Tramadol induces antidepressant-type effects in mice

The Behavioral and Neurochemical Changes Induced by Boldenone and/or Tramadol in Adult Male Rats

Boldenone and tramadol are abused among large sectors of adolescents. Therefore, the behavioral changes concerned with memory and cognitive functions and neurochemical variations were investigated in the cortex of rats treated with boldenone and/or tramadol. Rats were divided into control and rats treated with boldenone, tramadol, or both drugs. At the end of the treatment period, the memory and cognitive functions were evaluated by the Y-maze test (YMT) and elevated plus maze test (EPMT) and the motor activity was determined by the open field test (OFT). The cortex was dissected to carry out the neurochemical analyses. Rats treated with boldenone and/or tramadol showed impaired memory and cognitive functions and reduced motor activity. A significant increase in lipid peroxidation (MDA), nitric oxide (NO), and a significant decrease in reduced glutathione (GSH) were observed in the cortex of rats treated with boldenone and/or tramadol. The levels of acetylcholinesterase (AChE) and monoamine oxidase (MAO) decreased significantly. Western blot data showed a significant decrease in Bcl2 and a significant increase in caspase-3 and inducible nitric oxide synthase (iNOS) in rats treated with boldenone and/or tramadol. These changes were associated with neuronal death as indicated from the histopathological examination.

The present findings indicate that boldenone and/or tramadol induced impairment in memory and cognitive functions. These changes could be mediated by the increase in oxidative stress, neuroinflammation, reduced AChE level, and reduced number of survived neurons in the cortex as indicated from the decreased Bcl2 level and the histological examination.

Multiple Sclerosis and the Endogenous Opioid System

Multiple sclerosis (MS) is an autoimmune disease characterized by chronic inflammation, neuronal degeneration and demyelinating lesions within the central nervous system. The mechanisms that underlie the pathogenesis and progression of MS are not fully known and current therapies have limited efficacy. Preclinical investigations using the murine experimental autoimmune encephalomyelitis (EAE) model of MS, as well as clinical observations in patients with MS, provide converging lines of evidence implicating the endogenous opioid system in the pathogenesis of this disease. In recent years, it has become increasingly clear that endogenous opioid peptides, binding μ- (MOR), κ- (KOR) and δ-opioid receptors (DOR), function as immunomodulatory molecules within both the immune and nervous systems. The endogenous opioid system is also well known to play a role in the development of chronic pain and negative affect, both of which are common comorbidities in MS. As such, dysregulation of the opioid system may be a mechanism that contributes to the pathogenesis of MS and associated symptoms. Here, we review the evidence for a connection between the endogenous opioid system and MS. We further explore the mechanisms by which opioidergic signaling might contribute to the pathophysiology and symptomatology of MS.

The plausible mechanisms of tramadol for treatment of COVID-19

Currently, no single medication has been approved for the management of coronavirus disease-2019 (COVID-19) caused by the new severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Therefore, drug repositioningby investigating the use of existing drugs for management of COVID-19 patients is considered a desperate need. Tramadol is a commonly prescribed analgesic drug for treatment of moderate to severe pain with less potential for dependence and respiratory depression. Multiple evidence support that tramadol is a promising drug for treatment of COVID-19 patients. Herein, we discuss the possible beneficial effects of using tramadol against SARS-CoV-2 infection and their underlying mechanism of action. The anti-inflammatory effect of tramadol may help to suppress the COVID-19 related cytokine storm through decreasing interleukin (IL)-6, tumor necrosis factor-alpha (TNF-α), and C-reactive protein (CRP). Besides, tramadol activates natural killer (NK) and T-cells and enhances IL-2 secretion, which produce immune-enhancing effect against SARS-CoV-2. Recent studies confirmed that COVID-19 patients with acute respiratory failure showed increased fibrin formation and polymerization that may lead to thrombosis. Tramadol owing to its hypocoagulable effect may protect against venous thromboembolism in these patients. Moreover, tramadol can exert a cardioprotective effect via decreasing lactate dehydrogenase (LDH) level which is elevated in most of patients with COVID-19. Furthermore, the severity and mortality of COVID-19 have been correlated with old age patients, which may be due to the lack of antioxidant mechanisms and increased oxidative damage. Tramadol could protect COVID-19 patient from disease complications by increases the antioxidant enzymes superoxide dismutase and glutathione peroxidase while diminished malondialdehyde. More interestingly, tramadol as an effective analgesic and antitussive may have a beneficial effect on COVID-19 patients suffering from cough, headache, ache, and pain. The tramadol anti-psychotic effect may also protect against psychiatric disorders associated with SARS-CoV-2 infection. Moreover, tramadol has bactericidal activity against a wide range of pathogens including Pseudomonas aeruginosa which is common in severe COVID-19 patients leading to pneumonia with worse clinical outcomes. Therefore, we hypothesize that tramadol might be a promising adjuvant therapeutic option against SARS-CoV-2 infection. Based on that, tramadol should be considered as adjuvant therapy for COVID-19 clinical trials.

Low-Dose Tramadol as an Off-Label Antidepressant: A Data Mining Analysis from the Patients' Perspective

Objectives: The purpose of this analysis was to assess, from the patients' perspective, the effectiveness and relative safety of tramadol as an off-label antidepressant and to determine if patients' views and experiences are consistent with the biomedical literature. Method: A data mining approach was used to analyze databases available at drugs.com. Results: Tramadol was reported to be an effective or very effective antidepressant by 94.6% of patients (123/130) who provided ratings submitted to User Reviews for Tramadol to Treat Depression (https://www.drugs.com/comments/tramadol/for-depression.html). When compared to 34 other antidepressants in the database titled Drugs Used to treat Depression (https://www.drugs.com/condition/depression.html), for which there were ≥100 individual reviews for each drug, tramadol was rated as being the most effective (effectiveness rating = 9.1/10). Phenelzine (effectiveness rating = 8.7/10) was the only other antidepressant having ≥100 individual reviews coupled with a very high (8.0-10.0) effectiveness rating. Eleven patients reported significant symptoms of withdrawal upon cessation of tramadol, and five patients reported loss or reduction of libido as a side effect. Most (57/72, 79.2%) patients who reported a dose consumed experienced relief from depression at low therapeutic doses (25-150 mg/day). Fourteen patients reported taking this antidepressant for 5-10 years, and four patients reported taking tramadol for 10 or more years. Results demonstrated that most patients' comments and beliefs are consistent with the biomedical literature. Conclusions: Patients' reviews coupled with a survey of the biomedical literature indicate that at low therapeutic doses in the absence of interactions with other drugs, adult patients found tramadol to be a generally safe, effective, and fast-acting medication for relief from depression.

Tramadol-Induced Mood Elevation in a Patient with No Previous Psychiatric History

Tramadol is a powerful analgesic medication with antidepressant effects like venlafaxine. Hypomanic features were reported in patients with psychiatric history of mood disturbance when tramadol was prescribed for them. However, it is extremely rare to notice such mood-elevating effect in patients who have no previous psychiatric history. We report on the observation of a distressing mood-elevating effect for tramadol in a patient with no previous psychiatric history. We present the case of a 26-year-old female patient who developed accelerated flow of speech, overactivity, and difficulty in sleeping following intake of tramadol 50 mg dose. These symptoms resolved four hours later and recurred as she retook tramadol. The patient has no history of mood disorder or any other psychiatric comorbidity. Clinicians should exercise caution when prescribing tramadol.

