Chronic pain is a major healthcare concern that directly affects over one hundred million people in the United States alone. While current treatment options like opioids and NSAIDs are effective, they are with significant drawbacks that prevent long term use. It is important to identify and understand new druggable targets for the treatment of pain. Recent findings have demonstrated substance P functions in the RVM to maintain hypersensitivity to noxious heat stimuli in models of persistent peripheral inflammatory injury in a manner dependent on presynaptic NMDA receptors. What remains unclear is how substance P assumes this pronociceptive role following peripheral inflammatory injury. The experiments detailed in this thesis investigated whether the levels and or release of substance P in the RVM was altered following peripheral inflammatory injury.
The effect of peripheral inflammatory injury on levels of substance P in the RVM was tested at several time points. The data show that there were no changes in substance P levels in the ipsilateral or contralateral RVM of CFA injected rats compared to their saline controls at any of the time points tested. To assess whether changes in substance P levels occurred in a subset of neurons within the RVM, computer aided densitometry analysis was used to measure substance P immunoreactivity in sections from the RVM of rats treated with CFA or saline. Substance P immunoreactivity was increased in the ipsilateral RVM of the CFA group compared to the corresponding saline sections at the 4 day, but not the 2 week time point. No other changes were observed.
Electron microscopy was used to demonstrate the presence of the NMDA receptor and substance P on the same axon terminals within the RVMs of rats treated with either CFA or saline. This colocalization is significant because it identifies NMDA receptors in position to regulate the release of substance P from axon terminals in the RVM. There were no obvious differences in the degree of colocalization between CFA and saline groups. Functional experiments were devised that tested whether substance P release (basal and evoked) in the RVM was increased following peripheral inflammatory injury, and whether said release was regulated by NMDA receptors. The data show that neither basal nor evoked (potassium or veratridine) release was increased following peripheral inflammatory injury. NMDA was able to facilitate the release of substance P in both the CFA and saline treatment groups, but the facilitation was not different between groups. In the absence of any depolarization stimulus, NMDA was unable to elicit any release of substance P beyond basal values.
All told, the data show substance P levels in the RVM are not altered by peripheral inflammatory injury. Additionally, neither basal nor evoked release of substance P is altered by peripheral inflammatory injury. The data provide functional and anatomical evidence for modulation of substance P release by glutamate acting at presynaptic NMDA receptors, but do not support the idea of differential modulation of substance P release following peripheral inflammatory injury.