Lipid rafts in glial cells: role in neuroinflammation and pain processing

Activation of microglia and astrocytes secondary to inflammatory processes contributes to the development and perpetuation of pain with a neuropathic phenotype. This pain state presents as a chronic debilitating condition and affects a large population of patients with conditions like rheumatoid arthritis and diabetes, or after surgery, trauma, or chemotherapy. Here, we review the regulation of lipid rafts in glial cells and the role they play as a key component of neuroinflammatory sensitization of central pain signaling pathways. In this context, we introduce the concept of an inflammaraft (i-raft), enlarged lipid rafts harboring activated receptors and adaptor molecules and serving as an organizing platform to initiate inflammatory signaling and the cellular response. Characteristics of the inflammaraft include increased relative abundance of lipid rafts in inflammatory cells, increased content of cholesterol per raft, and increased levels of inflammatory receptors, such as toll-like receptor (TLR)4, adaptor molecules, ion channels, and enzymes in lipid rafts. This inflammaraft motif serves an important role in the membrane assembly of protein complexes, for example, TLR4 dimerization. Operating within this framework, we demonstrate the involvement of inflammatory receptors, redox molecules, and ion channels in the inflammaraft formation and the regulation of cholesterol and sphingolipid metabolism in the inflammaraft maintenance and disruption. Strategies for targeting inflammarafts, without affecting the integrity of lipid rafts in noninflammatory cells, may lead to developing novel therapies for neuropathic pain states and other neuroinflammatory conditions.

Unilateral Epidural Targeting of Resiniferatoxin Induces Bilateral Neurolysis of Spinal Nociceptive Afferents

OBJECTIVE

This study modeled image-guided epidural drug delivery to test whether intraprocedural distribution of pre-injected contrast reliably predicts the neuroanatomical reach of resiniferatoxin-mediated nociceptive neurolysis.

METHODS

Swine (N = 12) received unilateral L4-S2 computed tomography fluoroscopy injections by a blinded neuroradiologist; 0.25 mL of contrast was pre-injected to confirm dorsal periganglionic targeting, followed by a 0.5-mL injection of 5 µg of resiniferatoxin/Tween80 or vehicle control. Epidural contrast distribution was graded according to maximum medial excursion. Spinal cord substance P immunostaining quantified the magnitude and anatomical range of resiniferatoxin activity.

RESULTS

Periganglionic injection was well tolerated by all animals without development of neurological deficits or other complications. Swine were a suitable model of human clinical spinal intervention. The transforaminal approach was used at all L4 and 50% of L5 segments; the remaining segments were approached by the interlaminar route. All injections were successful with unilateral contrast distribution for all resiniferatoxin injections (N = 28). Immunohistochemistry showed bilateral ablation of substance P+ fibers entering the spinal cord of all resiniferatoxin-treated segments. The intensity of substance P immunostaining in treated segments fell below the lower 99% confidence interval of controls, defining the knockout phenotype. Substance P knockout occurred over a narrow range and was uncorrelated to the anatomical distribution of pre-injected contrast.

CONCLUSIONS

Periganglionic resiniferatoxin/Tween80 induced bilateral ablation of spinal cord substance P despite exclusively unilateral targeting. These data suggest that the location of pre-injected contrast is an imperfect surrogate for the neuroanatomical range of drugs delivered to the dorsal epidural compartment that may fail to predict contralateral drug effects.

