Trigeminal P2X 3 receptor expression differs from dorsal root ganglion and is modulated by deep tissue inflammation

Abstract

The distribution and modulation of the P2X 3 receptor was studied in trigeminal ganglion neurons 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.