Localization of 5-HT3 receptors in the rat spinal cord: immunohistochemistry and in situ hybridization

Structural basis for partial agonism in 5-HT3A receptors

Hyperactivity of serotonin 3 receptors (5-HT3R) underlies pathologies associated with irritable bowel syndrome and chemotherapy-induced nausea and vomiting. Setrons, a class of high-affinity competitive antagonists, are used in the treatment of these conditions. Although generally effective for chemotherapy-induced nausea and vomiting, the use of setrons for treating irritable bowel syndrome has been impaired by adverse side effects. Partial agonists are now being considered as an alternative strategy, with potentially less severe side effects than full antagonists. However, a structural understanding of how these ligands work is lacking. Here, we present high-resolution cryogenic electron microscopy structures of the mouse 5-HT3AR in complex with partial agonists (SMP-100 and ALB-148471) captured in pre-activated and open-like conformational states. Molecular dynamics simulations were used to assess the stability of drug-binding poses and interactions with the receptor over time. Together, these studies reveal mechanisms for the functional differences between orthosteric partial agonists, full agonists and antagonists of the 5-HT3AR.

Atypical antidepressant mirtazapine inhibits 5-hydroxytryptamine3 receptor currents in NCB-20 cells

Mirtazapine, an atypical antidepressant, is known to enhance serotonergic transmission by inhibiting the 5-hydroxytryptamine (5-HT)_1A, 5-HT_2C, and 5-HT₃ receptors. However, the mechanism of action on the 5-HT₃ receptor remains unclear. We investigated the inhibitory mechanisms of mirtazapine on 5-HT₃ receptors of NCB20 neuroblastoma cells using the whole-cell voltage-clamp method. Mirtazapine inhibited the 5-HT₃ receptor currents in a concentration-dependent manner, and the inhibitory effect was influenced by the concentration of 5-HT. When mirtazapine was co-applied to 5-HT, the maximal response of the 5-HT₃ receptor current was reduced and EC₅₀ was increased, suggesting that mirtazapine might act as a non-competitive inhibitor. Inhibition of 5-HT₃ current by mirtazapine was stronger in pre-application than in co-application, which suggests that mirtazapine might act as a closed state inhibitor. This finding was further supported by no use-dependency of the mirtazapine for 5-HT₃ receptor inhibition. Finally, mirtazapine accelerated the desensitization and deactivation process in a concentration-dependent manner. The difference in recovery time showed that mirtazapine drastically influences the desensitization process than the deactivation process. These mechanistic characteristics of mirtazapine support the understanding of the relationship between the 5-HT₃ receptor and atypical antidepressants.

5-HT3 Receptors in Rat Dorsal Root Ganglion Neurons: Ca2+ Entry and Modulation of Neurotransmitter Release

Rat dorsal root ganglion (DRG) neurons express 5-hydroxytryptamine receptors (5-HT3Rs). To elucidate their physiological role in the modulation of sensory signaling, we aimed to quantify their functional expression in newborn and adult rat DRG neurons, as well as their ability to modulate the Ca²⁺-dependent neurotransmitter release, by means of electrophysiological techniques combined with fluorescence-based Ca²⁺ imaging. The selective 5-HT3R agonist mCPBG (10 μM) elicited whole-cell currents in 92.5% of adult DRG neurons with a significantly higher density current than in responding newborn cells (52.2%), suggesting an increasing serotoninergic modulation on primary afferent cells during development. Briefly, 5-HT3Rs expressed by adult DRG neurons are permeable to Ca²⁺ ions, with a measured fractional Ca²⁺ current (i.e., the percentage of total current carried by Ca²⁺ ions, Pf) of 1.0%, similar to the value measured for the human heteromeric 5-HT3_A/B receptor (P_f = 1.1%), but lower than that of the human homomeric 5-HT3_A receptor (P_f = 3.5%). mCPBG applied to co-cultures of newborn DRG and spinal neurons significantly increased the miniature excitatory postsynaptic currents (mEPSCs) frequency in a subset of recorded spinal neurons, even in the presence of Cd²⁺, a voltage-activated Ca²⁺ channel blocker. Considered together, our findings indicate that the Ca²⁺ influx through heteromeric 5-HT3Rs is sufficient to increase the spontaneous neurotransmitter release from DRG to spinal neurons.

Acetaminophen Exerts an Analgesic Effect on Muscular Hyperalgesia in Repeated Cold-Stressed Rats through the Enhancement of the Descending Pain Inhibitory System Involving Spinal 5-HT3 and Noradrenergic α2 Receptors

Musculoskeletal and psychological complaints have increased with the widespread use of visual display terminals, and musculoskeletal pain is known to be closely related to stress. One method of experimentally inducing persistent muscle pain is repeated cold stress (RCS), and animals exposed to such stress exhibit a dysfunction in the descending pain inhibitory system. Acetaminophen (N-acetyl-p-aminophenol; APAP) is widely used to relieve several types of pain, including musculoskeletal pain, and is available as an OTC drug. However, the mechanism underlying its analgesic action has not yet been fully elucidated. In this study, we compared the analgesic effect of APAP on RCS-induced muscular hyperalgesia with those of other analgesics to identify its mechanism of action. The daily oral administration of APAP significantly suppressed the decrease in the mechanical withdrawal threshold caused by RCS, similar to the results for neurotropin but not for the cyclooxygenase inhibitor ibuprofen (IBP). Moreover, the intrathecal administration of antagonists of the 5-hydroxytryptamine (5-HT)3 receptor or α2-adrenoceptor significantly abolished the analgesic effect of APAP but not of IBP. These results suggest that the analgesic effect of APAP on RCS-induced muscular pain might be exerted due to the activation of the descending pathways involving the spinal 5-HT3 receptor or α2-adrenoceptor.

Zingiber officinale Roscoe Rhizomes Attenuate Oxaliplatin-Induced Neuropathic Pain in Mice

Oxaliplatin is a platinum derivative chemotherapeutic drug widely used against cancers, but even a single treatment can induce a severe allodynia that requires treatment interruption and dose diminution. The rhizome of Zingiber officinale roscoe (Z. officinale, ginger), has been widely used in traditional medicine to treat various diseases causing pain; however, its effect against oxaliplatin-induced neuropathic pain has never been assessed. In mice, a single oxaliplatin (6 mg/kg, i.p.) treatment induced significant cold and mechanical allodynia. Cold and mechanical allodynia were assessed by acetone drop and von Frey filament tests, respectively. Water extracts of Z. officinale (100, 300, and 500 mg/kg, p.o.) significantly attenuated both cold and mechanical allodynia induced by oxaliplatin. Intrathecal pre-treatment with the antagonist 5-HT1A (NAN-190, i.t., 1 μg), but not with the antagonist 5-HT2A (ketanserin, i.t., 1 μg), significantly blocked the analgesic effect of Z. officinale against both cold and mechanical allodynia. However, 5-HT3 antagonist (MDL-72222, i.t., 15 μg) administration only blocked the anti-allodynic effect of Z. officinale against cold allodynia. Real-time PCR analysis demonstrated that Z. officinale significantly increased the mRNA expression of the spinal 5-HT1A receptor that was downregulated after oxaliplatin injection. These results suggest that Z. officinale may be a viable treatment option for oxaliplatin-induced neuropathic pain.

High-resolution structures of multiple 5-HT3AR-setron complexes reveal a novel mechanism of competitive inhibition

Serotonin receptors (5-HT3AR) play a crucial role in regulating gut movement, and are the principal target of setrons, a class of high-affinity competitive antagonists, used in the management of nausea and vomiting associated with radiation and chemotherapies. Structural insights into setron-binding poses and their inhibitory mechanisms are just beginning to emerge. Here, we present high-resolution cryo-EM structures of full-length 5-HT3AR in complex with palonosetron, ondansetron, and alosetron. Molecular dynamic simulations of these structures embedded in a fully-hydrated lipid environment assessed the stability of ligand-binding poses and drug-target interactions over time. Together with simulation results of apo- and serotonin-bound 5-HT3AR, the study reveals a distinct interaction fingerprint between the various setrons and binding-pocket residues that may underlie their diverse affinities. In addition, varying degrees of conformational change in the setron-5-HT3AR structures, throughout the channel and particularly along the channel activation pathway, suggests a novel mechanism of competitive inhibition.

