Do alpha2-adrenoceptors play an integral role in the antinociceptive mechanism of action of antidepressant compounds?

European Headache Federation (EHF) critical re-appraisal and meta-analysis of oral drugs in migraine prevention-part 1: amitriptyline

OBJECTIVE

The aim of this paper is to critically re-appraise the published trials assessing amitriptyline for migraine prophylaxis.

METHODS

We report our methods and results following the Preferred Reporting Items for Systematic Reviews (PRISMA), by searching MEDLINE, EMBASE, Cochrane CENTRAL, and ClinicalTrials.gov for randomized trials of pharmacologic treatments for migraine prophylaxis. We included randomized trials that compared amitriptyline with placebo for migraine prophylaxis in adults. Our outcomes of interest were informed by the Outcome Set for preventive intervention trials in chronic and episodic migraine (COSMIG) and include the proportion of patients who experience a 50% or more reduction in migraine days per month, migraine days per month, and adverse events leading to discontinuation. We assessed risk of bias by using a modified Cochrane RoB 2.0 tool and the certainty of evidence by using the GRADE approach.

RESULTS

Our search yielded 10.826 unique records, of which three trials (n = 622) were eligible for data synthesis and analysis. We found moderate certainty evidence that amitriptyline increases the proportion of patients who experience a 50% or more reduction in monthly migraine days, compared to placebo (relative risk: 1.60 (95% CI 1.17 to 2.19); absolute risk difference: 165 more per 1,000 (95% CI 47 more to 327 more). We found moderate certainty evidence that amitriptyline increases the proportion of patients who discontinue due to adverse events compared to placebo (risk difference: 0.05 (95% CI 0.01 to 0.10); absolute risk difference: 50 more per 1,000 (95% CI 10 more to 100 more).

CONCLUSIONS

Our meta-analysis showed that amitriptyline may have a prophylactic role in migraine patients, however these results are far from robust. This warrants further large-scale research to evaluate the role of amitriptyline in migraine prevention.

New Insights on Metabolic and Genetic Basis of Migraine: Novel Impact on Management and Therapeutical Approach

Migraine is a hereditary disease, usually one-sided, sometimes bilateral. It is characterized by moderate to severe pain, which worsens with physical activity and may be associated with nausea and vomiting, may be accompanied by photophobia and phonophobia. The disorder can occur at any time of the day and can last from 4 to 72 h, with and without aura. The pathogenic mechanism is unclear, but extensive preclinical and clinical studies are ongoing. According to electrophysiology and imaging studies, many brain areas are involved, such as cerebral cortex, thalamus, hypothalamus, and brainstem. The activation of the trigeminovascular system has a key role in the headache phase. There also appears to be a genetic basis behind the development of migraine. Numerous alterations have been identified, and in addition to the genetic cause, there is also a close association with the surrounding environment, as if on the one hand, the genetic alterations may be responsible for the onset of migraine, on the other, the environmental factors seem to be more strongly associated with exacerbations. This review is an analysis of neurophysiological mechanisms, neuropeptide activity, and genetic alterations that play a fundamental role in choosing the best therapeutic strategy. To date, the goal is to create a therapy that is as personalized as possible, and for this reason, steps forward have been made in the pharmacological field in order to identify new therapeutic strategies for both acute treatment and prophylaxis.

Dermal delivery of amitriptyline for topical analgesia

Abstract

Amitriptyline, administered orally, is currently one of the treatment options for the management of neuropathic pain and migraine. Because of the physicochemical properties of the molecule, amitriptyline is also a promising candidate for delivery as a topical analgesic. Here we report the dermal delivery of amitriptyline from a range of simple formulations. The first stage of the work required the conversion of amitriptyline hydrochloride to the free base form as confirmed by nuclear magnetic resonance (NMR). Distribution coefficient values were measured at pH 6, 6.5, 7, and 7.4. Solubility and stability of amitriptyline were assessed prior to conducting in vitro permeation and mass balance studies. The compound demonstrated instability in phosphate-buffered saline (PBS) dependent on pH. Volatile formulations comprising of isopropyl alcohol (IPA) and isopropyl myristate (IPM) or propylene glycol (PG) were evaluated in porcine skin under finite dose conditions. Compared with neat IPM, the IPM:IPA vehicles promoted 8-fold and 5-fold increases in the amount of amitriptyline that permeated at 24 h. Formulations containing PG also appear to be promising vehicles for dermal delivery of amitriptyline, typically delivering higher amounts of amitriptyline than the IPM:IPA vehicles. The results reported here suggest that further optimization of topical amitriptyline formulations should be pursued towards development of a product for clinical investigational studies.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s13346-021-00982-x.

Monoamines as Drug Targets in Chronic Pain: Focusing on Neuropathic Pain

Monoamines are involved in regulating the endogenous pain system and indeed, peripheral and central monoaminergic dysfunction has been demonstrated in certain types of pain, particularly in neuropathic pain. Accordingly, drugs that modulate the monaminergic system and that were originally designed to treat depression are now considered to be first line treatments for certain types of neuropathic pain (e.g., serotonin and noradrenaline (and also dopamine) reuptake inhibitors). The analgesia induced by these drugs seems to be mediated by inhibiting the reuptake of these monoamines, thereby reinforcing the descending inhibitory pain pathways. Hence, it is of particular interest to study the monoaminergic mechanisms involved in the development and maintenance of chronic pain. Other analgesic drugs may also be used in combination with monoamines to facilitate descending pain inhibition (e.g., gabapentinoids and opioids) and such combinations are often also used to alleviate certain types of chronic pain. By contrast, while NSAIDs are thought to influence the monoaminergic system, they just produce consistent analgesia in inflammatory pain. Thus, in this review we will provide preclinical and clinical evidence of the role of monoamines in the modulation of chronic pain, reviewing how this system is implicated in the analgesic mechanism of action of antidepressants, gabapentinoids, atypical opioids, NSAIDs and histaminergic drugs.

