Role of peripheral 5-HT(4), 5-HT(6), and 5-HT(7) receptors in development and maintenance of secondary mechanical allodynia and hyperalgesia

Anti-allodynic and anti-hyperalgesic activity of (±)-licarin A in neuropathic rats via NO-cyclic-GMP-ATP-sensitive K+ channel pathway

The study aimed to examine the effect of intraperitoneal and intrathecal (±)-licarin A in neuropathic pain induced by L5 and L6 spinal nerve ligation (SNL) in male Wistar rats and the possible involvement of the NO-cGMP-ATP-sensitive K+ channel pathway. Neuropathic pain signs (allodynia and hyperalgesia) were evaluated on postoperative Day 14 using von Frey filaments. Single intraperitoneal (0.01, 0.1, 1, and 10 mg/kg) and intrathecal (0.01, 0.1, 1, and 10 µg/rat) administration of (±)-licarin A improved allodynia and hyperalgesia. The (±)-licarin A-induced anti-allodynic and anti-hyperalgesic activity was prevented by the intrathecal injection of  l-NAME (100 µg/rat; nonselective nitric oxide synthase inhibitor), ODQ (10 µg/rat; guanylate cyclase inhibitor), and glibenclamide (50 µg/rat; adenosine triphosphate (ATP)-sensitive K+ channel blocker). The data suggest that (±)-licarin A exerts its anti-allodynic and anti-hyperalgesic activity by activating the NO-cGMP-ATP-sensitive K+ channel pathway.

Metatranscriptome analysis reveals the putative venom toxin repertoire of the biofouling hydroid Ectopleura larynx

Cnidarians thriving in biofouling communities on aquaculture net pens represent a significant health risk for farmed finfish due to their stinging cells. The toxins coming into contact with the fish, during net cleaning, can adversely affect their behavior, welfare, and survival, with a particularly serious health risk for the gills, causing direct tissue damage such as formation of thrombi and increasing risks of secondary infections. The hydroid Ectopleura larynx is one of the most common fouling organisms in Northern Europe. However, despite its significant economic, environmental, and operational impact on finfish aquaculture, biological information on this species is scarce and its venom composition has never been investigated. In this study, we generated a whole transcriptome of E. larynx, and identified its putative expressed venom toxin proteins (predicted toxin proteins, not functionally characterized) based on in silico transcriptome annotation mining and protein sequence analysis. The results uncovered a broad and diverse repertoire of putative toxin proteins for this hydroid species. Its toxic arsenal appears to include a wide and complex selection of toxin proteins, covering a large panel of potential biological functions that play important roles in envenomation. The putative toxins identified in this species, such as neurotoxins, GTPase toxins, metalloprotease toxins, ion channel impairing toxins, hemorrhagic toxins, serine protease toxins, phospholipase toxins, pore-forming toxins, and multifunction toxins may cause various major deleterious effects in prey, predators, and competitors. These results provide valuable new insights into the venom composition of cnidarians, and venomous marine organisms in general, and offer new opportunities for further research into novel and valuable bioactive molecules for medicine, agronomics and biotechnology.

The 5-HT7 receptor system as a treatment target for mood and anxiety disorders: A systematic review

BACKGROUND

Preclinical animal and preliminary human studies indicate that 5-HT7 antagonists have the potential as a new treatment approach for mood and anxiety disorders. In this systematic review, we aimed to review the relationship between the 5-HT7 receptor system and mood and anxiety disorders, and to explore the pharmacology and therapeutic potential of medications that target the 5-HT7 receptor for their treatment.

METHODS

Medline, Cochrane Library, EMBASE, PsycINFO databases, the National Institute of Health website Clinicaltrials.gov, controlled-trials.com, and relevant grey literature were used to search for original research articles, and reference lists of included articles were then hand searched.

RESULTS

Sixty-four studies were included in the review: 52 animal studies and 12 human studies. Studies used a variety of preclinical paradigms and questionnaires to assess change in mood, and few studies examined sleep or cognition. Forty-four out of 47 (44/47) preclinical 5-HT7 modulation studies identified potential antidepressant effects and 20/23 studies identified potential anxiolytic effects. In clinical studies, 5/7 identified potential antidepressant effects in major depressive disorder, 1/2 identified potential anxiolytic effects in generalized anxiety disorder, and 3/3 identified potential antidepressant effects in bipolar disorders.

CONCLUSION

While there is some evidence that the 5-HT7 receptor system may be a potential target for treating mood and anxiety disorders, many agents included in the review also bind to other receptors. Further research is needed using drugs that bind specifically to 5-HT7 receptors to examine treatment proof of concept further.

Pramipexole inhibits formalin-induce acute and long-lasting mechanical hypersensitivity via NF-kB pathway in rats

Pain is one of the most frequent causes for patients to seek medical care. It interferes with daily functioning and affects the quality of life of the patient. There is a clear need to investigate nonopioid or non-nonsteroidal anti-inflammatory drug alternatives for the treatment of pain. In this study, we determined the effect of acute pre- and posttreatment with pramipexole (PPX), a dopamine D2/D3 selective agonist, on formalin 1%-induced acute and long-lasting nociceptive behavior sensitivity in rats. Moreover, we sought to investigate whether the antiallodynic and antihyperalgesic effect induced by PPX was mediated through the nuclear factor-κB (NF-kB) signaling pathway. Moreover, acute systemic pretreatment with PPX (1 and 3 mg/kg, ip) suppressed the formalin-induced nociceptive behavior during both phases of the formalin test and the development of formalin-induced secondary mechanical allodynia and hyperalgesia in both paws. Acute systemic posttreatment with PPX (3 mg/kg, ip) reverted the formalin-induced long-lasting secondary mechanical allodynia and hyperalgesia. Furthermore, PPX inhibits the protein expression of NF-κB-p65 and the levels of tumor necrosis factor-α and interleukin-1β in the spinal cord of animals with secondary mechanical allodynia and hyperalgesia induced by formalin. These data suggest that PPX has a potential role in producing anti-inflammatory activity. Moreover, the antiallodynic and antihyperalgesic effects induced by PPX can be mediated through the NF-kB signaling pathway.

The Influence of Sleep Disturbance on Chronic Pain

PURPOSE OF REVIEW

The purpose of this review is to present an overview of common sleep disturbance pathologies and their impact on chronic pain, while examining various factors that are implicit in the relationship between sleep disturbance and chronic pain, including neurobiochemistry, anatomy, and systemic mediators, and reviewing recent and landmark literature.

