Decrease in the descending inhibitory 5-HT system in rats with spinal nerve ligation

Influence of the descending pain-inhibiting serotonergic pathway on the antihyperalgesic effect of gabapentin in neuropathic pain model rats

Gabapentinoids are used worldwide as first-line agents for the treatment of neuropathic pain. Accumulating evidence indicates that one of the antihyperalgesic mechanisms of gabapentinoids is through activation of the noradrenergic pathway of the descending pain inhibition system. However, the involvement of the serotonin pathway is unclear. We investigated the effects of gabapentin (GBP) on the serotonergic pathway of the descending inhibitory system using the spinal nerve ligation (SNL) rat model. As in previous reports, administration of GBP to SNL rats improved paw withdrawal thresholds (PWT). Intrathecally administered serotonin receptor antagonists abolished GBP's amelioration in PWT. GBP did not ameliorate PWT in noradrenaline-depleted SNL rats by DSP-4. However, GBP ameliorated PWT in serotonin-depleted SNL rats by para-chlorophenylalanine, which was not inhibited by intrathecal administration of a serotonin receptor antagonist. Immunohistochemical analysis of serotonin in the spinal dorsal horn revealed a slight, albeit statistically insignificant, increase in 5-HT levels in SNL rats compared to naive rats. However, no apparent changes were observed before or after GBP administration in naive and SNL rats. In conclusion, the involvement of the serotonergic pathway in the antihyperalgesic effects of GBP on the spinal cord is secondary, although it cooperates with the noradrenergic system to produce analgesia.

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

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

A Hybrid Titanium-Softmaterial, High-Strength, Transparent Cranial Window for Transcranial Injection and Neuroimaging

A transparent and penetrable cranial window is essential for neuroimaging, transcranial injection and comprehensive understanding of cortical functions. For these applications, cranial windows made from glass coverslip, polydimethylsiloxane (PDMS), polymethylmethacrylate, crystal and silicone hydrogel have offered remarkable convenience. However, there is a lack of high-strength, high-transparency, penetrable cranial window with clinical application potential. We engineer high-strength hybrid Titanium-PDMS (Ti-PDMS) cranial windows, which allow large transparent area for in vivo two-photon imaging, and provide a soft window for transcranial injection. Laser scanning and 3D printing techniques are used to match the hybrid cranial window to different skull morphology. A multi-cycle degassing pouring process ensures a good combination of PDMS and Ti frame. Ti-PDMS cranial windows have a high fracture strength matching human skull bone, excellent light transmittance up to 94.4%, and refractive index close to biological tissue. Ti-PDMS cranial windows show excellent bio-compatibility during 21-week implantation in mice. Dye injection shows that the PDMS window has a "self-sealing" to keep liquid from leaking out. Two-photon imaging for brain tissues could be achieved up to 450 µm in z-depth. As a novel brain-computer-interface, this Ti-PDMS device offers an alternative choice for in vivo drug delivery, optical experiments, ultrasonic treatment and electrophysiology recording.

A systematic review on descending serotonergic projections and modulation of spinal nociception in chronic neuropathic pain and after spinal cord stimulation

Chronic neuropathic pain is a debilitating ordeal for patients worldwide and pharmacological treatment efficacy is still limited. As many pharmacological interventions for neuropathic pain often fail, insights into the underlying mechanism and role of identified receptors is of utmost importance. An important target for improving treatment of neuropathic pain is the descending serotonergic system as these projections modulate nociceptive signaling in the dorsal horn. Also with use of last resort treatments like spinal cord stimulation (SCS), the descending serotonergic projections are known to be involved in the pain relieving effect. This systematic review summarizes the involvement of the serotonergic system on nociceptive modulation in the healthy adult rodent and the chronic neuropathic rodent and summarizes all available literature on the serotonergic system in the SCS-treated neuropathic rodent. Medline, Embase and Pubmed databases were used in the search for articles. Descending serotonergic modulation of nociceptive signaling in spinal dorsal horn in normal adult rat is mainly inhibitory and mediated by 5-HT1a, 5-HT1b, 5-HT2c, 5-HT3 and 5-HT4 receptors. Upon injury and in the neuropathic rat, this descending serotonergic modulation becomes facilitatory via activation of the 5-HT2a, 5-HT2b and 5-HT3 receptors. Analgesia due to neuromodulatory intervention like SCS restores the inhibitory function of the descending serotonergic system and involves 5-HT2, 5-HT3 and 5-HT4 receptors. The results of this systematic review provide insights and suggestions for further pharmacological and or neuromodulatory treatment of neuropathic pain based on targeting selected serotonergic receptors related to descending modulation of nociceptive signaling in spinal dorsal horn. With the novel developed SCS paradigms, the descending serotonergic system will be an important target for mechanism-based stimulation induced analgesia.