Current Prophylactic Medications for Migraine and Their Potential Mechanisms of Action

A relatively high number of different medications is currently used for migraine prevention in clinical practice. Although these compounds were initially developed for other indications and differ in their mechanisms of action, some general themes can be identified from the mechanisms at play. Efficacious preventive drugs seem to either suppress excitatory nervous signaling via sodium and/or calcium receptors, facilitate GABAergic inhibition, reduce neuronal sensitization, block cortical spreading depression and/or reduce circulating levels of CGRP. We here review such mechanisms for the different compounds.

Bioactive chromone constituents from Vitex negundo alleviate pain and inflammation

Background

Vitex negundo L. has been widely studied for its beneficial effect in inflammatory and pain conditions. The present study describes the isolation of two new bioactive chromone constituents from V. negundo and their in vivo evaluation for anti-inflammatory and antinociceptive activities.

Methods

Two new chromone derivatives, namely, methyl 3-(2-(5-hydroxy-6-methoxy-4-oxo-4H-chromen-2-yl)ethyl)benzoate (1) and 3-(1-hydroxy-2-(5-hydroxy-6-methoxy-4-oxo-4H-chromen-2-yl)ethyl)benzoic acid (2) were isolated from V. negundo and their structures were determined through various spectroscopic techniques including mass spectrometry, UV, IR, 1H NMR, 13C NMR, and two-dimensional-NMR like correlation spectroscopy and heteronuclear multiple bond correlation techniques. The isolated compounds (1–2) were tested for their prospective antinociceptive activity in acetic acid-induced abdominal constriction assay and anti-inflammatory activity in the carrageenan-induced paw edema assay in mice.

Results

Significant attenuation (P<0.001) of tonic visceral nociception was demonstrated by compound 1 and 2 at doses of 50 and 100 mg/kg. At similar doses, these compounds (1–2) also showed potent amelioration (P<0.001) of carrageenan-induced paw swelling.

Conclusion

The isolated chromone derivatives (1–2) from V. negundo are able to alleviate nociception and inflammation and the findings corroborated that V. negundo may be used as a potential source of antinociceptive and anti-inflammatory candidates.

Repeated Administration of Amitriptyline in Neuropathic Pain: Modulation of the Noradrenergic Descending Inhibitory System

BACKGROUND

The tricyclic antidepressant amitriptyline, the serotonin and noradrenaline reuptake inhibitor duloxetine, and gabapentinoids are first-line drugs for treatment of neuropathic pain. The analgesic effect of these drugs relates to brainstem-spinal descending noradrenergic systems. However, amitriptyline utilizes a variety of mechanisms for analgesia in neuropathic pain, and it is unclear which mechanism is most important. In the present study, we investigated the role of descending noradrenergic systems in the analgesic effect of these drugs for treatment of neuropathic pain. We also examined whether amitriptyline modifies the descending noradrenergic systems.

METHODS

Seven days after L5 spinal nerve ligation (SNL), rats received N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4, 50 mg/kg) to degenerate noradrenergic fibers. The rats then received 5 daily intraperitoneal injections of amitriptyline (10 mg/kg), duloxetine (10 mg/kg), pregabalin (10 mg/kg), or gabapentin (50 mg/kg) from 21 days after SNL surgery. Paw withdrawal thresholds were determined to assess the effect of the drugs on hyperalgesia after SNL. To determine whether 5 daily injections of amitriptyline activated noradrenergic neurons in the locus coeruleus (LC) and spinal cord with or without DSP-4 treatment, we performed immunohistochemistry using antibodies for c-Fos and dopamine beta-hydroxylase (DβH).

RESULTS

Five daily injections of amitriptyline, duloxetine, pregabalin, and gabapentin exerted antihyperalgesic effects in SNL rats (P < .001; estimated treatment effect of amitriptyline [99% confidence interval]: 59.9 [35.1-84.7] g). The antihyperalgesic effects of duloxetine, pregabalin, and gabapentin were reversed by pretreatment with DSP-4 (P < .001, respectively). However, antihyperalgesia was still observed after treatment of amitriptyline in SNL rats with DSP-4 pretreatment (P < .001, 59.7 [30.0-89.3] g), and this analgesic effect was not reversed by the α2-adrenoceptor antagonist idazoxan (30 μg). Additionally, 5 daily injections of amitriptyline increased the ratio of c-Fos-immunoreactive (IR) cells in noradrenergic LC neurons in SNL rats with or without DSP-4 pretreatment (P < .001, respectively). Five daily injections of amitriptyline increased DβH-IR in the LC and the spinal dorsal horn of SNL rats (P < .001, respectively). With DSP-4 pretreatment, DβH-IR was dramatically decreased with or without 5 daily injections of amitriptyline (P < .001).

CONCLUSIONS

Five daily injections of amitriptyline produced antihyperalgesic effects against neuropathic pain despite suppression of noradrenergic descending inhibitory systems. Amitriptyline activated LC neurons and increased noradrenergic fibers density in SNL rats. These results suggest that amitriptyline could still produce analgesia under pathological dysfunction of the descending noradrenergic system. Amitriptyline may enhance the analgesic effect of drugs for neuropathic pain that require normal descending noradrenergic inhibition to produce analgesia, such as serotonin and noradrenaline reuptake inhibitors and gabapentinoids.

A multi-target therapeutic potential of Prunus domestica gum stabilized nanoparticles exhibited prospective anticancer, antibacterial, urease-inhibition, anti-inflammatory and analgesic properties

Background

Phytotherapeutics exhibit diverse pharmacological effects that are based on the combined action of a mixture of phytoconstituents. In this study, Prunus domestica gum-loaded, stabilized gold and silver nanoparticles (Au/Ag-NPs) were evaluated for their prospective anticancer, antibacterial, urease-inhibition, anti-inflammatory, and analgesic properties.

