Abstract
Musculoskeletal pain conditions, particularly those associated with temporomandibular disorders (TMD) affect a large percentage of the population. Identifying mechanisms underlying hyperalgesia could contribute to the development of new treatment strategies for the management of TMD and other muscle pain conditions. In this study, we provide evidence of functional interactions between two ligand-gated channels, P2X&sub3; and transient receptor potential V1 (TRPV1), in trigeminal sensory neurons, and propose that the interactions serve as an underlying mechanism for the development of mechanical hyperalgesia. Mechanical sensitivity of the masseter muscle was assessed in lightly anesthetized rats via an electronic anesthesiometer (Ro et al., 2009). Direct intramuscular injection of a selective P2X&sub3; agonist, alpha,beta-methylene adenosine triphosphate (αβmeATP), induced a dose- and time-dependent hyperalgesia. Mechanical sensitivity in the contralateral muscle was unaffected suggesting local P2X&sub3; mediate hyperalgesia. Anesthetizing the overlying skin had no effect on αβmeATP-induced hyperalgesia confirming the contribution of P2X&sub3; from the muscle. Importantly, the αβmeATP-induced hyperalgesia was prevented by pretreatment of the muscle with a TRPV1 antagonist, AMG9810. P2X&sub3; was co-expressed with TRPV1 in the masseter muscle afferents confirming the possibility for intracellular interactions. Additionally, in a subpopulation of P2Xv/TRPV1 positive neurons, capsaicin-induced Ca(2+) transients were significantly amplified following P2X&sub3; activation. Finally, activation of P2X&sub3; induced phosphorylation of serine, but not threonine, residues in TRPV1 in trigeminal ganglia cultures. Significant phosphorylation was observed at 15 min, the time point at which behavioral hyperalgesia was prominent. Previously, activation of either P2X&sub3; or TRPV1 had been independently implicated in the development of mechanical hyperalgesia. Our data propose P2X&sub3; and TRPV1 interact in a facilitatory manner, which could contribute to the peripheral sensitization known to underlie masseter hyperalgesia.