Acute Low Dose of Trazodone Recovers Glutamate Release Efficiency and mGlu2/3 Autoreceptor Impairments in the Spinal Cord of Rats Suffering From Chronic Sciatic Ligation

We investigated whether chronic sciatic ligation modifies the glutamate release in spinal cord nerve endings (synaptosomes) as well as the expression and the function of presynaptic release-regulating mGlu2/3 autoreceptors and 5-HT_2A heteroreceptors in these particles. Synaptosomes were from the spinal cord of animals suffering from the sciatic ligation that developed on day 6 post-surgery a significant decrease of the force inducing paw-withdrawal in the lesioned paw. The exocytosis of glutamate (quantified as release of preloaded [³H]D-aspartate, [³H]D-Asp) elicited by a mild depolarizing stimulus (15 mM KCl) was significantly increased in synaptosomes from injured rats when compared to controls (uninjured rats). The mGlu2/3 agonist LY379268 (1000 pM) significantly inhibited the 15 mM KCl-evoked [³H]D-Asp overflow from control synaptosomes, but not in terminals isolated from injured animals. Differently, a low concentration (10 nM) of (±) DOI, unable to modify the 15 mM KCl-evoked [³H]D-Asp overflow in control spinal cord synaptosomes, significantly reduced the glutamate exocytosis in nerve endings isolated from the injured rats. Acute oral trazodone (TZD, 0.3 mg/kg on day 7 post-surgery) efficiently recovered glutamate exocytosis as well as the efficiency of LY379268 in inhibiting this event in spinal cord synaptosomes from injured animals. The sciatic ligation significantly reduced the expression of mGlu2/3, but not of 5-HT_2A, receptor proteins in spinal cord synaptosomal lysates. Acute TZD recovered this parameter. Our results support the use of 5-HT_2A antagonists for restoring altered spinal cord glutamate plasticity in rats suffering from sciatic ligation.

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 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.

Citral-induced analgesia is associated with increased spinal serotonin, reduced spinal nociceptive signaling, and reduced systemic oxidative stress in arthritis

ETHNOPHARMACOLOGICAL RELEVANCE

Citral (3,7-dimethyl-2,6-octadienal) is the main component of Cymbopogon citratus (DC) Stapf, an herb with analgesic properties. Arthritic pain is the main unpleasant component of rheumatoid arthritis. The pharmacological approaches used to treat arthritic pain are often accompanied by adjuvant drugs or non-pharmacological treatments, showing a constant need in identifying new efficient analgesic drugs.

AIM OF THE STUDY

To test the hypothesis that citral, which is a monoterpenoid compound with therapeutic properties, reduces nociception, spinal pro-nociceptive and pro-inflammatory signaling, and systemic oxidative stress in arthritic rats.

MATERIALS AND METHODS

Complete Freund's adjuvant (CFA) was administrated in the left knee joint of rats. Oral treatment with citral was performed during eight days and mechanical allodynia was monitored during the period of treatment to evaluate the analgesic effect of citral. We assessed the levels of serotonin (5-hydroxytryptamine, 5-HT) in the lumbar dorsal horn of the spinal cord (DHSC) and the profiles of expression of the glycogen synthase kinase-3β (GSK3β), which is a 5-HT-regulated intracellular protein, and of the stress-activated protein kinase (SAPK)/jun N-terminal kinase (JNK) in the DHSC. Plasma levels of superoxide dismutase (SOD) were assessed as an indicator of oxidative stress.

RESULTS

Administration of CFA induced mechanical allodynia associated with reduced spinal GSK3β phosphorylation, increased spinal SAPK/JNK phosphorylation, and increased plasma SOD levels. Oral administration of citral reversed mechanical allodynia, increased endogenous spinal 5-HT levels, reduced spinal SAPK/JNK phosphorylation, and reduced plasma SOD levels.

CONCLUSION

Citral shows anti-nociceptive effects in an animal model of arthritic pain by modulating spinal nociceptive signaling.

Evaluation of clinical efficacy of silver-needle warm acupuncture in treating adults with acute low back pain due to lumbosacral disc herniation: study protocol for a randomized controlled trial

BACKGROUND

As a common cause of low back pain, lumbosacral disc herniation (LDH) is usually dealt with using non-surgical interventions. In the face of concerns about prescription opioid abuse, alternative and complementary treatments may be promising, among which silver-needle warm acupuncture is considered as an upgrading option for its potential anti-inflammatory and strong analgesic effect for patients with chronic pain. In this proposed study, we aim to assess its clinical efficacy in comparison with conventional stainless steel filiform-needle warm acupuncture.

METHODS/DESIGN

This is a randomized, two-armed, patient- and assessor-blinded trial. One hundred and sixty eligible patients recruited from December 2018 to June 2020 in three centers will be assigned for warm acupuncture treatment with either stainless steel filiform or silver needles. Nine sessions of 20-min treatment will be conducted during 3 consecutive weeks. Assessments with instruments including the Oswestry Disability Index, the visual analog scale, and the Japanese Orthopedic Association Back Pain Evaluation Questionnaire will be performed at four time points to explore the difference of clinical efficacy between two groups.

DISCUSSION

If the results show that participants treated with silver-needle warm acupuncture gain a greater improvement in terms of pain intensity, physical function, and quality of life, this study is expected to offer reliable evidence to widely push this treatment for LDH in clinical practice.

TRIAL REGISTRATION

Chinese Clinical Trial Registry, ChiCTR1800019051 . Registered on 24 October 2018.

