Resident glial cell activation in response to perispinal inflammation leads to acute changes in nociceptive sensitivity: Implications for the generation of neuropathic pain

Injury or disease affecting the spinal cord is often accompanied by abnormal, chronic pain. Recent estimates suggest that approximately 60% of patients with multiple sclerosis are affected by significant changes in pain sensitivity or experience ongoing neuropathic pain of unknown etiology. Chronic pain is also a significant concern after direct spinal cord trauma. Inflammatory events and the changes in astrocyte and microglia reactivity at the spinal level in response to injury or disease are now recognized as important processes that can initiate pain hypersensitivity. Changes in the structural integrity or permeability of the blood-brain barrier/blood-spinal cord barrier (BBB/BSCB) can facilitate the inflammatory events that result in these abnormal pain states. It remains unclear, however, whether chronic pain in these disorders is dependent on the influx of peripheral leukocytes or whether changes in the reactivity of resident glial cells within the central nervous system alone are sufficient. To address this question, we generated a model of perispinal inflammation that resulted in significant changes in the reactivity of resident astrocytes and microglia within the spinal cord but maintained the integrity of the BSCB. A number of similar changes at the behavioural and cellular level occur in this model that mimic the responses seen in animal models of multiple sclerosis or spinal cord injury (SCI). However, these changes are short lived and resolve over the course of a 2-week observation period. Our findings suggest that the chronicity of pain after injury or disease in the nervous system is dependent on the integrity of the BBB/BSCB.