High-flow oxygen and pro-serotonin agents for non-interventional treatment of post-dural-puncture headache

OBJECTIVE

Post dural puncture headache (PDPH) is a common complication in patients following diagnostic or therapeutic lumbar puncture, procedures requiring epidural access, and spinal surgery. Epidural blood patch (EBP), the gold standard for the treatment of this pathology requires training not provided to emergency physicians. In addition, the presence of concomitant pathology and abnormal laboratory values are contraindications to perform EBP. In presence of these limitations, we sought for a non-interventional management of PDPH utilizing high-flow oxygen and pro-serotonin agents. We reviewed the mechanism of action of this therapy METHODS: To illustrate our proposal, we report a series of twelve consecutive patients with PDPH treated with high-flow oxygen therapy at 12 L/min via a non-rebreathing mask and intravenous metoclopramide.

RESULTS

All patients were treated with this conservative therapy, no adverse reactions were observed. After the intervention, the headache resolved without further indications for PDPH.

CONCLUSION

Our series suggests that combining high-flow oxygen and pro-serotonin agents such metoclopramide in the ED might be a feasible option as effective as the invasive methods used in treating PDPH. This therapy appears to be efficient and to minimize risk, cost and side effects. It presents an easily accessible alternative that should be considered when PDPH is not a viable option.

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

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

Serotonergic Modulation of Nociceptive Circuits in Spinal Cord Dorsal Horn

Background:

Despite the extensive number of studies performed in the last 50 years, aimed at describing the role of serotonin and its receptors in pain modulation at the spinal cord level, several aspects are still not entirely understood. The interpretation of these results is often complicated by the use of different pain models and animal species, together with the lack of highly selective agonists and antagonists binding to serotonin receptors.

Method:

In this review, a search has been conducted on studies investigating the modulatory action exerted by serotonin on specific neurons and circuits in the spinal cord dorsal horn. Particular attention has been paid to studies employing electro-physiological techniques, both in vivo and in vitro.

Conclusion:

The effects of serotonin on pain transmission in dorsal horn depend on several factors, including the type of re-ceptors activated and the populations of neurons involved. Recently, studies performed by activating and/or recording from identified neurons have importantly contributed to the understanding of serotonergic modulation on dorsal horn circuits.

Molecular mechanism of setron-mediated inhibition of full-length 5-HT3A receptor

Serotonin receptor (5-HT_3AR) is the most common therapeutic target to manage the nausea and vomiting during cancer therapies and in the treatment of irritable bowel syndrome. Setrons, a class of competitive antagonists, cause functional inhibition of 5-HT_3AR in the gastrointestinal tract and brainstem, acting as effective anti-emetic agents. Despite their prevalent use, the molecular mechanisms underlying setron binding and inhibition of 5-HT_3AR are not fully understood. Here, we present the structure of granisetron-bound full-length 5-HT_3AR solved by single-particle cryo-electron microscopy to 2.92 Å resolution. The reconstruction reveals the orientation of granisetron in the orthosteric site with unambiguous density for interacting sidechains. Molecular dynamics simulations and electrophysiology confirm the granisetron binding orientation and the residues central for ligand recognition. Comparison of granisetron-bound 5-HT_3AR with the apo and serotonin-bound structures, reveals key insights into the mechanism underlying 5-HT_3AR inhibition.

Neuropathic Pain After Spinal Cord Injury: Challenges and Research Perspectives

Neuropathic pain is a debilitating consequence of spinal cord injury (SCI) that remains difficult to treat because underlying mechanisms are not yet fully understood. In part, this is due to limitations of evaluating neuropathic pain in animal models in general, and SCI rodents in particular. Though pain in patients is primarily spontaneous, with relatively few patients experiencing evoked pains, animal models of SCI pain have primarily relied upon evoked withdrawals. Greater use of operant tasks for evaluation of the affective dimension of pain in rodents is needed, but these tests have their own limitations such that additional studies of the relationship between evoked withdrawals and operant outcomes are recommended. In preclinical SCI models, enhanced reflex withdrawal or pain responses can arise from pathological changes that occur at any point along the sensory neuraxis. Use of quantitative sensory testing for identification of optimal treatment approach may yield improved identification of treatment options and clinical trial design. Additionally, a better understanding of the differences between mechanisms contributing to at- versus below-level neuropathic pain and neuropathic pain versus spasticity may shed insights into novel treatment options. Finally, the role of patient characteristics such as age and sex in pathogenesis of neuropathic SCI pain remains to be addressed.

5HT3 receptors: Target for new antidepressant drugs

5HT3 receptors (5HT3Rs) have long been identified as a potential target for antidepressants. Several studies have reported that antagonism of 5HT3Rs produces antidepressant-like effects. However, the exact role of 5HT3Rs and the mode of antidepressant action of 5HT3R antagonists still remain a mystery. Here, we provide a comprehensive overview of 5HT3Rs: (a) regional and subcellular distribution of 5HT3Rs in discrete brain regions, (b) preclinical and clinical evidence supporting the antidepressant effect of 5HT3R antagonists, and (c) neurochemical, biological and neurocellular signaling pathways associated with the antidepressant action of 5HT3R antagonists. 5HT3Rs located on the serotonergic and other neurotransmitter interneuronal projections control their release and affect mood and emotional behavior; however, new evidence suggests that apart from modulating the neurotransmitter functions, 5HT3R antagonists have protective effects in the pathogenic events including hypothalamic-pituitary-adrenal-axis hyperactivity, brain oxidative stress and impaired neuronal plasticity, pointing to hereby unknown and novel mechanisms of their antidepressant action. Nonetheless, further investigations are warranted to establish the exact role of 5HT3Rs in depression and antidepressant action of 5HT3R antagonists.

Galanin-Mediated Behavioural Hyperalgesia from the Dorsomedial Nucleus of the Hypothalamus Involves Two Independent Descending Pronociceptive Pathways

Activation of the dorsomedial nucleus of the hypothalamus (DMH) by galanin (GAL) induces behavioural hyperalgesia. Since DMH neurones do not project directly to the spinal cord, we hypothesized that the medullary dorsal reticular nucleus (DRt), a pronociceptive region projecting to the spinal dorsal horn (SDH) and/or the serotoninergic raphe-spinal pathway acting on the spinal 5-HT3 receptor (5HT3R) could relay descending nociceptive facilitation induced by GAL in the DMH. Heat-evoked paw-withdrawal latency (PWL) and activity of SDH neurones were assessed in monoarthritic (ARTH) and control (SHAM) animals after pharmacological manipulations of the DMH, DRt and spinal cord. The results showed that GAL in the DMH and glutamate in the DRt lead to behavioural hyperalgesia in both SHAM and ARTH animals, which is accompanied particularly by an increase in heat-evoked responses of wide-dynamic range neurons, a group of nociceptive SDH neurones. Facilitation of pain behaviour induced by GAL in the DMH was reversed by lidocaine in the DRt and by ondansetron, a 5HT3R antagonist, in the spinal cord. However, the hyperalgesia induced by glutamate in the DRt was not blocked by spinal ondansetron. In addition, in ARTH but not SHAM animals PWL was increased after lidocaine in the DRt and ondansetron in the spinal cord. Our data demonstrate that GAL in the DMH activates two independent descending facilitatory pathways: (i) one relays in the DRt and (ii) the other one involves 5-HT neurones acting on spinal 5HT3Rs. In experimental ARTH, the tonic pain-facilitatory action is increased in both of these descending pathways.

Inhibition of spinal 5-HT3R reverted diabetes-induced mechanical hypersensitivity in a GABAAR-mediated neurotransmission-dependent manner

Spinal 5-HT3 receptor (5-HT3R) has been implicated in chronic pain development. The extent to which 5-HT3R contributes to spinal sensitization and diabetic neuropathic pain (DNP) remains elusive and the mechanisms subserving the effects of 5-HT3R activation on spinal pain processing during chronic pain are still unclear. In this study, we evaluated the contribution of spinal 5-HT3R to pain facilitation and spinal sensitization during DNP, exploiting the role of GABAAR-mediated neurotransmission and glial activation in the effects elicited by intrathecal administration of a 5-HT3R antagonist. Mechanical nociception was evaluated by paw pressure test in streptozotocin (STZ)-diabetic and control rats after intrathecal (i.t.) administration of a 5-HT3R antagonist (Y25130). The spinal activation of extracellular signal-regulated kinases (ERKs) pathway and the expression of 5-HT3R, glial fibrillary acidic protein (GFAP; marker of astroglia activation) and ionized calcium binding adaptor molecule 1 (IBA-1; marker of microglia activation) were evaluated at the peak maximum effect of Y25130. The involvement of GABAAR-mediated neurotransmission in the behavioral pain effect of Y25130, was assessed in STZ-diabetic animals receiving i.t. administrations of muscimol (GABAAR agonist). Intrathecal administration of Y25130 reverted mechanical hyperalgesia and decreased the activation of ERKs in STZ-diabetic rats, while no effects were observed in control animals. The spinal activation of GABAAR by i.t. administration of muscimol abolished Y25130-driven antinociception. The expression of IBA-1, GFAP and 5-HT3R was unaltered by treatment. These findings point to a GABA-mediated pronociceptive role of spinal 5-HT3R during DNP.