Acute spinal cord injury (SCI) transforms how GABA affects nociceptive sensitization

Noxious input can sensitize pain (nociceptive) circuits within the spinal cord, inducing a lasting increase in spinal cord neural excitability (central sensitization) that is thought to contribute to chronic pain. The development of spinally-mediated central sensitization is regulated by descending fibers and GABAergic interneurons. The current study provides evidence that spinal cord injury (SCI) transforms how GABA affects nociceptive transmission within the spinal cord, recapitulating an earlier developmental state wherein GABA has an excitatory effect. In spinally transected rats, noxious electrical stimulation and inflammation induce enhanced mechanical reactivity (EMR), a behavioral index of nociceptive sensitization. Pretreatment with the GABAA receptor antagonist bicuculline blocked these effects. Peripheral application of an irritant (capsaicin) also induced EMR. Both the induction and maintenance of this effect were blocked by bicuculline. Cellular indices of central sensitization [c-fos expression and ERK phosphorylation (pERK)] were also attenuated. In intact (sham operated) rats, bicuculline had the opposite effect. Pretreatment with a GABA agonist (muscimol) attenuated nociceptive sensitization in intact, but not spinally injured, rats. The effect of SCI on GABA function was linked to a reduction in the Cl- transporter, KCC2, leading to a reduction in intracellular Cl- that would attenuate GABA-mediated inhibition. Pharmacologically blocking the KCC2 channel (with i.t. DIOA) in intact rats mimicked the effect of SCI. Conversely, a pharmacological treatment (bumetanide) that should increase intracellular Cl- levels blocked the effect of SCI. The results suggest that GABAergic neurons drive, rather than inhibit, the development of nociceptive sensitization after spinal injury.

Rat exposure in mice with neuropathic pain induces fear and antinociception that is not reversed by 5-HT2C receptor activation in the dorsal periaqueductal gray

Previous studies have demonstrated that serotonin 5-HT2C receptors in the dorsal periaqueductal gray (dPAG) mediate both anxiety and antinociception in mice submitted to the elevated plus maze. The present study examined the effects of intra-dPAG infusion of the serotonin 5-HT2C receptor agonist (MK-212) in the defensive reactions and antinociception in mice with neurophatic pain confronted by a predator. Neuropathic pain was induced by chronic constriction injury (CCI) of the sciatic nerve, and predator confrontation was performed using the rat exposure test (RET). Our results demonstrated that both sham-operated and CCI mice exhibited intense defensive reactions when confronted by rats. However, rat-exposed CCI mice showed reduced pain reactivity in comparison to CCI mice exposed to a toy rat. Intra-dPAG infusion of MK-212 prior to predator exposure did not significantly alter defensive or antinociceptive responses. To our knowledge, our results represent the first evidence of RET-induced antinociception in mice. Moreover, the results of the present study suggest that 5-HT2C receptor activation in the dPAG is not critically involved in the control of predator-evoked fearful or antinociceptive responses.

Activation of the Mammalian Target of Rapamycin in the Rostral Ventromedial Medulla Contributes to the Maintenance of Nerve Injury-Induced Neuropathic Pain in Rat

The mammalian target of rapamycin (mTOR), a serine-threonine protein kinase, integrates extracellular signals, thereby modulating several physiological and pathological processes, including pain. Previous studies have suggested that rapamycin (an mTOR inhibitor) can attenuate nociceptive behaviors in many pain models, most likely at the spinal cord level. However, the mechanisms of mTOR at the supraspinal level, particularly at the level of the rostral ventromedial medulla (RVM), remain unclear. Thus, the aim of this study was to elucidate the role of mTOR in the RVM, a key relay region for the descending pain control pathway, under neuropathic pain conditions. Phosphorylated mTOR was mainly expressed in serotonergic spinally projecting neurons and was significantly increased in the RVM after spared nerve injury- (SNI-) induced neuropathic pain. Moreover, in SNI rat brain slices, rapamycin infusion both decreased the amplitude instead of the frequency of spontaneous excitatory postsynaptic currents and reduced the numbers of action potentials in serotonergic neurons. Finally, intra-RVM microinjection of rapamycin effectively alleviated established mechanical allodynia but failed to affect the development of neuropathic pain. In conclusion, our data provide strong evidence for the role of mTOR in the RVM in nerve injury-induced neuropathic pain, indicating a novel mechanism of mTOR inhibitor-induced analgesia.

Mice Lacking Serotonin 2C Receptors Have increased Affective Responses to Aversive Stimuli

Although central serotonergic systems are known to influence responses to noxious stimuli, mechanisms underlying serotonergic modulation of pain responses are unclear. We proposed that serotonin 2C receptors (5-HT2CRs), which are expressed within brain regions implicated in sensory and affective responses to pain, contribute to the serotonergic modulation of pain responses. In mice constitutively lacking 5-HT2CRs (2CKO mice) we found normal baseline sensory responses to noxious thermal, mechanical and chemical stimuli. In contrast, 2CKO mice exhibited a selective enhancement of affect-related ultrasonic afterdischarge vocalizations in response to footshock. Enhanced affect-related responses to noxious stimuli were also exhibited by 2CKO mice in a fear-sensitized startle assay. The extent to which a brief series of unconditioned footshocks produced enhancement of acoustic startle responses was markedly increased in 2CKO mice. As mesolimbic dopamine pathways influence affective responses to noxious stimuli, and these pathways are disinhibited in 2CKO mice, we examined the sensitivity of footshock-induced enhancement of startle to dopamine receptor blockade. Systemic administration of the dopamine D2/D3 receptor antagonist raclopride selectively reduced footshock-induced enhancement of startle without influencing baseline acoustic startle responses. We propose that 5-HT2CRs regulate affective behavioral responses to unconditioned aversive stimuli through mechanisms involving the disinhibition of ascending dopaminergic pathways.

