The antiallodynic effect of intrathecal tianeptine is exerted by increased serotonin and norepinephrine in the spinal dorsal horn

Antinociceptive effects of nefopam activating descending serotonergic modulation via 5-HT2 receptors in the nucleus raphe magnus

BACKGROUND

Nefopam is a centrally acting antinociceptive drug; however, the underlying mechanisms are not fully understood. This study investigated the supraspinal mechanisms of nefopam.

METHODS

The effects of intraperitoneally administered nefopam were assessed in rats using the formalin test, and the mechanisms were investigated by intrathecal or intra-nucleus raphe magnus (NRM) pre-treatment with the serotonin (5-HT) receptor antagonist or 5-HT2 receptor antagonist. The change in extracellular 5-HT levels was measured by spinal cord microdialysis.

RESULTS

Intraperitoneally administered nefopam showed antinociceptive effects in the rat formalin test, which were reversed by intrathecal pre-treatment with 5-HT receptor antagonist dihydroergocristine. Microdialysis study revealed that systemic nefopam significantly increased 5-HT level in the spinal dorsal horn. Pretreatment of cinanserin, a 5-HT2 receptor antagonist, into the NRM blocked the antinociceptive effects of intraperitoneally delivered nefopam. Direct injection of nefopam into the NRM mimicked the effects of systemic nefopam, and this effect was reversed by intra-NRM cinanserin pre-treatment. The increase in spinal level of 5-HT by systemic nefopam was attenuated by intra-NRM cinanserin pre-treatment.

CONCLUSION

The antinociceptive effects of systemically administered nefopam are mediated by 5-HT2 receptors in the NRM, which recruit the descending serotonergic fibres to increase the release of 5-HT into the spinal dorsal horn.

SIGNIFICANCE

This study revealed supraspinal mechanisms of nefopam-produced analgesia mediated by 5-HT2 receptors in the NRM recruiting the descending serotonergic fibres to increase the release of 5-HT into the spinal dorsal horn. These observations support a potential role for nefopam in multimodal analgesia based on its distinct mechanisms of action that are not shared by the other analgesics.

Development of Dibenzothiazepine Derivatives as Multifunctional Compounds for Neuropathic Pain

Neuropathic pain is a chronic and sometimes intractable condition caused by lesions or diseases of the somatosensory nervous system. Many drugs are available but unfortunately do not provide satisfactory effects in patients, producing limited analgesia and undesirable side effects. Thus, there is an urgent need to develop new pharmaceutical agents to treat neuropathic pain. To date, highly specific agents that modulate a single target, such as receptors or ion channels, never progress to the clinic, which may reflect the diverse etiologies of neuropathic pain seen in the human patient population. Therefore, the development of multifunctional compounds exhibiting two or more pharmacological activities is an attractive strategy for addressing unmet medical needs for the treatment of neuropathic pain. To develop novel multifunctional compounds, key pharmacophores of currently used clinical pain drugs, including pregabalin, fluoxetine and serotonin analogs, were hybridized to the side chain of tianeptine, which has been used as an antidepressant. The biological activities of the hybrid analogs were evaluated at the human transporters of neurotransmitters, including serotonin (hSERT), norepinephrine (hNET) and dopamine (hDAT), as well as mu (μ) and kappa (κ) opioid receptors. The most advanced hybrid of these multifunctional compounds, 17, exhibited multiple transporter inhibitory activities for the uptake of neurotransmitters with IC₅₀ values of 70 nM, 154 nM and 2.01 μM at hSERT, hNET and hDAT, respectively. Additionally, compound 17 showed partial agonism (EC₅₀ = 384 nM) at the μ-opioid receptor with no influence at the κ-opioid receptor. In in vivo pain animal experiments, the multifunctional compound 17 showed significantly reduced allodynia in a spinal nerve ligation (SNL) model by intrathecal administration, indicating that multitargeted strategies in single therapy could considerably benefit patients with multifactorial diseases, such as pain.

Pharmacological interactions between intrathecal pregabalin plus tianeptine or clopidogrel in a rat model of neuropathic pain

Background

There is still unmet need in treating neuropathic pain and increasing awareness regarding the use of drug combinations to increase the effectiveness of treatment and reduce adverse effects in patients with neuropathic pain.

Methods

This study was performed to determine the individual and combined effects of pregabalin, tianeptine, and clopidogrel in a rat model of neuropathic pain. The model was created by ligation of the L5-L6 spinal nerve in male Sprague-Dawley rats; mechanical allodynia was confirmed using von Frey filaments. Drugs were administered to the intrathecal space and mechanical allodynia was assessed; drug interactions were estimated by isobolographic or fixed-dose analyses.

Results

Intrathecal pregabalin and tianeptine increased the mechanical withdrawal threshold in a dose-dependent manner, but intrathecal clopidogrel had little effect on the mechanical withdrawal threshold. An additive effect was noted between pregabalin and tianeptine, but not between pregabalin and clopidogrel.

