Antidepressants for chronic neuropathic pain

HDAC6-selective inhibitors decrease nerve-injury and inflammation-associated mechanical hypersensitivity in mice

BACKGROUND

HDAC6 is a class IIB histone deacetylase expressed at many levels of the nociceptive pathway. This study tested the ability of novel and selective HDAC6 inhibitors to alleviate sensory hypersensitivity behaviors in mouse models of peripheral nerve injury and peripheral inflammation.

METHODS

We utilized the murine spared nerve injury (SNI) model for peripheral nerve injury and the Complete Freund's Adjuvant (CFA) model of peripheral inflammation. We applied the Von Frey assay to monitor mechanical allodynia.

RESULTS

Using the SNI model, we demonstrate that daily administration of the brain-penetrant HDAC6 inhibitor, ACY-738, abolishes mechanical allodynia in male and in female mice. Importantly, there is no tolerance to the antiallodynic actions of these compounds as they produce a consistent increase in Von Frey thresholds for several weeks. We observed a similar antiallodynic effect when utilizing the HDAC6 inhibitor, ACY-257, which shows limited brain expression when administered systemically. We also demonstrate that ACY-738 and ACY-257 attenuate mechanical allodynia in the CFA model of peripheral inflammation.

CONCLUSIONS

Overall, our findings suggest that inhibition of HDAC6 provides a promising therapeutic avenue for the alleviation of mechanical allodynia associated with peripheral nerve injury and peripheral inflammation.

Enhancement of antinociceptive effect of morphine by antidepressants in diabetic neuropathic pain model

BACKGROUND

Recent studies have shown that influence of antidepressants on analgesic action of opioids is heterogeneous. The aim of this study was to investigate the effect of acute and repeated (21 days) antidepressant (amitriptyline, moclobemide and reboxetine) treatment on the antinociceptive action of morphine, an opioid agonist, in streptozotocin (STZ)-induced neuropathic pain model.

METHODS

The studies were performed on the male Wistar rats. The changes in nociceptive thresholds were determined by using mechanical stimuli (the Randall-Selitto and the von Frey tests). Diabetes was induced by intramuscular administration of STZ.

RESULTS

In this work we report that acute as well as repeated per os administration of antidepressants (amitriptyline, moclobemide and reboxetine) significantly potentiated the antihyperalgesic effect of morphine in STZ-induced neuropathic pain model.

CONCLUSION

Combination therapy, such as classical antidepressants (amitriptyline, moclobemide) with opioids, or agents with noradrenaline reuptake inhibition and μ-opioid receptor activation could be a new target for research into treatment of painful diabetic neuropathy.

Conotoxins targeting neuronal voltage-gated sodium channel subtypes: potential analgesics?

Voltage-gated sodium channels (VGSC) are the primary mediators of electrical signal amplification and propagation in excitable cells. VGSC subtypes are diverse, with different biophysical and pharmacological properties, and varied tissue distribution. Altered VGSC expression and/or increased VGSC activity in sensory neurons is characteristic of inflammatory and neuropathic pain states. Therefore, VGSC modulators could be used in prospective analgesic compounds. VGSCs have specific binding sites for four conotoxin families: μ-, μO-, δ- and ί-conotoxins. Various studies have identified that the binding site of these peptide toxins is restricted to well-defined areas or domains. To date, only the μ- and μO-family exhibit analgesic properties in animal pain models. This review will focus on conotoxins from the μ- and μO-families that act on neuronal VGSCs. Examples of how these conotoxins target various pharmacologically important neuronal ion channels, as well as potential problems with the development of drugs from conotoxins, will be discussed.

Altered associative learning and emotional decision making in fibromyalgia

OBJECTIVE

The present study examines the possibility that a chronic pain condition, such as fibromyalgia, was associated with deficits in decision making and associative learning.

METHODS

Fifteen patients with fibromyalgia (aged 42-59 years) and 15 healthy controls (aged 39-61 years) participated in the experiment. Subjects completed anxiety (STAI) and depression (BDI) questionnaires, as well as standardized neuropsychological tests (Stroop and WAIS subscales). In addition, an emotional decision-making task (Iowa Gambling Task) and a conditional associative learning task (CALT) were administered to all participants.