Spinal 5-HT3 receptors mediate descending facilitation and contribute to behavioral hypersensitivity via a reciprocal neuron-glial signaling cascade

BACKGROUND

It has been recently recognized that the descending serotonin (5-HT) system from the rostral ventromedial medulla (RVM) in the brainstem and the 5-HT3 receptor subtype in the spinal dorsal horn are involved in enhanced descending pain facilitation after tissue and nerve injury. However, the mechanisms underlying the activation of the 5-HT3 receptor and its contribution to facilitation of pain remain unclear.

RESULTS

In the present study, activation of spinal 5-HT3 receptors by intrathecal injection of a selective 5-HT3 receptor agonist SR 57227 induced spinal glial hyperactivity, neuronal hyperexcitability and pain hypersensitivity in rats. We found that there was neuron-to-microglia signaling via the chemokine fractalkine, microglia to astrocyte signaling via cytokine IL-18, astrocyte to neuronal signaling by IL-1β, and enhanced activation of NMDA receptors in the spinal dorsal horn. Glial hyperactivation in spinal dorsal horn after hindpaw inflammation was also attenuated by molecular depletion of the descending 5-HT system by intra-RVM Tph-2 shRNA interference.

CONCLUSIONS

These findings offer new insights into the cellular and molecular mechanisms at the spinal level responsible for descending 5-HT-mediated pain facilitation during the development of persistent pain after tissue and nerve injury. New pain therapies should focus on prime targets of descending facilitation-induced glial involvement, and in particular the blocking of intercellular signaling transduction between neurons and glia.

Identification of 5-HT receptor subtypes enhancing inhibitory transmission in the rat spinal dorsal horn in vitro

Background

5-hydroxytryptamine (5-HT) is one of the major neurotransmitters widely distributed in the CNS. Several 5-HT receptor subtypes have been identified in the spinal dorsal horn which act on both pre- and postsynaptic sites of excitatory and inhibitory neurons. However, the receptor subtypes and sites of actions as well as underlying mechanism are not clarified rigorously. Several electrophysiological studies have been performed to investigate the effects of 5-HT on excitatory transmission in substantia gelatinosa (SG) of the spinal cord. In the present study, to understand the effects of 5-HT on the inhibitory synaptic transmission and to identify receptor subtypes, the blind whole cell recordings were performed from SG neurons of rat spinal cord slices.

Results

Bath applied 5-HT (50 μM) increased the frequency but not amplitudes of spontaneous inhibitory postsynaptic currents (sIPSCs) in 58% of neurons, and both amplitude and frequency in 23% of neurons. The frequencies of GABAergic and glycinergic mIPSCs were both enhanced. TTX (0.5 μM) had no effect on the increasing frequency, while the enhancement of amplitude of IPSCs was eliminated. Evoked-IPSCs (eIPSCs) induced by focal stimulation near the recording neurons in the presence of CNQX and APV were enhanced in amplitude by 5-HT. In the presence of Ba²⁺ (1 mM), a potassium channel blocker, 5-HT had no effect on both frequency and amplitude. A 5-HT_2A receptor agonist, TCB-2 mimicked the 5-HT effect, and ketanserin, an antagonist of 5-HT_2A receptor, inhibited the effect of 5-HT partially and TCB-2 almost completely. A 5-HT_2C receptor agonist WAY 161503 mimicked the 5-HT effect and this effect was blocked by a 5-HT_2C receptor antagonist, N-desmethylclozapine. The amplitudes of sIPSCs were unaffected by 5-HT_2A or 5-HT_2C agonists. A 5-HT₃ receptor agonist mCPBG enhanced both amplitude and frequency of sIPSCs. This effect was blocked by a 5-HT₃ receptor antagonist ICS-205,930. The perfusion of 5-HT_2B receptor agonist had no effect on sIPSCs.

Conclusions

Our results demonstrated that 5-HT modulated the inhibitory transmission in SG by the activation of 5-HT_2A and 5-HT_2C receptors subtypes located predominantly at inhibitory interneuron terminals, and 5-HT₃ receptors located at inhibitory interneuron terminals and soma-dendrites, consequently enhanced both frequency and amplitude of IPSCs.

Analgesic mechanisms of Fagopyrum cymosum extracts in rats with irritable bowel syndrome

AIM: To observe the effect of Fagopyrum cymosum extracts (Fag) on visceral hypersensitivity in rats with irritable bowel syndrome (IBS)-like colon irritation (CI) and to explore the possible mechanisms involved.

METHODS: IBS-like models were created using neonatal CI method. CI adult rats were given Fag intragastrically for 2 wk, and visceral sensitivity was then evaluated using abdominal withdrawal reflex (AWR) score. The expression of 5-hydroxytryptamine (5-HT) in the spinal cord was detected by immunohistochemistry and that of 5-HT1A receptor (5-HT1AR) and 5-HT3A receptor (5-HT3AR) was detected by Western blot.

RESULTS: Compared to control rats, AWR scores in CI rats were significantly increased (20 mmHg: 0.625 ± 0.518 vs 1.333 ± 0.778; 30 mmHg: 0.750 ± 0.463 vs 1.667 ± 0.888; 40 mmHg: 1.125 ± 0.641 vs 2.000 ± 0.739; 50 mmHg: 1.500 ± 0.926 vs 2.583 ± 0.793; 60 mmHg: 2.125 ± 0.991 vs 3.083 ± 0.669; all P < 0.05). In the spinal cord, the expression of 5-HT and 5-HT3AR was enhanced and that of 5-HT1AR expression was reduced in CI rats compared to control rats (all P < 0.01). Fag at a high dose could markedly reduce AWR scores (20 mmHg: 0.250 ± 0.002; 30 mmHg: 0.875 ± 0.044; 40 mmHg: 1.250 ± 0.036; 50 mmHg: 1.875 ± 0.050; 60 mmHg: 2.625 ± 0.037; all P < 0.05), down-regulate the expression of 5-HT and 5-HT3AR, and up-regulate the expression of 5-HT1AR (all P < 0.05) in spinal cord (P < 0.05) in CI rats. Fag at a low dose had no significant effect on CI rats.

CONCLUSION: Fag can exert an analgesic role in IBS-like CI rats by regulating 5-HT and its receptors in the spinal cord.

Distinct mechanisms underlying pronociceptive effects of opioids

In addition to analgesia, opioids may also produce paradoxical pain amplification [opioid-induced hyperalgesia (OIH)] either on abrupt withdrawal or during continuous long-term application. Here, we assessed antinociceptive and pronociceptive effects of three clinically used opioids at C-fiber synapses in the rat spinal dorsal horn in vivo. During 60 min of intravenous infusions of remifentanil (450 μg·kg⁻¹·h⁻¹), fentanyl (48 μg·kg⁻¹·h⁻¹), or morphine (14 mg·kg⁻¹·h⁻¹), C-fiber-evoked field potentials were depressed and paired-pulse ratios (PPR) were increased, indicating a presynaptic inhibition by all three opioids. After withdrawal, postsynaptic responses were enhanced substantially for the remaining of the recording periods of at least 3 h. Withdrawal from remifentanil led to long-term potentiation (LTP) of synaptic strength in C-fibers via activation of spinal μ-opioid receptors (MORs) and spinal NMDA receptors (NMDARs). Fentanyl and morphine caused an enhancement of synaptic transmission at C-fibers, which involved two distinct mechanisms: (1) an opioid withdrawal LTP that also required activation of spinal MORs and NMDARs and that was associated with a decrease in PPR suggestive of a presynaptic mechanism of its expression, and (2) an immediate-onset, descending facilitation of C-fiber-evoked field potentials during and after intravenous infusion of fentanyl and morphine. Immediate-onset, descending facilitation was mediated by the activation of extraspinal MORs, descending serotonergic pathways, and spinal 5-hydroxytryptamine-3 receptors (5-HT₃Rs). Our study identified fundamentally different pronociceptive effects of clinically used opioids and suggests that OIH can be prevented by the combined use of NMDAR and 5-HT₃R antagonists.

Detection of mRNA encoding receptors by in situ and Northern hybridization

The ability to measure mRNA encoding protein is attractive, since, provided certain criteria are met, the investigator can be confident of highly selective detection. In addition, changes in mRNA expression as well as their precise cellular localization, provides the scientist with important information that may not be evident by the detection of the translated protein. Hence, the combined approach of assessing mRNA and protein expression will often allow a more precise hypothesis to be formulated. In the present chapter, we describe methods we have optimized for the detection of mRNA following extraction from tissue or cells (Northern hybridization) and the detection of specific mRNA transcripts within their synthesising cells (in situ hybridization).

