The antihyperalgesic effect of venlafaxine in diabetic rats does not involve the opioid system

Effect of Venlafaxine, Pramipexole, and Valsartan on Spermatogenesis in Male Rats

The study's aim was to explore the effect of venlafaxine, valsartan, and pramipexole on spermatogenesis. It was hypothesized that these drugs may affect the male fertility because of their long-term use in treatment of depression, hypertension, and Parkinson's diseases. Male rats were given venlafaxine, valsartan, and pramipexole at low- and high-dose levels orally once daily for 10 weeks. Testosterone (25 mg/kg) was given as a standard via an intramuscular route once weekly. Rats were sacrificed after blood collection by cardiac puncture, and testes were removed. Sperm parameters were examined from spermatozoa of the cauda epididymis, and testes were treated for histopathological analysis. Results showed nonsignificant effect of venlafaxine on the sperm count, whereas a decreased sperm count was noted in all the treatment groups as compared to that of the control except valsartan at a low dose, which significantly (p < 0.001) raised the sperm count (96.26 ± 2.4) in reference with the control value (49.13 ± 2.3). Treatments had variable effects on total sperm motility and morphological parameters, but valsartan at a low dose showed maximum sperm motility (71.55 ± 0.7) among all. DNA integrity of spermatozoa remained intact in all groups. Luteinizing hormone and follicle-stimulating hormone levels decreased, and testosterone levels increased in all treatment groups as compared to control values, which indicate fertility. Histopathology revealed normal texture of testes with venlafaxine and valsartan, but testicular damage occurred with high-dose pramipexole. It is concluded that the use of venlafaxine, valsartan, and pramipexole at a low dose is devoid of any harmful effect on spermatogenesis, whereas pramipexole at a high dose adversely affect it.

Psychotropic Drugs for the Management of Chronic Pain and Itch

Clinical observations have shown that patients with chronic neuropathic pain or itch exhibit symptoms of increased anxiety, depression and cognitive impairment. Such patients need corrective therapy with antidepressants, antipsychotics or anticonvulsants. It is known that some psychotropic drugs are also effective for the treatment of neuropathic pain and pruritus syndromes due to interaction with the secondary molecular targets. Our own clinical studies have identified antipruritic and/or analgesic efficacy of the following compounds: tianeptine (atypical tricyclic antidepressant), citalopram (selective serotonin reuptake inhibitor), mianserin (tetracyclic antidepressant), carbamazepine (anticonvulsant), trazodone (serotonin antagonist and reuptake inhibitor), and chlorprothixene (antipsychotic). Venlafaxine (serotonin-norepinephrine reuptake inhibitor) is known to have an analgesic effect too. The mechanism of such effect of these drugs is not fully understood. Herein we review and correlate the literature data on analgesic/antipruritic activity with pharmacological profile of these compounds.

Methanol extract of Cola nitida ameliorates inflammation and nociception in experimental animals

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Methanol extract Cola nitida possesses analgesic properties in mice.

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Methanol extract of Cola nitida showed anti-inflammatory activity.

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Methanol extract of cola nitida mediates its nociceptive action through cholinergic pathway.

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Opioid and beta adrenergic pathways do not mediate the analgesic potential of Cola nitida.

Methanol extract of Cola nitida (MECN) was evaluated for its anti-inflammatory and analgesic activities using rats and mice. Inflammatory activity of MECN was assessed by carrageenan-induced paw oedema while analgesic activity was evaluated by acetic acid –induced writhing and formalin paw lick test. Histological analyses of the paws were also carried out. There was evaluation of the mechanism(s) of action of MECN using naloxone, a blocker of opioid receptors; atropine, blocker of muscarinic receptors; and propranolol, blocker of beta adrenergic receptors. Findings from the study revealed that MECN has both anti-inflammatory and analgesic properties. These properties were found to be dose dependent with 200 mg/kg of MECN discovered to be the most potent dose. 200 mg/kg was able to cause statistically significant reduction in paw size (p < 0.001) when compared with the carrageenan group. Histological analysis revealed that rats treated with 200 mg/kg of MECN showed no inflammatory cells in the left paw compared to other groups treated with carrageenan. In the formalin test, the number of paw licking was significantly reduced by MECN at 50 mg/kg, 100 mg/kg and 200 mg/kg in both neurogenic and inflammatory pain responses (p < 0.001) even as 200 mg/kg showed the highest percentage inhibition of 98.17% while 100 mg/kg of aspirin showed percentage inhibition of 93.66%. In acetic acid-induced writhing test, 50 mg/kg, 100 mg/kg and 200 mg/kg of MECN produced significant inhibition of writhes when compared with control as highest inhibition is observed in mice that received 200 mg/kg which is similar to aspirin. Administration of propranolol and naloxone was unable to reverse the analgesic function of MECN. However, atropine administration blocked the analgesic function of MECN. This shows that MECN exhibits its analgesic property through cholinergic pathway and not opioid and adrenergic pathways. Phytochemical screening revealed that MECN contains flavonoids, steroids, saponins, tannins, anthraquinines, terpenoids, and alkanoids. These phytochemical contents may thus be responsible for its analgesic and anti-inflammatory properties.

