Research paperL5 spinal nerve axotomy induces sensitization of cutaneous L4 Aβ-nociceptive dorsal root ganglion neurons in the rat in vivo
Introduction
Chronic peripheral neuropathic pain (PNP) often results from partial injury of a peripheral nerve. It affects 6–8% of the general population [1], and is inadequately controlled by currently available drugs. PNP is characterized, in humans, by ongoing/spontaneous pain and hypersensitivity to normally painful (hyperalgesia) and non-painful (allodynia) stimuli [2]. Several animal models have been developed to investigate the pathophysiology of PNP including the widely used L5 spinal nerve ligation/axotomy (SNL/SNA) model [3]. The advantage of this model is that the directly axotomized L5-dorsal root ganglion (DRG) neurons are separated from the spared “uninjured“ L4-DRG neurons, thereby allowing investigation of the relative contributions to SNL/SNA-induced PNP of directly injured L5-DRG neurons and adjacent ipsilateral L4-neurons (with conducting fibers intermingling with the degenerating axotomized L5-fibers in the same peripheral nerve).
Preclinical studies using the SNL/SNA model suggest that not only injured/axotomized L5-DRG neurons are involved in the development of PNP, but the adjacent L4-DRG neurons also play an important role (see e.g. [4]). Indeed, there is growing and compelling evidence, from animal studies using the 5 SNL/SNA model, for a major contribution of L4-neurons to PNP including: (1) the necessity of the L4-afferent neurons for transmission of evoked pain signals to the CNS; (2) spontaneous firing in L4C- and A-fiber neurons [5], [6], [7], [8], [9]; (3) up-regulation and phenotypic changes in expression of neuropeptides and neurotransmitters/neuromodulators in L4-DRG neurons [10], [11], [12]; 4) attenuation of mechanical hypersensitivity following elimination of L4C-afferents with local capsaicin treatment [13]; 5) desensitization of L4 Aβ-low threshold mechanoreceptive (LTM) neurons to mechanical stimuli [14] and 6) sensitization of L4C-and Aδ-fiber nociceptive DRG neurons [14], [15]. However, whether cutaneous L4 Aβ-nociceptive DRG neurons also become sensitized after L5 spinal nerve injury remains unanswered.
Most Aβ-fiber DRG neurons are LTMs, but Aβ-nociceptors constitute a substantial group of these neurons (see [16]). Axotomized L5 Aβ-fiber neurons have been implicated in tactile allodynia following L5-spinal nerve injury, but these are believed to be Aβ-LTMs [6]. L4C-and Aδ-fiber nociceptive DRG neurons have also been implicated in development of PNP associated with spinal nerve injury [14], [15], but it is possible that the conducting/uninjured L4 Aβ-nociceptors also become sensitized after L5 SNL/SNA and thereby contribute to the symptoms of PNP. Therefore, the primary aim of the present study was to examine this hypothesis. It should be pointed out that sensitization of primary afferent nociceptors (peripheral sensitization) is characterized by spontaneous activity (SA), lowered stimulus threshold and/or enhanced response to suprathreshold stimuli (e.g. [17].
Section snippets
Experimental animals
The experiments were conducted on young female Wistar rats (180–220 g, Charles River, U.K.). Prior to the electrophysiological experiments, the rats were housed in a room maintained at room temperature between 22 and 24 °C while under a 12 h (h) dark and light cycle, with soft bedding and access to food and water at libitum. All experimental procedures were approved by the University of Liverpool Ethical review group, and complied with the 1986 UK Scientific Procedures Animals Act.
Animal model of peripheral neuropathic pain (PNP)
The L5 SNA model
SNA rats exhibit behavioral sign of mechanical hypersensitivity
Comparison of pain behavior values in rats (n = 12) 7 days after L5 SNA with pre-operated (baseline) values, showed significant decreases (P < 0.001, paired t-test) in paw withdrawal threshold to a mechanical stimulus, indicating development of mechanical hypersensitivity 7 days after SNA (Fig. 1). All the behavioral experiments were conducted in a blinded fashion.
Sampled cutaneous L4 Aβ-nociceptive DRG neurons
Intrasomal recordings were made from a total sample of 67 L4 Aβ-nociceptive DRG neurons with receptive fields in the glabrous skin of the
Discussion
The aim of the present study was to examine the hypothesis that cutaneous L4 Aβ-nociceptors become sensitized after L5 SNA. Consistent with this hypothesis, the results show, for the first time, that in this model of PNP, cutaneous L4 Aβ-nociceptors become sensitized 7 days after L5 SNA. This sensitization was evidenced by: (1) significant decreases in the mean mechanical and dorsal root electrical thresholds; (2) a significant increase in the mean response to a suprathreshold mechanical
Acknowledgements
The author is thankful to the College of Medicine Research Centre and Deanship of Scientific Research, King Saud University, Riyadh, Saudi Arabia, for supporting this work. This work was also supported by grants to LD from the UK MRC and BBSRC.
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