Spinal 5-HT(3) receptor activation induces behavioral hypersensitivity via a neuronal-glial-neuronal signaling cascade

Recent studies indicate that the descending serotonin (5-HT) system from the rostral ventromedial medulla (RVM) in the brainstem and the 5-HT(3) receptor subtype in the spinal dorsal horn are involved in enhanced descending pain facilitation after tissue and nerve injury. However, the mechanisms underlying the activation of the 5-HT(3) receptor and its contribution to facilitation of pain remain unclear. In the present study, activation of spinal 5-HT(3) receptor by intrathecal injection of a selective 5-HT(3) receptor agonist, SR57227, induced spinal glial hyperactivity, neuronal hyperexcitability, and pain hypersensitivity in rats. We found that there was neuron-to-microglia signaling via chemokine fractalkine, microglia to astrocyte signaling via the cytokine IL-18, astrocyte to neuronal signaling by IL-1β, and enhanced activation of GluN (NMDA) receptors in the spinal dorsal horn. In addition, exogenous brain-derived neurotrophic factor-induced descending pain facilitation was accompanied by upregulation of CD11b and GFAP expression in the spinal dorsal horn after microinjection in the RVM, and these events were significantly prevented by functional blockade of spinal 5-HT(3) receptors. Enhanced expression of spinal CD11b and GFAP after hindpaw inflammation was also attenuated by molecular depletion of the descending 5-HT system by intra-RVM Tph-2 shRNA interference. Thus, these findings offer new insights into the cellular and molecular mechanisms at the spinal level responsible for descending 5-HT-mediated pain facilitation during the development of persistent pain after tissue and nerve injury. New pain therapies should focus on prime targets of descending facilitation-induced glial involvement, and in particular the blocking of intercellular signaling transduction between neuron and glia.

5-hydroxytryptamine type 3 receptor modulates opioid-induced hyperalgesia and tolerance in mice

BACKGROUND

Opioid-induced hyperalgesia (OIH) and tolerance are challenging maladaptations associated with opioids in managing pain. Recent genetic studies and the existing literature suggest the 5-hydroxytryptamine type 3 (5-HT3) receptor participates in these phenomena. The location of the relevant receptor populations and the interactions between the 5-HT3 system and other systems controlling OIH and tolerance have not been explored, however. We hypothesized that 5-HT3 receptors modulate OIH and tolerance, and that this modulation involves the control of expression of multiple neurotransmitter and receptor systems.

METHODS

C57BL/6 mice were exposed to a standardized 4-day morphine administration protocol. The 5-HT3 antagonist ondansetron was administered either during or after the conclusion of morphine administration. Mechanical testing was used to quantify OIH, and thermal tail-flick responses were used to measure morphine tolerance. In other experiments spinal cord and dorsal root ganglion tissues were harvested for analysis of messenger RNA concentrations by real-time polymerase chain reaction or immunochemistry analysis.

RESULTS

The results showed that (1) systemic or intrathecal injection of ondansetron significantly prevented and reversed OIH, but not local intraplantar injection; (2) systemic or intrathecal injection of ondansetron prevented and reversed tolerance; and (3) ondansetron blocked morphine-induced increases of multiple genes relevant to OIH and tolerance in dorsal root ganglion and spinal cord.

CONCLUSIONS

Morphine acts via a 5-HT3-dependent mechanism to support multiple maladaptations to the chronic administration of morphine. Furthermore, the use of 5-HT3 receptor antagonists may provide a new avenue to prevent or reverse OIH and tolerance associated with chronic opioid use.

Spinal 5-HT receptors that contribute to the pain-relieving effects of spinal cord stimulation in a rat model of neuropathy

Spinal cord stimulation (SCS) is extensively employed in the management of neuropathic pain, but the underlying mechanisms are only partially understood. Recently, we demonstrated that the pain-relieving effect of SCS appears to involve the spinal serotonin system, and the present study aimed at identifying the types of the spinal serotonin receptors involved. Experiments were performed on rats with neuropathy produced by partial ligation of the sciatic nerve. Tactile sensitivity was assessed using von Frey filaments, and cold and heat sensitivity with cold spray and radiant heat, respectively. Selective 5-HT receptor antagonists, methiothepin (5-HT(1,6,7)), ketanserin tartrate (5-HT(2A)), TICM (5-HT(3)), SDZ-205,557 (5-HT(4)), as well as receptor agonists, α-m-5-HT (5-HT(2)), m-CPBG (5-HT(3)) in per se ineffective doses, or vehicle, were administrated intrathecally 5 minutes prior to the application of SCS. Ketanserin and SDZ-205,557 significantly attenuated the suppressive effect of SCS on tactile hypersensitivity, while methiothepin and TICM were ineffective. The suppressive effect on cold hypersensitivity of SCS was counteracted by ketanserin only. None of the 5-HT receptor antagonists attenuated the suppressive effect on heat hyperalgesia of SCS. Subeffective doses of α-m-5-HT and m-CPBG enhanced the suppressive effect of SCS on tactile hypersensitivity. The enhancing effect of m-CPBG was abolished by a γ-aminobutyric acid (GABA)(A) or GABA(B) antagonist intrathecally. These results suggest that the activation of 5-HT(2A), 5-HT(3), and 5-HT(4) receptors plays an important role in SCS-induced relief of neuropathic pain. The activation of 5-HT(3) receptors appears to operate via spinal GABAergic interneurons.

Role of 5-HT3 Receptors in the Antidepressant Response

Serotonin (5-HT)₃ receptors are the only ligand-gated ion channel of the 5-HT receptors family. They are present both in the peripheral and central nervous system and are localized in several areas involved in mood regulation (e.g., hippocampus or prefrontal cortex). Moreover, they are involved in regulation of neurotransmitter systems implicated in the pathophysiology of major depression (e.g., dopamine or GABA). Clinical and preclinical studies have suggested that 5-HT₃ receptors may be a relevant target in the treatment of affective disorders. 5-HT₃ receptor agonists seem to counteract the effects of antidepressants in non-clinical models, whereas 5-HT₃ receptor antagonists, such as ondansetron, present antidepressant-like activities. In addition, several antidepressants, such as mirtazapine, also target 5-HT₃ receptors. In this review, we will report major advances in the research of 5-HT₃ receptor's roles in neuropsychiatric disorders, with special emphasis on mood and anxiety disorders.

Spinal cord mechanisms mediating behavioral hyperalgesia induced by neurokinin-1 tachykinin receptor activation in the rostral ventromedial medulla

Hyperalgesia in animal injury models is linked to activation of descending raphespinal modulatory circuits originating in the rostral ventromedial medulla (RVM). A neurokinin-1 (NK-1) receptor antagonist microinjected into the RVM before or after inflammation produced by complete Freund's adjuvant (CFA) resulted in an attenuation of thermal hyperalgesia. A transient (acute) or a continuous infusion of Substance P (SP) microinjected into the RVM of non-inflamed animals led to similar pain hypersensitivity. Intrathecal pretreatment or post-treatment of a 5-HT3 receptor antagonist (Y-25130 or ondansetron) blocked the SP-induced hyperalgesia. The SP-induced hyperalgesia was both GABA(A) and NMDA receptor-dependent after pre- and post-treatment with selective antagonists at the spinal level. A microinjection of SP into the RVM also led to increased NMDA NR1 receptor subunit phosphorylation in spinal cord tissue. The GABA(A) receptor-mediated hyperalgesia involved a shift in the anionic gradient in dorsal horn nociceptive neurons and an increase in phosphorylated NKCC1 protein (isoform of the Na-K-Cl cotransporter). Following a low dose of SP infused into the RVM, intrathecal muscimol (GABA(A) agonist) increased SP-induced thermal hyperalgesia, phosphorylated NKCC1 protein expression, and NMDA NR1 subunit phosphorylation in the spinal cord. The thermal hyperalgesia was blocked by intrathecal gabazine, the GABA(A) receptor antagonist, and MK-801, the NMDA receptor channel blocker. These findings indicate that NK-1 receptors in the RVM are involved in SP-induced thermal hyperalgesia, this hyperalgesia is 5-HT3-receptor dependent at the spinal level, and involves the functional interaction of spinal GABA(A) and NMDA receptors.