Conclusions

These findings suggest that intrathecal coadministration of pregabalin and tianeptine effectively attenuated mechanical allodynia in the rat model of neuropathic pain. Thus, pregabalin plus tianeptine may be a valid option to enhance the efficacy of neuropathic pain treatment.

Systemically administered neurotensin receptor agonist produces antinociception through activation of spinally projecting serotonergic neurons in the rostral ventromedial medulla

Background

Supraspinal delivery of neurotensin (NTS), which may contribute to the effect of a systemically administered agonist, has been reported to be either pronociceptive or antinociceptive. Here, we evaluated the effects of systemically administered NTSR1 agonist in a rat model of neuropathic pain and elucidated the underlying supraspinal mechanism.

Methods

Neuropathic pain was induced by L5 and L6 spinal nerve ligation in male Sprague-Dawley rats. The effects of intraperitoneally administered NTSR1 agonist PD 149163 was assessed using von Frey filaments. To examine the role of 5-HT neurotransmission, a serotonin (5-HT) receptor antagonist dihydroergocristine was pretreated intrathecally, and spinal microdialysis studies were performed to measure the change in extracellular level of 5-HT in response to PD 149163 administration. To investigate the supraspinal mechanism, NTSR1 antagonist 48692 was microinjected into the rostral ventromedial medulla (RVM) prior to systemic PD 149163. Additionally, the effect of intrathecal DHE on intra-RVM PD 149163 was assessed.

Results

Intraperitoneally administered PD 149163 exhibited a dose-dependent attenuation of mechanical allodynia. This effect was partially reversed by intrathecal pretreatment with dihydroergocristine and was accompanied by an increased extracellular level of 5-HT in the spinal cord. The PD 149163-produced antinociception was also blocked by intra-RVM SB 48692. Direct injection of PD 149163 into the RVM mimicked the maximum effect of the same drug delivered intraperitoneally, which was reversed by intrathecal dihydroergocristine.

Conclusions

These observations indicate that systemically administered NTSR1 agonist produces antinociception through the NTSR1 in the RVM, activating descending serotonergic projection to release 5-HT into the spinal dorsal horn.

Antinociceptive effects of nefopam modulating serotonergic, adrenergic, and glutamatergic neurotransmission in the spinal cord

The present study investigated the effects of intrathecal nefopam on the pain behavior and on the extracellular levels of serotonin (5-HT), norepinephrine (NE), and glutamate in the spinal cord, in a rat model of pain induced by formalin. Nefopam was intrathecally administered 10 min prior to the formalin test to assess its antinociceptive effects. In another cohorts of animals, dihydroergocristine, yohimbine, or (RS)-α-Methylserine-O-phosphate (MSOP), a serotonergic, α-2 adrenergic receptor, or group III metabotropic glutamate receptor antagonist, respectively, were administered prior to the application of nefopam in the formalin test. Microdialysis studies were conducted to measure the extracellular levels of 5-HT, NE, and glutamate in the spinal cord following nefopam administration. Intrathecal nefopam reduced formalin-induced behavior in both phases of the test. The blockade of serotonergic or adrenergic receptors partially reversed the analgesic effects of nefopam in the first phase of the formalin test whereas MSOP reversed these effects in both phases. The microdialysis results revealed that intrathecal nefopam significantly increased 5-HT and NE levels and attenuated the formalin-induced release of glutamate in the spinal cord. Thus, the present data suggest that the increase in the extracellular levels of 5-HT and NE, and reductions in glutamate release in the spinal cord, may have contributed to the analgesic effects of nefopam.

Opioid Receptors Contribute to Antinociceptive Effect of Tianeptine on Colorectal Distension-Induced Visceral Pain in Rats

Tianeptine is a clinically effective atypical antidepressant with distinct neurochemical properties. In this study, we aimed to investigate the contribution of opioid receptors in the antinociceptive effect of tianeptine on visceral pain in awake rats and to differentiate the subtype and the localization (central and/or peripheral) of these opioid receptors involved in this antinociception. Visceromotor response to noxious colorectal distension (CRD) was quantified with electromyographic recordings, obtained from previously implanted electrodes into the external oblique musculature of rats under anesthesia, before and after tianeptine administration. The opioid receptor antagonist naloxone hydrochloride (NLX) and peripherally restricted opioid receptor antagonist naloxone methiodide (NLXM) were administered intravenously 10 min before tianeptine (10 mg/kg, i.v.). The antinociceptive effect of tianeptine was abolished by NLX (1 and 2 mg/kg, i.v.), but was partially reduced by NLXM (1 and 2 mg/kg, i.v.). A µ-opioid receptor-selective dose (0.03 mg/kg, i.v.) of NLX, but not NLXM, significantly inhibited the antinociceptive effect of tianeptine. Our results suggest that antinociceptive effect of tianeptine on CRD-induced visceral nociception in rats involves the activation of both central and peripheral opioid receptors.

