Elsevier

Neuroscience Letters

Volume 624, 15 June 2016, Pages 72-77
Neuroscience Letters

Research paper
L5 spinal nerve axotomy induces sensitization of cutaneous L4 Aβ-nociceptive dorsal root ganglion neurons in the rat in vivo

https://doi.org/10.1016/j.neulet.2016.05.008Get rights and content

Highlights

  • Cutaneous L4 Aβ-nociceptors became sensitized 7 days after L5-spinal nerve axotomy (SNA). They exhibited:

  • Decreased mechanical and electrical thresholds and enhanced responsiveness to suprathreshold mechanical stimulus.

  • An apparent increase in incidence of spontaneous activity.

  • These changes in L4 Aβ-nociceptors may contribute to peripheral neuropathic pain.

Abstract

Partial nerve injury often leads to peripheral neuropathic pain (PNP), a major health problem that lacks effective drug treatment. PNP is characterized by ongoing/spontaneous pain, and hypersensitivity to noxious (hyperalgesia) and innocuous (allodynia) stimuli. Preclinical studies using the L5 spinal nerve ligation/axotomy (SNL/SNA) model of PNP suggest that this type of chronic pain results partly from sensitization of ipsilateral L4C-and Aδ-fiber nociceptive dorsal root ganglion (DRG) neurons, but whether L4 β-nociceptors, which constitute a substantial group of DRG neurons, also become sensitized remains unanswered. To address this issue, intracellular recordings from somata of cutaneous Aβ-nociceptors (classified according to their dorsal root conduction velocities (>6.5 m/s), and physiologically based on their responses to noxious (but not innocuous) mechanical stimuli) were made from L4-DRGs in normal (control) rats and in rats seven days after L5 SNA in vivo. Compared with control, cutaneous L4 Aβ-nociceptive DRG neurons in SNA rats (that developed mechanical hypersensitivity) exhibited sensitization indicated by: a) decreased mean mechanical threshold (from 57.8 ± 7.1 to 10.3 ± 1.7 mN), b) decreased mean dorsal root electrical threshold (from 11.4 ± 0.7 to 4.3 ± 0.4 V), c) increased mean response to a suprathreshold mechanical stimulus (from 18.5 ± 1.8 to 34 ± 3.7 spikes/sec) and d) an obvious, but non-significant, increase in the incidence of ongoing/spontaneous activity (from 3% to 18%). These findings suggest that cutaneous L4 Aβ-nociceptors also become sensitized after L5 SNA, and that sensitization of this subclass of A-fiber nociceptors may contribute both directly and indirectly to nerve injury-induced PNP.

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.

References (28)

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