RESULTS

Results indicated that fibromyalgia had a poorer performance than healthy controls in both tasks, showing more perseveration errors in the learning task, and more disadvantageous decisions, as well as a more random behavior in the gambling task. Moreover, we observed that poor performance on the associative learning task was mediated by depression, whereas performance on the gambling task was not influenced by depression. No group differences were found on the standardized neuropsychological tests.

CONCLUSION

These findings indicate that pain and depressive symptoms in fibromyalgia might lead to significant deficits in emotionally charged cognitive tasks. Furthermore, it suggests that chronic pain might impose a high cost on executive control, undermining mainly affective processes involved in learning, memory, attention, and decision-making.

Functional somatic complaints in depression: An overview

Depression is a disorder of major public health importance which often manifests through functional somatic complaints. Concept of functional somatic complaints dates back to the time of Wernicke and is later substantiated by various authors. Although considered as an alternative 'idiom of distress' in certain culture, functional somatic complaints are universal. Various international, cross-cultural, inpatient and outpatient based studies have reported that about two-third of subjects of depression present to clinicians with functional somatic complaints which often leads to misrecognition of their illness and in turn leads to increased utilization of health services. These functional somatic complaints can be related to various organ systems but show remarkable homogeneity in their presentation across culture. Various instruments have attempted to tap the functional somatic complaints but are limited by their cross-cultural validity. Among important correlates of functional somatic complaints are female gender, severity of depression, subsyndromal anxiety, alexithymia, somatosensory amplification and hypochondriacal worry are to name a few. Neurobiological understanding implicates neurotransmitters serotonin and norepinephrine, resultantly Serotonin Norepinephrine Reuptake Inhibitors have been found to be effective in treating functional somatic complaints in depression. Future revisions in the nosological systems should consider giving proper importance to some of these symptoms for diagnosing depression.

Treatments for neuropathic pain differentially affect delayed matching accuracy by macaques: effects of amitriptyline and gabapentin

Current clinical treatments for neuropathic pain include amitriptyline, a tricyclic antidepressant with mixed pharmacology that is also clinically reported to impair cognitive performance; and gabapentin, a compound that selectively interacts with alpha2delta-1 calcium channel subunits. Since few assessments of cognitive performance have been made in non-human primates with these marketed treatments, the purpose of this study was to determine their relative abilities to alter working memory as measured in mature macaques in their performance of a delayed matching-to-sample task. Four delay intervals of increasing duration provided increasing impairment in task accuracies during vehicle sessions. Administration of clinically relevant doses of amitriptyline significantly decreased task accuracy at the highest dose tested (3mg/kg). Administration of gabapentin increased mean task accuracy, though the effect was not statistically significant until intra-subject variability was reduced by selecting the individual best dose for each animal (which averaged 12.8mg/kg). Most of the effect was obtained during the presentation of long delay trials (18.2% above vehicle). Task improvement was sustained during sessions run 24h after gabapentin administration. In a series that used a task-relevant distractor to determine gabapentin's effect on attention, drug treatment reversed distractor-impaired accuracy during long delay trials (25.4% above vehicle). The selective improvement in long delay accuracy in both paradigms suggests improvement in encoding or retention components of working memory. It is currently unclear whether the ability of acute administration of gabapentin to modestly improve working memory occurs by a mechanism that could be related to its anti-allodynic mechanism of action.

