Reversal of inflammatory and noninflammatory visceral pain by central or peripheral actions of sumatriptan

Molecular mechanisms of pain in acute pancreatitis: recent basic research advances and therapeutic implications

Although severe abdominal pain is the main symptom of acute pancreatitis, its mechanisms are poorly understood. An emerging body of literature evidence indicates that neurogenic inflammation might play a major role in modulating the perception of pain from the pancreas. Neurogenic inflammation is the result of a crosstalk between injured pancreatic tissue and activated neurons, which leads to an auto-amplification loop between inflammation and pain during the progression of acute pancreatitis. In this review, we summarize recent findings on the role of neuropeptides, ion channels, and the endocannabinoid system in acute pancreatitis-related pain. We also highlight potential therapeutic strategies that could be applied for managing severe pain in this disease.

Electroacupuncture Alleviates 46-Trinitrobenzene Sulfonic Acid-Induced Visceral Pain via the Glutamatergic Pathway in the Prefrontal Cortex

Visceral pain caused by inflammatory bowel disease (IBD) greatly diminishes the quality of life in affected patients. Yet, the mechanism of how IBD causes visceral pain is currently not fully understood. Previous studies have suggested that the central nervous system (CNS) and gut-brain axis (GBA) play an important role in IBD-inducing visceral pain. As one of the treatments for IBD, electroacupuncture (EA) has been used to treat various types of pain and gastrointestinal diseases in clinical practice. However, whether EA relieves the visceral pain of IBD through the gut-brain axis has not been confirmed. To verify the relationship between visceral pain and CNS, the following experiments were conducted. ¹H-NMR analysis was performed on the prefrontal cortex (PFC) tissue obtained from IBD rat models to determine the link between the metabolites and their role in EA treatment against visceral pain. Western blot assay was employed for detecting the contents of glutamate transporter excitatory amino acid transporters 2 (EAAT2) and the glutamate receptor N-methyl-D-aspartate (NMDA) to verify whether EA treatment can alleviate neurotoxic symptoms induced by abnormal increases of glutamate. Study results showed that the glutamate content was significantly increased in the PFC of TNBS-induced IBD rats. This change was reversed after EA treatment. This process was associated with increased EAAT2 expression and decreased expression of NMDA receptors in the PFC. In addition, an increase in intestinal glutamic-metabolizing bacteria was observed. In conclusion, this study suggests that EA treatment can relieve visceral pain by reducing glutamine toxicity in the PFC, and serves an alternative clinical utility.

Migraine: beyond pain

Most people who see, treat or experience migraine will be aware that its clinical manifestations exceed the symptom of head pain. However, available acute treatments so far have targeted migraine symptoms only in the context of the pain phase of an attack. The associated disability clearly involves more than just these symptoms, and the phenotype can include additional painless features, including alterations in mood, cognition and homeostasis and sensory sensitivities. Recognising these symptoms, understanding their neurobiological basis and systematically recording them prospectively in clinical therapeutic trials are likely to offer valuable pathophysiological and therapeutic insights into this complex brain disorder, ultimately helping to improve the quality of lives of sufferers. We aim to explore the multifaceted disorder that is migraine, with a particular focus on the non-painful non-aura symptoms.

Evaluation of Anti-inflammatory and Antioxidant Effects of Sumatriptan on Carbon Tetrachloride-induced Hepatotoxicity in Rats

The liver detoxifies and metabolizes many drugs and xenobiotics which may cause hepatotoxicity due to some toxic agents. Carbon tetrachloride (CCl₄) is metabolized in cytochrome P450 and its reactive radical metabolites cause lipid peroxidation, cellular injury, and apoptosis. Sumatriptan (SUM), 5-HT1B/1D receptor agonist, had anti-inflammatory and anti-oxidant effects. In this research the effect of SUM pre-treatment against CCl₄-induced hepatotoxicity was examined. Adult rats received SUM (0.1, 0.3 and 1 mg/kg; i.p.) for 3 consecutive days before CCl₄ (2 ml/kg; i.p. on the 3^rd day). The aminotransferases serum levels, tissue levels of anti-oxidant and pro-inflammatory markers and histopathological examination were evaluated. SUM (0.3 mg/kg) prevented significantly the elevation of aminotransferases versus the control group (CCl₄ group) (P<0.0001) and also, reversed meaningfully the changes of the MPO, MDA, SOD and CAT, IL-1β and TNF-α levels. Additionally, CCl₄-intoxication resulted to the disruption of lobular and cellular structures and inflammation in histopathological evaluation which is prevented by SUM (0.3 mg/kg). These data revealed that SUM (0.3 mg/kg), but no at doses 0.1 and 1 mg/kg, decreases the hepatotoxicity of induced by CCl₄ in rats.