Facilitatory actions of serotonin type 3 receptors on GABAergic inhibitory synaptic transmission in the spinal superficial dorsal horn

Analgesic effects of serotonin (5-hydroxytryptamine [5-HT]) type 3 (5-HT3) receptors may involve the release of gamma-aminobutyric acid (GABA) in the spinal dorsal horn. However, the precise synaptic mechanisms for 5-HT3 receptor-mediated spinal analgesia are not clear. In this study, we investigated whether GABAergic neurons in the superficial dorsal horn (SDH) express functional 5-HT3 receptors and how these 5-HT3 receptors affect GABAergic inhibitory synaptic transmission in the SDH, by using slice preparations from adult glutamate decarboxylase 67-green fluorescent protein (GAD67-GFP) knock-in mice. Tight-seal whole cell recordings from GFP-positive and -negative neurons showed that 5-HT3 receptor-specific agonist 2-methyl-serotonin (2-Me-5-HT) induced inward currents in a substantial population of both GFP-positive and -negative neurons. Additionally, we confirmed expression of 5-HT3 receptors in both types of neurons by single-cell reverse transcription-polymerase chain reaction (RT-PCR) analysis. Further, GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs)-both those evoked by electrical stimulation and those occurring spontaneously in tetrodotoxin (i.e., miniature IPSCs [mIPSCs])-were recorded from GFP-negative neurons. 2-Me-5-HT increased the amplitude of the evoked IPSCs and the frequency of mIPSCs. The amplitude of mIPSCs was not affected by 2-Me-5-HT, suggesting that 5-HT augments GABAergic synaptic transmission via presynaptic mechanisms. The present observations indicate that 5-HT3 receptors are expressed on both somadendritic regions and presynaptic terminals of GABAergic neurons and regulate GABAA receptor-mediated inhibitory synaptic transmission in the SDH. Taken together, these results provide clues for the underlying mechanisms of the antinociceptive actions of 5-HT3 receptors in the spinal dorsal horn.

The 5-HT3 receptor as a therapeutic target

The 5-HT3 receptor is a neurotransmitter-gated ion channel. It is a member of the Cys-loop family of receptors, which also includes nicotinic acetylcholine, glycine and GABAA receptors. Each member of the family consists of an arrangement of five subunits surrounding a central ion-conducting pore. The 5-HT3 receptor binding site is composed of six loops from two adjacent subunits, and the critical ligand binding residues within these loops are well documented. There are a range of 5-HT3 receptor agonists and competitive antagonists, but it is the antagonists that dominate their clinical use. Studies have proposed a range of disease symptoms that might be amenable to 5-HT3 receptor selective compounds; however, so far only the treatment of emesis and irritable bowel syndrome have been fully realised. In this review, the authors look at the structure, function and distribution of 5-HT3 receptors and how this may influence their role in disease. The authors also describe the existing clinical applications of 5-HT3 antagonists and the future potential of these drugs.

Identification of 5-HT3A and 5-HT3B receptor subunits in human hippocampus

The pentameric 5-HT(3) receptor complex is a ligand-gated ion channel that mediates fast synaptic transmission in the brain. Expression of two subunits (5-HT(3A) and 5-HT(3B) subunits) gives rise to at least two receptor isoforms (homomeric 5-HT(3A) and heteromeric 5-HT(3A/3B) receptors), which differ in their biophysical characteristics, although expression of these proteins has not been investigated in human brain. The expression of h5-HT(3A) and 5-HT(3B) subunits in the human hippocampus was investigated using selective polyclonal antibodies (SDS-PAGE/Western blotting, immunohistochemistry), with expression of each subunit verified by PCR detection of subunit transcripts. 5-HT(3A) and 5-HT(3B) subunit immunoreactivity was identified within the human hippocampus. The cellular pattern of expression for each subunit was similar, with predominant immunoreactivity associated with pyramidal neurones in CA fields 2 and 3, and also the relatively large neurones within the hilus (CA4 field). Transcripts for each subunit were also identified in human hippocampal tissue. These findings indicate that human hippocampal neurones are capable of forming at least two, functionally different, isoforms of the 5-HT(3) receptor. Furthermore the expression pattern of 5-HT(3A) and 5-HT(3B) subunits in human hippocampus appears to differ with the rodent counterpart, which may underlie the differences in some of the behavioural effects of 5-HT(3) receptor antagonists between these species.

Contribution of peripheral 5-HT2A or 5-HT3 receptors to Fos expression in the trigeminal spinal nucleus produced by acute injury to the masseter muscle during persistent temporomandibular joint inflammation in rats

We investigated the contribution of peripheral 5-HT2A or 5-HT3 receptors to Fos expression in the trigeminal spinal nucleus (VSP) following acute masseter muscle injury in male rats with or without temporomandibular joint (TMJ) inflammation persisting for 7 days. TMJ inflammation was evoked by an injection of complete Freund's adjuvant (CFA). Two hours after formalin injection into the masseter muscle produced Fos-like immunoreactivity (Fos-LI) in several regions of the VSP and upper cervical spinal cord (C2), such as ventrolateral (vl) area of the trigeminal subnucleus caudalis (Vc)/subnucleus interpolaris (Vi) transition (vl-Vi/Vc), paratrigeminal nucleus (dPa5), middle portion of the Vc (mid-Vc) and Vc/C2 transition (Vc/C2) regions in both groups. Significant increases in the number of Fos-LI were observed in these areas in CFA group compared with non-CFA group. TMJ inflammation alone did not induce a significant level of Fos-LI in the VSP. In order to assess the effect of antagonizing 5-HT2A or 5-HT3 receptors on formalin-induced Fos-LI, rats were pre-treated with local (masseter muscle) administration of ketanserin or tropisetron (0.01, 0.1 mg/rat) 20 min prior to formalin injection. In CFA group, these antagonists given locally reduced the Fos-LI response in the laminae I-II at the mid-Vc and Vc/C2 regions. These antagonists reduced the Fos-LI response in the dPa5, but not in the vl-Vi/Vc region. The Fos-LI response was not affected by i.v. administration of ketanserin (0.01, 0.1 mg/rat) or tropisetron (0.01 mg/rat). In non-CFA group, these antagonists given locally did not reduce the Fos-LI response. These results suggest that peripheral 5-HT2A and 5-HT3 receptors contribute to nociceptive processing in the masseter muscle in TMJ inflammatory conditions.

Descending serotonergic facilitation of spinal ERK activation and pain behavior

Serotonin (5-HT) derived from bulbo-spinal projection is released by nociceptive input into the spinal dorsal horn. Here we report that formalin injection in the paw produced pain behavior (flinching) and phosphorylation of spinal ERK1/2 (P-ERK1/2, indicating activation) in rats. Depletion of spinal 5-HT by intrathecal (IT) 5,7-DHT, a serotonergic neurotoxin, profoundly reduced formalin evoked flinching and the increase in P-ERK1/2. Ondansetron (a 5-HT3 receptor antagonist) at IT doses that inhibited flinching also attenuated spinal ERK activation. These findings reveal that primary afferent-evoked activation of spinal ERK requires the input from an excitatory 5-HT descending pathway.

Potentiation of transient receptor potential V1 functions by the activation of metabotropic 5-HT receptors in rat primary sensory neurons

5-Hydroxytryptamine (5-HT) is one of the major chemical mediators released in injured and inflamed tissue and is capable of inducing hyperalgesia in vivo. However, the cellular mechanisms of 5-HT-induced hyperalgesia remain unclear. Transient receptor potential V1 (TRPV1) plays a pivotal role in nociceptive receptors. In the present study, we determined whether 5-HT changes TRPV1 functions in cultured dorsal root ganglion (DRG) neurons isolated from neonatal rats, using Ca(2+) imaging and whole-cell patch-clamp techniques. In more than 70% of DRG neurons, 5-HT potentiated the increases of [Ca(2+)](i) induced by capsaicin, protons and noxious heat. Capsaicin-induced current and depolarizing responses, and proton-induced currents were also augmented by 5-HT. RT-PCR analysis revealed the expression of 5-HT(2A) and 5-HT(7) receptors in rat DRG neurons. Agonists for 5-HT(2A) and 5-HT(7) receptors mimicked the potentiating effect of 5-HT, and their antagonists decreased it. In DRG ipsilateral to the complete Freund's adjuvant-injected inflammation side, expression levels of 5-HT(2A) and 5-HT(7) mRNAs increased, and the potentiating effect of 5-HT was more prominent than in the contralateral control side. These results suggest that the PKC- and PKA-mediated signalling pathways are involved in the potentiating effect of 5-HT on TRPV1 functions through the activation of 5-HT(2A) and 5-HT(7) receptors, respectively. Under inflammatory conditions, the increases of the biosynthesis of these 5-HT receptors may lead to further potentiation of TRPV1 functions, resulting in the generation of inflammatory hyperalgesia in vivo.