Racemic intrathecal mirtazapine but not its enantiomers acts anti-neuropathic after chronic constriction injury in rats

The unique noradrenergic and specific serotonergic antidepressant mirtazapine acts antinociceptive. It is optically active and currently marketed as racemate. In an animal model of acute pain it has been shown that the enantiomers exhibit differential effects: the R(-)-enantiomer showed anti-, the S(+)-enantiomer pronociceptive properties while the racemate acted antinociceptive at low doses and profoundly pronociceptive after high-dose application. Aim of the present study was to evaluate potential enantioselective effects of mirtazapine in neuropathic pain. In a chronic constriction injury model of neuropathic pain, Wistar rats were injected (+/-)-mirtazapine and the enantiomers intrathecally. All substances were dosed between 0.001 and 1mg/kg and compared to vehicle in a randomized and blinded approach. Thermal hyperalgesia and mechanical allodynia were assessed. In contrast to the acute pain results, only racemic mirtazapine exerted significant sustained analgesic effects up to 48 h. Antinociception was observed at all dosages with a maximum in the range of 0.01 mg/kg. Surprisingly, neither enantiomer was pro- nor antinociceptive at any dose or time. Our findings suggest that the synergism of both enantiomers is required to evoke a significant analgesic effect for the treatment of neuropathic pain. Our study gained no evidence for the use of either R(-) nor S(+)-mirtazapine alone. Due to the unique characteristics of (+/-)-mirtazapine and its proven efficacy in acute pain our results suggest that racemic mirtazapine may be a particularly useful antidepressant in the adjunctive treatment of chronic neuropathic pain states and could provide additional benefit to current therapeutic options.

Post-surgical neuropathic pain

Surgeons and anaesthetists are involved in Pain Medicine, as they have a responsibility to contribute to postoperative pain management and are often consulted about longer-term pain problems as well. A large component of persistent pain after surgery can be defined as neuropathic pain (NP). Nerves are injured during surgery and pain can persist after the surgical wound has healed. NP is because of a primary lesion or dysfunction of the peripheral or central nervous system. Prevalence estimates indicate that 2-3% of the population in the developed world experience NP. Persistent post-surgical NP is a mostly unrecognized clinical problem. The chronicity and persistence of post-surgical NP is often severely debilitating and impinges on the psychosocial, physical, economic and emotional well-being of patients. Options for treatment of any neuropathic factors are based on understanding the pain mechanisms involved. The current understandings of the mechanisms involved are presented. There is reasonable evidence for the efficacy of pharmacological management for NP. The aim of this article was to appraise the prevention, diagnostic work-up, the physical and particularly the pharmacological management of post-surgical NP and to provide a glimpse of advances in the field. It is a practical approach to post-surgical NP for all surgeons and anaesthetists. The take-home message is that prevention is better than waiting for post-surgical NP to become persistent.

Mirtazapine decreases the pain feeling in healthy participants

OBJECTIVES

The treatment of neuropathic pain is mainly based on antiepileptics, tricyclic antidepressants, and opiates. These drugs have important side effects disturbing the patient's quality of life. Mirtazapine (MTZ) is a new and well-tolerated tricyclic antidepressants with both monoaminergic and opioid properties that might favorably influence pain. The aim of this study was to assess whether MTZ can reduce the pain induced by a standardized stimulus presented to healthy human participants. The nociceptive flexion reflex (NFR) to an electric stimulus was chosen to determine the pain threshold.

METHODS

The effect of MTZ compared to placebo was assessed on 10 healthy participants in a double-blinded cross-over design. The NFR was measured the day after a single oral dose of drug (30 mg) or placebo.

RESULTS

A significant increase in upper limb (+29%, P=0.006) NFR threshold was observed.

DISCUSSION

MTZ increases the pain tolerance in healthy participants. The potential benefit of this effect on pain should be investigated more thoroughly in chronic neuropathic pain patients. The NFR might serve as an additional tool for the monitoring of these patients.

Recent advances in the pharmacological management of pain

Pain is an unpleasant sensation that originates from ongoing or impending tissue damage. Management of different types of pain (acute, postoperative, inflammatory, neuropathic or cancer) is the most frequent issue encountered by clinicians and pharmacological therapy is the first line of approach for the treatment of pain. This review presents and discusses recent clinical advances regarding both the improvements in delivery of analgesic drugs and improvements in the design of analgesic molecules. The new modalities of administration of analgesics used in the clinic are reviewed, including skin patches, oral and mucosal sprays, transdermal delivery systems and intranasal administration. New insights are then presented on standard drugs used to relieve pain, such as opioids (including tramadol), NSAIDs including selective cyclo-oxygenase-2 inhibitors, paracetamol (acetaminophen), local anaesthetics and adjuvant analgesics such as antidepressants, anticonvulsants (gabapentin and pregabalin), cannabinoids, ketamine and others (e.g. nefopam). Although the understanding of pain mechanisms has improved significantly recently, much more is yet to be discovered and awaited. Broadening of our knowledge is needed to improve basic and clinical research in this field in order to better alleviate pain in millions of people.

