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

Vortioxetine reduces the development of pain-related behaviour in a knee osteoarthritis model in rats: Involvement of nerve growth factor (NGF) down-regulation

BACKGROUND AND PURPOSE

Vortioxetine, a multimodal-acting antidepressant, has recently shown analgesic properties. We aimed to investigate its prophylactic effect in the osteoarthritis (OA) model and gain insights into the underlying molecular mechanisms. Duloxetine was studied as a reference.

EXPERIMENTAL APPROACH

In the monoiodoacetate (MIA)-induced rat model of knee OA, pain-related behaviour was assessed in weight-bearing and Von Frey tests. Antidepressants were administered orally once daily for 28 days. Gene expression of pain-related mediators (Ngf, Il-1β, Tnf-α, Bdnf, and Tac1 encoding substance P) and oxidative stress parameters were determined after completion of the treatment/behavioural testing protocol.

KEY RESULTS

Vortioxetine and duloxetine dose dependently reduced weight-bearing asymmetry and mechanical hyperalgesia of the paw ipsilateral to the MIA-injected knee. Vortioxetine reduced the increased Ngf mRNA expression in the MIA-injected knees to the level in sham-injected counterparts. It reduced oxidative stress parameters in the affected knees, more effectively in females than males. Duloxetine showed no effect on Ngf mRNA expression and oxidative stress. Both antidepressants decreased mRNA expression of pain-related mediators in the lumbar L3-L5 ipsilateral DRGs and spinal cords, which were up-regulated in MIA-injected rats. This effect was male-specific.

CONCLUSION AND IMPLICATIONS

Vortioxetine may be effective against the development of chronic pain in OA. Its antihyperalgesic effect may be mediated, at least in part, by normalization of NGF expression in the affected joint. Decrease of localized oxidative stress and of expression of pain-related mediators that contribute to central sensitization are also involved in vortioxetine's antihyperalgesic effect, in a sex-specific pattern.

The immunomodulatory effects of classical psychedelics: A systematic review of preclinical studies

Emerging evidence suggests that classical psychedelics possess immunomodulatory and anti-inflammatory properties; however, these effects are yet to be well-established. This systematic review aims to provide a timely and comprehensive overview of the immunomodulatory effects of classical psychedelics in preclinical studies. A systematic search was conducted on six databases, including CINAHL, EMBASE, MEDLINE, PsychINFO, Scopus, and Web of Science. Eligible studies targeting classical psychedelics for evaluation of their effects on inflammatory markers and immunomodulation have been included for analysis. Data was extracted from 40 out of 2822 eligible articles, and their risk of bias was assessed using the Systematic Review Center for Laboratory Animal Experimentation (SYRCLE) tool and Quality Assessment Tool for In Vitro Studies (QUIN). Studies examined 2,5-dimethoxy-4-iodoamphetamine (DOI; n = 18); psilocybin (4-PO-DMT; n = 9); N,N-dimethyltryptamine (DMT; n = 8); lysergic acid diethylamide (LSD; n = 6); 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT; n = 3); psilocin (4-HO-DMT; n = 3); and mescaline (n = 2). In 36 studies where inflammatory cytokine levels were measured following psychedelic administration, a decrease in at least one inflammatory cytokine was observed in 29 studies. Immune cell activity was assessed in 10 studies and findings were mixed, with an equal number of studies (n = 5 out of 10) reporting either an increase or decrease in immune cell activity. Classical psychedelics were found to alleviate pre-existing inflammation but promote inflammation when administered under normal physiological conditions. This information is anticipated to inform future clinical trials, exploring classical psychedelics' potential to alleviate inflammation in various pathologies.

The roles of 5-HT in orofacial pain

OBJECTIVES

Acute and chronic orofacial pain are very common and remain a vexing health problem that has a negative effect on the quality of life. Serotonin (5-HydroxyTryptamine, 5-HT) is a kind of monoamine neurotransmitter that is involved in many physiological and pathological processes. However, its role in orofacial pain remains inconclusive. Therefore, this review aims to summarize the recent advances in understanding the effect exerted by 5-HT on the modulation of orofacial pain.

SUBJECTS AND METHODS

An extensive search was conducted on PubMed and Web of Science for pertinent studies focusing on the effects of 5-HT on the modulation of orofacial pain.

RESULTS

In this review, we concisely review how 5-HT mediates orofacial pain, how 5-HT is regulated and how we can translate these findings into clinical applications for the prevention and/or treatment of orofacial pain.

CONCLUSIONS

5-HT plays a key role in the modulation of orofacial pain, implying that 5-HT modulators may serve as effective treatment for orofacial pain. However, further research on the precise mechanisms underlying the modulation of orofacial pain is still warranted.

Non-hallucinogenic compounds derived from iboga alkaloids alleviate neuropathic and visceral pain in mice through a mechanism involving 5-HT2A receptor activation

The aim of this study was to determine the anti-hypersensitivity activity of novel non-hallucinogenic compounds derived from iboga alkaloids (i.e., ibogalogs), including tabernanthalog (TBG), ibogainalog (IBG), and ibogaminalog (DM506), using mouse models of neuropathic (Chronic Constriction Injury; CCI) and visceral pain (dextrane sulfate sodium; DSS). Ibogalogs decreased mechanical hyperalgesia and allodynia induced by CCI in a dose- and timeframe-dependent manner, where IBG showed the longest anti-hyperalgesic activity at a comparatively lower dose, whereas DM506 displayed the quickest response. These compounds also decreased hypersensitivity induced by colitis, where DM506 showed the longest activity. To understand the mechanisms involved in these effects, two approaches were utilized: ibogalogs were challenged with the 5-HT2A receptor antagonist ketanserin and the pharmacological activity of these compounds was assessed at the respective 5-HT2A, 5-HT6, and 5-HT7 receptor subtypes. The behavioral results clearly demonstrated that ketanserin abolishes the pain-relieving activity of ibogalogs without inducing any effect per se, supporting the concept that 5-HT2A receptor activation, but not inhibition, is involved in this process. The functional results showed that ibogalogs potently activate the 5-HT2A and 5-HT6 receptor subtypes, whereas they behave as inverse agonists (except TBG) at the 5-HT7 receptor. Considering previous studies showing that 5-HT6 receptor inhibition, but not activation, and 5-HT7 receptor activation, but not inhibition, relieved chronic pain, we can discard these two receptor subtypes as participating in the pain-relieving activity of ibogalogs. The potential involvement of 5-HT2B/2 C receptor subtypes was also ruled out. In conclusion, the anti-hypersensitivity activity of ibogalogs in mice is mediated by a mechanism involving 5-HT2A receptor activation.

Examination of the effects of vitexin and vitexin-loaded solid lipid nanoparticles on neuropathic pain and possible mechanisms of action

This research aims to investigate the possible antiallodynic and antihyperalgesic effects of pure vitexin and vitexin-loaded solid lipid nanoparticles (SLN) on neuropathic pain and the pathways mediating these effects. Chronic constriction nerve injury was induced in female rats, and the effects of vitexin at the doses of 5, 10, 20, 40 mg/kg were evaluated. Ketanserin, ondansetron, WAY-100635, yohimbine and bicuculin, which are antagonists of receptors on pain pathways. were used to examine the mechanisms of the effects of vitexin. Pure vitexin exhibited antiallodynic activity at all administered doses, whereas antihyperalgesic activity was not observed at 5 mg/kg vitexin dose. SLN formulation was prepared with 5 mg/kg vitexin, the lowest dose. Vitexin-loaded formulation significantly increased antiallodynic and antihyperalgesic effects. Ondansetron, WAY-100635, yohimbine, and bicuculine antagonized the antiallodynic and antihyperalgesic effects of vitexin. So, it was concluded that serotonin (5-hydroxtryptamine, 5-HT) receptor subtypes 5-HT3 and 5-HT1A, alpha-2 adrenergic, and γ-Aminobutyric acid type A (GABA-A) receptors are involved in the antiallodynic and antihyperalgesic activity of vitexin. In conclusion, vitexin and vitexin-loaded formulation have the potential for clinical use in neuropathic pain management, and different pain pathways contributed to this effect. And also, it is thought that vitexin-loaded SLN formulation is more effective than pure vitexin, which will provide an advantage in treatment.

Low-frequency (5-Hz) stimulation of ventrolateral periaqueductal gray modulates the descending serotonergic system in the peripheral neuropathic pain

ABSTRACT

Neuropathic pain is a type of chronic pain that entails severe prolonged sensory dysfunctions caused by a lesion of the somatosensory system. Many of those suffering from the condition do not experience significant improvement with existing medications, resulting in various side effects. In this study, Sprague-Dawley male rats were used, and long-term deep brain stimulation of the ventrolateral periaqueductal gray was conducted in a rat model of spared nerve injury. We found that 5-Hz deep brain stimulation effectively modulated mechanical allodynia and induced neuronal activation in the rostral ventromedial medulla, restoring impaired descending serotonergic system. At the spinal level, glial cells were still activated but only the 5-HT1a receptor in the spinal cord was activated, implying its inhibitory role in mechanical allodynia. This study found that peripheral neuropathy caused dysfunction in the descending serotonergic system, and prolonged stimulation of ventrolateral periaqueductal gray can modulate the pathway in an efficient manner. This work would provide new opportunities for the development of targeted and effective treatments for this debilitating disease, possibly giving us lower chances of side effects from repeated high-frequency stimulation or long-term use of medication.