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.

Venlafaxine and oxycodone have different effects on spinal and supraspinal activity in man: a somatosensory evoked potential study

AIMS

Opioids and antidepressants that inhibit serotonin and norepinephrine reuptake (SNRI) are recognized as analgesics to treat severe and moderate pain, but their mechanisms of action in humans remain unclear. The present study aimed to explore how oxycodone (an opioid) and venlafaxine (an SNRI) modulate spinal and supraspinal sensory processing.

METHODS

Twenty volunteers were included in a randomized, double-blinded, three-way (placebo, oxycodone, venlafaxine), crossover study. Spinal and full scalp cortical evoked potentials (EPs) to median nerve stimulation were recorded before and after 5 days of treatment. Assessment of the central effects of the three treatments involved: (i) amplitudes and latencies of spinal EPs (spinal level); (ii) amplitudes and latencies of the P14 potential (subcortical level); (iii) amplitudes and latencies of early and late cortical EPs (cortical level); (iv) brain sources underlying early cortical Eps; and (v) brain networks underlying the late cortical EPs.

RESULTS

In the venlafaxine arm, the spinal P11 and the late cortical N60-80 latencies were reduced by 1.8% [95% confidence interval (CI) 1.7%, 1.9%) and 5.7% (95% CI 5.3%, 6.1%), whereas the early cortical P25 amplitude was decreased by 7.1% (95%CI 6.1%, 8.7%). Oxycodone increased the subcortical P14 [+25% (95% CI 22.2%, 28.6%)], early cortical N30 [+12.9% (95% CI 12.5%, 13.2%)] amplitudes and the late cortical N60-80 latency [+2.9% (95% CI 1.9%, 4.0%)]. The brainstem and primary somatosensory cortex source strengths were increased by 66.7% (95% CI 62.5%, 75.0%) and 28.8% (95% CI 27.5%, 29.6%) in the oxycodone arm, whereas the primary somatosensory cortex strength was decreased in the venlafaxine arm by 18.3% (95% CI 12.0%, 28.1%).

CONCLUSIONS

Opioids and SNRI drugs exert different central effects. The present study contributed to the much-needed human models of the mechanisms of action of drugs with effects on the central nervous system.

Antinociceptive effects of venlafaxine in a rat model of peripheral neuropathy: role of alpha2-adrenergic receptors

This study was designed to determine whether acute or chronic venlafaxine administration was effective in alleviating symptoms of neuropathic pain in a rat model of neuropathic pain, and whether the effect of venlafaxine involved manipulation of α2-adrenoceptors,by determining the effect of yohimbine, a α2-adrenoceptor antagonist on its actions. Neuropathic pain was induced by chronic constriction injury (CCI) of the sciatic nerve in the rats that resulted in stimulus-evoked thermal hyperalgesia, tactile mechanical and cold allodynia. Acute venlafaxine injections (20 and 40 mg/kg i.p.) on the 7th, 14th and 21st postoperative days could not reduce tactile and cold hypersensitivity significantly compared to CCI group. But in these groups venlafaxine (40 mg/kg i.p.) blocked heat hyperalgesia. When venlafaxine (10 and 20mg/kg i.p.) administration was started on the first day after CCI and given daily until the 14th day, tactile hypersensitivity and heat hyperalgesia considerably were attenuated. But when venlafaxine (20mg/kg i.p.) treatment was initiated on the 10th day after CCI, once the model had been fully established, and given daily for 11 days, no differences in withdrawal thresholds were observed compared with CCI group however heat hyperalgesia significantly has been blocked. Also the effect of venlafaxine on heat hyperalgesia was reversed by pretreatment with yohimbine at all-time intervals. These results indicate that venlafaxine, when administered immediately after nerve injury, and for a sufficient period of time, can prevent the development and expression of neuropathic pain. Also we conclude that α2-adrenoceptors participate in the antinociceptive effects of venlafaxine.