RECENT FINDINGS

Earlier literature reviews and studies have introduced the bidirectional relationship between sleep disturbance and chronic pain; that is, impaired sleep may worsen chronic pain, and chronic pain causes sleep disturbance. However, more recent reviews and studies seem to show a more associative, rather than causative relationship. There have been recent studies that attempt to determine mechanisms that link sleep disturbance and chronic pain; the results of these studies were more varied, ultimately concluding that there may be a separate, yet-to-be discovered mechanism that shows the causative relationship between sleep disturbance and pain. There are several neurotransmitters that are involved in the mediation of chronic pain and sleep disturbance as separate entities, and some studies have shown that there may be mechanisms that govern both chronic pain and sleep disturbance as a single unit. Other neuroendocrine substances also serve to mediate chronic pain and sleep disturbance. All these substances are found to be associated with various sleep disorders and are also associated with chronic pain symptoms as well. Inflammation plays a role in chronic pain and sleep disturbance, with an increase in inflammatory substances and mediators associated with an increase or worsening in chronic pain symptoms and sleep disorders. The HPA axis plays a role in chronic pain and sleep disorders, influencing pain and sleep pathways through stress response, inflammation, and maintenance of homeostasis. There are several variables that influence both chronic pain and sleep disturbance, and more research into these variables may further our understanding into the complex pathways governing the influence of sleep disturbance on pain, and ultimately to improve treatment for this issue.

SZAP exerts analgesic effects on rheumatalgia in CIA rats by suppressing pain hyperalgesia and inhibiting TRPV1 and P2X3

ETHNOPHARMACOLOGICAL RELEVANCE

ShexiangZhuifeng Analgesic Plaster (SZAP) is a traditional Chinese medicine and transdermal formulation composed of many Chinese herbs and active compounds. SZAP was recently approved by the China Food and Drug Administration for the treatment of pain associated with osteoarticular diseases and is preferred by most rheumatoid arthritis patients in China. However, its mechanism has not been elucidated in detail.

AIM OF THE STUDY

We sought to determine the analgesic effect of SZAP in collagen-induced arthritis (CIA) rats and explore the underlying mechanisms of pain transmission, such as via the TRPV1 and P2X3 receptors.

METHODS

After CIA was established, rats were treated with SZAP for 7 days. Paw thickness, arthritis score, and haematoxylin and eosin staining were used to evaluate the effectiveness of SZAP. Paw withdrawal threshold (PWT) and tail-flick latency (TFL) were used to estimate the analgesic effect of SZAP. The levels of PGE₂, BK, 5-HT, SP, and CGRP in the serum and synovium were determined using ELISA kits, and ATP in the synovium was measured using HPLC. The expression of TRPV1 and P2X3 in the DRG was detected using western blotting and immunofluorescence. TRPV1 and P2X3 agonists were further used to determine the analgesic effects of SZAP on CIA rats based on PWT and TFL.

RESULTS

SZAP not only significantly ameliorated arthritis scores and paw thickness by improving the pathological damage of synovial joints, but also remarkably alleviated pain in CIA rats. Further, treatment with SZAP significantly reduced peripheral 5-HT, PGE₂ BK, SP, CGRP, and ATP. Additionally, the expression of TRPV1 and P2X3 in the DRG was markedly downregulated by SZAP. Interestingly, the analgesic effect of SZAP was weakened (reduction of PWT and TFL) when TRPV1 and P2X3 were activated by capsaicin or α,β-meATP, respectively.

CONCLUSION

SZAP ameliorates rheumatalgia by suppressing hyperalgesia and pain transmission through the inhibition of TRPV1 and P2X3 in the DRG of CIA rats.

Cardamonin Modulates Neuropathic Pain through the Possible Involvement of Serotonergic 5-HT1A Receptor Pathway in CCI-Induced Neuropathic Pain Mice Model

Cardamonin, a naturally occurring chalcone isolated from Alpinia species has shown to possess strong anti-inflammatory and anti-nociceptive activities. Previous studies have demonstrated that cardamonin exerts antihyperalgesic and antiallodynic properties in chronic constriction injury (CCI)-induced neuropathic pain animal model. However, the mechanisms underlying cardamonin's effect have yet to be fully understood. The present study aims to investigate the involvement of the serotonergic system in cardamonin induced antihyperalgesic and antiallodynic effects in CCI-induced neuropathic pain mice model. The neuropathic pain symptoms in the CCI mice model were assessed using Hargreaves Plantar test and von-Frey filament test on day 14 post-surgery. Central depletion of serotonin along the descending serotonergic pathway was done using ρ-chlorophenylalanine (PCPA, 100 mg/kg, i.p.), an inhibitor of serotonin synthesis for four consecutive days before cardamonin treatment, and was found to reverse the antihyperalgesic and antiallodynic effect produced by cardamonin. Pretreatment of the mice with several 5-HT receptor subtypes antagonists: methiothepin (5-HT1/6/77 receptor antagonist, 0.1 mg/kg), WAY 100635 (5-HT1A receptor antagonist, 1 mg/kg), isamoltane (5-HT1B receptor antagonist, 2.5 mg/kg), ketanserin (5-HT2A receptor antagonist, 0.3 mg/kg), and ondansetron (5-HT3 receptor antagonist, 0.5 mg/kg) were shown to abolish the effect of cardamonin induced antihyperalgesic and antiallodynic effects. Further evaluation of the 5-HT1A receptor subtype protein expressions reveals that cardamonin significantly upregulated its expression in the brainstem and spinal cord. Our results suggest that the serotonergic pathway is essential for cardamonin to exert its antineuropathic effect in CCI mice through the involvement of the 5-HT1A receptor subtype in the central nervous system.

