Dental malocclusion stimulates neuromuscular circuits associated with temporomandibular disorders.

Unilateral anterior crossbite (UAC) has been demonstrated to cause masseter hyperactivity via the periodontal trigeminal mesencephalic nucleus (Vme)-trigeminal motor nucleus circuit. Here, we studied activation of motor neurons of the facial nucleus (VII), hypoglossal nucleus (XII), nucleus ambiguus (Amb), and spinal nucleus of the accessory nerve (SNA) in rats with UAC via their similar connections with Vme. An anterograde tracer, biotinylated dextran amine (BDA), was injected into the Vme to identify the central axon terminals around the motor neurons of VII, XII, Amb, and SNA. The expression of vesicular glutamate transporter 1 (VGLUT1) in neurons of VII, XII, Amb, and SNA, and the expression of acetylcholinesterase (AChE) were measured in the stapedius, lingualis, palatopharyngeal, and sternocleidomastoid muscles. In BDA-treated rats, many BDA-labeled cell bodies in the Vme and terminals in VII, XII, Amb, and SNA were identified. Compared with control rats, rats with UAC showed higher expression of VGLUT1 in these nuclei, and statistically significantly higher expression of AChE in the stapedius, lingualis, and sternocleidomastoid muscles, but not in the palatopharyngeal muscle. These findings suggest that UAC activates orofacial, head, and cervical multimotor behaviors via connections between the Vme and the corresponding motor nuclei.