Time course of hyperalgesia in the rat chronic constriction injury model of neuropathic pain

Abstract

Background: Patients with insults to the somatosensory system may suffer from great debilitating pain with the presence of sharp and burning neuropathic pain that is largely resistant to treatment. This pain is spontaneous in most patients but may manifest as both hyperalgesia and/or allodynia. Objective: The aim of the current research was to investigate changes in the latency of paw withdrawal to noxious heat stimuli after nerve injury. Material and methods: The experiments were performed in six male Lewis rats (Charles River Laboratories, Wilmington, MA, USA). Chronic Constriction Injury (CCI) was performed on three rats and the other three rats were subjected to sham operation. The rats underwent three rounds of habituation and baseline measurements of latencies of ipsilateral paw withdrawal were taken one day before the operation (Day 1). After the operation, heat latencies of ipsilateral paw withdrawal were measured in CCI and sham rats on Days 1, 3, 6, 8, 10, 13, 15 and 21. Result: The results show that sham rats did not develop hyperalgesia but in CCI rats there was a significant decrease in the heat latencies between baseline and Day 8. Over the rest of the time points, the mean latency started to increase indicting recovery. There was some variability in both groups. Conclusions: These changes in the time course of hyperalgesia may be related to immune cell activation and cytokine production at different time points. This was an animal study, yet it may pave the way to understanding similar conditions in humans. Acknowledgements: Dr Karima Kahlat for supervising this project.