European journal of pain : EJP
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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. ⋯ 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.
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Neuropeptide-expressing small diameter sensory neurones are thought to be vital in generating inflammatory hyperalgesic responses. Within the dorsal root ganglion (DRG), both the levels of the neuropeptide calcitonin gene-related peptide (CGRP) and the numbers of CGRP-immunoreactive (CGRP-IR) DRG neurones have been shown to increase in a number of acute adjuvant-induced inflammatory pain models. The aim of this study was to look specifically at changes in numbers of CGRP-IR DRG neurones in a chronic model of inflammatory joint pain following complete Freund's adjuvant (CFA) injection into the rat knee. ⋯ Following dosing of CFA-injected rats with rofecoxib (Vioxx) or paracetamol, there was a significant decrease in the number of ipsilateral CGRP-IR small and medium DRG neurones in rofecoxib- but not paracetamol-treated rats. These data also correlated with behavioural readouts where hypersensitivity and knee joint inflammation were significantly reduced by rofecoxib but not paracetamol treatment. In conclusion, these data show that changes in ipsilateral CGRP expression within small DRG neurones are consistent with behavioural readouts in both time course, rofecoxib and paracetamol studies in this model of chronic inflammatory pain.