Characteristics of sensory DRG neurons innervating the wrist joint in rats.

BACKGROUND: Wrist pain can be the result of trauma, or inflammatory processes such as arthritis or synovitis. There is evidence that sensory nerve fibers are present in the wrist joints of animals and humans; however, the sensory innervation pattern of the wrist, as well as the types of nerves innervating it, have not been clarified. The purpose of this study was to characterize the types of sensory dorsal root ganglion (DRG) neurons innervating the wrist joint in the rat.

METHODS: In this study, retrograde neurotransport was combined with lectin affinity histochemistry and immunohistochemistry to characterize DRG neurons innervating the wrist joint in rats. We used 3 markers: calcitonin gene-related peptide (CGRP) as a marker of small, peptide-containing neurons associated with inflammatory pain; the glycoprotein binding the isolectin from Griffonia simplicifolia (IB4) for small, non-peptide-containing neurons related to transmission of pain following nerve injury; and neurofilament 200 (NF200) for small and large myelinated fibers. IB4-binding and CGRP-containing neurons are typically involved in pain sensation, whereas NF200 is associated with pain and proprioception.

RESULTS: Neurons innervating the wrist joints, retrogradely labeled with fluoro-gold (FG), were distributed throughout DRGs from C6 to T1. Of all of the FG labeled neurons, the percentage of NF200 immunoreactive (IR) neurons and CGRP-IR neurons were 26% and 45%, respectively. The percentage of IB4-binding neurons was 3%, significantly less than the ratio of CGRP-IR neurons to the total FG labeled neurons.

CONCLUSION: Under physiological conditions in rats, DRG neurons transmit several types of sensation from the wrist joint including proprioception and pain. Most of the labeled neurons were CGRP-IR peptide containing neurons. It is likely that these neurons are the predominant afferents for inflammatory pain signals from the wrist. Because peptide-containing neurons are associated with inflammatory pain, it is likely that the inflammation in the wrist joint causes wrist joint pain.