Abstract:

Objective To investigate the morphological and electrophysiological characteristics of muscular nociceptive neurons in adult na?ve rats. Methods Fluorescent dye 1,1’-dioctadecyl-3,3,3’,3’-tetrame-thylindocarbocyanine perchlorate（Dil,200 mg/L）was injected into the right anterior tibialis muscle to label the primary muscular sensory neurons in the dorsal root ganglions (DRGs). Immunohistochemical staining and in vivo electrophysiological recordings of DRG neurons were conducted 7-12 days after injection. Results Primary sensory neurons innervating the anterior tibialis muscle (including small, medium and large sized somata) were mostly located in the lumbar(L)3, L4 and L5 DRGs (number of DiI⁺ neurons: 19.8±7.1, 26.5±8.0 and 14.2±5.0, respectively，n=8). Immunohistochemistry results showed that calcitonia gene related peptide (CGRP)+ (peptidergic) neurons innervating the anterior tibial muscle accounted for 31.5%, 26.1% and 22.3% of DiI⁺ neurons in L3, L4 and L5 DRG, respectively; while isolectin B4(IB4)+ (non-peptidergic) neurons accounted for 45.8%, 45.1% and 40.6% of DiI+ neurons in L3-L5 DRG, respectively. The transient receptor potential vanilloid type-1 (TRPV1)⁺ neurons accounted for 27.2%, 26.3%, and 32.1%, of DiI⁺ neurons in L3-L5 DRG, respectively. Electrophysiological recordings revealed that most of the neurons responding to 40mN or higher force mechanical stimulation were medium and small neurons. The mechanical threshold of nociceptive neurons is generally larger than 80mN (nociceptive sensory neurons), with large and variable peripheral receptive fields. Most of the neurons (8/10) with conduction velocity measured by electrically stimulating the sciatic nerve were C-nociceptive ［conduction velocity<1.5 m/s, average mechanical threshold (124.6±40.5) mN］. Conclusion Most of the primary nociceptive neurons innervating the anterior tibial muscle from L3-L5 DRGs in the rat are small-and medium-sized somata, with high mechanical threshold and large peripheral receptive field. In addition, these neurons may express a number of nociceptive neuronal markers.

Key words: Muscular pain, Nociceptive sensory neuron, Peptidergic neuron, Non-peptidergic neuron, In vivoelectrophysiological recording, Immunohistochemistry, Rat