Beyond its anti-migraine properties, sumatriptan is an anti-inflammatory agent: A systematic review

Sumatriptan is the first available medication from triptans family that was approved by the U.S. Food and Drug Administration for migraine attacks and cluster headaches in 1991. Most of its action is mediated by selective 5-HT_1B/1D receptor agonism. Recent investigations raised the possibility of repositioning of this drug to other indications beyond migraine, as increasing evidence suggests for an anti-inflammatory property of sumatriptan. We performed a literature search using PubMed, Web of Science, Scopus, and Google Scholar using "inflammation AND sumatriptan" or "inflammation AND 5HT1B/D" as the keywords. Then, articles were screened for their relevance and those directly discussing the correlation between inflammation and sumatriptan or 5HT1B/D were included. Total references reviewed or inclusion/exclusion were 340 retrieved full-text articles (n = 340), then based on critical assessment 66 of them were included in this systematic review. Our literature review indicates that at low doses, sumatriptan can reduce inflammatory markers (e.g., interleukin-1β, tumor necrosis factor-α, and nuclear factor-κB), affects caspases and changes cells lifespan. Additionally, nitric oxide synthase and nitric oxide signaling seem to be regulated by this drug. It also inhibits the release of calcitonin gene-related peptide. Sumatriptan protects against many inflammatory conditions including cardiac and mesenteric ischemia/reperfusion, skin flap, pruritus, peripheral, and central nervous system injuries such as spinal cord injury, testicular torsion-detorsion, oral mucositis, and other experimental models. Considering the safety and potency of low dose sumatriptan compared to corticosteroids and other immunosuppressive medications, it is worth to take advantage of sumatriptan in inflammatory conditions.

Sumatriptan improves the locomotor activity and neuropathic pain by modulating neuroinflammation in rat model of spinal cord injury

OBJECTIVES

To investigate the therapeutic effects of sumatriptan in a rat model of spinal cord injury (SCI) and possible anti-inflammatory and analgesic mechanisms underlying this effect.

METHODS

Using an aneurysm mini-clip model of contusive SCI, T9-10 laminectomies were performed for 60 male rats. Animals were divided into six experimental groups (n = 10 per group) as follows: a minocycline administered positive control group, a saline-vehicle negative control group, a sham-operated group, and three experimental groups which received separate doses of sumatriptan (0.1, 0.3 and 1 mg/kg). Behavioural assessments were used to evaluate locomotor activity and neuropathic pain for 28 days. At the end of the study, spinal cord tissues were collected from sacrificed animals for histopathological analysis. Levels of calcitonin gene-related peptide (CGRP) and two pro-inflammatory cytokines (tumor necrosis factor [TNF]-α and interleukin [IL]-1β) were assessed by the enzyme-linked immunosorbent assay (ELISA).

RESULTS

Sumatriptan significantly (P < 0.001) improved the locomotor activity in SCI group. Sumatriptan was also more effective than the positive control, i.e. minocycline (0.3 mg/kg). Additionally, sumatriptan and minocycline similarly attenuated the mechanical and thermal allodynia in SCI (P < 0.001). TNF-α, IL-1β and CGRP levels in sumatriptan- and minocycline-treated groups significantly (P < 0.001) decreased compared to controls. Histopathological analysis also revealed a markedly improvement in hemorrhage followed by inflammatory cell invasion, neuronal vacuolation, and cyst formation in both sumatriptan- and minocycline-treated groups compared to control animals.

CONCLUSIONS

Sumatriptan improves functional recovery from SCI through its anti-inflammatory effects and reducing pro-inflammatory and pain mediators.

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.

Brain structure and function related to headache: Brainstem structure and function in headache

OBJECTIVE

To review and discuss the literature relevant to the role of brainstem structure and function in headache.

BACKGROUND

Primary headache disorders, such as migraine and cluster headache, are considered disorders of the brain. As well as head-related pain, these headache disorders are also associated with other neurological symptoms, such as those related to sensory, homeostatic, autonomic, cognitive and affective processing that can all occur before, during or even after headache has ceased. Many imaging studies demonstrate activation in brainstem areas that appear specifically associated with headache disorders, especially migraine, which may be related to the mechanisms of many of these symptoms. This is further supported by preclinical studies, which demonstrate that modulation of specific brainstem nuclei alters sensory processing relevant to these symptoms, including headache, cranial autonomic responses and homeostatic mechanisms.

REVIEW FOCUS

This review will specifically focus on the role of brainstem structures relevant to primary headaches, including medullary, pontine, and midbrain, and describe their functional role and how they relate to mechanisms of primary headaches, especially migraine.

Is selective 5-HT1F receptor agonism an entity apart from that of the triptans in antimigraine therapy?

Migraine is a neurovascular disorder that involves activation of the trigeminovascular system and cranial vasodilation mediated by release of calcitonin gene-related peptide (CGRP). The gold standard for acute migraine treatment are the triptans, 5-HT1B/1D/(1F) receptor agonists. Their actions are thought to be mediated through activation of: (i) 5-HT1B receptors in cranial blood vessels with subsequent cranial vasoconstriction; (ii) prejunctional 5-HT1D receptors on trigeminal fibers that inhibit trigeminal CGRP release; and (iii) 5-HT1B/1D/1F receptors in central nervous system involved in (anti)nociceptive modulation. Unfortunately, coronary arteries also express 5-HT1B receptors whose activation would produce coronary vasoconstriction; hence, triptans are contraindicated in patients with cardiovascular disease. In addition, since migraineurs have an increased cardiovascular risk, it is important to develop antimigraine drugs devoid of vascular (side) effects. Ditans, here defined as selective 5-HT1F receptor agonists, were developed on the basis that most of the triptans activate trigeminal 5-HT1F receptors, which may explain part of the triptans' antimigraine action. Amongst the ditans, lasmiditan: (i) fails to constrict human coronary arteries; and (ii) is effective for the acute treatment of migraine in preliminary Phase III clinical trials. Admittedly, the exact site of action is still unknown, but lasmiditan possess a high lipophilicity, which suggests a direct action on the central descending antinociceptive pathways. Furthermore, since 5-HT1F receptors are located on trigeminal fibers, they could modulate CGRP release. This review will be particularly focussed on the similarities and differences between the triptans and the ditans, their proposed sites of action, side effects and their cardiovascular risk profile.

