Antinociception in rat by sarpogrelate, a selective 5-HT(2A) receptor antagonist, is peripheral

Role of Descending Serotonergic Fibers in the Development of Pathophysiology after Spinal Cord Injury (SCI): Contribution to Chronic Pain, Spasticity, and Autonomic Dysreflexia

As the nervous system develops, nerve fibers from the brain form descending tracts that regulate the execution of motor behavior within the spinal cord, incoming sensory signals, and capacity to change (plasticity). How these fibers affect function depends upon the transmitter released, the receptor system engaged, and the pattern of neural innervation. The current review focuses upon the neurotransmitter serotonin (5-HT) and its capacity to dampen (inhibit) neural excitation. A brief review of key anatomical details, receptor types, and pharmacology is provided. The paper then considers how damage to descending serotonergic fibers contributes to pathophysiology after spinal cord injury (SCI). The loss of serotonergic fibers removes an inhibitory brake that enables plasticity and neural excitation. In this state, noxious stimulation can induce a form of over-excitation that sensitizes pain (nociceptive) circuits, a modification that can contribute to the development of chronic pain. Over time, the loss of serotonergic fibers allows prolonged motor drive (spasticity) to develop and removes a regulatory brake on autonomic function, which enables bouts of unregulated sympathetic activity (autonomic dysreflexia). Recent research has shown that the loss of descending serotonergic activity is accompanied by a shift in how the neurotransmitter GABA affects neural activity, reducing its inhibitory effect. Treatments that target the loss of inhibition could have therapeutic benefit.

Chronic pain and psychedelics: a review and proposed mechanism of action

The development of chronic pain is a complex mechanism that is still not fully understood. Multiple somatic and visceral afferent pain signals, when experienced over time, cause a strengthening of certain neural circuitry through peripheral and central sensitization, resulting in the physical and emotional perceptual chronic pain experience. The mind-altering qualities of psychedelics have been attributed, through serotonin 2A (5-HT2A) receptor agonism, to ‘reset’ areas of functional connectivity (FC) in the brain that play prominent roles in many central neuropathic states. Psychedelic substances have a generally favorable safety profile, especially when compared with opioid analgesics. Clinical evidence to date for their use for chronic pain is limited; however, several studies and reports over the past 50 years have shown potential analgesic benefit in cancer pain, phantom limb pain and cluster headache. While the mechanisms by which the classic psychedelics may provide analgesia are not clear, several possibilities exist given the similarity between 5-HT2A activation pathways of psychedelics and the nociceptive modulation pathways in humans. Additionally, the alterations in FC seen with psychedelic use suggest a way that these agents could help reverse the changes in neural connections seen in chronic pain states. Given the current state of the opioid epidemic and limited efficacy of non-opioid analgesics, it is time to consider further research on psychedelics as analgesics in order to improve the lives of patients with chronic pain conditions.

Serotonergic transmission after spinal cord injury

Changes in descending serotonergic innervation of spinal neural activity have been implicated in symptoms of paralysis, spasticity, sensory disturbances and pain following spinal cord injury (SCI). Serotonergic neurons possess an enhanced ability to regenerate or sprout after many types of injury, including SCI. Current research suggests that serotonine (5-HT) release within the ventral horn of the spinal cord plays a critical role in motor function, and activation of 5-HT receptors mediates locomotor control. 5-HT originating from the brain stem inhibits sensory afferent transmission and associated spinal reflexes; by abolishing 5-HT innervation SCI leads to a disinhibition of sensory transmission. 5-HT denervation supersensitivity is one of the key mechanisms underlying the increased motoneuron excitability that occurs after SCI, and this hyperexcitability has been demonstrated to underlie the pathogenesis of spasticity after SCI. Moreover, emerging evidence implicates serotonergic descending facilitatory pathways from the brainstem to the spinal cord in the maintenance of pathologic pain. There are functional relevant connections between the descending serotonergic system from the rostral ventromedial medulla in the brainstem, the 5-HT receptors in the spinal dorsal horn, and the descending pain facilitation after tissue and nerve injury. This narrative review focussed on the most important studies that have investigated the above-mentioned effects of impaired 5-HT-transmission in humans after SCI. We also briefly discussed the promising therapeutical approaches with serotonergic drugs, monoclonal antibodies and intraspinal cell transplantation.

