Elsevier

Experimental Neurology

Volume 214, Issue 2, December 2008, Pages 219-228
Experimental Neurology

Governing role of primary afferent drive in increased excitation of spinal nociceptive neurons in a model of sciatic neuropathy

https://doi.org/10.1016/j.expneurol.2008.08.003Get rights and content

Abstract

Previously we reported that the cuff model of peripheral neuropathy, in which a 2 mm polyethylene tube is implanted around the sciatic nerve, exhibits aspects of neuropathic pain behavior in rats similar to those in humans and causes robust hyperexcitation of spinal nociceptive dorsal horn neurons. The mechanisms mediating this increased excitation are not known and remain a key unresolved question in models of peripheral neuropathy. In anesthetized adult male Sprague–Dawley rats 2–6 weeks after cuff implantation we found that elevated discharge rate of single lumbar (L3–4) wide dynamic range (WDR) neurons persists despite acute spinal transection (T9) but is reversed by local conduction block of the cuff-implanted sciatic nerve; lidocaine applied distal to the cuff (i.e. between the cuff and the cutaneous receptive field) decreased spontaneous baseline discharge of WDR dorsal horn neurons ∼ 40% (n = 18) and when applied subsequently proximal to the cuff, i.e. between the cuff and the spinal cord, it further reduced spontaneous discharge by ∼ 60% (n = 19; P < 0.05 proximal vs. distal) to a level that was not significantly different from that of naive rats. Furthermore, in cuff-implanted rats WDR neurons (n = 5) responded to mechanical cutaneous stimulation with an exaggerated afterdischarge which was reversed entirely by proximal nerve conduction block. These results demonstrate that the hyperexcited state of spinal dorsal horn neurons observed in this model of peripheral neuropathy is not maintained by tonic descending facilitatory mechanisms. Rather, on-going afferent discharges originating from the sciatic nerve distal to, at, and proximal to the cuff maintain the synaptically-mediated gain in discharge of spinal dorsal horn WDR neurons and hyperresponsiveness of these neurons to cutaneous stimulation. Our findings reveal that ectopic afferent activity from multiple regions along peripheral nerves may drive CNS changes and the symptoms of pain associated with peripheral neuropathy.

Introduction

Neuropathic pain is a dysfunctional and debilitating pain that remains intractable to most current treatment strategies (Finnerup et al., 2007, Hawksley, 2006, Vissers, 2006). The etiology of neuropathic pain is varied in that the causes can include infection (e.g. varicella-zoster virus leading to post-herpetic neuralgia, neuritis, HIV), metabolic disorders (e.g. diabetes, vitamin deficiency), toxins/drugs (e.g. chemotherapy), autoimmune disease (e.g. Guillain–Barre syndrome), and physical trauma to the nerve and other structures. Neuropathic pain is distinguishable from acute pain in that it may be exaggerated (hyperalgesia), provoked by innocuous stimulation (dysesthesia, allodynia), and/or occurs spontaneously. It is commonly thought that pain resulting from peripheral neuropathy is triggered by discharge in the freshly injured nerve that generates multiple pathophysiological, transcriptional, translational, neurochemical, and structural changes in sensory processing in pain pathways. The current state of understanding of this type of pain is that ectopic discharge from both damaged and also neighboring intact/surviving primary afferent fibers, produced by the effects of Wallerian degeneration (Obata et al., 2004), develops (Devor and Wall, 1990, Kovalsky et al., 2008, Lee et al., 2003, Li et al., 2000, Obata et al., 2003, Wu et al., 2002, Yoon et al., 1996), and elevates discharge of spinal nociceptive neurons (Pitcher and Henry, 2000, Pitcher and Henry, 2004, Sotgiu et al., 1995a), inducing changes in sensory processing in the spinal dorsal horn and in supraspinal structures.

There is a lack of consensus, however, regarding the locus of the governing change that maintains neuropathic pain and, in fact, different components of the somatosensory axis have been proposed to maintain this pain. One hypothesis suggests that central sensitization at the level of the spinal dorsal horn is maintained independently of primary afferent input (Sandkuhler and Liu, 1998) since input from the periphery is reportedly insufficient to maintain spinal modifications in sensory processing and ultimately neuropathic pain (Burgess et al., 2002, Sun et al., 2005, Xie et al., 2005). Another view suggests that mechanisms at the spinal dorsal horn level require continuous facilitatory input from supraspinal structures (Bian et al., 1998, Carlson et al., 2007, Gardell et al., 2003, Kauppila et al., 1998, Kovelowski et al., 2000, Ossipov et al., 2000, Pertovaara et al., 1997, Pertovaara et al., 2001, Porreca et al., 2001, Saade et al., 2006a, Saade et al., 2006b, Saade et al., 2007, Suzuki et al., 2002, Suzuki et al., 2004a, Suzuki et al., 2004b, Vera-Portocarrero et al., 2006).