Neuraxial Cytokines in Pain States

A high-intensity potentially tissue-injuring stimulus generates a homotopic response to escape the stimulus and is associated with an affective phenotype considered to represent pain. In the face of tissue or nerve injury, the afferent encoding systems display robust changes in the input–output function, leading to an ongoing sensation reported as painful and sensitization of the nociceptors such that an enhanced pain state is reported for a given somatic or visceral stimulus. Our understanding of the mechanisms underlying this non-linear processing of nociceptive stimuli has led to our appreciation of the role played by the functional interactions of neural and immune signaling systems in pain phenotypes. In pathological states, neural systems interact with the immune system through the actions of a variety of soluble mediators, including cytokines. Cytokines are recognized as important mediators of inflammatory and neuropathic pain, supporting system sensitization and the development of a persistent pathologic pain. Cytokines can induce a facilitation of nociceptive processing at all levels of the neuraxis including supraspinal centers where nociceptive input evokes an affective component of the pain state. We review here several key proinflammatory and anti-inflammatory cytokines/chemokines and explore their underlying actions at four levels of neuronal organization: (1) peripheral nociceptor termini; (2) dorsal root ganglia; (3) spinal cord; and (4) supraspinal areas. Thus, current thinking suggests that cytokines by this action throughout the neuraxis play key roles in the induction of pain and the maintenance of the facilitated states of pain behavior generated by tissue injury/inflammation and nerve injury.

Inhibition of Neuroinflammation by AIBP: Spinal Effects upon Facilitated Pain States

Apolipoprotein A-I binding protein (AIBP) reduces lipid raft abundance by augmenting the removal of excess cholesterol from the plasma membrane. Here, we report that AIBP prevents and reverses processes associated with neuroinflammatory-mediated spinal nociceptive processing. The mechanism involves AIBP binding to Toll-like receptor-4 (TLR4) and increased binding of AIBP to activated microglia, which mediates selective regulation of lipid rafts in inflammatory cells. AIBP-mediated lipid raft reductions downregulate LPS-induced TLR4 dimerization, inflammatory signaling, and expression of cytokines in microglia. In mice, intrathecal injections of AIBP reduce spinal myeloid cell lipid rafts, TLR4 dimerization, neuroinflammation, and glial activation. Intrathecal AIBP reverses established allodynia in mice in which pain states were induced by the chemotherapeutic cisplatin, intraplantar formalin, or intrathecal LPS, all of which are pro-nociceptive interventions known to be regulated by TLR4 signaling. These findings demonstrate a mechanism by which AIBP regulates neuroinflammation and suggest the therapeutic potential of AIBP in treating preexisting pain states.

Characterization of Analgesic Actions of the Chronic Intrathecal Infusion of H-Dmt-D-Arg-Phe-Lys-NH2 in Rat

OBJECTIVES

DMT-DALDA (H-Dmt-D-Arg-Phe-Lys-NH2; Dmt = 2',6'-dimethyltyrosine) is a selective mu opioid agonist. We sought to characterize efficacy, tolerance, dependence and side-effect profile when given by continuous intrathecal infusion.

MATERIALS AND METHODS

Adult male Sprague Dawley rats were prepared with chronic intrathecal catheters and osmotic mini-pumps to deliver vehicle (saline), DMT-DALDA or morphine. Hind paw thermal escape latencies were assessed. In addition, effects upon intraplantar formalin-evoked flinching and withdrawal after 14 days of infusion were examined. The flare response after intradermal delivery was examined in the canine model.

RESULTS

1) Intrathecal infusion of 0.3 to 30 pmol/μL/hour of DMT-DALDA or 37.5 nmol/μL/hour of morphine more than 7 or 14 days resulted in a dose-dependent increase in thermal escape latency. The maximum antinociceptive effect was observed between 1 and 4 days after start of infusion with preserved cornea, blink, placing and stepping. By days 12 to 14, response latencies were below baseline. 2) On days 2 to 4 of DMT-DALDA infusion, the pan opioid receptor antagonist naloxone (Nx), but not the delta-preferring antagonist naltrindole, antagonized the analgesic effects. 3) Assessment of formalin flinching on day 1 following IT DMT-DALDA Infusion showed significant analgesia in phases 1 and 2. On day 6 of infusion there was minimal effect, while on day 13, there was an increase in flinching. 4) On days 7 and 14 of infusion Nx resulted in prominent withdrawal signs indicating dependence and withdrawal. 5) Intradermal morphine and DMT-DALDA both yield a naltrexone-insensitive, cromolyn-sensitive flare in the canine model at similar concentrations.