Methods

Au/Ag-NPs were biosynthesized and characterized with UV-Vis, FTIR, SEM, EDX, and XRD techniques. The effect of gum and metal ion concentration, reaction temperature, and time on the synthetic stability of nanoparticles was studied along with their post-synthetic stability against varying pH and salt concentrations, long-term storage and extremes of temperature. Nanoparticles were tested for anticancer (HeLa cervical cancer cells), antibacterial (Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa), urease inhibition (jack-bean urease), anti-inflammatory (carrageenan-induced paw edema), and antinociceptive (abdominal constriction response) activities.

Results

The nanoparticles were mostly spherical with an average particle size between 7 and 30 nm (Au-NPs) and 5–30 nm (Ag-NPs). Au/Ag-NPs maintained their colloidal stability and nanoscale characteristics against variations in physicochemical factors. Au/Ag-NPs have potent anticancer potential (IC₅₀ = 2.14 ± 0.15 μg/mL and 3.45 ± 0.23 μg/mL). Au/Ag-NPs selectively suppressed the growth of S. aureus (10.5 ± 0.6 mm, 19.7 ± 0.4 mm), E. coli (10 ± 0.4 mm, 14.4 ± 0.7 mm), and P. aeruginosa (8.2 ± 0.3 mm, 13.1 ± 0.2 mm), as well as showed preferential inhibition against jack-bean urease (19.2 ± 0.86%, 21.5 ± 1.17%). At doses of 40 and 80 mg/kg, Au-NPs significantly ameliorated the increase in paw edema during the 1st h (P < 0.05, P < 0.01) and 2–5 h (P < 0.001) of carrageenan-induced inflammation compared to the 200 and 400 mg/kg doses of P. domestica gum (P < 0.05, P < 0.001). At similar doses, Au-NPs also significantly abolished (P < 0.01) the tonic visceral, chemically-induced nociception, which was comparable to that of P. domestica gum (200 mg/kg; P < 0.05, 400 mg/kg; P < 0.01).

Topical Treatments for Localized Neuropathic Pain

Purpose of Review

Topical therapeutic approaches in localized neuropathic pain (LNP) syndromes are increasingly used by both specialists and general practitioners, with a potentially promising effect on pain reduction. In this narrative review, we describe the available compounds for topical use in LNP syndromes and address their potential efficacy according to the literature.

Recent Findings

Local anaesthetics (e.g., lidocaine, bupivacaine and mepivacaine), as well as general anaesthetic agents (e.g., ketamine), muscle relaxants (e.g., baclofen), capsaicin, anti-inflammatory drugs (e.g., diclofenac), salicylates, antidepressants (e.g., amitriptyline and doxepin), α2 adrenergic agents (e.g., clonidine), or even a combination of them have been tested in various applications for the treatment of LNP. Few of them have reached a sufficient level of evidence to support systematic use as treatment options.

Summary

Relatively few systemic side effects or drug–drug interactions and satisfactory efficacy seem to be the benefits of topical treatments. More well-organized and tailored studies are necessary for the further conceptualization of topical treatments for LNP.

Antinociceptive Effect of Mirtazapine in Rats with Diabetic Neuropathy

INTRODUCTION

To evaluate the antinociceptive effect of mirtazapine and the mechanisms mediating this effect in neuropathic pain in rats with diabetes.

METHODS

The experiments were performed in Sprague Dawley rats using a hot-plate device. Streptozotocin (STZ) was administered to the rats after taking control measurements. Rats with a blood glucose level of 240 mg/dL or above in the blood specimen obtained from the tail vein 3 days after STZ administration were considered as being diabetic. Three weeks after STZ administration, the hot-plate test was performed. Compared with the control measurements, rats that exhibited >20% decrease in the second hot-plate test measurements were considered to have developed neuropathy. Drugs [mirtazapine, naloxone (opioidergic antagonist), metergoline (serotonergic antagonist), and BRL44408 (adrenergic antagonist)] and drug combinations were administered to those rats that developed neuropathy. After administrating the drugs or drug combinations, the third hot-plate test was performed.

RESULTS

Mirtazapine at doses of 10 and 15 mg/kg exhibited a significant antinociceptive effect. Naloxone, metergoline, or BRL44408 alone did not cause an antinociceptive effect. However, combinations of these drugs with mirtazapine (15 mg/kg) significantly decreased the antinociceptive effect of mirtazapine.

CONCLUSION

It is suggested that mirtazapine has a significant antinociceptive effect in diabetic neuropathy and that opioidergic, serotonergic, and adrenergic systems have roles to play in this effect.

Topical treatment in pain medicine: from ancient remedies to modern usage

Over several millennia, substances have been applied to the skin for treatment of pain. Some ingredients are in current use; others have been discontinued. Mechanisms of action include interactions with nociceptive neural networks and inflammatory processes. Substances must penetrate the stratum corneum barrier and vehicles that enhance penetration have been developed. Topical drugs with links to the past include menthol, capsaicin, some opioids, local anesthetic agents and NSAIDs. Mandragora is also described as an example of a herbal remedy that has been discontinued due to its toxicity. The future for topical drugs is promising, with the advent of new drugs tailored for specific pain mechanisms and the development of both penetration enhancers and sterile preparation methods.

The search for novel analgesics: targets and mechanisms

The management of the pain state is of great therapeutic relevance to virtually every medical specialty. Failure to manage its expression has deleterious consequence to the well-being of the organism. An understanding of the complex biology of the mechanisms underlying the processing of nociceptive information provides an important pathway towards development of novel and robust therapeutics. Importantly, preclinical models have been of considerable use in determining the linkage between mechanism and the associated behaviorally defined pain state. This review seeks to provide an overview of current thinking targeting pain biology, the use of preclinical models and the development of novel pain therapeutics. Issues pertinent to the strengths and weaknesses of current development strategies for analgesics are considered.