The 5-HT6 Receptors in the Ventrolateral Orbital Cortex Attenuate Allodynia in a Rodent Model of Neuropathic Pain

Mechanical allodynia, characterized by a painful sensation induced by innocuous stimuli, is thought to be caused by disruption in pain-related regions. Identification and reversal of this pathologic neuroadaptation are therefore beneficial for clinical treatment. Previous evidence suggests that 5-HT₆ receptors in the ventrolateral orbital cortex (VLO) are involved in neuropathic pain, but their function is poorly understood. The aim of the present study is to unveil the role of 5-HT₆ receptors in the VLO and the underlying mechanisms in pain modulation. Here, by using the spared nerve injury (SNI) pain model, first, we report that 5-HT₆ receptor protein decreased in the contralateral VLO compared with the ipsilateral VLO in rats with allodynia. Second, microinjection of the selective 5-HT₆ receptor agonists EMD-386088 and WAY-208466 into the contralateral VLO consistently and significantly depressed allodynia. Third, microinjection of the selective antagonist SB-258585 blocked the agonist-induced anti-allodynic effect, while the antagonist applied alone to the VLO had no effect. Furthermore, the anti-nociceptive effect of EMD-386088 on neuropathic pain was prevented by the adenylate cyclase (AC) inhibitor SQ-22536, and protein kinase A (PKA) inhibitor H89, suggesting that AC/PKA signaling might underlie the antinociception of agonists. Finally, the 5-HT₆ receptors were found to be colocalized with a glutamate transporter (EAAC1) by immunofluorescent staining, and the glutamate receptor antagonist kynurenic acid was found to completely block antinociception. These findings indicated that the antinociceptive effect of 5-HT₆ receptor agonists might occur via interaction with the glutamatergic system. Altogether, the agonists activated 5-HT₆ receptors present in the glutamatergic neurons in the VLO to facilitate the AC/PKA cascade, which subsequently might evoke glutamate release, thus depressing allodynia. These findings suggest a potential therapeutic role of 5-HT₆ receptor agonists in treating neuropathic pain.

The Influence of Metoclopramide on Trigeminovascular Nociception: Possible Anti-migraine Mechanism of Action

Metoclopramide is widely used as an abortive migraine therapy due to the advantage of having not only antiemetic, but also analgesic properties. Despite the proven clinical efficacy of metoclopramide in acute migraine, the mechanism of its anti-cephalalgic action has not been entirely elucidated. Taking into account the key role of the trigeminovascular system activation in migraine pathophysiology, we aimed to investigate metoclopramide effects on the excitability of central trigeminovascular neurons and neurogenic dural vasodilation using valid electrophysiological and neurovascular models of trigeminovascular nociception. Extracellular recordings of the activity of second-order dura-sensitive neurons were made in the trigeminocervical complex (TCC) of 16 anaesthetised rats. Cumulative metoclopramide infusion (three steps in 30 min intervals, 5 mg/kg i.v. per step, n = 8) significantly and dose-dependently suppressed both ongoing firing of the TCC neurons and their responses to dural electrical stimulation, maximally to 30%[0-49%] (median[Q1-Q3]) and 4%[0-30%] of the initial level, respectively (both p = 0.001, compared to saline (n = 8)). By contrast, the neurogenic dural vasodilation studied in a separate group of 12 rats was not significantly affected by cumulative infusion of metoclopramide (5 mg/kg i.v. per step, n = 6) compared to both baseline values and the vehicle group (n = 6) (all p > 0.05). These results provide evidence that metoclopramide is unable to affect the peripheral response to trigeminovascular activation, but it does suppress the central response, which is highly predictive of anti-migraine action. Thus, here we show the neurophysiological mechanism underlying the therapeutic efficacy of metoclopramide in migraine.

The 5-HT6 receptor interactome: New insight in receptor signaling and its impact on brain physiology and pathologies

The serotonin (5-HT)₆ receptor is a Gs-coupled receptor exclusively expressed in the central nervous system. Highest receptor densities are found in brain regions implicated in mnemonic functions where the receptor is primarily but not exclusively located in the primary cilium of neurons. The 5-HT₆ receptor continues to raise particular interest for neuropharmacologists, given the pro-cognitive effects of antagonists in a wide range of cognitive impairment paradigms in rodents and human. The 5-HT₆ receptor also finely controls key neuro-developmental processes including neuron migration and differentiation. However, its influence upon neurodevelopment and cognition is not solely mediated by its coupling to the Gs-adenylyl cyclase pathway, suggesting alternative signal transduction mechanisms. This prompted studies aimed at characterizing the receptor interactome that identified 125 candidate receptor partners, making the 5-HT₆ receptor one of the G protein-coupled receptors with the most extensively characterized interactome. These studies showed that the receptor localization at the plasma membrane and, consequently, its signal transduction, are finely modulated by several receptor partners. They demonstrated that prefrontal 5-HT₆ receptors engage the mTOR pathway to compromise cognition in neurodevelopmental models of schizophrenia, and a role of the 5-HT₆-mTOR pathway in temporal epilepsy. Finally, they revealed that the receptor activates Cdk5 signaling in an agonist-independent manner through a mechanism involving receptor phosphorylation by the associated Cdk5 and highlighted its key role in the migration of neurons and neurite growth. These new receptor-operated signaling mechanisms should be considered in the future development of drugs acting on 5-HT₆ receptors. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Correlation of 5-HTR6 gene polymorphism with vestibular migraine

Objective

To investigate the correlation of 5‐hydroxy tryptamine receptor 6 (5‐HTR6) gene polymorphism with vestibular migraine (VM).

Methods

A total of 92 VM patients were enrolled as the observation group, and 100 healthy people receiving physical examinations as the control group. Their general clinical information was collected, and the level of 5‐HT in plasma and the vestibular function test indexes were detected. Moreover, the polymorphism of 5‐HTR6 rs770963777 was detected with the TaqMan‐MGB probe.

Results

The observation group had a lower level of 5‐HT than the control group (P < .05), and the abnormality rates of the vestibular function tests, including the caloric test, head‐shaking test, and vestibular autorotation test, were obviously higher than those in the control group (P < .01). The comparisons showed that the distribution frequencies of the genotypes and alleles were different between the two groups (P < .05). According to the analysis of the genetic mode, there were differences in recessive and additive modes between the two groups (P < .05), but the dominant mode was not different between the two groups (P > .05).

Conclusion

The level of 5‐HT and the vestibular function test indexes can serve as the effective indicators for observing VM, and the polymorphism of 5‐HTR6 rs770963777 site is correlated with VM onset.

