A preclinical model of hyperalgesia following spinal stenosis/compression.

BACKGROUND: Identification of mechanisms for pain/hyperalgesia following spinal cord injury requires long-term evaluation of individual subjects because of the variability in effect over time for humans. METHODS: Rats were trained on an operant escape task that determined their preference for occupancy of a brightly lit compartment versus a dark compartment with a floor preheated to 10, 32 or 44.5 °C. Following determination of baseline preferences, the animals received extradural implantation of a small piece of polymer in the thoracic spinal canal. The polymer narrowed the spinal canal and compressed the spinal cord. Post-operative tests of escape preference were conducted over 23 weeks (experiments 1 and 2) and 62 weeks (experiment 3), permitting statistical evaluation of individual effects. RESULTS: Spinal stenosis/compression produced hyperalgesia for cold and/or heat stimulation (17 animals; 77%), no post-operative change in sensitivity (4 animals) or hypoalgesia for cold or heat (2 animals). When hyperalgesia occurred, it developed gradually over 4 months. Following removal of the polymer in experiment 3, heat sensitivity returned to baseline levels for four of four animals that had been hyperalgesic when the polymer was in place, but cold hyperalgesia was retained for four of five animals. Overall, post-operative changes in cold and heat sensitivity were not strongly related, indicating that different mechanisms were responsible for enhanced sensitivity to 10 and 44.5 °C. CONCLUSIONS: Histology revealed that hyperalgesia occurred when there was: (1) damage to spinal white matter; or (2) cystic cavitation; or (3) compression and distortion of the spinal cord without an obvious loss of grey or white matter.

Effects of interleukin-10 (IL-10) on pain behavior and gene expression following excitotoxic spinal cord injury in the rat.

Intraspinal injection of quisqualic acid (QUIS) produces excitotoxic injury with pathophysiological characteristics similar to those associated with ischemic and traumatic spinal cord injury (SCI). Responses to QUIS-induced injury include an inflammatory component, as well as the development of spontaneous and evoked pain behaviors. We hypothesized that QUIS-induced inflammation and subsequent gene expression contribute to the development and progression of pain-related behaviors and that blockade of inflammation-related gene expression leads to the amelioration of these behaviors. Using the QUIS model of spinal cord injury, we examined whether interleukin-10 (IL-10), a potent anti-inflammatory cytokine, is able to reduce mRNA levels of inflammatory and cell death-related genes leading to a reduction of pain behaviors. The results demonstrate that animals receiving systemic injection of IL-10, 30 minutes following QUIS-induced SCI, showed a significant delay in the onset of excessive grooming behavior, a significant reduction in grooming severity, and a significant reduction in the longitudinal extent of a pattern of neuronal loss within the spinal cord characterized as "grooming-type damage." QUIS injections also resulted in an increase in mRNA levels of interleukin-1 beta (IL-1 beta), cyclooxygenase-2 (COX-2), inducible nitric oxide synthase (iNOS), CD95 ligand (CD95-L, also called FAS-L/APO-1L), and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Results of QUIS injury plus IL-10 treatment resulted in a significant downregulation of IL1-beta and iNOS mRNA and these results were supported by Western blot analysis of protein levels following IL-10 treatment. These data suggest that IL-10 reduces inflammation and that targeting injury-induced inflammation is an effective strategy for limiting the extent of neuronal damage following excitotoxic SCI and thus the onset and progression of injury-induced pain behaviors.

Beneficial effects of modest systemic hypothermia on locomotor function and histopathological damage following contusion-induced spinal cord injury in rats.

OBJECT: Local spinal cord cooling (LSCC) is associated with beneficial effects when applied following ischemic or traumatic spinal cord injury (SCI). However, the clinical application of LSCC is associated with many technical difficulties such as the requirement of special cooling devices, emergency surgery, and complicated postoperative management. If hypothermia is to be considered for future application in the treatment of SCI, alternative approaches must be developed. The objectives of the present study were to evaluate 1) the relationship between systemic and epidural temperature after SCI; 2) the effects of modest systemic hypothermia on histopathological damage at 7 and 44 days post-SCI; and 3) the effects of modest systemic hypothermia on locomotor outcome at 44 days post-SCI. METHODS: A spinal cord contusion (12.5 mm at T-10) was produced in adult rats that had been randomly divided into two groups. Group 1 rats (seven in Experiment 1; 12 in Experiment 2) received hypothermic treatment (epidural temperature 32-33 degrees C) 30 minutes postinjury for 4 hours; Group 2 rats (nine in Experiment 1; eight in Experiment 2) received normothermic treatment (epidural temperature 37 degrees C) 30 minutes postinjury for 4 hours. Blood pressure, blood gas levels, and temperatures (epidural and rectal) were monitored throughout the 4-hour treatment period. Twice weekly assessment of locomotor function was performed over a 6-week survival period by using the Basso-Beattie-Bresnahan locomotor rating scale. Seven (Experiment 1) and 44 (Experiment 2) days after injury, animals were killed, perfused, and their spinal cords were serially sectioned. The area of tissue damage was quantitatively analyzed from 16 longitudinal sections selected from the central core of the spinal cord. CONCLUSIONS: The results showed that 1) modest changes in the epidural temperature of the spinal cord can be produced using systemic hypothermia; 2) modest systemic hypothermia (32-33 degrees C) significantly protects against locomotor deficits following traumatic SCI; and 3) modest systemic hypothermia (32-33 degrees C) reduces the area of tissue damage at both 7 and 44 days postinjury. Although additional research is needed to study the therapeutic window and long-term benefits of systemic hypothermia, these data support the possible use of modest systemic hypothermia in the treatment of acute SCI.