Spinal cord stimulation modulates supraspinal centers of the descending antinociceptive system in rats with unilateral spinal nerve injury

BACKGROUND

The descending antinociceptive system (DAS) is thought to play crucial roles in the antinociceptive effect of spinal cord stimulation (SCS), especially through its serotonergic pathway. The nucleus raphe magnus (NRM) in the rostral ventromedial medulla is a major source of serotonin [5-hydroxytryptamine (5-HT)] to the DAS, but the role of the dorsal raphe nucleus (DRN) in the ventral periaqueductal gray matter is still unclear. Moreover, the influence of the noradrenergic pathway is largely unknown. In this study, we evaluated the involvement of these serotonergic and noradrenergic pathways in SCS-induced antinociception by behavioral analysis of spinal nerve-ligated (SNL) rats. We also investigated immunohistochemical changes in the DRN and locus coeruleus (LC), regarded as the adrenergic center of the DAS, and expression changes of synthetic enzymes of 5-HT [tryptophan hydroxylase (TPH)] and norepinephrine [dopamine β-hydroxylase (DβH)] in the spinal dorsal horn.

RESULTS

Intrathecally administered methysergide, a 5-HT1- and 5-HT2-receptor antagonist, and idazoxan, an α2-adrenergic receptor antagonist, equally abolished the antinociceptive effect of SCS. The numbers of TPH-positive serotonergic and phosphorylated cyclic AMP response element binding protein (pCREB)-positive neurons and percentage of pCREB-positive serotonergic neurons in the DRN significantly increased after 3-h SCS. Further, the ipsilateral-to-contralateral immunoreactivity ratio of DβH increased in the LC of SNL rats and reached the level seen in naïve rats, even though the number of pCREB-positive neurons in the LC was unchanged by SNL and SCS. Moreover, 3-h SCS did not increase the expression levels of TPH and DβH in the spinal dorsal horn.

CONCLUSIONS

The serotonergic and noradrenergic pathways of the DAS are involved in the antinociceptive effect of SCS, but activation of the DRN might primarily be responsible for this effect, and the LC may have a smaller contribution. SCS does not potentiate the synthetic enzymes of 5HT and norepinephrine in the neuropathic spinal cord.

BDNF contributes to the development of neuropathic pain by induction of spinal long-term potentiation via SHP2 associated GluN2B-containing NMDA receptors activation in rats with spinal nerve ligation

The pathogenic mechanisms underlying neuropathic pain still remain largely unknown. In this study, we investigated whether spinal BDNF contributes to dorsal horn LTP induction and neuropathic pain development by activation of GluN2B-NMDA receptors via Src homology-2 domain-containing protein tyrosine phosphatase-2 (SHP2) phosphorylation in rats following spinal nerve ligation (SNL). We first demonstrated that spinal BDNF participates in the development of long-lasting hyperexcitability of dorsal horn WDR neurons (i.e. central sensitization) as well as pain allodynia in both intact and SNL rats. Second, we revealed that BDNF induces spinal LTP at C-fiber synapses via functional up-regulation of GluN2B-NMDA receptors in the spinal dorsal horn, and this BDNF-mediated LTP-like state is responsible for the occlusion of spinal LTP elicited by subsequent high-frequency electrical stimulation (HFS) of the sciatic nerve in SNL rats. Finally, we validated that BDNF-evoked SHP2 phosphorylation is required for subsequent GluN2B-NMDA receptors up-regulation and spinal LTP induction, and also for pain allodynia development. Blockade of SHP2 phosphorylation in the spinal dorsal horn using a potent SHP2 protein tyrosine phosphatase inhibitor NSC-87877, or knockdown of spinal SHP2 by intrathecal delivery of SHP2 siRNA, not only prevents BDNF-mediated GluN2B-NMDA receptors activation as well as spinal LTP induction and pain allodynia elicitation in intact rats, but also reduces the SNL-evoked GluN2B-NMDA receptors up-regulation and spinal LTP occlusion, and ultimately alleviates pain allodynia in neuropathic rats. Taken together, these results suggest that the BDNF/SHP2/GluN2B-NMDA signaling cascade plays a vital role in the development of central sensitization and neuropathic pain after peripheral nerve injury.

The role of descending inhibitory pathways on chronic pain modulation and clinical implications

The treatment and management of chronic pain is a major challenge for clinicians. Chronic pain is often underdiagnosed and undertreated, and there is a lack of awareness of the pathophysiologic mechanisms that contribute to chronic pain. Chronic pain involves peripheral and central sensitization, as well as the alteration of the pain modulatory pathways. Imbalance between the descending facilitatory systems and the descending inhibitory systems is believed to be involved in chronic pain in pathological conditions. A pharmacological treatment that could restore the balance between these 2 pathways by diminishing the descending facilitatory pain pathways and enhancing the descending inhibitory pain pathways would be a valuable therapeutic option for patients with chronic pain. Due to the lack of evidence for pharmacological options that act on descending facilitation pathways, in this review we summarize the role of the descending inhibitory pain pathways in pain perception. This review will focus primarily on monoaminergic descending inhibitory pain pathways and their contribution to the mechanism of chronic pain and several pharmacological treatment options that enhance these pathways to reduce chronic pain. We describe anatomical structures and neurotransmitters of the descending inhibitory pain pathways that are activated in response to nociceptive pain and altered in response to sustained and persistent pain which leads to chronic pain in various pathological conditions.

