Development of opioid analgesic tolerance in rat to extended-release buprenorphine formulated for laboratory subjects.

Buprenorphine in an extended-release formulation intended for use in laboratory subjects is frequently administered to rats to provide extended analgesia without repeated handling. While levels of buprenorphine may persist in serum once extended-release buprenorphine has been introduced, exposure to opioids can cause opioid tolerance or opioid-induced hypersensitivity. This work examined the analgesic duration and efficacy of a single administration of extended-release buprenorphine intended for use in laboratory subjects in models of inflammatory pain and post-operative pain and the development of opioid tolerance in rat. After subcutaneous administration of 1 mg/kg extended-release buprenorphine, analgesic efficacy did not persist for the expected 72 hours. No changes were observed in mechanical thresholds in the hindpaws that were contralateral to the injury, suggesting a lack of centrally mediated opioid-induced hypersensitivity. To determine whether opioid tolerance arose acutely after one exposure to extended-release buprenorphine, we conducted the warm water tail flick assay; on Day 1 we administered either saline or extended-release buprenorphine (1 mg/kg) and on Day 3 we quantified the standard buprenorphine dose-response curve (0.1-3 mg/kg). Rats previously given extended-release buprenorphine displayed decreased analgesic responses after administration of standard buprenorphine as compared to the robust efficacy of standard buprenorphine in control subjects. Males appeared to show evidence of acute opioid tolerance, while females previously exposed to opioid did not demonstrate a decreased response at the doses examined. Taken together, these results suggest that opioid tolerance arises quickly in male rats after exposure to the extended-release formulation of buprenorphine. This tolerance may account for the brief period of antinociception observed.

Comparative dose effectiveness of intravenous and intrathecal AAV9.CB7.hIDS, RGX-121, in mucopolysaccharidosis type II mice.

Mucopolysaccharidosis type II (MPS II) is an X-linked recessive lysosomal disease caused by iduronate-2-sulfatase (IDS) deficiency, leading to accumulation of glycosaminoglycans (GAGs) and the emergence of progressive disease. Enzyme replacement therapy is the only currently approved treatment, but it leaves neurological disease unaddressed. Cerebrospinal fluid (CSF)-directed administration of AAV9.CB7.hIDS (RGX-121) is an alternative treatment strategy, but it is unknown if this approach will affect both neurologic and systemic manifestations. We compared the effectiveness of intrathecal (i.t.) and intravenous (i.v.) routes of administration (ROAs) at a range of vector doses in a mouse model of MPS II. While lower doses were completely ineffective, a total dose of 1 × 109 gc resulted in appreciable IDS activity levels in plasma but not tissues. Total doses of 1 × 1010 and 1 × 1011 gc by either ROA resulted in supraphysiological plasma IDS activity, substantial IDS activity levels and GAG reduction in nearly all tissues, and normalized zygomatic arch diameter. In the brain, a dose of 1 × 1011 gc i.t. achieved the highest IDS activity levels and the greatest reduction in GAG content, and it prevented neurocognitive deficiency. We conclude that a dose of 1 × 1010 gc normalized metabolic and skeletal outcomes, while neurologic improvement required a dose of 1 × 1011 gc, thereby suggesting the prospect of a similar direct benefit in humans.

Biodistribution of Agmatine to Brain and Spinal Cord after Systemic Delivery.

Agmatine, an endogenous polyamine, has been shown to reduce chronic pain behaviors in animal models and in patients. This reduction is due to inhibition of the GluN2B subunit of the N-methyl-D-aspartate receptor (NMDAR) in the central nervous system (CNS). The mechanism of action requires central activity, but the extent to which agmatine crosses biologic barriers such as the blood-brain barrier (BBB) and intestinal epithelium is incompletely understood. Determination of agmatine distribution is limited by analytical protocols with low sensitivity and/or inefficient preparation. This study validated a novel bioanalytical protocol using high-performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS) for quantification of agmatine in rat biologic matrices. These protocols were then used to determine the plasma pharmacokinetics of agmatine and the extent of distribution to the CNS. Precision and accuracy of the protocol met US Food and Drug Administration (FDA) standards in surrogate matrix as well as in corrected concentrations in appropriate matrices. The protocol also adequately withstood stability and dilution conditions. Upon application of this protocol to pharmacokinetic study, intravenous agmatine showed a half-life in plasma ranging between 18.9 and 14.9 minutes. Oral administration led to a prolonged plasma half-life (74.4-117 minutes), suggesting flip-flop kinetics, with bioavailability determined to be 29%-35%. Intravenous administration led to a rapid increase in agmatine concentration in brain but a delayed distribution and lower concentrations in spinal cord. However, half-life of agmatine in both tissues is substantially longer than in plasma. These data suggest that agmatine adequately crosses biologic barriers in rat and that brain and spinal cord pharmacokinetics can be functionally distinct. SIGNIFICANCE STATEMENT: Agmatine has been shown to be an effective nonopioid therapy for chronic pain, a significantly unmet medical necessity. Here, using a novel bioanalytical protocol for quantification of agmatine, we present the plasma pharmacokinetics and the first report of agmatine oral bioavailability as well as variable pharmacokinetics across different central nervous system tissues. These data provide a distributional rationale for the pharmacological effects of agmatine as well as new evidence for kinetic differences between brain and spinal cord.

