Review: 5-HT1, 5-HT2, 5-HT3 and 5-HT7 Receptors and their Role in the Modulation of Pain Response in the Central Nervous System

Ventral tegmental area serotonin 5-HT1A receptors and corticolimbic cFos/BDNF/GFAP signaling pathways mediate dextromethorphan/morphine anti-allodynia

AIMS

The present study examined the role of ventral tegmental area (VTA) serotonergic 5HT_1A receptors in dextromethorphan/morphine-induced anti-allodynia and the possible changes of corticolimbic cFos, brain-derived neurotrophic factor (BDNF), and glial fibrillary acidic protein (GFAP) following the treatments.

MATERIALS AND METHODS

The VTA cannulation and the chronic constriction of the sciatic nerve were performed in male Wistar rats. Flexion withdrawal thresholds to mechanical stimulation in the hind-limb were determined using von Frey hairs. The expressions of cFos, BDNF, and GFAP were evaluated using the Western blotting technique.

KEY FINDINGS

BDNF (in the hippocampus), and GFAP (in the targeted sites) levels were increased following neuropathic pain. Morphine administration induced an anti-allodynic effect with a decrease in the amygdala BDNF level. Dextromethorphan/morphine-induced anti-allodynia was accompanied by the decrease of hippocampus/amygdala/PFC GFAP and amygdala cFos expressions. The PFC BDNF expression level was increased in dextromethorphan/morphine-treated rats. Intra-VTA microinjection of (S)-WAY100135 (1 µg/rat), a selective 5-HT_1A receptor antagonist, inhibited the anti-allodynic effect of dextromethorphan/morphine. This treatment increased the cFos level in the hippocampus and the amygdala while decreased the PFC level of cFos. The hippocampal BDNF expression was significantly increased, while the amygdala and the PFC expressions of BDNF were decreased under treatment. (S)-WAY100135 plus dextromethorphan/morphine increased the hippocampal/amygdala and PFC levels of GFAP.

SIGNIFICANCE

These findings indicate that dextromethorphan could potentiate the analgesic effect of morphine via the implication of the VTA serotonin 5-HT_1A receptors. It seems that the changes in the corticolimbic cFos/BDNF/GFAP signaling pathway may be involved in the observed anti-allodynic effect.

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.

5-HT1A Serotonergic, α-Adrenergic and Opioidergic Receptors Mediate the Analgesic Efficacy of Vortioxetine in Mice

Vortioxetine is a multimodal antidepressant drug that affects several brain neurochemicals and has the potential to induce various pharmacological effects on the central nervous system. Therefore, we investigated the centrally mediated analgesic efficacy of this drug and the mechanisms underlying this effect. Analgesic activity of vortioxetine (5, 10 and 20 mg/kg, p.o.) was examined by tail-clip, tail-immersion and hot-plate tests. Motor performance of animals was evaluated using Rota-rod device. Time course measurements (30-180 min) showed that vortioxetine (10 and 20 mg/kg) administrations significantly increased the response latency, percent maximum possible effect and area under the curve values in all of the nociceptive tests. These data pointed out the analgesic effect of vortioxetine on central pathways carrying acute thermal and mechanical nociceptive stimuli. Vortioxetine did not alter the motor coordination of mice indicating that the analgesic activity of this drug was specific. In mechanistic studies, pre-treatments with p-chlorophenylalanine (serotonin-synthesis inhibitor), NAN-190 (serotonin 5-HT_1A receptor antagonist), α-methyl-para-tyrosine (catecholamine-synthesis inhibitor), phentolamine (non-selective α-adrenoceptor blocker), and naloxone (non-selective opioid receptor blocker) antagonised the vortioxetine-induced analgesia. Obtained findings indicated that vortioxetine-induced analgesia is mediated by 5-HT_1A serotonergic, α-adrenergic and opioidergic receptors, and contributions of central serotonergic and catecholaminergic neurotransmissions are critical for this effect.

Discovery of Nonracemic Amisulpride to Maximize Benefit/Risk of 5-HT7 and D2 Receptor Antagonism for the Treatment of Mood Disorders

In contrast to the dose‐occupancy relationship in the treatment of schizophrenia, the minimal effective level of dopamine receptor 2 (D2R) blockade for antipsychotics in the treatment of bipolar depression is unknown. Lower doses aimed at reducing extrapyramidal side effects must be balanced against the need to retain the therapeutic benefit of D2R blockade on emergent cycling, mixed, manic, anxiety, and/or psychotic symptoms. Dose‐reductions intended to lower D2R blockade, however, could also decrease concomitant serotonin receptor antagonism and its potential benefit on depressive symptoms. Here, we uncoupled the potential antidepressant activity in amisulpride, driven by 5‐HT7 receptor (5‐HT7R) antagonism, from the D2R‐mediated antipsychotic activity by discovering that each enantiomer favors a different receptor. Aramisulpride was more potent at 5‐HT7R relative to esamisulpride (Ki 47 vs. 1,900 nM, respectively), whereas esamisulpride was more potent at D2R (4.0 vs. 140 nM). We hypothesized that a nonracemic ratio might achieve greater 5‐HT7R‐mediated antidepressant effects at a lower level of D2R blockade. The dose‐occupancy relationship of esamisulpride at D2R was determined by positron emission tomography (PET) imaging in human volunteers. Separately the dose‐relationship of aramisulpride was established in humans using suppression of rapid eye movement (REM) sleep as a marker of 5‐HT7R antagonism. These results led to the discovery of an 85:15 ratio of aramisulpride to esamisulpride (SEP‐4199) that maximizes the potential for antidepressant benefit of aramisulpride via 5‐HT7R and reduces esamisulpride to minimize D2R‐related extrapyramidal side effects while still retaining D2R‐mediated effects predicted to provide benefit in bipolar depression. The antidepressant efficacy of SEP‐4199 was recently confirmed in a proof‐of‐concept trial for the treatment of bipolar depression (NCT03543410).