Isolation rearing reduces mechanical allodynia in a mouse model of chronic inflammatory pain

Social isolation rearing in mice after weaning reduces pain sensitivity to acute pain, and this hypoalgesia is mediated by the descending serotonergic pain inhibitory system in which the spinal serotonin (5-HT)1A receptor is involved. However, it is not known whether isolation rearing affects pain sensitivity to neuropathic or inflammatory chronic pain. In this study, we examined the effects of isolation rearing on chronic pain induced by Freund's complete adjuvant (FCA) and partial sciatic nerve ligation using the von Frey test (to assess mechanical allodynia) and the plantar test (to assess thermal hyperalgesia). In the FCA model, isolation rearing reduced mechanical allodynia, but not thermal hyperalgesia. In contrast, isolation rearing had no effect on allodynia or hyperalgesia in the sciatic nerve ligation model. The isolation rearing-induced inhibition of allodynia was alleviated by intrathecal injection of WAY100635, a selective 5-HT1A receptor antagonist. FCA increased 5-HT turnover and decreased 5-HT1A receptor expression in the spinal cord of group-reared mice, while it did not have these effects in isolation-reared mice. These results suggest that FCA suppresses the serotonergic pain inhibitory system selectively in group-reared mice. Moreover, systemic administration of osemozotan, a selective 5-HT1A receptor agonist, inhibited FCA-induced mechanical allodynia in group-reared mice, and this effect of the drug was suppressed by intrathecal injection of WAY100635. Collectively, these findings suggest that isolation rearing selectively reduces FCA-induced mechanical allodynia in mice and that this effect is mediated by the activation of spinal 5-HT1A receptors.

Associations between serotonin transporter gene polymorphisms and heat pain perception in adults with chronic pain

BACKGROUND

The triallelic serotonin transporter gene linked polymorphic region (5-HTTLPR) has been associated with alterations in thermal pain perception. The primary aim of this study was to investigate the associations between heat pain (HP) perception and the triallelic 5-HTTLPR in a large cohort of adults with chronic pain.

METHODS

The cohort included 277 adults with chronic pain who met inclusion criteria, and were consecutively admitted to an outpatient pain rehabilitation program from March 2009 through March 2010. Individuals were genotyped for the triallelic 5-HTTLPR (including rs25531) and categorized as high, intermediate, or low expressors of the serotonin transporter. Standardized measures of HP perception were obtained using a validated quantitative sensory test method of levels.

RESULTS

The distribution of the high, intermediate, and low expressing genotypes was 61 (22%), 149 (54%) and 67 (24%), respectively. The Hardy-Weinberg P-value was 0.204 which indicated no departure from equilibrium. A significant effect of genotype was observed for values of HP threshold (P = 0.029). Individual group comparisons showed that values of HP threshold were significantly greater in the intermediate compared to the high expressing group (P = 0.009) but not the low expressing group (P > 0.1). In a multiple variable linear regression model, the intermediate group (P = 0.034) and male sex (P = 0.021) were associated with significantly greater values of HP 0.5, but no significant genotype-by-sex interaction effect was observed.

CONCLUSIONS

In this study that involved adults with chronic pain, the intermediate triallelic 5-HTTLPR expressing group, but not the low expressing group, was associated with greater HP thresholds compared to the high expressing group.

The antinociceptive effect of reversible monoamine oxidase-A inhibitors in a mouse neuropathic pain model

Neuropathic pain is a debilitating condition that is often resistant to common analgesics, such as opioids, but is sensitive to some antidepressants, an effect that seems to be mediated by spinal cord 5-HT3 receptors. Because the analgesic potential of monoamine oxidase-A (MAO-A) inhibitors is understudied, we evaluated the potential antinociceptive effect of the reversible MAO-A inhibitors moclobemide and 2-(3,4-dimethoxy-phenyl)-4,5-dihydro-1H-imidazole (2-DMPI) in a mouse neuropathic pain model induced by chronic constriction injury (CCI) of the sciatic nerve. Neuropathic mice showed a decreased mechanical paw withdrawal threshold (PWT) 7 days after lesion compared with the baseline PWT, characterizing the development of hyperalgesia. Moclobemide (100-300 μmol/kg, s.c.) and 2-DMPI (30-300 μmol/kg, s.c.) treatments were able to reverse the CCI-induced hyperalgesia, with 50% inhibitory dose (ID50) values of 39 (18-84) and 11 (4-33) μmol/kg, and maximum inhibition (Imax) values of 88±14 and 98±15%, respectively, at the 300 μmol/kg dose. In addition, we observed a significant increase in the MAO-A activity in the lumbar spinal cord of CCI-submitted mice compared with sham-operated animals. Furthermore, the antihyperalgesic effects of both 2-DMPI and moclobemide were largely reversed by intrathecal injection of the 5-HT3 receptor antagonist ondansetron (10 μg/site). These results suggest a possible involvement of MAO-A in the mechanisms of neuropathic pain and a potential utility of the reversible inhibitors of MAO-A in the development of new therapeutic approaches to treat it.

Lysine-specific demethylase 1 in breast cancer cells contributes to the production of endogenous formaldehyde in the metastatic bone cancer pain model of rats

Background

Bone cancer pain seriously affects the quality of life of cancer patients. Our previous study found that endogenous formaldehyde was produced by cancer cells metastasized into bone marrows and played an important role in bone cancer pain. However, the mechanism of production of this endogenous formaldehyde by metastatic cancer cells was unknown in bone cancer pain rats. Lysine-specific demethylase 1 (LSD1) is one of the major enzymes catalyzing the production of formaldehyde. The expression of LSD1 and the concentration of formaldehyde were up-regulated in many high-risk tumors.