Tramadol's Inhibitory Effects on Sexual Behavior: Pharmacological Studies in Serotonin Transporter Knockout Rats

Tramadol is an effective pharmacological intervention in human premature ejaculation (PE). To investigate whether the inhibitory action of tramadol is primarily caused by its selective serotonin reuptake inhibitory (SSRI) effects we tested the dose–response effects of tramadol on sexual behavior in serotonin transporter wild type (SERT^+/+), heterozygous (SERT^+/-), and knockout (SERT^-/-) rats. To investigate whether other mechanisms contribute to the inhibitory effects, WAY100,635, a 5-HT_1A receptor antagonist and naloxone, a μ-opioid receptor antagonist, were tested on sexual behavior together with tramadol. Tramadol dose-dependently decreases sexual activity in all genotypes. In all studies, SERT^+/- rats did not respond differently from SERT^+/+ rats. WAY100,635 did not affect sexual activity in SERT^+/+, but dose-dependently reduced sexual activity in SERT^-/- rats. WAY100,635 (0.3 mg/kg) combined with tramadol (20 mg/kg) significantly reduced sexual activity in SERT^+/+ and even stronger in SERT^-/- rats. Naloxone did not affect sexual behavior consistently in SERT^+/+ rats, while in SERT^-/- rats all doses reduced ejaculation frequency mildly. Combining naloxone (20 mg/kg) and tramadol (20 mg/kg) decreased ejaculation frequencies in both genotypes. Interestingly, combining tramadol (20 mg/kg), WAY100,635 (0.3 mg/kg) and naloxone (20 mg/kg) led to complete elimination of all sexual activity in both SERT^+/+ and SERT^-/- rats. These findings suggest that the inhibitory effects of tramadol on male sexual behavior in SERT^+/+ rats is mainly, if not exclusively, due to SERT inhibition, with an important role for 5-HT_1A receptors, although influence of other systems (e.g., noradrenergic) cannot be excluded. As SSRIs exert their sexual inhibition after chronic administration, tramadol may be therapeutically attractive as “on demand” therapy for PE.

Contrasting Regulation of Catecholamine Neurotransmission in the Behaving Brain: Pharmacological Insights from an Electrochemical Perspective

Catecholamine neurotransmission plays a key role in regulating a variety of behavioral and physiologic processes, and its dysregulation is implicated in both neurodegenerative and neuropsychiatric disorders. Over the last four decades, in vivo electrochemistry has enabled the discovery of contrasting catecholamine regulation in the brain. These rapid and spatially resolved measurements have been conducted in brain slices, and in anesthetized and freely behaving animals. In this review, we describe the methods enabling in vivo measurements of dopamine and norepinephrine, and subsequent findings regarding their release and regulation in intact animals. We thereafter discuss key studies in awake animals, demonstrating that these catecholamines are not only differentially regulated, but are released in opposition of each other during appetitive and aversive stimuli.

Cortical excitability in tramadol dependent patients: A transcranial magnetic stimulation study

BACKGROUND

Addiction to tramadol, a widely used analgesic, is becoming increasingly common. Tramadol can also induce seizures even after a single clinical dose. We tested whether the epileptogenicity of tramadol was associated with any changes in cortical excitability and inhibitory transmission using transcranial magnetic stimulation (TMS).

METHODS

The study included 16 tramadol dependent patients and 15 age and sex matched healthy volunteers. Clinical evaluation was conducted using an addiction severity index. TMS assessment of excitability was conducted on the motor cortex since the response to each TMS pulse at that site is easily measured in terms of the amplitude of the twitches it evokes in contralateral muscles. Measures included resting and active motor threshold (RMT and AMT respectively), motor evoked potential (MEP) amplitude, cortical silent period (CSP) duration, transcallosal inhibition (TCI), and short interval intracortical inhibition and facilitation (SICI and ICF respectively). Urinary level of tramadol was measured immediately before assessing cortical excitability in each patient.

RESULTS

RMT and AMT were significantly lower, the duration of the CSP was shorter and SICI was reduced in patients compared with the control group. These findings are suggestive of increased neural excitability and reduced GABAergic inhibition following exposure to tramadol. Also there were negative correlations between the severity of tramadol dependence and a number of cortical excitability parameters (AMT, RMT, and CSP with P=0.002, 0.005, and 0.04 respectively).

CONCLUSIONS

The results provide evidence for hyperexcitability of the motor cortex coupled with inhibitory deficits in tramadol dependent patients.

Drug Use and Misuse in the Mountains: A UIAA MedCom Consensus Guide for Medical Professionals

Donegani, Enrico, Peter Paal, Thomas Küpper, Urs Hefti, Buddha Basnyat, Anna Carceller, Pierre Bouzat, Rianne van der Spek, and David Hillebrandt. Drug use and misuse in the mountains: a UIAA MedCom consensus guide for medical professionals. High Alt Med Biol. 17:157-184, 2016.-Aims: The aim of this review is to inform mountaineers about drugs commonly used in mountains. For many years, drugs have been used to enhance performance in mountaineering. It is the UIAA (International Climbing and Mountaineering Federation-Union International des Associations d'Alpinisme) Medcom's duty to protect mountaineers from possible harm caused by uninformed drug use. The UIAA Medcom assessed relevant articles in scientific literature and peer-reviewed studies, trials, observational studies, and case series to provide information for physicians on drugs commonly used in the mountain environment. Recommendations were graded according to criteria set by the American College of Chest Physicians.

RESULTS

Prophylactic, therapeutic, and recreational uses of drugs relevant to mountaineering are presented with an assessment of their risks and benefits.

CONCLUSIONS

If using drugs not regulated by the World Anti-Doping Agency (WADA), individuals have to determine their own personal standards for enjoyment, challenge, acceptable risk, and ethics. No system of drug testing could ever, or should ever, be policed for recreational climbers. Sponsored climbers or those who climb for status need to carefully consider both the medical and ethical implications if using drugs to aid performance. In some countries (e.g., Switzerland and Germany), administrative systems for mountaineering or medication control dictate a specific stance, but for most recreational mountaineers, any rules would be unenforceable and have to be a personal decision, but should take into account the current best evidence for risk, benefit, and sporting ethics.