AAV-mediated gene transfer to colon-innervating primary afferent neurons.

Investigation of neural circuits underlying visceral pain is hampered by the difficulty in achieving selective manipulations of individual circuit components. In this study, we adapted a dual AAV approach, used for projection-specific transgene expression in the CNS, to explore the potential for targeted delivery of transgenes to primary afferent neurons innervating visceral organs. Focusing on the extrinsic sensory innervation of the mouse colon, we first characterized the extent of dual transduction following intrathecal delivery of one AAV9 vector and intracolonic delivery of a second AAV9 vector. We found that if the two AAV9 vectors were delivered one week apart, dorsal root ganglion (DRG) neuron transduction by the second vector was greatly diminished. Following delivery of the two viruses on the same day, we observed colocalization of the transgenes in DRG neurons, indicating dual transduction. Next, we delivered intrathecally an AAV9 vector encoding the inhibitory chemogenetic actuator hM4D(Gi) in a Cre-recombinase dependent manner, and on the same day injected an AAV9 vector carrying Cre-recombinase in the colon. DRG expression of hM4D(Gi) was demonstrated at the mRNA and protein level. However, we were unable to demonstrate selective inhibition of visceral nociception following hM4D(Gi) activation. Taken together, these results establish a foundation for development of strategies for targeted transduction of primary afferent neurons for neuromodulation of peripheral neural circuits.

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.

Adeno-associated virus-mediated gene transfer of arginine decarboxylase to the central nervous system prevents opioid analgesic tolerance.

Agmatine, a decarboxylated form of L-arginine, prevents opioid analgesic tolerance, dependence, and self-administration when given by both central and systemic routes of administration. Endogenous agmatine has been previously detected in the central nervous system. The presence of a biochemical pathway for agmatine synthesis offers the opportunity for site-specific overexpression of the presumptive synthetic enzyme for local therapeutic effects. In the present study, we evaluated the development of opioid analgesic tolerance in ICR-CD1 mice pre-treated with either vehicle control or intrathecally delivered adeno-associated viral vectors (AAV) carrying the gene for human arginine decarboxylase (hADC). Vehicle-treated or AAV-hADC-treated mice were each further divided into two groups which received repeated delivery over three days of either saline or systemically-delivered morphine intended to induce opioid analgesic tolerance. Morphine analgesic dose-response curves were constructed in all subjects on day four using the warm water tail flick assay as the dependent measure. We observed that pre-treatment with AAV-hADC prevented the development of analgesic tolerance to morphine. Peripheral and central nervous system tissues were collected and analyzed for presence of hADC mRNA. In a similar experiment, AAV-hADC pre-treatment prevented the development of analgesic tolerance to a high dose of the opioid neuropeptide endomorphin-2. Intrathecal delivery of anti-agmatine IgG (but not normal IgG) reversed the inhibition of endomorphin-2 analgesic tolerance in AAV-hADC-treated mice. To summarize, we report here the effects of AAV-mediated gene transfer of human ADC (hADC) in models of opioid-induced analgesic tolerance. This study suggests that gene therapy may contribute to reducing opioid analgesic tolerance.

Targeting the somatosensory system with AAV9 and AAV2retro viral vectors.