Objective

This study aimed to investigate whether LSD1 in metastasized MRMT-1 breast cancer cells in bone marrows participated in the production of endogenous formaldehyde in bone cancer pain rats.

Methodology/Principal Findings

Concentration of the endogenous formaldehyde was measured by high performance liquid chromatography (HPLC). Endogenous formaldehyde dramatically increased in cultured MRMT-1 breast cancer cells in vitro, in bone marrows and sera of bone cancer pain rats, in tumor tissues and sera of MRMT-1 subcutaneous vaccination model rats in vivo. Formaldehyde at a concentration as low as the above measured (3 mM) induced pain behaviors in normal rats. The expression of LSD1 which mainly located in nuclei of cancer cells significantly increased in bone marrows of bone cancer pain rats from 14 d to 21 d after inoculation. Furthermore, inhibition of LSD1 decreased the production of formaldehyde in MRMT-1 cells in vitro. Intraperitoneal injection of LSD1 inhibitor pargyline from 3 d to 14 d after inoculation of MRMT-1 cancer cells reduced bone cancer pain behaviors.

Conclusion

Our data in the present study, combing our previous report, suggested that in the endogenous formaldehyde-induced pain in bone cancer pain rats, LSD1 in metastasized cancer cells contributed to the production of the endogenous formaldehyde.

Multiple roles of serotonin in pain control mechanisms--implications of 5-HT₇ and other 5-HT receptor types

Among monoamine neurotransmitters, serotonin (5-HT) is known to play complex modulatory roles in pain signaling mechanisms since the first reports, about forty years ago, on its essentially pro-nociceptive effects at the periphery and anti-nociceptive effects when injected directly at the spinal cord level. The discovery of multiple 5-HT receptor subtypes allowed possible explanations to this dual action at the periphery versus the central nervous system (CNS) since both excitatory and inhibitory effects can be exerted through 5-HT activation of different 5-HT receptors. However, it also appeared that activation of the same receptor subtype at CNS level might induce variable effects depending on the physiological or pathophysiological status of the animal administered with agonists. In particular, the marked neuroplastic changes induced by nerve lesion, which account for sensitization of pain signaling mechanisms, can contribute to dramatic changes in the effects of a given 5-HT receptor agonist in neuropathic rats versus intact healthy rats. This has notably been observed with 5-HT₇ receptor agonists which exert a pronociceptive action in healthy rats but alleviate hyperalgesia consecutive to nerve lesion in neuropathic animals. Analysis of cellular mechanisms underlying such dual 5-HT actions mediated by a single receptor subtype indicates that the neuronal phenotype which expresses this receptor also plays a key role in determining which modulatory action 5-HT would finally exert on pain signaling mechanisms.

Effects of milnacipran, a 5-HT and noradrenaline reuptake inhibitor, on C-fibre-evoked field potentials in spinal long-term potentiation and neuropathic pain

BACKGROUND AND PURPOSE

The analgesic action of 5-HT and noradrenaline reuptake inhibitors (SNRIs) on nociceptive synaptic transmission in the spinal cord is poorly understood. We investigated the effects of milnacipran, an SNRI, on C-fibre-evoked field potentials (FPs) in spinal long-term potentiation (LTP), a proposed synaptic mechanism of hypersensitivity, and on the FPs in a neuropathic pain model.

EXPERIMENTAL APPROACH

C-fibre-evoked FPs by electrical stimulation of the sciatic nerve fibres were recorded in the spinal dorsal horn of anaesthetized adult rats, and LTP was induced by high-frequency stimulation of the sciatic nerve fibres. A rat model of neuropathic pain was produced by L5 spinal nerve ligation and transection.

KEY RESULTS

Milnacipran produced prolonged inhibition of C-fibre-evoked FPs when applied spinally after the establishment of LTP of C-fibre-evoked FPs in naïve animals. In the neuropathic pain model, spinal administration of milnacipran clearly reduced the basal C-fibre-evoked FPs. These inhibitory effects of milnacipran were blocked by spinal administration of methysergide, a 5-HT½ receptor antagonist, and yohimbine or idazoxan, α₂-adrenoceptor antagonists. However, spinal administration of milnacipran in naïve animals did not affect the basal C-fibre-evoked FPs and the induction of spinal LTP.

CONCLUSION AND IMPLICATIONS

Milnacipran inhibited C-fibre-mediated nociceptive synaptic transmission in the spinal dorsal horn after the establishment of spinal LTP and in the neuropathic pain model, by activating both spinal 5-hydroxytryptaminergic and noradrenergic systems. The condition-dependent inhibition of the C-fibre-mediated transmission by milnacipran could provide novel evidence regarding the analgesic mechanisms of SNRIs in chronic pain.