CONCLUSIONS

These data suggest that DMT-DALDA is a potent, spinally active agonist with a propensity to produce tolerance dependence and mast cell degranulation. While it was equiactive to morphine in producing mast cell degranulation, it was >1000 fold more potent in producing analgesia, suggesting a possible lower risk in producing a spinal mass at equianalgesic doses.

Characterization of the antinociceptive effects of intrathecal DALDA peptides following bolus intrathecal delivery

Background and aims We systematically characterized the potency and side effect profile of a series of small opioid peptides with high affinity for the mu opioid receptor. Methods Male Sprague Dawley rats were prepared with intrathecal (IT) catheters, assessed with hind paw thermal escape and evaluated for side effects including Straub tail, truncal rigidity, and pinnae and corneal reflexes. In these studies, DMT-DALDA (dDAL) (H-Dmt-D-Arg-Phe-Lys-NH2 MW=981), dDALc (H-Dmt-Cit-Phe-Lys-NH2 MW=868), dDALcn (H-Dmt-D-Cit-Phe-Nle-NH2 MW=739), TAPP (H-Tyr-D-Ala-Phe-Phe-NH2 MW=659), dDAL-TICP ([Dmt1]DALDA-(CH2)2-NH-TICP[psi]; MW=1519), and dDAL-TIPP (H-Dmt-D-Arg-Phe-Lys(Nε-TIPP)-NH2 were examined. In separate studies, the effects of approximately equiactive doses of IT DMT DALDA (10 pmol), morphine (30 nmol) and fentanyl (1 nmol) were examined on formalin-induced flinching at different pretreatment intervals (15 min - 24 h). Results (1) All agents resulted in a dose-dependent reversible effect upon motor function (Straub Tail>Truncal rigidity). (2) The ordering of analgesic activity (%MPE) at the highest dose lacking reliable motor signs after bolus delivery was: DMT-DALDA (80%±6/3 pmol); dDALc (75%±8/1 pmol); dDALcn (84%±10/300 pmol); TAPP (56%±12/10 nmol); dDAL-TICP (52%±27/300 pmol). (3) All analgesic effects were reversed by systemic (IP) naloxone (1 mg/kg). Naltrindole (3 mg/kg, IP) had no significant effect upon the maximum usable peptide dose. (4) Tolerance and cross-tolerance development after 5 daily boluses of DMT-DALDA (3 pmol) and morphine (30 nmol) revealed that both agents displayed a progressive decline over 5 days. (5) Cross-tolerance assessed at day 5 revealed a reduction in response to morphine in DMT-DALDA treated animal but not DMT-DALDA in the morphine treated animal, indicating an asymmetric cross-tolerance. (6) IT DMT-DALDA, morphine and fentanyl resulted in significant reductions in phase 1 and phase 2 flinching. With a 15 min pretreatment all drugs resulted in comparable reductions in flinching. However, at 6 h, the reduction in flinching after DMT-DALDA and morphine were comparably reduced while fentanyl was not different from vehicle. All effects on flinching were lost by 24 h. Conclusions These results emphasize the potent mu agonist properties of the DALDA peptidic structure series, their persistence similar to morphine and their propensity to produce tolerance. The asymmetric cross-tolerance between equiactive doses may reflect the relative intrinsic activity of morphine and DMT-DALDA. Implications These results suggest that the DALDA peptides with their potency and duration of action after intrathecal delivery suggest their potential utility for their further development as a spinal therapeutic to manage pain.

