Together, these studies suggest that burst SCS does not act via spinal GABAergic mechanisms, despite its attenuation of spinal hyperexcitability and allodynia similar to that of tonic SCS; understanding other potential spinal inhibitory mechanisms may lead to enhanced analgesia during burst stimulation.

No abstract

Chronic neck pain affects up to 70% of persons, with the facet joint being the most common source. Intra-articular injection of the non-steroidal anti-inflammatory drug ketorolac reduces post-operative joint-mediated pain; however, the mechanism of its attenuation of facet-mediated pain has not been evaluated. Protease-activated receptor-1 (PAR1) has differential roles in pain maintenance depending on the type and location of painful injury. This study investigated if the timing of intra-articular ketorolac injection after painful cervical facet injury affects behavioral hypersensitivity by modulating spinal astrocyte activation and/or PAR1 expression. Rats underwent a painful joint distraction and received an injection of ketorolac either immediately or 1 day later. Separate control groups included injured rats with a vehicle injection at day 1 and sham operated rats. Forepaw mechanical allodynia was measured for 7 days, and spinal cord tissue was immunolabeled for glial fibrillary acidic protein (GFAP) and PAR1 expression in the dorsal horn on day 7. Ketorolac administered on day 1 after injury significantly reduced allodynia ( p = 0.0006) to sham levels, whereas injection immediately after the injury had no effect compared with vehicle. Spinal astrocytic activation followed behavioral responses and was significantly decreased ( p = 0.009) only for ketorolac given at day 1. Spinal PAR1 ( p = 0.0025) and astrocytic PAR1 ( p = 0.012) were significantly increased after injury. Paralleling behavioral data, astrocytic PAR1 was returned to levels in sham only when ketorolac was administered on day 1. Yet, spinal PAR1 was significantly reduced ( p < 0.0001) by ketorolac independent of timing. Spinal astrocyte expression of PAR1 appears to be associated with the maintenance of facet-mediated pain.

Many neurological disorders are initiated by blood-brain barrier breakdown, which potentiates spinal neuroinflammation and neurodegeneration. Peripheral neuropathic injuries are known to disrupt the blood-spinal cord barrier (BSCB) and to potentiate inflammation. But, it is not known whether BSCB breakdown facilitates pain development. In this study, a neural compression model in the rat was used to evaluate relationships among BSCB permeability, inflammation and pain-related behaviors. BSCB permeability increases transiently only after injury that induces mechanical hyperalgesia, which correlates with serum concentrations of pro-inflammatory cytokines, IL-7, IL-12, IL-1α and TNF-α. Mammalian thrombin dually regulates vascular permeability through PAR1 and activated protein C (APC). Since thrombin protects vascular integrity through APC, directing its affinity towards protein C, while still promoting coagulation, might be an ideal treatment for BSCB-disrupting disorders. Salmon thrombin, which prevents the development of mechanical allodynia, also prevents BSCB breakdown after neural injury and actively inhibits TNF-α-induced endothelial permeability in vitro, which is not evident the case for human thrombin. Salmon thrombin’s production of APC faster than human thrombin is confirmed using a fluorogenic assay and APC is shown to inhibit BSCB breakdown and pain-related behaviors similar to salmon thrombin. Together, these studies highlight the impact of BSCB on pain and establish salmon thrombin as an effective blocker of BSCB, and resulting nociception, through its preferential affinity for protein C.

Chronic neck pain is a major problem with common causes including disc herniation and spondylosis that compress the spinal nerve roots. Cervical nerve root compression in the rat produces sustained behavioral hypersensitivity, due in part to the early upregulation of pro-inflammatory cytokines, the sustained hyperexcitability of neurons in the spinal cord and degeneration in the injured nerve root. Through its activation of the protease-activated receptor-1 (PAR1), mammalian thrombin can enhance pain and inflammation; yet at lower concentrations it is also capable of transiently attenuating pain which suggests that PAR1 activation rate may affect pain maintenance. Interestingly, salmon-derived fibrin, which contains salmon thrombin, attenuates nerve root-induced pain and inflammation, but the mechanisms of action leading to its analgesia are unknown. This study evaluates the effects of salmon thrombin on nerve root-mediated pain, axonal degeneration in the root, spinal neuronal hyperexcitability and inflammation compared to its human counterpart in the context of their enzymatic capabilities towards coagulation substrates and PAR1. Salmon thrombin significantly reduces behavioral sensitivity, preserves neuronal myelination, reduces macrophage infiltration in the injured nerve root and significantly decreases spinal neuronal hyperexcitability after painful root compression in the rat; whereas human thrombin has no effect. Unlike salmon thrombin, human thrombin upregulates the transcription of IL-1β and TNF-α and the secretion of IL-6 by cortical cultures. Salmon and human thrombins cleave human fibrinogen-derived peptides and form clots with fibrinogen with similar enzymatic activities, but salmon thrombin retains a higher enzymatic activity towards coagulation substrates in the presence of antithrombin III and hirudin compared to human thrombin. Conversely, salmon thrombin activates a PAR1-derived peptide more weakly than human thrombin. These results are the first to demonstrate that salmon thrombin has unique analgesic, neuroprotective and anti-inflammatory capabilities compared to human thrombin and that PAR1 may contribute to these actions.