Review article: the many potential roles of intestinal serotonin (5-hydroxytryptamine, 5-HT) signalling in inflammatory bowel disease

BACKGROUND

Serotonin (5-hydroxytryptamine, 5-HT) is an important mediator of every major gut-related function. Recent investigations also suggest that 5-HT can influence the development and severity of inflammation within the gut, particularly in the setting of inflammatory bowel disease (IBD).

AIM

To review the roles that the intestinal serotonin signalling system plays in gut function, with a specific focus on IBD.

METHODS

We reviewed manuscripts from 1952 to 2017 that investigated and discussed roles for 5-HT signalling in gastrointestinal function and IBD, as well as the influence of inflammation on 5-HT signalling elements within the gut.

RESULTS

Inflammation appears to affect every major element of intestinal 5-HT signalling, including 5-HT synthesis, release, receptor expression and reuptake capacity. Importantly, many studies (most utilising animal models) also demonstrate that modulation of selective serotonergic receptors (via agonism of 5-HT₄ R and antagonism of 5-HT₃ R) or 5-HT signal termination (via serotonin reuptake inhibitors) can alter the likelihood and severity of intestinal inflammation and/or its complicating symptoms. However, there are few human studies that have studied these relationships in a targeted manner.

CONCLUSIONS

Insights discussed in this review have strong potential to lead to new diagnostic and therapeutic tools to improve the management of IBD and other related disorders. Specifically, strategies that focus on modifying the activity of selective serotonin receptors and reuptake transporters in the gut could be effective for controlling disease activity and/or its associated symptoms. Further studies in humans are required, however, to more completely understand the pathophysiological mechanisms underlying the roles of 5-HT in this setting.

Pathophysiology of Migraine: A Disorder of Sensory Processing

Plaguing humans for more than two millennia, manifest on every continent studied, and with more than one billion patients having an attack in any year, migraine stands as the sixth most common cause of disability on the planet. The pathophysiology of migraine has emerged from a historical consideration of the "humors" through mid-20th century distraction of the now defunct Vascular Theory to a clear place as a neurological disorder. It could be said there are three questions: why, how, and when? Why: migraine is largely accepted to be an inherited tendency for the brain to lose control of its inputs. How: the now classical trigeminal durovascular afferent pathway has been explored in laboratory and clinic; interrogated with immunohistochemistry to functional brain imaging to offer a roadmap of the attack. When: migraine attacks emerge due to a disorder of brain sensory processing that itself likely cycles, influenced by genetics and the environment. In the first, premonitory, phase that precedes headache, brain stem and diencephalic systems modulating afferent signals, light-photophobia or sound-phonophobia, begin to dysfunction and eventually to evolve to the pain phase and with time the resolution or postdromal phase. Understanding the biology of migraine through careful bench-based research has led to major classes of therapeutics being identified: triptans, serotonin 5-HT1B/1D receptor agonists; gepants, calcitonin gene-related peptide (CGRP) receptor antagonists; ditans, 5-HT1F receptor agonists, CGRP mechanisms monoclonal antibodies; and glurants, mGlu5 modulators; with the promise of more to come. Investment in understanding migraine has been very successful and leaves us at a new dawn, able to transform its impact on a global scale, as well as understand fundamental aspects of human biology.

Animal models of gastrointestinal and liver diseases. Animal models of visceral pain: pathophysiology, translational relevance, and challenges

Visceral pain describes pain emanating from the thoracic, pelvic, or abdominal organs. In contrast to somatic pain, visceral pain is generally vague, poorly localized, and characterized by hypersensitivity to a stimulus such as organ distension. Animal models have played a pivotal role in our understanding of the mechanisms underlying the pathophysiology of visceral pain. This review focuses on animal models of visceral pain and their translational relevance. In addition, the challenges of using animal models to develop novel therapeutic approaches to treat visceral pain will be discussed.

Serotonin, 5HT1 agonists, and migraine: new data, but old questions still not answered

PURPOSE OF REVIEW

The serotonergic system has long been linked to migraine but recent studies highlight how much is still unclear about this link. And recent data add to the uncertainty of where/how triptans act and why they are headache specific.

RECENT FINDINGS

Markers of 5HT levels in the brains of migraine patients show no changes between attacks. Several recent meta-analyses show the most convincing data on genetic differences in the serotonergic system for 5HT transporters. Findings of additional triptan actions on peripheral trigeminovascular neurons and in the hypothalamus add more fuel to the debate on where these drugs act. A growing list of studies show efficacy of multiple triptans and other 5HT1b/1d agonists in preclinical models of nonheadache pain arguing for reevaluation of whether these drugs have efficacy in other pain states. Despite these issues, serotonergic drugs continue to be the gold standard for abortive agents with new members on the horizon (5HT1f agonists).

SUMMARY

Given the clear efficacy of serotonergic drugs for migraine, continued study on the role of the endogenous 5HT system may lead to more novel therapies. And with the list of studies demonstrating efficacy triptans in models of nonheadache, clinical studies should address whether these drugs work for other types of pain.

Effect of DA-9701 on colorectal distension-induced visceral hypersensitivity in a rat model

BACKGROUND/AIMS

DA-9701 is a newly developed drug made from the vegetal extracts of Pharbitidis semen and Co-rydalis tuber. The aim of this study was to evaluate the effect of DA-9701 on colorectal distension (CRD)-induced visceral hypersensitivity in a rat model.

