Effects of the NMDA-receptor antagonist ketamine on perceptual correlates of long-term potentiation within the nociceptive system

Electrical matrix stimulation suppresses acute itch independently of activation of sleeping nociceptors

INTRODUCTION

Itch can be reduced by pain. Activation of sleeping nociceptors (CMi) is a crucial mechanism for the peripheral component of intense and long-lasting pain. Thus, activation of CMi might be especially effective in itch reduction. Electrical stimulation using sinusoidal pulses activates CMi with tolerable pain intensity, whereas short rectangular pulses with low intensity do not. In humans, histaminergic itch is mediated by histamine-sensitive CMi, whereas other pruritogens activate polymodal nociceptors (CM).

METHODS

In a psychophysical approach in a balanced crossover repeated-measures design in healthy volunteers, we activated nociceptors by two different electrical stimulation paradigms via a matrix electrode: 4 Hz sinusoidal pulses that activate C-nociceptors including CMi or 4 Hz rectangular stimuli to activate nociceptors excluding CMi. After 5-min stimulation, itch was induced by either histamine iontophoresis or application of cowhage spicules. Itch ratings were assessed via a numerical rating scale (NRS).

RESULTS

Electrical 4 Hz sine wave stimulation (0.1 mA) with low pain ratings of 1.5 (NRS; 0-10) induced an axon reflex erythema (3 cm2 ), indicating activation of CMi, whereas rectangular 0.2 ms pulses (average 0.91 mA) with the same pain rating did not. Both electrical stimulation paradigms reduced itch magnitude over time evoked by either histamine or cowhage to a similar extent. Peak maximum itch evoked by histamine was reduced by both stimulation paradigms, but not cowhage maximum itch.

DISCUSSION

Since electrical stimulation with the rectangular pulse paradigm reduces itch to a similar extent as the sine wave stimulation paradigm, the input of CMi is not necessarily required for itch suppression. The input of A-fibres and polymodal nociceptors, similarly, as also achieved by scratching, seems to be sufficient for both forms of chemically evoked itch.

SIGNIFICANCE

Since activation of CMi does not provide additional benefit for itch suppression, spinal pain pathways transmitted via CM versus CMi have differential effects on itch-processing circuits. This is important knowledge for using electrical matrix stimulation as itch suppressor since activation of sleeping nociceptors either requires significantly painful stimulation paradigms or specialized stimulation paradigms as sinusoidal pulses. An alternative approach using half-sine wave pulses with low pain intensity activating specifically polymodal nociceptors to suppress itch via matrix electrode stimulation may be considered.

A back-translational study of descending interactions with the induction of hyperalgesia by high-frequency electrical stimulation in rat and human

ABSTRACT

In humans and animals, high-frequency electrocutaneous stimulation (HFS) induces an "early long-term potentiation-like" sensitisation, where synaptic plasticity is underpinned by an ill-defined interaction between peripheral input and central modulatory processes. The relative contributions of these processes to the initial pain or nociceptive response likely differ from those that underpin development of the heightened response. To investigate the impact of HFS-induced hyperalgesia on pain and nociception in perception and neural terms, respectively, and to explore the impact of descending inhibitory pathway activation on the development of HFS-induced hyperalgesia, we performed parallel studies utilising identical stimuli to apply HFS concurrent to (1) a conditioned pain modulation paradigm during psychophysical testing in healthy humans or (2) a diffuse noxious inhibitory controls paradigm during in vivo electrophysiological recording of spinal neurones in healthy anaesthetised rats. High-frequency electrocutaneous stimulation alone induced enhanced perceptual responses to pinprick stimuli in cutaneous areas secondary to the area of electrical stimulation in humans and increased the excitability of spinal neurones which exhibited stimulus intensity-dependent coded responses to pinprick stimulation in a manner that tracked with human psychophysics, supporting their translational validity. Application of a distant noxious conditioning stimulus during HFS did not alter perceived primary or secondary hyperalgesia in humans or the development of primary or secondary neuronal hyperexcitability in rats compared with HFS alone, suggesting that, upon HFS-response initiation in a healthy nervous system, excitatory signalling escapes inhibitory control. Therefore, in this model, dampening facilitatory mechanisms rather than augmenting top-down inhibitions could prevent pain development.

Narrative Review: Low-Dose Ketamine for Pain Management

Pain is the leading cause of medical consultations and occurs in 50-70% of emergency department visits. To date, several drugs have been used to manage pain. The clinical use of ketamine began in the 1960s and it immediately emerged as a manageable and safe drug for sedation and anesthesia. The analgesic properties of this drug were first reported shortly after its use; however, its psychomimetic effects have limited its use in emergency departments. Owing to the misuse and abuse of opioids in some countries worldwide, ketamine has become a versatile tool for sedation and analgesia. In this narrative review, ketamine's role as an analgesic is discussed, with both known and new applications in various contexts (acute, chronic, and neuropathic pain), along with its strengths and weaknesses, especially in terms of psychomimetic, cardiovascular, and hepatic effects. Moreover, new scientific evidence has been reviewed on the use of additional drugs with ketamine, such as magnesium infusion for improving analgesia and clonidine for treating psychomimetic symptoms. Finally, this narrative review was refined by the experience of the Pain Group of the Italian Society of Emergency Medicine (SIMEU) in treating acute and chronic pain with acute manifestations in Italian Emergency Departments.