Activation of 5-HT6 Receptors in the Ventrolateral Orbital Cortex Produces Anti-Anxiodepressive Effects in a Rat Model of Neuropathic Pain

The comorbidity of anxiety and depression frequently occurs in patients with neuropathic pain. The ventrolateral orbital cortex (VLO) plays a critical role in mediating neuropathic pain and anxiodepression in rodents. Previous studies suggested that 5-HT6 receptors in the VLO are involved in neuropathic pain. Strong evidence supports a close link between 5-HT6 receptors and affective disorders such as depression and anxiety disorders. However, it remains unclear whether the 5-HT6 receptors in the VLO are involved in neuropathic pain-induced anxiodepression. Using a rat neuropathic pain model of spared nerve injury (SNI), we demonstrated that rats exhibited significant anxiodepression-like behaviors and the expression of VLO 5-HT6 receptors obviously decreased four weeks after SNI surgery. Microinjection of the 5-HT6 receptor agonist EMD-386088 into the VLO or overexpression of VLO 5-HT6 receptors alleviated anxiodepression-like behaviors. These effects were blocked by pre-microinjection of a selective 5-HT6 receptor antagonist (SB-258585) or inhibitors of AC (SQ-22536), PKA (H89), and MEK1/2 (U0126) respectively. Meanwhile, the expression of p-ERK, p-CREB, and BDNF in the VLO decreased four weeks after SNI surgery. Furthermore, administration of EMD-386088 upregulated the expression of BDNF, p-ERK, and p-CREB in the VLO of SNI rats, which were reversed by pre-injection of SB-258585. These findings suggest that activating 5-HT6 receptors in the VLO has anti-anxiodepressive effects in rats with neuropathic pain via activating AC-cAMP-PKA-MERK-CREB-BDNF signaling pathway. Accordingly, 5-HT6 receptor in the VLO could be a potential target for the treatment of the comorbidity of neuropathic pain and anxiodepression.

Analgesic effects of oral Yokukansan on acute postoperative pain and involvement of the serotonin nervous system: a mouse model study

BACKGROUND

Yokukansan, a traditional Japanese medicine (Kampo), has been widely used to treat neurosis, dementia, and chronic pain. Previous in vitro studies have suggested that Yokukansan acts as a partial agonist of the 5-HT1A receptor, resulting in amelioration of chronic pain through inhibition of nociceptive neuronal activity. However, its effectiveness for treating postoperative pain remains unknown, although its analgesic mechanism of action has been suggested to involve serotonin and glutamatergic neurotransmission. This study aimed to investigate the effect of Yokukansan on postoperative pain in an animal model.

METHODS

A mouse model of postoperative pain was created by plantar incision, and Yokukansan was administered orally the day after paw incision. Pain thresholds for mechanical and heat stimuli were examined in a behavioral experiment. In addition, to clarify the involvement of the serotonergic nervous system, we examined the analgesic effects of Yokukansan in mice that were serotonin-depleted by para-chlorophenylalanine (PCPA) treatment and intrathecal administration of NAN-190, 5-HT1A receptor antagonist.

RESULTS

Orally administered Yokukansan increased the pain threshold dose-dependent in postoperative pain model mice. Pretreatment of para-chlorophenylalanine dramatically suppressed serotonin immunoreactivity in the spinal dorsal horn without changing the pain threshold after the paw incision. The analgesic effect of Yokukansan tended to be attenuated by para-chlorophenylalanine pretreatment and significantly attenuated by intrathecal administration of 2.5 µg of NAN-190 compared to that in postoperative pain model mice without para-chlorophenylalanine treatment and NAN-190 administration.

CONCLUSION

This study demonstrated that oral administration of Yokukansan has acute analgesic effects in postoperative pain model mice. Behavioral experiments using serotonin-depleted mice and mice intrathecally administered with a 5-HT1A receptor antagonist suggested that Yokukansan acts as an agonist at the 5-HT1A receptor, one of the serotonin receptors, to produce analgesia.

Therapeutic potential of psychedelics: History, advancements, and unexplored frontiers

Psychedelics (serotonergic hallucinogens) are psychoactive substances that can alter perception and mood, and affect cognitive functions. These substances activate 5-HT2A receptors and may exert therapeutic effects. Some of the disorders for which psychedelic-assisted therapy have been studied include depression, addiction, anxiety and post-traumatic stress disorder. Despite the increasing number of studies reporting clinical effectiveness, with fewer negative symptoms and, additionally, minimal side effects, questions remain to be explored in the field of psychedelic medicine. Although progress has been achieved, there is still little understanding of the relationship among human brain and the modulation induced by these drugs. The present article aimed to describe, review and highlight the most promising findings in the literature regarding the (putative) therapeutic effects of psychedelics.

Chronic pain as an emergent property of a complex system and the potential roles of psychedelic therapies

Despite research advances and urgent calls by national and global health organizations, clinical outcomes for millions of people suffering with chronic pain remain poor. We suggest bringing the lens of complexity science to this problem, conceptualizing chronic pain as an emergent property of a complex biopsychosocial system. We frame pain-related physiology, neuroscience, developmental psychology, learning, and epigenetics as components and mini-systems that interact together and with changing socioenvironmental conditions, as an overarching complex system that gives rise to the emergent phenomenon of chronic pain. We postulate that the behavior of complex systems may help to explain persistence of chronic pain despite current treatments. From this perspective, chronic pain may benefit from therapies that can be both disruptive and adaptive at higher orders within the complex system. We explore psychedelic-assisted therapies and how these may overlap with and complement mindfulness-based approaches to this end. Both mindfulness and psychedelic therapies have been shown to have transdiagnostic value, due in part to disruptive effects on rigid cognitive, emotional, and behavioral patterns as well their ability to promote neuroplasticity. Psychedelic therapies may hold unique promise for the management of chronic pain.

Multiple reports on the causal relationship between various chronic pain and gut microbiota: a two-sample Mendelian randomization study

Background

Previous evidence suggests a link between gut microbiota and chronic pain, but the causal relationship is not yet fully understood.

Methods

We categorized gut microbiota based on phylum, class, order, family, and genus levels and gathered pain-related information from the UKB and FinnGen GWAS project. Then, we conducted MR analysis to explore the potential causal relationship between gut microbiota and chronic pain at 12 specific locations.

Results

We have discovered a direct connection between genetic susceptibility in the gut microbiota (gut metabolites) and pain experienced at 12 specific locations. Notably, Serotonin (5-HT) and Glycine were found to be associated with a higher risk of pain in the extremities. On the other hand, certain microbial families and orders were found to have a protective effect against migraines. Specifically, the family Bifidobacteriaceae (IVW, FDR p = 0.013) was associated with a lower risk of migraines. Furthermore, the genus Oxalobacter (IVW, FDR p = 0.044) was found to be linked to an increased risk of low back pain. Importantly, these associations remained significant even after applying the Benjamini-Hochberg correction test. Our analysis did not find any heterogeneity in the data (p > 0.05), as confirmed by the Cochrane's Q-test. Additionally, both the MR-Egger and MR-PRESSO tests indicated no significant evidence of horizontal pleiotropy (p > 0.05).

Conclusion

Our MR analysis demonstrated a causal relationship between the gut microbiota and pain, highlighting its potential significance in advancing our understanding of the underlying mechanisms and clinical implications of microbiota-mediated pain.

Microbiological and metabolic pathways analysing the mechanisms of alfalfa polysaccharide and sulfated alfalfa polysaccharide in alleviating obesity

Alfalfa polysaccharide (AP) and sulfated alfalfa polysaccharide (SAP) exhibit potential for alleviating obesity. This study aimed to analyze the mechanism of action of AP and SAP in alleviating obesity through combined microbiomics and metabolomics. The research selected validated optimal AP and SAP concentration for experiment. The results showed that AP and SAP down-regulated colonic inflammatory gene expression, regulated intestinal pH to normal, and restored intestinal growth. Microbial sequencing showed that AP and SAP altered the microbial composition ratio. AP increased the relative abundance of Muribaculaceae and Romboutsia. SAP increased the relative abundance of Dubosiella, Fecalibaculum and Desulfovibrionaceae. Metabolomic analysis showed that AP regulated steroid hormone biosynthesis, neuroactive ligand-receptor interactions and bile secretion pathways. SAP focuses more on pathways related to amino acid metabolism. Meanwhile, AP and SAP down-regulated the mRNA expression of colonic COX-2, PepT-1 and HK2 and up-regulated the mRNA expression of TPH1. Correlation analysis showed a strong correlation between metabolites and gut bacteria. Dubosiella, Faecalibaculum may be the critical marker flora for polysaccharides to alleviate obesity. This study indicates that AP and SAP alleviate obesity through different pathways and that specific polysaccharide modifications affect characteristic microbial and metabolic pathways, providing new insights into polysaccharide modifications.