Spinal-supraspinal and intrinsic μ-opioid receptor agonist-norepinephrine reuptake inhibitor (MOR-NRI) synergy of tapentadol in diabetic heat hyperalgesia in mice

Tapentadol is a μ-opioid receptor (MOR) agonist and norepinephrine reuptake inhibitor (NRI) with established efficacy in neuropathic pain in patients and intrinsic synergistic interaction of both mechanisms as demonstrated in rodents. In diabetic mice, we analyzed the central antihyperalgesic activity, the occurrence of site-site interaction, as well as the spinal contribution of opioid and noradrenergic mechanisms in a hotplate test. Tapentadol (0.1-3.16 µg/animal) showed full efficacy after intrathecal as well as after intracerebroventricular administration (ED50 0.42 µg/animal i.t., 0.18 µg/animal i.c.v.). Combined administration of equianalgesic doses revealed spinal-supraspinal synergy (ED50 0.053 µg/animal i.t. + i.c.v.). Morphine (0.001-10 µg/animal) also showed central efficacy and synergy (ED50 0.547 µg/animal i.t., 0.004 µg/animal i.c.v., 0.014 µg/animal i.t. + i.c.v.). Supraspinal potencies of tapentadol and morphine correlated with the 50-fold difference in their MOR affinities. In contrast, spinal potencies of both drugs were similar and correlated with their relative systemic potencies (ED50 0.27 mg/kg i.p. tapentadol, 1.1 mg/kg i.p. morphine). Spinal administration of the opioid antagonist naloxone or the α2-adrenoceptor antagonist yohimbine before systemic administration of equianalgesic doses of tapentadol (1 mg/kg i.p.) or morphine (3.16 mg/kg i.p.) revealed pronounced influence on opioidergic and noradrenergic pathways for both compounds. Tapentadol was more sensitive toward both antagonists than was morphine, with median effective dose values of 0.75 and 1.72 ng/animal i.t. naloxone and 1.56 and 2.04 ng/animal i.t. yohimbine, respectively. It is suggested that the antihyperalgesic action of systemically administered tapentadol is based on opioid spinal-supraspinal synergy, as well as intrinsic spinally mediated MOR-NRI synergy.

Modification of morphine analgesia by venlafaxine in diabetic neuropathic pain model

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

Venlafaxine and neuropathic pain

The possible mechanisms involved in the antinociceptive effect of venlafaxine (VFX), a selective serotonin and noradrenaline reuptake inhibitor, after a single administration and chronic treatment were investigated in a diabetic neuropathic pain (DNP) model. VFX produced a significant antihyperalgesic effect after a single and repeated administration. This effect was reversed by pretreatment with yohimbine (a relatively selective α(2)-adrenergic antagonist) and p-chloroamphetamine (a neurotoxin which destroys serotonergic neurons). Conversely, naloxone (a nonselective opioid antagonist) did not reverse the effect of VFX in a DNP model. It is concluded that both noradrenergic and serotonergic mechanisms participate in the antinociceptive effect of VFX in the DNP model. However, the noradrenergic mechanism probably plays a more important role.

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

OBJECTIVE

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

METHODS

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

RESULTS

Not applicable.

CONCLUSIONS

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

Evidence for a differential opioidergic involvement in the analgesic effect of antidepressants: prediction for efficacy in animal models of neuropathic pain?

BACKGROUND AND PURPOSE

Antidepressants are one of the recommended treatments for neuropathic pain. However, their analgesic action remains unpredictable, and there are no selection criteria for clinical use. Better knowledge of their mechanism of action could help highlight differences underlying their unequal efficacy.

EXPERIMENTAL APPROACH

We compared the activity of a tricyclic antidepressant (clomipramine) with selective 5-HT and noradrenaline reuptake inhibitors (milnacipran and duloxetine) in streptozocin-induced diabetic and chronic constriction nerve injury-induced neuropathic rats, after repeated injections. We looked for an opioidergic mechanism in their action.

KEY RESULTS

Abolition of mechanical hyperalgesia was observed in mononeuropathic rats after five injections of clomipramine (5 mg·kg(-1) , s.c.) and milnacipran (10 or 20 mg·kg(-1) , i.p.) and in diabetic rats after clomipramine. An additional antinociceptive effect was obtained with five injections of duloxetine (3 mg·kg(-1) , i.p.) in both models and milnacipran (10 mg·kg(-1) , i.p.) in diabetic rats. These effects were observed with plasma antidepressant concentrations similar to those found in patients treated for neuropathic pain. Naloxone (1 mg·kg(-1) , i.v.) only suppressed the anti-hyperalgesic effects of clomipramine in both models of pain and of milnacipran in the traumatic model.