The pronociceptive role of 5-HT6 receptors in ventrolateral orbital cortex in a rat formalin test model

Recent studies have shown the 5-HT₆ receptors are expressed in regions which are important in pain processing such as the cortex, amygdala, thalamus, PAG, spinal cord and dorsal root ganglia (DRG), suggesting a putative role of 5-HT₆ receptors in pain modulation. The ventrolateral orbital cortex (VLO) is part of an endogenous analgesic system, consisting of the spinal cord - thalamic nucleus submedius (Sm) - VLO - periaqueductal gray (PAG) - spinal cord loop. The present study assessed the possible role of 5-HT₆ receptors in the VLO in formalin-induced inflammatory pain model. Firstly we found that microinjection of selective 5-HT₆ receptor agonists EMD-386088 (5 μg in 0.5 μl) and WAY-208466 (8 μg in 0.5 μl) both augmented 5% formalin-induced nociceptive behavior. Microinjection of selective 5-HT₆ receptor antagonist SB-258585 (1,2 and 4 μg in 0.5 μl) significantly reduced formalin-induced flinching. Besides, the pronociceptive effects of EMD-386088 and WAY-208466 were dramatically reduced by SB-258585, implicating 5-HT₆ receptor mechanisms in mediating these responses. In addition, the pronociceptive effect of EMD-386088 was also prevented by the adenylate cyclase (AC) inhibitor SQ-22536 (2 nmol in 0.5 μl) and the protein kinase A (PKA) inhibitor H89 (10 nmol in 0.5 μl), respectively. We further confirmed the above results with quantification of spinal c-fos expression. Taken together, our results suggested that 5-HT₆ receptors play a pronociceptive role in the VLO in the rat formalin test due to its activation of AC - PKA pathway. Therefore, cerebral cortical 5-HT₆ receptors could be a new target to develop analgesic drugs.

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.

Cyanobacterial extract with serotonin receptor subtype 7 (5-HT7 R) affinity modulates depression and anxiety-like behavior in mice

Marine cyanobacteria represent a unique source in the field of drug discovery due to the secondary metabolites they produce and the structural similarity these compounds have to endogenous mammalian receptor ligands. A series of cyanobacteria were subjected to extraction, fractionation by column chromatography and screened for affinity against CNS targets with a focus on serotonin receptors (5-HTRs). Out of 276 fractions screened, 21% had activity at 5-HTRs and/or the 5-HT transporter (SERT). One sample, a cyanobacterium identified by 16S rRNA sequencing as Leptolyngbya from Las Perlas archipelago in Panama, contained a fraction with noted affinity for the 5-HT₇ receptor (5-HT₇ R). This fraction (DUQ0002I) was screened via intracerebroventricular (ICV) injections in mice using depression and anxiety assays including the forced swim, tail suspension, elevated zero maze, and light-dark preference tests. DUQ0002I decreased depression and anxiety-like behaviors in males and did not have effects in 5-HT₇ R knockout or female mice. Administration of DUQ0002I to the CA1 of the hippocampus induced antidepression-like, but not anxiolytic-like behaviors. Testing of further purified materials showed no behavioral effects, leading us to hypothesize that the behavioral effects are likely caused by a synergistic effect between multiple compounds in the fraction. Finally, DUQ0002I was used in a model of neuropathic pain with comorbid depression (spared nerve injury-SNI). DUQ0002I had a similar antidepressant effect in animals with SNI, suggesting a role for the 5-HT₇ R in the development of comorbid pain and depression. These results demonstrate the potential that cyanobacterial metabolites have in the field of neuropharmacognosy.

Contribution of spinal 5-HT5A receptors to the antinociceptive effects of systemically administered cannabinoid agonist WIN 55,212-2 and morphine

The antinociceptive effects of cannabinoids and opioids have been known for centuries. Serotonin and its receptors are also known to play important roles in nociception. However, the contribution of spinal 5-HT_5A receptors in antinociceptive effects of cannabinoids and opioids has not been studied. We conducted this study to clarify spinal mechanisms of the actions of the antinociceptive effects of cannabinoids and opioids. Hot plate and tail flick tests were used to assess the antinociceptive activity in Balb/c mice. WIN 55,212-2, a nonselective CB1 and CB2 agonist, and morphine exerted significant antinociceptive effects at 1, 3, and 10 mg/kg doses administered intraperitoneally in both hot plate and tail flick tests. The selective 5-HT_5A receptor antagonist SB-699551 (10 nmol/mouse) was administered intrathecally 10 min before the agonists. SB-699551 significantly reduced the antinociceptive effect of both WIN 55,212-2 and morphine. In the rotarod test, WIN 55,212-2 disrupted the motor coordination at a dose of 10 mg/kg, while morphine did not affect this function at any dose. Our findings show that spinal 5-HT_5A receptors are involved in the antinociceptive effects of WIN 55,212-2 and morphine.

Peripheral 5-HT3 Receptors Are Involved in the Antinociceptive Effect of Bunodosine 391

Bunodosine 391 (BDS 391), a low molecular weight compound isolated from the sea anemone Bunodosoma cangicum, increases the nociceptive threshold and inhibits inflammatory hyperalgesia. Serotonin receptors are involved in those effects. In this study, we have expanded the characterization of the antinociceptive effect of BDS 391 demonstrating that, in rats: (a) the compound inhibits (1.2–12 ng/paw) overt pain, in the formalin test, and mechanical hyperalgesia (0.6–6.0 ng/paw) detected in a model of neuropathic pain; (b) intraplantar administration of ondansetron, a selective 5-HT3 receptor antagonist, blocks the effect of BDS 391, whereas ketanserin, a 5-HT2 receptor antagonist, partially reversed this effect, indicating the involvement of peripheral 5-HT2 and 5-HT3 receptors in BDS 391 antinociception; and (c) in binding assay studies, BDS 391 was not able to displace the selective 5-HT receptor antagonists, suggesting that this compound does not directly bind to these receptors. The effect of biguanide, a selective 5-HT3 receptor agonist, was also evaluated. The agonist inhibited the formalin’s nociceptive response, supporting an antinociceptive role for 5-HT3 receptors. Our study is the first one to show that a non-peptidic low molecular weight compound obtained from a sea anemone is able to induce antinociception and that activation of peripheral 5-HT3 receptors contributes to this effect.