Altered functional connectivity of brainstem nuclei in new daily persistent headache: Evidence from resting-state functional magnetic resonance imaging

OBJECTIVES

The new daily persistent headache (NDPH) is a rare primary headache disorder. However, the underlying mechanisms of NDPH remain incompletely understood. This study aims to apply seed-based analysis to explore the functional connectivity (FC) of brainstem nuclei in patients with NDPH using resting-state functional magnetic resonance imaging (MRI).

METHODS

The FC analysis from the region of interest (ROI) to whole brain voxels was used to investigate 29 patients with NDPH and 37 well-matched healthy controls (HCs) with 3.0 Tesla MRI. The 76 nuclei in the brainstem atlas were defined as ROIs. Furthermore, we explored the correlations between FC and patients' clinical characteristics and neuropsychological evaluations.

RESULTS

Patients with NDPH exhibited reduced FC in multiple brainstem nuclei compared to HCs (including right inferior medullary reticular formation, right mesencephalic reticular formation, bilateral locus coeruleus, bilateral laterodorsal tegmental nucleus-central gray of the rhombencephalon, median raphe, left medial parabrachial nucleus, periaqueductal gray, and bilateral ventral tegmental area-parabrachial pigmented nucleus complex) and increased FC in periaqueductal gray. No significant correlations were found between the FC of these brain regions and clinical characteristics or neuropsychological evaluations after Bonferroni correction (p > 0.00016).

CONCLUSIONS

Our results demonstrated that patients with NDPH have abnormal FC of brainstem nuclei involved in the perception and regulation of pain and emotions.

Role of 5-hydroxytryptamine type 3 receptors in the regulation of anxiety reactions

5-Hydroxytryptamine (5-HT) type 3 receptor (5-HT3R) is the only type of ligand-gated ion channel in the 5-HT receptor family. Through the high permeability of Na+, K+, and Ca2+ and activation of subsequent voltage-gated calcium channels (VGCCs), 5-HT3R induces a rapid increase of neuronal excitability or the release of neurotransmitters from axon terminals in the central nervous system (CNS). 5-HT3Rs are widely expressed in the medial prefrontal cortex (mPFC), amygdala (AMYG), hippocampus (HIP), periaqueductal gray (PAG), and other brain regions closely associated with anxiety reactions. They have a bidirectional regulatory effect on anxiety reactions by acting on different types of cells in different brain regions. 5-HT3Rs mediate the activation of the cholecystokinin (CCK) system in the AMYG, and the γ‍-aminobutyric acid (GABA) "disinhibition" mechanism in the prelimbic area of the mPFC promotes anxiety by the activation of GABAergic intermediate inhibitory neurons (IINs). In contrast, a 5-HT3R-induced GABA "disinhibition" mechanism in the infralimbic area of the mPFC and the ventral HIP produces anxiolytic effects. 5-HT2R-mediated regulation of anxiety reactions are also activated by 5-HT3R-activated 5-HT release in the HIP and PAG. This provides a theoretical basis for the treatment of anxiety disorders or the production of anxiolytic drugs by targeting 5-HT3Rs. However, given the circuit specific modulation of 5-HT3Rs on emotion, systemic use of 5-HT3R agonism or antagonism alone seems unlikely to remedy anxiety, which deeply hinders the current clinical application of 5-HT3R drugs. Therefore, the exploitation of circuit targeting methods or a combined drug strategy might be a useful developmental approach in the future.

Research of the analgesic effects and central nervous system impact of electroacupuncture therapy in rats with knee osteoarthritis

It aimed to observe the effects of TongDu TiaoShen (TDTS) electroacupuncture (EA) on the analgesia and central system of knee osteoarthritis (KOA) rats and explore its mechanism. SD rats were rolled into the blank group, model group (KOA), control group (duloxetine 500 mg/kg/d, Ctrl), conventional EA group, and TDTS-EA group. Radiometric pain measurements and the Lequesne MG scale were used to evaluate the behavioral performance of the rats. Dopamine (DA), norepinephrine (NE), 5-hydroxytryptamine (5-HT), β-endorphin (β-EP), and leucine-enkephalin (L-ENK) were detected in the midbrain and spinal cord of lumbar enlargement. Interleukin (IL)-1β protein expression was detected by Western blot. The incubation period of thermal pain and foot contraction was decreased in the KOA group versus blank group, the Lequesne MG score was increased, DA, NE, 5-HT, β-EP, and L-ENK in the midbrain and spinal cord were increased, and synovial tissue IL-1β protein expression was increased (P < 0.05). EA group and TDTS-EA group had an increased incubation period of thermal pain contraction, decreased Lequesne MG score, decreased DA, NE, etc. In the midbrain, increased 5-HT and NE in the spinal cord, and decreased IL-1β in the synovial tissue versus KOA group (P < 0.05). The Lequesne MG score and midbrain DA, NE, 5-HT, β-EP, and synovial tissue IL-1β expression were decreased in TDTS-EA group versus EA group (P < 0.05). EA can effectively improve the behavioral score of KOA and participate in central analgesia by regulating central DA, NE, 5-HT, β-EP, and L-ENK.

Technological Trends Involving Probiotics in the Treatment of Diabetic Neuropathy: A Patent Review (2009-2022)

BACKGROUND

Diabetic neuropathy (DN) causes neuropathic pain, and current treatments are unsatisfactory. Recently studies have demonstrated an assertive correlation between gut microbiota and pain modulation.

OBJECTIVE

Considering the emerging search for new therapies for the control of DN and the growing commercial interest in the probiotics market, this study aimed to provide patents on the use of probiotics in the control of DN.

METHODS

This is a patent prospection performed in the Espacenet Patent database, using the association of keywords and IPC related to probiotics in medical preparations and foods, from 2009 to December 2022.

RESULTS

Results have shown that in 2020, there was a boom in patent filing in the area. Asian countries accounted for more than 50% of all 48 inventions (n = 48), with Japan as the only applicant in 2021. Products being developed in recent years point to effects that may represent an advancement in DN treatment, such as reduced concentration of pro-inflammatory mediators, metabolites and neurotransmitters release, and hypoglycemic potential. All effects were more related to the Lactobacillus and Bifidobacterium genera, associated with more than one property mentioned.

CONCLUSION

The mechanisms attributed to the microorganisms suggest the therapeutic potential of probiotics in the non-pharmacological treatment of pain. New applications for probiotics have resulted from great research interest by academia, but also reflect commercial interests despite the paucity of clinical trials. Thus, the present work supports the evolution of research to explore the benefits of probiotics and their clinical use in DN.

Regulation of Pain Perception by Microbiota in Parkinson Disease

Pain perception involves current stimulation in peripheral nociceptive nerves and the subsequent stimulation of postsynaptic excitatory neurons in the spinal cord. Importantly, in chronic pain, the neural activity of both peripheral nociceptors and postsynaptic neurons in the central nervous system is influenced by several inflammatory mediators produced by the immune system. Growing evidence has indicated that the commensal microbiota plays an active role in regulating pain perception by either acting directly on nociceptors or indirectly through the modulation of the inflammatory activity on immune cells. This symbiotic relationship is mediated by soluble bacterial mediators or intrinsic structural components of bacteria that act on eukaryotic cells, including neurons, microglia, astrocytes, macrophages, T cells, enterochromaffin cells, and enteric glial cells. The molecular mechanisms involve bacterial molecules that act directly on neurons, affecting their excitability, or indirectly on non-neuronal cells, inducing changes in the production of proinflammatory or anti-inflammatory mediators. Importantly, Parkinson disease, a neurodegenerative and inflammatory disorder that affects mainly the dopaminergic neurons implicated in the control of voluntary movements, involves not only a motor decline but also nonmotor symptomatology, including chronic pain. Of note, several recent studies have shown that Parkinson disease involves a dysbiosis in the composition of the gut microbiota. In this review, we first summarize, integrate, and classify the molecular mechanisms implicated in the microbiota-mediated regulation of chronic pain. Second, we analyze the changes on the commensal microbiota associated to Parkinson disease and propose how these changes affect the development of chronic pain in this pathology. SIGNIFICANCE STATEMENT: The microbiota regulates chronic pain through the action of bacterial signals into two main locations: the peripheral nociceptors and the postsynaptic excitatory neurons in the spinal cord. The dysbiosis associated to Parkinson disease reveals increased representation of commensals that potentially exacerbate chronic pain and reduced levels of bacteria with beneficial effects on pain. This review encourages further research to better understand the signals involved in bacteria-bacteria and bacteria-host communication to get the clues for the development of probiotics with therapeutic potential.

Association between single-nucleotide polymorphisms in serotonin receptor 2A and melatonin receptor 1A genes and pain after root canal treatment

AIM

This study aimed to investigate whether single-nucleotide polymorphisms (SNPs) in the genes encoding 5-HTR2A (5-Hydroxytryptamine (serotonin) receptor 2A) and MTNR1A (melatonin receptor 1A) may contribute to postoperative pain perception after root canal treatment. We hypothesised that SNPs in HTR2A and MTNR1A genes were associated with postoperative pain after root canal treatment.