CONCLUSIONS AND IMPLICATIONS

The opioid system appears to be involved in the mechanism of action of antidepressants that only have an anti-hyperalgesic effect but not in those that have a stronger (i.e. antinociceptive) effect. These differences between the antidepressants occurred whatever the aetiology of the neuropathy and, if confirmed in clinical trials, could be used to decide which antidepressant is administered to a patient with neuropathic pain.

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

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

The antinociceptive and antihyperalgesic effect of tapentadol is partially retained in OPRM1 (μ-opioid receptor) knockout mice

Activation of the μ-opioid receptor (MOR) and noradrenaline reuptake inhibition (NRI) are well recognized as analgesic principles in acute and chronic pain indications. The novel analgesic tapentadol combines MOR agonism and NRI in a single molecule. The present study used OPRM1 (MOR) knockout (KO) mice to determine the relative contribution of MOR activation to tapentadol-induced analgesia in models of acute (nociceptive) and chronic (neuropathic) pain. Antinociceptive efficacy was inferred from paw withdrawal latencies on a 48 °C hot plate in naive animals. Antihyperalgesic efficacy was inferred from the number of nocifensive reactions in diabetic animals (streptozotocin-induced) and non-diabetic controls on a 50 °C hot plate. The effect of tapentadol (0.316-31.6 mg/kg IP) and the MOR agonist morphine (3-10 mg/kg IP) was determined in OPRM1 KO- and congenic wildtype mice. At baseline, diabetic OPRM1 KO mice showed reduced nocifensive reactions as compared to diabetic wildtype mice. In both pain models, morphine and tapentadol were effective in wildtype mice. In the KO mice, however, morphine failed to produce analgesia in either model. On the other hand, tapentadol still had clear effects, and when tested at a dose that was fully efficacious in wildtype mice, showed reduced but still significant antinociceptive efficacy in non-diabetic, and antihyperalgesic efficacy in diabetic OPRM1 KO mice. The remaining antinociceptive activity of tapentadol in OPRM1 KO mice was abolished by the α₂-adrenoceptor antagonist yohimbine. In OPRM1 wildtype mice, the antihyperalgesic effect of tapentadol was 10 times more potent in diabetic animals (ED₅₀=1.10 mg/kg) than its antinociceptive effect in naïve animals (ED₅₀=10.8 mg/kg). This study supports the conclusion that the analgesic effect of tapentadol is only partly due to the activation of MOR, both under acute and chronic pain conditions, and that the efficacy of tapentadol against acute and chronic pain is based on its combined mechanism of action.

Antidepressant treatment of neuropathic pain: looking for the mechanism

Neuropathic pain arises as a direct consequence of a lesion or disease affecting the somatosensory system. Among the recommended first-line treatments are antidepressant drugs – that is, molecules that were initially developed to treat other disorders of the nervous system. While their clinical efficacy against neuropathic pain was established more than 30 years ago, there is little information on the mechanism underlying their antidepressant action. However, understanding the therapeutic mechanism of these treatments could help to improve them, or even lead to new therapeutic approaches. In this article, we discuss the difficulties in conducting relevant preclinical research on neuropathic pain treatment with antidepressant drugs and we present the most recent findings on the putative mechanism, which highlight the role of β2-adrenoceptors and δ-opioid receptors.

Mu-opioid receptors are not necessary for nortriptyline treatment of neuropathic allodynia

Tricyclic antidepressants (TCAs) are among the first line treatments clinically recommended against neuropathic pain. However, the mechanism by which they alleviate pain is still unclear. Pharmacological and genetic approaches evidenced a critical role of delta-opioid receptors (DORs) in the therapeutic action of chronic TCA treatment. It is however unclear whether mu-opioid receptors (MORs) are also necessary to the pain-relieving action of TCAs. The lack of highly selective MOR antagonists makes difficult to conclude based on pharmacological studies. In the present work, we thus used a genetic approach and compared mutant mice lacking MORs and their wild-type littermates. The neuropathy was induced by unilateral sciatic nerve cuffing. The threshold for mechanical response was evaluated using von Frey filaments. MOR-deficient mice displayed the same baseline for mechanical sensitivity as their wild-type littermates. After sciatic nerve cuffing, both wild-type and MOR-deficient mice displayed an ipsilateral mechanical allodynia. After about 10 days of treatment, nortriptyline suppressed this allodynia in both wild-type and MOR-deficient mice. MORs are thus not critical for nortriptyline action against neuropathic pain. An acute injection of the DOR antagonist naltrindole induced a relapse of neuropathic allodynia in both wild-type and MOR-deficient mice, thus confirming the critical role of DORs in nortriptyline action. Moreover, morphine induced an acute analgesia in control and in neuropathic wild-type mice, but was without effect in MOR-deficient mice. While MORs are crucial for morphine action, they are not critical for nortriptyline action. Our results highlight the functional difference between DORs and MORs in mechanisms of pain relief.

