A novel animal model of graded neuropathic pain: Utility to investigate mechanisms of population heterogeneity

Abstract

The mechanisms underlying neuropathic pain are not well understood, resulting in unsatisfactory treatment outcomes for many patients. Animal models underpin much of the current understanding of pain mechanisms due to their perceived ability to mimic pain hypersensitivities; however, are limited by their binomial approach (pain vs. control), which does not reflect the clinical heterogeneity in nociceptive hypersensitivity. We modified the chronic constriction injury model by varying the number of sciatic nerve chromic gut sutures. Each Sprague Dawley rat received 4 pieces of chromic gut to control for the inflammatory challenge posed by the gut. Treatment groups were neuronal sutures (N), subcutaneous sutures (S) N0S0, N0S4, N1S3, N2S2 and N4S0. At postoperative (PO) day 29, there was a ‘dose–response’ relationship between the number of perineural sutures and von Frey threshold (N0S4 < N1S3 < N2S2 < N4S0, P < 0.05). This graded model was applied to investigate lumbar dorsal spinal cord glial activation marker expression. Microglial CD11b expression was positively correlated with graded allodynia in the ipsilateral dorsal horn (P < 0.05, r² > 0.9) and associated in the dorsolateral funiculus (DLF; P = 0.10, r² > 0.8) at PO day 14. Astrocyte GFAP expression was positively associated with graded allodynia in the ipsilateral dorsal horn (P = 0.18, r² > 0.6) and ipsilateral DLF (P < 0.05, r² > 0.9). DLF glial activation may represent a contributor to contralateral pain. Our novel graded model has a dynamic range, allowing sensitive detection of interactions and subtle influences on neuropathic pain processing.

Introduction

Chronic pain is a debilitating condition affecting millions of people world wide (Dworkin et al., 2007) and poses a significant financial burden to society (Access Economics, 2007). Neuropathic pain differs from other pain conditions in that it principally results from primary damage of the nervous system and may be unaccompanied by ongoing injury. Symptoms include hypersensitivity to noxious (hyperalgesia) and non-noxious (allodynia) stimuli. The pathophysiological mechanisms underlying this kind of pain are incompletely understood, though recent evidence suggests a central nervous system immune-like component (Milligan and Watkins, 2009). At present, there are no approved disease-modifying treatments, only symptomatic therapies such as opioids, antidepressants and anticonvulsants. For these reasons, neuropathic pain remains the subject of intense current investigation.

Many animal models of neuropathic pain have been shown to mimic certain aspects of human neuropathic pain, including allodynia and hyperalgesia, and have been valuable in pain research and drug development (Costigan et al., 2009). The most well-established of these include either a loose ligation or chronic constriction of the entire sciatic nerve using chromic gut sutures designed to produce damage to only some of the sciatic axons by inducing swelling and then strangulation (Bennett and Xie, 1988) or a tight ligation of half the proximal sciatic nerve (Seltzer et al., 1990). The other commonly used model involves a tight ligation of two spinal segmental nerves, L5 and L6, close to the dorsal root ganglion (Kim and Chung, 1992). Whilst these animal models do mimic symptoms associated with human pain, they are not without limitations. One major limitation is that clinically, patients present with heterogeneous pain sensitivities, ranging from mild to severe. However, to our knowledge, there are no well accepted animal models that adequately capture this range of increased pain sensitivity.

Hence, we aimed to modify an existing model of neuropathic pain, the chronic constriction injury (CCI) model (Bennett and Xie, 1988) to develop a graded behavioral neuropathic pain model of the clinical heterogeneous phenotype. As an example of the wider application of our novel model, our secondary aim was to investigate expression of glial activation markers in the dorsal lumbar spinal cord, as a quantifiable biological process of neuropathic pain.