Adeno-associated viral (AAV) vectors allow for site-specific and time-dependent genetic manipulation of neurons. However, for successful implementation of AAV vectors, major consideration must be given to the selection of viral serotype and route of delivery for efficient gene transfer into the cell type being investigated. Here we compare the transduction pattern of neurons in the somatosensory system following injection of AAV9 or AAV2retro in the parabrachial complex of the midbrain, the spinal cord dorsal horn, the intrathecal space, and the colon. Transduction was evaluated based on Cre-dependent expression of tdTomato in transgenic reporter mice, following delivery of AAV9 or AAV2retro carrying identical constructs that drive the expression of Cre/GFP. The pattern of distribution of tdTomato expression indicated notable differences in the access of the two AAV serotypes to primary afferent neurons via peripheral delivery in the colon and to spinal projections neurons via intracranial delivery within the parabrachial complex. Additionally, our results highlight the superior sensitivity of detection of neuronal transduction based on reporter expression relative to expression of viral products.

Central Nervous System Distribution of an Opioid Agonist Combination with Synergistic Activity.

Novel combinations of specific opioid agonists like loperamide and oxymorphindole targeting the µ- and δ-opioid receptors, respectively, have shown increased potency with minimized opioid-associated risks. However, whether their interaction is pharmacokinetic or pharmacodynamic in nature has not been determined. This study quantitatively determined whether these drugs have a pharmacokinetic interaction that alters systemic disposition or central nervous system (CNS) distribution. We performed intravenous and oral in vivo pharmacokinetic assessments of both drugs after discrete dosing and administration in combination to determine whether the combination had any effect on systemic pharmacokinetic parameters or CNS exposure. Drugs were administered at 5 or 10 mg/kg i.v. or 30 mg/kg orally to institute for cancer research (ICR) mice and 5 mg/kg i.v. to Friend leukemia virus strain B mice of the following genotypes: wild-type, breast cancer resistance protein (Bcrp-/- ) (Bcrp knockout), Mdr1a/b-/- [P-glycoprotein (P-gp) knockout], and Bcrp-/- Mdr1a/b-/- (triple knockout). In the combination, clearance of oxymorphindole (OMI) was reduced by approximately half, and the plasma area under the concentration-time curve (AUC) increased. Consequently, brain and spinal cord AUCs for OMI in the combination also increased proportionately. Both loperamide and OMI are P-gp substrates, but administration of the two drugs in combination does not alter efflux transport at the CNS barriers. Because OMI alone shows appreciable brain penetration but little therapeutic efficacy on its own, and because loperamide's CNS distribution is unchanged in the combination, the mechanism of action for the increased potency of the combination is most likely pharmacodynamic and most likely occurs at receptors in the peripheral nervous system. This combination has favorable characteristics for future development. SIGNIFICANCE STATEMENT: Opioids have yet to be replaced as the most effective treatments for moderate-to-severe pain and chronic pain, but their side effects are dangerous. Combinations of opioids with peripheral activity, such as loperamide and oxymorphindole, would be valuable in that they are effective at much lower doses and have reduced risks for dangerous side effects because the µ-opioid receptor agonist is largely excluded from the CNS.

Biodistribution of Adeno-Associated Virus Serotype 5 Viral Vectors Following Intrathecal Injection.

The pharmacokinetic profile of AAV particles following intrathecal delivery has not yet been clearly defined. The present study evaluated the distribution profile of adeno-associated virus serotype 5 (AAV5) viral vectors following lumbar intrathecal injection in mice. After a single bolus intrathecal injection, viral DNA concentrations in mouse whole blood, spinal cord, and peripheral tissues were determined using quantitative polymerase chain reaction (qPCR). The kinetics of AAV5 vector in whole blood and the concentration over time in spinal and peripheral tissues were analyzed. Distribution of the AAV5 vector to all levels of the spinal cord, dorsal root ganglia, and into systemic circulation occurred rapidly within 30 min following injection. Vector concentration in whole blood reached a maximum 6 h postinjection with a half-life of approximately 12 h. Area under the curve data revealed the highest concentration of vector distributed to dorsal root ganglia tissue. Immunohistochemical analysis revealed AAV5 particle colocalization with the pia mater at the spinal cord and macrophages in the dorsal root ganglia (DRG) 30 min after injection. These results demonstrate the widespread distribution of AAV5 particles through cerebrospinal fluid and preferential targeting of DRG tissue with possible clearance mechanisms via DRG macrophages.

Agmatine requires GluN2B-containing NMDA receptors to inhibit the development of neuropathic pain.