Diffusion-weighted MR imaging in a rat model of syringomyelia after excitotoxic spinal cord injury.

BACKGROUND AND PURPOSE: Recent experimental data have shown that an increase of excitatory amino acids and the initiation of inflammatory responses within the injured spinal cord may play a role in post-traumatic syringomyelia. The purpose of this study was to determine whether diffusion-weighted MR imaging with apparent diffusion coefficient (ADC) maps could provide earlier evidence of spinal cord cavitation in a rat model of syringomyelia than available with conventional MR imaging. METHODS: The spinal cord gray matter of four rats was injected with the alpha-amino-3 hydroxy-5 methyl-4 isoxazole propionic acid/metabotropic receptor agonist quisqualic acid. Animals were sacrificed at 1, 4, or 8 weeks after injection, and the spinal cords were fixed in formalin for 1 week and imaged with T1-, T2-, and diffusion-weighted sequences. One control specimen was also imaged. ADC maps were constructed from the diffusion-weighted data. Histopathologic analyses of sections stained with cresyl violet were compared with the MR images. RESULTS: By 1 week after injection, ADC maps at the level of injection showed areas within the gray matter of increased intensity and increased ADC values as compared with the control specimen. These bright areas corresponded to cysts or cavities within the cord parenchyma on the histopathologic sections. The ADC values within affected gray matter areas progressively increased at 4 and 8 weeks, also corresponding to cyst formation. Conventional T1- and T2-weighted images showed corresponding lesions with cystic characteristics at 4 and 8 weeks, but not at 1 week. CONCLUSION: In an animal model of syringomyelia, diffusion-weighted imaging with ADC maps detected cystic lesions within spinal cord gray matter before they were seen on conventional T1- and T2-weighted images.

Effects of adrenal medullary transplants on pain-related behaviors following excitotoxic spinal cord injury.

Previous studies have shown that intraspinal injection of quisqualic acid (QUIS) produces excitotoxic injury with pathological characteristics similar to those associated with ischemic and traumatic spinal cord injury (SCI). Significant changes in the functional properties of sensory neurons adjacent to the site of injury have also been observed in this model. Additionally, following QUIS injections, mechanical and cold allodynia, combined with excessive grooming behavior have been shown to be the behavioral correlates of these pathological and physiological changes. These behaviors are believed to be related to the clinical conditions of spontaneous and evoked pain following SCI. Given the therapeutic properties of adrenal chromaffin cell transplantation in conditions of neuropathic and cancer pain, it is proposed that the neuroactive substances released from chromaffin cells can alter or prevent the onset and progression of QUIS-induced behavioral changes. The effects of adrenal transplants were evaluated in 14 male Long-Evans rats that received intraspinal injections of QUIS. Pain behaviors, including the progression of excessive grooming behavior (n=8) and hypersensitivity to mechanical and thermal stimuli (n=6) were evaluated following transplantation. A 53% increase in mechanical thresholds was observed following adrenal transplants along with a 70% reduction in the area of skin targeted for excessive grooming. These behaviors were not affected in 11 animals receiving transplants of skeletal muscle. The effects of adrenal transplants on cold allodynia consisted of a stabilization of response latencies in contrast to the continued decrease in latencies, i.e., increased sensitivity, following transplants of skeletal muscle. The results are consistent with previous studies showing the therapeutic efficacy of adrenal chromaffin cell transplants in neuropathic pain, and support the use of this treatment strategy for the alleviation of chronic pain following spinal cord injury.

Intraspinal injection of adenosine agonists protect against L-NAME induced neuronal loss in the rat.