Long-term reversal of chronic pain behavior in rodents through elevation of spinal agmatine

Chronic pain remains a significant burden worldwide, and treatments are often limited by safety or efficacy. The decarboxylated form of L-arginine, agmatine, antagonizes N-methyl-d-aspartate receptors, inhibits nitric oxide synthase, and reverses behavioral neuroplasticity. We hypothesized that expressing the proposed synthetic enzyme for agmatine in the sensory pathway could reduce chronic pain without motor deficits. Intrathecal delivery of an adeno-associated viral (AAV) vector carrying the gene for arginine decarboxylase (ADC) prevented the development of chronic neuropathic pain as induced by spared nerve injury in mice and rats and persistently reversed established hypersensitivity 266 days post-injury. Spinal long-term potentiation was inhibited by both exogenous agmatine and AAV-human ADC (hADC) vector pre-treatment but was enhanced in rats treated with anti-agmatine immunoneutralizing antibodies. These data suggest that endogenous agmatine modulates the neuroplasticity associated with chronic pain. Development of approaches to access this inhibitory control of neuroplasticity associated with chronic pain may yield important non-opioid pain-relieving options.

Multichannel transcranial direct current stimulation over the left dorsolateral prefrontal cortex may modulate the induction of secondary hyperalgesia, a double-blinded cross-over study in healthy volunteers

Background

Central sensitization is thought to play a critical role in the development of chronic pain, and secondary mechanical hyperalgesia is considered one of its hall-mark features. Consequently, interventions capable of modulating its development could have important therapeutic value. Non-invasive neuromodulation of the left dorsolateral prefrontal cortex (DLPFC) has shown potential to reduce pain, both in healthy volunteers and in patients. Whether it can modulate the induction of central sensitization, however, is less well known.

Objective

To determine whether multifocal transcranial direct current stimulation (tDCS) targeting the left DLPFC affects the development of secondary mechanical hyperalgesia.

Methods

In this within-subjects, cross-over, double-blinded study, eighteen healthy volunteers participated in three experimental sessions. After 20 minutes of either anodal, cathodal, or sham multichannel tDCS over the left DLPFC, secondary mechanical hyperalgesia was induced using high-frequency electrical stimulation (HFS) of the volar forearm. We assessed intensity of perception to 128 mN mechanical pinprick stimuli at baseline and up to 240 minutes after HFS. We also mapped the area of mechanical hyperalgesia.

Results

HFS resulted in a robust and unilateral increase in the intensity of perception to mechanical pinprick stimuli at the HFS arm, which was not different between tDCS stimulation conditions. However, the area of hyperalgesia was reduced after anodal tDCS compared to sham.

Conclusion

Anodal tDCS over the left DLPFC modestly modulates the size of the HFS-induced area of secondary mechanical hyperalgesia, suggesting that non-invasive neuromodulation targeting the left DLPFC may be a potential intervention to limit the development of central sensitization.

Human surrogate models of central sensitization: A critical review and practical guide

Background

As in other fields of medicine, development of new medications for management of neuropathic pain has been difficult since preclinical rodent models do not necessarily translate to the clinics. Aside from ongoing pain with burning or shock‐like qualities, neuropathic pain is often characterized by pain hypersensitivity (hyperalgesia and allodynia), most often towards mechanical stimuli, reflecting sensitization of neural transmission.

Data treatment

We therefore performed a systematic literature review (PubMed‐Medline, Cochrane, WoS, ClinicalTrials) and semi‐quantitative meta‐analysis of human pain models that aim to induce central sensitization, and generate hyperalgesia surrounding a real or simulated injury.

Results

From an initial set of 1569 reports, we identified and analysed 269 studies using more than a dozen human models of sensitization. Five of these models (intradermal or topical capsaicin, low‐ or high‐frequency electrical stimulation, thermode‐induced heat‐injury) were found to reliably induce secondary hyperalgesia to pinprick and have been implemented in multiple laboratories. The ability of these models to induce dynamic mechanical allodynia was however substantially lower. The proportion of subjects who developed hypersensitivity was rarely provided, giving rise to significant reporting bias. In four of these models pharmacological profiles allowed to verify similarity to some clinical conditions, and therefore may inform basic research for new drug development.

Conclusions

While there is no single “optimal” model of central sensitization, the range of validated and easy‐to‐use procedures in humans should be able to inform preclinical researchers on helpful potential biomarkers, thereby narrowing the translation gap between basic and clinical data.

