β 2 -adrenergic receptor activation decreases the mechanical sensitivity of rat masticatory muscle afferent fibres.

Background: Activation of β ₂ adrenergic receptors reduces cutaneous mechanical pain thresholds in rats. While β ₂ adrenergic receptor activation may contribute to mechanisms that underlie temporomandibular joint pain, its effect on masticatory muscle pain sensitivity is uncertain.
Objectives: The current study sought to determine the extent to which β adrenergic receptors are expressed by masticatory muscle afferent fibres, and to assess the effect of local activation of these receptors on the mechanical sensitivity of masticatory muscle afferent fibres in rats.
Methods: Trigeminal ganglion neurons that innervate the rat (n = 12) masseter muscle and lower lip were identified by tissue injection of fluorescent dyes and were then stained with antibodies against β ₁ or β ₂ adrenergic receptors. Extracellular recordings from 60 trigeminal ganglion neurons that innervate the masticatory muscle were undertaken in a second group of anaesthetised rats of both sexes (n = 37) to assess afferent mechanical activation thresholds. Thresholds were assessed before and after injection of the β adrenergic receptor agonists into masticatory muscle.
Results: β ₁ and β ₂ adrenergic receptor expression was greater in labial skin than in masticatory muscle ganglion neurons (p < .05, one-way ANOVA, Holm-Sidak test). There was a higher expression of β ₂ adrenergic receptors in masticatory muscle ganglion neurons in males than in females. The mixed β agonist isoproterenol increased afferent mechanical activation threshold in male but not female rats (p < .05, Mann-Whitney test). In male rats, salbutamol, a β ₂ selective agonist, also increased afferent mechanical activation threshold but hydralazine, a vasodilator, did not (p < .05, Mann-Whitney test).
Conclusion: Activation of β ₂ adrenergic receptors decreases the mechanical sensitivity of masticatory muscle afferent fibres in a sex-related manner.
(© 2024 The Author(s). Journal of Oral Rehabilitation published by John Wiley & Sons Ltd.)