Toluene exposure enhances acute and chronic formalin-induced nociception in rats: Participation of 5-HT3 receptors

The purpose of this study was to evaluate the effect of acute toluene exposure on formalin (0.5% and 1%)-induced acute and long-lasting nociceptive hypersensitivity in rats. In addition, we sought to investigate the role of peripheral 5-HT₃ receptors in the pronociceptive effect of toluene. Toluene exposure (6000ppm) for 30min enhanced 0.5% or 1% formalin-induced acute nociception and long-lasting secondary allodynia and hyperalgesia. In contrast, exposition to toluene for 30min in rats previously injected (six days before) with 1% formalin did not affect long-lasting hypersensitivy. Local peripheral pre-treatment with alosetron (5-HT₃ receptor antagonist, 10-100 nmol) reduced the pronociceptive effect of toluene in acute nociception and long-lasting secondary allodynia and hyperalgesia. Alosetron (100nmol) was also able to reduce the nociceptive effects of 1% formalin in absence of toluene. Moreover, local peripheral injection of m-CPBG (5-HT₃ receptor agonist, 300 nmol) enhanced 0.5% formalin-induced acute and long-lasting nociception in air- and toluene-exposed rats. Alosetron (10nmol) blocked the pronociceptive effects of m-CPBG (300nmol) on 0.5% formalin-induced acute and long-lasting hypersensitivity in rats exposed to toluene. Alosetron (at 10nmol) did not modify formalin-induced nociceptive behaviors. Finally, local peripheral pre-treatment with methiothepin (non-selective 5-HT receptor antagonist, 1.5nmol), did not affect the pronociceptive effect of toluene on 1% formalin-induced acute and long-lasting hypersensitivity. Our data demonstrate that acute exposure to toluene has pronociceptive effects in formalin-induced acute nociception and long-lasting hypersensitivity. Our data suggest that this pronociceptive effect depend on activation of peripheral 5-HT₃, but not methiothepin-sensitive 5-HT, receptors.

Selective 5-HT7 receptor agonists LP 44 and LP 211 elicit an analgesic effect on formalin-induced orofacial pain in mice

The most recently identified serotonin (5-HT) receptor is the 5-HT7 receptor. The antinociceptive effects of a 5-HT7 receptor agonist have been shown in neuropathic and inflammatory animal models of pain. A recent study demonstrated the functional expression of 5-HT7 receptors in the substantia gelatinosa (SG) of the trigeminal subnucleus caudalis, which receives and processes orofacial nociceptive inputs.

The α5 subunit containing GABAA receptors contribute to chronic pain

It has been recently proposed that α5-subunit containing GABAA receptors (α5-GABAA receptors) that mediate tonic inhibition might be involved in pain. The purpose of this study was to investigate the contribution of α5-GABAA receptors in the loss of GABAergic inhibition and in formalin-induced, complete Freund's adjuvant (CFA)-induced and L5 and L6 spinal nerve ligation-induced long-lasting hypersensitivity. Formalin or CFA injection and L5 and L6 spinal nerve ligation produced long-lasting allodynia and hyperalgesia. Moreover, formalin injection impaired the rate-dependent depression of the Hofmann reflex. Peripheral and intrathecal pretreatment or post-treatment with the α5-GABAA receptor antagonist, L-655,708 (0.15-15 nmol), prevented and reversed, respectively, these long-lasting behaviors. Formalin injection increased α5-GABAA receptor mRNA expression in the spinal cord and dorsal root ganglia (DRG) mainly at 3 days. The α5-GABAA receptors were localized in the dorsal spinal cord and DRG colabeling with NeuN, CGRP, and IB4 which suggests their presence in peptidergic and nonpeptidergic neurons. These receptors were found mainly in small and medium sized neurons. Formalin injection enhanced α5-GABAA receptor fluorescence intensity in spinal cord and DRG at 3 and 6 days. Intrathecal administration of L-655,708 (15 nmol) prevented and reversed formalin-induced impairment of rate-dependent depression. These results suggest that α5-GABAA receptors play a role in the loss of GABAergic inhibition and contribute to long-lasting secondary allodynia and hyperalgesia.

The potential role of serotonergic mechanisms in the spinal oxytocin-induced antinociception

The role of oxytocin (OXT) in pain modulation has been suggested. Indeed, hypothalamic paraventricular nuclei (PVN) electrical stimuli reduce the nociceptive neuronal activity (i.e., neuronal discharge associated with activation of Aδ- and C-fibers) of the spinal dorsal horn wide dynamic range (WDR) cells and nociceptive behavior. Furthermore, raphe magnus nuclei lesion reduces the PVN-induced antinociception, suggesting a functional interaction between the OXT and the serotoninergic system. The present study investigated in Wistar rats the potential role of spinal serotonergic mechanisms in the OXT- and PVN-induced antinociception. In long-term secondary mechanical allodynia and hyperalgesia induced by formalin or extracellular unitary recordings of the WDR cells we evaluated the role of 5-hydroxytryptamine (5-HT) effect on the OXT-induced antinociception. All drugs were given intrathecally (i.t.). OXT (1×10^-5-1×10^-4nmol) or 5-HT (1×10^-3-1×10^-1nmol) prevented the formalin-induced sensitization, an effect mimicked by PVN stimulation. Moreover, administration of OXT (1×10^-5nmol) plus 5-HT (1×10^-3nmol) at ineffective doses, produced antinociception. This effect was antagonized by: (i) d(CH₂)₅[Tyr(Me)²,Thr⁴,Tyr-NH₂⁹]OVT (oxytocin receptor antagonist; 2×10^-2nmol); or (ii) methiothepin (a non-specific 5-HT_1/2/5/6/7 receptor antagonist; 80nmol). Similar results were obtained with PVN stimulation plus 5-HT (5×10^-5nmol). In WDR cell recordings, the PVN-induced antinociception was enhanced by i.t. 5-HT and partly blocked when the spinal cord was pre-treated with methiothepin (80nmol). Taken together, these results suggest that serotonergic mechanisms at the spinal cord level are partly involved in the OXT-induced antinociception.

Analgesic Effects of Danggui-Shaoyao-San on Various "Phenotypes" of Nociception and Inflammation in a Formalin Pain Model

Danggui-Shaoyao-San (DSS) is a traditional Chinese medicine, which has long been used for pain treatment and has been demonstrated to possess anti-oxidative, cognitive enhancement, and anti-depressant effects. In the present study, the effects of aqueous extracts of DSS on spontaneous pain behaviors and long-term hyperalgesia were examined to investigate the anti-nociceptive effects and underlying mechanisms. Single pretreatment of DSS dose-dependently reduced spontaneous flinches/licking time in the second, rather than the first, phase after subcutaneous injection of 5 % formalin into one hindpaw, in doses of 2.4 and 9.6 g/kg. DSS also dose-dependently inhibited FOS and cyclooxygenase-2 (COX-2) expression in both superficial and deep layers within the spinal dorsal horn. Further, DSS reduced hypoalgesia in the injected paw from 1 to 3 days and produced anti-hyperalgesic actions in both the injected paw after 3 days and non-injected paw. These data suggest involvement of enhancement of descending pain inhibition by suppression of 5-HTT levels in the spinal dorsal horn and reduction of peripheral long-term inflammation, including paw edema and ulcers. These findings suggest that DSS may be a useful therapeutic agent for short- and long-term inflammation induced pain, through both anti-inflammatory and suppression of central sensitization mechanisms.