Central involvement of 5-HT1A receptors in antinociception induced by photobiomodulation in animal model of neuropathic pain

This study aimed to investigate the central involvement of 5-HT1A receptors in the nociceptive behavior of mice submitted to the chronic constriction injury (CCI) of sciatic nerve and the subsequent application of photobiomodulation (PBM). Male mice (Swiss-albino) were submitted to CCI and subsequently received an infusion of WAY100635 (5-HT1A receptor antagonist) or intracerebroventricular saline (ICV), followed by infrared laser irradiation (808 nm), in continuous mode, with the power of 100 mW and a dose of 0 J/cm² (control group) or 50 J/cm². The thermal hyperalgesia was evaluated by hot plate test, while mechanical allodynia was evaluated by von Frey filaments. After CCI, animals showed a reduction in the nociceptive threshold (p<0.001) when compared to the sham group. In von Frey test, the CCI + saline + PBM 50 J/cm² group showed an increase in nociceptive threshold (p<0.001) in all measurement moments in comparison with groups CCI + SALINE + PBM 0 J/cm², CCI + WAY100635 + PBM 50 J/cm², and CCI + WAY100635 + PBM 0 J/cm². Similarly, in hot plate test, CCI + SALINE + PBM 50 J/cm² group showed an increase in nociceptive threshold after application of PBM at 120 and 180 min. Because of the results found, it can be suggested the involvement of 5-HT1A receptors in the central nervous system, since WAY100635 was able to reverse the antinociceptive effect provided by PBM in animals submitted to CCI.

Association of serotonin transporter-linked polymorphic region (5-HTTLPR) with heat pain stimulation and postoperative pain in gastric cancer patients

Background

The aim of this study was to assess whether the genotype of the serotonin transporter-linked polymorphic region (5-HTTLPR) in gastric cancer patients is associated with postoperative pain and pain threshold.

Methods

We conducted a prospective cohort study of 251 patients scheduled for gastrectomy from May to September 2019. All patients enrolled in the study were asked to complete the Hospital Anxiety and Depression Scale questionnaire. Heat pain threshold (HPT), cold pain threshold (CPT) and Pressure pain threshold (PPT) were measured for all participants one day prior to surgery. Blood samples were collected for genetic testing. All patients were connected to a patient-controlled intravenous analgesia (PCIA) pump at the end of the surgery. After exclusion of 15 patients, the postoperative conditions of 236 patients were recorded.

Results

Distribution of homozygous long (L/L), heterozygous (L/S), and homozygous short (S/S) 5-HTTLPR genotypes among participants were 26 (11.0%), 91 (38.6%), and 119 (50.4%), respectively. Heat pain threshold (P = 0.038) and Numerical rating scale (NRS) in the 1st postoperative 24 h (P = 0.026) were significantly different between long (L/L) and short (S/S) genotype carriers.

Conclusions

In patients with gastric cancer, heat pain stimulation is associated with 5-HTTLPR polymorphism, and postoperative pain may be related to 5-HTTLPR polymorphism.

On the therapeutic targets and pharmacological treatments for pain relief following spinal cord injury: A mechanistic review

Spinal cord injury (SCI) is globally considered as one of the most debilitating disorders, which interferes with daily activities and life of the affected patients. Despite many developments in related recognizing and treating procedures, post-SCI neuropathic pain (NP) is still a clinical challenge for clinicians with no distinct treatments. Accordingly, a comprehensive search was conducted in PubMed, Medline, Scopus, Web of Science, and national database (SID and Irandoc). The relevant articles regarding signaling pathways, therapeutic targets and pharmacotherapy of post-SCI pain were also reviewed. Data were collected with no time limitation until November 2020. The present study provides the findings on molecular mechanisms and therapeutic targets, as well as developing the critical signaling pathways to introduce novel neuroprotective treatments of post-SCI pain. From the pathophysiological mechanistic point of view, post-SCI inflammation activates the innate immune system, in which the immune cells elicit secondary injuries. So, targeting the critical signaling pathways for pain management in the SCI population has significant importance in providing new treatments. Indeed, several receptors, ion channels, excitatory neurotransmitters, enzymes, and key signaling pathways could be used as therapeutic targets, with a pivotal role of n-methyl-D-aspartate, gamma-aminobutyric acid, and inflammatory mediators. The current review focuses on conventional therapies, as well as crucial signaling pathways and promising therapeutic targets for post-SCI pain to provide new insights into the clinical treatment of post-SCI pain. The need to develop innovative delivery systems to treat SCI is also considered.

In Silico Study Examining New Phenylpropanoids Targets with Antidepressant Activity

BACKGROUND

Natural products, such as phenylpropanoids, which are found in essential oils derived from aromatic plants, have been explored during non-clinical psychopharmacology studies, to discover new molecules with relevant pharmacological activities in the central nervous system, especially antidepressant and anxiolytic activities. Major depressive disorder is a highly debilitating psychiatric disorder and is considered to be a disabling public health problem, worldwide, as a primary factor associated with suicide. Current clinically administered antidepressants have late-onset therapeutic actions, are associated with several side effects, and clinical studies have reported that some patients do not respond well to treatment or reach complete remission.

OBJECTIVE

To review important new targets for antidepressant activity and to select phenylpropanoids with antidepressant activity, using Molegro Virtual Docker and Ossis Data Warris, and to verify substances with more promising antidepressant activity.