Synthetic and Receptor Signaling Explorations of the Mitragyna Alkaloids: Mitragynine as an Atypical Molecular Framework for Opioid Receptor Modulators

Mu-opioid receptor agonists represent mainstays of pain management. However, the therapeutic use of these agents is associated with serious side effects, including potentially lethal respiratory depression. Accordingly, there is a longstanding interest in the development of new opioid analgesics with improved therapeutic profiles. The alkaloids of the Southeast Asian plant Mitragyna speciosa, represented by the prototypical member mitragynine, are an unusual class of opioid receptor modulators with distinct pharmacological properties. Here we describe the first receptor-level functional characterization of mitragynine and related natural alkaloids at the human mu-, kappa-, and delta-opioid receptors. These results show that mitragynine and the oxidized analogue 7-hydroxymitragynine, are partial agonists of the human mu-opioid receptor and competitive antagonists at the kappa- and delta-opioid receptors. We also show that mitragynine and 7-hydroxymitragynine are G-protein-biased agonists of the mu-opioid receptor, which do not recruit β-arrestin following receptor activation. Therefore, the Mitragyna alkaloid scaffold represents a novel framework for the development of functionally biased opioid modulators, which may exhibit improved therapeutic profiles. Also presented is an enantioselective total synthesis of both (-)-mitragynine and its unnatural enantiomer, (+)-mitragynine, employing a proline-catalyzed Mannich-Michael reaction sequence as the key transformation. Pharmacological evaluation of (+)-mitragynine revealed its much weaker opioid activity. Likewise, the intermediates and chemical transformations developed in the total synthesis allowed the elucidation of previously unexplored structure-activity relationships (SAR) within the Mitragyna scaffold. Molecular docking studies, in combination with the observed chemical SAR, suggest that Mitragyna alkaloids adopt a binding pose at the mu-opioid receptor that is distinct from that of classical opioids.

Effect of Tramadol/Acetaminophen on Motivation in Patients with Chronic Low Back Pain

Background. The contribution of apathy, frequently recognized in individuals with neurodegenerative diseases, to chronic low back pain (LBP) remains unclear. Objectives. To investigate levels of apathy and clinical outcomes in patients with chronic LBP treated with tramadol-acetaminophen. Methods. A retrospective case-control study involving 73 patients with chronic LBP (23 male, 50 female; mean age 71 years) treated with tramadol-acetaminophen (n = 36) and celecoxib (n = 37) was performed. All patients were assessed using the self-reported questionnaires. A mediation model was constructed using a bootstrapping method to evaluate the mediating effects of pain relief after treatment. Results. A total of 35 (55.6%) patients met the criteria for apathy. A four-week treatment regimen in the tramadol group conferred significant improvements in the Apathy scale and numerical rating scale but not in the Rolland-Morris Disability Questionnaire, Pain Disability Assessment Scale, or Pain Catastrophizing Scale. The depression component of the Hospital Anxiety and Depression Scale was lower in the tramadol group than in the celecoxib group. The mediation analysis found that the impact of tramadol-acetaminophen on the change in apathy was not mediated by the pain relief. Conclusions. Tramadol-acetaminophen was effective at reducing chronic LBP and conferred a prophylactic motivational effect in patients with chronic LBP.

Medical and Psychiatric Effects of Long-Term Dependence on High Dose of tramadol

BACKGROUND

Tramadol dependence has been studied recently after large-scale exposure. Although tramadol dependence has increased rapidly in Egypt since 2004, no studies have evaluated the effect of high dose long-term tramadol dependence.

OBJECTIVES

To address the chronic sequel of tramadol dependence over at least 5 years duration with a large dose (more than 675 mg/day, three tablets or more, each tablet of 225 mg). The study was aimed to check the physical and psychiatric status during tramadol dependence and 3 months after complete treatment.

METHODS

The present study was applied on 79 patients with single tramadol-dependence dose of 675 mg or more for 5 years or more. We examined the physical and psychological impact of tramadol abuse before and after 3 months of stoppage of the drug.

RESULTS

The blood chemistry was nearly within normal parameters, although slight nonsignificant rise in liver enzymes was reported in some cases. Patients during tramadol dependence period were angry, hostile, and aggressive. On the other hand, after treatment the main problem observed was the significant increase in comorbid anxiety, depressive, and obsessive-compulsive symptoms, but no increase was found in psychotic symptoms. Tramadol-dependence dose was more important than duration of use in psychiatric illness.

CONCLUSIONS

Tramadol dependence on high dose could be physically safe to some limit, but psychiatrically it has many side effects.

Efficacy of tramadol-acetaminophen tablets in low back pain patients with depression

BACKGROUND

Tramadol-acetaminophen tablets are currently used to treat pain, including that of degenerative lumbar disease. Although there are many reports on tramadol-acetaminophen tablets, treatment outcomes in low back pain (LBP) patients with depression remain uncertain. This study investigated the outcomes of LBP patients with depression treated with tramadol-acetaminophen tablets.

METHODS

Of 95 patients with chronic LBP, 70 (26 men, 44 women; mean age 64 years) who were judged as having depression by the Self-Rating Depression Scale (SDS) were included in this study. In this trial, patients received one of two randomly assigned 8-week treatment regimes: tramadol-acetaminophen (Tramadol group, n = 35) and non-steroidal anti-inflammatory drugs (NSAIDs) (NSAID group, n = 35). In addition to completing self-report questionnaires, patients provided demographic and clinical information. All patients were assessed using a Numerical Rating Scale (NRS), Oswestry Disability Index (ODI), Pain Disability Assessment Scale (PDAS), Hospital Anxiety and Depression Scale (HADS), SDS, and Pain Catastrophizing Scale (PCS).

RESULTS

After 8 weeks' treatment, the NRS and SDS scores were lower in the Tramadol group than in the NSAID group (p < 0.05). There were no significant differences in the ODI, PDAS, and PCS scores between the groups (p = 0.47, 0.09, 0.47). Although there was no difference in the anxiety component of the HADS between the groups (p = 0.36), the depression component was lower in the Tramadol group than in the NSAID group (p < 0.05). There was no significant difference between groups in the percentage of patients with treatment-associated adverse events.

CONCLUSIONS

This investigation found that tramadol-acetaminophen is effective for reducing LBP and provided a prophylactic antidepressant effect in chronic LBP patients with depression.

Construction of drug network based on side effects and its application for drug repositioning

Drugs with similar side-effect profiles may share similar therapeutic properties through related mechanisms of action. In this study, a drug-drug network was constructed based on the similarities between their clinical side effects. The indications of a drug may be inferred by the enriched FDA-approved functions of its neighbouring drugs in the network. We systematically screened new indications for 1234 drugs with more than 2 network neighbours, 36.87% of the drugs achieved a performance score of Normalized Discounted Cumulative Gain in the top 5 positions (NDCG@5)≥0.7, which means most of the known FDA-approved indications were well predicted at the top 5 positions. In particular, drugs for diabetes, obesity, laxatives and antimycobacterials had extremely high performance with more than 80% of them achieving NDCG@5≥0.7. Additionally, by manually checking the predicted 1858 drug-indication pairs with Expression Analysis Systematic Explorer (EASE) score≤10⁻⁵ (EASE score is a rigorously modified Fisher exact test p value), we found that 80.73% of such pairs could be verified by preclinical/clinical studies or scientific literature. Furthermore, our method could be extended to predict drugs not covered in the network. We took 98 external drugs not covered in the network as the test sample set. Based on our similarity criteria using side effects, we identified 41 drugs with significant similarities to other drugs in the network. Among them, 36.59% of the drugs achieved NDCG@5≥0.7. In all of the 106 drug-indication pairs with an EASE score≤0.05, 50.94% of them are supported by FDA approval or preclinical/clinical studies. In summary, our method which is based on the indications enriched by network neighbors may provide new clues for drug repositioning using side effects.