METHODS

Male Sprague-Dawley rats were subjected to neonatal colon irritation (CI) using CRD at 1 week after birth (CI group). At 6 weeks after birth, CRD was applied to these rats with a pressure of 20 to 90 mm Hg, and changes in the mean arterial pressure (MAP) were measured at baseline (i.e., without any drug administration) and after the administration of different doses of DA-9701.

RESULTS

In the absence of DA-9701, the MAP changes after CRD were significantly higher in the CI group than in the control group at all applied pressures. In the control group, MAP changes after CRD were not significantly affected by the administration of DA-9701. In the CI group, however, the administration of DA-9701 resulted in a significant decrease in MAP changes after CRD. The administration of DA-9701 at a dose of 1.0 mg/kg produced a more significant decrease in MAP changes than the 0.3 mg/kg dose.

CONCLUSIONS

The administration of DA-9701 resulted in a significant increase in pain threshold in rats with CRD-induced visceral hypersensitivity.

A systematic review of the evidence for central nervous system plasticity in animal models of inflammatory-mediated gastrointestinal pain

BACKGROUND

Abdominal pain frequently accompanies inflammatory disorders of the gastrointestinal tract (GIT), and animal models of GIT inflammation have been developed to explore the role of the central nervous system (CNS) in this process. Here, we summarize the evidence from animal studies for CNS plasticity following GIT inflammation.

METHODS

A systematic review was conducted to identify studies that: (1) used inflammation of GIT organs, (2) assessed pain or visceral hypersensitivity, and (3) presented evidence of CNS involvement. Two hundred and eight articles were identified, and 79 were eligible for analysis.

RESULTS

Rats were most widely used (76%). Most studies used adult animals (42%) with a bias toward males (74%). Colitis was the most frequently used model (78%) and 2,4,6-trinitrobenzenesulfonic acid the preferred inflammatory agent (33%). Behavioral (58%), anatomical/molecular (44%), and physiological (24%) approaches were used alone or in combination to assess CNS involvement during or after GIT inflammation. Measurement times varied widely (<1 h-> 2 wk after inflammation). Blinded outcomes were used in 42% studies, randomization in 10%, and evidence of visceral inflammation in 54%. Only 3 studies fulfilled our criteria for high methodological quality, and no study reported sample size calculations.

CONCLUSIONS

The included studies provide strong evidence for CNS plasticity following GIT inflammation, specifically in the spinal cord dorsal horn. This evidence includes altered visceromotor responses and indices of referred pain, elevated neural activation and peptide content, and increased neuronal excitability. This evidence supports continued use of this approach for preclinical studies; however, there is substantial scope to improve study design.

The TRPM8 channel forms a complex with the 5-HT(1B) receptor and phospholipase D that amplifies its reversal of pain hypersensitivity

Effective relief from chronic hypersensitive pain states remains an unmet need. Here we report the discovery that the TRPM8 ion channel, co-operating with the 5-HT(1B) receptor (5-HT(1B)R) in a subset of sensory afferents, exerts an influence at the spinal cord level to suppress central hypersensitivity in pain processing throughout the central nervous system. Using cell line models, ex vivo rat neural tissue and in vivo pain models, we assessed functional Ca(2+) fluorometric responses, protein:protein interactions, immuno-localisation and reflex pain behaviours, with pharmacological and molecular interventions. We report 5-HT(1B)R expression in many TRPM8-containing afferents and direct interaction of these proteins in a novel multi-protein signalling complex, which includes phospholipase D1 (PLD1). We provide evidence that the 5-HT(1B)R activates PLD1 to subsequently activate PIP 5-kinase and generate PIP2, an allosteric enhancer of TRPM8, achieving a several-fold increase in potency of TRPM8 activation. The enhanced activation responses of synaptoneurosomes prepared from spinal cord and cortical regions of animals with a chronic inflammatory pain state are inhibited by TRPM8 activators that were applied in vivo topically to the skin, an effect potentiated by co-administered 5-HT(1B)R agonists and attenuated by 5-HT(1B)R antagonists, while 5-HT(1B)R agents alone had no detectable effect. Corresponding results are seen when assessing reflex behaviours in inflammatory and neuropathic pain models. Control experiments with alternative receptor/TRP channel combinations reveal no such synergy. Identification of this novel receptor/effector/channel complex and its impact on nociceptive processing give new insights into possible strategies for enhanced analgesia in chronic pain.

Butyrate-induced colonic hypersensitivity is mediated by mitogen-activated protein kinase activation in rat dorsal root ganglia

OBJECTIVE

Increased faecal butyrate levels have been reported in irritable bowel syndrome. Rectal instillation of sodium butyrate (NaB) increases visceral sensitivity in rats by an unknown mechanism. We seek to examine the signal transduction pathways responsible for the enhanced neuronal excitability in the dorsal root ganglion (DRG) following NaB enemas and demonstrate that this is responsible for the colonic hypersensitivity reported in this animal model.

DESIGN

Colorectal distention (CRD) studies were performed in rats treated with NaB rectal instillation with/without intrathecal or intravenous administration of mitogen-activated protein (MAP) kinase kinase inhibitor U0126. Western blot analysis and immunocytochemistry studies elucidated intracellular signalling pathways that modulate IA. Patch-clamp recordings were performed on isolated DRG neurons treated with NaB, with/without U0126.

