Glutamate receptor ligands attenuate allodynia and hyperalgesia and potentiate morphine effects in a mouse model of neuropathic pain

Mechanism of Action of Tibial Nerve Stimulation in the Treatment of Lower Urinary Tract Dysfunction

BACKGROUND AND OBJECTIVE

Tibial nerve stimulation (TNS) has long been used to effectively treat lower urinary tract dysfunction (LUTD). Although numerous studies have concentrated on TNS, its mechanism of action remains elusive. This review aimed to concentrate on the mechanism of action of TNS against LUTD.

MATERIALS AND METHODS

A literature search was performed in PubMed on October 31, 2022. In this study, we introduced the application of TNS for LUTD, summarized different methods used in exploring the mechanism of TNS, and discussed the next direction to investigate the mechanism of TNS.

RESULTS AND CONCLUSIONS

In this review, 97 studies, including clinical studies, animal experiments, and reviews, were used. TNS is an effective treatment for LUTD. The study of its mechanisms primarily concentrated on the central nervous system, tibial nerve pathway, receptors, and TNS frequency. More advanced equipment will be used in human experiments to investigate the central mechanism, and diverse animal experiments will be performed to explore the peripheral mechanism and parameters of TNS in the future.

Opioids Induce Bidirectional Synaptic Plasticity in a Brainstem Pain Center in the Rat

Opioids are powerful analgesics commonly used in pain management. However, opioids can induce complex neuroadaptations, including synaptic plasticity, that ultimately drive severe side effects, such as pain hypersensitivity and strong aversion during prolonged administration or upon drug withdrawal, even following a single, brief administration. The lateral parabrachial nucleus (LPBN) in the brainstem plays a key role in pain and emotional processing; yet, the effects of opioids on synaptic plasticity in this area remain unexplored. Using patch-clamp recordings in acute brainstem slices from male and female Sprague Dawley rats, we demonstrate a concentration-dependent, bimodal effect of opioids on excitatory synaptic transmission in the LPBN. While a lower concentration of DAMGO (0.5 µM) induced a long-term depression of synaptic strength (low-DAMGO LTD), abrupt termination of a higher concentration (10 µM) induced a long-term potentiation (high-DAMGO LTP) in a subpopulation of cells. LTD involved a metabotropic glutamate receptor (mGluR)-dependent mechanism; in contrast, LTP required astrocytes and N-methyl-D-aspartate receptor (NMDAR) activation. Selective optogenetic activation of spinal and periaqueductal gray matter (PAG) inputs to the LPBN revealed that, while LTD was expressed at all parabrachial synapses tested, LTP was restricted to spino-parabrachial synapses. Thus, we uncovered previously unknown forms of opioid-induced long-term plasticity in the parabrachial nucleus that potentially modulate some adverse effects of opioids. PERSPECTIVE: We found a previously unrecognized site of opioid-induced plasticity in the lateral parabrachial nucleus, a key region for pain and emotional processing. Unraveling opioid-induced adaptations in parabrachial function might facilitate the identification of new therapeutic measures for addressing adverse effects of opioid discontinuation such as hyperalgesia and aversion.

MMG22 Potently Blocks Hyperalgesia in Cisplatin-treated Mice

MMG22 is a bivalent ligand containing MOR agonist and mGluR5 antagonist pharmacophores connected by a 22-atom linker. Intrathecal (i.t.) administration of MMG22 to inflamed mice has been reported to produce fmol-range antinociception in the reversal of LPS-induced hyperalgesia. MMG22 reduced hyperalgesia in the spared nerve injury (SNI) model of neuropathic pain at 10 days after injury but not at 30 days after injury, perhaps related to the inflammation that occurs early after injury but subsequently subsides. The present study determined the efficacy of MMG22 in cisplatin-treated male mice in order to provide data relating to the efficacy of MMG22 in the treatment of neuropathic pain that is associated with inflammation. Groups of eight mice each received daily intraperitoneal (i.p.) injections of cisplatin for seven days to produce robust mechanical allodynia defined by the decrease in withdrawal threshold using an electronic von Frey applied to the plantar surface of the hind paw. Intrathecal administration of MMG22 potently reduced mechanical hyperalgesia (ED50 0.04 fmol/mouse) without tolerance, whereas MMG10 was essentially inactive. Morphine was less potent than MMG22 by >5-orders of magnitude and displayed tolerance. Subcutaneous MMG22 was effective (ED50 = 2.41 mg/kg) and devoid of chronic tolerance. We propose that MMG22 induces the formation of a MOR-mGluR5 heteromer through selective interaction with the upregulated NR2B subunit of activated NMDAR, in view of the 4600-fold reduction of i.t. MMG22 antinociception by the selective NR2B antagonist, Ro25-6981. A possible explanation for the substantially reduced potency for MMG22 in the SNI model is discussed.

Recent Advances in the Modulation of Pain by the Metabotropic Glutamate Receptors

Metabotropic glutamate receptors (mGluR or mGlu) are G-protein coupled receptors activated by the binding of glutamate, the main classical neurotransmitter of the nervous system. Eight different mGluR subtypes (mGluR1-8) have been cloned and are classified in three groups based on their molecular, pharmacological and signaling properties. mGluRs mediate several physiological functions such as neuronal excitability and synaptic plasticity, but they have also been implicated in numerous pathological conditions including pain. The availability of new and more selective allosteric modulators together with the canonical orthosteric ligands and transgenic technologies has led to significant advances in our knowledge about the role of the specific mGluR subtypes in the pathophysiological mechanisms of various diseases. Although development of successful compounds acting on mGluRs for clinical use has been scarce, the subtype-specific-pharmacological manipulation might be a compelling approach for the treatment of several disorders in humans, including pain; this review aims to summarize and update on preclinical evidence for the roles of different mGluRs in the pain system and discusses knowledge gaps regarding mGluR-related sex differences and neuroimmune signaling in pain.

Multifunctional Opioid-Derived Hybrids in Neuropathic Pain: Preclinical Evidence, Ideas and Challenges

When the first- and second-line therapeutics used to treat neuropathic pain (NP) fail to induce efficient analgesia—which is estimated to relate to more than half of the patients—opioid drugs are prescribed. Still, the pathological changes following the nerve tissue injury, i.a. pronociceptive neuropeptide systems activation, oppose the analgesic effects of opiates, enforcing the use of relatively high therapeutic doses in order to obtain satisfying pain relief. In parallel, the repeated use of opioid agonists is associated with burdensome adverse effects due to compensatory mechanisms that arise thereafter. Rational design of hybrid drugs, in which opioid ligands are combined with other pharmacophores that block the antiopioid action of pronociceptive systems, delivers the opportunity to ameliorate the NP-oriented opioid treatment via addressing neuropathological mechanisms shared both by NP and repeated exposition to opioids. Therewith, the new dually acting drugs, tailored for the specificity of NP, can gain in efficacy under nerve injury conditions and have an improved safety profile as compared to selective opioid agonists. The current review presents the latest ideas on opioid-comprising hybrid drugs designed to treat painful neuropathy, with focus on their biological action, as well as limitations and challenges related to this therapeutic approach.

Median Nerve Stimulation as a Nonpharmacological Approach to Bypass Analgesic Tolerance to Morphine: A Proof-of-Concept Study in Mice

Analgesic tolerance to opioids contributes to the opioid crisis by increasing the quantity of opioids prescribed and consumed. Thus, there is a need to develop non-opioid-based pain-relieving regimens as well as strategies to circumvent opioid tolerance. Previously, we revealed a non-opioid analgesic mechanism induced by median nerve electrostimulation at the overlaying PC6 (Neiguan) acupoint (MNS-PC6). Here, we further examined the efficacy of MNS-PC6 in morphine-tolerant mice with neuropathic pain induced by chronic constriction injury (CCI) of the sciatic nerve. Daily treatments of MNS-PC6 (2 Hz, 2 mA), but not electrostimulation at a nonmedian nerve-innervated location, for a week post-CCI induction significantly suppressed established mechanical allodynia in CCI-mice in an orexin-1 (OX₁) and cannabinoid-1 (CB₁) receptor-dependent fashion. This antiallodynic effect induced by repeated MNS-PC6 was comparable to that induced by repeated gabapentin (50 mg/kg, i.p.) or single morphine (10 mg/kg, i.p.) treatments, but without tolerance, unlike repeated morphine-induced analgesia. Furthermore, single and repeated MNS-PC6 treatments remained fully effective in morphine-tolerant CCI-mice, also in an OX₁ and CB₁ receptor-dependent fashion. In CCI-mice receiving escalating doses of morphine for 21 days (10, 20 and 50 mg/kg), single and repeated MNS-PC6 treatments remained fully effective. Therefore, repeated MNS-PC6 treatments induce analgesia without tolerance, and retain efficacy in opioid-tolerant mice via a mechanism that involves OX₁ and CB₁ receptors. This study suggests that MNS-PC6 is an alternative pain management strategy that maybe useful for combatting the opioid epidemic, and opioid-tolerant patients receiving palliative care. PERSPECTIVE: Median nerve stimulation relieves neuropathic pain in mice without tolerance and retains efficacy even in mice with analgesic tolerance to escalating doses of morphine, via an opioid-independent, orexin-endocannabinoid-mediated mechanism. This study provides a proof of concept for utilizing peripheral nerve stimulating devices for pain management in opioid-tolerant patients.

