properties of the masticatory muscle primary afferent neurons from rat trigeminal ganglion

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Masticatory muscle pain is one of the important symptoms of the tempolomandibular disorders. In this project, properties of the masticatory muscle primary afferent neurons from the rat trigeminal ganglion were investigated. The distribution and modulation of the P2X_3 receptor was studied to provide insight into the role of ATP in craniofacial sensory mechanisms. Binding to the D-galactose specific lectin IB4 was found in 73% of P2X_3-positive neurons while only 16% of IB4 neurons expressed P2X_3. Neurons expressing P2X_3 alone were significantly larger than IB4-or IB4/P2X_3-positive neurons. Investigation of target-specificity revealed that 22% of trigeminal ganglion muscle afferent neurons were positive for P2X_3 versus 16% of cutaneous afferent neurons. Muscle P2X_3 afferents were significantly smaller than the overall muscle afferent population while P2X_3 cutaneous afferent neurons were not. Presumptive heteromeric (P2X_<2/3>) muscle afferent neurons were also identified and comprised 77% of the P2X_3 muscle afferent population. Muscle afferent neurons co-expressed P2X_3 with either calcitonin gene-related peptide (15%) or substance P (4%). The number of P2X_3-positive muscle afferent neurons significantly increased one and four days following complete Freund's adjuvant-induced masseter muscle inflammation, but significantly decreased after 12 days. These results indicate that within trigeminal ganglia : 1.the P2X_3 receptor is expressed in both small and medium-sized neurons ; 2.the P2X_3 receptor is not exclusively expressed in IB4 neurons ; 3.P2X_3 is co-expressed with neuropeptides ; 4.differences in the proportion of cutaneous versus muscle P2X_3 afferents are not apparent. Trigeminal P2X_3 neurons therefore differ markedly from dorsal root ganglion P2X_3 afferents. This study also shows that deep tissue inflammation modulates expression of the P2X_3 receptor and thus may warrant exploration as a target for therapeutic intervention.