Design, synthesis, and biological evaluation of a series of bifunctional ligands of opioids/SSRIs

A series of opioid and serotonin re-uptake inhibitors (SSRIs) bifunctional ligands have been designed, synthesized, and tested for their activities and efficacies at μ-, δ- and κ opioid receptors and SSRIs receptors. Most of the compounds showed high affinities for μ- and δ-opioid receptors and lower affinities for SSRIs and κ opioid receptors. A docking study on the μ-opioid receptor binding pocket has been carried out for ligands 3-11. The ligands 7 and 11 have displayed the highest binding profiles for the μ-opioid receptor binding site with ΔGbind (-12.14kcal/mol) and Ki value (1.0nM), and ΔGbind (-12.41kcal/mol) and Ki value (0.4nM), respectively. Ligand 3 was shown to have the potential of dual acting serotonin/norepinephrine re-uptake inhibitor (SNRI) antidepressant activity in addition to opioid activities, and thus could be used for the design of multifunctional ligands in the area of a novel approach for the treatment of pain and depression.

Exploring the possible mechanisms of action behind the antinociceptive activity of Bacopa monniera

AIM

Earlier studies have demonstrated that Bacopa monniera (BM), a plant described in Ayurveda for many CNS actions was found to exhibit antidepressant (methanolic extract at 20mg/kg and 40mg/kg p.o.) as well as antinociceptive activity (aqueous extract (AE) at 80 mg/kg, 120 mg/kg and 160 mg/kg p.o.). The present study sought to explore the possible mechanisms of antinociceptive effects of aqueous extract of Bacopa monniera (AEBM) at 80 mg/kg, 120 mg/kg and 160 mg/kg given orally.

MATERIALS AND METHODS

AEBM was given singly as well as with selective α2 receptor blocker Yohimbine, selective β1 receptor blocker Atenolol, serotonin receptor antagonist Cyproheptadine and a non-selective opioid receptor antagonist naloxone in experimental groups of mice and rats under strict protocols and conditions.

RESULTS

We observed that the antinociceptive effects of AEBM in the acetic acid writhing test was prevented by prior treatment with the selective Yohimbine (1 mg/kg, i.p; 14.50 ± 2.26 and 37.17 ± 2.14 writhes in the AEBM-treated and yohimbine pre-treated AEBM groups, respectively) and selective β1 Atenolol receptor blocker (1 mg/kg, i.p; 14.50 ± 2.26 and 31.00 ± 5.44 writhes in the AEBM-treated and yohimbine pre-treated AEBM groups, respectively). In the formalin test, the reduction in licking time with AEBM was found to be reversed by prior treatment with serotonin receptor antagonist Cyproheptadine (1 mg/kg, i.p; 47.33 ± 2.25s and 113.50 ± 3.83s (during phase I i.e. 0-5 min) and 26.67 ± 3.83s and 88.17 ± 7.27s (during phase II i.e. 20-30 min) in the AEBM-treated and Cyproheptadine pre-treated AEBM groups, respectively). The % increase in tail flick latency with AEBM was prevented by prior treatment with the non-selective opioid receptor antagonist naloxone (2mg/kg, i.p; 282.35 and 107.35 in the AEBM-treated and naloxone-treated groups, respectively).

CONCLUSIONS

Our results indicate, that the endogenous adrenergic, serotonergic and opioidergic systems are involved in the analgesic mechanism of action of the aqueous extract of Bacopa monniera.

The antinociceptive effects of intravenous tianeptine in colorectal distension-induced visceral pain in rats: the role of 5-HT₃ receptors

Tianeptine is an unusual tricyclic antidepressant drug. In this study, we aimed to investigate the antinociceptive effect of tianeptine on visceral pain in rats and to determine whether possible antinociceptive effect of tianeptine is mediated by serotonergic (5-HT(2,3)) and noradrenergic (α(1,2)) receptor subtypes. Male Sprague Dawley rats (250-300 g) were supplied with a venous catheter, for drug administrations, and enameled nichrome electrodes, for electromyography, at external oblique musculature. Colorectal distension (CRD) was employed as the noxious visceral stimulus and the visceromotor response (VMR) to CRD was quantified electromyographically before and 5, 15, 30, 60, 90 and 120 min after tianeptine administration. Antagonists were administered 10 min before tianeptine for their ability to change tianeptine antinociception. Intravenous administration of tianeptine (2.5-20 mg/kg) produced a dose-dependent reduction in VMR. Administration of 5-HT(3) receptor antagonist ondansetron (0.5, 1 and 2 mg/kg), but not 5-HT(2) receptor antagonist ketanserine (0.5, 1 and 2 mg/kg), reduced the antinociceptive effect of tianeptine (10mg/kg). In addition, administration of α(1)-adrenoceptor antagonist prazosin (1 mg/kg) or α(2)-adrenoceptor antagonist yohimbine (1 mg/kg) did not cause any significant effect on the tianeptine-induced antinociception. Our data indicate that intravenous tianeptine exerts a pronounced antinociception against CRD-induced visceral pain in rats, and suggests that the antinociceptive effect of tianeptine appears to be mediated in part by 5-HT(3) receptors, but does not involve 5-HT(2) receptors or α-adrenoceptors.

Study of antinociceptive activity of SSRI (fluoxetine and escitalopram) and atypical antidepressants (venlafaxine and mirtazepine) and their interaction with morphine and naloxone in mice

Objective:

to study the probable site of antinociceptive action of SSRI (fluoxetine, escitalopram) and atypical antidepressants (mirtazapine, venlafaxine) and their interaction with morphine and naloxone.

Materials and Methods:

the study was conducted on albino mice (25-35 grams) of either sex. Different doses of morphine (0.5 and 1 mg/kg), fluoxetine (2, 5 and 10 mg/kg), venlafaxine (30, 40 and 50 mg/kg), mirtazapine (3, 5 and 7 mg/kg) and escitalopram (2.5, 5 and 10 mg/kg) were administered subcutaneously to obtain their subanalgesic doses using tail flick analgesiometer. Tail flick latencies were obtained at 15, 30, 60 and 120 min. after drug administration. Naloxone (1 mg/kg) was administered 10 minutes prior to test drug for testing antagonism.