The bivalent ligand, MMG22, reduces neuropathic pain after nerve injury without the side effects of traditional opioids

ABSTRACT

Functional interactions between the mu opioid receptor (MOR) and the metabotropic glutamate receptor 5 (mGluR5) in pain and analgesia have been well established. MMG22 is a bivalent ligand containing MOR agonist (oxymorphamine) and mGluR5 antagonist (MPEP) pharmacophores tethered by a 22-atom linker. MMG22 has been shown to produce potent analgesia in several models of chronic inflammatory and neuropathic pain (NP). This study assessed the efficacy of systemic administration of MMG22 at reducing pain behavior in the spared nerve injury (SNI) model of NP in mice, as well as its side-effect profile and abuse potential. MMG22 reduced mechanical hyperalgesia and spontaneous ongoing pain after SNI, with greater potency early (10 days) as compared to late (30 days) after injury. Systemic administration of MMG22 did not induce place preference in naive animals, suggesting absence of abuse liability when compared to traditional opioids. MMG22 also lacked the central locomotor, respiratory, and anxiolytic side effects of its monomeric pharmacophores. Evaluation of mRNA expression showed the transcripts for both receptors were colocalized in cells in the dorsal horn of the lumbar spinal cord and dorsal root ganglia. Thus, MMG22 reduces hyperalgesia after injury in the SNI model of NP without the typical centrally mediated side effects associated with traditional opioids.

Pain-related behavior is associated with increased joint innervation, ipsilateral dorsal horn gliosis, and dorsal root ganglia activating transcription factor 3 expression in a rat ankle joint model of osteoarthritis

Supplemental Digital Content is Available in the Text.

In a rat model of osteoarthritis, we found increased joint sensory and sympathetic innervation and glia changes in dorsal horn, accompanying pain-related behavior onset.

Introduction:

Osteoarthritis (OA)-associated pain is often poorly managed, as our understanding of the underlying pain mechanisms remains limited. The known variability from patient to patient in pain control could be a consequence of a neuropathic component in OA.

Methods:

We used a rat monoiodoacetate model of the ankle joint to study the time-course of the development of pain-related behavior and pathological changes in the joint, dorsal root ganglia (DRG), and spinal cord, and to investigate drug treatments effects.

Results:

Mechanical hypersensitivity and loss of mobility (as assessed by treadmill) were detected from 4 weeks after monoiodoacetate. Cold allodynia was detected from 5 weeks. Using histology and x-ray microtomography, we confirmed significant cartilage and bone degeneration at 5 and 10 weeks. We detected increased nociceptive peptidergic and sympathetic fiber innervation in the subchondral bone and synovium at 5 and 10 weeks. Sympathetic blockade at 5 weeks reduced pain-related behavior. At 5 weeks, we observed, ipsilaterally only, DRG neurons expressing anti-activating transcription factor 3, a neuronal stress marker. In the spinal cord, there was microgliosis at 5 and 10 weeks, and astrocytosis at 10 weeks only. Inhibition of glia at 5 weeks with minocycline and fluorocitrate alleviated mechanical allodynia.

Conclusion:

Besides a detailed time-course of pathology in this OA model, we show evidence of contributions of the sympathetic nervous system and dorsal horn glia to pain mechanisms. In addition, late activating transcription factor 3 expression in the DRG that coincides with these changes provides evidence in support of a neuropathic component in OA pain.

The Modulation of Pain by Metabotropic Glutamate Receptors 7 and 8 in the Dorsal Striatum

The dorsal striatum, apart from controlling voluntary movement, displays a recently demonstrated pain inhibition. It is connected to the descending pain modulatory system and in particular to the rostral ventromedial medulla through the medullary dorsal reticular nucleus. Diseases of the basal ganglia, such as Parkinson's disease, in addition to being characterized by motor disorders, are associated with pain and hyperactivation of the excitatory transmission. A way to counteract glutamatergic hyperactivation is through the activation of group III metabotropic glutamate receptors (mGluRs), which are located on presynaptic terminals inhibiting neurotransmitter release. So far the mGluRs of group III have been the least investigated, owing to a lack of selective tools. More recently, selective ligands for each mGluR of group III, in particular positive and negative allosteric modulators, have been developed and the role of each subtype is starting to emerge. The neuroprotective potential of group III mGluRs in pathological conditions, such as those characterized by elevate glutamate, has been recently shown. In the dorsal striatum, mGluR7 and mGluR8 are located at glutamatergic corticostriatal terminals and their stimulation inhibits pain in pathological conditions such as neuropathic pain. The two receptors in the dorsal striatum have instead a different role in pain control in normal conditions. This review will discuss recent results focusing on the contribution of mGluR7 and mGluR8 in the dorsal striatal control of pain. The role of mGluR4, whose antiparkinsonian activity is widely reported, will also be addressed.

Neuropathic Pain Dysregulates Gene Expression of the Forebrain Opioid and Dopamine Systems

Disturbances in the function of the mesostriatal dopamine system may contribute to the development and maintenance of chronic pain, including its sensory and emotional/cognitive aspects. In the present study, we assessed the influence of chronic constriction injury (CCI) of the sciatic nerve on the expression of genes coding for dopamine and opioid receptors as well as opioid propeptides in the mouse mesostriatal system, particularly in the nucleus accumbens. We demonstrated bilateral increases in mRNA levels of the dopamine D1 and D2 receptors (the latter accompanied by elevated protein level), opioid propeptides proenkephalin and prodynorphin, as well as delta and kappa (but not mu) opioid receptors in the nucleus accumbens at 7 to 14 days after CCI. These results show that CCI-induced neuropathic pain is accompanied by a major transcriptional dysregulation of molecules involved in dopaminergic and opioidergic signaling in the striatum/nucleus accumbens. Possible functional consequences of these changes include opposite effects of upregulated enkephalin/delta opioid receptor signaling vs. dynorphin/kappa opioid receptor signaling, with the former most likely having an analgesic effect and the latter exacerbating pain and contributing to pain-related negative emotional states.

Potentiation of morphine-induced antinociception by harmaline: involvement of μ-opioid and ventral tegmental area NMDA receptors

RATIONAL

Morphine is one of the most well-known and potent analgesic agents; however, it can also induce various side effects. Thus, finding drugs and mechanisms which can potentiate the analgesic effects of low doses of morphine will be a good strategy for pain management.

OBJECTIVE

The involvement of μ-opioid receptors and ventral tegmental area (VTA) glutamatergic system in harmaline and morphine combination on the nociceptive response were investigated. Also, we examined reward efficacy and tolerance expression following the drugs.

METHODS

Animals were bilaterally cannulated in the VTA by stereotaxic instrument. A tail-flick (TF) apparatus and conditioned place preference (CPP) paradigm were used to measure nociceptive response and rewarding effects in male NMRI mice respectively.

RESULTS

Morphine (2 mg/kg, i.p.) had no effect in TF test. Also, harmaline (1.25 and 5 mg/kg, i.p.) could not change pain threshold. Combination of a non-effective dose of harmaline (5 mg/kg) and morphine (2 mg/kg) produced antinociception and also prevented morphine tolerance but had no effect on the acquisition of CPP. Systemic administration of naloxone (0.5 and 1 mg/kg) and intra-VTA microinjection of NMDA (0.06 and 0.1 μg/mouse) before harmaline (5 mg/kg) plus morphine (2 mg/kg) prevented antinociception induced by the drugs. D-AP5 (0.5 and 1 μg/mouse, intra-VTA) potentiated the effect of low-dose harmaline (1.25 mg/kg) and morphine (2 mg/kg) and induced antinociception. Microinjection of the same doses of NMDA or D-AP5 into the VTA alone had no effect on pain threshold.