Active behaviours produced by antidepressants and opioids in the mouse tail suspension test

Most classical preclinical tests to predict antidepressant activity were initially developed to detect compounds that influenced noradrenergic and/or serotonergic activity, in accordance with the monoaminergic hypothesis of depression. However, central opioid systems are also known to influence the pathophysiology of depression. While the tail suspension test (TST) is very sensitive to several types of antidepressant, the traditional form of scoring the TST does not distinguish between different modes of action. The present study was designed to compare the behavioural effects of classical noradrenergic and/or serotonergic antidepressants in the TST with those of opioids. We developed a sampling technique to differentiate between behaviours in the TST, namely, curling, swinging and immobility. Antidepressants that inhibit noradrenaline and/or serotonin re-uptake (imipramine, venlafaxine, duloxetine, desipramine and citalopram) decreased the immobility of mice, increasing their swinging but with no effect on their curling behaviour. No differences were observed between antidepressants that act on noradrenergic or serotoninergic transmission. While opioid compounds also decreased the immobility of the mice [morphine, codeine, levorphanol, (-)-methadone, (±)-tramadol and (+)-tramadol], they selectively increased curling behaviour. Blocking opioid receptors with naloxone prevented the antidepressant-like effect of codeine, and μ-opioid receptor knockout decreased normal curling behaviour and blocked (±)-tramadol-induced curling, further demonstrating the reliability and validity of this approach. These results show that at least two behaviourally distinct processes occur in the TST, highlighting the antidepressant-like effects of opioids evident in this test. Furthermore, our data suggest that swinging and curling behaviours are mediated by enhanced monoamine and opioid neurotransmission, respectively.

Evaluation of antidepressant activity of tramadol in mice

OBJECTIVE

To evaluate antidepressant like effect of tramadol in mice.

MATERIALS AND METHODS

Tramadol was administered at three different doses (10,20 and 40 mg/kg, i.p) once daily for 7 days to Swiss albino mice of either sex. The immobility period of control and drug treated mice were recorded in tail suspension test (TST).The antidepressant effect of tramadol was compared to that of fluoxetine (20 mg/kg, i.p), administered for seven days.

RESULTS

Tramadol produced significant antidepressant effect at all the doses, as indicated by reduction in immobility times as compared to control. The efficacy of tramadol at doses of 20 and 40 mg/kg was comparable with that of fluoxetine. Tramadol at 10 mg/kg dose showed significantly less antidepressant activity compared to fluoxetine.

CONCLUSION

The results of the present study indicate antidepressant like activity of tramadol.

Proof of concept trials in bipolar disorder and major depressive disorder: a translational perspective in the search for improved treatments

A better understanding of the neurobiology of mood disorders, informed by preclinical research and bi-directionally translated to clinical research, is critical for the future development of new and effective treatments. Recently, diverse new targets/compounds have been specifically tested in preclinical models and in proof-of-concept studies, with potential relevance as treatments for mood disorders. Most of the evidence comes from case reports, case series, or controlled proof-of-concept studies, some with small sample sizes. These include (1) the opioid neuropeptide system, (2) the purinergic system, (3) the glutamatergic system, (4) the tachykinin neuropeptide system, (5) the cholinergic system (muscarinic system), and (6) intracellular signaling pathways. These targets may be of substantial interest in defining future directions in drug development, as well as in developing the next generation of therapeutic agents for the treatment of mood disorders. Overall, further study of these and similar drugs may lead to a better understanding of relevant and clinically useful drug targets in the treatment of these devastating illnesses.

Tramadol and another atypical opioid meperidine have exaggerated serotonin syndrome behavioural effects, but decreased analgesic effects, in genetically deficient serotonin transporter (SERT) mice

The serotonin syndrome is a potential side-effect of serotonin-enhancing drugs, including antidepressants such as selective serotonin reuptake inhibitors (SSRIs) and monoamine oxidase inhibitors (MAOIs). We recently reported a genetic mouse model for the serotonin syndrome, as serotonin transporter (SERT)-deficient mice have exaggerated serotonin syndrome behavioural responses to the MAOI tranylcypromine and the serotonin precursor 5-hydroxy-l-tryptophan (5-HTP). As numerous case reports implicate the atypical opioids tramadol and meperidine in the development of the human serotonin syndrome, we examined tramadol and meperidine as possible causative drugs in the rodent model of the serotonin syndrome in SERT wild-type (+/+), heterozygous (+/-) and knockout (-/-) mice. Comparisons were made with SERT mice treated with either vehicle or morphine, an opioid not implicated in the serotonin syndrome in humans. Here we show that tramadol and meperidine, but not morphine, induce serotonin syndrome-like behaviours in mice, and we show that this response is exaggerated in mice lacking one or two copies of SERT. The exaggerated response to tramadol in SERT-/- mice was blocked by pretreatment with the 5-HT1A antagonist WAY 100635. Further, we show that morphine-, meperidine- and tramadol-induced analgesia is markedly decreased in SERT-/- mice. These studies suggest that caution seems warranted in prescribing or not warning patients receiving SSRIs or MAOIs that dangerous side-effects may occur during concurrent use of tramadol and similar agents. These findings suggest that it is conceivable that there might be increased vulnerability in individuals with SERT polymorphisms that may reduce SERT by more than 50%, the level in SERT+/- mice.

Cooperative opioid and serotonergic mechanisms generate superior antidepressant-like effects in a mice model of depression

Although complete remission of symptoms is the goal of any depression treatment, many patients fail to attain or maintain a long-term, symptom-free status. The opioid system has been implicated in the aetiology of depression, and some preclinical and clinical data suggest that opioids possess a genuine antidepressant-like effect. This study aimed to investigate a potential antidepressant strategy combining different classes of monoaminergic compounds with the weak mu-opioid agonist codeine in the tail suspension test in mice, a paradigm aimed at screening potential antidepressants. The results showed that codeine produced an antidepressant-like effect when administered alone, that was effectively antagonized by the opioid antagonist naloxone. The combination of subeffective doses of codeine with the selective serotonin reuptake inhibitors (fluoxetine or citalopram) lead to an accentuated reduction in immobility time. In contrast, immobility time remained unchanged when codeine was combined with a noradrenaline reuptake inhibitor (desipramine) or with a noradrenaline/serotonin reuptake inhibitor (duloxetine). The immobility time also remained unchanged with the combination of subeffective doses of codeine plus (+/-)-tramadol (weak mu-opioid agonist with serotonin/noradrenaline reuptake inhibitor properties) or (-)-tramadol (noradrenaline reuptake inhibitor). Conversely, the combination with (+)-tramadol (mu-opioid agonist with serotonin reuptake inhibitor properties) produced a large decrease in the immobility time. All these combinations were without effects on motor behaviour in mice. These data support the hypothesis that a combination of classical serotonergic antidepressants and weak opioid receptor agonists may be a helpful new strategy in the treatment of refractory depression.

