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The Journal of Neuroscience, January 25, 2006, 26(4):1281-1292; doi:10.1523/JNEUROSCI.3388-05.2006

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Neurobiology of Disease
Spontaneous Pain, Both Neuropathic and Inflammatory, Is Related to Frequency of Spontaneous Firing in Intact C-Fiber Nociceptors

Laiche Djouhri, Stella Koutsikou, Xin Fang, Simon McMullan, and Sally N. Lawson

Department of Physiology, Medical School, University of Bristol, Bristol BS8 1TD, United Kingdom

Correspondence should be addressed to Dr. Laiche Djouhri, Department of Physiology, University of Bristol, Medical School, University Walk, Bristol BS8 1TD, UK. Email: L.Djouhri{at}bristol.ac.uk

Spontaneous pain, a poorly understood aspect of human neuropathic pain, is indicated in animals by spontaneous foot lifting (SFL). To determine whether SFL is caused by spontaneous firing in nociceptive neurons, we studied the following groups of rats: (1) untreated; (2) spinal nerve axotomy (SNA), L5 SNA 1 week earlier; (3) mSNA (modified SNA), SNA plus loose ligation of the adjacent L4 spinal nerve with inflammation-inducing chromic gut; and (4) CFA (complete Freund’s adjuvant), intradermal complete Freund’s adjuvant-induced hindlimb inflammation 1 and 4 d earlier. In all groups, recordings of SFL and of spontaneous activity (SA) in ipsilateral dorsal root ganglion (DRG) neurons (intracellularly) were made. Evoked pain behaviors were measured in nerve injury (SNA/mSNA) groups. Percentages of nociceptive-type C-fiber neurons (C-nociceptors) with SA increased in intact L4 but not axotomized L5 DRGs in SNA and mSNA (to 35%), and in L4/L5 DRGs 1–4 d after CFA (to 38–25%). SFL occurred in mSNA but not SNA rats. It was not correlated with mechanical allodynia, extent of L4 fiber damage [ATF3 (activation transcription factor 3) immunostaining], or percentage of L4 C-nociceptors with SA. However, L4 C-nociceptors with SA fired faster after mSNA (1.8 Hz) than SNA (0.02 Hz); estimated L4 total firing rates were 5.0 and 0.6 kHz, respectively. Similarly, after CFA, faster L4 C-nociceptor SA after 1 d was associated with SFL, whereas slower SA after 4 d was not. Thus, inflammation causes L4 C-nociceptor SA and SFL. Overall, SFL was related to SA rate in intact C-nociceptors. Both L5 degeneration and chromic gut cause inflammation. Therefore, both SA and SFL/spontaneous pain after nerve injury (mSNA) may result from cumulative neuroinflammation.

Key words: dorsal root ganglion; DRG; pain; neuropathic pain; neuroinflammation; C-fiber; spontaneous pain

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Received Aug. 11, 2005; revised Dec. 5, 2005; accepted Dec. 11, 2005.

Correspondence should be addressed to Dr. Laiche Djouhri, Department of Physiology, University of Bristol, Medical School, University Walk, Bristol BS8 1TD, UK. Email: L.Djouhri{at}bristol.ac.uk

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