RESULTS AND CONCLUSION

An in silico molecular modeling study, based on homology, was conducted to determine the three-dimensional structure of the 5-hydroxytryptamine 2A receptor (5- HT2AR), then molecular docking studies were performed and the predisposition for cytotoxicity risk among identified molecules was examined. A model for 5-HT2AR homology, with satisfactory results, was obtained indicating the good stereochemical quality of the model. The phenylpropanoid 4-allyl-2,6-dimethoxyphenol showed the lowest binding energy for 5-HT2AR, with results relevant to the L-arginine/nitric oxide (NO)/cGMP pathway, and showed no toxicity within the parameters of mutagenicity, carcinogenicity, reproductive system toxicity, and skin-tissue irritability, when evaluated in silico; therefore, this molecule can be considered promising for the investigation of antidepressant activity.

Pivotal mental states

This paper introduces a new construct, the 'pivotal mental state', which is defined as a hyper-plastic state aiding rapid and deep learning that can mediate psychological transformation. We believe this new construct bears relevance to a broad range of psychological and psychiatric phenomena. We argue that pivotal mental states serve an important evolutionary function, that is, to aid psychological transformation when actual or perceived environmental pressures demand this. We cite evidence that chronic stress and neurotic traits are primers for a pivotal mental state, whereas acute stress can be a trigger. Inspired by research with serotonin 2A receptor agonist psychedelics, we highlight how activity at this particular receptor can robustly and reliably induce pivotal mental states, but we argue that the capacity for pivotal mental states is an inherent property of the human brain itself. Moreover, we hypothesize that serotonergic psychedelics hijack a system that has evolved to mediate rapid and deep learning when its need is sensed. We cite a breadth of evidences linking stress via a variety of inducers, with an upregulated serotonin 2A receptor system (e.g. upregulated availability of and/or binding to the receptor) and acute stress with 5-HT release, which we argue can activate this primed system to induce a pivotal mental state. The pivotal mental state model is multi-level, linking a specific molecular gateway (increased serotonin 2A receptor signaling) with the inception of a hyper-plastic brain and mind state, enhanced rate of associative learning and the potential mediation of a psychological transformation.

Role of serotonin in regulation of pancreatic and mesenteric arterial function in diabetic mice

The aim of this study was to investigate the reaction of pancreatic and mesenteric artery to 5-hydroxytryptamine (5-HT, serotonin) and the mechanism of nitric oxide in diabetes. Diabetic mice were induced by streptozotocin through intraperitoneal injection. The vascular tension of the pancreatic, mesenteric and brain basilar arteries in diabetic and control mice were measured by myograph in the applications of angiotensin II, 5-HT, 5-HT_2A receptor agonist 2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI), 5-HT_1B/1D receptor agonist sumatriptan, 5-HT_2B receptor agonist BW723C86, 5-HT_1D receptor antagonist Palonosetron and 5-HT₂ receptor antagonist Sarpogrelate. The effect of 5-HT on arteries pretreated with L-NAME and sodium nitroprusside (SNP) on arteries pretreated with norepinephrine were measured. The mRNA expressions of eNOS, 5-HT_1B, 5-HT_1D, 5-HT_2A and 5-HT_2B in pancreatic and mesenteric arteries were measured by Real-time PCR. The concentration of 5-HT in plasma and eNOS in pancreatic and mesenteric arteries were tested. Our results showed that the tension of pancreatic and mesenteric arteries in diabetic mice impaired to 5-HT, but not Ang II, and to DOI and sumatriptan, but normalized by incubation with L-NAME. Pancreatic and mesenteric arteries showed no differences to SNP after pretreated with NE between diabetic and control mice. The mRNA of eNOS and 5-HT receptors in pancreatic and mesenteric artery showed no difference between control and diabetic mice. We conclude that the effect of 5-HT on the tension of pancreatic and mesenteric arteries decrease in diabetic mice. It may due to the decreased activity of 5-HT receptors and the activation of eNOS, which causes nitric oxide to release more and makes the tension of vessels decreased.

Association between polymorphisms in catechol-O-methyl transferase, opioid receptor Mu 1 and serotonin receptor genes with postoperative pain following root canal treatment

AIM

To evaluate the effect of single nucleotide polymorphisms in the COMT, OPRM1, 5HT1A, 5HT2A and 5HTR3B genes on the intensity of postoperative pain following root canal treatment.

METHODOLOGY

Ninety-five patients with mandibular and maxillary molar teeth diagnosed with symptomatic apical periodontitis and a level of preoperative pain greater than 50 on a 100 mm visual analogue scale (VAS) were included. Salivary DNA was collected from the participants and stored in Eppendorf tubes at -80 °C. Preoperative percussion pain values were recorded before the root canal treatment procedures. After completion of root canal treatment, the participants were given instructions to record their postoperative pain intensity levels at 24, 48 and 72 h, 5 days and 1 week after treatment, using the VAS. A second visit for the patients after seven days was planned to record their intensity levels of percussion pain on VAS. The percussion test was performed by tapping on the occlusal surface of the tooth with a blunt instrument. A QIAamp DNA Mini Kit was used to isolate DNA from saliva, and SNP Genotyping Analysis software version 1 was used to analyse the genotypes by calculating FAM and HEX signals. The Kruskal-Wallis and Mann-Whitney U-tests were used to evaluate pain intensity values amongst the genotypes, alleles, haplotypes and allele combinations. Nominal data (gender, intake and tooth number) were analysed using a Chi-square test. Bonferroni correction was performed. Thus, the significance level was set at 1.6% (P = 0.016), 2.5% (P = 0.025) and 1.25% (P = 0.0125) for genotype, allele and haplotype comparisons, respectively.

RESULTS

There was no significant difference amongst the genotypes and alleles in terms of pre- and postoperative pain intensity. There was no significant difference amongst the haplotypes formed for the COMT gene in terms of pain intensity. Additionally, there was no significant association between the allelic combination formed for 5HT1A + 5HT2A genes and the intensity of postoperative pain.

CONCLUSION

The findings indicate that none of the evaluated SNPs for COMT, OPRM1, 5HT1A, 5HT2A and 5HTR3B genes were associated with the intensity of postoperative pain.