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

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

Serotonergic neuromodulation of peripheral nociceptors

Nociception, the encoding and processing of noxious environmental stimuli by sensory neurons, functions to protect an organism from bodily damage. Activation of the terminal endings of certain sensory neurons, termed nociceptors, triggers a train of impulses to neurons in the spinal cord. Signals are integrated and processed in the dorsal spinal cord and then projected to the brain where they elicit the perception of pain. A number of neuromodulators that can affect nociceptors are released in the periphery during the inflammation that follows an initial injury. Serotonin (5-HT) is a one such proinflammatory mediator. This review discusses our current understanding of the neuromodulatory role of 5-HT, and specifically how this monoamine activates and sensitizes nociceptors. Potential therapeutic targets to treat pain are described.

Role of peripheral versus spinal 5-HT(7) receptors in the modulation of pain undersensitizing conditions

Several studies have suggested that 5-HT(7) receptors are involved in nociceptive processing but the exact contribution of peripheral versus central 5-HT(7) receptors still needs to be elucidated. In the present study, the respective roles of peripheral and spinal 5-HT(7) receptors in the modulation of mechanical hypersensitivity were investigated under two different experimental pain conditions. In a first set of experiments, the selective 5-HT(7) receptor agonist, E-57431, was systemically, intrathecally or peripherally (intraplantarly) administered to rats sensitized by intraplantar injection of capsaicin. Oral administration of E-57431 (1.25-10 mg/kg) was found to exert a clear-cut dose-dependent reduction of capsaicin-induced mechanical hypersensitivity. Interestingly, intrathecal administration of E-57431 (100 μg) also inhibited mechanical hypersensitivity secondary to capsaicin injection. In contrast, a dose-dependent enhancement of capsaicin-induced mechanical hypersensitivity was observed after local intraplantar injection of E-57431 (0.01-1 μg). In a second set of experiments, E-57431 was systemically or intrathecally administered to rats submitted to neuropathic pain (spared nerve injury model). Significant inhibition of nerve injury-induced mechanical hypersensitivity was found after intraperitoneal (10 mg/kg) as well as intrathecal (100 μg) administration of E-57431 in this chronic pain model. These studies provide evidence that, under sensitizing neurogenic/neuropathic conditions, activation of 5-HT(7) receptors exerts antinociceptive effects at the level of the spinal cord and pronociceptive effects at the periphery. The antinociceptive effect mediated by central 5-HT(7) receptors seems to predominate over the pronociceptive effect at the periphery when a selective 5-HT(7) receptor agonist is systemically administered.

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.

A spinal mechanism of action for duloxetine in a rat model of painful diabetic neuropathy

BACKGROUND AND PURPOSE

This study was designed to clarify mechanisms responsible for the anti-allodynic effects of duloxetine in diabetes.

EXPERIMENTAL APPROACH

The streptozotocin-induced diabetic rat model was used to compare the efficacy of duloxetine, 5-HT, the 5-HT(2A) receptor agonist [1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI)] and two antagonists (ketanserin and pruvanserin) on tactile allodynia.

KEY RESULTS

Systemic or intrathecal injection of duloxetine alleviated tactile allodynia in diabetic rats. The effect of systemic duloxetine was reduced by intrathecal administration of ketanserin or pruvanserin, indicating participation of spinal 5-HT(2A) receptors in the mechanism of action of duloxetine. In contrast to spinal delivery, systemic and local peripheral injections of ketanserin or pruvanserin alleviated tactile allodynia in diabetic rats. This effect was reversed immediately after systemic or local DOI injection.

CONCLUSIONS AND IMPLICATIONS

These results support the involvement of spinal 5-HT(2A) receptors in the ability of duloxetine to ameliorate painful diabetic neuropathy. Our data also suggest that the role of 5-HT(2A) receptors depends on the level of the neuraxis at which activation takes place, with peripheral activation contributing to tactile allodynia in diabetic rats, whereas spinal activation of this receptor alleviates tactile allodynia. The development of selective peripheral 5-HT(2A) receptor antagonists may offer a novel approach for the treatment of diabetic neuropathic pain.