Mast Cell Degranulation and Fibroblast Activation in the Morphine-induced Spinal Mass: Role of Mas-related G Protein-coupled Receptor Signaling

BACKGROUND

As the meningeally derived, fibroblast-rich, mass-produced by intrathecal morphine infusion is not produced by all opiates, but reduced by mast cell stabilizers, the authors hypothesized a role for meningeal mast cell/fibroblast activation. Using the guinea pig, the authors asked: (1) Are intrathecal morphine masses blocked by opiate antagonism?; (2) Do opioid agonists not producing mast cell degranulation or fibroblast activation produce masses?; and (3) Do masses covary with Mas-related G protein-coupled receptor signaling thought to mediate mast cell degranulation?

METHODS

In adult male guinea pigs (N = 66), lumbar intrathecal catheters connected to osmotic minipumps (14 days; 0.5 µl/h) were placed to deliver saline or equianalgesic concentrations of morphine sulfate (33 nmol/h), 2',6'-dimethyl tyrosine-(Tyr-D-Arg-Phe-Lys-NH2) (abbreviated as DMT-DALDA; 10 pmol/h; μ agonist) or PZM21 (27 nmol/h; biased μ agonist). A second pump delivered subcutaneous naltrexone (25 µg/h) in some animals. After 14 to 16 days, animals were anesthetized and perfusion-fixed. Drug effects on degranulation of human cultured mast cells, mouse embryonic fibroblast activation/migration/collagen formation, and Mas-related G protein-coupled receptor activation (PRESTO-Tango assays) were determined.

RESULTS

Intrathecal infusion of morphine, DMT-DALDA or PZM21, but not saline, comparably increased thermal thresholds for 7 days. Spinal masses proximal to catheter tip, composed of fibroblast/collagen type I (median: interquartile range, 0 to 4 scale), were produced by morphine (2.3: 2.0 to 3.5) and morphine plus naltrexone (2.5: 1.4 to 3.1), but not vehicle (1.2: 1.1 to 1.5), DMT-DALDA (1.0: 0.6 to 1.3), or PZM21 (0.5: 0.4 to 0.8). Morphine in a naloxone-insensitive fashion, but not PZM21 or DMT-DALDA, resulted in mast cell degranulation and fibroblast proliferation/collagen formation. Morphine-induced fibroblast proliferation, as mast cell degranulation, is blocked by cromolyn. Mas-related G protein-coupled receptor activation was produced by morphine and TAN67 (∂-opioid agonist), but not by PZM21, TRV130 (mu biased ligand), or DMT-DALDA.

CONCLUSIONS

Opiates that activate Mas-related G protein-coupled receptor will degranulate mast cells, activate fibroblasts, and result in intrathecal mass formation. Results suggest a mechanistically rational path forward to safer intrathecal opioid therapeutics.

Characterization of Effect of Repeated Bolus or Continuous Intrathecal Infusion of Morphine on Spinal Mass Formation in the Dog

BACKGROUND

We determined whether intrathecally delivering the same daily dose of morphine (MS) at a fixed concentration of 25 mg/mL by periodic boluses versus continuous infusion would reduce intrathecal mass (IMs) formation in dogs.

METHODS

Adult dogs (hound cross, n = 32) were implanted with intrathecal catheters connected to SynchroMed II infusion pumps. Animals were randomly assigned to receive infusion of 0.48 mL/day of saline or MS dosing (12 mg/day at 25 mg/mL) as boluses: x1 (q24hour), x2 (q12hour), x4 (q6hour), or x8 (q3hour) given at the rate of 1000 μL/hour, or as a continuous infusion (25 mg/mL/20 μL/hour).

RESULTS

With IT saline, minimal pathology was noted. In contrast, animals receiving morphine displayed spinally compressing durally derived masses with the maximal cross-sectional area being greatest near the catheter tip. Histopathology showed that IMs consisted of fibroblasts in a collagen (type 1) matrix comprised of newly formed collagen near the catheter and mature collagen on the periphery of the mass. The rank order of median cross-sectional mass area (mm² ) was: Saline: 0.7 mm² ; x2: 1.8 mm² ; x4: 2.7 mm² ; x1: 2.7 mm² ; x8: 4.2 mm² ; Continuous: 8.1 mm² , with statistical difference from saline being seen with continuous (p < 0.0001) and x8 (p < 0.05). Bench studies with a 2D diffusion chamber confirmed an increase in dye distribution and lower peak concentrations after bolus delivery versus continuous infusion of dye.