METHODOLOGY

This genetic cohort study enrolled patients with single-rooted teeth diagnosed with pulp necrosis and asymptomatic apical periodontitis before root canal treatment. Root canal treatment was performed in one session using a standardized protocol. Postoperative pain and tenderness were assessed using a visual analogue scale (recorded every day for 7 days and on the 14th and 30th days after root canal treatment). Genomic DNA was extracted from saliva and used to genotype the SNPs in HTR2A (rs4941573 and rs6313) and MTNR1A (rs6553010, rs6847693 and rs13140012) using real-time polymerase chain reaction. Genotypes were compared using univariate and multivariate Poisson regression with generalized estimating equations (p < .05).

RESULTS

In total, 108 patients were enrolled in this study. The SNPs rs6553010 (MTNR1A), rs4941573 and rs6313 (HTR2A) were associated with an increased risk of developing pain after root canal treatment (p < .05).

CONCLUSIONS

This study suggests that SNPs in HTR2A and MTNR1A influence pain response after root canal treatment.

Efficacy and safety of duloxetine in chronic musculoskeletal pain: a systematic review and meta-analysis

BACKGROUND

Chronic musculoskeletal pain (CMP) is a complex condition that is mainly treated with analgesic drugs. However, antidepressant intervention is also an important factor in the treatment of CMP. Duloxetine is an effective treatment option for patients with CMP as its antidepressant effect. The purpose of this article is to evaluate the efficacy and safety of duloxetine in treating CMP.

DATABASES AND DATA TREATMENT

We searched PubMed, Web of Science, Embase, Cochrane Library from inception to May, 2022. Randomized controlled trials (RCTs) evaluating the efficacy and safety of duloxetine versus placebo in patients with CMP were included. We identified 13 articles and studied a population of 4201 participants in 4 countries.

RESULTS

This meta-analysis showed that the duloxetine has statistically significant compared with the placebo control, benefits on 24-hour average pain, living quality, physical function, and global impressions and there was no difference in the incidence of serious adverse event. In general, duloxetine can improve mood and pain level at the same time.

CONCLUSIONS

This review shows a significant contribution of duloxetine to CMP symptom relief. This meta-analysis improved that duloxetine can significantly reduce the pain level of patients, improve depressive symptoms and global impression, and has no obvious serious adverse reactions. However, additional studies are required to confirm the relationship between psychological diseases and chronic pain and explore their internal links.

Multiple modulatory roles of serotonin in chronic pain and injury-related anxiety

Chronic pain is long-lasting pain that often persists during chronic diseases or after recovery from disease or injury. It often causes serious side effects, such as insomnia, anxiety, or depression which negatively impacts the patient's overall quality of life. Serotonin (5-HT) in the central nervous system (CNS) has been recognized as an important neurotransmitter and neuromodulator which regulates various physiological functions, such as pain sensation, cognition, and emotions-especially anxiety and depression. Its widespread and diverse receptors underlie the functional complexity of 5-HT in the CNS. Recent studies found that both chronic pain and anxiety are associated with synaptic plasticity in the anterior cingulate cortex (ACC), the insular cortex (IC), and the spinal cord. 5-HT exerts multiple modulations of synaptic transmission and plasticity in the ACC and the spinal cord, including activation, inhibition, and biphasic actions. In this review, we will discuss the multiple actions of the 5-HT system in both chronic pain and injury-related anxiety, and the synaptic mechanisms behind them. It is likely that the specific 5-HT receptors would be new promising therapeutic targets for the effective treatment of chronic pain and injury-related anxiety in the future.

Glia Signaling and Brain Microenvironment in Migraine

Migraine is a complicated neurological disorder affecting 6% of men and 18% of women worldwide. Various mechanisms, including neuroinflammation, oxidative stress, altered mitochondrial function, neurotransmitter disturbances, cortical hyperexcitability, genetic factors, and endocrine system problems, are responsible for migraine. However, these mechanisms have not completely delineated the pathophysiology behind migraine, and they should be further studied. The brain microenvironment comprises neurons, glial cells, and vascular structures with complex interactions. Disruption of the brain microenvironment is the main culprit behind various neurological disorders. Neuron-glia crosstalk contributes to hyperalgesia in migraine. In the brain, microenvironment and related peripheral regulatory circuits, microglia, astrocytes, and satellite cells are necessary for proper function. These are the most important cells that could induce migraine headaches by disturbing the balance of the neurotransmitters in the nervous system. Neuroinflammation and oxidative stress are the prominent reactions glial cells drive during migraine. Understanding the role of cellular and molecular components of the brain microenvironment on the major neurotransmitters engaged in migraine pathophysiology facilitates the development of new therapeutic approaches with higher effectiveness for migraine headaches. Investigating the role of the brain microenvironment and neuroinflammation in migraine may help decipher its pathophysiology and provide an opportunity to develop novel therapeutic approaches for its management. This review aims to discuss the neuron-glia interactions in the brain microenvironment during migraine and their potential role as a therapeutic target for the treatment of migraine.

The mechanistic basis for the effects of electroacupuncture on neuropathic pain within the central nervous system

Nociceptive signaling responses to painful stimuli are transmitted to the central nervous system (CNS) from the afferent nerves of the periphery through a series of neurotransmitters and associated signaling mechanisms. Electroacupuncture (EA) is a pain management strategy that is widely used, with clinical evidence suggesting that a frequency of 2-10 Hz is better able to suppress neuropathic pain in comparison to higher frequencies such as 100 Hz. While EA is widely recognized as a viable approach to alleviating neuralgia, the mechanistic basis underlying such analgesic activity remains poorly understood. The present review offers an overview of current research pertaining to the mechanisms whereby EA can alleviate neuropathic pain in the CNS, with a particular focus on the serotonin/norepinephrine, endogenous opioid, endogenous cannabinoid, amino acid neurotransmitter, and purinergic pathways. Moreover, the corresponding neurotransmitters, neuromodulatory compounds, neuropeptides, and associated receptors that shape these responses are discussed. Together, this review seeks to provide a robust foundation for further studies of the EA-mediated alleviation of neuropathic pain.

Development and challenges in the discovery of 5-HT1A and 5-HT7 receptor ligands

Serotonin (5-hydroxytryptamine) is a small molecule that acts both in the central and peripheral nervous system as a neurotransmitter and a hormone, respectively. Serotonin is synthesized via a multi-stage pathway beginning with l-tryptophan, which is converted by an enzyme called tryptophan hydroxylase into L-5-Hydroxytryptophan. It is well-known for its significance in the control of mood, anxiety, depression, and insomnia as well as in normal human functions such as sleep, sexual activity, and appetite. Thus, for medical chemists and pharmaceutical firms, serotonin is one of the most desirable targets. Among the seven different classes of serotonin receptors, the 5-HT_1A was one of the first discovered serotonin receptors, and the 5-HT₇ was the last addition to the serotonin receptor family. Both the classes were thoroughly examined. 5-HT_1A neurotransmission-related dysfunctions are linked to many psychological conditions such as anxiety, depression, and movement disorders. 5-HT₇ is a member of the cell surface receptor GPCR superfamily and is regulated by the serotonin neurotransmitter. It has been the focus of intensive research efforts since its discovery, which was prompted by its presence in functionally important regions of the brain. The thalamus and hypothalamus have the highest 5-HT₇ receptor densities. They are also found in the hippocampus and cortex at higher densities. Thermoregulation, circadian rhythm, learning and memory, and sleep are all associated with the 5-HT₇ receptor. It is also suspected that this receptor may be involved in the control of mood, indicating that it may be a beneficial target for depression treatment. Several differently structured molecules such as aminotetralins, ergolines, arylpiperazines, indolylalkylamines, aporphines, and aryloxyalkyl-amines are known to bind to 5-HT_1A and 5-HT₇ receptor sites. In brain serotonin receptors 5-HT_1A and 5-HT₇ are strongly co-expressed in regions involved in depression. However, their functional interaction has not been identified. An overview of the 5-HT_1A and 5-HT₇ receptor ligands belonging to different chemical groups is mentioned in this review.

Potential Roles of 5-HT3 Receptor Antagonists in Reducing Chemotherapy-induced Peripheral Neuropathy (CIPN)

5-HT₃ receptor antagonists corresponding to ondansetron, granisetron, tropisetron, and palonosetron are clinically accustomed to treating nausea and emesis in chemotherapy patients. However, current and previous studies reveal novel potentials of those ligands in other diseases involving the nervous system, such as addiction, pruritus, and neurological disorders, such as anxiety, psychosis, nociception, and cognitive function. This review gathers existing studies to support the role of 5-HT₃ receptors in CIPN modulation. It has been reported that chemotherapy drugs increase the 5-HT content that binds with the 5-HT₃ receptor, which later induces pain. As also shown in pre-clinical and clinical studies that various neuropathic pains could be blocked by the 5-HT₃ receptor antagonists, we proposed that 5-HT₃ receptor antagonists via 5- HT₃ receptors may also inhibit neuropathic pain induced by chemotherapy. Our review suggests that future studies focus more on the 5-HT₃ receptor antagonists and their modulation in CIPN to reduce the gap in the current pharmacotherapy for cancer-related pain.