CONCLUSION

The findings showed that harmaline potentiated the analgesic effect of morphine and reduced morphine tolerance. Glutamatergic and μ-opioidergic system interactions in the VTA seem to have a modulatory role in harmaline plus morphine-induced analgesia.

Pharmacological Blockade of Spinal CXCL3/CXCR2 Signaling by NVP CXCR2 20, a Selective CXCR2 Antagonist, Reduces Neuropathic Pain Following Peripheral Nerve Injury

Recently, the role of CXCR2 in nociception has been noted. Our studies provide new evidence that the intrathecal administration of its CINC ligands (Cytokine-Induced Neutrophil Chemoattractant; CXCL1-3) induces pain-like behavior in naïve mice, and the effect occurring shortly after administration is associated with the neural location of CXCR2, as confirmed by immunofluorescence. RT-qPCR analysis showed, for the first time, raised levels of spinal CXCR2 after chronic constriction injury (CCI) of the sciatic nerve in rats. Originally, on day 2, we detected escalated levels of the spinal mRNA of all CINCs associated with enhancement of the protein level of CXCL3 lasting until day 7. Intrathecal administration of CXCL3 neutralizing antibody diminished neuropathic pain on day 7 after CCI. Interestingly, CXCL3 is produced in lipopolysaccharide-stimulated microglial, but not astroglial, primary cell cultures. We present the first evidence that chronic intrathecal administrations of the selective CXCR2 antagonist, NVP CXCR2 20, attenuate neuropathic pain symptoms and CXCL3 expression after CCI. Moreover, in naïve mice, this antagonist prevented CXCL3-induced hypersensitivity. However, NVP CXCR2 20 did not diminish glial activation, thus not enhancing morphine/buprenorphine analgesia. These results provide novel insight into the crucial role of CXCR2 in neuropathy based on CXCL3 modulation, which may become a potential therapeutic target in pain treatment.

Targeting MOR-mGluR5 heteromers reduces bone cancer pain by activating MOR and inhibiting mGluR5

Pain is among the most common symptoms in cancer and approximately 90% of patients experience end-stage cancer pain. The management of cancer pain is challenging due to the significant side effects associated with opioids, and novel therapeutic approaches are needed. MMG22 is a bivalent ligand containing MOR agonist and mGluR₅ antagonist pharmacophores joined by a 22-atom spacer. MMG22 exhibited extraordinary analgesia following intrathecal administration in a mouse model of bone cancer pain. Here, we assessed the effectiveness of systemic administration of MMG22 in reducing cancer pain and evaluated whether MMG22 displays side effects associated with opioids. Fibrosarcoma cells were injected into and around the calcaneus bone in C3H mice. Mechanical hyperalgesia was defined as an increase in the paw withdrawal frequencies (PWFs) evoked by application of a von Frey monofilament (3.9 mN bending force) applied to the plantar surface of the hind paw Subcutaneous (s.c.), intramuscular (i.m.), and oral (p.o.) administration of MMG22 produced robust dose-dependent antihyperalgesia, whose ED₅₀ was orders of magnitude lower than morphine. Moreover, the ED₅₀ for MMG22 decreased with disease progression. Importantly, s.c. administration of MMG22 did not produce acute (24 h) or long-term (9 days) tolerance, was not rewarding (conditioned place preference test), and did not produce naloxone-induced precipitated withdrawal or alter motor function. A possible mechanism of action of MMG22 is discussed in terms of inhibition of spinal NMDAR via antagonism of its co-receptor, mGluR₅, and concomitant activation of neuronal MOR. We suggest that MMG22 may be a powerful alternative to traditional opioids for managing cancer pain. This article is part of the Special Issue entitled 'New Vistas in Opioid Pharmacology'.

Histamine, histamine receptors, and neuropathic pain relief

Histamine, acting via distinct histamine H₁, H₂, H₃, and H₄ receptors, regulates various physiological and pathological processes, including pain. In the last two decades, there has been a particular increase in evidence to support the involvement of H₃ receptor and H₄ receptor in the modulation of neuropathic pain, which remains challenging in terms of management. However, recent data show contrasting effects on neuropathic pain due to multiple factors that determine the pharmacological responses of histamine receptors and their underlying signal transduction properties (e.g., localization on either the presynaptic or postsynaptic neuronal membranes). This review summarizes the most recent findings on the role of histamine and the effects mediated by the four histamine receptors in response to the various stimuli associated with and promoting neuropathic pain. We particularly focus on mechanisms underlying histamine‐mediated analgesia, as we aim to clarify the analgesic potential of histamine receptor ligands in neuropathic pain.

Linked Articles

This article is part of a themed section on New Uses for 21st Century. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.3/issuetoc

Metabotropic Glutamate Receptor 5 and 8 Modulate the Ameliorative Effect of Ultramicronized Palmitoylethanolamide on Cognitive Decline Associated with Neuropathic Pain

This study investigated whether metabotropic glutamate receptor (mGluR) 5 and 8 are involved in the effect of ultramicronizedpalmitoylethanolamide (um-PEA) on the cognitive behavior and long term potentiation (LTP) at entorhinal cortex (LEC)-dentate gyrus (DG) pathway in mice rendered neuropathic by the spare nerve injury (SNI). SNI reduced discriminative memory and LTP. Um-PEA treatment started after the development of neuropathic pain had no effects in sham mice, whereas it restored cognitive behavior and LTP in SNI mice. 2-Methyl-6-(phenylethynyl) pyridine (MPEP), a selective mGluR5 antagonist, improved cognition in SNI mice and produced a chemical long term depression of the field excitatory postsynaptic potentials (fEPSPs) in sham and SNI mice. After theta burst stimulation (TBS) MPEP restored LTP in SNI mice. In combination with PEA, MPEP antagonized the PEA effect on discriminative memory and decreased LTP in SNI mice. The (RS)-4-(1-amino-1-carboxyethyl)phthalic acid (MDCPG), a selective mGluR8 antagonist, did not affect discriminative memory, but it induced a chemical LTP and prevented the enhancement of fEPSPs after TBS in SNI mice which were treated or not treated with PEA. The effect of PEA on LTP and cognitive behavior was modulated by mGluR5 and mGluR8. In particular in the SNI conditions, the mGluR5 blockade facilitated memory and LTP, but prevented the beneficial effects of PEA on discriminative memory while the mGluR8 blockade, which was ineffective in itself, prevented the favorable action of the PEA on LTP. Thus, although their opposite roles (excitatory/inhibitory of the two receptor subtypes on the glutamatergic system), they appeared to be required for the neuroprotective effect of PEA in conditions of neuropathic pain.

Emerging Trends in Pain Modulation by Metabotropic Glutamate Receptors

Pain is an essential protective mechanism meant to prevent tissue damages in organisms. On the other hand, chronic or persistent pain caused, for example, by inflammation or nerve injury is long lasting and responsible for long-term disability in patients. Therefore, chronic pain and its management represents a major public health problem. Hence, it is critical to better understand chronic pain molecular mechanisms to develop innovative and efficient drugs. Over the past decades, accumulating evidence has demonstrated a pivotal role of glutamate in pain sensation and transmission, supporting glutamate receptors as promising potential targets for pain relieving drug development. Glutamate is the most abundant excitatory neurotransmitter in the brain. Once released into the synapse, glutamate acts through ionotropic glutamate receptors (iGluRs), which are ligand-gated ion channels triggering fast excitatory neurotransmission, and metabotropic glutamate receptors (mGluRs), which are G protein-coupled receptors modulating synaptic transmission. Eight mGluRs subtypes have been identified and are divided into three classes based on their sequence similarities and their pharmacological and biochemical properties. Of note, all mGluR subtypes (except mGlu6 receptor) are expressed within the nociceptive pathways where they modulate pain transmission. This review will address the role of mGluRs in acute and persistent pain processing and emerging pharmacotherapies for pain management.