Observations:

fluoxetine (5 and 10 mg/kg), mirtazapine (5 and 7 mg/kg) and venlafaxine (40 and 50 mg/kg) were found to have antinociceptive activity but not at lower doses. Escitalopram failed to show any antinociceptive activity at any of the doses used. The antinociceptive effect of all the drugs was antagonized by naloxone (1 mg/kg). Further, subanalgesic doses of fluoxetine, mirtazapine and venlafaxine showed analgesic activity with suboptimal dose of morphine (0.5 mg/kg).

Result and conclusion:

fluoxetine, mirtazapine and venlafaxine have antinociceptive activity whereas escitalopram doesn’t; their site of action seems to be the same as that of opioid analgesics (‘mue’ receptors). However, other pathways (cholinergic, histaminic, noradrenergic, GABAergic) may be involved in mediation of their analgesic activity, deserving further elucidation. Results apparently show that these drugs may be useful in the management of pain as monotherapy or in combination with other opioids.

Beta2-adrenoceptor agonists alleviate neuropathic allodynia in mice after chronic treatment

BACKGROUND AND PURPOSE

Antidepressants are a first-line treatment against neuropathic pain. We previously demonstrated that beta(2)-adrenoceptors are necessary for antidepressants to exert their anti-allodynic action. The aim of the present study was to assess whether beta(2)-adrenoceptor agonists could be sufficient to alleviate neuropathic allodynia.

EXPERIMENTAL APPROACH

We used a murine model of neuropathy induced by unilateral sciatic nerve cuffing in C57BL/6J mice. We previously demonstrated that this animal model is sensitive to chronic, but not to acute, treatment with antidepressant drugs, which is clinically relevant. The mechanical allodynia was evaluated using the von Frey filaments.

KEY RESULTS

We showed that chronic but not acute treatment with the beta-adrenoceptor agonists, bambuterol, isoprenaline, fenoterol, salbutamol, salmeterol, terbutaline or ritodrine suppressed mechanical allodynia. We confirmed that the action of these beta-adrenoceptor agonists was mediated through beta(2)-adrenoceptors by blocking it with intraperitoneal or intrathecal, but not intracerebroventricular or intraplantar, injections of the antagonist ICI118551. We also showed that chronic treatments with the beta-adrenoceptor antagonists, propranolol or ICI118551 did not suppress the allodynia.

CONCLUSIONS AND IMPLICATIONS

Our data show that chronic treatment with beta-adrenoceptor agonists has the same antiallodynic properties as treatments with antidepressant drugs. This study was, however, conducted in an animal model, and a clinical validation will be required to confirm the value of the present findings in patients.

Serotonin-norepinephrine reuptake inhibitors for pain control: premise and promise

The precise mechanisms of pain perception and transmission in the central nervous system have not been fully elucidated. However, extensive data support a role for the monoamine neurotransmitters, serotonin and norepinephrine, in the modulation of pain. Experiments with animal models of pain indicate that noradrenergic interventions, and to a lesser extent serotonergic interventions, reduce pain-related behavior. This is supported by data from clinical trials in humans in which antidepressants have been shown to reduce pain and functional impairment in central and neuropathic pain conditions. These effects are particularly well-studied in trials with serotonin-norepinephrine reuptake inhibitors (SNRIs), which have provided a useful tool in the clinician's arsenal, particularly considering the limitations of other classes of pain medications such as opioids, anti-inflammatories, and anticonvulsants (i.e., limited efficacy, safety and tolerability issues). Moreover, painful physical symptoms are frequently comorbid with major psychiatric disorders such as major depressive disorder and anxiety disorders. This paper reviewed and summarized the rationale and potential role of SNRIs for the control of pain including clinical and preclinical background. Currently evidence does not definitely support a role of the SNRIs, while limited data propose a putative promise of SNRIs in the treatment of pain related disorders including fibromyalgia and depressed patients with multiple somatic complaints. More researches are warranted to generalize currently available preliminary evidences.

Antidepressants as analgesics

Depression is a common accompaniment of pain, particularly when pain is unremitting. The use of a variety of antidepressant medications is associated with pain reduction, an effect that is independent of the mood-enhancing qualities of these drugs. This pain relief is a consequence of a wide variety of actions of antidepressants on the neuroregulatory mechanisms associated with pain perception and transmission. The older tricyclic antidepressants (TCAs) and the newer 'balanced' reuptake inhibitors (such as duloxetine) seem to be more efficacious in terms of providing pain relief than the selective serotonin reuptake inhibitors (SSRIs). Unfortunately, adverse effects are not uncommon during antidepressant use, particularly with TCAs. It is now becoming apparent that TCAs can have an analgesic effect when applied topically and that this effect is produced by peripheral mechanisms rather than systemic uptake. Antidepressants remain a major therapeutic tool in the management of chronic pain.

New developments in the use of tricyclic antidepressants for the management of pain

In recent years, tricyclic antidepressant drugs have experienced a resurgence in their use as valuable pharmacological tools in the treatment of pain. Along with the evolution in our understanding of their analgesic mechanisms of action, there have been concurrent breakthroughs regarding their indications for use and modes of administration. This review focuses on recent advances in our understanding of how antidepressant drugs exert their antinociceptive effects, and new developments regarding their clinical application.

Amitriptyline is a potent blocker of human Kv1.1 and Kv7.2/7.3 channels

BACKGROUND

Kv1.1 and Kv7.2/7.3 channels control excitability of neuronal cells. As hyperexcitability is a sign of neuropathic pain, epilepsy, and anxiety disorders, these channels may be important molecular targets of amitriptyline that cause pharmacological as well as toxicological effects by altering neuronal excitability. Since the molecular mechanisms underlying these effects of amitriptyline have not been fully elucidated, we aimed to characterize the interaction of amitriptyline with human Kv1.1 and Kv7.2/7.3 channels. We also intended to establish the interaction of amitriptyline with the Kv7.2/7.3 channel opener, retigabine.

METHODS

Kv1.1 and Kv7.2/7.3 channels were expressed in human embryonic kidney cells and in Chinese hamster ovary cells. The effects of amitriptyline and retigabine were studied with the patch-clamp technique.

