Contribution of injured and uninjured dorsal root ganglion neurons to pain behavior and the changes in gene expression following chronic constriction injury of the sciatic nerve in rats

Abstract

Neuropathic pain models, such as the chronic constriction injury (CCI) model, are partial nerve injury models where there exist both intact and injured peripheral axons. Recent studies suggested that dorsal root ganglion (DRG) neurons with intact axons also show the alteration of excitability and gene expression and might have some role in the pathophysiological mechanisms of neuropathic pain. The incidence of pain-related behavior after the CCI is unstable and variable. In the present study, we used activating transcription factor 3 (ATF3) expression as a neuronal injury marker, and analyzed a relationship between the number of axotomized neurons and the incidence of pain-related behavior. We divided all rats into three groups according to the percentage of ATF3-immunoreactive (IR) neurons, group 1 (<12.5%), group 2 (12.5-25%), and group 3 (>25%). We found that rats in groups 2 and 3 showed thermal hyperalgesia, whereas only the rats in group 2 developed tactile allodynia from the third day to the fourteenth day after surgery. Rats in group 1 did not show thermal hyperalgesia or tactile allodynia. The DRG neurons in group 2 contained ATF3-IR neurons mainly in medium- and large-sized neurons. In order to investigate brain-derived neurotrophic factor (BDNF) and gamma-aminobutyric acid(A)-receptor (GABA(A)-R) regulation in both intact and injured primary afferent neurons after the CCI, we used a double-labeling method with immunohistochemistry and in situ hybridization, as well as double immunofluorescent staining. The CCI induced an increased number of BDNF-labeled neurons in the ipsilateral DRG and the increase in BDNF expression was observed mainly in small- and medium-sized neurons that were mainly ATF3-negative. On the other hand, the number of GABA(A)-Rgamma2 subunit mRNA-positive neurons decreased in the ipsilateral DRG and GABA(A)-R- and ATF3-labeled neurons rarely overlapped. These changes in molecular phenotype in intact and injured primary afferents may be involved in the pathophysiological mechanisms of neuropathic pain produced by partial nerve injury.

Differential Implication of Proinflammatory Cytokine Interleukin-6 in the Development of Cephalic versus Extracephalic Neuropathic Pain in Rats

“…These results confirm and extend previous data on this model (Chudler et al, 1997;Tsujino et al, 2000;Sweitzer et al, 2001;Obata et al, 2003;Lee et al, 2004). We also report a similar induction of these genes in the TG of IoN-CCI rats.…”

“…Moreover, both the overexpression of these cytokines and the associated pain behavior could be prevented in injured animals by minocycline, a semisynthetic second-generation tetracycline described as an inhibitor of microglial activation in animal models of neuropathic pain (Raghavendra et al, 2003;Ledeboer et al, 2005;Piao et al, 2006;Scholz and Woolf, 2007), further accrediting the idea that this cell type plays a key role in neuropathic pain induction. However, upregulation of activating transcription factor 3 (ATF3), a marker of neuronal damage/injury (Tsujino et al, 2000;Hai and Hartman, 2001), is also associated with the establishment of longlasting pain conditions (Obata et al, 2003;Shortland et al, 2006), indicating that neuronal injury is also involved in neuropathic pain.…”

Dorsal root compression produces myelinated axonal degeneration near the biomechanical thresholds for mechanical behavioral hypersensitivity

“…Within the first week after neuronal compression, neurons exhibit decreased firing thresholds and increased action potential durations . During that time frame, the progression of Wallerian degeneration can alter electrophysiology and membrane properties in compressed neurons and increase signaling of leukocytes and chemokines, enhancing neuronal excitability in adjacent, surviving axons (Boucher et al, 2000;Li et al, 2000;Brack and Stein, 2003;Obata et al, 2003;Song et al, 2003;White et al, 2005;Tan et al, 2006). While surviving axons can produce aberrant signals after compression, regenerating A-fibers may also re-map to the superficial laminae of the spinal cord (Ramer et al, 1997;Myers, 1999, 2000;Watanabe et al, 2007).…”

“…Axonal degeneration and inflammation can lead to increased electrical activity in adjacent intact axons, and can result in persistent allodynia-like behavioral hypersensitivity (Li et al, 2000;Obata et al, 2003). Axonal injury can also produce a breakdown in fast axonal transport, producing accumulations of transported proteins such as neurofilament (NF) and β-amyloid precursor protein (βAPP) (Chen et al, 1999;Chung et al, 2005;Singh et al, 2006).…”

An evolving cellular pathology occurs in dorsal root ganglia, peripheral nerve and spinal cord following intravenous administration of paclitaxel in the rat

“…The cell size distribution of ATF3 in neuronal subsets was determined by measuring the soma area (Image Pro Plus v 3.0) of individual ATF3 and NeuN positive profiles in three paclitaxel-treated rats per each time point. Results are reported as the percentage of labeled cells within each size population according to previously established size criteria (Obata et al, 2003) small (<600 μm 2 ), medium (600-1200 μm 2 ) and large (>1200 μm 2 ) neurons. The number of nodules of Nageotte was determined by counting the total number of clusters (> 8 adjacent nuclei) of ATF3-IR satellite cells in the entire area of a DRG section from a total of 4 sections per ganglion (Peters et al, 2007).…”