An integrated review on new targets in the treatment of neuropathic pain

Neuropathic pain is a complex chronic pain state caused by the dysfunction of somatosensory nervous system, and it affects the millions of people worldwide. At present, there are very few medical treatments available for neuropathic pain management and the intolerable side effects of medications may further worsen the symptoms. Despite the presence of profound knowledge that delineates the pathophysiology and mechanisms leading to neuropathic pain, the unmet clinical needs demand more research in this field that would ultimately assist to ameliorate the pain conditions. Efforts are being made globally to explore and understand the basic molecular mechanisms responsible for somatosensory dysfunction in preclinical pain models. The present review highlights some of the novel molecular targets like D-amino acid oxidase, endoplasmic reticulum stress receptors, sigma receptors, hyperpolarization-activated cyclic nucleotide-gated cation channels, histone deacetylase, Wnt/β-catenin and Wnt/Ryk, ephrins and Eph receptor tyrosine kinase, Cdh-1 and mitochondrial ATPase that are implicated in the induction of neuropathic pain. Studies conducted on the different animal models and observed results have been summarized with an aim to facilitate the efforts made in the drug discovery. The diligent analysis and exploitation of these targets may help in the identification of some promising therapies that can better manage neuropathic pain and improve the health of patients.

Deletion of the type 2 metabotropic glutamate receptor increases heroin abuse vulnerability in transgenic rats

Opioid abuse is a rapidly growing public health crisis in the USA. Despite extensive research in the past decades, little is known about the etiology of opioid addiction or the neurobiological risk factors that increase vulnerability to opioid use and abuse. Recent studies suggest that the type 2 metabotropic glutamate receptor (mGluR2) is critically involved in substance abuse and addiction. In the present study, we evaluated whether low-mGluR2 expression may represent a risk factor for the development of opioid abuse and addiction using transgenic mGluR2-knockout (mGluR2-KO) rats. Compared to wild-type controls, mGluR2-KO rats exhibited higher nucleus accumbens (NAc) dopamine (DA) and locomotor responses to heroin, higher heroin self-administration and heroin intake, more potent morphine-induced analgesia and more severe naloxone-precipitated withdrawal symptoms. In contrast, mGluR2-KO rats displayed lower motivation for heroin self-administration under high price progressive-ratio (PR) reinforcement conditions. Taken together, these findings suggest that mGluR2 may play an inhibitory role in opioid action, such that deletion of this receptor results in an increase in brain DA responses to heroin and in acute opioid reward and analgesia. Low-mGluR2 expression in the brain may therefore be a risk factor for the initial development of opioid abuse and addiction.

Median nerve stimulation induces analgesia via orexin-initiated endocannabinoid disinhibition in the periaqueductal gray

Adequate pain management remains an unmet medical need. We previously revealed an opioid-independent analgesic mechanism mediated by orexin 1 receptor (OX1R)-initiated 2-arachidonoylglycerol (2-AG) signaling in the ventrolateral periaqueductal gray (vlPAG). Here, we found that low-frequency median nerve stimulation (MNS) through acupuncture needles at the PC6 (Neiguan) acupoint (MNS-PC6) induced an antinociceptive effect that engaged this mechanism. In mice, MNS-PC6 reduced acute thermal nociceptive responses and neuropathy-induced mechanical allodynia, increased the number of c-Fos-immunoreactive hypothalamic orexin neurons, and led to higher orexin A and lower GABA levels in the vlPAG. Such responses were not seen in mice with PC6 needle insertion only or electrical stimulation of the lateral deltoid, a nonmedian nerve-innervated location. Directly stimulating the surgically exposed median nerve also increased vlPAG orexin A levels. MNS-PC6-induced antinociception (MNS-PC6-IA) was prevented by proximal block of the median nerve with lidocaine as well as by systemic or intravlPAG injection of an antagonist of OX1Rs or cannabinoid 1 receptors (CB1Rs) but not by opioid receptor antagonists. Systemic blockade of OX1Rs or CB1Rs also restored vlPAG GABA levels after MNS-PC6. A cannabinoid (2-AG)-dependent mechanism was also implicated by the observations that MNS-PC6-IA was prevented by intravlPAG inhibition of 2-AG synthesis and was attenuated in Cnr1 ^-/- mice. These findings suggest that PC6-targeting low-frequency MNS activates hypothalamic orexin neurons, releasing orexins to induce analgesia through a CB1R-dependent cascade mediated by OX1R-initiated 2-AG retrograde disinhibition in the vlPAG. The opioid-independent characteristic of MNS-PC6-induced analgesia may provide a strategy for pain management in opioid-tolerant patients.

CD11b-activated Src signal attenuates neuroinflammatory pain by orchestrating inflammatory and anti-inflammatory cytokines in microglia

Neuroinflammation plays an important role in the induction and maintenance of chronic pain. Orchestra of pattern-recognition receptor-induced pro-inflammatory and anti-inflammatory cytokines is critical for inflammation homeostasis. CD11b on macrophages could inhibit toll-like receptor (TLR) activation-induced inflammatory responses. However, the function of CD11b on microglia remains unknown. In the current study, we demonstrated that CD11b-deficient microglia cells produced more inflammatory cytokines, such as interleukin-6 and tumor necrosis factor alpha, while less anti-inflammatory cytokines. Signal transduction assay confirmed that nuclear factor-κB activation was increased in CD11b-deficient microglia cells, which resulted from decreased activation of Src. Inhibition of Src by PP1 increased inflammation in wild-type microglia cells significantly, but not in CD11b-deficient microglia cells. In vivo, CD11b-deficient mice were more susceptible to chronic constrictive injury-induced allodynia and hyperalgesia with significantly more inflammatory cytokines expression. All these results indicated that the regulatory function of CD11b-Src signal pathway on both inflammatory and anti-inflammatory cytokines in microglia cells is a potential target in neuropathic pain treatment.

Zaprinast diminished pain and enhanced opioid analgesia in a rat neuropathic pain model

The mechanism of neuropathic pain is complex and unclear. Based on our results, we postulate that an intensification of the kynurenine pathway occurs as a consequence of nerve injury. The G protein-coupled receptor 35 (GPR35) is important for kynurenine pathway activation. Cyclic GMP-specific phosphodiesterase inhibitors have also been shown to have beneficial effects on neuropathic pain. Therefore, the aims of our research were to elucidate how a substance that acts as both an agonist of GPR35 and an inhibitor of phosphodiesterase influences neuropathic pain in a rat model. Here, we demonstrated that preemptive and repeated intrathecal (i.t.) administration (16 h and 1 h before injury and then after nerve ligation daily for 7 days) of zaprinast (1 μg/5 μl) significantly attenuated mechanical (von Frey test) and thermal (cold plate test) hypersensitivity measured on day 7 after chronic constriction injury, and the effect of even a single injection lasted up to 24 h. Our data indicate that zaprinast diminished the number of IBA1-positive cells and consequently attenuated the levels of IL-1beta, IL-6, IL-18, and NOS2 in the lumbar spinal cord and/or dorsal root ganglia. Our results also demonstrated that zaprinast potentiated the analgesic properties of morphine and buprenorphine. In summary, in a neuropathic pain model, zaprinast significantly reduced pain symptoms and enhanced the effectiveness of opioids. Our data provide new evidence that modulation of both GPR35 and phosphodiesterase could be an important strategy for innovative pharmacological treatments designed to decrease hypersensitivity evoked by nerve injury.

GpTx-1 and [Ala5 , Phe6 , Leu26 , Arg28 ]GpTx-1, two peptide NaV 1.7 inhibitors: analgesic and tolerance properties at the spinal level

BACKGROUND AND PURPOSE

The voltage-gated sodium channel Na_V 1.7 is considered a therapeutic target for pain treatment based on human genetic evidence. GpTx-1 and its potent analogue [Ala⁵ , Phe⁶ , Leu²⁶ , Arg²⁸ ]GpTx-1 (GpTx-1-71) were recently characterized as Na_V 1.7 inhibitors in vitro. Furthermore, the present work was conducted to investigate the analgesic properties of these two peptides in different pain models after spinal administration.

EXPERIMENTAL APPROACH

The antinociceptive activities of both GpTx-1 and GpTx-1-71 were investigated in mouse models of acute, visceral, inflammatory and neuropathic pain. Furthermore, the side effects of GpTx-1 and GpTx-1-71 were evaluated in rotarod, antinociceptive tolerance, acute hyperlocomotion and gastrointestinal transit tests.