Significance

Being able to mimic aspects of pathological pain directly in humans has a huge potential to understand pathophysiology and provide animal research with translatable biomarkers for drug development. One group of human surrogate models has proven to have excellent predictive validity: they respond to clinically active medications and do not respond to clinically inactive medications, including some that worked in animals but failed in the clinics. They should therefore inform basic research for new drug development.

Perceptual correlates of homosynaptic long-term potentiation in human nociceptive pathways: a replication study

Animal studies have shown that high-frequency stimulation (HFS) of peripheral C-fibres induces long-term potentiation (LTP) within spinal nociceptive pathways. The aim of this replication study was to assess if a perceptual correlate of LTP can be observed in humans. In 20 healthy volunteers, we applied HFS to the left or right volar forearm. Before and after applying HFS, we delivered single electrical test stimuli through the HFS electrode while a second electrode at the contra-lateral arm served as a control condition. Moreover, to test the efficacy of the HFS protocol, we quantified changes in mechanical pinprick sensitivity before and after HFS of the skin surrounding both electrodes. The perceived intensity was collected for both electrical and mechanical stimuli. After HFS, the perceived pain intensity elicited by the mechanical pinprick stimuli applied on the skin surrounding the HFS-treated site was significantly higher compared to control site (heterotopic effect). Furthermore, we found a higher perceived pain intensity for single electrical stimuli delivered to the HFS-treated site compared to the control site (homotopic effect). Whether the homotopic effect reflects a perceptual correlate of homosynaptic LTP remains to be elucidated.

Activation of liver x receptors prevents the spinal LTP induced by skin/muscle retraction in the thigh via SIRT1/NF-Κb pathway

It has been reported that skin/muscle incision and retraction (SMIR) in the thigh, produces mechanical allodynia in the hind paw, far from the site of incision/retraction. The mechanical allodynia lasts about 22 days, indicating chronic post-operative pain develops. The precise mechanisms, however, are largely unclear. In the current study, we further found that SMIR surgery induced LTP of c-fiber evoked field potentials that lasted at least 4 h. The mRNA and protein level of tumor necrosis factor-alpha (TNFα) and acetylated nuclear factor-kappaB p65 (ac-NF-κB p65) in the lumbar spinal dorsal horn was gradually increased during LTP development, while pretreatment with either TNFα neutralization antibody or NF-κB inhibitor PDTC completely prevented the induction of LTP. Moreover, the expression of Silent information regulator 1 (SIRT1) in the lumbar spinal dorsal horn was decreased and activation of SIRT1 by SRT1720 also prevented the induction of LTP. Importantly, the spinal expression of Liver X receptors (LXRs) was increased, both at mRNA and protein level following SMIR. Application of LXRs agonist T0901317 to the spinal dorsal horn prevented LTP induction following SMIR. Mechanistically, T0901317 enhanced the expression of SIRT1 and decreased the expression of ac-NF-κB p65 and TNFα. Spinal application of SIRT1 antagonist EX-527, 30 min before T0901317 administration, completely blocked the inhibiting effect of T0901317 on LTP, and on expression of ac-NF-κB p65 and TNFα. These results indicated that activation of LXRs prevented SMIR-induced LTP by inhibiting NF-κB/TNFα pathway via increasing SIRT1 expression.

Prostaglandin Signaling Governs Spike Timing-Dependent Plasticity at Sensory Synapses onto Mouse Spinal Projection Neurons

Highly correlated presynaptic and postsynaptic activity evokes spike timing-dependent long-term potentiation (t-LTP) at primary afferent synapses onto spinal projection neurons. While prior evidence indicates that t-LTP depends upon an elevation in intracellular Ca²⁺ within projection neurons, the downstream signaling pathways that trigger the observed increase in glutamate release from sensory neurons remain poorly understood. Using in vitro patch-clamp recordings from female mouse lamina I spino-parabrachial neurons, the present study demonstrates a critical role for prostaglandin synthesis in the generation of t-LTP. Bath application of the selective phospholipase A₂ (PLA₂) inhibitor arachidonyl trifluoromethyl ketone (AACOCF₃) or the cyclooxygenase 2 (Cox-2) inhibitor nimesulide prevented t-LTP at sensory synapses onto spino-parabrachial neurons. Similar results were observed following the block of the EP2 subtype of prostaglandin E₂ (PGE₂) receptor with PF 04418948. Meanwhile, perfusion with PGE₂ or the EP2 agonist butaprost potentiated the amplitude of monosynaptic primary afferent-evoked EPSCs while decreasing the paired-pulse ratio, suggesting a presynaptic site of action. Cox-2 was constitutively expressed in both spinal microglia and lamina I projection neurons within the superficial dorsal horn (SDH). Suppression of microglial activation with minocycline had no effect on the production of t-LTP, suggesting the possibility that prostaglandins produced within projection neurons could contribute to an enhanced probability of glutamate release at primary afferent synapses. Collectively, the results suggest that the amplification of ascending nociceptive transmission by the spinal SDH network is governed by PLA₂-Cox-2-PGE₂ signaling.SIGNIFICANCE STATEMENT Long-term potentiation (LTP) of primary afferent synapses contributes to the sensitization of spinal nociceptive circuits and has been linked to greater pain sensation in humans. Prior work has implicated elevated glutamate release in the generation of spike timing-dependent LTP (t-LTP) at sensory synapses onto ascending spinal projection neurons, but the underlying mechanisms remain unknown. Here we provide evidence that the activation of EP2 prostaglandin receptors by prostaglandin E₂, occurring downstream of phospholipase A₂ and cyclooxygenase 2 activation, mediates t-LTP at these synapses via changes in presynaptic function. This suggests that prostaglandins can increase the flow of nociceptive information from the spinal cord to the brain independently of their known ability to suppress synaptic inhibition within the dorsal horn.