Molecular and cellular mechanisms that initiate pain and itch

Somatosensory neurons mediate our sense of touch. They are critically involved in transducing pain and itch sensations under physiological and pathological conditions, along with other skin-resident cells. Tissue damage and inflammation can produce a localized or systemic sensitization of our senses of pain and itch, which can facilitate our detection of threats in the environment. Although acute pain and itch protect us from further damage, persistent pain and itch are debilitating. Recent exciting discoveries have significantly advanced our knowledge of the roles of membrane-bound G protein-coupled receptors and ion channels in the encoding of information leading to pain and itch sensations. This review focuses on molecular and cellular events that are important in early stages of the biological processing that culminates in our senses of pain and itch.

Therapeutic Potential of 5-HT6 Receptor Agonists

Given its predominant expression in the central nervous system (CNS), 5-hydroxytryptamine (5-HT: serotonin) subtype 6 receptor (5-HT6R) has been considered as a valuable target for the development of CNS drugs with limited side effects. After 2 decades of intense research, numerous selective ligands have been developed to target this receptor; this holds potential interest for the treatment of neuropathological disorders. In fact, some agents (mainly antagonists) are currently undergoing clinical trial. More recently, a series of potent and selective agonists have been developed, and preclinical studies have been conducted that suggest the therapeutic interest of 5-HT6R agonists. This review details the medicinal chemistry of these agonists, highlights their activities, and discusses their potential for treating cognitive issues associated with Alzheimer's disease (AD), depression, or obesity. Surprisingly, some studies have shown that both 5-HT6R agonists and antagonists exert similar procognitive activities. This article summarizes the hypotheses that could explain this paradox.

Participation of peripheral P2Y1, P2Y6 and P2Y11 receptors in formalin-induced inflammatory pain in rats

Metabotropic P2Y receptors subfamily consists of eight functional mammalian receptors. Specifically, P2Y1, P2Y6 and P2Y11 receptors have been described in the sensory nervous system, but their participation, at peripheral level, in behavioral pain models is scarcely understood. This study assessed the role of peripheral P2Y1, P2Y6 and P2Y11 receptors in formalin-induced inflammatory pain. Ipsilateral, but not contralateral peripheral pre-treatment with the endogenous P2Y1 (ADP, 100-1000nmol/paw), P2Y6 (UDP, 180-300nmol/paw) and P2Y11 (ATP, 100-1000nmol/paw), or selective P2Y1 (MRS2365, 0.1-10nmol/paw), P2Y6 (PSB0474, 0.1-0.10pmol/paw) and P2Y11 (NF546, 0.3-3nmol/paw) receptor agonists increased 0.5% formalin-induced flinching behavior. Concordantly, peripheral pre-treatment with the selective P2Y1 (MRS2500, 0.01-10pmol/paw), P2Y6 (MRS2578, 3-30nmol/paw) and P2Y11 (NF340, 1-10nmol/paw) receptor antagonists significantly decreased 1% formalin-induced flinching behavior. Furthermore, the pronociceptive effect of ADP (100nmol/paw) or MRS2365 (10nmol/paw), UDP (300nmol/paw) or PSB0474 (10pmol/paw) and ATP (1000nmol/paw) or NF546 (3nmol/paw) was blocked by the selective P2Y1 (MRS2500, 0.01nmol/paw), P2Y6 (MRS2578, 3nmol/paw), and P2Y11 (NF340, 1nmol/paw) receptor antagonists, respectively. Western blot analysis confirmed the presence of P2Y1 (66kDa), P2Y6 (36kDa) and P2Y11 (75kDa) receptors in dorsal root ganglia (DRG) and sciatic nerve. Results suggest that peripheral activation of P2Y1, P2Y6 and P2Y11 receptors plays a pronociceptive role in formalin-induced pain.

Antineuropathic effect of 7-hydroxy-3,4-dihydrocadalin in streptozotocin-induced diabetic rodents

Background

Painful neuropathy is the most common and debilitating complication of diabetes and results in hyperalgesia and allodynia. Hyperglycemia clearly plays a key role in the development and progression of diabetic neuropathy. Current therapeutic approaches are only partially successful and they are only thought to reduce the pain associated with peripheral neuropathy. Some natural products offer combined antioxidant, anti-inflammatory and antinociceptive properties that may help to treat in a more integrative manner this condition. In this regard, the purpose of this study was to investigate the antineuropathic effect of 7-hydroxy-3,4-dihydrocadalin in streptozotocin-induced diabetic rats and mice without glucose control as well as the possible mechanism of action involved in this effect.

Methods

Rats and mice were injected with 50 or 200 mg/kg streptozotocin, respectively, to produce hyperglycemia. The formalin test and von Frey filaments were used to assess the nociceptive activity. Rota-rod was utilized to measure motor activity and malondialdehyde assay to determine anti-oxidative properties.

Results

After 3 weeks of diabetes induction, chemical hyperalgesia was observed in streptozotocin-injected rats. Oral acute administration of 7-hydroxy-3,4-dihydrocadalin (0.3–30 mg/kg) decreased in a dose-dependent manner formalin-evoked hyperalgesia in diabetic rats. In addition, methiothepin (non-selective 5-HT receptor antagonist, 1 mg/kg, i.p.) and ODQ (guanylyl cyclase inhibitor, 2 mg/kg, i.p.), but not naltrexone (opioid receptor antagonist, 1 mg/kg, s.c.), prevented 7-hydroxy-3,4-dihydrocadalin-induced antihyperalgesic effect. The anti-hyperalgesic effect of 7-hydroxy-3,4-dihydrocadalin was similar to that produced by pregabalin (10 mg/kg, p.o.). Furthermore, oral acute administration of 7-hydroxy-3,4-dihydrocadalin (30 mg/kg) reduced streptozotocin-induced changes in malondialdehyde concentration from plasma samples. Unlike pregabalin, 7-hydroxy-3,4-dihydrocadalin did not affect motor activity. Six weeks after diabetes induction, tactile allodynia was observed in the streptozotocin-injected rats. At this time, oral administration of 7-hydroxy-3,4-dihydrocadalin (30 mg/kg) or pregabalin (10 mg/kg) reduced in a similar way tactile allodynia in diabetic rats. Finally, chronic oral administration of 7-hydroxy-3,4-dihydrocadalin (30-300 mg/kg, 3 times/week, during 6 weeks), significantly prevented the development of mechanical hyperalgesia and allodynia in streptozotocin-induced diabetic mice.