RESULTS

Visceromotor responses (VMR) were markedly enhanced in NaB-treated rats. Western blot analysis of DRG neurons from NaB-treated rats showed a 2.2-fold increase in phosphorylated ERK1/2 (pEKR1/2) and 1.9-fold increase in phosphorylated voltage-gated potassium channel subunit 4.2 (pKv4.2). Intrathecal or intravenous administration of U0126 reduced VMR to CRD in NaB-treated rats and prevented increases in pERK1/2 and pKv4.2. Patch-clamp recordings of isolated DRG neurons showed that NaB caused a reduction in IA to 48.9%±1.4% of control and an increase in neuronal excitability, accompanied by a twofold increase in pERK1/2 and pKv4.2. Concurrent U0126 administration prevented these changes.

CONCLUSIONS

Visceral hypersensitivity induced by colonic NaB treatment is mediated by activation of the MAP kinase-ERK1/2 pathway, which phosphorylates Kv4.2. This results in a reduction in IA and an enhancement of DRG neuronal excitability.

Capturing the aversive state of cephalic pain preclinically

OBJECTIVE

Preclinical evaluation of headache by behavioral assessment of reward from pain relief.

METHODS

Inflammatory mediators (IMs) or control solution were applied to the rat dura mater to elicit a presumed state of cephalic pain. Hind paw incision was used in separate groups of animals to model noncephalic postsurgical pain. Drugs were given systemically or microinjected within the rostral ventromedial medulla (RVM), nucleus accumbens (NAc), or rostral anterior cingulate cortex (rACC). Peripheral nerve block was produced at the level of the popliteal fossa, and behavior was assessed using evoked sensory stimuli or conditioned place preference (CPP). Immunohistochemistry and brain microdialysis measurements were performed.

RESULTS

Dural IMs produced long-lasting generalized cutaneous allodynia. RVM lidocaine produced CPP, increased NAc c-Fos, and dopamine release selectively in rats receiving dural IMs; CPP was blocked by intra-NAc α-flupenthixol, a dopaminergic antagonist. Intravenous α-calcitonin gene-related peptide (αCGRP)(8-37) produced CPP and elicited NAc dopamine release selectively in rats treated with dural IMs. Prior lesion of the rACC or treatment with systemic sumatriptan or αCGRP(8-37) abolished RVM lidocaine-induced CPP in IM-treated rats. Sumatriptan treatment blocked NAc dopamine release in IM-treated rats receiving RVM lidocaine. Systemic sumatriptan did not alter pain relief-induced CPP in rats with incisional injury.

INTERPRETATION

Cephalic pain was unmasked in rats by assessment of motivated behavior to seek relief. Relief of pain activates the dopaminergic reward pathway to elicit negative reinforcement of behavior. Medications clinically effective for migraine headache selectively elicit relief of ongoing cephalic, but not postsurgical, noncephalic pain. These studies provide a platform for exploring migraine pathophysiology and for the discovery of new headache therapies.

What would 5-HT do? Regional diversity of 5-HT(1) receptor modulation of primary afferent neurotransmission

5-HT (serotonin) is a significant modulator of sensory input to the CNS, but the only analgesics that selectively target G-protein-coupled 5-HT receptors are highly specific for treatment of headache. Two recent papers in BJP shed light on this puzzling situation by showing that primary afferent neurotransmission to the superficial layers of the spinal and trigeminal dorsal is inhibited by different subtypes of the 5-HT(1) receptor - 5-HT(1B(and 1D) ) in the trigeminal dorsal horn and 5-HT(1A) in the spinal dorsal horn. The inputs being studied probably include nociceptive afferents, and the similarities of the methods employed in the two studies minimize the possibility that the different findings are an experimental artefact. Rather, the findings raise interesting questions about the possible anatomical or functional basis for the apparent regional selectivity of 5-HT(1) receptor actions, and whether these differences could be exploited for therapy. The results also emphasize the relative lack of information we have about the molecular details of the pro- or anti-nociceptive actions of 5-HT itself on primary afferent neurotransmission.

Abdominal migraine reviewed from both central and peripheral aspects

Despite the 2%-5% prevalence of abdominal migraine (AM) during childhood, the precise mechanism remains unknown. In this review, we present recent studies on AM and speculate its mechanism from both peripheral and central nervous system aspects. Although the main symptoms of AM exist at the peripheral level, previous studies have reported possible dysfunction of central nervous system, including photophobia, phonophobia and abnormal visual evoked responses. Recently, a case has been reported with AM combined with “Alice in Wonderland” syndrome with visual and/or bodily distortions, which serves as another piece of evidence of central dysfunction. Another case reported an AM patient having peculiar stereotypical ecchymosis in the legs and buttocks associated with pain attack, which implied possible involvement of peripheral nervous system. Although further investigations and accumulation of AM cases are still needed, we believe that the schema hypothesized here is helpful to plan further experimental approach to clarify the mechanism of this peculiar disease.

Activation of TRPA1 on dural afferents: a potential mechanism of headache pain

Activation of transient receptor potential ankyrin-1 (TRPA1) on meningeal nerve endings has been suggested to contribute to environmental irritant-induced headache, but this channel may also contribute to other forms of headache, such as migraine. The preclinical studies described here examined functional expression of TRPA1 on dural afferents and investigated whether activation of TRPA1 contributes to headache-like behaviors. Whole-cell patch-clamp recordings were performed in vitro with 2 TRPA1 agonists, mustard oil (MO), and the environmental irritant umbellulone (UMB) on dural-projecting trigeminal ganglion neurons. Application of MO and UMB to dural afferents produced TRPA1-like currents in approximately 42% and 38% of cells, respectively. By means of an established in vivo behavioral model of migraine-related allodynia, dural application of MO and UMB produced robust time-related tactile facial and hind paw allodynia that was attenuated by pretreatment with the TRPA1 antagonist HC-030031. Additionally, MO or UMB were applied to the dura, and exploratory activity was monitored for 30min with an automated open-field activity chamber. Dural MO and UMB decreased the number of vertical rearing episodes and the time spent rearing in comparison to vehicle-treated animals. This change in activity was prevented in rats pretreated with HC-030031 as well as sumatriptan, a clinically effective antimigraine agent. These data indicate that TRPA1 is expressed on a substantial fraction of dural afferents, and activation of meningeal TRPA1 produces behaviors consistent with those observed in patients during migraine attacks. Further, they suggest that activation of meningeal TRPA1 via endogenous or exogenous mechanisms can lead to afferent signaling and headache.