A paradigm shift has emerged in Parkinson’s disease (PD) highlighting the prominent role of CD4+ Tregs in pathogenesis and treatment. Bench to bedside research, conducted by others and our own laboratories, advanced a neuroprotective role for Tregs making pharmacologic transformation of immediate need. Herein, a vasoactive intestinal peptide receptor-2 (VIPR2) peptide agonist, LBT-3627, was developed as a neuroprotectant for PD-associated dopaminergic neurodegeneration. Employing both 6-hydroxydopamine (6-OHDA) and α-synuclein (α-Syn) overexpression models in rats, the sequential administration of LBT-3627 increased Treg activity without altering cell numbers both in naïve animals and during progressive nigrostriatal degeneration. LBT-3627 administration was linked to reductions of inflammatory microglia, increased survival of dopaminergic neurons, and improved striatal densities. While α-Syn overexpression resulted in reduced Treg activity, LBT-3627 rescued these functional deficits. This occurred in a dose-dependent manner closely mimicking neuroprotection. Taken together, these data provide the basis for the use of VIPR2 agonists as potent therapeutic immune modulating agents to restore Treg activity, attenuate neuroinflammation, and interdict dopaminergic neurodegeneration in PD. The data underscore an important role of immunity in PD pathogenesis.

Both traumatic and slow-onset disc herniation can directly compress and/or chemically irritate cervical nerve roots, and both types of root injury elicit pain in animal models of radiculopathy. This study investigated the relative contributions of mechanical compression and chemical irritation of the nerve root to spinal regulation of neuronal activity using several outcomes. Modifications of two proteins known to regulate neurotransmission in the spinal cord, the neuropeptide calcitonin gene-related peptide (CGRP) and glutamate transporter 1 (GLT-1), were assessed in a rat model after painful cervical nerve root injuries using a mechanical compression, chemical irritation or their combination of injury. Only injuries with compression induced sustained behavioral hypersensitivity (p≤0.05) for two weeks and significant decreases (p<0.037) in CGRP and GLT-1 immunoreactivity to nearly half that of sham levels in the superficial dorsal horn. Because modification of spinal CGRP and GLT-1 is associated with enhanced excitatory signaling in the spinal cord, a second study evaluated the electrophysiological properties of neurons in the superficial and deeper dorsal horn at day 7 after a painful root compression. The evoked firing rate was significantly increased (p=0.045) after compression and only in the deeper lamina. The painful compression also induced a significant (p=0.002) shift in the percentage of neurons in the superficial lamina classified as lowthreshold mechanoreceptive (sham 38%; compression 10%) to those classified as wide dynamic range neurons (sham 43%; compression 74%). Together, these studies highlight mechanical compression as a key modulator of spinal neuronal signaling in the context of radicular injury and pain.

Study Design Temporal immunohistochemistry analysis of spinal cord tissue from a rat model of cervical radiculopathy. Objective The goal was to measure spinal endothelial and astrocytic vimentin expression following a painful nerve root compression to define spinal cellular expression of vimentin in the context of pain. Summary of Background Data The intermediate filament, vimentin, is expressed in a variety of cell types in the spinal cord and is modulated in response to neural pathologies. Early after nerve root compression spinal astrocytes become activated and blood-spinal cord barrier (BSCB) breakdown occurs in parallel with development of pain-related behaviors; these spinal responses remain activated as does the presence of pain. In addition to vimentin, glial fibrillary acidic protein (GFAP) expression is a hallmark of astrocyte activation. In contrast, vascular endothelial cells down-regulate vimentin expression in parallel with vascular breakdown. It is not known whether spinal astrocytes and endothelial cells modulate their expression of vimentin in response to a painful neural injury. Methods Mechanical hyperalgesia was measured and spinal cord tissue was harvested at days 1 and 7 following a unilateral nerve root compression in rats. Vimentin was co-immunolabeled with GFAP to label astrocytes and von Willebrand factor (VWF) for endothelial cells in the spinal cord on the side of injury. Results Spinal astrocytic vimentin increases by day 7 after nerve root compression, corresponding to when mechanical hyperalgesia is maintained. Spinal endothelial vimentin increases as early as day 1 after a painful compression and is even more robust at day 7. Conclusions The delayed elevation in spinal astrocytic vimentin corresponding to sustained mechanical hyperalgesia supports its having a relationship with pain maintenance. Further, since BSCB integrity has been shown to be reestablished by day 7 after a painful compression, endothelial expressed vimentin may help to fortify spinal vasculature contributing to BSCB stability.