Modulation of Purinergic Receptors in a Model for TMJ Hyperalgesia

Objectives: To develop an animal model for persistent temporomandibular joint (TMJ) pain that causes minimal damage to joint tissues and improves translational value. To assess the role of purinergic P2 receptor subtypes in TMJ nociception.
Methods: Adult male, ovariectomized (OvX) female, OvX+estradiol (OvXE) rats (3-7/group) receive a single low non-tissue damaging dose (10µg) of Complete Freund’s Adjuvant (CFA) in the TMJ and survive 4, 7, or 10 days before testing and tissue collection for P2 receptor analyses. Jaw muscle function test = intraTMJ ATP (0.01-1mM) -evoked masticatory muscle electromyography (MMemg). P2 receptor expression by qPCR in samples of: TMJ tissues, trigeminal ganglion (TG), trigeminal brainstem (Vc), isolated microglia in Vc.
Results: ATP-evoked MMemg increases: OvX>Male at 7 and 10 days; OvXE>Male at 4,7,10 days; OvXE>OvX at 4, 7 days after CFA. P2 receptor expression: TMJ=increase in P2X3, P2X5, P2Y2 at 4,7,10d (all groups); TG = increase in P2X2, P2X3, P2Y12, decrease in P2Y1, P2Y2; Vc= increase in P2X7, decrease in P2X3, P2X5, P2Y2; isolated microglia in Vc = increase in P2X4, P2X7, P2Y12, P2Y13 and IL-1beta. Sex differences isolated microglia in Vc: OvXE>Male for P2X4 and P2X7, IL-1beta and NLRP3 inflammasome expression.
Conclusions: Low grade TMJ inflammation causes persistent ATP-evoked hyperalgesia with minimal signs of damage for 10 days. Females display greater MMemg responses than males at all times after CFA. Increase in expression level of P2 receptors associated with neurons (P2X2, P2X3) coupled with decreased P2Y1, P2Y2 suggests that peripheral TG sensitization plays a role in TMJ hyperalgesia, while increases in P2X4, P2X7, P2Y12, and P2Y13 suggest activation of microglia in brainstem contribute to central sensitization by amplifying and maintaining hyperalgesia after even minor joint insult. Purinergic P2 receptors and secretory products of microglia may be significant therapeutic targets for pain management in TMD.