Pre-emptive multimodal analgesia with tramadol and ketamine-lidocaine infusion for suppression of central sensitization in a dog model of ovariohysterectomy

Objectives

The effects of pre-emptive infusion of ketamine-lidocaine with tramadol on the suppression of central sensitization were investigated in a dog ovariohysterectomy model.

Patients and methods

Twelve dogs were randomly assigned to two groups: ketamine-lidocaine-tramadol (KLT) and tramadol (T) groups. Both groups received intravenous tramadol 4 mg/kg body weight as premedication. Immediately after induction, the KLT group received ketamine and lidocaine at 0.5 and 2 mg/kg loading dose, followed by continuous rate infusion of 50 and 100 µg/kg/min, respectively, for 2 hours. Dogs in T group received saline bolus and continuous rate infusion at equi-volume. Intraoperatively, hemodynamic responses to surgical stimulation were recorded, whereas postoperative pain was evaluated using an algometer and short form of the Glasgow composite measure pain scale.

Results

Intraoperatively, hemodynamic responses to surgical stimulation were obtunded to a greater degree in KLT compared to T group. Postoperatively, the pain scores increased only for the first hour in KLT group, compared to 12 hours in T group. Mechanical thresholds at the abdomen decreased postoperatively between 12 and 60 hours in KLT group versus the entire 72 hours in T group. Thresholds at tibia and radius in both groups increased in the immediate 1 hour postoperatively, but decreased thereafter. Significant decrement of thresholds from baseline were detected in the tibia at 24, 42, and 60 hours in KLT group compared to 24-72 hours in T group, and in the radius between 36 and 48 hours in T group, but none in KLT group.

Conclusion

Addition of pre-emptive ketamine-lidocaine infusion to single intravenous dose of tramadol enhanced attenuation of central sensitization and improved intra- and postoperative analgesia.

The human pain system exhibits higher-order plasticity (metaplasticity)

The human pain system can be bidirectionally modulated by high-frequency (HFS; 100 Hz) and low-frequency (LFS; 1 Hz) electrical stimulation of nociceptors leading to long-term potentiation or depression of pain perception (pain-LTP or pain-LTD). Here we show that priming a test site by very low-frequency stimulation (VLFS; 0.05 Hz) prevented pain-LTP probably by elevating the threshold (set point) for pain-LTP induction. Conversely, prior HFS-induced pain-LTP was substantially reversed by subsequent VLFS, suggesting that preceding HFS had primed the human nociceptive system for pain-LTD induction by VLFS. In contrast, the pain elicited by the pain-LTP-precipitating conditioning HFS stimulation remained unaffected. In aggregate these experiments demonstrate that the human pain system expresses two forms of higher-order plasticity (metaplasticity) acting in either direction along the pain-LTD to pain-LTP continuum with similar shifts in thresholds for LTD and LTP as in synaptic plasticity, indicating intriguing new mechanisms for the prevention of pain memory and the erasure of hyperalgesia related to an already established pain memory trace. There were no apparent gender differences in either pain-LTP or metaplasticity of pain-LTP. However, individual subjects appeared to present with an individual balance of pain-LTD to pain-LTP (a pain plasticity "fingerprint").

Influence of transcutaneous spinal stimulation on human LTP-like pain amplification. A randomized, double-blind study in volunteers

OBJECTIVE

Transcutaneous spinal direct current stimulation (tsDCS) has been proven to affect nociceptive signal processing. We designed a randomized, double-blind, cross-over study to investigate whether tsDCS applied before or after inducing long-term potentiation-(LTP)-like hyperalgesia may decrease nociceptive sensitivity.

METHODS

In healthy volunteers, tsDCS (2.5mA, 15min) was applied to the thoracic spine prior (n=14) or immediately following (n=12) electrical high-frequency stimulation (HFS) to the thigh, inducing hyperalgesia. Mechanical and electrical perception were assessed before HFS stimulation and at three time points following HFS stimulation (all within 90min of HFS). Subjects took part in three separate sessions to test effects of anodal, cathodal, or sham tsDCS.