KEY RESULTS

The i.t. administration of both GpTx-1 and GpTx-1-71 dose-dependently produced powerful antinociception in the different pain models. This effect was attenuated by the opioid receptor antagonist naloxone, suggesting the involvement of the opioid system. In this study, repeated administration of these two_peptides produced spinal analgesia without a loss of potency over 8 days in mouse models of acute, inflammatory and neuropathic pain. Moreover, spinal administration of GpTx-1 and GpTx-1-71 did not induce significant effects on motor coordination, evoke acute hyperlocomotion or increase gastrointestinal transit time.

CONCLUSIONS AND IMPLICATIONS

Our data indicate that the Na_V 1.7 peptide inhibitors GpTx-1 and GpTx-1-71 produce powerful, nontolerance-forming analgesia in preclinical pain models, which might be dependent on the endogenous opioid system. In addition, at the spinal level, the limited side effects imply that these Na_V 1.7 peptide inhibitors could be potentially developed as GpTx-1-based drugs for pain relief.

Bivalent ligand that activates mu opioid receptor and antagonizes mGluR5 receptor reduces neuropathic pain in mice

The mu opioid receptor (MOR) and metabotropic glutamate receptor 5 (mGluR5) are well-established pharmacological targets in the management of chronic pain. Both receptors are expressed in the spinal cord. MMG22, a bivalent ligand containing 2 pharmacophores separated by 22 atoms, which simultaneously activates MOR and antagonizes mGluR5, has been shown to produce potent reversal of tactile hypersensitivity in rodent models of lipopolysaccharide (LPS)-and bone cancer-induced chronic pain. This study assessed whether intrathecal MMG22 also is effective in reducing pain of neuropathic origin. Furthermore, we theorized that MMG22 should reduce hyperalgesia in nerve-injured mice in a manner consistent with a synergistic interaction between MOR and mGluR5. Several weeks after spared nerve injury, tactile hypersensitivity was reversed in mice by the intrathecal injection of MMG22 (0.01-10 nmol) but also by its shorter spacer analog, MMG10, with similar potency. The potencies of the bivalent ligands were 10- to 14-fold higher than those of the compounds upon which the bivalent structure was based, the MOR agonist oxymorphone and the mGluR5 antagonist MPEP. Coadministration of oxymorphone and MPEP demonstrated analgesic synergism, an interaction confirmed by isobolographic analysis. This study indicates that in the spared nerve injury-induced model of neuropathic pain, the 2 pharmacophores of the bivalent ligands MMG22 and MMG10 target MOR and mGluR5 as separate receptor monomers. The observed increase in the potency of MMG22 and MMG10, compared with oxymorphone and MPEP, may reflect the synergistic interaction of the 2 pharmacophores of the bivalent ligand acting at their respective separate receptor monomers.

Pharmacological blockade of CXCR3 by (±)-NBI-74330 reduces neuropathic pain and enhances opioid effectiveness - Evidence from in vivo and in vitro studies

It has been suggested that CXCR3 is important for nociception. Our experiments were conducted to evaluate involvement of CXCR3 and its ligands (CXCL4, CXCL9, CXCL10, CXCL11/CCL21) in neuropathic pain. Our studies give new evidence that intrathecal administration of each CXCR3 ligand induces pain-like behaviour in naive mice that occurs shortly after injection due to its location of neurons, which is confirmed by immunofluorescent staining. Moreover, intrathecal administrations of CXCL9, CXCL10, CCL21 neutralizing antibodies diminished pain-related behaviour. RT-PCR/Western blot analysis unprecedentedly showed spinal elevated levels of CXCR3 after chronic constriction injury of the sciatic nerve in rats in parallel with different time-course changes of its endogenous ligands. Initially, on day 2 we observed spinal increased levels of CXCL10 and CXCL11 indicating that these chemokines have important roles in triggering neuropathy. Then, on day 7, we observed increased levels of CXCL4, CXCL9, CXCL10. Interestingly, changes in CXCL9 level persisted until day 28, suggesting that these chemokines are responsible for long-term, persistent neuropathy. Additionally, in DRG the CXCL4, CXCL9 were elevated. The results obtained from primary glial cultures, suggests that all CXCR3 ligands can be produced in microglia, but also, except for CXCL4, in astrocytes. We provide the first evidence that in neuropathy chronic intrathecal administration of CXCR3 antagonist, (±)-NBI-74330, attenuates hypersensitivity with concomitant occurrence of microglial and some of CXCR3 ligands activation observed in the spinal cord and/or DRG level. This paper underlies the significance of CXCR3 in neuropathic pain and shows therapeutic potential of its blockade for enhancement of morphine analgesia as the major novelty of this work.

Metabotropic glutamate receptor subtype 7 in the dorsal striatum oppositely modulates pain in sham and neuropathic rats

The study investigated the role of the metabotropic glutamate receptor subtype 7 (mGluR7) in pain signalling in the dorsal striatum of sham and neuropathic rats. Supraspinal circuitries involved in the dorsal striatum control of pain were also explored. In the sham rats, microinjection of N,N'-bis(diphenylmethyl)-1,2-ethanediamine (AMN082), a selective mGluR7 positive allosteric modulator, into the dorsal striatum, facilitated pain, increased the activity of the ON cells and inhibited the activity of the OFF cells in the rostral ventromedial medulla, and decreased glutamate levels in the dorsal striatum. Conversely, AMN082 inhibited pain and the activity of the ON cells while increased the activity of the OFF cells in rats with spared nerve injury (SNI) of the sciatic nerve. AMN082 also decreased glutamate levels in the dorsal striatum of SNI rats. The effect of AMN082 on mechanical allodynia and glutamate release was blocked by 6-(2,4-dimethylphenyl)-2-ethyl-6,7-dihydro-4(5H)-benzoxazolone (ADX71743), a selective mGluR7 negative allosteric modulator. Moreover, in the sham rats, AMN082 increased the activity of total nociceptive convergent neurons in the dorsal reticular nucleus while in the SNI rats, such activity was decreased. The administration of lidocaine into the subthalamic nucleus abolished the effect of AMN082 on the total nociceptive convergent neurons in the sham rats but not in the SNI rats. Thus, the dual effect of mGluR7 in facilitating or inhibiting pain responses may be due to the recruitment of different pathways of the basal ganglia, the indirect or direct pathway, in physiological or pathological conditions, respectively.

Metabotropic Glutamate Receptor 2/3 (mGluR2/3) Activation Suppresses TRPV1 Sensitization in Mouse, But Not Human, Sensory Neurons

The use of human tissue to validate putative analgesic targets identified in rodents is a promising strategy for improving the historically poor translational record of preclinical pain research. We recently demonstrated that in mouse and human sensory neurons, agonists for metabotropic glutamate receptors 2 and 3 (mGluR2/3) reduce membrane hyperexcitability produced by the inflammatory mediator prostaglandin E2 (PGE2). Previous rodent studies indicate that mGluR2/3 can also reduce peripheral sensitization by suppressing inflammation-induced sensitization of TRPV1. Whether this observation similarly translates to human sensory neurons has not yet been tested. We found that activation of mGluR2/3 with the agonist APDC suppressed PGE2-induced sensitization of TRPV1 in mouse, but not human, sensory neurons. We also evaluated sensory neuron expression of the gene transcripts for mGluR2 (Grm2), mGluR3 (Grm3), and TRPV1 (Trpv1). The majority of Trpv1+ mouse and human sensory neurons expressed Grm2 and/or Grm3, and in both mice and humans, Grm2 was expressed in a greater percentage of sensory neurons than Grm3. Although we demonstrated a functional difference in the modulation of TRPV1 sensitization by mGluR2/3 activation between mouse and human, there were no species differences in the gene transcript colocalization of mGluR2 or mGluR3 with TRPV1 that might explain this functional difference. Taken together with our previous work, these results suggest that mGluR2/3 activation suppresses only some aspects of human sensory neuron sensitization caused by PGE2. These differences have implications for potential healthy human voluntary studies or clinical trials evaluating the analgesic efficacy of mGluR2/3 agonists or positive allosteric modulators.