An unexpected connection: A narrative review of the associations between Gut Microbiome and Musculoskeletal Pain

PURPOSE

Multiple diverse factors contribute to musculoskeletal pain, a major cause of physical dysfunction and health-related costs worldwide. Rapidly growing evidence demonstrates that the gut microbiome has overarching influences on human health and the body's homeostasis and resilience to internal and external perturbations. This broad role of the gut microbiome is potentially relevant and connected to musculoskeletal pain, though the literature on the topic is limited. Thus, the literature on the topic of musculoskeletal pain and gut microbiome was explored.

METHODS

This narrative review explores the vast array of reported metabolites associated with inflammation and immune-metabolic response, which are known contributors to musculoskeletal pain. Moreover, it covers known modifiable (e.g., diet, lifestyle choices, exposure to prescription drugs, pollutants, and chemicals) and non-modifiable factors (e.g., gut architecture, genetics, age, birth history, and early feeding patterns) that are known to contribute to changes to the gut microbiome. Particular attention is devoted to modifiable factors, as the ultimate goal of researching this topic is to implement gut microbiome health interventions into clinical practice.

RESULTS

Overall, numerous associations exist in the literature that could converge on the gut microbiome's pivotal role in musculoskeletal health. Particularly, a variety of metabolites that are either directly produced or indirectly modulated by the gut microbiome have been highlighted.

CONCLUSION

The review highlights noticeable connections between the gut and musculoskeletal health, thus warranting future research to focus on the gut microbiome's role in musculoskeletal conditions.

Bibliometric and visual analysis of research on the links between the gut microbiota and pain from 2002 to 2021

BACKGROUND AND AIMS

The gut microbiota is involved in the regulation of pain, which is proved by plenty of evidence. Although a substantial quantity of research on the link between the gut microbiota and pain has emerged, no study has focused on the bibliometric analysis of this topic. We aim to present a bibliometric review of publications over the past 20 years and predict research hot spots.

METHODS

Relevant publications between 2002 and 2021 were extracted from the Science Citation Index-Expanded (SCI-EXPANDED) of the Web of Science Core Collection (WoSCC) database on April 22, 2022. CiteSpace (version 5.8 R3c), VOSviewer, the Online Analysis Platform of Literature Metrology, and the R package bibliometrix were used to analyze and visualize.

RESULTS

A total of 233 articles have been published between 2002 and 2021. The number of publication outputs increased rapidly since 2016. The collaboration network revealed that the USA, Baylor College of Medicine, and Vassilia Theodorou were the most influential country, institute, and scholar, respectively. Alimentary pharmacology and therapeutics and Gut were the most co-cited journal and Neurogastroenterology and Motility was the most productive journal. Visceral sensitivity, fibromyalgia, gastrointestinal, chronic pain, stress, gut microbiome, LGG, brain-gut axis, SLAB51, and sequencing were the top 10 clusters in co-occurrence cluster analysis. Keyword burst detection indicated that the brain-gut axis and short-chain fatty acid were the current research hot spots.

CONCLUSION

Research on the links between the gut microbiota and pain has increased rapidly since 2016. The current research focused on the brain-gut axis and short-chain fatty acid. Accordingly, the SCFAs-mediated mechanism of pain regulation will be a research direction of great importance on the links between the gut microbiota and pain. This study provided instructive assistance to direct future research efforts on the links between the gut microbiota and pain.

The neurobiology of pain and facial movements in rodents: Clinical applications and current research

One of the most controversial aspects of the use of animals in science is the production of pain. Pain is a central ethical concern. The activation of neural pathways involved in the pain response has physiological, endocrine, and behavioral consequences, that can affect both the health and welfare of the animals, as well as the validity of research. The strategy to prevent these consequences requires understanding of the nociception process, pain itself, and how assessment can be performed using validated, non-invasive methods. The study of facial expressions related to pain has undergone considerable study with the finding that certain movements of the facial muscles (called facial action units) are associated with the presence and intensity of pain. This review, focused on rodents, discusses the neurobiology of facial expressions, clinical applications, and current research designed to better understand pain and the nociceptive pathway as a strategy for implementing refinement in biomedical research.

The Effect of Ginger and Its Sub-Components on Pain

Zingiber officinale Roscoe (ginger) has long been used as an herbal medicine to treat various diseases, and its main sub-components, [6]-gingerol and [6]-shogaol, were also reported to have anti-inflammatory, anti-oxidant, and anti-tumor effects. However, their effects on various types of pain and their underlying mechanisms of action have not been clearly analyzed and understood yet. Thus, in this review, by analyzing 16 studies that used Z. officinale, [6]-gingerol, and [6]-shogaol on mechanical, spontaneous and thermal pain, their effects and mechanisms of action have been analyzed. Pain was induced by either nerve injury or chemical injections in rodents. Nine studies analyzed the analgesic effect of Z. officinale, and four and three studies focused on [6]-gingerol and [6]-shogaol, respectively. Seven papers have demonstrated the underlying mechanism of action of their analgesic effects. Studies have focused on the spinal cord and one on the dorsal root ganglion (DRG) neurons. Involvement and change in the function of serotonergic receptors (5-HT_1A, _B, _D, and _5A), transient receptor potential vanilloid 1 (TRPV1), N-methyl-D-aspartate (NMDA) receptors, phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2), histone deacetylase 1 (HDAC1), voltage-gated sodium channel 1.8 (Na_v1.8), substance P (SP), and sciatic nerve’s morphology have been observed.

Comparative transcriptome analysis of brain and gonad reveals reproduction-related miRNAs in the giant prawn, Macrobrachium rosenbergii

Macrobrachium rosenbergii (M. rosenbergii), as a species of common prawn, is a delicacy that is consumed all over the world. By interacting with the target gene 3′-untranslated region (3'-UTR), microRNAs (miRNAs) regulate its expression and ultimately participate in the regulation of reproductive development. However, research focusing on miRNA regulation during gonadal development in M. rosenbergii received very little attention. To explore the association between miRNA and reproduction, we performed RNA sequencing (RNA-seq) on brain and gonad organs in male and female M. rosenbergii. A total of 494 miRNAs were obtained in RNA-seq, including 31 and 59 differentially expressed (DE) miRNAs in the brain and gonads, respectively. Furthermore, 9 DE miRNAs were randomly selected from the brain and gonads, and qRT-PCR was conducted to validate the results of RNA-seq. Interestingly, dpu-miR-133 was found to be substantially expressed in the male brain and testis but poorly expressed in the female brain, ovary, and other organs. Analysis of dpu-miR-133 by Targetscan and MiRanda predicted to target 5-HT1. Furthermore, the dual-luciferase reporter assay manifested that dpu-miR-133 can combine with 5-HT1. Overall, our research work provides basic data for further study on the miRNA-mediated regulation of brain, gonad, and reproductive development of study M. rosenbergii.

Role of 5-HT2A receptor in modulating glutamatergic activity in the ventrolateral orbital cortex: implication in trigeminal neuralgia

5-HT_2A receptors (5-HT_2ARs) are widely expressed in the central nervous system, including in the ventrolateral orbital cortex (VLO). The VLO is an important cortical component for pain processing. Brain 5-HT_2ARs are implicated in both pro- and anti- nociceptive functions. However, the roles of 5-HT_2ARs in the VLO in trigeminal neuralgia and neuronal synaptic function remain to be understood. We used chronic constriction injury of infraorbital nerve (IoN-CCI) model and shRNA mediated gene knockdown in mice to investigate the role of 5-HT_2ARs in the VLO in trigeminal neuralgia. We found that knockdown of 5-HT_2ARs in the VLO aggravated spontaneous pain and mechanical allodynia in mice after IoN-CCI. At the synaptic level, decreasing 5-HT_2AR expression by shRNA or inhibition of 5-HT_2AR activity by its antagonist ketanserin decreased the frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) of the neurons in the VLO, whereas 5-HT_2AR partial agonist 2,5-Dimethoxy-4-iodoamphetamine (DOI) enhanced sEPSCs of the neurons in the VLO. In summary, 5-HT_2ARs in the VLO modulate the trigeminal pain by regulating neuronal glutamatergic activity.

Switch of serotonergic descending inhibition into facilitation by a spinal chloride imbalance in neuropathic pain

Descending control from the brain to the spinal cord shapes our pain experience, ranging from powerful analgesia to extreme sensitivity. Increasing evidence from both preclinical and clinical studies points to an imbalance toward descending facilitation as a substrate of pathological pain, but the underlying mechanisms remain unknown. We used an optogenetic approach to manipulate serotonin (5-HT) neurons of the nucleus raphe magnus that project to the dorsal horn of the spinal cord. We found that 5-HT neurons exert an analgesic action in naïve mice that becomes proalgesic in an experimental model of neuropathic pain. We show that spinal KCC2 hypofunction turns this descending inhibitory control into paradoxical facilitation; KCC2 enhancers restored 5-HT-mediated descending inhibition and analgesia. Last, combining selective serotonin reuptake inhibitors (SSRIs) with a KCC2 enhancer yields effective analgesia against nerve injury-induced pain hypersensitivity. This uncovers a previously unidentified therapeutic path for SSRIs against neuropathic pain.