Pharmacological activation of 5-HT7 receptors reduces nerve injury-induced mechanical and thermal hypersensitivity

The involvement of the 5-HT(7) receptor in nociception and pain, particularly chronic pain (i.e., neuropathic pain), has been poorly investigated. In the present study, we examined whether the 5-HT(7) receptor participates in some modulatory control of nerve injury-evoked mechanical hypersensitivity and thermal (heat) hyperalgesia in mice. Activation of 5-HT(7) receptors by systemic administration of the selective 5-HT(7) receptor agonist AS-19 (1 and 10mg/kg) exerted a clear-cut reduction of mechanical and thermal hypersensitivities that were reversed by co-administering the selective 5-HT(7) receptor antagonist SB-258719. Interestingly, blocking of 5-HT(7) receptors with SB-258719 (2.5 and 10mg/kg) enhanced mechanical (but not thermal) hypersensitivity in nerve-injured mice and induced mechanical hypersensitivity in sham-operated mice. Effectiveness of the treatment with a 5-HT(7) receptor agonist was maintained after repeated systemic administration: no tolerance to the antiallodynic and antihyperalgesic effects was developed following treatment with the selective 5-HT(7) receptor agonist E-57431 (10mg/kg) twice daily for 11 days. The 5-HT(7) receptor co-localized with GABAergic cells in the dorsal horn of the spinal cord, suggesting that the activation of spinal inhibitory GABAergic interneurons could contribute to the analgesic effects of 5-HT(7) receptor agonists. In addition, a significant increase of 5-HT(7) receptors was found by immunohistochemistry in the ipsilateral dorsal horn of the spinal cord after nerve injury, suggesting a "pain"-triggered regulation of receptor expression. These results support the idea that the 5-HT(7) receptor subtype is involved in the control of pain and point to a new potential use of 5-HT(7) receptor agonists for the treatment of neuropathic pain.

Roles of the hippocampal formation in pain information processing

Pain is a complex experience consisting of sensory-discriminative, affective-motivational, and cognitive-evaluative dimensions. Now it has been gradually known that noxious information is processed by a widely-distributed, hierarchically- interconnected neural network, referred to as neuromatrix, in the brain. Thus, identifying the multiple neural networks subserving these functional aspects and harnessing this knowledge to manipulate the pain response in new and beneficial ways are challenging tasks. Albeit with elaborate research efforts on the cortical responses to painful stimuli or clinical pain, involvement of the hippocampal formation (HF) in pain is still a matter of controversy. Here, we integrate previous animal and human studies from the viewpoint of HF and pain, sequentially representing anatomical, behavioral, electrophysiological, molecular/biochemical and functional imaging evidence supporting the role of HF in pain processing. At last, we further expound on the relationship between pain and memory and present some unresolved issues.

5-HT7 receptor activation inhibits mechanical hypersensitivity secondary to capsaicin sensitization in mice

This work aimed to evaluate the potential role of the 5-HT(7) receptor in nociception secondary to a sensitizing stimulus in mice. For this purpose, the effects of relevant ligands (5-HT(7) receptor agonists: AS-19, MSD-5a, E-55888; 5-HT(7) receptor antagonists: SB-258719, SB-269970; 5-HT(1A) receptor agonist: F-13640; 5-HT(1A) receptor antagonist: WAY-100635) were assessed on capsaicin-induced mechanical hypersensitivity, a pain behavior involving hypersensitivity of dorsal horn neurons (central sensitization). For the 5-HT(7) receptor agonists used, binding profile and intrinsic efficacy to stimulate cAMP formation in HEK-293F cells expressing the human 5-HT(7) receptor were also evaluated. AS-19 and E-55888 were selective for 5-HT(7) receptors. E-55888 was a full agonist whereas AS-19 and MSD-5a behaved as partial agonists, with maximal effects corresponding to 77% and 61%, respectively, of the cAMP response evoked by the full agonist 5-HT. Our in vivo results revealed that systemic administration of 5-HT(7) receptor agonists exerted a clear-cut dose-dependent antinociceptive effect that was prevented by 5-HT(7) receptor antagonists, but not by the 5-HT(1A) receptor antagonist. The order of efficacy (E-55888>AS-19>MSD-5a) matched their in vitro efficacy as 5-HT(7) receptor agonists. Contrary to agonists, a dose-dependent promotion of mechanical hypersensitivity was observed after administration of 5-HT(7) receptor antagonists, substantiating the involvement of the 5-HT(7) receptor in the control of capsaicin-induced mechanical hypersensitivity. These findings suggest that serotonin exerts an inhibitory role in the control of nociception through activation of 5-HT(7) receptors, and point to a new potential therapeutic use of 5-HT(7) receptor agonists in the field of analgesia.

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.

Sarpogrelate hydrochloride, a 5-HT2A receptor antagonist, attenuates neurogenic pain induced by nucleus pulposus in rats

STUDY DESIGN

An in vivo study using a rat lumbar disc herniation model.