Intraspinal injection of the nonspecific inhibitor of nitric oxide synthase N-nitro-L-arginine methyl ester (L-NAME) results in a dose-dependent loss of neurons in the rat spinal cord. This effect is thought to result from a reduction in basal levels of nitric oxide (NO), thereby producing an ischemic reaction secondary to vasoconstriction and reduced spinal cord blood flow (SCBF). An important component of this ischemic reaction is the release of excitatory amino acids and the initiation of an excitotoxic cascade. In the present study, microinjections of adenosine A1 and A2 receptor agonists were made in the spinal cord to evaluate the neuroprotective effects of these drugs against neuronal loss produced by L-NAME. Animals were divided into six groups based on the composition of injected solutions: (a) L-NAME; (b) L-NAME + N6-cyclopentyladenosine (CPA, A1 agonist); (c) L-NAME + 5'-(N-cyclopropyl)-carboxamidoadenosine (CPCA, A2 agonist); (d) L-NAME + CPA + CPCA; (e) N-methyl D-aspartate (NMDA); and (f) NMDA + CPA. Injections of L-NAME or NMDA produced a unilateral loss of spinal neurons, a local inflammatory response, and darkly stained pyknotic nuclei surrounding the area of neuronal loss. CPA and CPCA significantly reduced the area of L-NAME-induced neuronal loss, and a synergistic effect was observed when ineffective doses of these agonists were co-injected with L-NAME. The excitotoxic effects of NMDA were not affected by CPA. The results have shown that A1 and A2 receptor agonists provide significant neuroprotection against L-NAME induced neuronal loss, presumably by inhibiting ischemia induced release of excitatory amino acids (A1 agonist), or by restoring SCBF secondary to vasodilation (A2 agonist). It is suggested by these results that the intraspinal injection of L-NAME is an effective model to study the pathological consequences of vasoconstriction, reduced SCBF, and ischemia secondary to decreased NO production in the rat spinal cord. Finally, the results provide support for the continued investigation of specific adenosine agonists as therapeutic agents directed against the ischemic and excitotoxic components of spinal injury.

Traumatic spinal cord injury induces nuclear factor-kappaB activation.

Inflammatory responses are a major component of secondary injury and play a central role in mediating the pathogenesis of acute and chronic spinal cord injury (SCI). The nuclear factor-kappaB (NF-kappaB) family of transcription factors is required for the transcriptional activation of a variety of genes regulating inflammatory, proliferative, and cell death responses of cells. In this study we examined the temporal and cellular expression of activated NF-kappaB after traumatic SCI. We used a contusion model (N.Y.U. Impactor) to initiate the early biochemical and molecular changes that occur after traumatic injury to reproduce the pathological events associated with acute inflammation after SCI. The activation and cellular distribution of activated NF-kappaB was evaluated by using a monoclonal antibody that selectively recognizes activated p65 in a NF-kappaB dimer. Immunohistochemical and Western blot analyses demonstrated that NF-kappaB activation occurred as early as 0.5 hr postinjury and persisted for at least 72 hr. Using electrophoretic mobility shift assays (EMSA), we demonstrate that NF-kappaB is activated after SCI. In our immunohistochemical, Western, and EMSA experiments there are detectable levels of activated NF-kappaB in our control animals. Using double-staining protocols, we detected activated NF-kappaB in macrophages/microglia, endothelial cells, and neurons within the injured spinal cord. Colocalization of activated NF-kappaB with the NF-kappaB-dependent gene product, inducible nitric oxide synthase (iNOS), suggests functional implications for this transcription factor in the pathogenesis of acute spinal cord injury. Although there is considerable evidence for the involvement of an inflammatory reaction after traumatic SCI, this is the first evidence for the activation of NF-kappaB after trauma. Strategies directed at blocking the initiation of this cascade may prove beneficial as a therapeutic approach for the treatment of acute SCI.

Syringomyelia: clinical observations and experimental studies.

Although cavitary lesions of the spinal cord have been recognized for centuries, only recently have effective, noninvasive imaging techniques allowed antemortem diagnosis of this clinical syndrome. Methods of treatment have not been consistently successful in alleviating or reversing the clinical symptoms caused by these cystic lesions. Incomplete understanding of the underlying pathologic basis for the syringes has impeded the development of effective methods of treatment. This review documents historical considerations regarding clinical observations and experimental studies of this entity and the animal models that have been reported for each of the major types of syringomyelia. Recent studies have suggested that development of a relevant animal model of posttraumatic syringomyelia is imminent. Successful development of an experimental model will not only permit definition of the pathogenesis of cyst formation but also provide methods for testing of therapeutic interventions.