CONCLUSIONS

Using multiple bolus dosing, IMs were reduced as compared to continuous infusion, suggesting relevance of bolus delivery in yielding reduced intrathecal masses.

Neuraxial TNF and IFN-beta co-modulate persistent allodynia in arthritic mice

In rheumatoid arthritis, joint pain can persist despite resolution of swelling. Similarly, in the murine K/BxN serum transfer model, a persistent tactile allodynia is observed after the resolution of joint inflammation (post-inflammatory pain) in male mice. Here, we found female wild type (WT) mice show inflammatory, but reduced post-inflammatory tactile allodynia. The transition to the post-inflammatory phenotype is dependent on TLR4 signaling. At the spinal level, we found differences in TNF and IFNβ mRNA expression in WT and TLR4 deficient males. In wild type male and female mice, there is differential temporal spinal expression of TNF and IFNβ. In WT males, blockade of TNF or administration of IFNβ was insufficient to affect the persistent allodynia. However, co-administration of intrathecal (IT) IFNβ and anti-TNF antibodies in male WT mice permanently reversed tactile allodynia. IT IFNβ treatment induces expression of anti-inflammatory proteins, contributing to the beneficial effect. Together, these experiments illustrated differences in the transition to chronic tactile allodynia in male and female animals and the complexities of effective pharmacologic interventions.

Role of Toll-like receptor 4 signaling in mast cell-mediated migraine pain pathway

Degranulation of meningeal mast cells leading to the sensitization of trigeminal vascular afferent processing is believed to be one of the mechanisms underlying the migraine pain pathway. Recent work suggests that Toll-like receptor 4 (TLR4) may be involved in signaling states of central sensitization. Using a murine model of light aversion produced by compound 48/80 (2 mg/kg, intraperitoneal) mast cell degranulation, employed as a surrogate marker for photophobia observed in migraineurs, we examined the role of TLR4 in migraine-like behavior and neuronal activation. Using a two-chambered light/dark box, we found that compound 48/80 administration in male and female C57Bl/6 mice produced light aversion lasting up to 2 h, and that pre-treatment with sumatriptan (1 mg/kg, i.p.) reliably prevented this effect. Genetic deletion and pharmacological blockade of TLR4 with TAK-242 (3 mg/kg, i.p.) reversed the light aversive effects of compound 48/80 in males but not in females. Assessing the downstream signaling pathway in mutant mice, we found that the TLR4-mediated, light aversion was dependent upon myeloid differentiation primary response gene 88 but not Toll-interleukin-1 receptor domain-containing adapter-inducing interferon-β signaling. In separate groups, male mice sacrificed at 10 min following compound 48/80 revealed a significant increase in the incidence of evoked p-extracellular signal–regulated kinases (+) neurons in the nucleus caudalis of wild type but not Tlr4−/− mice or in mice pre-treated with sumatriptan. This study thus provides the first evidence for involvement of TLR4 signaling through MyD88 in initiating and maintaining migraine-like behavior and nucleus caudalis neuronal activation in the mouse.

Botulinum toxin blocks mast cells and prevents rosacea like inflammation

BACKGROUND

Rosacea is a chronic inflammatory skin condition whose etiology has been linked to mast cells and the antimicrobial peptide cathelicidin LL-37. Individuals with refractory disease have demonstrated clinical benefit with periodic injections of onabotulinum toxin, but the mechanism of action is unknown.

OBJECTIVES

To investigate the molecular mechanism by which botulinum toxin improves rosacea lesions.