OBJECTIVES

To evaluate the effects of sarpogrelate hydrochloride on neurogenic pain induced by nucleus pulposus translocation and to elucidate its mechanism.

SUMMARY OF BACKGROUND DATA

Sarpogrelate, an antiplatelet agent with selective 5-hydroxytryptamine (5-HT) receptor 2A antagonist activity, has been reported to improve low back pain, sciatica, and numbness of lower extremities in patients with lumbar disc herniation. However, the efficacy of sarpogrelate for pain relief in this situation has not been established by clinical studies and its mechanism remains unknown.

METHODS

The autologous nucleus pulposus was placed onto the left L4 and L5 nerve roots of 30 Sprague-Dawley rats allotted to sarpogrelate (100 mg/kg, n = 15) and control (vehicle, n = 15) treatment groups. Sarpogrelate or vehicle was administered orally once daily between days 7 and 14 after surgery. Mechanical allodynia was measured before and after treatment. The right and left nerve roots and dorsal root ganglions were isolated from 5 animals in each group to assay 5-HT, 5-hydroxyindoleacetic acid (5-HIAA), and norepinephrine on day 5 of administration (= day 11 after surgery).

RESULTS

Sarpogrelate treatment significantly reduced mechanical allodynia on days 5 and 8 of administration. The placement of the nucleus pulposus onto nerve roots increased norepinephrine but not 5-HT and 5-HIAA contents in inflamed nerve roots or dorsal root ganglions. Sarpogrelate did not affect these levels.

CONCLUSIONS

Sarpogrelate attenuated pain-related behavior induced by the nucleus pulposus in the animal model. Although further investigation is needed concerning the mechanism of action, this study supported the hypothesis that sarpogrelate is efficacious for treating the pain of lumbar disc herniation.

Is serotonin hyperalgesic or analgesic?

Serotonin (5-hydroxtryptamine, 5-HT) is an important molecule in pain processing and modulation. Whether 5-HT has an analgesic or hyperalgesic action depends on the cell type and type of receptor it acts on. In the periphery, 5-HT sensitizes afferent nerve fibers, thus contributing to hyperalgesia in inflammation and nerve injury. In the trigeminal system, agonism at 5-HT1B/D receptors reduces neurotransmitter release, but actions through the 5-HT2A receptor may underlie chronic headache. Furthermore, genetic alterations in the 5-HT system may influence the susceptibility to migraine. In the central nervous system, 5-HT is involved in descending inhibition, but facilitatory serotonergic pathways may be functionally more important.

Microinjection of ritanserin into the dorsal hippocampal CA1 and dentate gyrus decrease nociceptive behavior in adult male rat

Prenatal 5HT depletion causes a significant decrease in the level of nociceptive sensitivity during the second phase of the formalin test behavioral response. These experiments were designed to test whether blocking 5HT2A/2c receptors in the CA1 region of the hippocampus and dentate gyrus would decrease nociceptive behaviors induced by a peripheral noxious stimulus formalin as an animal model of unremitting human being. The 5HT2A/2c receptor antagonist ritanserin (2, 4 and 8 microg/0.5 microl) was injected into the CA1 area and dentate gyrus of behaving rats 5 min before subcutaneous injection of formalin irritant. Nociceptive behaviors in both phases of the formalin test were significantly decreased by ritanserin (4 and 8 microg/0.5 microl) and ritanserin had no effect at 2 microg/0.5 microl. These results support the hypothesis that the hippocampal formation may modify the processing of incoming nociceptive information and that 5HT2A/2c receptor-sensitive mechanisms in the hippocampus may play a role in nociception and/or the expression of related behaviors.

Peripheral 5-HT2A receptor antagonism attenuates primary thermal hyperalgesia and secondary mechanical allodynia after thermal injury in rats