Differential effects of calcitonin gene-related peptide receptor blockade by olcegepant on mechanical allodynia induced by ligation of the infraorbital nerve vs the sciatic nerve in the rat

Previous studies showed that 5-hydroxytryptamine (5-HT)(1B/1D) receptor stimulation by triptans alleviates neuropathic pain caused by chronic constriction injury to the infraorbital nerve (CCI-ION) but not the sciatic nerve (CCI-SN) in rats. To assess whether such differential effects in the cephalic vs extracephalic territories is a property shared by other antimigraine drugs, we used the same models to investigate the effects of olcegepant, which has an antimigraine action mediated through calcitonin gene-related peptide (CGRP) receptor blockade. Adult male rats underwent unilateral CCI to the ION or the SN, and subsequent allodynia and/or hyperalgesia were assessed in ipsilateral vibrissal territory or hindpaw, respectively, using von Frey filaments and validated nociceptive tests. c-Fos expression was quantified by immunohistochemistry and interleukin 6 and activating transcription factor 3 (ATF3) mRNAs by real-time quantitative reverse transcriptase-polymerase chain reaction. Like naratriptan (0.1 to 0.3mg/kg, subcutaneously), olcegepant (0.3 to 0.9mg/kg, intravenously) markedly reduced mechanical allodynia in CCI-ION rats. In contrast, in CCI-SN rats, mechanical allodynia was completely unaffected and hyperalgesia was only marginally reduced by these drugs. A supra-additive antiallodynic effect was observed in CCI-ION rats treated with olcegepant (0.3mg/kg intravenously) plus naratriptan (0.1mg/kg subcutaneously), whereas this drug combination remained inactive in CCI-SN rats. Olcegepant (0.6mg/kg, intravenously) significantly reduced the number of c-Fos immunolabeled cells in spinal nucleus of the trigeminal nerve and upregulation of ATF3 transcript (a marker of neuron injury) but not that of interleukin-6 in trigeminal ganglion of CCI-ION rats. These findings suggest that CGRP receptor blockade might be of potential interest to alleviate trigeminal neuropathic pain.

Brain-Gut Interactions in IBS

Irritable bowel syndrome (IBS) is a common gastrointestinal disorder with an estimated prevalence of 10–20%. Current understanding of the pathophysiology of IBS is incomplete due to the lack of a clearly identified pathological abnormality and due to the lack of reliable biomarkers. Possible mechanisms believed to contribute to IBS development and IBS like symptoms include physical stressors, such as infection or inflammation, psychological, and environmental factors, like anxiety, depression, and significant negative life events. Some of these mechanisms may involve the brain-gut axis (BGA). In this article we review the current knowledge on the possible involvement of the BGA in IBS and discuss new directions for potential future therapies of IBS.

The complex actions of sumatriptan on rat dural afferents

AIM

To test the hypothesis that the clinical efficacy of triptans reflects convergent modulation of ion channels also involved in inflammatory mediator (IM)-induced sensitization of dural afferents.

METHODS

Acutely dissociated retrogradely labeled rat dural afferents were studied with whole cell and perforated patch techniques in the absence and presence of sumatriptan and/or IM (prostaglandin E2, bradykinin, and histamine).

RESULTS

Sumatriptan dose-dependently suppressed voltage-gated Ca²⁺ currents. Acute (2 min) sumatriptan application increased dural afferent excitability and occluded further IM-induced sensitization. In contrast, pre-incubation (30 min) with sumatriptan had no influence on dural afferent excitability and partially prevented IM-induced sensitization of dural afferents. The sumatriptan-induced suppression of voltage-gated Ca²⁺ currents and acute sensitization and pre-incubation-induced block of IM-induced sensitization were blocked by the 5-HT(1D) antagonist BRL 15572. Pre-incubation with sumatriptan failed to suppress the IM-induced decrease in action potential threshold and overshoot (which results from modulation of voltage-gated Na⁺ currents) and activation of Cl⁻ current, and had no influence on the Cl⁻ reversal potential. However, pre-incubation with sumatriptan caused a dramatic hyperpolarizing shift in the voltage dependence of K⁺ current activation.

DISCUSSION

These results indicate that although the actions of sumatriptan on dural afferents are complex, at least two distinct mechanisms underlie the antinociceptive actions of this compound. One of these mechanisms, the shift in the voltage dependence of K⁺ channel activation, may suggest a novel strategy for future development of anti-migraine agents.