Striatal dopamine release and genetic variation of the serotonin 2C receptor in humans

Mesoaccumbal and nigrostriatal projections are sensitive to stress, and heightened stress sensitivity is thought to confer risk for neuropsychiatric disorders. Serotonin 2C (5-HT(2C)) receptors mediate the inhibitory effects of serotonin on dopaminergic circuitry in experimental animals, and preclinical findings have implicated 5-HT(2C) receptors in motivated behaviors and psychotropic drug mechanisms. In humans, a common missense single-nucleotide change (rs6318, Cys23Ser) in the 5-HT(2C) receptor gene (HTR2C) has been associated with altered activity in vitro and with clinical mood disorders. We hypothesized that dopaminergic circuitry would be more sensitive to stress in humans carrying the Ser23 variant. To test this hypothesis, we studied 54 healthy humans using positron emission tomography and the displaceable D(2)/D(3) receptor radiotracer [(11)C]raclopride. Binding potential (BP(ND)) was quantified before and after a standardized stress challenge consisting of 20 min of moderate deep muscular pain, and reduction in BP(ND) served as an index of dopamine release. The Cys23Ser variant was genotyped on a custom array, and ancestry informative markers were used to control for population stratification. We found greater dopamine release in the nucleus accumbens, caudate nucleus, and putamen among Ser23 carriers, after controlling for sex, age, and ancestry. Genotype accounted for 12% of the variance in dopamine release in the nucleus accumbens. There was no association of Cys23Ser with baseline BP(ND). These findings indicate that a putatively functional HTR2C variant (Ser23) is associated with greater striatal dopamine release during pain in healthy humans. Mesoaccumbal stress sensitivity may mediate the effects of HTR2C variation on risk of neuropsychiatric disorders.

Spinal cord stimulation: neurophysiological and neurochemical mechanisms of action

Chronic neuropathic pain can significantly reduce quality of life and place an economic burden on individuals and society. Spinal cord stimulation (SCS) is an alternative approach to the treatment of neuropathic pain when standard pharmacological agents have failed. However, an improved understanding of the mechanisms by which SCS inhibits pain is needed to enhance its clinical utility. This review summarizes important findings from recent studies of SCS in animal models of neuropathic pain, highlights current understanding of the spinal neurophysiological and neurochemical mechanisms by which SCS produces an analgesic effect, and discusses the potential clinical applicability of these findings and future directions for research.

Involvement of descending serotonergic and noradrenergic pathways in CB1 receptor-mediated antinociception

Cannabinoids produce antinociceptive and antihyperalgesic effects mainly through activation of the inhibitory CB1 receptors. The demonstration that antinociceptive effects of systemic cannabinoids are significantly diminished following surgical dorsolateral funiculus lesion provides evidence that supraspinal sites and descending pain modulatory pathways play crucial roles in systemic cannabinoid analgesia. In this review, we will firstly provide a background, brief overview of descending modulatory pathways followed by descending pathways implicated in cannabinoid analgesia. We will then describe the recent evidence of the involvement of descending serotonergic and noradrenergic pathways in CB1 receptor-mediated antinociception. This review will provide evidences that systemically administered cannabinoids reinforce the descending serotonergic and noradrenergic pathways to produce acute antinociceptive effects via spinal 5-HT7, 5-HT2A and alpha-2 adrenoceptors activation.

5-HT7 receptor activation attenuates thermal hyperalgesia in streptozocin-induced diabetic mice

The role of 5-HT7 receptors in the nociceptive processing received most attention during the last few years. The involvement of 5-HT₇ receptors in nerve injury-induced neuropathic pain states have been reported only recently; however, there are no reports on its contribution in diabetic neuropathic pain. We therefore planned to investigate the effect of 5-HT₇ receptor activation on the changes of nociceptive threshold in diabetic mice. Diabetes was induced by a single intraperitoneal injection of streptozocin (150 mg/kg, i.p.). The nociceptive responses in normal and diabetic animals were tested in the hot-plate and tail-flick assays. Both hot-plate and tail-flick latencies significantly shortened at 1-3/4 weeks (thermal hyperalgesia) and prolonged at 6-7 weeks (thermal hypoalgesia) after streptozocin administration. At the dose of 10 mg/kg, systemic injections of AS-19, a selective 5-HT₇ receptor agonist, reduced thermal hyperalgesia at early stage of diabetes, but did not influence thermal hypoalgesia at late stage. Co-administration of SB-258719, a selective 5-HT₇ receptor antagonist, at a dose that had no effect on its own (10 mg/kg), reversed the anti-hyperalgesic effect of AS-19. Our results indicate that systemic administration of 5-HT₇ receptor agonists may have clinical utility in treating diabetic neuropathic pain.

An increase in spinal cord noradrenaline is a major contributor to the antihyperalgesic effect of antidepressants after peripheral nerve injury in the rat

Antidepressants are often used for the treatment of neuropathic pain. Clinical studies suggest that the efficacy of serotonin (5-HT) and noradrenaline (NA) reuptake inhibitors (SNRIs) for neuropathic pain is greater than that of selective 5-HT reuptake inhibitors (SSRIs). In the present study, we determined the efficacy and mechanisms involved in the antihyperalgesic effects of milnacipran, an SNRI, compared with paroxetine, an SSRI, and maprotiline, a selective NA reuptake inhibitor, using a rat model of neuropathic pain. Male Sprague-Dawley rats underwent spinal nerve ligation (SNL), and the withdrawal threshold to paw pressure was measured. Intraperitoneal injection of milnacipran (3-30mg/kg) produced a dose-dependent antihyperalgesic effect. The effect was reversed by intrathecal injection of the α(2)-adrenoceptor antagonist idazoxan (30μg), but not by various 5-HT receptor antagonists. Paroxetine produced an antihyperalgesic effect only at the highest dose tested (10mg/kg). This effect was reversed by intrathecal injection of both idazoxan and ondansetron (30μg), a 5-HT3 receptor antagonist. Maprotiline produced an antihyperalgesic effect (10 and 30mg/kg), and the effect was reversed by intrathecal idazoxan. In microdialysis studies, NA and 5-HT concentrations in the spinal dorsal horn were increased after injection of either milnacipran or paroxetine, and only NA was increased after maprotiline. Furthermore, the NA content in the spinal cord of SNL rats was greater than that in normal animals. These findings suggest that an increase in NA in the spinal cord plays an important role in the antihyperalgesic effects of not only NA reuptake inhibitors but also SSRIs.