Tapentadol, but not morphine, selectively inhibits disease-related thermal hyperalgesia in a mouse model of diabetic neuropathic pain

Neuropathic pain in diabetic patients is a common distressing symptom and remains a challenge for analgesic treatment. Selective inhibition of pathological pain sensation without modification of normal sensory function is a primary aim of analgesic treatment in chronic neuropathic pain. Tapentadol is a novel analgesic with two modes of action, mu-opioid receptor (MOR) agonism and noradrenaline (NA) reuptake inhibition. Mice were rendered diabetic by means of streptozotocin, and neuropathic hyperalgesia was assessed in a 50 degrees C hot plate test. Normal nociception was determined in control mice. Tapentadol (0.1-1mg/kg i.v.) and morphine (0.1-3.16 mg/kg i.v.) dose-dependently attenuated heat-induced nociception in diabetic animals with full efficacy, reaching >80% at the highest doses tested. Tapentadol was more potent than morphine against heat hyperalgesia, with ED(50) (minimal effective dose) values of 0.32 (0.316) and 0.65 (1)mg/kg, respectively. Non-diabetic controls did not show significant anti-nociception with tapentadol up to the highest dose tested (1mg/kg). In contrast, 3.16 mg/kg morphine, the dose that resulted in full anti-hyperalgesic efficacy under diabetic conditions, produced significant anti-nociception in non-diabetic controls. Selective inhibition of disease-related hyperalgesia by tapentadol suggests a possible advantage in the treatment of chronic neuropathic pain when compared with classical opioids, such as morphine. It is hypothesized that this superior efficacy profile of tapentadol is due to simultaneous activation of MOR and inhibition of NA reuptake.

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

RATIONALE

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

OBJECTIVES

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

Antinociceptive efficacy of lacosamide in a rat model for painful diabetic neuropathy

Lacosamide was tested in the streptozotocin rat model of diabetic neuropathic pain in comparison to drugs which are commonly used in the treatment of diabetic neuropathic pain, i.e. antidepressants and anticonvulsants. In diabetic rats, lacosamide attenuated cold (10, 30 mg/kg, i.p.), warm (3, 10, 30 mg/kg, i.p.) and mechanical allodynia (30 mg/kg, i.p.). Streptozotocin-induced thermal and mechanical hyperalgesia were reduced by lacosamide at doses of 10 and 30 mg/kg, i.p. Morphine (3 mg/kg) showed similar efficacy on allodynia and hyperalgesia. Amitriptyline (10 mg/kg), venlafaxine (15 mg/kg), levetiracetam (180 mg/kg) and pregabalin (100 mg/kg) exhibited significant effects on thermal allodynia and mechanical hyperalgesia. Only treatment with amitriptyline (30 mg/kg, i.p.) produced full reversal of thermal allodynia comparable to lacosamide. Lamotrigine (45 mg/kg, i.p.) had no effect on both behavioral readouts. Lacosamide's potency and efficacy in reversing pain behavior might be due to its new, yet unknown mechanism of action.

The Roles of the Opioidergic System and Nitric Oxide in the Analgesic Effect of Venlafaxine

The noradrenalin and serotonin re-uptake inhibitor venlafaxine has an analgesic effect that is independent of its antidepressant activity; however, the mechanism of this effect remains to be elucidated. This study was performed to investigate the possible roles of the opioidergic system and nitric oxide (NO) pathway in the analgesic effect of venlafaxine. Eighty Wistar rats of both sexes were allocated to 10 groups. The hot plate test was used to assess the antinociceptive/analgesic effect. The temperature of the hot plate was adjusted to 52.5+/-1 degrees C, the cut-off period was set to be 50 sec; licking of the hind paw was used as a sign of pain perception. Venlafaxine alone (25 mg/kg) showed marked analgesic activity (p<0.05). N-omega-nitro-L-arginine (L-NOARG) alone (20 mg/kg) and naloxone alone (2 mg/kg and 4 mg/kg) showed no analgesic activity (p>0.05). Coadministration of low-dose naloxone (2 mg/kg) and both doses of L-NOARG (20 and 40 mg/kg) with venlafaxine (25 mg/kg) did not modify the analgesic effect but high-dose naloxone (4 mg/kg) decreased it significantly (p<0.05). In conclusion, these results suggest that the opioidergic system but not the NO pathway has a role in the analgesic effect of venlafaxine.

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.