Ptychodiscus brevis toxin-induced depression of spinal reflexes involves 5-HT via 5-HT3 receptors modulated by NMDA receptor

The involvement of 5-hydroxytryptaminergic (5-HT) system for the Ptychodiscus brevis toxin (PbTx)-induced depression of spinal reflexes was evaluated. The reflex potentials were recorded at ventral root by stimulating the corresponding dorsal root in neonatal rat spinal cord in vitro. Superfusion of PbTx (2.8-84microM) depressed the monosynaptic (MSR) and polysynaptic (PSR) reflexes in a concentration-dependent manner. The depression of the reflexes was maximal with 84microM of the toxin. Ondansetron (0.1microM), a 5-HT(3) receptor antagonist, blocked the PbTx-induced depression of MSR and PSR. Spiperone (a 5-HT(2A) antagonist) or ketanserin (5-HT(2A/2C) antagonist and also at 5-HT(1B/1D)) failed to block the PbTx-induced depression of the reflexes. The 5-HT concentration of the cords was increased by four-fold after exposure to PbTx (28microM) and the increase was not seen in the cords pretreated with dl-2 amino-5-phosphonovaleric acid (APV, a NMDA receptor antagonist). Superfusion of 5-HT or phenylbiguanide (PBG, a 5-HT(3) receptor agonist) also produced depression of the spinal reflexes in a concentration-dependent manner. The 5-HT-induced depression of reflexes was blocked by ondansetron but not by spiperone. The results demonstrate that the PbTx-induced depression of spinal reflexes involves 5-hydroxytryptamine via 5-HT(3) receptors modulated by NMDA receptor.

Spinal-supraspinal serotonergic circuits regulating neuropathic pain and its treatment with gabapentin

Not all neuropathic pain patients gain relief from current therapies that include the anticonvulsant, gabapentin, thought to modulate calcium channel function. We report a neural circuit that is permissive for the effectiveness of gabapentin. Substance P-saporin (SP-SAP) was used to selectively ablate superficial dorsal horn neurons expressing the neurokinin-1 receptor for substance P. These neurons project to the brain as shown by retrograde labelling and engage descending brainstem serotonergic influences that enhance spinal excitability via a facilitatory action on 5HT(3) receptors. We show the integrity of this pathway following nerve injury contributes to the behavioural allodynia, neuronal plasticity of deep dorsal horn neurons and the injury-specific actions of gabapentin. Thus SP-SAP attenuated the tactile and cold hypersensitivity and abnormal neuronal coding (including spontaneous activity, expansion of receptive field size) seen after spinal nerve ligation. Furthermore the powerful actions of gabapentin after neuropathy were blocked by either ablation of NK-1 expressing neurones or 5HT(3) receptor antagonism using ondansetron. Remarkably, 5HT(3) receptor activation provided a state-dependency (independent of that produced by neuropathy) allowing GBP to powerfully inhibit in normal uninjured animals. This circuit is therefore a crucial determinant of the abnormal neuronal and behavioural manifestations of neuropathy and importantly, the efficacy of gabapentin. As this spino-bulbo-spinal circuit contacts areas of the brain implicated in the affective components of pain, this loop may represent a route by which emotions can influence the degree of pain in a patient, as well as the effectiveness of the drug treatment. These hypotheses are testable in patients.

Central serotonin 3 receptors play an important role in the modulation of nociceptive neural activity of trigeminal subnucleus caudalis and nocifensive orofacial behavior in rats with persistent temporomandibular joint inflammation

The role of central serotonin 3 receptors on neural activities recorded from superficial laminae of trigeminal subnucleus caudalis/upper cervical spinal cord junction region was investigated using rats with (Complete Freund's Adjuvant day 7 group) or without (non-Complete Freund's Adjuvant group) persistent temporomandibular joint inflammation evoked by Complete Freund's Adjuvant for 7 days. We identified two types of units, Deep-wide dynamic range units and Skin-wide dynamic range units from extracellular recordings. Deep-wide dynamic range units have mechanoreceptive fields in the deep craniofacial tissues including masseter muscle but do not have cutaneous mechanoreceptive fields. Deep-wide dynamic range unit discharges evoked by the formalin injection into masseter muscle were significantly enhanced in the late phase in Complete Freund's Adjuvant day 7 group. Discharges of Skin-wide dynamic range units evoked by the noxious pinch stimulation to facial skin in Complete Freund's Adjuvant day 7 group were significantly enhanced compared with those in non-Complete Freund's Adjuvant group. Topical administration of central serotonin 3 receptor antagonist, tropisetron, onto trigeminal subnucleus caudalis/upper cervical spinal cord junction region significantly reduced both formalin-evoked Deep-wide dynamic range unit and pinch-evoked Skin-wide dynamic range unit discharges in non-Complete Freund's Adjuvant and Complete Freund's Adjuvant day 7 groups significantly. The inhibitory effects of tropisetron on pinch-evoked Skin-wide dynamic range unit discharges were prolonged in Complete Freund's Adjuvant day 7 group compared with those in non-Complete Freund's Adjuvant group. The role of central serotonin 3 receptors in trigeminal subnucleus caudalis/upper cervical spinal cord junction region was also tested by orofacial formalin test in Complete Freund's Adjuvant day 7 group. Intracisternal administration of tropisetron decreased the orofacial nocifensive behavior in the late phase evoked by the injection of formalin into the masseter muscle. These results suggest that central serotonin 3 receptors in trigeminal subnucleus caudalis/upper cervical spinal cord junction region are involved in mediating pronociceptive effects in both superficial and deep craniofacial tissues nociception during persistent temporomandibular joint inflammation.

Serotonin-related enhancement of recovery of hind limb motor functions in spinal rats after grafting of embryonic raphe nuclei

Recently, we demonstrated improvements in hind limb locomotor-like movements following grafting of embryonic raphe nuclei cells into the spinal cord below the level of total transection in adult rats. The purpose of the present study was to clarify whether this improvement was due to newly established serotonergic innervation between the graft and the host. Two months after intraspinal grafting of the embryonic raphe nuclei, the spinalized rats, when put on a treadmill, could be induced to walk with regular alternating hind limb movements with the plantar contact with the ground during the stance phase, and ankle dorsiflexion during the swing phase of each step cycle. In the same situation the spinal rats, that did not receive the graft, were not able to initiate the dorsiflexion of the ankle joint during the swing phase and very often the dorsal surface of the foot was dragged along the ground. Intraperitoneal application of directly acting 5-HT2 antagonist Cyproheptadine (1 mg/kg) impaired reversibly the hind limb locomotor-like movements in grafted rats. This impairment lasted for 2-3 h. The same procedure in control rats did not markedly alter the hind limb locomotor-like movements. The effect of Cyproheptadine in grafted rats was reversed by i.p. injections of the 5-HT2 agonist Quipazine (0.5 mg/kg). These results show that the graft-induced restitution of hind limb locomotor abilities in adult spinal rats is brought about by the new serotonergic innervation of the host spinal cord circuitry from the grafted neurons and is mediated by 5-HT2 receptors.

Serotonin excites neurons in the human submucous plexus via 5-HT3 receptors

BACKGROUND & AIMS

Serotonin (5-hydroxytryptamine [5-HT]) is a key signaling molecule in the gut. Recently, the neural 5-HT3 receptor received a lot of attention as a possible target in functional bowel diseases. Yet, the 5-HT3 receptor-mediated changes in properties of human enteric neurons is unknown.

METHODS

We used a fast imaging technique in combination with the potentiometric dye 1-(3-sulfonatopropyl)-4-[beta[2-(di-n-octylamino)-6-naphthyl]vinyl]pyridinium betaine to monitor directly the membrane potential changes in neurons of human submucous plexus from surgical specimens of 21 patients. An Ussing chamber technique was used to study 5-HT3 receptor involvement in chloride secretion.

RESULTS

Local microejection of 5-HT directly onto ganglion cells resulted in a transient excitation of enteric neurons characterized by increased spike discharge. This response was mimicked by the 5-HT3 receptor agonist, 2-methyl-5-HT, and blocked by the 5-HT3 receptor antagonist, tropisetron. The proportions of 5-HT-responsive nerve cells per ganglion ranged from 25.5% +/- 18.4% in the duodenum to 54.2% +/- 46.9% in the colon. Interestingly, 2-methyl-5-HT did not evoke chloride secretion in the human intestine but it did in the guinea-pig intestine. Specific 5-HT3A and 5-HT3B receptor subunit immunoreactivity as well as 5-HT3A and 5-HT3B receptor-specific messenger RNA were detected in the tissue samples. Based on co-labeling with the pan-neuronal marker HuC/D we conclude that submucous nerve cells potentially express heteromeric 5-HT3A/B receptors.