METHODS

Primary human and murine mast cells were pretreated with onabotulinum toxin A or B or control. Mast cell degranulation was evaluated by β-hexosaminidase activity. Expression of botulinum toxin receptor Sv2 was measured by qPCR. The presence of SNAP-25 and VAMP2 was established by immunofluorescence. In vivo rosacea model was established by intradermally injecting LL-37 with or without onabotulinum toxin A pretreatment. Mast cell degranulation was assessed in vivo by histologic counts. Rosacea biomarkers were analyzed by qPCR of mouse skin sections.

RESULTS

Onabotulinum toxin A and B inhibited compound 48/80-induced degranulation of both human and murine mast cells. Expression of Sv2 was established in mouse mast cells. Onabotulinum toxin A and B increased cleaved SNAP-25 and decreased VAMP2 staining in mast cells respectively. In mice, injection of onabotulinum toxin A significantly reduced LL-37-induced skin erythema, mast cell degranulation, and mRNA expression of rosacea biomarkers.

CONCLUSIONS

These findings suggest that onabotulinum toxin reduces rosacea-associated skin inflammation by directly inhibiting mast cell degranulation. Periodic applications of onabotulinum toxin may be an effective therapy for refractory rosacea and deserves further study.

Structural homology of myelin basic protein and muscarinic acetylcholine receptor: Significance in the pathogenesis of complex regional pain syndrome

Complex regional pain syndrome is an extremely painful condition that develops after trauma to a limb. Complex regional pain syndrome exhibits autoimmune features in part mediated by autoantibodies against muscarinic‐2 acetylcholine (M2) receptor. The mechanisms underlying the M2 receptor involvement in complex regional pain syndrome remain obscure. Based on our recent work demonstrating that limb nerve trauma releases a potent proalgesic, immunodominant myelin basic protein fragment, our present sequence database analyses reveal an unexpected and previously undescribed structural homology of the proalgesic myelin basic protein fragment with the M2 receptor. As both complex regional pain syndrome and the proalgesic myelin basic protein activity are prevalent in females, this myelin basic protein/M2 homology presents an inviting hypothesis explaining the mechanisms of autoimmune pathogenesis and sexual dimorphism that underlies vulnerability toward developing complex regional pain syndrome and other pain states with neuropathic features. This hypothesis may aid in the development of novel diagnostic and therapeutic strategies to chronic pain.

Rapid continuous 3D printing of customizable peripheral nerve guidance conduits

Engineered nerve guidance conduits (NGCs) have been demonstrated for repairing peripheral nerve injuries. However, there remains a need for an advanced biofabrication system to build NGCs with complex architectures, tunable material properties, and customizable geometrical control. Here, a rapid continuous 3D-printing platform was developed to print customizable NGCs with unprecedented resolution, speed, flexibility, and scalability. A variety of NGC designs varying in complexity and size were created including a life-size biomimetic branched human facial NGC. In vivo implantation of NGCs with microchannels into complete sciatic nerve transections of mouse models demonstrated the effective directional guidance of regenerating sciatic nerves via branching into the microchannels and extending toward the distal end of the injury site. Histological staining and immunostaining further confirmed the progressive directional nerve regeneration and branching behavior across the entire NGC length. Observational and functional tests, including the von Frey threshold test and thermal test, showed promising recovery of motor function and sensation in the ipsilateral limbs grafted with the 3D-printed NGCs.