The influence of AMN082, metabotropic glutamate receptor 7 (mGlu7) allosteric agonist on the acute and chronic antinociceptive effects of morphine in the tail-immersion test in mice: Comparison with mGlu5 and mGlu2/3 ligands

Preclinical data indicated that the metabotropic glutamate receptors 5 (mGlu5) and glutamate receptors 2/3 (mGlu2/3) are involved in modulating morphine antinociception. However, little is known about the role of metabotropic glutamate receptors 7 (mGlu7) in this phenomenon. We compared the effects of AMN082 (0.1, 1 or 5mg/kg, ip), a selective mGlu7 allosteric agonist, LY354740 (0.1, 1 or 5mg/kg, ip), an mGlu2/3 agonist and MTEP (0.1, 1 or 5mg/kg, ip), a selective mGlu5 antagonist, on the acute antinociceptive effect of morphine (5mg/kg, sc) and also on the development and expression of tolerance to morphine analgesia in the tail-immersion test in mice. To determine the role of mGlu7 in morphine tolerance, and the association of the mGlu7 effect with the N-methyl-d-aspartate (NMDA) receptors regulation, we used MMPIP (10mg/kg, ip), a selective mGlu7 antagonist and MK-801, a NMDA antagonist. Herein, the acute administration of AMN082, MTEP or LY354740 alone failed to evoked antinociception, and did not affect morphine (5mg/kg, sc) antinociception. However, these ligands inhibited the development of morphine tolerance, and we indicated that MMPIP reversed the inhibitory effect of AMN082. When given together, the non-effective doses of AMN082 and MK-801 did not alter the tolerance to morphine. Thus, mGlu7, similarly to mGlu2/3 and mGlu5, are involved in the development of tolerance to the antinociceptive effects of morphine, but not in the acute morphine antinociception. Furthermore, while mGlu7 are engaged in the development of morphine tolerance, no interaction exists between mGlu7 and NMDA receptors in this phenomenon.

Possible modulation of PPAR-γ cascade against depression caused by neuropathic pain in rats

BACKGROUND

Sciatic nerve ligation causes neuropathic pain with chronic constriction injury (CCI). However, there is no published report on the effect of pioglitazone as an antidepressant in the treatment of depression induced by neuropathic pain with CCI in rats. The aim of this study was to evaluate the effect of pioglitazone as an antidepressant by targeting oxidative stress by the peripheral neuropathic pain model using the CCI of the sciatic nerve.

METHODS

Behavioral studies were carried out to measure thermal hyperalgesia and cold allodynia as markers of neuropathic pain and force swim test for depression. These were followed by estimation of biochemical parameters which include lipid peroxidation (LPO), reduced glutathione, catalase, nitrite and superoxide dismutase (SOD) in the rat brains as a measure of oxidative stress. We administered two intraperitoneal doses of pioglitazone (4.5 and 9.0 mg/kg, i.p.) to the treated group for 28 consecutive days from the day of injury and behavioral as well as biochemical evaluations were performed.

RESULTS

The results suggested that the administration of pioglitazone significantly countered the neuropathic pain induced depression as interpreted through elevated pain threshold of tactile allodynia and thermal hyperalgesia followed by decreased immobility time in the 9.0 mg/kg dose group.

CONCLUSIONS

It may be concluded that the oxidative stress plays a critical role in the pathogenesis of neuropathic pain and depression as evidenced by the behavioral studies and the changes in the levels of lipid peroxidase, nitrite, catalase, and glutathione and SOD.

Basic/Translational Development of Forthcoming Opioid- and Nonopioid-Targeted Pain Therapeutics

Opioids represent an efficacious therapeutic modality for some, but not all pain states. Singular reliance on opioid therapy for pain management has limitations, and abuse potential has deleterious consequences for patient and society. Our understanding of pain biology has yielded insights and opportunities for alternatives to conventional opioid agonists. The aim is to have efficacious therapies, with acceptable side effect profiles and minimal abuse potential, which is to say an absence of reinforcing activity in the absence of a pain state. The present work provides a nonexclusive overview of current drug targets and potential future directions of research and development. We discuss channel activators and blockers, including sodium channel blockers, potassium channel activators, and calcium channel blockers; glutamate receptor-targeted agents, including N-methyl-D-aspartate, α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, and metabotropic receptors. Furthermore, we discuss therapeutics targeted at γ-aminobutyric acid, α2-adrenergic, and opioid receptors. We also considered antagonists of angiotensin 2 and Toll receptors and agonists/antagonists of adenosine, purine receptors, and cannabinoids. Novel targets considered are those focusing on lipid mediators and anti-inflammatory cytokines. Of interest is development of novel targeting strategies, which produce long-term alterations in pain signaling, including viral transfection and toxins. We consider issues in the development of druggable molecules, including preclinical screening. While there are examples of successful translation, mechanistically promising preclinical candidates may unexpectedly fail during clinical trials because the preclinical models may not recapitulate the particular human pain condition being addressed. Molecular target characterization can diminish the disconnect between preclinical and humans' targets, which should assist in developing nonaddictive analgesics.

Biphalin, a Dimeric Enkephalin, Alleviates LPS-Induced Activation in Rat Primary Microglial Cultures in Opioid Receptor-Dependent and Receptor-Independent Manners

Neuropathic pain is relatively less responsive to opioids than other types of pain, which is possibly due to a disrupted opioid system partially caused by the profound microglial cell activation that underlines neuroinflammation. We demonstrated that intrathecally injected biphalin, a dimeric enkephalin analog, diminished symptoms of neuropathy in a preclinical model of neuropathic pain in rats (CCI, chronic constriction injury of the sciatic nerve) at day 12 postinjury. Using primary microglial cell cultures, we revealed that biphalin did not influence cell viability but diminished NO production and expression of Iba1 in LPS-stimulated cells. Biphalin also diminished MOP receptor level, as well as pronociceptive mediators (iNOS, IL-1β, and IL-18) in an opioid receptor-dependent manner, and it was correlated with diminished p-NF-κB, p-IκB, p-p38MAPK, and TRIF levels. Biphalin reduced IL-6, IL-10, TNFα, p-STAT3, and p-ERK1/2 and upregulated SOCS3, TLR4, and MyD88; however, this effect was not reversed by naloxone pretreatment. Our study provides evidence that biphalin diminishes neuropathy symptoms, which might be partially related to reduced pronociceptive mediators released by activated microglia. Biphalin may be a putative drug for future pain therapy, especially for the treatment of neuropathic pain, when the lower analgesic effects of morphine are correlated with profound microglial cell activation.

Metabotropic Glutamate Receptor 7: From Synaptic Function to Therapeutic Implications

Metabotropic glutamate receptor 7 (mGluR7) is localized presynaptically at the active zone of neurotransmitter release. Unlike mGluR4 and mGluR8, which share mGluR7's presynaptic location, mGluR7 shows low affinity for glutamate and is activated only by high glutamate concentrations. Its wide distribution in the central nervous system (CNS) and evolutionary conservation across species suggest that mGluR7 plays a primary role in controlling excitatory synapse function. High mGluR7 expression has been observed in several brain regions that are critical for CNS functioning and are involved in neurological and psychiatric disorder development. Until the recent discovery of selective ligands for mGluR7, techniques to elucidate its role in neural function were limited to the use of knockout mice and gene silencing. Studies using these two techniques have revealed that mGluR7 modulates emotionality, stress and fear responses. N,N`-dibenzhydrylethane-1,2-diamine dihydrochloride (AMN082) was reported as the first selective mGluR7 allosteric agonist. Pharmacological effects of AMN082 have not completely confirmed the mGluR7-knockout mouse phenotype; this has been attributed to rapid receptor internalization after drug treatment and to the drug's apparent lack of in vivo selectivity. Therefore, the more recently developed mGluR7 negative allosteric modulators (NAMs) are crucial for understanding mGluR7 function and for exploiting its potential as a target for therapeutic interventions. This review presents the main findings regarding mGluR7's effect on modulation of synaptic function and its role in normal CNS function and in models of neurologic and psychiatric disorders.

Current and Future Issues in the Development of Spinal Agents for the Management of Pain

Targeting analgesic drugs for spinal delivery reflects the fact that while the conscious experience of pain is mediated supraspinally, input initiated by high intensity stimuli, tissue injury and/or nerve injury is encoded at the level of the spinal dorsal horn and this output informs the brain as to the peripheral environment. This encoding process is subject to strong upregulation resulting in hyperesthetic states and downregulation reducing the ongoing processing of nociceptive stimuli reversing the hyperesthesia and pain processing. The present review addresses the biology of spinal nociceptive processing as relevant to the effects of intrathecally-delivered drugs in altering pain processing following acute stimulation, tissue inflammation/injury and nerve injury. The review covers i) the major classes of spinal agents currently employed as intrathecal analgesics (opioid agonists, alpha 2 agonists; sodium channel blockers; calcium channel blockers; NMDA blockers; GABA A/B agonists; COX inhibitors; ii) ongoing developments in the pharmacology of spinal therapeutics focusing on less studied agents/targets (cholinesterase inhibition; Adenosine agonists; iii) novel intrathecal targeting methodologies including gene-based approaches (viral vectors, plasmids, interfering RNAs); antisense, and toxins (botulinum toxins; resniferatoxin, substance P Saporin); and iv) issues relevant to intrathecal drug delivery (neuraxial drug distribution), infusate delivery profile, drug dosing, formulation and principals involved in the preclinical evaluation of intrathecal drug safety.