Conclusions

Data suggests that 7-hydroxy-3,4-dihydrocadalin has acute and chronic effects in painful diabetic neuropathy. This effect seems to involve antioxidant properties as well as activation of 5-HT receptors and inhibition of guanylyl cyclase enzyme.

Comprehensive RNA-Seq expression analysis of sensory ganglia with a focus on ion channels and GPCRs in Trigeminal ganglia

The specific functions of sensory systems depend on the tissue-specific expression of genes that code for molecular sensor proteins that are necessary for stimulus detection and membrane signaling. Using the Next Generation Sequencing technique (RNA-Seq), we analyzed the complete transcriptome of the trigeminal ganglia (TG) and dorsal root ganglia (DRG) of adult mice. Focusing on genes with an expression level higher than 1 FPKM (fragments per kilobase of transcript per million mapped reads), we detected the expression of 12984 genes in the TG and 13195 in the DRG. To analyze the specific gene expression patterns of the peripheral neuronal tissues, we compared their gene expression profiles with that of the liver, brain, olfactory epithelium, and skeletal muscle. The transcriptome data of the TG and DRG were scanned for virtually all known G-protein-coupled receptors (GPCRs) as well as for ion channels. The expression profile was ranked with regard to the level and specificity for the TG. In total, we detected 106 non-olfactory GPCRs and 33 ion channels that had not been previously described as expressed in the TG. To validate the RNA-Seq data, in situ hybridization experiments were performed for several of the newly detected transcripts. To identify differences in expression profiles between the sensory ganglia, the RNA-Seq data of the TG and DRG were compared. Among the differentially expressed genes (> 1 FPKM), 65 and 117 were expressed at least 10-fold higher in the TG and DRG, respectively. Our transcriptome analysis allows a comprehensive overview of all ion channels and G protein-coupled receptors that are expressed in trigeminal ganglia and provides additional approaches for the investigation of trigeminal sensing as well as for the physiological and pathophysiological mechanisms of pain.

Multiple roles of serotonin in pain control mechanisms--implications of 5-HT₇ and other 5-HT receptor types

Among monoamine neurotransmitters, serotonin (5-HT) is known to play complex modulatory roles in pain signaling mechanisms since the first reports, about forty years ago, on its essentially pro-nociceptive effects at the periphery and anti-nociceptive effects when injected directly at the spinal cord level. The discovery of multiple 5-HT receptor subtypes allowed possible explanations to this dual action at the periphery versus the central nervous system (CNS) since both excitatory and inhibitory effects can be exerted through 5-HT activation of different 5-HT receptors. However, it also appeared that activation of the same receptor subtype at CNS level might induce variable effects depending on the physiological or pathophysiological status of the animal administered with agonists. In particular, the marked neuroplastic changes induced by nerve lesion, which account for sensitization of pain signaling mechanisms, can contribute to dramatic changes in the effects of a given 5-HT receptor agonist in neuropathic rats versus intact healthy rats. This has notably been observed with 5-HT₇ receptor agonists which exert a pronociceptive action in healthy rats but alleviate hyperalgesia consecutive to nerve lesion in neuropathic animals. Analysis of cellular mechanisms underlying such dual 5-HT actions mediated by a single receptor subtype indicates that the neuronal phenotype which expresses this receptor also plays a key role in determining which modulatory action 5-HT would finally exert on pain signaling mechanisms.

High-frequency transcutaneous electrical nerve stimulation reduces pain and cardio-respiratory parameters in an animal model of acute pain: participation of peripheral serotonin

The objective of this study was to investigate the effect of high-frequency transcutaneous electrical nerve stimulation (HF-TENS) in antihyperalgesia, assessed through changes of sciatic nerve activity and its effects on cardiorespiratory parameters, using formalin-induced nociception in anesthetized rats. The animals were divided into formalin (FORM) and HF-TENS groups. All rats received injections of 5% formalin (50 μl, right hind-paw). The sciatic nerve activity and cardiopulmonary parameters (mean arterial pressure, heart rate, and respiratory frequency) were measured, and then the serum levels of serotonin (5-HT) were determined by an enzyme-linked immunosorbent assay kit. The formalin injection was able to increase the sciatic nerve activity, heart rate, and respiratory frequency. The treatment with HF-TENS significantly reduced the sciatic nerve activity and respiratory frequency 20 minutes after formalin injection and was able to increase serum 5-HT. Furthermore, when comparing the groups, reductions in the mean arterial pressure, heart rate, respiratory frequency, and sciatic nerve activity were shown at different times. Thus, we concluded that HF-TENS was capable of inducing analgesia, which was most likely related to increased serotonin release. Moreover, we demonstrated that TENS was able to block the adverse cardiovascular and respiratory changes induced by pain. Further neurophysiological studies are necessary to clarify the intrinsic mechanisms underlying HF-TENS-induced analgesia.

Mast cell stabilization promotes antinociceptive effects in a mouse model of postoperative pain

BACKGROUND

Nerve injury and consequent inflammatory responses produced by surgical incision result in a complicated pain status which still affects half of all surgical patients. Therefore, it is essential for anesthesiologists to identify the mechanisms of postoperative pain. Mast cells are resident cells of connective tissue and the mucosa that participate in the immune response. Degranulation of mast cells is involved in the development of postoperative pain and can be induced by surgical incision. The aim of this study was to investigate whether stabilization of mast cells causes an antinociceptive effect in a mouse model of postoperative pain.

METHODS

Postoperative pain was induced by making an incision in the hind paw of BALB/c mice. The mast cell membrane stabilizer cromoglycate (200 μg/20 μL) was injected before incision of the paw, and postoperative pain responses were measured by assessing guarding behavior, withdrawal threshold to mechanical stimuli, and latency of heat pain behavior 1, 2, and 7 days after the incision.