Targeting toll-like receptor-4 (TLR4)-an emerging therapeutic target for persistent pain states

Toll-like receptors (TLRs) are a family of pattern recognition receptors that initiate signaling in innate and adaptive immune pathways. The highly conserved family of transmembrane proteins comprises an extracellular domain that recognizes exogenous and endogenous danger molecules and an ectodomain that activates downstream pathways in response. Recent studies suggest that continuous activation or dysregulation of TLR signaling may contribute to chronic disease states. The receptor is located not only on inflammatory cells (meningeal and peripheral macrophages) but on neuraxial glia (microglia and astrocytes), Schwann cells, fibroblasts, dorsal root ganglia, and dorsal horn neurons. Procedures blocking TLR functionality have shown pronounced effects on pain behavior otherwise observed in models of chronic inflammation and nerve injury. This review addresses the role of TLR4 as an emerging therapeutic target for the evolution of persistent pain and its role in noncanonical signaling, mediating anomalous pro-algesic actions of opiates. Accordingly, molecules targeting inhibition of this receptor have promise as disease-modifying and opioid-sparing alternatives for persistent pain states.

Editor's Highlight: Formulation and Toxicology Evaluation of the Intrathecal AYX1 DNA-Decoy in Sprague Dawley Rats

The longevity of pain after surgery is debilitating and limits the recovery of patients. AYX1 is a double-stranded, unprotected, 23 base-pair oligonucleotide designed to reduce acute post-surgical pain and prevent its chronification with a single intrathecal perioperative dose. AYX1 mimics the DNA sequence normally bound by EGR1 on chromosomes, a transcription factor transiently induced in the dorsal root ganglia-spinal cord network following a noxious input. AYX1 binds to EGR1 and prevents it from launching waves of gene regulation that are necessary to maintain pain over time. A formulation suitable for an intrathecal injection of AYX1 was developed, including a specific ratio of AYX1 and calcium so the ionic homeostasis of the cerebrospinal fluid is maintained and no impact on neuromuscular control is produced upon injection. A GLP toxicology study in naïve Sprague Dawley rats was conducted using 3 dose levels up to the maximum feasible dose. Clinical observations, neurobehavioral observations, clinical pathology and histopathology of the nervous system and peripheral tissues were conducted. An additional nonGLP study was conducted in the spared nerve injury model of chronic neuropathic pain in which EGR1 is induced in the dorsal root ganglia and spinal cord. Similar testing was performed, including a modified Irwin test to assess a potential impact of AYX1 on autonomic nervous system responses, locomotion, activity, arousal, sensorimotor, and neuromuscular function. No AYX1-related adverse events were observed in any of the studies and the no-observed-adverse-effect-level was judged to be the maximum feasible dose.

Origins of antidromic activity in sensory afferent fibers and neurogenic inflammation

Neurogenic inflammation results from the release of biologically active agents from the peripheral primary afferent terminal. This release reflects the presence of releasable pools of active product and depolarization-exocytotic coupling mechanisms in the distal afferent terminal and serves to alter the physiologic function of innervated organ systems ranging from the skin and meninges to muscle, bone, and viscera. Aside from direct stimulation, this biologically important release from the peripheral afferent terminal can be initiated by antidromic activity arising from five anatomically distinct points of origin: (i) afferent collaterals at the peripheral-target organ level, (ii) afferent collaterals arising proximal to the target organ, (iii) from mid-axon where afferents lacking myelin sheaths (C fibers and others following demyelinating injuries) may display crosstalk and respond to local irritation, (iv) the dorsal root ganglion itself, and (v) the central terminals of the afferent in the dorsal horn where local circuits and bulbospinal projections can initiate the so-called dorsal root reflexes, i.e., antidromic traffic in the sensory afferent.

Effects of opioid and nonopioid analgesics on canine wheal formation and cultured human mast cell degranulation

Mast cell (MC) degranulation has been implicated in the side effect profile of a variety of clinically useful agents. Thus, after intrathecal delivery, formation of space-occupying, meningeally-derived masses may be related to local MC degranulation. We systematically characterized degranulating effects of opioid and nonopioid analgesics on cutaneous flares in the dog and in primary human MC (hMC) cultures.

METHODS

Dogs were anesthetized with IV propofol and received intradermal (ID) injections (50μL). Flare diameters were measured at 30min. Drugs showing flare responses were tested after intramuscular (IM) cromolyn (10mg/kg), a MC stabilizer. Human primary MCs (human cord blood CD34⁺/CD45⁺ cells) were employed and β-hexosaminidase in cell-free supernatants were measured to assess degranulation.