The Roles of Serotonin in Neuropsychiatric Disorders

The serotonergic system extends throughout the central nervous system (CNS) and the gastrointestinal (GI) tract. In the CNS, serotonin (5-HT, 5-hydroxytryptamine) modulates a broad spectrum of functions, including mood, cognition, anxiety, learning, memory, reward processing, and sleep. These processes are mediated through 5-HT binding to 5-HT receptors (5-HTRs), are classified into seven distinct groups. Deficits in the serotonergic system can result in various pathological conditions, particularly depression, schizophrenia, mood disorders, and autism. In this review, we outlined the complexity of serotonergic modulation of physiologic and pathologic processes. Moreover, we provided experimental and clinical evidence of 5-HT's involvement in neuropsychiatric disorders and discussed the molecular mechanisms that underlie these illnesses and contribute to the new therapies.

The cellular mechanism by which the rostral ventromedial medulla acts on the spinal cord during chronic pain

Clinical therapies for chronic pain are limited. While targeted drugs are promising therapies for chronic pain, they exhibit insufficient efficacy and poor targeting. The occurrence of chronic pain partly results from central changes caused by alterations in neurons in the rostral ventromedial medulla (RVM) in the brainstem regulatory pathway. The RVM, which plays a key role in the descending pain control pathway, greatly contributes to the development and maintenance of pain. However, the exact roles of the RVM in chronic pain remain unclear, making it difficult to develop new drugs targeting the RVM and related pathways. Here, we first discuss the roles of the RVM and related circuits in chronic pain. Then, we analyze synaptic transmission between RVM neurons and spinal cord neurons, specifically focusing on the release of neurotransmitters, to explore the cellular mechanisms by which the RVM regulates chronic pain. Finally, we propose some ideas for the development of drugs targeting the RVM.

Chemical Mediators' Expression Associated with the Modulation of Pain in Rheumatoid Arthritis

BACKGROUND

The management of pain in patients with rheumatoid arthritis (RA) is a complex subject due to the autoimmune nature of the pathology. Studies have shown that chemical mediators play a fundamental role in the determination, susceptibility and modulation of pain at different levels of the central and peripheral nervous system, resulting in interesting novel molecular targets to mitigate pain in patients with RA. However, due to the complexity of pain physiology in RA cand the many chemical mediators, the results of several studies are controversial.

OBJECTIVE

The aim of this study was to identify the chemical mediators that are able to modulate pain in RA.

METHOD

In this review, a search was conducted on PubMed, ProQuest, EBSCO, and the Science Citation index for studies that evaluated the expression of chemical mediators on the modulation of pain in RA.

RESULTS

Few studies have highlighted the importance of the expression of some chemical mediators that modulate pain in patients with rheumatoid arthritis. The expression of TRPV1, ASIC-3, and TDV8 encode ionic channels in RA and modulates pain, likewise, the transcription factors in RA, such as TNFα, TGF-β1, IL-6, IL-10, IFN-γ, IL-1b, mTOR, p21, caspase 3, EDNRB, CGRPCALCB, CGRP-CALCA, and TAC1 are also directly involved in pain perception.

CONCLUSION

The expression of all chemical mediators is directly related to RA and the modulation of pain by a complex intra and extracellular signaling pathway, however, transcription factors are involved in modulating acute pain, while the ionic channels are involved in chronic pain in RA.

Drug Development in Channelopathies: Allosteric Modulation of Ligand-Gated and Voltage-Gated Ion Channels

Ion channels have been characterized as promising drug targets for treatment of numerous human diseases. Functions of ion channels can be fine-tuned by allosteric modulators, which interact with channels and modulate their activities by binding to sites spatially discrete from those of orthosteric ligands. Positive and negative allosteric modulators have presented a plethora of potential therapeutic advantages over traditionally orthosteric agonists and antagonists in terms of selectivity and safety. This thematic review highlights the discovery of representative allosteric modulators for ligand-gated and voltage-gated ion channels, discussing in particular their identifications, locations, and therapeutic uses in the treatment of a range of channelopathies. Additionally, structures and functions of selected ion channels are briefly described to aid in the rational design of channel modulators. Overall, allosteric modulation represents an innovative targeting approach, and the corresponding modulators provide an abundant but challenging landscape for novel therapeutics targeting ligand-gated and voltage-gated ion channels.

Synergistic interaction between B vitamins and statins to counter nociception in rats

Evidence suggests that the antinociceptive activity of various drugs can be increased when administered in combination with B vitamins (BVs). The aim of this study was to examine the potential interaction between statins and BVs to counter nociception, the latter measured by the formalin test. Rats were orally administered atorvastatin (1, 3, 10 and 30 mg/kg), pravastatin (1, 3, 10 and 30 mg/kg), rosuvastatin (1, 3, 10 and 30 mg/kg), BVs (31, 56, 100 and 180 mg/kg) or calculated combinations of BVs with each drug. The effective dose 30 (ED₃₀ ) was calculated for each statin and BVs and subjected to isobolographic analysis, thus finding the ED₃₀ of the combinations. The antinociceptive experimental ED₃₀ values for BVs administered with atorvastatin, pravastatin or rosuvastatin were 1.53 ± 0.38, 6.74 ± 0.04 and 4.26 ± 0.39, respectively, being lower (p < .05) than the corresponding theoretical ED₃₀ : 28.02 ± 2.20, 28.17 ± 2.20 and 29.86 ± 2.21. Since BVs likely boost the antinociceptive effect of statins, these combinations could possibly be advantageous in pain management.