Defensive and Emotional Behavior Modulation by Serotonin in the Periaqueductal Gray

Serotonin 5-hydroxytryptamine (5-HT) is a key neurotransmitter for the modulation and/or regulation of numerous physiological processes and psychiatric disorders (e.g., behaviors related to anxiety, pain, aggressiveness, etc.). The periaqueductal gray matter (PAG) is considered an integrating center for active and passive defensive behaviors, and electrical stimulation of this area has been shown to evoke behavioral responses of panic, fight-flight, freezing, among others. The serotonergic activity in PAG is influenced by the activation of other brain areas such as the medial hypothalamus, paraventricular nucleus of the hypothalamus, amygdala, dorsal raphe nucleus, and ventrolateral orbital cortex. In addition, activation of other receptors within PAG (i.e., CB1, Oxytocin, µ-opioid receptor (MOR), and γ-aminobutyric acid (GABA_A)) promotes serotonin release. Therefore, this review aims to document evidence suggesting that the PAG-evoked behavioral responses of anxiety, panic, fear, analgesia, and aggression are influenced by the activation of 5-HT_1A and 5-HT_2A/C receptors and their participation in the treatment of various mental disorders.

5-HT7 Receptors Regulate Excitatory-Inhibitory Balance in Mouse Spinal Cord Dorsal Horn

Serotonergic receptors of the 5-HT₇ type (5-HT₇Rs) are widely expressed in the central nervous system (CNS), where they modulate several functions, such as pain. Behavioral experiments in vivo have shown both anti- and pro-nociceptive actions of 5-HT₇Rs, although an analgesic effect seems to be prevalent. In the spinal cord dorsal horn, the mechanisms involved in 5-HT₇R-mediated synaptic modulation are still poorly understood, especially those regarding the control of synaptic inhibition. The present study investigated the modulation exerted by 5-HT₇Rs on dorsal horn excitatory and inhibitory synaptic circuits, by performing patch-clamp recordings from lamina II neurons in mouse spinal cord slices. Our results show that applying the selective 5-HT₇ agonist LP-211 facilitates glutamatergic release by enhancing the frequency of spontaneous postsynaptic currents (sEPSCs) and increasing the peak amplitude of excitatory postsynaptic currents (EPSCs) evoked by dorsal root stimulation. The effects on sEPSCs were still observed in the presence of the 5-HT_1A antagonist WAY-100635, while the 5-HT₇ antagonist SB-269970 blocked them. LP-211 was also able to increase the release of gamma-aminobutyric acid (GABA) and glycine, as shown by the increase of spontaneous inhibitory currents (sIPSC) frequency and evoked inhibitory postsynaptic currents (IPSC) amplitude. LP-211 was proved to be more effective in potentiating synaptic inhibition as compared to excitation: consistently, 5-HT₇R activation significantly enhanced the excitability of tonic firing neurons, mainly corresponding to inhibitory interneurons. Our data bring new insights into the mechanisms of synaptic modulation mediated by 5-HT₇Rs in the dorsal horn. Stronger impact on synaptic inhibition supports the hypothesis that these receptors may play an anti-nociceptive role in the spinal cord of naïve animals.

[6]-Shogaol Attenuates Oxaliplatin-Induced Allodynia through Serotonergic Receptors and GABA in the Spinal Cord in Mice

Although oxaliplatin is a well-known anti-cancer agent used for the treatment of colorectal cancer, treated patients often experience acute cold and mechanical allodynia as side effects. Unfortunately, no optimal treatment has been developed yet. In this study, [6]-shogaol (10 mg/kg, i.p.), which is one of the major bioactive components of Zingiber officinale roscoe (Z. officinale), significantly alleviated allodynia induced by oxaliplatin (6 mg/kg, i.p.) injection. Cold and mechanical allodynia were assessed by acetone drop and von Frey filament tests, respectively. The analgesic effect of [6]-shogaol was blocked by the intrathecal injection of 5-HT1A, 5-HT3, and GABAB receptor antagonists, NAN-190 (1 μg), MDL-72222 (15 μg), and CGP 55845 (10 μg), respectively. Furthermore, oxaliplatin injection lowered the GABA concentration in the superficial laminae of the spinal dorsal horn, whereas [6]-shogaol injection significantly elevated it. The GAD (glutamic acid decarboxylase) 65 concentration also increased after [6]-shogaol administration. However, pre-treatment of NAN-190 completely inhibited the increased GABA induced by [6]-shogaol in the spinal dorsal horn, whereas MDL-72222 partially blocked the effect. Altogether, these results suggest that [6]-shogaol could attenuate oxaliplatin-induced cold and mechanical allodynia through 5-HT_1A and 5-HT₃ receptor antagonists located in the GABAergic neurons in the spinal dorsal horn in mice.

If the doors of perception were cleansed, would chronic pain be relieved? Evaluating the benefits and risks of psychedelics

Psychedelic substances have played important roles in diverse cultures, and ingesting various plant preparations to evoke altered states of consciousness has been described throughout recorded history. Accounts of the subjective effects of psychedelics typically focus on spiritual and mystical-type experiences, including feelings of unity, sacredness, and transcendence. Over the past two decades, there has been increasing interest in psychedelics as treatments for various medical disorders, including chronic pain. Although concerns about adverse medical and psychological effects contributed to their controlled status, contemporary knowledge of psychedelics suggests that risks are relatively rare when patients are carefully screened, prepared, and supervised. Clinical trial results have provided support for the effectiveness of psychedelics in different psychiatric conditions. However, there are only a small number of generally uncontrolled studies of psychedelics in patients with chronic pain (e.g., cancer pain, phantom limb pain, migraine, and cluster headache). Challenges in evaluating psychedelics as treatments for chronic pain include identifying neurobiologic and psychosocial mechanisms of action and determining which pain conditions to investigate. Truly informative proof-of-concept and confirmatory randomized clinical trials will require careful selection of control groups, efforts to minimize bias from unblinding, and attention to the roles of patient mental set and treatment setting. Perspective: There is considerable promise for the use of psychedelic therapy for pain, but evidence-based recommendations for the design of future studies are needed to ensure that the results of this research are truly informative.

Sex-specific characteristics of cells expressing the cannabinoid 1 receptor in the dorsal horn of the lumbar spinal cord

It is becoming increasingly clear that robust sex differences exist in the processing of acute and chronic pain in both rodents and humans. However, the underlying mechanism has not been well characterized. The dorsal horn of the lumbar spinal cord is the fundamental building block of ascending and descending pain pathways. It has been shown that numerous neurotransmitter and neuromodulator systems in the spinal cord, including the endocannabinoid system and its main receptor, the cannabinoid 1 receptor (CB₁ R), play vital roles in processing nociceptive information. Our previous findings have shown that CB₁ R mRNA is widely expressed in the brain in sex-dependent patterns. However, the sex-, lamina-, and cell-type-specific characteristics of CB₁ R expression in the spinal cord have not been fully described. In this study, the CB₁ R-iCre-EGFP mouse strain was generated to label and identify CB₁ R-positive (CB₁ R^GFP ) cells. We reported no sex difference in CB₁ R expression in the lumbar dorsal horn of the spinal cord, but a dynamic distribution within superficial laminae II and III in female mice between estrus and nonestrus phases. Furthermore, the cell-type-specific CB₁ R expression pattern in the dorsal horn was similar in both sexes. Over 50% of CB₁ R^GFP cells were GABAergic neurons, and approximately 25% were glycinergic and 20-30% were glutamatergic neurons. The CB₁ R-expressing cells also represented a subset of spinal projection neurons. Overall, our work indicates a highly consistent distribution pattern of CB₁ R^GFP cells in the dorsal horn of lumbar spinal cord in males and females.

Chronic Pain in Musculoskeletal Diseases: Do You Know Your Enemy?

Musculoskeletal pain is a condition that characterises several diseases and represents a constantly growing issue with enormous socio-economic burdens, highlighting the importance of developing treatment algorithms appropriate to the patient’s needs and effective management strategies. Indeed, the algic condition must be assessed and treated independently of the underlying pathological process since it has an extremely negative impact on the emotional and psychic aspects of the individual, leading to isolation and depression. A full understanding of the pathophysiological mechanisms involved in nociceptive stimulation and central sensitization is an important step in improving approaches to musculoskeletal pain. In this context, the bidirectional relationship between immune cells and neurons involved in nociception could represent a key point in the understanding of these mechanisms. Therefore, we provide an updated overview of the magnitude of the musculoskeletal pain problem, in terms of prevalence and costs, and summarise the role of the most important molecular players involved in the development and maintenance of pain. Finally, based on the pathophysiological mechanisms, we propose a model, called the “musculoskeletal pain cycle”, which could be a useful tool to counteract resignation to the algic condition and provide a starting point for developing a treatment algorithm for the patient with musculoskeletal pain.