Effects of tianeptine on the development and maintenance of mechanical allodynia in a rat model of neuropathic pain

We validate the analgesic efficacy of tianeptine by different administration routes and timing in a rat model of neuropathic pain. Neuropathic pain was induced by ligating the L5 and L6 spinal nerves in male Sprague-Dawley rats, and mechanical allodynia was assessed using von Frey filaments. The effects of orally administered tianeptine and pretreatment with tianeptine (intrathecally or intraperitoneally) on mechanical allodynia were assessed. Oral and preemptive intrathecal administration of tianeptine significantly increased the paw withdrawal threshold but preemptive intraperitoneal administration did not. Nevertheless, intraperitoneal pretreatment of tianeptine potentiated the antiallodynic effects of subsequently administered tianeptine. These findings suggest that tianeptine may be effective for preventing and treating neuropathic pain and that it can be used more widely in clinical pain practice.

The Role of Spinal Dopaminergic Transmission in the Analgesic Effect of Nefopam on Rat Inflammatory Pain

Background

Nefopam has been known as an inhibitor of the reuptake of monoamines, and the noradrenergic and/or serotonergic system has been focused on as a mechanism of its analgesic action. Here we investigated the role of the spinal dopaminergic neurotransmission in the antinociceptive effect of nefopam administered intravenously or intrathecally.

Methods

The effects of intravenously and intrathecally administered nefopam were examined using the rat formalin test. Then we performed a microdialysis study to confirm the change of extracellular dopamine concentration in the spinal dorsal horn by nefopam. To determine whether the changes of dopamine level are associated with the nefopam analgesia, its mechanism was investigated pharmacologically via pretreatment with sulpiride, a dopaminergic D2 receptor antagonist.

Results

When nefopam was administered intravenously the flinching responses in phase I of the formalin test were decreased, but not those in phase II of the formalin test were decreased. Intrathecally injected nefopam reduced the flinching responses in both phases of the formalin test in a dose dependent manner. Microdialysis study revealed a significant increase of the level of dopamine in the spinal cord by intrathecally administered nefopam (about 3.8 fold the baseline value) but not by that administered intravenously. The analgesic effects of intrathecally injected nefopam were not affected by pretreatment with sulpiride, and neither were those of the intravenous nefopam.

Conclusions

Both the intravenously and intrathecally administered nefopam effectively relieved inflammatory pain in rats. Nefopam may act as an inhibitor of dopamine reuptake when delivered into the spinal cord. However, the analgesic mechanism of nefopam may not involve the dopaminergic transmission at the spinal level.

Analgesic effect of Chinese herbal formula Hua-Jian-Ba-Du ointment on visceral pain in mice induced by acetic acid

INTRODUCTION

Visceral pain is one of the most important pains caused by cancer or other diseases, and most of the medications may lead to tolerance, addiction, and toxic side effects. Hua-Jian-Ba-Du Ointment (HJBDO), which is a commonly used conjugate based on traditional Chinese medicine theory, has been effective against visceral pain. Here, we verify the efficacy and underlying mechanism of HJBDO in an acetic-acid induced visceral pain model.

METHODS

Mice were subjected to acetic acid with or without HJBDO. Hua-Jian-Ba-Du Ointment at low (7.5 mL/kg•d), moderate (15 mL/kg•d), and high (30 mL/kg•d) dosages was applied on the abdomen, 3 times per day for 3 days. The acetic acid writhing test was used to evaluate antinociception. Interleukin-2 (IL-2) in serum, tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and prostaglandin E2 (PGE2) in peritoneal fluid were detected by ELISA. 5-hydroxytryptamine (5-HT), norepinephrine (NE), dopamine (DA), and β-endorphin (β-EP) were examined by high performance liquid chromatography and radioimmunoassay, respectively. N-methyl-D-aspartic acid receptor (NMDAR1) and c-fos expressions in both rostral ventromedial medulla (RVM) and spinal dorsal horn were determined by western blot.

RESULTS

Hua-Jian-Ba-Du Ointment at 3 dosage levels produced dose-dependent antinociception and shortened the latent time. Hua-Jian-Ba-Du Ointment at high or moderate dosage inhibited the release of TNF-α, IL-6, and PGE2, as well as increased the release of IL-2. Hua-Jian-Ba-Du Ointment could also increase NE and 5-HT contents and decrease the NE content. No effect of HJBDO at 3 dosages on the DA system was detected. Furthermore, HJBDO could suppress the expressions of NMDAR and c-fos in both RVM and spinal dorsal horn.

CONCLUSION

Our results exhibited the analgesic effect of HJBDO on visceral pain in mice, and this effect might be mediated by the regulation of inflammation and neurotransmitters.