Inflammation or injury of peripheral tissue causes release of chemical mediators, including 5-hydroxytryptamine (5-HT), which is involved in the facilitation of nociceptive transmission and the induction of hyperalgesia. The present study examined the effect of a selective 5-HT2A receptor antagonist, sarpogrelate, on hyperalgesia and allodynia induced by thermal injury in rats. Mild thermal injury to the hindpaw produces thermal hyperalgesia in the injured area (primary thermal hyperalgesia) and mechanical allodynia in sites adjacent to the primary area (secondary mechanical allodynia). Mechanical allodynia was assessed by paw withdrawal thresholds using von Frey filaments, and thermal hyperalgesia was assessed by paw withdrawal latencies upon exposure to a radiant heat source. Intraperitoneal administration (30-100 mg/kg) or local injection (30-300 microg) of sarpogrelate 10 min prior to thermal injury attenuated secondary mechanical allodynia in a dose-dependent manner. Intraperitoneal administration (3-100 mg/kg) or local injection (30-300 microg) of sarpogrelate 10 min prior to thermal injury attenuated primary thermal hyperalgesia in a dose-dependent manner. Intraplantar injection of sarpogrelate (300 microg) to the contralateral hindpaw had no effect on primary thermal hyperalgesia or secondary mechanical allodynia in the ipsilateral paw. The tissue concentration of 5-HT was measured using microdialysis. Concentrations of 5-HT increased after thermal injury in both primary and secondary areas, and the increase was not attenuated by pretreatment with sarpogrelate (100 mg/kg, i.p.). These data suggest that 5-HT released in peripheral tissues after thermal injury sensitizes primary afferent neurons and produces mechanical allodynia and thermal hyperalgesia via peripheral 5-HT2A receptors.

Effects of a 5-HT2A receptor antagonist, sarpogrelate on thermal or inflammatory pain

The effects of intrathecally and systemically administered 5-hydroxytriptamine (5-HT)(2A) receptor antagonist, sarpogrelate on acute thermal or formalin induced pain were examined. Male Sprague-Dawley rats with lumbar intrathecal catheters were tested with their tail withdrawal response to thermal stimulation (tail flick test) or their paw flinching and shaking response by subcutaneous formalin injection into the hind paw (formalin test) after intrathecal or intraperitoneal administration of sarpogrelate. 5-HT(2A) receptor agonist was used to antagonize the effects of sarpogrelate. In the tail flick test, only intraperitoneal administration induced analgesia. In the formalin test, both intrathecal and intraperitoneal administration were analgesic. The analgesic effects were inhibited by pretreatment with 5-HT(2A) receptor agonist. Motor disturbance and behavioral side effects were not observed. In conclusion, sarpogrelate might be analgesic on inflammatory induced acute and facilitated pain by intrathecal or systemic administration. However, only systemic administration could be effective on thermal induced acute pain.

Serotonin in pain and analgesia: actions in the periphery

The purpose of this article is to summarize recent findings on the role of serotonin in pain processing in the peripheral nervous system. Serotonin (5-hydroxtryptamine [5-HT]) is present in central and peripheral serotonergic neurons, it is released from platelets and mast cells after tissue injury, and it exerts algesic and analgesic effects depending on the site of action and the receptor subtype. After nerve injury, the 5-HT content in the lesioned nerve increases. 5-HT receptors of the 5-HT3 and 5-HT2A subtype are present on C-fibers. 5-HT, acting in combination with other inflammatory mediators, may ectopically excite and sensitize afferent nerve fibers, thus contributing to peripheral sensitization and hyperalgesia in inflammation and nerve injury.

Functions of 5-HT2A receptor and its antagonists in the cardiovascular system

The serotonin (5-hydroxytryptamine, 5-HT) receptors have conventionally been divided into seven subfamilies, most of which have several subtypes. Among them, 5-HT(2A) receptor is associated with the contraction of vascular smooth muscle, platelet aggregation and thrombus formation and coronary artery spasms. Accordingly, selective 5-HT(2A) antagonists may have potential in the treatment of cardiovascular diseases. Sarpogrelate, a selective 5-HT(2A) antagonist, has been introduced clinically as a therapeutic agent for the treatment of ischemic diseases associated with thrombosis. Molecular modeling studies also suggest that sarpogrelate is a 5-HT(2A) selective antagonist and is likely to have pharmacological effects beneficial in the treatment of cardiovascular diseases. This review describes the above findings as well as the signaling linkages of the 5-HT(2A) receptors and the mode of agonist binding to 5-HT(2A) receptor using data derived from molecular modeling and site-directed mutagenesis.

Association of the -1438 G/A and 102 T/C polymorphism of the 5-Ht2A receptor gene with irritable bowel syndrome 5-Ht2A gene polymorphism in irritable bowel syndrome

GOALS

The aim of this study is to investigate whether there were any association between the 102 T/C and -1438 G/A polymorphisms of the 5-HT2A receptor gene and IBS, and abdominal pain, anxiety and depression.