RESULTS

The incision produced guarding pain, mechanical allodynia, and heat hypersensitivity. Cromoglycate decreased the guarding pain score (day 1) and the withdrawal threshold to mechanical stimuli (days 1, 2, and 7). However, the withdrawal latency to heat was not affected by cromoglycate treatment.

CONCLUSION

Cromoglycate significantly attenuated the pain response expressed as guarding pain and mechanical allodynia in a mouse model of postoperative pain. Thus, mast cell activation is likely a mechanism of postoperative pain and is an interesting target for the development of new therapies.

Evidence for the participation of peripheral 5-HT₂A, 5-HT₂B, and 5-HT₂C receptors in formalin-induced secondary mechanical allodynia and hyperalgesia

The role of 5-HT₂A/₂B/₂C receptors in formalin-induced secondary allodynia and hyperalgesia in rats was assessed. Formalin produced acute nociceptive behaviors (flinching and licking/lifting) followed by long-term secondary mechanical allodynia and hyperalgesia. Pre-treatment for five consecutive days with compound 48/80 (1, 3, 10, 10, and 10 μg/paw) prevented formalin-induced secondary allodynia and hyperalgesia. Ipsilateral, but not contralateral, peripheral pre-treatment (nmol/paw) with the 5-HT₂ receptor agonist DOI (3-30), 5-HT (10-100) or fluoxetine (0.3-3) significantly increased 0.5% formalin-induced secondary allodynia and hyperalgesia in both paws. The pronociceptive effect of DOI (10 nmol/paw), 5-HT (100 nmol/paw) and fluoxetine (1 nmol/paw) was blocked by selective 5-HT₂A (ketanserin), 5-HT₂B (RS-127445), and 5-HT₂C (RS-102221) receptor antagonists. Furthermore, ipsilateral pre-treatment (nmol/paw) with ketanserin (1, 10, and 100), RS-127445 (0.01, 0.1 and 1) or RS-102221 (1, 10 and 100) prevented while post-treatment reversed 1% formalin-induced secondary allodynia and hyperalgesia in both paws. In marked contrast, contralateral injection of the greatest tested dose of 5-HT₂A/₂B/₂C receptor antagonists did not modify long-lasting secondary allodynia and hyperalgesia. These results suggest that 5-HT released from mast cells after formalin injection sensitizes primary afferent neurons via 5-HT₂A/₂B/₂C receptors leading to the development and maintenance of secondary allodynia and hyperalgesia.

Secondary mechanical allodynia and hyperalgesia depend on descending facilitation mediated by spinal 5-HT₄, 5-HT₆ and 5-HT₇ receptors

In the present study we determined the role of spinal 5-hydroxytriptamine (5-HT) and 5-HT(4/6/7) receptors in the long-term secondary mechanical allodynia and hyperalgesia induced by formalin in the rat. Formalin produced acute nociceptive behaviors (flinching and licking/lifting) followed by long-term secondary mechanical allodynia and hyperalgesia in both paws. In addition, formalin increased the tissue content of 5-HT in the ipsilateral, but not contralateral, dorsal part of the spinal cord compared to control animals. Intrathecal (i.t.) administration of 5,7-dihydroxytriptamine (5,7-DHT), a serotonergic neurotoxin, diminished tissue 5-HT content in the ipsilateral and contralateral dorsal parts of the spinal cord. Accordingly, i.t. 5,7-DHT prevented formalin-induced secondary allodynia and hyperalgesia in both paws. I.t. pre-treatment (-10 min) with ML-10302 (5-HT(4) agonist), EMD-386088 (5-HT(6) agonist) and LP-12 (5-HT(7) agonist) significantly increased secondary mechanical allodynia and hyperalgesia in both paws. In contrast, i.t. pre-treatment (-20 min) with GR-125487 (5-HT(4) antagonist), SB-258585 (5-HT(6) antagonist) and SB-269970 (5-HT(7) antagonist) significantly prevented formalin-induced long-term effects in both paws. In addition, these antagonists prevented the pro-nociceptive effect of ML-10302, EMD-386088 and LP-12, respectively. The i.t. post-treatment (6 days after formalin injection) with GR-125487, SB-258585 and SB-269970 reversed formalin-induced secondary allodynia and hyperalgesia in both paws. These results suggest that spinal 5-HT, released from the serotonergic projections in response to formalin injection, activates pre- or post-synaptic 5-HT(4/6/7) receptors at the dorsal root ganglion/spinal cord promoting the development and maintenance of secondary allodynia and hyperalgesia.

Sertraline inhibits formalin-induced nociception and cardiovascular responses

The objective of the present study was to determine the antihyperalgesic effect of sertraline, measured indirectly by the changes of sciatic afferent nerve activity, and its effects on cardiorespiratory parameters, using the model of formalin-induced inflammatory nociception in anesthetized rats. Serum serotonin (5-HT) levels were measured in order to test their correlation with the analgesic effect. Male Wistar rats (250-300 g) were divided into 4 groups (N = 8/per group): sertraline-treated group (Sert + Saline (Sal) and Sert + Formalin (Form); 3 mg·kg-1·day-1, ip, for 7 days) and saline-treated group (Sal + Sal and Sal + Form). The rats were injected with 5% (50 µL) formalin or saline into the right hind paw. Sciatic nerve activity was recorded using a silver electrode connected to a NeuroLog apparatus, and cardiopulmonary parameters (mean arterial pressure, heart rate and respiratory frequency), assessed after arterial cannulation and tracheotomy, were monitored using a Data Acquisition System. Blood samples were collected from the animals and serum 5-HT levels were determined by ELISA. Formalin injection induced the following changes: sciatic afferent nerve activity (+50.8 ± 14.7%), mean arterial pressure (+1.4 ± 3 mmHg), heart rate (+13 ± 6.8 bpm), respiratory frequency (+4.6 ± 5 cpm) and serum 5-HT increased to 1162 ± 124.6 ng/mL. Treatment with sertraline significantly reduced all these parameters (respectively: +19.8 ± 6.9%, -3.3 ± 2 mmHg, -13.1 ± 10.8 bpm, -9.8 ± 5.7 cpm) and serum 5-HT level dropped to 634 ± 69 ng/mL (P < 0.05). These results suggest that sertraline plays an analgesic role in formalin-induced nociception probably through a serotonergic mechanism.