RESULTS

Within 60minHFS led to unilateral changes on the conditioned side: mechanical pain thresholds tended to decrease and electrical detection thresholds significantly decreased (p<0.001); pain ratings measured using the numerical rating scale (NRS) increased for electrical stimuli (p<0.01) and two categories of mechanical stimuli ("Light(8-64mN)": p=ns; "Heavy(128-512mN)": p<0.01). Irrespective of stimulation order or polarity, tsDCS could not influence nociceptive sensitivity.

CONCLUSION

Hyperalgesia was adequately induced, but tsDCS had no effect on HFS-induced sensitization.

SIGNIFICANCE

While tsDCS has been shown to affect pain measures, our results suggest irrespective of time of stimulation or polarity that tsDCS may be less effective in modulating pain in a sensitized state in healthy subjects.

Rewiring of Developing Spinal Nociceptive Circuits by Neonatal Injury and Its Implications for Pediatric Chronic Pain

Significant evidence now suggests that neonatal tissue damage can evoke long-lasting changes in pain sensitivity, but the underlying cellular and molecular mechanisms remain unclear. This review highlights recent advances in our understanding of how injuries during a critical period of early life modulate the functional organization of synaptic networks in the superficial dorsal horn (SDH) of the spinal cord in a manner that favors the excessive amplification of ascending nociceptive signaling to the brain, which likely contributes to the generation and/or maintenance of pediatric chronic pain. These persistent alterations in synaptic function within the SDH may also contribute to the well-documented "priming" of developing pain pathways by neonatal tissue injury.

Test-Retest Reliability of 10 Hz Conditioning Electrical Stimulation Inducing Long-Term Potentiation (LTP)-Like Pain Amplification in Humans

Background

10 Hz conditioning electrical stimulation (CES) has been shown to induce long-term potentiation (LTP)-like pain amplification similar to traditional 100 Hz CES in healthy humans. The aim of this study was to assess the test-retest reliability and to estimate sample sizes required for future crossover and parallel study designs.

Methods

The 10 Hz paradigm (500 rectangular pulses lasting 50 s) was repeated on two separate days with one week interval in twenty volunteers. Perceptual intensities to single electrical stimulation (SES) at the conditioned skin site and to mechanical stimuli (pinprick and light stroking) in immediate vicinity to the conditioned skin site were recorded. Superficial blood flow (SBF) was assessed as indicator of neurogenic inflammation. All outcome measures were assessed with 10 min interval three times before and six times after the CES. The coefficient of variation and intra-class correlation coefficient were calculated within session and between sessions. Sample sizes were estimated for future crossover (Ncr) and parallel (Np) drug testing studies expected to detect a 30% decrease for the individual outcome measure following 10 Hz CES.

Results

Perceptual intensity ratings to light stroking (Ncr = 2, Np = 33) and pinprick stimulation (491 mN) (Ncr = 6, Np = 54) increased after CES and showed better reliability in crossover than parallel design. The SBF increased after CES, and then declined until reaching a plateau 20 minutes postCES. SBF showed acceptable reliability both in crossover and parallel designs (Ncr = 3, Np = 13). Pain ratings to SES were reliable, but with large estimated sample sizes (Ncr = 634, Np = 11310) due to the minor pain amplification.

Conclusions

The reliability of 10 Hz CES was acceptable in inducing LTP-like effects in the assessments of superficial blood flow, heterotopic mechanical hyperalgesia, and dysesthesia in terms of sample sizes for future crossover study designs.

Opioid-induced hyperalgesia: Cellular and molecular mechanisms

Opioids produce strong analgesia but their use is limited by a paradoxical hypersensitivity named opioid-induced hyperalgesia (OIH) that may be associated to analgesic tolerance. In the last decades, a significant number of preclinical studies have investigated the factors that modulate OIH development as well as the cellular and molecular mechanisms underlying OIH. Several factors have been shown to influence OIH including the genetic background and sex differences of experimental animals as well as the opioid regimen. Mu opioid receptor (MOR) variants and interactions of MOR with different proteins were shown important. Furthermore, at the cellular level, both neurons and glia play a major role in OIH development. Several neuronal processes contribute to OIH, like activation of neuroexcitatory mechanisms, long-term potentiation (LTP) and descending pain facilitation. Increased nociception is also mediated by neuroinflammation induced by the activation of microglia and astrocytes. Neurons and glial cells exert synergistic effects, which contribute to OIH. The molecular actors identified include the Toll-like receptor 4 and the anti-opioid systems as well as some other excitatory molecules, receptors, channels, chemokines, pro-inflammatory cytokines or lipids. This review summarizes the intracellular and intercellular pathways involved in OIH and highlights some mechanisms that may be challenged to limit OIH in the future.