RESULTS

Amitriptyline inhibited Kv1.1 and Kv7.2/7.3 channels in a concentration-dependent and reversible manner. The IC50-value was 22 +/- 3 microM (n = 33) and 10 +/- 1 microM (n = 40), respectively. Deactivating inward currents of Kv7.2/7.3 channels were inhibited with an IC50-value of 4.2 +/- 0.6 microM (n = 32). Inhibition of Kv7.2/7.3 channels by amitriptyline reversibly depolarized the resting membrane potential. Retigabine reversed both the inhibitory action of amitriptyline on Kv7.2/7.3 channels as well as the depolarization of the membrane potential.

CONCLUSIONS

Since amitriptyline inhibited Kv1.1 and Kv7.2/7.3 channels only at toxicologically relevant plasma concentrations, our results suggest a role for these channels in the neuroexcitatory side effects of amitriptyline. As the inhibitory effects of amitriptyline were reversed by retigabine, a combination of amitriptyline and retigabine could be of additional benefit in the therapy of neuropathic pain.

Phase 1A safety assessment of intravenous amitriptyline

The antidepressant amitriptyline is used as an adjuvant in the treatment of chronic pain. Among its many actions, amitriptyline blocks Na+ channels and nerves in several animal and human models. As perioperative intravenous lidocaine has been suggested to decrease postoperative pain, amitriptyline, because of its longer half-life time, might be more useful than lidocaine. However, the use of intravenous amitriptyline is not approved by the US Food and Drug Administration. We therefore investigated the adverse effects of preoperative intravenous amitriptyline in a typical phase 1A trial. After obtaining written Food and Drug Administration and institutional review board approval, we obtained written consent for preoperative infusion of amitriptyline in an open-label, dose-escalating design (25, 50, and 100 mg, n=5 per group). Plasma levels of amitriptyline/nortriptyline were determined, and adverse effects were recorded in a predetermined symptom list. Infusion of 25 and 50 mg amitriptyline appears to be well tolerated; however, the study was terminated when 1 subject in the 100-mg group developed severe bradycardia. Intravenous infusion of amitriptyline (25 to 50 mg over 1 hour) did not create side effects beyond dry mouth and drowsiness, or dizziness, in 2 of our 10 otherwise healthy participants receiving the 25- to 50-mg dose. An appropriately powered future trial is necessary to determine a potential role of amitriptyline in decreasing postoperative pain.

PERSPECTIVE

Amitriptyline potently blocks the persistently open Na+ channels, which are known to be instrumental in various pain states. As this occurs at very low plasma concentrations, a single preoperative intravenous infusion of amitriptyline could provide long-lasting pain relief and decrease the incidence of chronic pain.

Anti-convulsants and anti-depressants

Damage to a nerve should only lead to sensory loss. While this is common, the incidence of spontaneous pain, allodynia and hyperalgesia indicate marked changes in the nervous system that are possible compensations for the loss of normal function that arises from the sensory loss. Neuropathic pain arises from changes in the damaged nerve which then alter function in the spinal cord and the brain and lead to plasticity in areas adjacent to those directly influenced by the neuropathy. The peripheral changes drive central compensations so that the mechanisms involved are multiple and located at a number of sites. Nerve damage increases the excitability of both the damaged and undamaged nerve fibres, neuromas and the cell bodies in the dorsal root ganglion. These peripheral changes are substrates for the ongoing pain and the efficacy of excitability blockers such as carbamazepine, lamotrigine and mexiletine, all anti-convulsants. A better understanding of ion channels at the sites of injury has shown important roles of particular sodium, potassium and calcium channels in the genesis of neuropathic pain. Within the spinal cord, increases in the activity of calcium channels and the receptors for glutamate, especially the N-methyl-D-aspartate (NMDA) receptor, trigger wind-up and central hyperexcitability. Increases in transmitter release, neuronal excitability and receptive field size result from the damage to the peripheral nerves. Ketamine and gabapentin/pregabalin, again with anti-convulsant activity, may interact with these mechanisms. Ketamine acts on central spinal mechanisms of excitability whereas gabapentin acts on a subunit of calcium channels that is responsible for the release of pain transmitters into the spinal cord. In addition to these spinal mechanisms of hyperexcitability, spinal cells participate in a spinal-supraspinal loop that involves parts of the brain involved in affective responses to pain but also engages descending excitatory and inhibitory systems that use the monoamines. These pathways become more active after nerve injury and are the site of action of anti-depressants. This chapter reviews the evidence and mechanisms of drugs, both anti-depressants and anti-convulsants, that are believed to be effective in pain control, with a major emphasis on the neuropathic state.

Possible involvement of opioidergic systems in the antinociceptive effect of the selective serotonin reuptake inhibitors in sciatic nerve-injured mice

The involvement of opioid receptor activation in the antinociceptive effect of either fluvoxamine, a selective serotonin reuptake inhibitor, or serotonin (5-HT) on thermal hyperalgesia and mechanical allodynia in a model of neuropathic pain in mice induced by sciatic nerve ligation was examined. The experiments were conducted 2 or 6 weeks after unilateral sciatic nerve ligation. Ipsilateral thermal hyperalgesia and mechanical allodynia were observed both 2 and 6 weeks after sciatic nerve ligation. Neither s.c. fluvoxamine nor i.t. 5-HT affected sciatic nerve ligation-induced thermal hyperalgesia or mechanical allodynia in mice 2 weeks after sciatic nerve ligation. However, the same dose of either fluvoxamine or 5-HT significantly reduced mechanical allodynia but not thermal hyperalgesia in sciatic nerve ligated mice 6 weeks after surgery. The antinociceptive effect of fluvoxamine on sciatic nerve ligation-induced mechanical allodynia in mice 6 weeks after surgery was completely abolished by pretreatment with either naloxone, a nonselective opioid receptor antagonist, or beta-funaltrexamine, a selective mu-opioid receptor antagonist. Furthermore, pretreatment with naltrindole, a selective delta-opioid receptor antagonist, partially but significantly inhibited the antinociceptive effect of fluvoxamine in sciatic nerve ligated mice at the 6th postoperative week. The antinociceptive effect induced by i.t. 5-HT was also completely antagonized by either naloxone or beta-funaltrexamine, and partially inhibited by naltrindole. However, pretreatment with nor-binaltorphimine, a selective kappa-opioid receptor antagonist, had no effect against either s.c. fluvoxamine- or i.t. 5-HT-induced antinociception. These results suggest that the antinociceptive effect of s.c. fluvoxamine or i.t. 5-HT in the chronic state of sciatic nerve ligation-induced neuropathic pain may be related to opioidergic activity, mainly through the activation of spinal mu- and delta-opioid receptors.