CONCLUSIONS

We show that 5-HT excited human enteric neurons via 5-HT3 receptors, which may comprise both 5-HT3A and 5-HT3B receptor subunits.

Evidence for a spinal serotonergic control of the peripheral inflammation in the rat

We investigated the effect of serotonergic agonists and antagonists injected intrathecally by direct punction of the spinal cord at the lumbar level (between L5-L6) on peripheral inflammatory edema. Edema was induced by carrageenan injected subcutaneously in one hindpaw 30 min after spinal treatments. Serotonin (0.1, 1, 10 pmol) caused a graded-inhibition of the inflammatory paw edema. The corticosteroid inhibitor aminoglutethimide (100 mg/kg, p.o. 1.5 h before spinal treatment) did not modify this effect. The 5-HT1A agonist buspirone and the 5-HT1B/1D agonist sumatriptan (0.1, 1.0 and 10 nmol) also inhibited paw edema. The 5-HT1,2 antagonist methysergide (10 and 100 pmol) enhanced edema, but higher doses ( 4 and 8 nmol) diminished edema. NAN-190 (5-HT1 antagonist; 1 and 10 nmol) increased paw edema, while ritanserin (5-HT2 antagonist; 1 nmol) inhibited paw edema. Ondansetron (5-HT3 antagonist; up to 10 nmol) did not affect edema, but metoclopramide (5-HT3 antagonist / 5-HT4 agonist; 5, 10 and 30 pmol) inhibited edema. These data suggest that a tonic release of serotonin in the spinal cord may occurs during ongoing peripheral inflammation, modulating the neurogenic component of edema either by an inhibitory action on 5-HT1 receptors or by a stimulatory action on 5-HT2 receptors. A disfunction in such mechanism may be involved in the pathophysiology of certain types of headaches or migraine, which seem to depend on neurogenic vasodilation, and may also help to explain the therapeuthic effectiveness of some serotonergic agents in these conditions.

Potentiation of 5-HT3 receptor function by the activation of coexistent 5-HT2 receptors in trigeminal ganglion neurons of rats

5-HT receptor subtypes are widely expressed in primary sensory neurons, yet so far little is known about the interaction among them. This study aimed to investigate whether the activation of 5-HT2 and 5-HT1 receptors could modulate 5-HT3 receptor mediated current in rat trigeminal ganglion (TG) neurons using whole-cell patch clamp technique. The majority of TG neurons examined responded to 5-HT (10(-7)-10(-3) M) with a fast activating and rapid desensitizing inward current (77.2%, 71/92). This 5-HT activated current (I(5-HT)) was blocked by ICS 205-930 and mimicked by 2-methyl-5-HT, indicating that it was mediated by 5-HT3 receptor. With alpha-methyl-5-HT applied prior to 5-HT application, I(5-HT) was potentiated in a concentration-dependent manner, with the maximal modulatory effect at 10(-9) M of alpha-methyl-5-HT. The concentration-response curve for I(5-HT) pretreated with alpha-methyl-5-HT shifts upwards compared with that for I(5-HT) without alpha-methyl-5-HT pretreatment, the maximal I(5-HT) value having increased by (60.3 +/- 5.7)% of its control while the EC50 values of the two curves being very close, i.e. (2.0 +/- 0.3) x 10(-5) M vs (1.7 +/- 0.2) x 10(-5) M, respectively. The alpha-methyl-5-HT potentiation of I(5-HT) was removed by intracellular dialysis of either GDP-beta-S, a non-hydrolyzable GDP analog, or GF109203X, a selective PKC inhibitor, almost completely. Preapplication of R-(+)-UH-301, a selective agonist of 5-HT(1A) receptor, had no modulatory effect on I(5-HT). These results suggest that in the membrane of TG neurons, the activation of 5-HT2 receptors can exert an enhancing effect on the function of coexistent 5-HT3 receptors while that of 5-HT(1A) receptors cannot.

Transmitter content, origins and connections of axons in the spinal cord that possess the serotonin (5-hydroxytryptamine) 3 receptor

Recent evidence suggests that serotonin has pronociceptive actions in the spinal cord when it acts through 5-hydroxytryptamine (5-HT)(3) receptors. Cells and axon terminals which are concentrated in the superficial dorsal horn possess this receptor. We performed a series of immunocytochemical studies with an antibody raised against the 5-HT(3A) subunit in order to address the following questions: 1) Are axons that possess 5-HT(3) receptors excitatory? 2) Are 5-HT(3) receptors present on terminals of myelinated primary afferents? 3) What is the chemical nature of dorsal horn cells that possess 5-HT(3) receptors? 4) Do axons that possess 5-HT(3) receptors target lamina I projection cells? Approximately 45% of 5-HT(3A) immunoreactive boutons were immunoreactive for the vesicular glutamate transporter 2 and almost 80% formed synapse-like associations with GluR2 subunits of the AMPA receptor therefore it is principally glutamatergic axons that possess the receptor. Immunoreactivity was not present on myelinated primary afferent axons labeled with the B-subunit of cholera toxin or those containing the vesicular glutamate transporter 1. Calbindin (which is associated with excitatory interneurons) was found in 44% of 5-HT(3A) immunoreactive cells but other markers for inhibitory and excitatory cells were not present. Lamina I projection cells that possessed the neurokinin-1 receptor were associated with 5-HT(3A) axons but the density of contacts on individual neurons varied considerably. The results suggest that 5-HT(3) receptors are present principally on terminals of excitatory axons, and at least some of these originate from dorsal horn interneurons. The relationship between lamina I projection cells and axons possessing the 5-HT(3) receptor indicates that this receptor has an important role in regulation of ascending nociceptive information.

Uropathic observations in mice expressing a constitutively active point mutation in the 5-HT3A receptor subunit

Mutant mice with a hypersensitive serotonin (5-HT)3A receptor were generated through targeted exon replacement. A valine to serine mutation (V13'S) in the channel-lining M2 domain of the 5-HT3A receptor subunit rendered the 5-HT3 receptor 70-fold more sensitive to serotonin and produced constitutive activity when combined with the 5-HT3B subunit. Mice homozygous for the mutant allele (5-HT3Avs/vs) had decreased levels of 5-HT3A mRNA. Measurements on sympathetic ganglion cells in these mice showed that whole-cell serotonin responses were reduced, and that the remaining 5-HT3 receptors were hypersensitive. Male 5-HT3Avs/vs mice died at 2-3 months of age, and heterozygous (5-HT3Avs/+) males and homozygous mutant females died at 4-6 months of age from an obstructive uropathy. Both male and female 5-HT3A mutant mice had urinary bladder mucosal and smooth muscle hyperplasia and hypertrophy, whereas male mutant mice had additional prostatic smooth muscle and urethral hyperplasia. 5-HT3A mutant mice had marked voiding dysfunction characterized by a loss of micturition contractions with overflow incontinence. Detrusor strips from 5-HT3Avs/vs mice failed to contract to neurogenic stimulation, despite overall normal responses to a cholinergic agonist, suggestive of altered neuronal signaling in mutant mouse bladders. Consistent with this hypothesis, decreased nerve fiber immunoreactivity was observed in the urinary bladders of 5-HT3Avs/vs compared with 5-HT3A wild-type (5-HT3A+/+) mice. These data suggest that persistent activation of the hypersensitive and constitutively active 5-HT3A receptor in vivo may lead to excitotoxic neuronal cell death and functional changes in the urinary bladder, resulting in bladder hyperdistension, urinary retention, and overflow incontinence.

The 5-HT3 receptor facilitates at-level mechanical allodynia following spinal cord injury

Spinal cord injury (SCI) results in the development of mechanical allodynia immediately rostral to the lesion site, within the dermatome border of normal sensation and sensory loss (at-level mechanical allodynia). We propose that an observed threefold increase in serotonergic fibre immunoreactivity within spinal segments corresponding to these allodynic dermatomes facilitates the maintenance of chronic neuropathic pain via activation of the 5-HT(3) receptor (5-HT(3)-R). Serotonin (5-HT), the non-selective 5-HT(1)/5-HT(2) receptor antagonist, methysergide, the 5-HT(3)-R agonist, m-chlorophenylbiguanide (m-CPBG) or the 5-HT(3)-R antagonist, ondansetron were intrathecally administered five weeks following SCI in rats. Ondansetron produced a robust, long-term reduction of at-level mechanical allodynia, while m-CPBG exacerbated allodynia. Exogenous 5-HT transiently reduced at-level mechanical allodynia. This effect was opposed by methysergide, which enhanced mechanical allodynia. Co-administration of 5-HT and ondansetron produced a short-lasting partial summation of effects, further decreasing mechanical allodynia while co-administration of methysergide attenuated the anti-allodynic effect of ondansetron. Depletion of spinal 5-HT via 5,7-dihydroxytryptamine (5,7-DHT) resulted in decreased at-level mechanical allodynia. The reduction of allodynia by ondansetron was lost following 5,7-DHT administration, suggesting that reduced allodynia following intrathecal ondansetron is via blockade of 5-HT-induced excitation of the 5-HT(3)-R. These results suggest that increased 5-HT fibre density immediately rostral to the SCI lesion site could have transient effects to reduce mechanical allodynia via actions at 5-HT(1) and/or 5-HT(2) receptors. However, the more long-lasting effects of this enhanced serotonergic input may facilitate chronic, at-level allodynia via the 5-HT(3)-R.