Advances in the Treatment of Chronic Pain by Targeting GPCRs

Pain is an essential protective mechanism that the body uses to alert or prevent further damage. Pain sensation is a complex event involving perception, transmission, processing, and response. Neurons at different levels (peripheral, spinal cord, and brain) are responsible for these pro- or antinociceptive activities to ensure an appropriate response to external stimuli. The terminals of these neurons, both in the peripheral endings and in the synapses, are equipped with G protein-coupled receptors (GPCRs), voltage- and ligand-gated ion channels that sense structurally diverse stimuli and inhibitors of neuronal activity. This review will focus on the largest class of sensory proteins, the GPCRs, as they are distributed throughout ascending and descending neurons and regulate activity at each step during pain transmission. GPCR activation also directly or indirectly controls the function of co-localized ion channels. The levels and types of some GPCRs are significantly altered in different pain models, especially chronic pain states, emphasizing that these molecules could be new targets for therapeutic intervention in chronic pain.

An integrated strategy for discovering effective components of Shaoyao Gancao decoction for treating neuropathic pain by the combination of partial least-squares regression and multi-index comprehensive method

ETHNOPHARMACOLOGICAL RELEVANCE

Neuropathic pain, the incidence of which ranges from 5 to 8% in the general population, remains challenge in the treatment. Shaoyao Gancao decoction (SGD) is a Chinese classical formula used to relieve pain for thousands of years and has been applied for neuropathic pain nowadays. However, the effective components of SGD for the treatment of neuropathic pain remains unclear.

AIMS OF STUDY

To investigate the effect and potential mechanism of SGD against neuropathic pain and further reveal the effective components of SGD in the treatment of neuropathic pain.

MATERIALS AND METHODS

Spared nerve injury (SNI) model rats of neuropathic pain were orally given SGD to intervene, the components in vivo after SGD administration were determined, behavior indicators, biochemical parameters, and metabolomics were applied for assessing the efficacy. Then correlation between components and biomarkers was analyzed by pearson correlation method. To further measure the contribution of components to efficacy, the combination of partial least-squares regression (PLSR) and multi-index comprehensive method was carried out, according to the corresponding contribution degree of the results, the components with large contribution degree were considered as the effective components.

RESULTS

SGD exhibited a significant regulatory effect on neuropathic pain, which could increase the pain threshold and decrease the levels of SP, β-EP, PGE2 and NO. With the high resolution of UPLC-Q-TOF/MS technology, a total of 128 compounds from SGD were identified and 44 of them were absorbed in blood. Besides, 40 serum biomarkers were identified after intervention of SGD and the metabolic pathways were constructed. The key metabolic pathways including Glycerophospholipid metabolism, Linoleic acid metabolism, Alpha-linolenic acid metabolism, Glycosylphosphatidylinositol-anchor biosynthesis and Arachidonic acid metabolism may be related to the regulation of neuropathic pain. Metabolomics combined with PLSR and multi-index comprehensive method was utilized to discover 5 components including paeonol, DL-Arabinose, benzoic acid, hispaglabridin A and paeonilactone C as effective components of SGD in the treatment of neuropathic pain. This strategy was used to explore the effective components of SGD and elucidate its possible analgesic mechanism.

CONCLUSION

This study demonstrate that SGD significantly relieved neuropathic pain and elucidated the effective components of SGD for treating neuropathic pain, the strategy as an illustrative case study can be applied to other classical formula and is beneficial to improve the quality and efficacy.

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.

Gut microbiota regulates neuropathic pain: potential mechanisms and therapeutic strategy

Neuropathic pain (NP) is a sustained and nonreversible condition characterized by long-term devastating physical and psychological damage. Therefore, it is urgent to identify an effective treatment for NP. Unfortunately, the precise pathogenesis of NP has not been elucidated. Currently, the microbiota-gut-brain axis has drawn increasing attention, and the emerging role of gut microbiota is investigated in numerous diseases including NP. Gut microbiota is considered as a pivotal regulator in immune, neural, endocrine, and metabolic signaling pathways, which participates in forming a complex network to affect the development of NP directly or indirectly. In this review, we conclude the current understanding of preclinical and clinical findings regarding the role of gut microbiota in NP and provide a novel therapeutic method for pain relief by medication and dietary interventions.

Pain pharmacogenetics

Pain is a significant problem in medicine. The use of PGx markers to personalize postoperative analgesia can increase its effectiveness and avoid undesirable reactions. This article describes the mechanisms of nociception and antinociception and shows the pathophysiological mechanisms of pain in the human body. The main subject of this article is pharmacogenetic approach to the selection of anesthetics. Current review presents data for local and general anesthetics, opioids, and non-steroidal anti-inflammatory drugs. None of the anesthetics currently has clinical guidelines for pharmacogenetic testing. This literature review summarizes the results of original research available, to date, and draws attention to this area.

Contribution of central sensitization to stress-induced spreading hyperalgesia in rats with orofacial inflammation

Temporomandibular disorder (TMD) is commonly comorbid with fibromyalgia syndrome (FMS). The incidence of these pain conditions is prevalent in women and prone to mental stress. Chronic pain symptoms in patients with FMS and myofascial TMD (mTMD) are severe and debilitating. In the present study, we developed a new animal model to mimic the comorbidity of TMD and FMS. In ovariectomized female rats, repeated forced swim (FS) stress induced mechanical allodynia and thermal hyperalgesia in the hindpaws of the 17β-estradiol (E2) treated rats with orofacial inflammation. Subcutaneous injection of E2, injection of complete Freund’s adjuvant (CFA) into masseter muscles or FS alone did not induce somatic hyperalgesia. We also found that the somatic hyperalgesia was accompanied by upregulation of GluN1 receptor and serotonin (5-hydroxytryptamine, 5-HT)_3A receptor expression in the dorsal horn of spinal cord at L4-L5 segments. Intrathecal injection of N-methyl-D-aspartic acid receptor (NMDAR) antagonist 2-amino-5-phosphonovaleric acid (APV) or 5-HT₃ receptor antagonist Y-25130 blocked stress-induced wide-spreading hyperalgesia. These results suggest that NMDAR-dependent central sensitization in the spinal dorsal horn and 5-HT-dependent descending facilitation contribute to the development of wide-spreading hyperalgesia in this comorbid pain model.