The four-plates test: anxiolytic or analgesic paradigm?

The four-plates test (FPT) is an animal model of anxiety in which the exploration of the novel surroundings is suppressed by the delivery of a mild electric foot shock. The anti-nociceptive system has been reported to be activated by a variety of stressful stimuli such as footshock. The present study was thus designed to compare effects of drugs in the FPT and in the hot-plate test (an animal model of pain), in order to disambiguate the drug-induced anti-punishment effects obtained in the FPT from alterations in pain sensitivity. Various compounds, known to be implicated in anxiety states as well as nociception, have been studied. Although morphine induced a strong anti-nociceptive effect, it did not modify the number of shocks received in the FPT. Alprazolam and diazepam induced an anxiolytic-like effect in the FPT, at doses that did not induce any effect in the hot-plate test. The antidepressants previously reported anxiogenic (desipramine, maprotiline) in the FPT were found to be analgesic at the same doses. Milnacipran, venlafaxine and paroxetine did not modify the pain threshold, whereas they have previously been shown to induce anxiolytic-like effects in the FPT. The dopaminergic antidepressant agent nomifensine was without effect on both tests. Our results suggest that the reported drug-induced anti-punishment effects in the FPT are not related to modifications of pain threshold but to a pure anxiolytic-like effect.

Norepinephrine transporter knockout-induced up-regulation of brain alpha2A/C-adrenergic receptors

The norepinephrine transporter (NET) is responsible for the rapid removal of norepinephrine released from sympathetic neurons; this release is controlled by inhibitory alpha(2)-adrenergic receptors (alpha(2)ARs). Long-term inhibition of the NET by antidepressants has been reported to change the density and function of pre- and postsynaptic ARs, which may contribute to the antidepressant effects of NET inhibitors such as desipramine. NET-deficient (NET-KO) mice have been described to behave like antidepressant-treated mice. By means of quantitative real-time PCR we show that mRNAs encoding the alpha(2A)-adrenergic receptor (alpha(2A)AR) and the alpha(2C)-adrenergic receptor (alpha(2C)AR) are up-regulated in the brainstem, and that alpha(2C)AR mRNA is also elevated in the hippocampus and striatum of NET-KO mice. These results were confirmed at the protein level by quantitative autoradiography. The NET-KO mice showed enhanced binding of the selective alpha(2)AR antagonist [(3)H]RX821002 in several brain regions. Most robust increases (20-25%) in alpha(2)AR expression were observed in the hippocampus and in the striatum. Significant increases (16%) were also seen in the extended amygdala and thalamic structures. In an 'in vivo' test, the alpha(2)AR agonist clonidine (0.1 mg/kg) caused a significantly greater reduction of locomotor activity in NET-KO mice than in wild-type mice, showing the relevance of our findings at the functional level.

The Different Roles of 5-HT2 and 5-HT3 Receptors on Antinociceptive Effect of Paroxetine in Chemical Stimuli in Mice

Serotonin (5-HT) is known to be an important mediator in pain modulation. Some centrally acting agents, like selective serotonin reuptake inhibitors (SSRIs), modulate pain. Activation of the endogenous opioid mechanisms or potentiation of analgesic effect by serotonergic and/or noradrenergic pathways might be involved in antinociception of SSRIs. However, peripheral mechanisms of nociception are not clear. In this study, the antinociceptive effect of paroxetine, its interaction with the opioidergic system and serotonin receptors were tested using the writhing test in mice. Paroxetine (5, 10, 20 mg/kg) induced an antinociceptive effect following i.p. administration in writhing test. For the groups in which the antagonists were tested, the dose of paroxetine that caused a significant and equipotent analgesic effect similar to 0.5 mg/kg morphine was selected. Naloxone significantly antagonized the antinociceptive effects of both paroxetine and morphine in a similar pattern and magnitude. Ketanserin (5-HT(2)-receptor antagonist) or ondansetron (5-HT(3)-receptor antagonist) alone did not alter the nociceptive action of acetic acid. While the antinociceptive effect of paroxetine was highly potentiated by ketanserin, ondansetron reduced that antinociception. In conclusion, our results indicate that the antinociceptive effect of paroxetine mainly depends on central opioidergic and serotonergic mechanisms. Peripheral serotonergic mechanisms/receptors may contribute to this antinociceptive effect, especially by 5-HT(3)-receptor subtypes.

Possible involvement of opioidergic and serotonergic mechanisms in antinociceptive effect of paroxetine in acute pain

Antidepressant drugs, especially tricyclics have been widely used in the treatment of chronic pain, but not in acute pain. Because of numerous undesirable side effects, the selective serotonin reuptake inhibitors (SSRIs), with their favorable side effect profile, are preferred nowadays. An activation of the endogenous opioid mechanisms or potentiation of the analgesic effect mediated by serotonergic and/or noradrenergic pathways are thought to be involved in the antinociceptive action of SSRIs. In this study, the potential antinociceptive effect of paroxetine and its interaction with opioidergic system and serotonin receptors were evaluated. The antinociceptive effect of paroxetine was tested using a hot plate test in mice. Paroxetine, a SSRI antidepressant drug, induced an antinociceptive effect following i.p. administration. This antinociception was significantly inhibited by naloxone, an opioid receptor antagonist, suggesting the involvement of opioidergic mechanisms. While ondansetron (a 5-HT(3)-receptor antagonist) inhibited the effect of paroxetine, ketanserin (a 5-HT(2)-receptor antagonist) could not. In conclusion, paroxetine-induced antinociception, similar to morphine, suggests an involvement of direct or indirect action (via an increase in release of endogenous opioid peptide(s)) at opioid receptor sites and an involvement of serotonergic mechanisms mainly at the receptor level.