NMDARs in neurological diseases: a potential therapeutic target

N-methyl-D-aspartate ionotropic glutamate receptor (NMDARs) is a ligand-gated ion channel that plays a critical role in excitatory neurotransmission, brain development, synaptic plasticity associated with memory formation, central sensitization during persistent pain, excitotoxicity and neurodegenerative diseases in the central nervous system (CNS). Within iGluRs, NMDA receptors have been the most actively investigated for their role in neurological diseases, especially neurodegenerative pathologies such as Alzheimer's and Parkinson's diseases. It has been demonstrated that excessive activation of NMDA receptors (NMDARs) plays a key role in mediating some aspects of synaptic dysfunction in several CNS disorders, so extensive research has been directed on the discovery of compounds that are able to reduce NMDARs activity. This review discusses the role of NMDARs on neurological pathologies and the possible therapeutic use of agents that target this receptor. Additionally, we delve into the role of NMDARs in Alzheimer's and Parkinson's diseases and the receptor antagonists that have been tested on in vivo models of these pathologies. Finally, we put into consideration the importance of antioxidants to counteract oxidative capacity of the signaling cascade in which NMDARs are involved.

Hyperalgesia by synaptic long-term potentiation (LTP): an update

Long-term potentiation of synaptic strength (LTP) in nociceptive pathways shares principle features with hyperalgesia including induction protocols, pharmacological profile, neuronal and glial cell types involved and means for prevention. LTP at synapses of nociceptive nerve fibres constitutes a contemporary cellular model for pain amplification following trauma, inflammation, nerve injury or withdrawal from opioids. It provides a novel target for pain therapy. This review summarizes recent progress which has been made in unravelling the properties and functions of LTP in the nociceptive system and in identifying means for its prevention and reversal.

Long-term potentiation in spinal nociceptive pathways as a novel target for pain therapy

Long-term potentiation (LTP) in nociceptive spinal pathways shares several features with hyperalgesia and has been proposed to be a cellular mechanism of pain amplification in acute and chronic pain states. Spinal LTP is typically induced by noxious input and has therefore been hypothesized to contribute to acute postoperative pain and to forms of chronic pain that develop from an initial painful event, peripheral inflammation or neuropathy. Under this assumption, preventing LTP induction may help to prevent the development of exaggerated postoperative pain and reversing established LTP may help to treat patients who have an LTP component to their chronic pain. Spinal LTP is also induced by abrupt opioid withdrawal, making it a possible mechanism of some forms of opioid-induced hyperalgesia. Here, we give an overview of targets for preventing LTP induction and modifying established LTP as identified in animal studies. We discuss which of the various symptoms of human experimental and clinical pain may be manifestations of spinal LTP, review the pharmacology of these possible human LTP manifestations and compare it to the pharmacology of spinal LTP in rodents.

Analysis of hyperalgesia time courses in humans after painful electrical high-frequency stimulation identifies a possible transition from early to late LTP-like pain plasticity

Electrical high-frequency stimulation (HFS) of skin afferents elicits long-term potentiation (LTP)-like hyperalgesia in humans. Time courses were evaluated in the facilitating (homotopic) or facilitated (heterotopic) pathways to delineate the relative contributions of early or late LTP-like pain plasticity. HFS in healthy subjects (n=55) elicited highly significant pain increases to electrical stimuli via the conditioning electrode (to 145% of control, homotopic pain LTP) and to pinprick stimuli in adjacent skin (to 190% of control, secondary hyperalgesia). Individual time courses in subjects expressing a sufficient magnitude of hyperalgesia (>20% pain increase, n=28) revealed similar half-lives of homotopic pain LTP and secondary hyperalgesia of 6.9 h and 4.9 h (log(10) mean 0.839±0.395 and 0.687±0.306) and times to full recovery of 48 h and 24 h (log(10) mean 1.679±0.790 and 1.373±0.611). Time course and peak magnitudes were not correlated between (r=-0.19to+0.21, NS), nor within both readout (r=0.29 and 0.31, NS). In most subjects, time courses were consistent with early LTP1. Notably, in some subjects (10 of 28), estimated times to full recovery were much longer (>10 days), possibly indicating development of late LTP2-like pain plasticity. Dynamic mechanical allodynia (only present in 16 of 55 subjects) lasted for a shorter time than secondary hyperalgesia. Three different readouts of nociceptive central sensitization suggest that brief intense nociceptive input elicits early LTP1 of pain sensation (based on posttranslational modifications), but susceptible subjects may already develop longer-lasting late LTP2 (based on transcriptional modifications). These findings support the hypothesis that LTP may contribute to the development of persistent pain disorders.

Synaptic Plasticity and Pain: Role of Ionotropic Glutamate Receptors

Pain hypersensitivity that develops after tissue or nerve injury is dependent both on peripheral processes in the affected tissue and on enhanced neuronal responses in the central nervous system, including the dorsal horn of the spinal cord. It has become increasingly clear that strengthening of glutamatergic sensory synapses, such as those established in the dorsal horn by nociceptive thin-caliber primary afferent fibers, is a major contributor to sensitization of neuronal responses that leads to pain hypersensitivity. Here, the authors review recent findings on the roles of ionotropic glutamate receptors in synaptic plasticity in the dorsal horn in relation to acute and persistent pain.