Serotonin-Mediated Activation of Serotonin Receptor Type 1 Oppositely Modulates Voltage-Gated Calcium Channel Currents in Rat Sensory Neurons Innervating Hindlimb Muscle

Serotonin (5-HT) is a multifaceted neurotransmitter that has been described to play a role as a peripheral inflammatory mediator when released in ischemic or injured muscle. Dorsal root ganglia (DRG) neurons are key sensors of noxious stimuli that are released under inflammatory conditions or mechanical stress. Little information is available on the specific 5-HT receptor subtypes expressed in primary afferents that help regulate reflex pressor responses. In the present study, the whole-cell patch-clamp technique was employed to examine the modulation of voltage-gated calcium channel (CaV) 2.2 currents by 5-HT and to identify the 5-HT receptor subtype(s) mediating this response in acutely dissociated rat DRG neurons innervating triceps surae muscle. Our results indicate that exposure of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI)-labeled DRG neurons to 5-HT can exert three modulatory effects on CaV currents: high inhibition, low inhibition, and enhancement. Both 5-HT-mediated inhibition responses were blocked after pretreatment with pertussis toxin (PTX), indicating that 5-HT receptors are coupled to CaV2.2 via Gα i/o protein subunits. Application of selective serotonin receptor type 1 (5-HT1) agonists revealed that modulation of CaV2.2 currents occurs primarily after 5-HT1A receptor subtype stimulation and minimally from 5-HT1D activation. Finally, the intrathecal administration of the selective 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), significantly (P < 0.05) decreased the pressor response induced by intra-arterial administration of lactic acid. This suggests that 5-HT1A receptors are expressed presynaptically on primary afferent neurons innervating triceps surae muscle. Our findings indicate that preferential stimulation of 5-HT1 receptors, expressed on thin fiber muscle afferents, serves to regulate the reflex pressor response to metabolic stimuli. SIGNIFICANCE STATEMENT: The monoamine serotonin (5-HT), released under ischemic conditions, can contribute to the development of inflammation that negatively affects the exercise pressor reflex. The 5-HT receptor subtype and signaling pathway that underlies calcium channel modulation in dorsal root ganglia afferents, innervating hindlimb muscles, are unknown. We show that 5-HT can either block (primarily via serotonin receptor type 1 (5-HT1)A subtypes) or enhance voltage-gated calcium channel (CaV2.2) currents. Our findings suggest 5-HT exhibits receptor subtype selectivity, providing a complexity of cellular responses.

Recent Advances of Pyridinone in Medicinal Chemistry

Pyridinones have been adopted as an important block in medicinal chemistry that could serve as hydrogen bond donors and acceptors. With the help of feasible synthesis routes via established condensation reactions, the physicochemical properties of such a scaffold could be manipulated by adjustment of polarity, lipophilicity, and hydrogen bonding, and eventually lead to its wide application in fragment-based drug design, biomolecular mimetics, and kinase hinge-binding motifs. In addition, most pyridinone derivatives exhibit various biological activities ranging from antitumor, antimicrobial, anti-inflammatory, and anticoagulant to cardiotonic effects. This review focuses on recent contributions of pyridinone cores to medicinal chemistry, and addresses the structural features and structure–activity relationships (SARs) of each drug-like molecule. These advancements contribute to an in-depth understanding of the potential of this biologically enriched scaffold and expedite the development of its new applications in drug discovery.

Microbiota and Pain: Save Your Gut Feeling

Recently, a growing body of evidence has emerged regarding the interplay between microbiota and the nervous system. This relationship has been associated with several pathological conditions and also with the onset and regulation of pain. Dysregulation of the axis leads to a huge variety of diseases such as visceral hypersensitivity, stress-induced hyperalgesia, allodynia, inflammatory pain and functional disorders. In pain management, probiotics have shown promising results. This narrative review describes the peripheral and central mechanisms underlying pain processing and regulation, highlighting the role of the gut-brain axis in the modulation of pain. We summarized the main findings in regard to the stress impact on microbiota’s composition and its influence on pain perception. We also focused on the relationship between gut microbiota and both visceral and inflammatory pain and we provided a summary of the main evidence regarding the mechanistic effects and probiotics use.

Altered Pain Processing Associated with Administration of Dopamine Agonist and Antagonist in Healthy Volunteers

Striatal dopamine dysfunction is associated with the altered top-down modulation of pain processing. The dopamine D2-like receptor family is a potential substrate for such effects due to its primary expression in the striatum, but evidence for this is currently lacking. Here, we investigated the effect of pharmacologically manipulating striatal dopamine D2 receptor activity on the anticipation and perception of acute pain stimuli in humans. Participants received visual cues that induced either certain or uncertain anticipation of two pain intensity levels delivered via a CO₂ laser. Rating of the pain intensity and unpleasantness was recorded. Brain activity was recorded with EEG and analysed via source localisation to investigate neural activity during the anticipation and receipt of pain. Participants completed the experiment under three conditions, control (Sodium Chloride), D2 receptor agonist (Cabergoline), and D2 receptor antagonist (Amisulpride), in a repeated-measures, triple-crossover, double-blind study. The antagonist reduced an individuals’ ability to distinguish between low and high pain following uncertain anticipation. The EEG source localisation showed that the agonist and antagonist reduced neural activations in specific brain regions associated with the sensory integration of salient stimuli during the anticipation and receipt of pain. During anticipation, the agonist reduced activity in the right mid-temporal region and the right angular gyrus, whilst the antagonist reduced activity within the right postcentral, right mid-temporal, and right inferior parietal regions. In comparison to control, the antagonist reduced activity within the insula during the receipt of pain, a key structure involved in the integration of the sensory and affective aspects of pain. Pain sensitivity and unpleasantness were not changed by D2R modulation. Our results support the notion that D2 receptor neurotransmission has a role in the top-down modulation of pain.

Spinal 5-HT2A Receptor is Involved in Electroacupuncture Inhibition of Chronic Pain

Knee osteoarthritis (KOA) is a highly prevalent, chronic joint disorder, and it is a typical disease which can develop chronic pain. Our previous study has proved that endocannabinoid (2-AG)-CB1R-GABA-5-HT pathway is involved in electroacupuncture (EA) mediated inhibition of chronic pain. However, it is still unclear which among the 5-HT receptor subtype is involved in EA evoked 5-HT mediated inhibition of chronic pain in the dorsal spinal cord. 5-HT_2A is a G protein-coupled receptor and it is involved in 5-HT descending pain modulation system. We found that EA treatment at frequency of 2 Hz +1 mA significantly increased the expression of 5-HT_2A receptor in the dorsal spinal cord and intrathecal injection of 5-HT_2A receptor antagonist or agonist reversed or mimicked the analgesic effect of EA in each case respectively. Intrathecal injection of a selective GABA_A receptor antagonist Bicuculline also reversed the EA effect on pain hypersensitivity. Additionally, EA treatment reversed the reduced expression of GABA_A receptor and KCC2 in the dorsal spinal cord of KOA mice. Furthermore, we demonstrated that intrathecal 5-HT_2A receptor antagonist/agonist reversed or mimicked the effect of EA up-regulate of KCC2 expression, respectively. Similarly, intrathecal injection of PLC and PKC inhibitors prevented both anti-allodynic effect and up-regulation of KCC2 expression by EA treatment. Our data suggest that EA treatment up-regulated KCC2 expression through activating 5-HT_2A-Gq-PLC-PKC pathway and enhanced the inhibitory function of GABA_A receptor, thereby inhibiting chronic pain in a mouse model of KOA.

Dietary supplementation of gingerols- and shogaols-enriched ginger root extract attenuate pain-associated behaviors while modulating gut microbiota and metabolites in rats with spinal nerve ligation

Neuroinflammation is a central factor in neuropathic pain (NP). Ginger is a promising bioactive compound in NP management due to its anti-inflammatory property. Emerging evidence suggests that gut microbiome and gut-derived metabolites play a key role in NP. We evaluated the effects of two ginger root extracts rich in gingerols (GEG) and shogaols (SEG) on pain sensitivity, anxiety-like behaviors, circulating cell-free mitochondrial DNA (ccf-mtDNA), gut microbiome composition, and fecal metabolites in rats with NP. Sixteen male rats were divided into four groups: sham, spinal nerve ligation (SNL), SNL+0.75%GEG in diet, and SNL+0.75%SEG in diet groups for 30 days. Compared to SNL group, both SNL+GEG and SNL+SEG groups showed a significant reduction in pain- and anxiety-like behaviors, and ccf-mtDNA level. Relative to the SNL group, both SNL+GEG and SNL+SEG groups increased the relative abundance of Lactococcus, Sellimonas, Blautia, Erysipelatoclostridiaceae, and Anaerovoracaceae, but decreased that of Prevotellaceae UCG-001, Rikenellaceae RC9 gut group, Mucispirillum and Desulfovibrio, Desulfovibrio, Anaerofilum, Eubacterium siraeum group, RF39, UCG-005, Lachnospiraceae NK4A136 group, Acetatifactor, Eubacterium ruminantium group, Clostridia UCG-014, and an uncultured Anaerovoracaceae. GEG and SEG had differential effects on gut-derived metabolites. Compared to SNL group, SNL+GEG group had higher level of 1'-acetoxychavicol acetate, (4E)-1,7-Bis(4-hydroxyphenyl)-4-hepten-3-one, NP-000629, 7,8-Dimethoxy-3-(2-methyl-3-buten-2-yl)-2H-chromen-2-one, 3-{[4-(2-Pyrimidinyl)piperazino]carbonyl}-2-pyrazinecarboxylic acid, 920863, and (1R,3R,7R,13S)-13-Methyl-6-methylene-4,14,16-trioxatetracyclo[11.2.1.0∼1,10∼.0∼3,7∼]hexadec-9-en-5-one, while SNL+SEG group had higher level for (±)-5-[(tert-Butylamino)-2'-hydroxypropoxy]-1_2_3_4-tetrahydro-1-naphthol and dehydroepiandrosteronesulfate. In conclusion, ginger is a promising functional food in the management of NP, and further investigations are necessary to assess the role of ginger on gut-brain axis in pain management.