A decarboxylated form of L-arginine, agmatine, preferentially antagonizes NMDArs containing Glun2B subunits within the spinal cord and lacks motor side effects commonly associated with non-subunit-selective NMDAr antagonism, namely sedation and motor impairment. Spinally delivered agmatine has been previously shown to reduce the development of tactile hypersensitivity arising from spinal nerve ligation. The present study interrogated the dependence of agmatine's alleviation of neuropathic pain (spared nerve injury (SNI) model) on GluN2B-containing NMDArs. SNI-induced hypersensitivity was induced in mice with significant reduction of levels of spinal GluN2B subunit of the NMDAr and their floxed controls. Agmatine reduced development of SNI-induced tactile hypersensitivity in controls but had no effect in subjects with reduced levels of GluN2B subunits. Ifenprodil, a known GluN2B-subunit-selective antagonist, similarly reduced tactile hypersensitivity in controls but not in the GluN2B-deficient mice. In contrast, MK-801, an NMDA receptor channel blocker, reduced hypersensitivity in both control and GluN2B-deficient mice, consistent with a pharmacological pattern expected from a NMDAr antagonist that does not have preference for GluN2B subtypes. Additionally, we observed that spinally delivered agmatine, ifenprodil and MK-801 inhibited nociceptive behaviors following intrathecal delivery of NMDA in control mice. By contrast, in GluN2B-deficient mice, MK-801 reduced NMDA-evoked nociceptive behaviors, but agmatine had a blunted effect and ifenprodil had no effect. These results demonstrate that agmatine requires the GluN2B subunit of the NMDA receptor for inhibitory pharmacological actions in pre-clinical models of NMDA receptor-dependent hypersensitivity.

Comparative Effectiveness of Intracerebroventricular, Intrathecal, and Intranasal Routes of AAV9 Vector Administration for Genetic Therapy of Neurologic Disease in Murine Mucopolysaccharidosis Type I.

Mucopolysaccharidosis type I (MPS I) is an inherited metabolic disorder caused by deficiency of the lysosomal enzyme alpha-L-iduronidase (IDUA). The two current treatments [hematopoietic stem cell transplantation (HSCT) and enzyme replacement therapy (ERT)], are insufficiently effective in addressing neurologic disease, in part due to the inability of lysosomal enzyme to cross the blood brain barrier. With a goal to more effectively treat neurologic disease, we have investigated the effectiveness of AAV-mediated IDUA gene delivery to the brain using several different routes of administration. Animals were treated by either direct intracerebroventricular (ICV) injection, by intrathecal (IT) infusion into the cerebrospinal fluid, or by intranasal (IN) instillation of AAV9-IDUA vector. AAV9-IDUA was administered to IDUA-deficient mice that were either immunosuppressed with cyclophosphamide (CP), or immunotolerized at birth by weekly injections of human iduronidase. In animals treated by ICV or IT administration, levels of IDUA enzyme ranged from 3- to 1000-fold that of wild type levels in all parts of the microdissected brain. In animals administered vector intranasally, enzyme levels were 100-fold that of wild type in the olfactory bulb, but enzyme expression was close to wild type levels in other parts of the brain. Glycosaminoglycan levels were reduced to normal in ICV and IT treated mice, and in IN treated mice they were normalized in the olfactory bulb, or reduced in other parts of the brain. Immunohistochemical analysis showed extensive IDUA expression in all parts of the brain of ICV treated mice, while IT treated animals showed transduction that was primarily restricted to the hind brain with some sporadic labeling seen in the mid- and fore brain. At 6 months of age, animals were tested for spatial navigation, memory, and neurocognitive function in the Barnes maze; all treated animals were indistinguishable from normal heterozygous control animals, while untreated IDUA deficient animals exhibited significant learning and spatial navigation deficits. We conclude that IT and IN routes are acceptable and alternate routes of administration, respectively, of AAV vector delivery to the brain with effective IDUA expression, while all three routes of administration prevent the emergence of neurocognitive deficiency in a mouse MPS I model.

Sustained-release buprenorphine induces acute opioid tolerance in the mouse.