Ionotropic glutamate receptors in spinal nociceptive processing

Glutamate is the predominant excitatory transmitter used by primary afferent synapses and intrinsic neurons in the spinal cord dorsal horn. Accordingly, ionotropic glutamate receptors mediate basal spinal transmission of sensory, including nociceptive, information that is relayed to supraspinal centers. However, it has become gradually more evident that these receptors are also crucially involved in short- and long-term plasticity of spinal nociceptive transmission, and that such plasticity have an important role in the pain hypersensitivity that may result from tissue or nerve injury. This review will cover recent findings on pre- and postsynaptic regulation of synaptic function by ionotropic glutamate receptors in the dorsal horn and how such mechanisms contribute to acute and chronic pain.

Outpatient intravenous ketamine for the treatment of complex regional pain syndrome: a double-blind placebo controlled study

Complex regional pain syndrome (CRPS) is a severe chronic pain condition that most often develops following trauma. The pathophysiology of CRPS is not known but both clinical and experimental evidence demonstrate the important of the NMDA receptor and glial activation in its induction and maintenance. Ketamine is the most potent clinically available safe NMDA antagonist that has a well established role in the treatment of acute and chronic pain. This randomized double-blind placebo controlled trial was designed to evaluate the effectiveness of intravenous ketamine in the treatment of CRPS. Before treatment, after informed consent was obtained, each subject was randomized into a ketamine or a placebo infusion group. Study subjects were evaluated for at least 2 weeks prior to treatment and for 3 months following treatment. All subjects were infused intravenously with normal saline with or without ketamine for 4h (25ml/h) daily for 10 days. The maximum ketamine infusion rate was 0.35mg/kg/h, not to exceed 25mg/h over a 4h period. Subjects in both the ketamine and placebo groups were administered clonidine and versed. This study showed that intravenous ketamine administered in an outpatient setting resulted in statistically significant (p<0.05) reductions in many pain parameters. It also showed that subjects in our placebo group demonstrated no treatment effect in any parameter. The results of this study warrant a larger randomized placebo controlled trial using higher doses of ketamine and a longer follow-up period.

Effects of low-dose intranasal (S)-ketamine in patients with neuropathic pain

BACKGROUND

NMDA receptors are involved in the development and maintenance of neuropathic pain. We evaluated the efficacy and safety of intranasal (S)-ketamine, one of the most potent clinically available NMDA receptor antagonists.

METHODS

Sixteen patients with neuropathic pain of various origins were randomized into two treatment groups: (S)-ketamine 0.2mg/kg (group 1); (S)-ketamine 0.4mg/kg (group 2). Plasma concentrations of (S)-ketamine and (S)-norketamine were measured over 6h by High Performance Liquid Chromatography combined with mass spectrometry. Quantitative sensory testing (QST) was conducted before, during and after treatment. Side effects and amount of pain reduction were recorded.

RESULTS

Intranasal (S)-ketamine administration lead to peak plasma concentrations of 27.7+/-5.9ng/ml at 10+/-6.3min (group 1) and 34.3+/-22.2ng/ml at 13.8+/-4.8min after application (group 2). Maximal plasma concentrations of (S)-norketamine were 18.3+/-14.9ng/ml at 81+/-59min (group 1) and 34.3+/-5.5ng/ml at 75+/-40min (group 2). Pain scores decreased significantly in both groups with minimal pain at 60min after drug administration (70+/-10% and 61+/-13% of initial pain in groups 1 and 2). The time course of pain decrease was significantly correlated with plasma concentrations of (S)-ketamine and (S)-norketamine (partial correlations: (S)-norketamine: -0.90 and -0.86; (S)-ketamine: -0.72 and -0.71 for group 1 and group 2, respectively). Higher dosing elicited significantly more side effects. Intranasal (S)-ketamine had no significant impact on thermal or mechanical detection and pain thresholds in normal or symptomatic skin areas.

CONCLUSIONS

Intranasal administration of low dose (S)-ketamine rapidly induces adequate plasma concentrations of (S)-ketamine and subsequently of its metabolite (S)-norketamine. The time course of analgesia correlated with plasma concentrations.

Xenon-induced changes in CNS sensitization to pain

Electrophysiological investigations of the spinal cord in animals have shown that pain sensitizes the central nervous system via glutamate receptor dependent long-term potentiation (LTP) related to an enhancement of pain perception. To expand these findings, we used functional magnetic resonance (fMRI), blood oxygen level dependent (BOLD) and perfusion imaging in combination with repeated electrical stimulation in humans. Specifically we monitored modulation of somatosensory processing during inhibition of excitatory transmission by ocular application of the glutamate receptor antagonist xenon. BOLD responses upon secondary stimulation increased in mid insular and in primary/secondary sensory cortices under placebo and decreased under xenon treatments. Xenon-induced decreases in regional perfusion were confined to stimulation responsive brain regions and correlated with time courses of xenon concentrations in the cranial blood. Moreover, effects of xenon on behavioral, fMRI and perfusion data scaled with stimulus intensity. The dependence of pain sensitization on sufficient pre-activation reflects a multistage process which is characteristic for glutamate receptor related processes of LTP. This study demonstrates how LTP related processes known from the cellular level can be investigated at the brain systems level.