Joint manipulation reduces hyperalgesia by activation of monoamine receptors but not opioid or GABA receptors in the spinal cord

Joint manipulation has long been used for pain relief. However, the underlying mechanisms for manipulation-related pain relief remain largely unexplored. The purpose of the current study was to determine which spinal neurotransmitter receptors mediate manipulation-induced antihyperalgesia. Rats were injected with capsaicin (50 microl, 0.2%) into one ankle joint and mechanical withdrawal threshold measured before and after injection. The mechanical withdrawal threshold decreases 2 h after capsaicin injection. Two hours after capsaicin injection, the following drugs were administered intrathecally: bicuculline, blocks gamma-aminobutyric acid (GABAA) receptors; naloxone, blocks opioid receptors; yohimbine blocks, alpha2-adrenergic receptors; and methysergide, blocks 5-HT(1/2) receptors. In addition, NAN-190, ketanserin, and MDL-72222 were administered to selectively block 5-HT1A, 5-HT2A, and 5-HT3 receptors, respectively. Knee joint manipulation was performed 15 min after administration of drug. The knee joint was flexed and extended to end range of extension while the tibia was simultaneously translated in an anterior to posterior direction. The treatment group received three applications of manipulation, each 3 min in duration separated by 1 min of rest. Knee joint manipulation after capsaicin injection into the ankle joint significantly increases the mechanical withdrawal threshold for 45 min after treatment. Spinal blockade of 5-HT(1/2) receptors with methysergide prevented, while blockade of alpha2-adrenergic receptors attenuated, the manipulation-induced antihyperalgesia. NAN-190 also blocked manipulation-induced antihyperalgesia suggesting that effects of methysergide are mediated by 5-HT1A receptor blockade. However, spinal blockade of opioid or GABAA receptors had no effect on manipulation induced-antihyperalgesia. Thus, the antihyperalgesia produced by joint manipulation appears to involve descending inhibitory mechanisms that utilize serotonin and noradrenaline.

Periaqueductal gray-evoked dorsal root reflex is frequency dependent

The dorsal root reflex (DRR) is an antidromic action potential originating in the spinal cord that propagates toward the periphery. Given that both GABA(A) and 5-HT(3) receptors are involved in the generation of DRRs and stimulation of the periaqueductal gray (PAG) can induce the release of GABA and serotonin within the spinal cord, we investigated the modulation of DRRs by the PAG descending system. The central end of the cut left L5 dorsal root in adult Sprague-Dawley rats was tested with single fiber recording. Stimulating electrodes were placed in the PAG, sciatic nerve, or transcutaneously across hindpaws. Fifty-seven DRRs were recorded for the effect of PAG stimulation in 19 rats, and 51 DRRs from 26 rats and nine DRRs from seven rats were recorded for an effect of ipsilateral and contralateral peripheral stimulation, respectively. The results were expressed as a percentage of the number of DRRs over the number of stimuli. PAG stimulation at 0.2, 0.5, 5, 20, and 50 Hz produced ratio's of 113.16+/-9.84, 114.54+/-12.22, 24.6+/-3.23, 17.77+/-4.76, and 12.62+/-3.44 (%), respectively. Stimulation at ipsilateral peripheral nerve evoked DRRs of 103.26+/-8.93, 95.27+/-10.57, 37.66+/-7.55, 11.32+/-4.96, and 5.32+/-3.82 (%), respectively. Stimulation of the contralateral peripheral nerve evoked DRRs of 90.88+/-15.59, 44.30+/-10.77, 6.29+/-1.63, 0.45+/-0.19, and 0.29+/-0.15 (%), respectively. Transection at the thoracic spinal level completely eliminated PAG-induced DRRs. In conclusion, both PAG and peripheral stimulation produced DRRs in a frequency dependent manner. Stimulus intensity has no significant effect on DRRs.

Neuronal location of 5-hydroxytryptamine3 receptor-like immunoreactivity in the rat colon

The presence of 5-hydroxytryptamine(3) receptors on enteric neurons is known from pharmacological data that date back more than 40 years. However, an adequate account of which neurons bear these receptors has not been made because suitable antisera have not been available. We have found that the majority of antisera that have been raised against sequences from the 5-hydroxytryptamine(3) receptor also recognize pre-prosomatostatin. We report that this source of false labeling can be eliminated by pre-incubating the antisera with a peptide designed for this purpose. We have used the pre-absorbed antiserum to localize 5-hydroxytryptamine(3) receptors in the rat colon. Immunoreactive nerve cell bodies occurred in the myenteric and submucosal ganglia. The majority had smooth cell bodies and long, smooth processes, that is, Dogiel type II morphology. The initial segments of the long processes of the Dogiel type II neurons were strongly immunoreactive. About 12% of immunoreactive myenteric nerve cells were of the same or smaller size, and had multiple short filamentous processes. Some of the immunoreactive Dogiel type II neurons were also immunoreactive for calretinin in both plexuses, and the majority were immunoreactive for calbindin in submucosal ganglia. Specific immunoreactivity occurred in non-varicose, but not in varicose, fibers in the myenteric and submucosal ganglia, and in fiber bundles that traversed the longitudinal and circular muscle layers. Immunoreactive varicose fibers were observed only in the mucosa. It is concluded that 5-hydroxytryptamine(3) receptors occur on intrinsic sensory neurons in the rat colon, and on extrinsic sensory nerve fibers that innervate the colon.

Characterisation of axon terminals in the rat dorsal horn that are immunoreactive for serotonin 5-HT3A receptor subunits

Serotonin 5-HT(3) receptors are abundant in the superficial dorsal horn and are likely to have an involvement in processing of nociceptive information. It has been shown previously that 5-HT(3) receptors are present on primary afferent terminals and some dorsal horn cells. The primary aim of the present study was to determine what classes of primary afferent possess 5-HT(3)A receptor subunits. We performed a series of double- and triple-labelling immunofluorescence experiments. Subunits were labelled with an anti-peptide antibody and primary afferent axons were identified by the presence of calcitonin gene-related peptide (CGRP) and binding of the lectin IB4. Quantitative confocal microscopic analysis revealed that approximately 10% of axons displaying 5-HT(3)A immunoreactivity were also labelled for CGRP but that only 3% of these fibres bind IB4. We also investigated the relationship between immunoreactivity for the subunit and descending serotoninergic systems, axons originating from inhibitory neurons that contain glutamic acid decarboxylase, and axons of a subpopulation of excitatory neurons that contain neurotensin. None of these types of axon was associated with immunoreactivity for receptor subunits. Ultrastructural studies confirmed that punctate immunoreactive structures observed with the light microscope were axon terminals. These terminals invariably formed asymmetric synaptic junctions with dendritic profiles and often contained a mixture of granular and agranular vesicles. Some terminals formed glomerular-like arrangements. Immunoreactive cells were also examined and were found to contain intense patches of reaction product within the cytoplasm. We conclude that the majority (about 87%) of dorsal horn axons that are immunoreactive for 5-HT(3)A receptor subunits do not originate from the subtypes of primary afferent fibres that bind IB4 or contain CGRP. It is likely that most of these axons have an excitatory action and they may originate from dorsal horn interneurons and/or fine myelinated primary afferent fibres.

Superficial NK1-expressing neurons control spinal excitability through activation of descending pathways

The increase in pain sensitivity that follows injury is regulated by superficially located projection neurons in the dorsal horn of the spinal cord that express the neurokinin-1 (NK1) receptor. After selective ablation of these neurons in rats, we identified changes in receptive field size, mechanical and thermal coding and central sensitization of deeper dorsal horn neurons that are important for both pain sensations and reflexes. We were able to reproduce these changes by pharmacological block of descending serotonergic facilitatory pathways. Using Fos histochemistry, we found changes in the activation of serotonergic neurons in the brainstem as well as evidence for a loss of descending control of spinal excitability. We conclude that NK1-positive spinal projection neurons, activated by primary afferent input, project to higher brain areas that control spinal excitability--and therefore pain sensitivity--primarily through descending pathways from the brainstem.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.