Stress and visceral pain: from animal models to clinical therapies

Epidemiological studies have implicated stress (psychosocial and physical) as a trigger of first onset or exacerbation of irritable bowel syndrome (IBS) symptoms of which visceral pain is an integrant landmark. A number of experimental acute or chronic exteroceptive or interoceptive stressors induce visceral hyperalgesia in rodents although recent evidence also points to stress-related visceral analgesia as established in the somatic pain field. Underlying mechanisms of stress-related visceral hypersensitivity may involve a combination of sensitization of primary afferents, central sensitization in response to input from the viscera and dysregulation of descending pathways that modulate spinal nociceptive transmission or analgesic response. Biochemical coding of stress involves the recruitment of corticotropin releasing factor (CRF) signaling pathways. Experimental studies established that activation of brain and peripheral CRF receptor subtype 1 plays a primary role in the development of stress-related delayed visceral hyperalgesia while subtype 2 activation induces analgesic response. In line with stress pathways playing a role in IBS, non-pharmacologic and pharmacologic treatment modalities aimed at reducing stress perception using a broad range of evidence-based mind-body interventions and centrally-targeted medications to reduce anxiety impact on brain patterns activated by visceral stimuli and dampen visceral pain.

Anti-hyperalgesic effects of anti-serotonergic compounds on serotonin- and capsaicin-evoked thermal hyperalgesia in the rat

The peripheral serotonergic system has been implicated in the modulation of an array of pain states, from migraine to fibromyalgia; however, the mechanism by which serotonin (5HT) induces pain is unclear. Peripherally released 5HT induces thermal hyperalgesia, possibly via modulation of the transient receptor potential V1 (TRPV1) channel, which is gated by various noxious stimuli, including capsaicin. We previously reported in vitro that 5HT increases calcium accumulation in the capsaicin-sensitive population of sensory neurons with a corresponding increase in proinflammatory neuropeptide release, and both are antagonized by pretreatment with 5HT(2A) and 5HT(3) antagonists, as well as the anti-migraine drug sumatriptan. In the current study, we extended these findings in vivo using the rat hind paw thermal assay to test the hypothesis that peripheral 5HT enhances TRPV1-evoked thermal hyperalgesia that can be attenuated with 5HT(2A) and 5HT(3) receptor antagonists, as well as sumatriptan. Thermal hyperalgesia and edema were established by 5HT injection (0.1-10 nmol/100 μl) into the rat hind paw, and the latency to paw withdrawal (PWL) from noxious heat was determined. Rats were then pretreated with either 5HT before capsaicin (3 nmol/10 μl), the 5HT(2A) receptor antagonist ketanserin or the 5HT(3) receptor antagonist granisetron (0.0001-0.1 nmol/100 μl) before 5HT and/or capsaicin, or the 5HT(1B/1D) receptor agonist sumatriptan (0.01-1 nmol/100 μl) before capsaicin, and PWL was determined. We report that 5HT pretreatment enhances TRPV1-evoked thermal hyperalgesia, which is attenuated with local pretreatment with ketanserin, granisetron, or sumatriptan. We also report that peripheral 5HT induced a similar magnitude of thermal hyperalgesia in male and female rats. Overall, our results provide in vivo evidence supporting an enhancing role of 5HT on TRPV1-evoked thermal hyperalgesia, which can be attenuated by peripheral serotonergic intervention.

Colonic butyrate- algesic or analgesic?

Irritable bowel syndrome (IBS) is a common health issue that is characterized by abdominal pain, abnormal bowel movements, and altered visceral perception. The complexity and variability in symptoms pose serious challenges in treating IBS. Current therapy for IBS is primarily focused on reducing the abdominal pain, thereby improving the quality of life to a significant extent. Although the use of fiber rich diet is widely recommended in treating IBS, some studies have questioned its use. Intra-colonic butyrate, a short-chain fatty acid, is primarily produced by the fermentation of dietary fibers in the colon. In the existing literature there are conflicting reports about the function of butyrate. In rats it is known to induce visceral hypersensitivity without altered pathology, whereas in humans it has been reported to reduce visceral pain. Understanding the molecular mechanisms responsible for this contrasting effect of butyrate is important before recommending fiber rich diet to IBS patients.

The lack of peripheral pathology in migraine headache

This article reviews the baffling problem of the pathophysiology behind a peripheral genesis of migraine pain--or more particularly the baffling problem of its absence. I examine a number of pathophysiological states and the effector mechanisms for these states and find most of them very plausible and that they are all supported by abundant evidence. However, this evidence is mostly indirect; to date the occurrence of any of the presumed pathological states has not been convincingly demonstrated. Furthermore, there is little evidence of increased trigeminal sensory traffic into the central nervous system during a migraine attack. The article also examines a number of observations and experimental programs used to bolster a theory of peripheral pathology and suggests reasons why they may in fact not bolster it. I suggest that a pathology, if one exists, may be in the brain and even that it may not be a pathology at all. Migraine headache might just happen because of random noise in an exquisitely sensitive and complex network. The article suggests an experimental program to resolve these issues.

Stress and visceral pain: from animal models to clinical therapies

Epidemiological studies have implicated stress (psychosocial and physical) as a trigger of first onset or exacerbation of irritable bowel syndrome (IBS) symptoms of which visceral pain is an integrant landmark. A number of experimental acute or chronic exteroceptive or interoceptive stressors induce visceral hyperalgesia in rodents although recent evidence also points to stress-related visceral analgesia as established in the somatic pain field. Underlying mechanisms of stress-related visceral hypersensitivity may involve a combination of sensitization of primary afferents, central sensitization in response to input from the viscera and dysregulation of descending pathways that modulate spinal nociceptive transmission or analgesic response. Biochemical coding of stress involves the recruitment of corticotropin releasing factor (CRF) signaling pathways. Experimental studies established that activation of brain and peripheral CRF receptor subtype 1 plays a primary role in the development of stress-related delayed visceral hyperalgesia while subtype 2 activation induces analgesic response. In line with stress pathways playing a role in IBS, non-pharmacologic and pharmacologic treatment modalities aimed at reducing stress perception using a broad range of evidence-based mind-body interventions and centrally-targeted medications to reduce anxiety impact on brain patterns activated by visceral stimuli and dampen visceral pain.