Role of 5-HT1A and 5-HT3 receptors in serotonergic activation of sensory neurons in relation to itch and pain behavior in the rat

Serotonin (5-hydroxytryptamine, 5-HT) released by platelets, mast cells, and immunocytes is a potent inflammatory mediator which modulates pain and itch sensing in the peripheral nervous system. The serotonergic receptors expressed by primary afferent neurons involved in these sensory functions are not fully identified and appear to be to a large extent species dependent. Moreover, the mechanisms through which 5-HT receptor activation is coupled to changes in neuronal excitability have not been completely revealed. Using a combination of in vitro (calcium and voltage imaging and patch-clamp) and in vivo behavioral methods, we used both male and female Wistar rats to provide evidence for the involvement of two 5-HT receptor subtypes, 5-HT1A and 5-HT3, in mediating the sustained and transient effects, respectively, of 5-HT on rat primary afferent neurons involved in pain and itch processing. In addition, our results are consistent with a model in which sustained serotonergic responses triggered via the 5-HT1A receptor are due to closure of background potassium channels, followed by membrane depolarization and action potentials, during which the activation of voltage-gated calcium channels leads to calcium entry. Our results may provide a better understanding of mammalian serotonergic itch signaling.

Down-regulation of Spinal 5-HT2A and 5-HT2C Receptors Contributes to Somatic Hyperalgesia induced by Orofacial Inflammation Combined with Stress

Patients suffering with functional somatic pain syndromes such as temporomandibular disorders (TMD) and fibromyalgia syndrome (FMS) have some similar symptoms, but the underlying cause is still unclear. The purpose of this study was to investigate whether 5-HT_2A and 5-HT_2C receptors in the spinal cord contribute to somatic hyperalgesia induced by orofacial inflammation combined with different modes of stress. Ovariectomized rats were injected subcutaneously with estradiol and bilateral masseter muscles were injected with complete Freund's adjuvant followed by stress. Somatic sensitivity was assessed with thermal and mechanical stimulation. The anxiety- and depression-like behaviors were measured by immobility time, sucrose preference, elevated plus maze and open field tests. The expression of 5-HT_2A and 5-HT_2C receptors in the spinal cord was examined by Western blot. Orofacial inflammation combined with 11 day forced swim stress (FSS) induced persistent mechanical allodynia for 15 days and thermal hyperalgesia for 2 days. The mechanical and thermal hyperalgesia lasted for 43 days and 30 days respectively following orofacial inflammation combined with 11 day heterotypic stress. Orofacial inflammation combined with stress induced anxiety- and depression-like behaviors. The expression of 5-HT_2A and 5-HT_2C receptors significantly decreased in the orofacial inflammation combined with stress groups. Intrathecal injection of 5-HT_2A or 5-HT_2C receptor agonist reversed somatic hyperalgesia. The results suggest that down-regulation of 5-HT_2A and 5-HT_2C receptors in the spinal cord contributes to somatic hyperalgesia induced by orofacial inflammation combined with stress, indicating that 5-HT_2A and 5-HT_2C receptors may be potential targets in the treatment of TMD comorbid with FMS.

Role of 5-HT receptors in neuropathic pain: potential therapeutic implications

5-HT plays a crucial role in the progress and adjustment of pain both centrally and peripherally. The therapeutic action of the 5-HT receptors` agonist and antagonist in neuropathic pain have been widely reported in many studies. However, the specific roles of 5-HT subtype receptors have not been reviewed comprehensively. Therefore, we summarized the recent findings on multiple subtypes of 5-HT receptors in both central and peripheral nervous system in neuropathic pain, particularly, 5-HT1, 5-HT2, 5-HT3 and 5-HT7 receptors. In addition, 5-HT4, 5-HT5 and 5-HT6 receptors were also reviewed. Most of studies focused on the function of 5-HT subtype receptors in spinal level compared to brain areas. Based on these evidences, the pain process can be facilitated or inhibited that depending on the specific subtypes and the distribution of 5-HT receptors. Therefore, this review may provide potential therapeutic implications in treatment of neuropathic pain.

Serotonin signaling by maternal neurons upon stress ensures progeny survival

Germ cells are vulnerable to stress. Therefore, how organisms protect their future progeny from damage in a fluctuating environment is a fundamental question in biology. We show that in Caenorhabditis elegans, serotonin released by maternal neurons during stress ensures the viability and stress resilience of future offspring. Serotonin acts through a signal transduction pathway conserved between C. elegans and mammalian cells to enable the transcription factor HSF1 to alter chromatin in soon-to-be fertilized germ cells by recruiting the histone chaperone FACT, displacing histones, and initiating protective gene expression. Without serotonin release by maternal neurons, FACT is not recruited by HSF1 in germ cells, transcription occurs but is delayed, and progeny of stressed C. elegans mothers fail to complete development. These studies uncover a novel mechanism by which stress sensing by neurons is coupled to transcription response times of germ cells to protect future offspring.