Spinal cord long-term potentiation (LTP) is associated with increased dorsal horn gene expression of IL-1beta, GDNF and iNOS

Previous data show that spinal cord long-term potentiation (LTP) can be induced by electrical high-frequency stimulation (HFS) conditioning applied to the sciatic nerve. It has been suggested that the cellular events leading to this form of plasticity may contribute to central hyperalgesia. In the present study, extracellular recordings from single dorsal horn neurons and quantitative real-time reverse-transcriptase polymerase chain reaction (RT-PCR) on rat dorsal horn tissue were used to examine whether maintenance of spinal LTP is associated with changes in gene expression of the proinflammatory interleukin-1beta (IL-1beta), glial cell-line derived neurotrophic factor (GDNF), inducible nitric oxide synthase (iNOS), p38 mitogen-activated protein kinase (p38 MAPK), cyclooxygenase 2 (COX2) and tumor necrosis factor alpha (TNFalpha). The data demonstrated that the HFS conditioning induced a robust increase in the dorsal horn C-fibre responses, which outlasted the duration of the experiments of 6h (p<0.05, HFS vs. control). Moreover, a significant increase in the expression of mRNA for IL-1beta, GDNF and iNOS were observed 6h following the HFS conditioning (p<0.05, HFS vs. control). For the first time we show that spinal cord LTP is associated with an increased dorsal horn expression of the genes for IL-1beta, GDNF and iNOS.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Translating nociceptive processing into human pain models

As volunteers can easily communicate quality and intensity of painful stimuli, human pain models appear to be ideally suited to test analgesic compounds, but also to study pain mechanisms. Acute stimulation of nociceptors under physiologic conditions has proven not to be of particular use as an experimental pain model. In contrast, if the experimental models include sensitization of the peripheral or central pain processing they may indeed mimic certain aspects of chronic pain conditions. Peripheral inflammatory conditions can be induced experimentally with sensitization patterns correlating to clinical inflammatory pain. There are also well-characterized models of central sensitization, which mimic aspects of neuropathic pain patients such as touch evoked allodynia and punctate hyperalgesia. The main complaint of chronic pain patients, however, is spontaneous pain, but currently there is no human model available that would mimic chronic inflammatory or neuropathic pain. Thus, although being helpful for proof of concept studies and dose finding, current human pain models cannot replace patient studies for testing efficacy of analgesic compounds.

Ketamine reduces the cell death following inflammatory pain in newborn rat brain

Premature infants experience untreated repetitive pain that may alter their brain development. Effects of ketamine and repetitive pain on cellular death and subsequent behavior were studied in neonatal rats. Rat pups were randomized to undisturbed controls (C), 4% formalin injection (F), ketamine alone (K, 5 mg/kg) or formalin plus ketamine (KF) and were assessed for neuroactivation with Fos protein, cellular death with FluoroJade-B, cognition with the radial arm maze, and pain thresholds with the hot-plate. Greater Fos expression and cell death occurred in F vs. C groups in defined brain areas at 1 and 4 h in F compared with other groups. Cell death was accentuated 3.3-fold in cortical areas and 1.6-fold in subcortical areas in the F compared with the C group following repetitive pain and sacrifice 18-20 h later. These effects were ameliorated by ketamine. Compared with the F group, all other groups demonstrated greater exploratory and rearing behaviors and decreased time for bait consumption at 1-h and 3-h intervals. Significantly greater thermal pain latencies occurred in the KF and F groups. Repetitive neonatal pain accentuates neuronal excitation and cell death in developmentally regulated cortical and subcortical areas, which decreases the acquisition of visual-spatial clues, short-term and long-term memory, and increases pain latencies. Ketamine analgesia mitigates most of these effects.

Understanding LTP in pain pathways

Long-term potentiation (LTP) at synapses of nociceptive nerve fibres is a proposed cellular mechanism underlying some forms of hyperalgesia. In this review fundamental properties of LTP in nociceptive pathways are described. The following topics are specifically addressed: A concise definition of LTP is given and a differentiation is made between LTP and "central sensitisation". How to (and how not to) measure and how to induce LTP in pain pathways is specified. The signal transduction pathways leading to LTP at C-fibre synapses are highlighted and means of how to pre-empt and how to reverse LTP are delineated. The potential functional roles of LTP are evaluated at the cellular level and at the behavioural level in experimental animals. Finally, the impact of LTP on the perception of pain in human subjects is discussed.