However, in both animal models and humans effects of peripheral neuropathy including peripheral neurodegeneration, altered Na+ and K+ ion channel expression (Coward et al., 2001a, Coward et al., 2001b, Hong et al., 2004, Hong and Wiley, 2006, Joshi et al., 2006, Matthews et al., 2006, Shembalkar et al., 2001), and abnormal impulse discharge (Nordin et al., 1984, Nystrom and Hagbarth, 1981) in sensory afferents have been shown to persist several weeks/months (Casula et al., 2004, Coward et al., 2000, Fried et al., 1991, Kretschmer et al., 2002, Lin et al., 2001, Pan et al., 2001, Pertin et al., 2005, Roytta et al., 1999, Seltzer et al., 1991b). This raises the question what role, if any, do such multiple and long lasting modifications in peripheral sensory input play in maintaining the persistent changes in the central nervous system and in neuropathic pain?

In earlier studies on a rat model of neuropathic pain, in which a polyethylene cuff had been placed around the sciatic nerve, we observed spontaneous pain and tactile hypersensitivity that lasted several weeks (Pitcher et al., 1999a). Electrophysiological recordings made in the same neuropathic animals showed a gain in rate of spontaneous on-going discharge of wide dynamic range (WDR) dorsal horn neurons and gain in magnitude and duration of the afterdischarge of these neurons in response to stimulation of the peripheral cutaneous receptive field at the same time points weeks after cuff implantation correlating with neuropathic pain behaviors (Pitcher and Henry, 2000, Pitcher and Henry, 2004). This increased excitation of WDR neurons was not mediated by descending supraspinal input as the rats in these experiments were spinalized. What was not clear was whether this increased excitation was due to post-synaptic changes in the spinal dorsal horn or whether it was maintained by abnormal input from primary afferents. Thus, the present study was aimed to answer whether primary afferent input at time points well after the onset of peripheral neuropathy (i.e. several weeks) plays a role in mediating increased excitation of spinal dorsal horn neurons. We investigated this by recording from single lumbar nociceptive dorsal horn neurons in spinalized cuff-implanted rats and applying lidocaine directly to the sciatic nerve during spontaneous on-going activity or during the afterdischarge provoked by mechanical cutaneous stimulation of the cuff-implanted hind paw. We provide evidence that hyperexcited nociceptive dorsal horn neurons in rats with chronic peripheral neuropathy is reversed by application of lidocaine to the sciatic nerve proximal as well as distal to the cuff.

Section snippets

Materials and methods

Experiments were done using adult, male Sprague–Dawley rats (375–425 g) from Harlan Sprague Dawley, Inc. (Indianapolis, Indiana, USA). They were housed in plastic cages containing wood chip bedding (Hardwood Laboratory Bedding, Northeastern Products Corp., Warrensburg, NY, USA) and maintained on a 12:12 h light:dark cycle (lights on at 07:00 h) with access to food and water ad libitum. Guidelines in The Care and Use of Experimental Animals by the Canadian Council on Animal Care were followed

Results

All cuff-implanted rats used in this study exhibited increased tactile sensitivity in the von Frey filament test from day 14 and onward (data not shown) similar to that reported previously (Pitcher et al., 1999a, Pitcher and Henry, 2000, Pitcher and Henry, 2004).

Discussion

This study demonstrates that local anesthetic block of the sciatic nerve in the cuff model of peripheral neuropathy in rats reverses the abnormally increased spontaneous on-going discharge activity in lumbar dorsal horn WDR neurons and also reverses the abnormally exaggerated afterdischarge of these neurons following a noxious mechanical stimulus to the hind paw cutaneous receptive field. The data thus suggest that a constitutively active afferent input originating from multiple ectopic regions

Acknowledgments

This study was supported by grants from the Canadian Institutes of Health Research to JLH. GMP was a student supported by awards from the Royal Victoria Hospital Research Institute, the McGill Faculty of Medicine, and the Fonds pour la formation de chercheurs et l'aide a la recherche (Province of Quebec). The authors are grateful to Dr. Ze'ev Seltzer for critical reading of the manuscript and to Jennifer Ritchie for excellent technical assistance.

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