Sustained-release buprenorphine is widely used in mice with the intention of providing long-lasting analgesia. Statements about duration of therapeutic efficacy are based on persistence of serum buprenorphine levels over a minimum threshold, but behavioral data demonstrating sustained efficacy is not established. Additionally, chronic opioid exposure can induce tolerance and/or hyperalgesia; mice receiving sustained-release buprenorphine have not been evaluated for these effects. This study assessed clinical efficacy and duration of sustained-release buprenorphine in inflammatory, post-operative, and cancer pain; and screened for centrally-mediated opioid-induced hyperalgesia as well as opioid tolerance. At 1-2 mg/kg sustained-release buprenorphine, statistically significant analgesic efficacy occurred only at time points up to 2 h. These animals showed no changes in von Frey thresholds on the contralateral side, i.e. no centrally-mediated opioid hyperalgesia. To establish whether acute onset opioid tolerance resulted from a single sustained-release buprenorphine administration, we used the tail flick assay, exposing mice to sustained-release buprenorphine or saline on Day 1 and buprenorphine on Day 2. We measured duration and efficacy of 1 mg/kg buprenorphine after 1 mg/kg sustained-release buprenorphine, and also quantified a dose-response curve of buprenorphine (0.1-3 mg/kg) after 2 mg/kg sustained-release buprenorphine. Compared to control animals, mice previously exposed to sustained-release buprenorphine showed diminished analgesic response to buprenorphine; the resultant dose-response curve showed decreased efficacy. Pretreatment with naloxone, an opioid receptor antagonist, blocked sustained-release buprenorphine analgesic action. The short duration of antinociception following administration of sustained-release buprenorphine in mice is caused by the rapid development of tolerance.

Inhibition of HINT1 Modulates Spinal Nociception and NMDA Evoked Behavior in Mice.

The interactions between the mu-opioid (MOR) and N-methyl-d-aspartate receptor (NMDAR) constitute an area of intense investigation because of their contributions to maladaptive neuroplasticity. Recent evidence suggests that their association requires the involvement of histidine triad nucleotide-binding protein (HINT1) with the enzyme's active site being critical in its regulatory role. Since it is known that spinal blockade of NMDA receptors prevents the development of opioid analgesic tolerance, we hypothesized that spinal inhibition of the HINT1 enzyme may similarly inhibit opioid tolerance. To address these questions, we evaluated novel HINT1 active-site inhibitors in two models of NMDAR and MOR interaction, namely, MOR inhibition of spinal NMDA activation and acute endomorphin-2 tolerance. These studies revealed that while the tryptamine carbamate of guanosine inhibitor, TrpGc, blocked both the development of opioid tolerance and the inhibitory effect of opioids on NMDA activation of the NMDA receptor, acyl-sulfamate analogues could only block the latter. Thermodynamic binding and X-ray crystallographic studies suggested that there are key differences between the bound HINT1-inhibitor surfaces that may be responsible for their differential ability to probe the ability of HINT1 to regulate cross talk between the mu-opioid receptor and NMDA receptor in the spinal cord.

Combination of a δ-opioid Receptor Agonist and Loperamide Produces Peripherally-mediated Analgesic Synergy in Mice.

BACKGROUND: The long-term use of opioids for analgesia carries significant risk for tolerance, addiction, and diversion. These adverse effects are largely mediated by µ-opioid receptors in the central nervous system. Based on the authors' previous observation that morphine and δ-opioid receptor agonists synergize in spinal cord in a protein kinase Cε-dependent manner, they predicted that this µ-opioid receptor-δ-opioid receptor synergy would take place in the central terminals of nociceptive afferent fibers and generalize to their peripheral terminals. Therefore, the authors hypothesized that loperamide, a highly efficacious µ-opioid receptor agonist that is excluded from the central nervous system, and oxymorphindole, a δ-opioid receptor agonist that was shown to synergize with morphine spinally, would synergistically reverse complete Freund's adjuvant-induced hyperalgesia. METHODS: Using the Hargreaves assay for thermal nociception, the von Frey assay for mechanical nociception and the complete Freund's adjuvant-induced model of inflammatory pain, we tested the antinociceptive and antihyperalgesic effect of loperamide, oxymorphindole, or the loperamide-oxymorphindole combination. Animals (Institute for Cancer Research [ICR] CD1 strain mice; n = 511) received drug by systemic injection, intraplantar injection to the injured paw, or a transdermal solution on the injured paw. Dose-response curves for each route of administration and each nociceptive test were generated, and analgesic synergy was assessed by isobolographic analysis. RESULTS: In naïve animals, the loperamide-oxymorphindole combination ED50 value was 10 times lower than the theoretical additive ED50 value whether given systemically or locally. In inflamed animals, the combination was 150 times more potent systemically, and 84 times more potent locally. All combinations showed statistically significant synergy when compared to the theoretical additive values, as verified by isobolographic analysis. The antihyperalgesia was ablated by a peripherally-restricted opioid antagonist. CONCLUSIONS: From these data we conclude that the loperamide-oxymorphindole combination synergistically reverses complete Freund's adjuvant-induced inflammatory hyperalgesia. The authors also conclude that this interaction is mediated by opioid receptors located in the peripheral nervous system.