Cognitive function impairment in patients with neuropathic pain under standard conditions of care

The objective of this study is to analyze the prevalence to cognitive function impairment (CI) in a pragmatic cohort of subjects with different types of neuropathic pain under standard conditions of care by means of a cross-sectional baseline analysis of a sample of patients with NeP enrolled in a pragmatic, prospective, multicenter study evaluating the effectiveness of gabapentin. A total of 1519 patients (58.8% women), aged > or =18 years (mean [SD]=56.0 [13.7]), with neuropathic or mixed neuropathic and nociceptive pain for a mean (SD) of 1.1 (2.8) years were enrolled in the study. Pain was assessed with the short form of the McGill Pain Questionnaire, and cognitive function was measured with the Mini-Mental State Examination (MMSE). A score < or =24 on the MMSE was considered as CI. Multivariate logistic regression models were used to estimate CI prevalence. CI prevalence was substantially higher in patients with NeP than that reported in the general Spanish population, and significantly higher than in patients with mixed neuropathic and nociceptive pain: 11.4% (8.5%-14.3%) vs. 6.4% (4.6%-8.7%), P=0.006 (adjusted odds ratio=1.88 [1.21-2.91]). Prevalence significantly increased with age, up to 32.2% (20.6%-45.6%) in those with NeP, and to 28.2% (15.0%-44.9%) in those with mixed syndromes, for the age group > or =75 years, P<0.001 within age groups. Symptoms of anxiety and obesity, and to a lesser extent, symptoms of depression, were also factors associated with a higher prevalence rate of CI. This analysis showed that, after adjusting for confounding factors, the prevalence of CI was substantially higher in patients with NeP than those with mixed pain and the reference general population. Age, anxiety, depression, and obesity were factors found to be significantly associated with CI.

Mirtazapine and its enantiomers differentially modulate acute thermal nociception in rats

The antidepressant mirtazapine is an optically active drug and currently marketed as a racemic compound consisting of its S(+) and R(-)-enantiomers in a 50:50 mixture. As stereochemistry of antidepressants has become increasingly important to consider for the relevance of their analgesic properties, we investigated the effect of (+/-)-mirtazapine and its enantiomers in an animal model of acute thermal nociception. Wistar rats were injected intrathecal with either (+/-)-mirtazapine, R(-)-mirtazapine, S(+)-mirtazapine from 1 to 0.001 mg/kg and vehicle (0.9% NaCl), respectively. The effects on thermal paw withdrawal thresholds were monitored using the Hargreaves test. (+/-)-Mirtazapine exerted pro- and antinociceptive effects in acute thermal nociception, whereas R(-)-mirtazapine showed solely antinociceptive and S(+)-mirtazapine pronociceptive properties. These results clearly demonstrate a differential effect of (+/-)-mirtazapine and its enantiomers on nociception. As R(-)-mirtazapine exerts the antinociceptive activity of the racemic mixture it may be a putative candidate for an enantioselective use as analgesic.

Pharmacogenetics of monoamine transporters

The monoamine transporters are the sites of action of the most commonly used psychoactive compounds in therapeutic use today as well as the psychostimulant drugs of abuse. These transporters have been the focus of a large number of genetic association studies of complex behavioral phenomena. More recently, pharmacogenetic studies have suggested an association between a functional regulatory polymorphism in the serotonin transporter gene and antidepressant response. This review will discuss the clinical pharmacology of the monoamine transporters, their molecular genetic variability and the results of several association studies of the transporters and psychoactive drug response.