Background & Aims
Neuropathic pain, observed in approximately 40-60% of individuals following spinal cord injury (SCI), is marked by neuroimmune dysfunction. Early after SCI, immune cells such as macrophages, neutrophils, and dendritic cells, infiltrate at the lesion due to mechanical blood spinal cord barrier (BSCB) disruption. These infiltrating cells then adopt either anti- or pro-inflammatory activation states. Chronically, pro-inflammatory cytokine release may impair vascular structures at the lesion or cortical pain processing regions. Our hypothesis links neuropathic pain and ongoing vascular dysfunction after SCI with continued infiltration of monocyte-derived macrophages. Additionally, we expect that enhancing peripheral macrophage response via the chemoattractant CCL2 will diminish pain-like behaviors and alter supraspinal expressions of pain unpleasantness. Immune modulatory therapies and rehabilitation post-SCI shift immune cell polarization, offering promise in mitigating neuropathic pain.
Methods
To explore the connection between BSCB permeability and neuropathic pain, female Sprague-Dawley rats underwent a moderate C5 unilateral SCI. 2-Hrs before euthanasia, at 24-Hrs or 6-Wks post-SCI, rats received a 0.2 mL I.V. injection of Evan’s Blue, a marker of vascular permeability. To study the effects of modulating peripheral macrophages, LysM-eGFP C57bl6 mice underwent a C5 unilateral SCI or laminectomy followed by a C7-8 intraganglionic injection of CCL2 or vehicle with Naive mice as controls. Mechanical allodynia and thermal hyperalgesia were assessed preoperatively and for 6-Wks after injury using von Frey, mechanical conflict avoidance paradigm and Hargreaves’ tests. Anxiodepressive-like behaviors were assessed using sucrose preference, open field, and forced swim tests. For both studies, tissue samples were taken from the lesion, C7-8 spinal cord and DRGs, S1 and anterior cingulate cortices. Assessments of immune cell infiltration and BSCB permeability will be performed.
Results
In our analysis of barrier permeability, we found significantly greater BSCB permeability at the lesion site 24-hours post injury. SCI rats showed tactile allodynia in both forepaws and hindpaw at 6 weeks post injury indicating central sensitization. Preliminary data suggests a subset of animals displayed increased lesion site permeability at 6 weeks. A subset of SCI mice treated with CCL2 displayed less mechanical allodynia, but displayed an exacerbated depressive-like phenotype. We are currently processing and analyzing spinal cord, DRG, and cortical tissue for barrier permeability, microglia, and macrophage expression.
Conclusions
Our data suggests that early after SCI, immune cells infiltrate via a disrupted BSCB, however chronically, the barrier returns to a mostly intact state. At accessory pain regions, resident immune cells such as microglia may be responsible for the propagation and maintenance of local inflammation. Future experiments will explore the polarization states of infiltrating and resident immune cells as well as mechanisms of vascular permeability.
References
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Presenting Author
Grace A. Giddings
Poster Authors
Topics
- Models: Chronic Pain - Neuropathic