Involvement of L-arginine-nitric oxide-cyclic guanosine monophosphate pathway in the antidepressant-like effect of tramadol in the rat forced swimming test

Tramadol is a centrally acting analgesic which is used mainly for the treatment of moderate or severe pain. It is a synthetic opioid in the aminocyclohexanol group that binds weakly to micro-opioid receptors. Since it has been suggested that both opioid and monoaminergic systems play a role in depressive disorders, tramadol has been studied in the forced swimming test (FST). The present study was designed to explore the antidepressant activity of tramadol in rat FST and its possible mechanisms of action. The involvement of L-arginine-nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) signaling pathway in the antidepressant action of tramadol was investigated. Treatment with tramadol, given (30 min earlier) by oral route (p.o.) at the doses of 10, 20 and 40 mg/kg, decreased immobility time in the FST. Pretreatment of rats with L-arginine (250 mg/kg, intraperitoneal, i.p., a nitric oxide precursor) or sildenafil (5 mg/kg, i.p., a phosphodiesterase 5 inhibitor, PDE5) significantly reversed the reduction in immobility time elicited by tramadol (20 mg/kg, p.o.) in the FST. Treatment of animals with a sub-effective dose of tramadol (5 mg/kg, p.o.) produced a synergistic antidepressant-like effect with N(G)-nitro-L-arginine (L-NNA, 3 mg/kg, i.p., an inhibitor of nitric oxide synthase) or with 7-nitroindazole (7-NI, 9 mg/kg i.p., a specific neuronal nitric oxide synthase inhibitor) in the FST. Pretreatment of animals with methylene blue (3.75 mg/kg i.p., an inhibitor of NO synthase and soluble guanylate cyclase - sGC) or (1H-[1,2,4] oxadiazolo[4,3-a]quinoxalin-1-one) (ODQ, 2 mg/kg, i.p., a specific inhibitor of sGC) significantly caused a synergistic effect with a sub-effective dose of tramadol (5 mg/kg, p.o.) in the FST. In the present study, different doses of tramadol and the combination with the L-arginine-NO-cGMP pathway modulators had no effect on the locomotor activity of rats in the open-field test. Thus, our findings suggest that the acute administration of tramadol produces antidepressant-like effect in the rat FST by a mechanism that involves the inhibition of L-arginine-NO-cGMP pathway.

Effects of 5,7-dihydroxytryptamine lesion of the dorsal raphe nucleus on the antidepressant-like action of tramadol in the unpredictable chronic mild stress in mice

RATIONALE

Tramadol is a centrally acting clinically effective analgesic, with a weak opioid receptor affinity. It shows antidepressant-like effects in animal models such as forced swimming test, learned helplessness, and unpredictable chronic mild stress (UCMS) and enhances the concentrations of noradrenaline (NA) and serotonin (5-HT) by interfering with their reuptake and release mechanisms, like some antidepressants.

OBJECTIVES

The aim of this study was to explore whether the antidepressant-like effects of tramadol is affected by the serotonergic system. For this purpose, the effects of a lesion of the dorsal raphe nucleus (DRN) by 5,7-dihydroxytryptamine (5,7-DHT) on the action of tramadol (20 mg/kg, i.p.) on depression-related behavior and neurochemical correlates were investigated in mice. From the third week onward, we administered tramadol chronically during 4 weeks.

RESULTS

Tramadol reversed the physical and behavioral abnormalities induced by the UCMS. Furthermore, the lesion of the DRN by 5,7-DHT antagonized the antidepressant-like effects of tramadol on the coat state, in the splash test but not in the resident-intruder test. The results obtained by high-pressure liquid chromatography showed that the level of 5-HT was reduced by the lesion in some brain regions without affecting the level of NA. Moreover, while the UCMS regimen diminished the level of 5-HT, tramadol increased the level of this neurotransmitter in certain regions.

CONCLUSIONS

These results seem to indicate that the serotonergic system is critically involved in the antidepressant-like effects of tramadol in the UCMS in mice.

Antidepressant-like effect of tramadol in the unpredictable chronic mild stress procedure: possible involvement of the noradrenergic system

Tramadol, which inhibits the reuptake of noradrenaline and serotonin, is effective in animal models of depression. Its antidepressant-like effects may be mediated mainly by the noradrenergic system. This study investigated the role of the noradrenergic system in the antidepressant-like effects of tramadol and desipramine in the unpredictable chronic mild stress model. We assessed the involvement of beta-adrenoreceptors, particularly beta2-receptors in the activity of these drugs. In addition, we measured the level of noradrenaline and its metabolite 3-methoxy-4-hydroxy-phenylglycol (MHPG) in the locus coeruleus, hypothalamus, hippocampus and cerebellum in stressed mice. Unpredictable chronic mild stress induced a degradation of coat state and decreased grooming behaviour in the splash test, which was reversed by the chronic administration of tramadol (20 mg/kg) and desipramine (10 mg/kg). The nonselective beta-adrenoreceptor antagonist propranolol (5 mg/kg, intraperitoneally) and the selective beta2-receptor antagonist ICI 118,551 (2 mg/kg, intraperitoneally) reversed the antidepressant-like effects of tramadol and desipramine. Moreover, chronic tramadol and desipramine treatment increased the level of noradrenaline (NA) and MHPG in the locus coeruleus but not in the cerebellum, whereas only MHPG level was increased in the hypothalamus. Tramadol, however, increased the levels of MHPG and NA in the hippocampus, whereas desipramine only increased NA level. These data support the view that the noradrenergic system plays an important role in the antidepressant-like action of tramadol.

Antidepressant-like activity of tramadol in mice

OBJECTIVE

To evaluate antidepressant-like effect of tramadol in mice.

MATERIALS AND METHODS

Tramadol was administered at three different doses (10, 20, and 40 mg/kg, i.p.) once daily for 7 days to Swiss albino mice of either sex. The immobility period of control and drug-treated mice was recorded in forced swim test (FST). The antidepressant effect of tramadol was compared to that of fluoxetine (20 mg/kg, i.p.), administered for seven successive days.

RESULTS

Tramadol produced significant antidepressant effect at all the three (10, 20, and 40 mg/kg) doses, as indicated by reduction in immobility times of drug-treated mice compared to control mice. The efficacy of tramadol at doses of 10 and 20 mg/kg was comparable to that of fluoxetine, but antidepressant activity in animals administered with tramadol 40 mg/kg was significantly less as compared to fluoxetine-pretreated mice.

CONCLUSION

The results of the present study indicate antidepressant-like activity of tramadol.