Distribution of 5-HT(1B), 5-HT(1D) and 5-HT(1F) receptor expression in rat trigeminal and dorsal root ganglia neurons: relevance to the selective anti-migraine effect of triptans

Triptans, acting as serotonin, 5-HT(1B/1D/1F), receptor agonists, provide an effective and established treatment option in migraine and cluster headache. Clinical observations suggest a relatively specific effect of these compounds on primary headache disorders, but not in other pain syndromes. The mechanism of this specificity, however, is not well understood. Hence, we systematically studied primary sensory ganglia in rat to determine if the peripheral distribution of 5HT(1B/1D/1F) receptors showed any anatomical difference that would account for the specificity of clinical effect. Rat primary afferent and sensory ganglia neurons--trigeminal ganglia (Vg), and dorsal root ganglia (DRG): C(2), C(5), T(5), L(5)--were examined using paraffin-embedded, slide-bound tissue sections reacted with specific primary antibodies for rat 5-HT(1B, 1D) and (1F) receptors in a peroxidase-based immunohistochemical method. Immunoreactivity specific for all three serotonergic receptor subtypes was demonstrated in the five peripheral nervous system regions examined and quantitated. There was a good agreement for 5-HT(1B) and 5-HT(1D) receptors to that previously demonstrated in Vg and DRG L(5), while this was the first characterisation for 5-HT(1F) receptor in any of the five regions, as well as for 5-HT(1B) and 5HT(1D) receptors in DRG C(2), C(5) and T(5). In summary, all three 5-HT receptors are equally represented in Vg and the DRGs examined. We conclude that the triptans are theoretically able to bind to receptors at each level of the peripheral neuraxis without any apparent anatomical preference for the head.

Serotonin and the GI tract

Serotonin (5-hydroxytryptamine, 5-HT) participates in several functions of the gastrointestinal tract. Receptors in seven families (5-HT(1)-5-HT(7)) were identified, many of which are present on enterocytes, intrinsic and extrinsic neurons, interstitial cells, and gut myocytes. Most 5-HT is released from enterochromaffin cells in response to physiologic and pathologic stimuli. Roles of 5-HT in health include control of normal gut motor activity, secretion, and sensation, and regulation of food intake and cell growth. Abnormalities of serotonergic function contribute to symptom genesis in functional bowel disorders, inflammatory and infectious diseases of the gut, emetic responses to varied stimuli, obesity, and dysregulation of cell growth. Therapies acting as agonists or antagonists of 5-HT receptors or that modulate 5-HT reuptake play prominent roles in managing these conditions, although use of many agents is hampered by cardiopulmonary complications. Novel agents are in testing, which may exhibit efficacy without significant toxicity.

Migraine pain: reflections against vasodilatation

The original Wolff’s vascular theory of migraine was supported by the discovery of a class of drugs, the triptans, developed as a selective cephalic vasoconstrictor agents. Even in the neurovascular hypothesis of Moskowitz, that is the neurogenic inflammation of meningeal vessels provoked by peptides released from trigeminal sensory neurons, the vasodilatation provoked by calcitonin gene-related peptide (CGRP) is considered today much more important than oedema. The role of cephalic vasodilatation as a cause of migraine pain was recently sustained by studies showing the therapeutic effect of CGRP receptor antagonists. We discuss the evidence against vasodilatation as migraine pain generator and some findings which we suggest in support of a central (brain) origin of pain.

Limb remote-preconditioning protects against focal ischemia in rats and contradicts the dogma of therapeutic time windows for preconditioning

Remote ischemic preconditioning is an emerging concept for stroke treatment, but its protection against focal stroke has not been established. We tested whether remote preconditioning, performed in the ipsilateral hind limb, protects against focal stroke and explored its protective parameters. Stroke was generated by a permanent occlusion of the left distal middle cerebral artery (MCA) combined with a 30 min occlusion of the bilateral common carotid arteries (CCA) in male rats. Limb preconditioning was generated by 5 or 15 min occlusion followed with the same period of reperfusion of the left hind femoral artery, and repeated for two or three cycles. Infarct was measured 2 days later. The results showed that rapid preconditioning with three cycles of 15 min performed immediately before stroke reduced infarct size from 47.7+/-7.6% of control ischemia to 9.8+/-8.6%; at two cycles of 15 min, infarct was reduced to 24.7+/-7.3%; at two cycles of 5 min, infarct was not reduced. Delayed preconditioning with three cycles of 15 min conducted 2 days before stroke also reduced infarct to 23.0+/-10.9%, but with two cycles of 15 min it offered no protection. The protective effects at these two therapeutic time windows of remote preconditioning are consistent with those of conventional preconditioning, in which the preconditioning ischemia is induced in the brain itself. Unexpectedly, intermediate preconditioning with three cycles of 15 min performed 12 h before stroke also reduced infarct to 24.7+/-4.7%, which contradicts the current dogma for therapeutic time windows for the conventional preconditioning that has no protection at this time point. In conclusion, remote preconditioning performed in one limb protected against ischemic damage after focal cerebral ischemia.