α₁ adrenergic receptor activation has a dynamic effect on masticatory muscle afferent fibers

Temporomandibular Disorder (TMD) patients report amplification of pain in the masticatory muscles after psychological trauma or stressful conditions. The mechanisms underlying this phenomenon are yet to be elucidated. This study combined immunohistochemistry with single cell in vivo electrophysiology recordings of masticatory muscle afferent fibers to investigate the role of α₁-adrenergic receptors in muscle nociception. It was found that a subset of trigeminal afferent fibers which innervate the masseter and temporal muscles expressed α_1a, α_1b and α_1d receptors, including a smaller number of putative nociceptors which co-expressed TrpV₁ receptors. Local injection of the selective α₁ adrenergic receptor agonist phenylephrine into masticatory muscle decreased and increased the mechanical activation threshold of slow and fast conducting afferent fibers, respectively. This effect was reversed by co-administration of the α₁ selective antagonist terazosin. To rule out the possibility that local ischemia was responsible for the observed effect of phenylephrine on masticatory muscle afferent fibers, additional experiments were conducted where blood flow to the masticatory muscle was reduced by common carotid artery occlusion. This investigation found that muscle blood flow occlusion increased the mechanical activation threshold of the majority of masticatory muscle afferent fibers unrelated to conduction velocity. These findings suggest that under conditions of increased sympathetic tone, such as those related to stress, noradrenaline may sensitize masticatory muscle nociceptors to increase pain and desensitize muscle proprioceptors to alter muscle tone, through activation of α₁ receptors.