SAHA Inhibits Somatic Hyperalgesia Induced by Stress Combined with Orofacial Inflammation Through Targeting Different Spinal 5-HT Receptor Subtypes

Epigenetic regulation of gene expression has been implicated in the development of chronic pain. However, little is known about whether this regulation is involved in the development and treatment of chronic pain comorbidities such as fibromyalgia syndrome (FMS) and temporomandibular disorder (TMD), a comorbidity predominantly occurring among women. Here we explored the impact of the histone deacetylase (HDAC) inhibitor suberoylanilide hydroxamic acid (SAHA) on somatic hyperalgesia induced by stress or stress combined with orofacial inflammation, which mimicked the comorbidity of FMS and TMD in rats. Our data showed that somatic thermal hyperalgesia and mechanical allodynia induced by both conditions were completely prevented by intrathecal injection of SAHA, which upregulated 5-HT_2C receptors but downregulated 5-HT₃ receptors in the spinal dorsal horn. Subsequent spinal administration of RS102221 to inhibit 5-HT_2C receptors or SR57227 to activate 5-HT₃ receptors reversed the analgesic effect of SAHA under both conditions. These results indicate that SAHA attenuates the pro-nociceptive effects of stress combined with orofacial inflammation and the effects of stress alone. This likely occurs through epigenetic regulation of spinal 5-HT_2C and 5-HT₃ receptor expression, suggesting that SAHA has potential therapeutic value in FMS or comorbid FMS-TMD patients with somatic hyperalgesia.

The interplay between 5-HT2C and 5-HT3A receptors in the dorsal periaqueductal gray mediates anxiety-like behavior in mice

The monoamine neurotransmitter serotonin (5-HT) modulates anxiety by its activity on 5-HT_2C receptors (5-HT_2CR) expressed in the dorsal periaqueductal gray (dPAG). Here, we investigated the presence of 5-HT_3A receptors (5-HT_3AR) in the dPAG, and the interplay between 5-HT_2CR and 5-HT_3AR in the dPAG in mediating anxiety-like behavior in mice. We found that 5-HT_3AR is expressed in the dPAG and the blockade of these receptors using intra-dPAG infusion of ondansetron (5-HT_3AR antagonist; 3.0 nmol) induced an anxiogenic-like effect. The activation of 5-HT_3ABR by the infusion of mCPBG [1-(m-Chlorophenyl)-biguanide; 5-HT₃R agonist] did not alter anxiety-like behaviors. In addition, blockade of 5-HT_3AR (1.0 nmol) prevented the anxiolytic-like effect induced by the infusion of the 5-HT_2CR agonist mCPP (1-(3-chlorophenyl) piperazine; 0.03 nmol). None of the treatment effects on anxiety-like behaviors altered the locomotor activity levels. The present results suggest that the anxiolytic-like effect exerted by serotonin activity on 5-HT_2CR in the dPAG is modulated by 5-HT_3AR expressed in same region.

The Cannabidiol Analog PECS-101 Prevents Chemotherapy-Induced Neuropathic Pain via PPARγ Receptors

Chemotherapy-induced peripheral neuropathy (CIPN) is the main dose-limiting adverse effect of chemotherapy drugs such as paclitaxel (PTX). PTX causes marked molecular and cellular damage, mainly in the peripheral nervous system, including sensory neurons in the dorsal root ganglia (DRG). Several studies have shown the therapeutic potential of cannabinoids, including cannabidiol (CBD), the major non-psychotomimetic compound found in the Cannabis plant, to treat peripheral neuropathies. Here, we investigated the efficacy of PECS-101 (former HUF-101), a CBD fluorinated analog, on PTX-induced neuropathic pain in mice. PECS-101, administered after the end of treatment with PTX, did not reverse mechanical allodynia. However, PECS-101 (1 mg/kg) administered along with PTX treatment caused a long-lasting relief of the mechanical and cold allodynia. These effects were blocked by a PPARγ, but not CB1 and CB2 receptor antagonists. Notably, the effects of PECS-101 on the relief of PTX-induced mechanical and cold allodynia were not found in macrophage-specific PPARγ-deficient mice. PECS-101 also decreased PTX-induced increase in Tnf, Il6, and Aif1 (Iba-1) gene expression in the DRGs and the loss of intra-epidermal nerve fibers. PECS-101 did not alter motor coordination, produce tolerance, or show abuse potential. In addition, PECS-101 did not interfere with the chemotherapeutic effects of PTX. Thus, PECS-101, a new fluorinated CBD analog, could represent a novel therapeutic alternative to prevent mechanical and cold allodynia induced by PTX potentially through the activation of PPARγ in macrophages.

5-HT Receptors and Temperature Homeostasis

The present review summarizes the data concerning the influence of serotonin (5-HT) receptors on body temperature in warm-blooded animals and on processes associated with its maintenance. This review includes the most important part of investigations from the first studies to the latest ones. The established results on the pharmacological activation of 5-HT1A, 5-HT3, 5-HT7 and 5-HT2 receptor types are discussed. Such activation of the first 3 type of receptors causes a decrease in body temperature, whereas the 5-HT2 activation causes its increase. Physiological mechanisms leading to changes in body temperature as a result of 5-HT receptors' activation are discussed. In case of 5-HT1A receptor, they include an inhibition of shivering and non-shivering thermogenesis, as well simultaneous increase of peripheral blood flow, i.e., the processes of heat production and heat loss. The physiological processes mediated by 5-HT2 receptor are opposite to those of the 5-HT1A receptor. Mechanisms of 5-HT3 and 5-HT7 receptor participation in these processes are yet to be studied in more detail. Some facts indicating that in natural conditions, without pharmacological impact, these 5-HT receptors are important links in the system of temperature homeostasis, are also discussed.

Connections between Immune-Derived Mediators and Sensory Nerves for Itch Sensation

Although histamine is a well-known itch mediator, histamine H1-receptor blockers often lack efficacy in chronic itch. Recent molecular and cellular based studies have shown that non-histaminergic mediators, such as proteases, neuropeptides and cytokines, along with their cognate receptors, are involved in evocation and modulation of itch sensation. Many of these molecules are produced and secreted by immune cells, which act on sensory nerve fibers distributed in the skin to cause itching and sensitization. This understanding of the connections between immune cell-derived mediators and sensory nerve fibers has led to the development of new treatments for itch. This review summarizes current knowledge of immune cell-derived itch mediators and neuronal response mechanisms, and discusses therapeutic agents that target these systems.

5-HT7 Receptor Is Involved in Electroacupuncture Inhibition of Chronic Pain in the Spinal Cord

Knee osteoarthritis (KOA) is a common and disabling condition characterized by attacks of pain around the joints, and it is a typical disease that develops chronic pain. Previous studies have proved that 5-HT₁, 5-HT₂, and 5-HT₃ receptors in the spinal cord are involved in electroacupuncture (EA) analgesia. The 5-HT₇ receptor plays antinociceptive role in the spinal cord. However, it is unclear whether the 5-HT₇ receptor is involved in EA analgesia. The 5-HT₇ receptor is a stimulatory G-protein (Gs)-coupled receptor that activates adenylyl cyclase (AC) to stimulate cyclic adenosine monophosphate (cAMP) formation, which in turn activates protein kinase A (PKA). In the present study, we found that EA significantly increased the tactile threshold and the expression of the 5-HT₇ receptor in the dorsal spinal cord. Intrathecal injection of 5-HT₇ receptor agonist AS-19 mimicked the analgesic effect of EA, while a selective 5-HT₇ receptor antagonist reversed this effect. Moreover, intrathecal injection of AC and PKA antagonists prior to EA intervention prevented its anti-allodynic effect. In addition, GABA_A receptor antagonist bicuculline administered (intrathecal, i.t.) prior to EA intervention blocked the EA effect on pain hypersensitivity. Our data suggest that the spinal 5-HT₇ receptor activates GABAergic neurons through the Gs–cAMP–PKA pathway and participates in EA-mediated inhibition of chronic pain in a mouse model of KOA.

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.