Taltirelin, a thyrotropin-releasing hormone analog, alleviates mechanical allodynia through activation of descending monoaminergic neurons in persistent inflammatory pain

Thyrotropin-releasing hormone (TRH) and its analogs have been reported to modulate descending monoaminergic inhibitory neurons, resulting in antinociception. However, it remains unknown whether TRH exerts an antiallodynic effect during persistent pain. Here, we investigated the action of taltirelin, a stable TRH analog, on mechanical allodynia in mice with inflammatory persistent pain induced by an injection of complete Freund's adjuvant into the hindpaw. Systemic administration of 1.0 mg/kg taltirelin markedly reduced mechanical allodynia. This effect was abolished by the 6-hydroxydopamine (6-OHDA)-induced depletion of central noradrenaline. While intraperitoneal injection of the α₁-adrenoceptor antagonist prazosin had no effect, intraperitoneal and intrathecal administration of the α₂-adrenoceptor antagonist yohimbine prevented the antiallodynic action of taltirelin. In addition, DL-p-chlorophenylalanine (PCPA)-induced depletion of serotonin (5-HT) and intraperitoneal and intrathecal injection of the 5-HT(1A) receptor antagonist WAY-100635 blocked the effect of taltirelin on allodynia. These findings suggest that taltirelin alleviates mechanical allodynia in inflammatory persistent pain by modulating the descending noradrenergic and serotonergic neuronal pathways via indirect activation of spinal α₂-adrenergic and 5-HT(1A) receptors.

Electrophysiological properties of spinal wide dynamic range neurons in neuropathic pain rats following spinal nerve ligation

OBJECTIVE

The present study aimed to investigate the electrophysiological properties of wide dynamic range (WDR) neurons in spinal dorsal horn of rats with neuropathic pain induced by lumber 5 (L5) spinal nerve ligation (SNL) in a large size of samples.

METHODS

Adult Sprague-Dawley rats were divided into normal and SNL groups. Electrophysiological technique was used to record the characteristics of WDR neurons in the spinal dorsal horn.

RESULTS

Compared with the WDR neurons in normal rats, the WDR neurons in SNL rats showed an increase in excitability, manifested by an enlargement of the receptive field size, an increase in the proportion of neurons that exhibited spontaneous activities, decreases in the C-response threshold and latency, and an increase in the C-response duration. In addition, the numbers of Aβ- and C-fiber-evoked discharges were smaller in SNL rats than in normal rats.

CONCLUSION

The excitability of spinal WDR neurons increased in rats with neuropathic pain induced by L5 SNL. The increase in excitability of WDR neurons may contribute to the development of neuropathic pain.

Lack of analgesic efficacy of spinal ondansetron on thermal and mechanical hypersensitivity following spinal nerve ligation in the rat

The balance between descending inhibition and facilitation is thought to be disturbed in chronic pain states. Increased facilitation by spinally released serotonin has been suggested by demonstration that mechanically evoked neuronal responses of wide dynamic range neurons are inhibited by 5-HT3 receptor antagonists in rats following spinal nerve ligation (SNL) but not sham operation. Despite these physiologic data, the effects of spinal 5-HT3 receptor blockade on behavioral hypersensitivity and neurochemical alterations in spinal serotonergic system have not been thoroughly investigated following spinal nerve ligation in the rat. To test this, we acutely injected intrathecal ondansetron in rats between 14 and 30 days after SNL and assessed effects on thermal and mechanical hypersensitivity. We also determined the density of serotonergic nerve fibers, serotonin content and the levels of 5-HT3 receptors within the spinal cord at this time point. Intrathecal ondansetron (1, 3, 10, 30, and 100microg) produced no effect on behavioral measures of thermal or mechanical hypersensitivity whereas intrathecal morphine (1microg) and gabapentin (200microg) partially reversed thermal and mechanical hypersensitivity following SNL. In addition, SNL did not alter the density of serotonergic fibers or 5-HT3 receptor immunoreactivity or spinal tissue content of 5-HT within the dorsal horn. These results do not support anatomic plasticity of descending serotonergic pathways or tonic 5-HT3 receptor activity in maintaining hypersensitivity after nerve injury and in contrast to previous studies fail to demonstrate an anti-hypersensitivity effect of intrathecal injection of the 5-HT3 receptor antagonist ondansetron. Importantly, behavioral measures of mechanical hypersensitivity assess threshold responses whereas physiological studies of mechanically evoked neuronal responses involve application of suprathreshold stimuli. Thus, suprathreshold or more intense stimuli may be necessary to recruit descending serotonergic facilitatory drive required to observe the inhibitory effects of ondansetron on spinal neuronal excitability and behavioral hypersensitivity.