Pharmacological evidence for the involvement of the opioid system in the antidepressant-like effect of adenosine in the mouse forced swimming test

This study investigated the involvement of the opioid system in the antidepressant-like effect of adenosine in the forced swimming test. The effect of adenosine (10 mg/kg, i.p.) was prevented by the pretreatment of mice with naloxone (1 mg/kg, i.p., a nonselective opioid receptor antagonist), naltrindole (3 mg/kg, i.p., a selective delta-opioid receptor antagonist), clocinnamox (1 mg/kg, i.p., an irreversible mu-opioid receptor antagonist), and 2-(3,4-dichlorophenyl)-Nmethyl-N-[(1S)-1-(3-isothiocyanatophenyl)-2-(1-pyrrolidinyl)ethyl]acetamide (DIPPA; 1 mg/kg, i.p., a selective kappa-opioid receptor antagonist), but not with naloxone methiodide (1 mg/kg, s.c., a nonselective opioid receptor antagonist that does not cross the blood-brain barrier). Naloxone also prevented the anti-immobility effect of cyclohexyladenosine (CHA, 0.1 mg/kg, i.p., a selective adenosine A(1) receptor agonist) and N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)ethyl]adenosine (DPMA, 1 mg/kg, i.p., a selective adenosine A(2A) receptor agonist). The administration of DIPPA (0.1 mg/kg, i.p.) or morphine (1 mg/kg, s.c., a nonselective opioid receptor agonist), but not naltrindole (0.3 mg/kg, i.p.) and clocinnamox (0.1 mg/kg, i.p.) potentiated the effect of a subeffective dose of adenosine (1 mg/kg, i.p.) in the forced swimming test, without affecting the locomotor activity. No additive effect in the immobility time was observed when mice were treated with morphine (5 mg/kg, s.c.) plus adenosine (10 mg/kg, i.p.). These results indicate that the anti-immobility effect of adenosine in the forced swimming test, via adenosine A(1) and A(2A) receptors, is mediated by an interaction with the opioid system, likely dependent on an activation of mu- and delta-opioid receptors and an inhibition of kappa-opioid receptors.

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

RATIONALE

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

OBJECTIVES

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

RESULTS

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

CONCLUSIONS

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

Role of atypical opiates in OCD. Experimental approach through the study of 5-HT(2A/C) receptor-mediated behavior

RATIONALE

The selective serotonin (5-HT) reuptake inhibitors (SSRIs) represent the first-line pharmacotherapy for obsessive-compulsive disorder (OCD), and atypical antipsychotic drugs, which block 5-HT2A receptors, are used in augmentation strategies. Opiate drugs are also effective in treatment-refractory OCD and Tourette syndrome. The 5-HT2A-related behavior (i.e., head twitch) has been related with tics, stereotypes, and compulsive symptoms observed in Tourette syndrome and OCD.

OBJECTIVES

The aim of this study was to explore whether 5-HT2A-related behavior is affected by atypical opiate drugs.

MATERIALS AND METHODS

Head-twitch response was induced in mice by administration of either 5-hydroxytryptophan (5-HTP) or the 5-HT2A/C agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). Dose-effect curves of atypical opiate drugs [(+/-)-tramadol, (-)-methadone and levorphanol], morphine, and other psychoactive drugs (fluvoxamine, desipramine, nefazodone, and clozapine) were performed. Opioid mechanisms were investigated by administration of naloxone.

RESULTS

All the opiates tested reduced both 5-HTP and DOI-induced behavior in a naloxone-reversible fashion, atypical opiates being more effective. The effects of the other drugs depended on the protocol, clozapine being the most effective.

CONCLUSIONS

Combined 5-HT and opioid properties result in a greater efficacy in antagonizing 5-HT2A-related behavior. These results provide behavioral evidence to support convergent effects of the 5-HT and opioid systems in discrete brain areas, offering the potential for therapeutic advances in the management of refractory stereotypes and compulsive behaviors.

In vivo effect of tramadol on locus coeruleus neurons is mediated by alpha2-adrenoceptors and modulated by serotonin

Tramadol is a centrally-acting analgesic endowed with opioid, noradrenergic and serotonergic properties. Various data suggest that, in addition to its analgesic effect, tramadol may have antidepressant and anxiolytic-like effects. This study investigates, through single-unit extracellular recording techniques, the in vivo effects of tramadol on locus coeruleus (LC) neurons and its possible effects on alpha(2)-adrenoceptors, opioid receptors and the 5-HT system. Tramadol produced a dose-dependent and complete inhibition of LC activity (ED(50)=2.1mg/kg). This inhibitory effect was prevented and reversed by the selective alpha(2)-adrenoceptor antagonist, idazoxan, but not by the opioid receptor antagonist, naloxone. The inhibition of the synthesis of 5-HT by p-chlorophenylalanine and the pre-administration of the 5-HT(1A) receptor agonist, 8-OH-DPAT at 40microg/kg, caused a significant potentiation of the tramadol effect decreasing the ED(50) by 53% and 67% respectively. Lower doses of 8-OH-DPAT, of 1 and 4microg/kg, did not significantly modify the tramadol effect. In summary, the results indicate that tramadol elicits an inhibitory effect on LC neurons in vivo through alpha(2)-adrenoceptors. Moreover, this effect is modulated by the 5-HT system and particularly by 5-HT(1A) receptors.

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.

Potentiating effect of tramadol on methamphetamine-induced behavioral sensitization in mice

RATIONALE

Polydrug abuse is a common phenomenon in human drug addicts. Previous studies have shown that both tramadol (TRAM) and methamphetamine (METH) share the ability to modulate brain monoaminergic (dopamine, 5-hydroxytryptamine, and noradrenaline) systems that may be involved in behavioral sensitization to METH. Therefore, we hypothesized that there would be an interaction between TRAM and METH on behavioral sensitization induced by METH.

OBJECTIVES

To investigate whether TRAM affects METH-induced behavioral sensitization.

METHODS

Male Kunming mice were subjected to two regimens of drugs: (1) Mice were injected with TRAM (1-16 mg/kg, i.p.) alone or a combination of TRAM and METH (1 mg/kg, i.p.) once daily for 7 days. After 7 drug-free days (on day 15), animals were challenged with the corresponding TRAM dose or METH (1 mg/kg, i.p.). On days 1, 7, and 15, locomotion was monitored in the open field test after the last injection. (2) Mice received METH (1 mg/kg, i.p.) once daily for 7 days, followed by 7 drug-free days. On day 15, a challenge of TRAM (1-16 mg/kg, i.p.) or TRAM plus METH (1 mg/kg, i.p.) was given and then locomotor activity was quantified.

RESULTS

TRAM or METH challenge did not induce hyperlocomotion in mice chronically treated with TRAM, and TRAM challenge was insufficient to induce subsequent hyperlocomotion in METH-sensitized mice. However, TRAM significantly increased METH-induced hyperlocomotion. TRAM plus METH-sensitized mice showed a significantly greater hyperlocomotor response to METH challenge than METH-sensitized mice. Furthermore, TRAM (16 mg/kg, i.p.) plus METH (1 mg/kg, i.p.) challenge enhanced the sensitized locomotor response compared to METH-alone (1 mg/kg, i.p.) challenge in METH-sensitized mice.

CONCLUSIONS

TRAM fails to produce behavioral sensitization, and there is no apparent cross-sensitization between TRAM and METH. However, TRAM can increase METH-induced hyperlocomotion and potentiate the development and expression of behavioral sensitization to METH.

