pain syndromes. The use of venlafaxine for treatment of pain associated with MDD or GAD, neuropathic pain, headache, fibromyalgia, and postmastectomy pain syndrome is reviewed. Currently, no antidepressants, including venlafaxine, are approved for the treatment of chronic pain syndromes. Additional randomized, controlled trials are necessary to fully elucidate the role of venlafaxine in the treatment of chronic pain.