Potential role of selected antiepileptics used in neuropathic pain as human GABA transporter isoform 1 (GAT1) inhibitors-Molecular docking and pharmacodynamic studies

The chemical interaction of nine antiepileptic drugs (tiagabine, gabapentin, pregabalin, lamotrigine, zonisamide, valproic acid, valpromide, vigabatrin, progabide) and two endogenous metabolites (4-aminobutanoic acid, 4-hydroxybutanoic acid) with a model of human GABA transporter 1 (hGAT1) is described using the molecular docking method. To establish the role of hGAT1 in chronic pain, tiagabine, a selective hGAT1 inhibitor, was assessed in the in vivo experiments for its antiallodynic properties in two mouse models of neuropathic pain. Docking analyses performed in this study provided the complex binding energies, specific hydrogen bond components, and hydrogen bond properties such as energies, distances and angles. The data of the docking studies strongly support the assumption that the antiepileptic and analgesic actions of the studied drugs can be at least in part related to the strength of their chemical interactions with hGAT1. In vivo experiments with tiagabine confirmed the involvement of hGAT1 in the regulation of the mechanical nociceptive threshold in neuropathic pain.

The multiplicity of spinal AA-5-HT anti-nociceptive action in a rat model of neuropathic pain

There is considerable evidence to support the role of anandamide (AEA), an endogenous ligand of cannabinoid receptors, in neuropathic pain modulation. AEA also produces effects mediated by other biological targets, of which the transient receptor potential vanilloid type 1 (TRPV1) has been the most investigated. Both, inhibition of AEA breakdown by fatty acid amide hydrolase (FAAH) and blockage of TRPV1 have been shown to produce anti-nociceptive effects. Recent research suggests the usefulness of dual-action compounds, which may afford greater anti-allodynic efficacy. Therefore, in the present study, we examined the effect of N-arachidonoyl-serotonin (AA-5-HT), a blocker of FAAH and TRPV1, in a rat model of neuropathic pain after intrathecal administration. We found that treatment with AA-5-HT increased the pain threshold to mechanical and thermal stimuli, with highest effect at the dose of 500nM, which was most strongly attenuated by AM-630, CB2 antagonist, administration. The single action blockers PF-3845 (1000nM, for FAAH) and I-RTX (1nM, for TRPV1) showed lower efficacy than AA-5-HT. Moreover AA-5-HT (500nM) elevated AEA and palmitoylethanolamide (PEA) levels. Among the possible targets of these mediators, only the mRNA levels of CB2, GPR18 and GPR55, which are believed to be novel cannabinoid receptors, were upregulated in the spinal cord and/or DRG of CCI rats. It was previously reported that AA-5-HT acts in CB1 and TRPV1-dependent manner after systemic administration, but here for the first time we show that AA-5-HT action at the spinal level involves CB2, with potential contributions from GRP18 and/or GPR55 receptors.

MMPIP, an mGluR7-selective negative allosteric modulator, alleviates pain and normalizes affective and cognitive behavior in neuropathic mice

This study investigated the effects of a single administration of 6-(4-methoxyphenyl)-5-methyl-3-pyridinyl-4-isoxazolo[4,5-c]pyridin-4(5H)-one (MMPIP), a negative allosteric modulator (NAM) of metabotropic glutamate receptor 7 (mGluR7), on pain and on affective and cognitive behavior in neuropathic mice. The activity of pyramidal neurons in the prelimbic cortex (PLC), which respond to stimulation of the basolateral amygdala (BLA) with either excitation or inhibition, was also investigated. The spared nerve injury (SNI) of the sciatic nerve induced, 14 days after surgery, thermal hyperalgesia and mechanical allodynia, reduced open-arm choice in the elevated plus-maze, increased time of immobility in the tail suspension, and increased digging and burying in the marble burying test. Cognitive performance was also significantly compromised in the SNI mice. Spared nerve injury induced phenotypic changes on pyramidal neurons of the PLC; excitatory responses increased, whereas inhibitory responses decreased after BLA stimulation. mGluR7 expression, mainly associated with vesicular glutamate transporter, increased in the hippocampus and decreased in the BLA, PLC, and dorsal raphe in SNI mice. MMPIP increased thermal and mechanical thresholds and open-arm choice. It reduced the immobility in the tail suspension test and the number of marbles buried and of digging events in the marble burying test. MMPIP also improved cognitive performance and restored the balance between excitatory and inhibitory responses of PLC neurons in SNI mice. 7-hydroxy-3-(4-iodophenoxy)-4H-chromen-4-one, XAP044, another selective mGluR7 NAM, reproduced the effects of MMPIP on thermal hyperalgesia, mechanical allodynia, tail suspension, and marble burying test. Altogether, these findings show that mGluR7 NAMs reduce pain responses and affective/cognitive impairments in neuropathic pain conditions.

Botulinum neurotoxin: Progress in negating its neurotoxicity; and in extending its therapeutic utility via molecular engineering. MiniReview

While the poisonous effects of botulinum neurotoxin (BoNT) have been recognized since antiquity, the overall actions and mechanisms of effects of BoNT have been elucidated primarily over the past several decades. The general utility of BoNT is described in the paper, but the focus is mainly on the approaches towards negating the toxic effects of BoNT, and on the projection of an engineered BoNT molecule serving as a Trojan Horse to deliver a therapeutic load for treatment of a host of medical disorders. The BoNT molecule is configured with a binding domain, a zinc-dependent protease with specificity primarily for vesicular proteins, and a translocation domain for delivery of the metalloprotease into the cytoplasm. The anti-toxin approaches for BoNT include the use of vaccines, antibodies, block of BoNT binding or translocation, inhibition of metalloprotease activity, impeded translocation of the protease/catalytic domain, and inhibition of the downstream Src signaling pathway. Projections of BoNT as a therapeutic include its targeting to non-cholinergic nerves, also targeting to non-neuronal cells for treatment of hypersecretory disorders (e.g., cystic fibrosis), and treatment of hormonal disorders (e.g., acromegaly). Still in the exploratory phase, there is the expectation of major advances in BoNT neuroprotective strategies and burgeoning utility of engineered BoNTs as therapeutics.

The search for novel analgesics: targets and mechanisms

The management of the pain state is of great therapeutic relevance to virtually every medical specialty. Failure to manage its expression has deleterious consequence to the well-being of the organism. An understanding of the complex biology of the mechanisms underlying the processing of nociceptive information provides an important pathway towards development of novel and robust therapeutics. Importantly, preclinical models have been of considerable use in determining the linkage between mechanism and the associated behaviorally defined pain state. This review seeks to provide an overview of current thinking targeting pain biology, the use of preclinical models and the development of novel pain therapeutics. Issues pertinent to the strengths and weaknesses of current development strategies for analgesics are considered.

Parthenolide Relieves Pain and Promotes M2 Microglia/Macrophage Polarization in Rat Model of Neuropathy

Neuropathic pain treatment remains a challenge because pathomechanism is not fully understood. It is believed that glial activation and increased spinal nociceptive factors are crucial for neuropathy. We investigated the effect of parthenolide (PTL) on the chronic constriction injury to the sciatic nerve (CCI)-induced neuropathy in rat. We analyzed spinal changes in glial markers and M1 and M2 polarization factors, as well as intracellular signaling pathways. PTL (5 µg; i.t.) was preemptively and then daily administered for 7 days after CCI. PTL attenuated the allodynia and hyperalgesia and increased the protein level of IBA1 (a microglial/macrophage marker) but did not change GFAP (an astrocyte marker) on day 7 after CCI. PTL reduced the protein level of M1 (IL-1β, IL-18, and iNOS) and enhanced M2 (IL-10, TIMP1) factors. In addition, it downregulated the phosphorylated form of NF-κB, p38MAPK, and ERK1/2 protein level and upregulated STAT3. In primary microglial cell culture we have shown that IL-1β, IL-18, iNOS, IL-6, IL-10, and TIMP1 are of microglial origin. Summing up, PTL directly or indirectly attenuates neuropathy symptoms and promotes M2 microglia/macrophages polarization. We suggest that neuropathic pain therapies should be shifted from blanketed microglia/macrophage suppression toward maintenance of the balance between neuroprotective and neurotoxic microglia/macrophage phenotypes.