Involvement of 5-HT1A receptors in the antidepressant-like effect of adenosine in the mouse forced swimming test

This study investigated the involvement of 5-HT1 and 5-HT2 receptors in the antidepressant-like effect of adenosine in the mouse forced swimming test (FST). The pre-treatment of mice with PCPA (100mg/kg, i.p., an inhibitor of serotonin synthesis, for four consecutive days), NAN-190 (0.5mg/kg, i.p., a 5-HT1A receptor antagonist), pindolol (32 mg/kg, i.p., a 5-HT1A/1B receptor/beta-adrenoceptor antagonist) or WAY100635 (0.1 and 0.3mg/kg, s.c., a selective 5-HT1A receptor antagonist), but not with ketanserin (5mg/kg, i.p., a 5-HT2A/2C receptor antagonist), prevented the antidepressant-like effect of adenosine (10mg/kg, i.p.) in the FST. Moreover, the pre-treatment of animals with WAY100635 (0.1mg/kg, s.c.) blocked the decrease in immobility time in the FST elicited by adenosine (5 or 10mg/kg, i.p.), but produced a synergistic effect with a sub-effective dose of adenosine (1mg/kg, i.p.) and did not cause any alteration at the highest dose of adenosine administered (50mg/kg, i.p.). Adenosine (1mg/kg, i.p.) produced a synergistic antidepressant-like effect with pindolol (32 mg/kg), NAN-190 (0.5mg/kg, i.p.), WAY100635 (0.03 mg/kg, s.c.), 8-OH-DPAT (1mg/kg, i.p., a 5-HT1A receptor agonist), but not with DOI (1mg/kg, i.p., a preferential 5-HT2A receptor agonist) or ketanserin. The pre-treatment of mice with DPCPX (2mg/kg, i.p., a selective adenosine A1 receptor antagonist) or ZM241385 (1mg/kg, i.p., a selective adenosine A2A receptor antagonist) did not prevent the effect of fluoxetine (32 mg/kg, i.p., a preferential serotonin reuptake inhibitor) in the FST. Besides that, adenosine (1mg/kg, i.p.) did not produce a synergistic antidepressant-like effect with fluoxetine (10mg/kg, i.p.). Taken together, the results indicate that the antidepressant-like effect of adenosine in the FST appears to be mediated, at least in part, by an interaction with 5-HT1A receptors.

Effects of desipramine and tramadol in a chronic mild stress model in mice are altered by yohimbine but not by pindolol

This study investigated the antidepressant-like effects of a chronic treatment with either tramadol (20 mg/kg, i.p.) or desipramine (10 mg/kg, i.p.) in the unpredictable chronic mild stress model of depression in BALB/c mice. Mice were first submitted to a 2 week drug-free unpredictable chronic mild stress before the onset of the treatments. The unpredictable chronic mild stress regimen induced a degradation of the state of the coat and decreased the grooming behaviour in the splash test. These physical and behavioural abnormalities were counteracted by tramadol and desipramine. Furthermore, we observed neither a significant acceleration nor diminution by pindolol (5-HT1A/1B receptor antagonist, 10 mg/kg, i.p.) on the antidepressant-like actions of desipramine and tramadol whereas yohimbine (alpha2-adrenergic receptor antagonist, 2 mg/kg, i.p.) antagonized the antidepressant-like effects of both drugs during the unpredictable chronic mild stress regimen. The results of the study support the suggestion that antidepressant-like effect of tramadol and desipramine in mice in the unpredictable chronic mild stress model is mediated by the noradrenergic system rather than the serotonergic system.

Role of 5-HT1A and 5-HT1B receptors in the antinociceptive effect of tramadol

Tramadol, (1RS,2RS)-2-[(dimethylamine)-methyl]-1-(3-methoxyphenyl)-cyclohexanol hydrochloride, is an atypical centrally acting analgesic agent with relatively weak opioid receptor affinity and which, like some antidepressants, is able to inhibit the reuptake of serotonin (5-hydroxytryptamine, 5-HT) in the raphe nucleus. We have previously demonstrated that pindolol, a beta-adrenoceptor blocker/5-hydroxytryptamine(1A/1B) receptor antagonist, enhanced tramadol antinociception and that the selective 5-HT1A agonist 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) reduced it. These effects were related to the negative feedback control that regulates raphe region neurones. The current study examines the ability of the selective antagonist at somatodendritic 5-HT1A receptors, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl] ethyl]-N-(2-pyridinyl) cyclohexane carboxamide (WAY100635, 0.8 mg/kg), the selective antagonist at terminal 5-HT1B receptors, N-[3-(2-dimethylamino) ethoxy-4-methoxyphenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-(1,1'-biphenyl)-4-carboxamide (SB216641, 0.1-0.8 mg/kg) and the selective agonist at 5-HT1B receptors, 1,4-tDihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrolo[3,2-b] pyridin-5-one (CP93129, 0.2-0.4 mg/kg), to modify the antinociceptive effect of 4-64 mg/kg of tramadol in the hot plate test in mice. The results show that 0.8 mg/kg of WAY100635 enhanced antinociceptive effect of tramadol while neither agonism nor antagonism at the 5-HT1B receptor modifies it significantly at the doses tested. These results account for involvement of the somatodendritic 5-HT1A receptors in the analgesic effect of tramadol and support the supraspinal interaction of serotonin and the opioid system in the regulation of pain.

Current trends in neuropathic pain treatments with special reference to fibromyalgia

Neuropathic pain and fibromyalgia are prevalent diseases which have major consequences on healthcare resources and the individual. From the clinical point of view neuropathic pains represent a heterogeneous group of aetiologically different diseases ranging from cancer to diabetes. Patients with fibromyalgia also display clinical features common in neuropathic pain suggesting that there might be some overlap. The mechanisms responsible for symptoms and signs in both diseases are still unknown. Recently, there have been numerous reports of various pharmacological treatments of neuropathic pain and fibromyalgia with often disappointing results. Most of the studies were of short duration, had high attrition rates, and displayed other methodological problems. Some compounds had high rates of adverse effects which makes it often difficult for the patients to tolerate the treatment, especially in the long-term. At present, the best options for medication treatment are tricyclic antidepressants in lower dosage than usual in psychiatric disorders and a wide range of anticonvulsants. Opioids are sometimes recommended but have been found to have minor efficacy. Recently, there have been more controlled trials, which are reported here if they had been published between 2002 and 2004. Various compounds have been tested in different studies. Treatment of fibromyalgia, which has many features in common with depressive symptoms, became the focus of interest. New promising studies with dual serotonin-norepinephrine reuptake inhibitors (duloxetine and milnacipram) and a newer antiepileptic drug (pregabalin) are in progress. Future research will have to apply new approaches (e.g., using a mechanism-based classification of neuropathic pain and carrying out studies in populations with the same symptom but not necessarily the same disease) in order to find effective treatments for these common and often debilitating diseases.

Effect of the antidepressant nefazodone on the density of cells expressing mu-opioid receptors in discrete brain areas processing sensory and affective dimensions of pain

RATIONALE

The principal use of antidepressants is in the treatment of depression and affective disorders. Antidepressants have also been used as an adjuvant to analgesics in pain treatment. However, in chronic treatment, their antinociceptive and antidepressive effects coexist simultaneously. Antidepressants can interact with the opioid system, which is also involved in regulating nociceptive processing and affective state. Chronic antidepressants could act by increasing mu-opioid receptor expression in many brain areas involved in the regulation of nociception and affective state.

OBJECTIVES

The aim of this study was to evaluate the antinociceptive and antidepressant-like effects and the possible variations in mu-opioid receptor expression induced by a chronic nefazodone treatment in brain areas related to pain and affective state.