Loss of amitriptyline analgesia in alpha 2A-adrenoceptor deficient mice

Tricyclic antidepressants have analgesic and sedative effects in addition to their antidepressive properties. We tested the acute analgesic and locomotor inhibitory effects of the tricyclic antidepressant amitriptyline and the alpha(2)-adrenoceptor agonist clonidine in wild-type control and in alpha(2A)-adrenoceptor knockout mice in hot-plate and tail-flick tests. Amitriptyline-induced analgesia was lost in alpha(2A)-adrenoceptor knockout mice. The locomotor inhibitory effect of amitriptyline was reduced, but not fully abolished in alpha(2A)-adrenoceptor knockout mice. Similar results were obtained with clonidine. We conclude that alpha(2A)-adrenoceptors appear to have a significant role in amitriptyline-induced acute analgesia in mice, and that alpha(2A)-adrenoceptors also participate in the sedative effects of amitriptyline.

Local Anesthetic Properties of a Novel Derivative, N-Methyl Doxepin, Versus Doxepin and Bupivacaine

Among various tricyclic antidepressants, doxepin and amitriptyline are also long-acting local anesthetics. We synthesized a new compound, N-methyl doxepin, and investigated whether this derivative possesses local anesthetic properties. N-methyl doxepin and doxepin were tested in a rat sciatic nerve model at 2.5, 5.0, and 10 mM. Proprioceptive, motor, and nociceptive blockade were evaluated and compared with those induced by 0.5% bupivacaine. Block of Na(+) channels by N-methyl doxepin and doxepin was assessed in cultured pituitary tumor cells under voltage clamp conditions. N-methyl doxepin elicited complete nociceptive blockade that generally lasted longer than that caused by doxepin (e.g., approximately 7.4 h versus 5.3 h at 10 mM). Significant differences were observed for full recovery of function at all concentrations and for the duration of complete blockade except at 2.5 mM. Bupivacaine at 0.5% (15.4 mM) was less effective in producing complete blockade (approximately 1.5 h) than N-methyl doxepin and doxepin. Both doxepin and N-methyl doxepin were potent Na(+) channel blockers, although N-methyl doxepin displayed a slower wash-in rate. No morphological alterations were detected in cross-sectioned sciatic nerve specimens with these three drugs. We conclude that N-methyl doxepin is a potent Na(+) channel blocker and a long-acting local anesthetic for rat sciatic nerve blockade.

IMPLICATIONS

N-methyl doxepin and doxepin are both potent Na(+) channel blockers; they elicit rat sciatic nerve block lasting longer than that induced by bupivacaine and seem to be nontoxic to peripheral nerves at concentrations up to 10 mM.

Effects of imipramine on the expression and development of morphine dependence in mice

In the present study, the effects of imipramine and/or alpha-adrenoceptor agents on naloxone-induced jumping in morphine-dependent mice were examined. In the first set of experiments, the drugs were used before naloxone injection, to test their effects on the expression of jumping. Administration of imipramine (10-60 mg/kg) 15 min before naloxone increased the number of jumping in mice. Injection of the alpha2-adrenoceptor agonist, clonidine (0.1 mg/kg), or alpha1-adrenoceptor agonist, phenylephrine (4 mg/kg), themselves neither altered naloxone-induced jumping nor influenced the imipramine response. The alpha2-adrenoceptor antagonist, yohimbine (4 mg/kg), itself but not the alpha1-adrenoceptor antagonist, prazosin (1 mg/kg), increased jumping and decreased the imipramine effect. In the second set of experiments, imipramine and/or the alpha-adrenoceptor drugs were injected during the development of morphine dependence. Imipramine (10-40 mg/kg) increased the development of dependence and increased jumping was seen. Clonidine did not influence the imipramine effect. Phenylephrine was lethal in combination with imipramine. Both yohimbine and prazosin decreased the effect of imipramine. Imipramine and phenylephrine but not clonidine, yohimbine or prazosin decreased locomotion. It is concluded an alpha2-adrenoceptor mechanism may be involved in the influence of imipramine on the expression and development of naloxone-induced withdrawal signs in mice.

Mechanistic stratification of antineuralgic agents

Current treatment options in neuropathic pain include antidepressants, antiepileptics, antiarrhythmics, and analgesics. However, stratification of treatments based on their original therapeutic class is inadequate, as drugs belonging to a particular class may have distinct antineuralgic modes of action. It is therefore useful to review the mechanisms of action of these drugs and determine which of these mechanisms is most likely responsible for the drugs' efficacy in the symptomatic treatment of neuropathic pain. Switching from the traditional therapeutic class stratification to one based on putative antineuralgic mechanisms of action will allow more rational selection of therapies, and aid evaluation of the additive or synergistic effects of drugs when used in combination.

Antidepressants for chronic neuropathic pain

Tricyclic antidepressants have been used to manage pain for several decades, and are superior treatments for some patients suffering from neuropathic pain. Unfortunately, older antidepressants have dose-limiting side effects that can lead to drug intolerance. The most common are anticholinergic side effects, although some patients experience sexual dysfunction. Cognitive impairment, sedation, and orthostatic hypotension also are relatively common. Taking an overdose of tricyclic antidepressants can be lethal in overdose. Several weeks of therapy may be required before antinociception occurs, but tricyclic antidepressants in optimal doses appear to be the most effective treatment for neuropathic pain; this is supported by systematic reviews comparing them with other agents. Newer medications such as atypical antidepressants and anticonvulsants may be overtaking older antidepressants, but they should not be overlooked as important options for the management of pain.