Behavior of neuropathic pain in mice following chronic constriction injury comparing silk and catgut ligatures

Introduction

Neuropathic pain is defined as pain arising as a direct consequence of a lesion or disease affecting the somatosensory system and is common after surgery. Neuropathic pain can persist without an obvious injury. In this study we aim to validate a murine chronic constriction injury model as a model for neuropathic pain research and determine if silk or catgut ligatures induced most stable neuropathic pain behavior.

Methods

In this study mice underwent chronic constriction or sham surgery. Mice were tested on cutaneous hyperalgesia with the cumulative reaction time in the acetone test, on allodynia with the cumulative reaction time and number of lifts in the cold plate test and the maximal force before withdrawal in von Frey test.

Results

In the acetone test neuropathic pain was seen in CCI mice, but not in sham mice. Hyperalgesia was present postoperatively in CCI mice compared with preoperatively. In the cold plate test cumulative reaction time and number of lifts were higher in the ipsilateral hind paw than in the contralateral hind paw and sham mice. Postoperative measurements were higher than preoperatively. In the von Frey test the postoperative measurements were lower in the ipsilateral hind paw than preoperatively, while the contralateral hind paw showed an increase in maximal force before withdrawal. The contralateral hind paw showed more difference with sham mice than the ipsilateral hind paw. Silk ligatures showed more stable neuropathic pain behavior. In the acetone test, the cold plate test and the von Frey test the mice scored higher on neuropathic pain having silk ligatures, compared with catgut ligatures.

Conclusion

In this study we validated a murine CCI model for neuropathic pain behavior. In the murine CCI model it appears that silk ligatures demonstrate more stable neuropathic pain behaviors than catgut ligatures in de CCI model.

Electronic supplementary material

The online version of this article (doi:10.1186/s40064-015-1009-4) contains supplementary material, which is available to authorized users.

Effects of chronic doxepin and amitriptyline administration in naïve mice and in neuropathic pain mice model

Neuropathic pain is a severe clinical problem, often appearing as a co-symptom of many diseases or manifesting as a result of damage to the nervous system. Many drugs and agents are currently used for the treatment of neuropathic pain, such as tricyclic antidepressants (TCAs). The aims of this paper were to test the effects of two classic TCAs, doxepin and amitriptyline, in naïve animals and in a model of neuropathic pain and to determine the role of cytokine activation in the effects of these drugs. All experiments were carried out with Albino-Swiss mice using behavioral tests (von Frey test and the cold plate test) and biochemical analyses (qRT-PCR and Western blot). In the mice subjected to chronic constriction injury (CCI), doxepin and amitriptyline attenuated the symptoms of neuropathic pain and diminished the CCI-induced increase in the levels of spinal interleukin (IL)-6 and -1β mRNA, but not the protein levels of these cytokines, measured on day 12. Unexpectedly, chronic administration of doxepin or amitriptyline for 12 days produced allodynia and hyperalgesia in naïve mice. The treatment with these drugs did not influence the spinal levels of IL-1β and IL-6 mRNA, however, the protein levels of these pronociceptive factors were increased. The administration of ondansetron (5-HT3 receptor antagonist) significantly weakened the allodynia and hyperalgesia induced by both antidepressants in naïve mice; in contrast, yohimbine (α2-adrenergic receptors antagonist) did not influence these effects. Allodynia and hyperalgesia induced in naïve animals by amitriptyline and doxepin may be associated with an increase in the levels of pronociceptive cytokines resulting from 5-HT3-induced hypersensitivity. Our results provide new and important information about the possible side effects of antidepressants. Further investigation of these mechanisms may help to guide decisions about the use of classic TCAs for therapy.

Nerve demyelination increases metabotropic glutamate receptor subtype 5 expression in peripheral painful mononeuropathy

Wallerian degeneration or nerve demyelination, arising from spinal nerve compression, is thought to bring on chronic neuropathic pain. The widely distributed metabotropic glutamate receptor subtype 5 (mGluR5) is involved in modulating nociceptive transmission. The purpose of this study was to investigate the potential effects of mGluR5 on peripheral hypersensitivities after chronic constriction injury (CCI). Sprague-Dawley rats were operated on with four loose ligatures around the sciatic nerve to induce thermal hyperalgesia and mechanical allodynia. Primary afferents in dermis after CCI exhibited progressive decreases, defined as partial cutaneous denervation; importantly, mGluR5 expressions in primary afferents were statistically increased. CCI-induced neuropathic pain behaviors through the intraplantar injections of 2-methyl-6-(phenylethynyl)-pyridine (MPEP), a selective mGluR5 antagonist, were dose-dependently attenuated. Furthermore, the most increased mGluR5 expressions in primary afferents surrounded by reactive Schwann cells were observed at the distal CCI stumps of sciatic nerves. In conclusion, these results suggest that nerve demyelination results in the increases of mGluR5 expression in injured primary afferents after CCI; and further suggest that mGluR5 represents a main therapeutic target in developing pharmacological strategies to prevent peripheral hypersensitivities.

The importance of TRPV1-sensitisation factors for the development of neuropathic pain

Transient receptor potential vanilloid type 1 (TRPV1), classically associated with transduction of high-temperature and low-pH pain, underlies pain hypersensitivity in neuropathic pain. The molecular regulation of TRPV1 channel activity is not yet fully understood. Therefore, we investigated factors regulating sensitisation of this receptor during development of neuropathic pain in a rat model of chronic construction injury (CCI) in the dorsal root ganglia (DRG). In the rat CCI model, elevated levels of pro-inflammatory cytokines (TNFα, IL-1β and IL-6) in DRG corresponded to development of neuropathic pain. We assessed the expression of known kinases influencing TRPV1 sensitisation at the mRNA and/or protein level. Protein kinase C ε (PKCε) showed the strongest upregulation at the mRNA and protein levels among all tested kinases. Co-expression of PKCε and TRPV1 in L5 DRG of CCI animals was high during the development of neuropathic pain. The number of neurons expressing PKCε increased throughout the experiment. We provide complex data on the expression of a variety of factors involved in TRPV1 sensitisation in a CCI model of neuropathic pain. Our study supports evidence for involvement of TRPV1 in the development of neuropathic pain, by showing increased expression of interleukins and kinases responsible for the channel sensitisation. TNFα and NGF seem to play a role in the transition from acute to neuropathic pain, while PKCε in its maintenance. Further studies might confirm their significance as novel targets for the treatment of neuropathic pain.

Assessment of knee joint pain in experimental rodent models of osteoarthritis

Pain assessment in animal models of osteoarthritis is integral to interpretation of a model's utility in representing the clinical condition, and enabling accurate translational medicine. Here we describe two methods for behavioral pain assessments available for use in animal models of experimental osteoarthritic pain: Von Frey filaments and spontaneous activity monitoring.

Minocycline enhances the effectiveness of nociceptin/orphanin FQ during neuropathic pain

Nociceptin/orphanin FQ (N/OFQ) antinociception, which is mediated selectively by the N/OFQ peptide receptor (NOP), was demonstrated in pain models. In this study, we determine the role of activated microglia on the analgesic effects of N/OFQ in a rat model of neuropathic pain induced by chronic constriction injury (CCI) to the sciatic nerve. Repeated 7-day administration of minocycline (30 mg/kg i.p.), a drug that affects microglial activation, significantly reduced pain in CCI-exposed rats and it potentiates the analgesic effects of administered N/OFQ (2.5-5 μg i.t.). Minocycline also downregulates the nerve injury-induced upregulation of NOP protein in the dorsal lumbar spinal cord. Our in vitro study showed that minocycline reduced NOP mRNA, but not protein, level in rat primary microglial cell cultures. In [(35)S]GTPγS binding assays we have shown that minocycline increases the spinal N/OFQ-stimulated NOP signaling. We suggest that the modulation of the N/OFQ system by minocycline is due to the potentiation of its neuronal antinociceptive activity and weakening of the microglial cell activation. This effect is beneficial for pain relief, and these results suggest new targets for the development of drugs that are effective against neuropathic pain.