Acute administration of levetiracetam in tonic pain model modulates gene expression of 5HT1A and 5HT7 receptors in the thalamus of rats (Rattus norvergicus)

The nociceptive effect of Levetiracetam (LEV) on the expression of 5-HT_1A and 5-HT₇ receptors found in the thalamus was evaluated. Thirty-six male rats (Wistar) were randomized into six groups: in the Control group without treatment; LEV50 group LEV was administered in a single dose of 50 mg/kg i.g.; in the LEV300 group LEV dose of 300 mg/kg i.g.; in the FORMALIN group the formalin test was performed; in the LEV50/FORMALIN group LEV dose of 50 mg/kg i.g and the formalin test was performed; in the LEV300/FORMALIN group LEV dose of 300 mg/kg i.g and the formalin test was performed, subsequently the thalamus was dissected in all groups. In the formalin tests LEV exhibited an antinociceptive effect in the LEV300/FORMALIN group (p < 0.05) and a pronociceptive effect in the LEV50/FORMALIN group (p < 0.001). The results obtained by Real-time PCR confirmed the expression of the 5-HT_1A and 5-HT₇ receptors in the thalamus, 5-HT_1A receptors increased significantly in the FORMALIN group and the LEV300/FORMALIN group (p < 0.05). 5-HT7 receptors are only over expressed at a dose of 300 mg/Kg of LEV with formalin (p < 0.05). This suggests that LEV modulates the sensation of pain by controlling the expression of 5-HT_1A and 5-HT₇ in a tonic pain model, and that changes in the expression of 5-HT_1A and 5-HT₇ receptors are associated with the sensation of pain, furthermore its possibility to be used in clinical treatments for pain.

Pentameric Ligand-Gated Ion Channels as Pharmacological Targets Against Chronic Pain

Chronic pain is a common detrimental condition that affects around 20% of the world population. The current drugs to treat chronic pain states, especially neuropathic pain, have a limited clinical efficiency and present significant adverse effects that complicates their regular use. Recent studies have proposed new therapeutic strategies focused on the pharmacological modulation of G-protein-coupled receptors, transporters, enzymes, and ion channels expressed on the nociceptive pathways. The present work intends to summarize recent advances on the pharmacological modulation of pentameric ligand-gated ion channels, which plays a key role in pain processing. Experimental data have shown that novel allosteric modulators targeting the excitatory nicotinic acetylcholine receptor, as well as the inhibitory GABA_A and glycine receptors, reverse chronic pain-related behaviors in preclinical assays. Collectively, these evidences strongly suggest the pharmacological modulation of pentameric ligand-gated ion channels is a promising strategy towards the development of novel therapeutics to treat chronic pain states in humans.

Uncarialins A-I, Monoterpenoid Indole Alkaloids from Uncaria rhynchophylla as Natural Agonists of the 5-HT1A Receptor

Nine new monoterpenoid indole alkaloids, uncarialins A-I (1-9), were isolated from Uncaria rhynchophylla as well as 14 known analogues (10-23). Their structures were determined by HRESIMS, 1D and 2D NMR, and experimental and calculated electronic circular dichroism data. Compounds 5, 7, 15, and 22 displayed significant agonistic effects against the 5-HT_1A receptor with EC₅₀ values of 2.2 ± 0.1, 0.1 ± 0.1, 1.6 ± 0.3, and 2.0 ± 0.5 μM, respectively. The mechanisms of action of these four compounds with the 5-HT_1A receptor were investigated by molecular docking, and the results suggested that amino acid residues Asp116, Thr196, Asn386, and Tyr390 played critical roles in the observed activity of the above-mentioned compounds.

Valproate reverses stress-induced somatic hyperalgesia and visceral hypersensitivity by up-regulating spinal 5-HT2C receptor expression in female rats

Sodium valproate (VPA) has analgesic effects in clinical and experimental studies, but the mechanisms are still unclear. The present study examined the effects of VPA on stress-induced somatic hyperalgesia and visceral hypersensitivity and the role of 5-HT_2C receptors in the spinal cord. Repeated 3 day forced swim (FS) significantly reduced the thermal withdrawal latency and mechanical withdrawal threshold, and increased the magnitude of the visceromotor response to colorectal distention compared to the baseline values in rats. The somatic hyperalgesia and visceral hypersensitivity were accompanied by significant down-regulation of 5-HT_2C receptor expression in the L4-L5 and L6-S1 dorsal spinal cord. Intraperitoneal administration of VPA (300 mg/kg) before each FS and 1 day post FS prevented the development of somatic hyperalgesia and visceral hypersensitivity induced by FS stress, as well as down-regulation of 5-HT_2C receptors in the spinal cord. The reversal of somatic hyperalgesia and visceral hypersensitivity by VPA in FS rats was blocked by intrathecal administration of the selective 5-HT_2C receptor antagonist RS-102221 (30 μg/10 μL) 30 min after each VPA injection. The results suggest that VPA attenuates FS-induced somatic hyperalgesia and visceral hypersensitivity by restoring down-regulated function of 5-HT_2C receptors in the spinal cord.

The Role of Descending Pain Modulation in Chronic Primary Pain: Potential Application of Drugs Targeting Serotonergic System

Chronic primary pain (CPP) is a group of diseases with long-term pain and functional disorders but without structural or specific tissue pathologies. CPP is becoming a serious health problem in clinical practice due to the unknown cause of intractable pain and high cost of health care yet has not been satisfactorily addressed. During the past decades, a significant role for the descending pain modulation and alterations due to specific diseases of CPP has been emphasized. It has been widely established that central sensitization and alterations in neuroplasticity induced by the enhancement of descending pain facilitation and/or the impairment of descending pain inhibition can explain many chronic pain states including CPP. The descending serotonergic neurons in the raphe nuclei target receptors along the descending pain circuits and exert either pro- or antinociceptive effects in different pain conditions. In this review, we summarize the possible underlying descending pain regulation mechanisms in CPP and the role of serotonin, thus providing evidence for potential application of analgesic medications based on the serotonergic system in CPP patients.

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.