RESULTS

A significant skin flare for several classes of agents was observed including opioids, ziconotide, ketamine, ST-91, neostigmine, adenosine, bupivacaine, lidocaine, MK-801 and 48/80. Tizanidine, fentanyl, alfentanil, gabapentin and baclofen produced no flare. Flare produced by all ID agents, except adenosine, bupivacaine and lidocaine, was reduced by cromolyn. Naloxone had no effect upon opiate or 48/80 evoked flares. In hMC studies, 48/80 resulted in a concentration-dependent release of β-hexosaminidase. The rank order of drug-induced hMC β-hexosaminidase release was similar to that for flares.

CONCLUSIONS

A variety of therapeutically useful drugs degranulate MCs. This action may account for side effects such as the intrathecal granuloma resulting from spinally-delivered opioids. This degranulating effect may be useful in predicting potential intrathecal toxicity in the development of novel agents.

Toxicology Evaluation of Drugs Administered via Uncommon Routes: Intranasal, Intraocular, Intrathecal/Intraspinal, and Intra-Articular

As the need for nasal, ocular, spinal, and articular therapeutic compounds increases, toxicology assessments of drugs administered via these routes play an important role in human safety. This symposium outlined the local and systemic evaluation to support safety during the development of these drugs in nonclinical models with some case studies. Discussions included selection of appropriate species for the intended route; conducting nonclinical studies that closely mimic the intended use with adequate duration; functional assessment, if deemed necessary; evaluation of local tissues with special histological staining procedure; and evaluations of safety margins based on local and systemic toxicity.

Target engagement and histopathology of neuraxial resiniferatoxin in dog

OBJECTIVE

To evaluate target engagement of intracisternally (IC) delivered TRPV1 agonist, resiniferatoxin (RTX), as measured by primary afferent and dorsal horn substance P immunoreactivity (sP-IR), histopathology and thermal escape latencies in dogs.

STUDY DESIGN

Prospective experimental trial.

ANIMALS

Fourteen adult male Beagle dogs, weighing 10.3-13.2 kg; 11 dogs surviving to scheduled euthanasia.

METHODS

Anesthetized dogs were randomly assigned to be administered IC RTX (3.6 μg, 0.1 mL kg^-1) in a hyperbaric (hRTX, n = 6), normobaric (nRTX, n = 4) vehicle or a hyperbaric vehicle (hVehicle, n = 4). Over 16 days, animals were examined for thoracic and pelvic limb paw thermal withdrawal latencies and neurologic function. Spinal cords, trigeminal ganglia and dorsal root ganglia (DRGs) were assessed for morphologic changes and sP-IR.

RESULTS

IC RTX in anesthetized dogs resulted in a < 1 hour increase in blood pressure. Acute reactions leading to euthanasia within 8 hours occurred in three dogs (two hRTX, one nRTX). All other animals recovered with normal neurologic, bowel and bladder function. Final groups were: vehicle n = 4, hRTX n = 4 and nRTX n = 3. Animals in nRTX and hRTX showed increases in escape latencies in thoracic paws and, to a lesser extent, in pelvic paws, correlating to a loss of sP-IR in cervical cord with smaller reductions in thoracic and lumbar cord. In animals surviving to euthanasia, thickening of the arachnoid membrane (predominantly in the cervical region) was the most consistent change. This change, present in controls, was interpreted to be vehicle related. There was no evidence of structural changes in brain and spinal cord.

CONCLUSIONS AND CLINICAL RELEVANCE

IC RTX produced localized loss of spinal and DRG sP with a corresponding thermal analgesia, absent motor impairment or spinal pathology. Loss of three animals emphasizes the need to refine the use of this promising therapeutic modality in managing companion animal pain.