BACKGROUND

Genes involved in serotonin (5-HT) metabolism are good candidates for the pathogenesis of irritable bowel syndrome (IBS). Recently, a silent polymorphism in the 5-HT2A receptor gene was identified that is defined by a T to C transition at position 102. Also, a novel G to A base change at position -1438 of the promoter region has been detected in 5-HT2A receptor gene.

STUDY

Fifty-four patients with IBS diagnosed according to the Rome 1 criteria and 107 healthy individuals were included in the study. PCR was used to amplify a 468-bp (G-->A) and 342-bp (T-->C) fragment of genomic DNA containing the polymorphism. Hospital anxiety and depression scale was used to assess the risk of depression and anxiety. Severity of chronic abdominal pain was determined by visual analogue scale (VAS).

RESULTS

It was shown that there was a high incidence of homozygote C allele of the 102T/C polymorphism (%22.2; OR: 7.89, P = 0.04) and homozygote A allele of the -1438 G/A promoter region (%%37; OR: 11.14, P = 0.01) in patients with IBS. The risk of having an anxiety disorder was 83.3% in patients with C/C genotype, which was higher than other allele carrying patients, and overall mean (%52.7). (chi = 8.56, P = 0.014). The patients with T/T genotype had a VAS score of 54.93 +/- 2.59 mm, which was significantly higher than that of the patients with other genotypes (p1 = 0.02, p2 = 0.001).

CONCLUSION

This study suggests that the patients with homozygote C allele of the 102 T/C polymorphisms or homozygote A allele of the -1438 G/A polymorphism of the 5-HT2A receptor gene, have a high risk of IBS. On the other hand, T/T genotype of 102 T/C polymorphism may be associated with more severe pain in patient with IBS.

Absence of thermal hyperalgesia in serotonin transporter-deficient mice

Antidepressants in the treatment of neuropathic pain are thought to partially exert their effect by inhibition of serotonin (5-HT) reuptake and thus activation of central antinociceptive pathways. Mice deficient for the 5-HT transporter (5-HTT-/- mice) are regarded as a model of lifelong treatment with a serotonin reuptake inhibitor. Here we investigated 5-HTT-/- mice and compared their pain-related behavior after a unilateral chronic constrictive sciatic nerve injury (CCI) with that of wild-type littermates. Wild-type mice reproducibly developed ipsilateral thermal hyperalgesia and mechanical allodynia after CCI. 5-HTT-/- mice did not develop thermal hyperalgesia, but showed bilateral mechanical allodynia after the nerve injury. 5-HT levels as measured with HPLC increased after CCI in the injured nerve in both genotypes and decreased in the lumbar spinal cord in wild-type mice. 5-HTT-/- mice had significantly lower 5-HT concentrations than wild-type mice in all tissues investigated. Thus, in 5-HTT-/- mice, reduced 5-HT levels in the injured peripheral nerves correlate with diminished behavioral signs of thermal hyperalgesia, a pain-related symptom caused by peripheral sensitization. In contrast, bilateral mechanical allodynia, a centrally mediated phenomenon, was associated with decreased spinal 5-HT concentrations in 5-HTT-/- mice and may possibly be caused by a lack of spinal inhibition.

FR143166 attenuates spinal pain transmission through activation of the serotonergic system

We investigated the antinociceptive effect of 1-(4-fluorophenyl)-3-methyl-5-[4-(methylsulfinyl)phenyl]pyrazole (FR143166) in the tail-pinch test in mice. The p.o. and i.t. injection of FR143166 exerted dose-dependent antinociceptive actions with ED(50) values of 24 mg/kg and 15 micro g/mouse, respectively. However, i.c.v. injection of FR143166 at a maximum dose of 128 micro g/mouse did not show any antinociceptive effect. The antinociceptive effect of FR143166 injected i.t. was abolished by co-administration of the nonselective serotonin (5-hydroxytryptamine, 5-HT) receptor antagonist, methysergide, but not by the adrenoceptor antagonists, phentolamine and propranolol. Moreover, the effect of FR143166 was also reversed by the 5-HT(2A) receptor antagonist, ketanserin, and the 5-HT(3) receptor antagonist, MDL-72222 (3-tropanyl-3,5-dichlorobenzoate). The effect of FR143166 was attenuated by p-chlorophenylalanine, but not by 6-hydroxydopamine plus nomifensine pretreatment. These results suggest that the descending serotonergic system, especially spinal 5-HT(2A) and 5-HT(3) receptors, is involved in the antinociceptive activity of spinally administered FR143166 on noxious mechanical stimuli.