An Association of Serotonin with Pain Disorders and Its Modulation by Estrogens

Ovarian hormones play an important role in pain perception, and are responsible, at least in part, for the pain threshold differences between the sexes. Modulation of pain and its perception are mediated by neurochemical changes in several pathways, affecting both the central and peripheral nervous systems. One of the most studied neurotransmitters related to pain disorders is serotonin. Estrogen can modify serotonin synthesis and metabolism, promoting a general increase in its tonic effects. Studies evaluating the relationship between serotonin and disorders such as irritable bowel syndrome, fibromyalgia, migraine, and other types of headache suggest a clear impact of this neurotransmitter, thereby increasing the interest in serotonin as a possible future therapeutic target. This literature review describes the importance of substances such as serotonin and ovarian hormones in pain perception and illustrates the relationship between those two, and their direct influence on the presentation of the aforementioned pain-related conditions. Additionally, we review the pathways and receptors implicated in each disorder. Finally, the objective was to stimulate future pharmacological research to experimentally evaluate the potential of serotonin modulators and ovarian hormones as therapeutic agents to regulate pain in specific subpopulations.

Non-Dopaminergic Therapies

Currently all aspects of Parkinson’s disease (PD) treatment are less than ideal and would benefit from new interventions. We focus on problems associated with brain neurotransmitters amenable to more specific subreceptor and temporal manipulation. In addition, we consider potentially treatable CNS non-PD co-pathologies or co-morbidities that may exacerbate progression of various aspects of PD.

Pain regulation by gut microbiota: molecular mechanisms and therapeutic potential

The relationship between gut microbiota and neurological diseases, including chronic pain, has received increasing attention. The gut microbiome is a crucial modulator of visceral pain, whereas recent evidence suggests that gut microbiota may also play a critical role in many other types of chronic pain, including inflammatory pain, headache, neuropathic pain, and opioid tolerance. We present a narrative review of the current understanding on the role of gut microbiota in pain regulation and discuss the possibility of targeting gut microbiota for the management of chronic pain. Numerous signalling molecules derived from gut microbiota, such as by-products of microbiota, metabolites, neurotransmitters, and neuromodulators, act on their receptors and remarkably regulate the peripheral and central sensitisation, which in turn mediate the development of chronic pain. Gut microbiota-derived mediators serve as critical modulators for the induction of peripheral sensitisation, directly or indirectly regulating the excitability of primary nociceptive neurones. In the central nervous system, gut microbiota-derived mediators may regulate neuroinflammation, which involves the activation of cells in the blood-brain barrier, microglia, and infiltrating immune cells, to modulate induction and maintenance of central sensitisation. Thus, we propose that gut microbiota regulates pain in the peripheral and central nervous system, and targeting gut microbiota by diet and pharmabiotic intervention may represent a new therapeutic strategy for the management of chronic pain.

Adaptation in 5-HT2 receptors-CaMKII signaling in lateral habenula underlies increased nociceptive-sensitivity in ethanol-withdrawn rats

Alcoholics often experience hyperalgesia, especially during abstinence, yet the underlying cellular and molecular bases are unclear. Recent evidence suggests that 5-HT type 2 receptors (5-HT₂Rs) at glutamatergic synapses on lateral habenula (LHb) neurons may play a critical role. We, therefore, measured paw withdrawal responses to thermal and mechanical stimuli, and alcohol intake in a rat model of intermittent drinking paradigm, as well as spontaneous glutamatergic transmission (sEPSCs), and firing of LHb neurons in brain slices. Here, we report that nociceptive sensitivity was higher in rats at 24 h withdrawal from chronic alcohol consumption than that of alcohol-naive counterparts. The basal frequency of sEPSCs and firings was higher in slices of withdrawn rats than that of Naïve rats, and 5-HT2R antagonists attenuated the enhancement. Also, an acute ethanol-induced increase of sEPSCs and firings was smaller in withdrawal than in Naïve rats; it was attenuated by 5-HT₂R antagonists but mimicked by 5-HT₂R agonists. Importantly, intra-LHb infusion of 5-HT₂R agonists increased nociceptive sensitivity in Naïve rats, while antagonists or 5-HT reuptake blocker decreased nociceptive sensitivity and alcohol intake in withdrawn rats. Additionally, KN-62, a CaMKII inhibitor, attenuated the enhancement of EPSCs and firing induced by acute alcohol and by 5-HT₂R agonist. Furthermore, intra-LHb KN-62 reduced nociceptive sensitivity and alcohol intake. Quantitative real-time PCR assay detected mRNA of 5-HT2A and 2C in the LHb. Thus adaptation in 5-HT₂R-CaMKII signaling pathway contributes to the hyper-glutamatergic state, the hyperactivity of LHb neurons as well as the higher nociceptive sensitivity in rats withdrawn from chronic alcohol consumption.

Pharmacological interaction between NSAIDS with clomipramine and risperidone in mice visceral pain

Nonsteroidal anti-inflammatory drugs (NSAIDs) possess as primary action mechanism the inhibition of cyclooxygenases (COX-1, COX-2, and COX-3), thus producing a decreasing prostaglandin synthesis. This study was designed to evaluate whether the antinociception induced by NSAIDs could be modulated by clomipramine or risperidone using a chemical model of inflammatory acute visceral pain, the abdominal acetic acid induced a writhing test in mice. Dose-response curves, intraperitoneal, or intrathecal for the antinociceptive activity displayed by ketoprofen, piroxicam, nimesulide, parecoxib, and paracetamol were analyzed in order to obtain the ED₅₀ of each drug. Pretreatment of mice with either clomipramine or risperidone, increased antinociceptive potency of ketoprofen, piroxicam, nimesulide, parecoxib, and paracetamol, expressed by a decrease in the values of antinociceptive ED_50. The results that were obtained are in line with those where the inhibition of COXs provides a justification for most of the pharmacological actions. Nevertheless, several findings suggest other molecular mechanisms, among which may be mentioned, L-selecting shedding; inhibition of i-NOS; inhibition of NF-Kappa B; suppression metaloproteinasas; inhibition of ß2 integrin activation; activation of α₂ -adrenoceptor; increase of IL-1ß; upregulation IL-6. In conclusion, the data generated in this study demonstrated that risperidone and clomipramine, separately, increase antinociceptive potency of NSAIDs in a chemical model of inflammatory acute visceral tonic pain.