AAV-Mediated Gene Delivery to the Spinal Cord by Intrathecal Injection.

Gene therapy targeting the spinal cord is an important tool for analyzing mechanisms of nervous system diseases and the development of gene therapies. Analogous to a lumbar puncture in humans, the rodent spinal cord can be accessed through an efficient, noninvasive injection. Here we describe a method for AAV-mediated gene transfer to cells of the spinal cord by intrathecal injection of small quantities of AAV vector.

AAV-Mediated Gene Delivery to the Enteric Nervous System by Intracolonic Injection.

The enteric nervous system of the lower gastrointestinal tract comprises intrinsic neural circuits as well as extrinsic afferent and efferent innervation. The development of strategies for neuronal gene transfer has created new opportunities for functional analysis, circuit mapping, and neuromodulation in the enteric nervous system. Studies of AAV-mediated gene transfer to enteric neurons and dorsal root ganglion neurons (DRG) have provided proofs-of-concept for the utility of AAV vectors for genetic manipulations of the intrinsic and extrinsic components of the enteric nervous system. Here we describe a method for AAV-mediated gene transfer to enteric neurons of the descending colon as well as colon-innervating DRG neurons by injection within the intestinal wall (intracolonic injection).

Detailed Method for Intrathecal Delivery of Gene Therapeutics by Direct Lumbar Puncture in Mice.

Delivery of viral vectors directly into the central nervous system (CNS) has emerged as an important tool for the refinement of gene therapy. Intrathecal delivery by direct lumbar puncture in conscious rodents offers a minimally invasive approach that avoids tissue damage and/or destruction. Here we describe delivery of small quantities of viral vector product to the intrathecal space of rodents via direct lumbar puncture aided by a catheter.

Involvement of the VGF-derived peptide TLQP-62 in nerve injury-induced hypersensitivity and spinal neuroplasticity.

Neuroplasticity in the dorsal horn after peripheral nerve damage contributes critically to the establishment of chronic pain. The neurosecretory protein VGF (nonacronymic) is rapidly and robustly upregulated after nerve injury, and therefore, peptides generated from it are positioned to serve as signals for peripheral damage. The goal of this project was to understand the spinal modulatory effects of the C-terminal VGF-derived peptide TLQP-62 at the cellular level and gain insight into the function of the peptide in the development of neuropathic pain. In a rodent model of neuropathic pain, we demonstrate that endogenous levels of TLQP-62 increased in the spinal cord, and its immunoneutralization led to prolonged attenuation of the development of nerve injury-induced hypersensitivity. Using multiphoton imaging of submaximal glutamate-induced Ca responses in spinal cord slices, we demonstrate the ability of TLQP-62 to potentiate glutamatergic responses in the dorsal horn. We further demonstrate that the peptide selectively potentiates responses of high-threshold spinal neurons to mechanical stimuli in singe-unit in vivo recordings. These findings are consistent with a function of TLQP-62 in spinal plasticity that may contribute to central sensitization after nerve damage.

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.

Complement 3a receptor in dorsal horn microglia mediates pronociceptive neuropeptide signaling.

The complement 3a receptor (C3aR1) participates in microglial signaling under pathological conditions and was recently shown to be activated by the neuropeptide TLQP-21. We previously demonstrated that TLQP-21 elicits hyperalgesia and contributes to nerve injury-induced hypersensitivity through an unknown mechanism in the spinal cord. Here we determined that this mechanism requires C3aR1 and that microglia are the cellular target for TLQP-21. We propose a novel neuroimmune signaling pathway involving TLQP-21-induced activation of microglial C3aR1 that then contributes to spinal neuroplasticity and neuropathic pain. This unique dual-ligand activation of C3aR1 by a neuropeptide (TLQP-21) and an immune mediator (C3a) represents a potential broad-spectrum mechanism throughout the CNS for integration of neuroimmune crosstalk at the molecular level.