METHODS

Wistar rats were chronically treated with nefazodone (10 and 25 mg/kg IP, twice a day, for 14 days). Twelve hours after the last day 14 dose of nefazodone, a tail-flick test was performed. After the administration of a daily dose of nefazodone, Porsolt's test was carried out 12 h after last dose. Two hours after completion of 14 days treatment, other animals were processed for mu-opioid receptor immunocytochemistry using polyclonal antisera raised in rabbits. Several brain regions were analyzed: the frontal and cingulate cortex, the dorsal raphe nucleus and the periaqueductal gray.

RESULTS

Chronic nefazodone treatment induced a significant increase in tail-flick latency and a significant decrease in immobility time at total doses of 20 and 50 mg/kg per day ( P<0.05). In treated animals, the density of neural cells immunostained for mu-opioid receptor in the frontal and cingulate cortices, dorsal raphe nucleus and periaqueductal gray had increased after chronic nefazodone compared to controls.

CONCLUSION

Therefore, chronic nefazodone induces antinociceptive and antidepressant-like effects in rats and increases mu-opioid receptor expression in brain areas related to pain and affective state. These results suggest that antidepressants could be effective on somatic and affective dimensions of pain and this action could be related to its influence on the opioid system.

Evidence for serotonin receptor subtypes involvement in agmatine antidepressant like-effect in the mouse forced swimming test

This study investigated the involvement of 5-HT(1) and 5-HT(2) receptors in the antidepressant-like effect of agmatine in the mouse forced swimming test (FST). Pretreatment with p-chlorophenylalanine methyl ester (PCPA; 100 mg/kg, intraperitoneally (i.p.), an inhibitor of serotonin synthesis, for 4 consecutive days), methysergide (5 mg/kg, i.p., a serotonin (5-HT) antagonist), pindolol (32 mg/kg, i.p., a 5-HT(1A/1B) receptor/beta-adrenoceptor antagonist), N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridynyl)cyclohexanecarboxamide (WAY 100635; 0.3 mg/kg, subcutaneously (s.c.), a selective 5-HT(1A) receptor antagonist), 1-(2-methoxyphenyl)-4[-(2-phthalimido)butyl]piperazine) (NAN-190; 0.5 mg/kg, i.p., a 5-HT(1A) receptor antagonist), 1-(2-(1-pyrrolyl)-phenoxy)-3-isopropylamino-2-propanol (isamoltane; 2.5 mg/kg, i.p., a 5-HT(1B) receptor antagonist), cyproheptadine (3 mg/kg, i.p., a 5-HT(2) antagonist) or ketanserin (5 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist), but not with propranolol (2 mg/kg, i.p., a beta-adrenoceptor antagonist), prevented the effect of agmatine (10 mg/kg, i.p.) in the FST. A subeffective dose of agmatine (0.001 mg/kg, i.p.) produced a synergistic antidepressant-like effect with pindolol (32 mg/kg), NAN-190 (0.5 mg/kg, i.p.), WAY 100635 (0.03 mg/kg, s.c.), (+)-8-hydroxy-2-(di-n-propylamino)tetralin HBr (8-OH-DPAT; 0.01 mg/kg, i.p., a 5-HT(1A) receptor agonist), R(-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI; 1 mg/kg, i.p., a preferential 5-HT(2A) receptor agonist), or fluoxetine (10 mg/kg, i.p., a selective serotonin reuptake inhibitor, SSRI) but not with isamoltane (2.5 mg/kg, i.p.), ritanserin (4 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist) or ketanserin (5 mg/kg, i.p.). Taken together, the results firstly demonstrate that agmatine antidepressant-like effects in the FST seem to be mediated, at least in part, by an interaction with 5-HT(1A/1B) and 5-HT(2) receptors.

Long-term exposure of rats to tramadol alters brain dopamine and α1-adrenoceptor function that may be related to antidepressant potency

The aim of the present study was to determine whether tramadol, which has a potential antidepressant efficacy, evokes, when administered repeatedly, changes similar to the alterations induced by conventional antidepressant drugs. Repeated administration of tramadol (20 mg/kg i.p. for 21 days) enhanced the d-amphetamine-induced locomotor hyperactivity and increased the density of alpha(1)-adrenoceptors in the rat brain cortex, as measured by saturation analysis of [(3)H]prazosin binding. Autoradiographic analysis of [(3)H]7-OH-DPAT and [(3)H]raclopride binding revealed a significant up-regulation of dopamine D2 and D3 receptors in the rat nucleus accumbens upon repeated treatment with tramadol. All the above-mentioned effects induced by repeated administration of tramadol resemble the effects induced by conventional antidepressants. However, tramadol when administered repeatedly did not increase the levels of mRNA encoding for brain-derived neurotrophic factor (BDNF) and its receptor, TrkB. This is what differs tramadol from conventional antidepressants, since neurotrophic effects of these drugs have recently been postulated.

Opioid and monoamine systems mediate the discriminative stimulus of tramadol in rats

We analyzed the ability of the mu opioid peptide receptor ligands morphine and naloxone and several antidepressant drugs that are serotonin (fluoxetine), noradrenaline (reboxetine), mixed serotonin and noradrenaline (milnacipram and venlafaxine), dopamine (nomifensine) reuptake inhibitors, as well as roxindole (a nonselective drug) to substitute for, or alter, tramadol discrimination. Male Wistar rats were trained to discriminate tramadol (20 mg/kg) from saline in a two-choice water-reinforced paradigm. Out of the drugs studied, only morphine substituted for tramadol. In combination experiments, naloxone (0.1-1 mg/kg) attenuated the stimulus effects of tramadol (20 mg/kg) and the substitution evoked by morphine (2 mg/kg). Milnacipram (10 mg/kg) or reboxetine (10 mg/kg) enhanced the effects of tramadol (2.5-10 mg/kg); the other antidepressant drugs used failed to modulate tramadol discrimination. Our results indicate that tramadol can be used as a stimulus cue in rats, and that mu opioid peptide mechanisms are involved in its effects, while noradrenergic uptake inhibitors can enhance tramadol discrimination.

Non-selective opioid receptor antagonism of the antidepressant-like effect of venlafaxine in the forced swimming test in mice

The opioid system has been implicated in mood disorders as well as in the mechanism of action of antidepressants. Since the opioid component in venlafaxine (VLX) is still a matter of discussion, we investigated the role of opioid receptors in the antidepressant-like effect of VLX in the forced swimming test in mice. The non-selective opiate antagonist naloxone at high dose (2 mg/kg, s.c.) antagonized the effect of VLX. In contrast, beta-funaltrexamine (40 mg/kg, s.c.), which preferentially blocks mu(1)/mu(2) opioid receptors, naloxonazine (35 mg/kg, s.c.), a selective mu(1) opioid antagonist, naltrindole (10 mg/kg, s.c.), a selective delta opioid antagonist, and Nor-binaltorphimine (10 mg/kg, s.c.), which selectively blocks kappa-opioid receptors, were all ineffective. Thus, although apparently mediated by the opioid system, the behavioural effect of VLX does not involve specific opioid receptors.