A Preliminary Study Of The Central Mechanism Of The Trigeminal Proprioception In The Temporomandibular Disorder Syndrome

Temporomandibular disorder(TMD),known as temporomandibular joint disorder syndrome(TMJDS),that is,Costen syndrome,is a common oral disease,closely related with occlusion.The main symptoms of TMD are click,muscle pain,mandibular movement disorder and anxiety emotion.The feeling of occlusion is related to the structure of the chewing system,such as teeth,temporomandibular joint,masticatory muscles,and the most closely related periodontal sensations is mainly from the receptors of periodontal tissues.The feeling can be expressed as pain,touch pressure,temperature sensation,cold sensation and proprioceptive sensation and so on.The effect of proprioceptive sensation is feeling the food hardness,thickness and the size and direction of the tooth pressure.The primary neurons of the periodontal proprioceptive sensation are located in the trigeminal mesencephalic nucleus(Vme),as the primary afferent neurons,which are located in the central nervous system.The neurons are mainly large pseudounipolare neurons,which form specific nerve nucleus in brainstem.Whether the abnormal occlusion can lead to excitability changes of Vme neuron,and whether it can cause masseter hypercontraction is still lack of research.Temporomandibular joint disorders have been known as temporomandibular joint disorder syndrome(TMJDS),that is,Costen syndrome,in which there are clinical symptoms of temporomandibular joint pain,the radioactive pain from the temporomandibular joint to the head side when chewing,mandibular joint movement disorders and snapping,mandibular bias to the affected side,masticatory muscle spasms and pain.There also are earplugs,tinnitus,earache,dizziness,nystagmus,throat and tongue burning sensation and other symptoms.The central axons of the trigeminal mesencephalic nucleus can project to the motor nucleus which located in the brainstem,including the trigeminal motor nucleus,the facial nerve nucleus(dominate facial muscles,pedicle muscle,etc.),sublingual nucleus(mainly distribute to the genioglossus),solitary nucleus,ambiguus nucleus,and even cerebellum,cervical spinal cord and so on.Abnormal occlusion affects these functions with the synaptic neurons associated with trigeminal mesencephalic nucleus in the central nervous system,and exhibits corresponding symptoms,which are currently lacking in research.TMD patients not only suffer from a disease with clear pathological changes,but also are accompanied by psychological(such as anxiety,depression,etc.)problem.Among which,anxiety and depression play an important role in the occurrence and development of TMD.The lateral habenular nucleus(LHb)is an important brain area between the brainstem and limbic system,which is associated with the physiological symptoms of anxiety and depression.LHb neurons can be activated by anxious conditioned stimuli.At the same time,LHb has a direct projection to Vme.Current studies of the relation of TMD and emotion are limited in clinical investigation and animal behavioral tests.Whether abnormal occlusion can lead to the emergence of LHb-mediated anxiety and the effect of LHb activation on Vme remains to be studied.Recently,our lab has developed a unilateral anterior crossbite(UAC)model that could induce serious OA-like changes in TMJ cartilage of mice and rats.Based on this animal model,we first used CTb tract-tracing,in situ hybridization,immunofluorescence,real-time quantitative PCR and Western blot to observe whether the central axon of the trigeminal mesencephalic nucleus corresponding to the periodontal tissue has a direct projection to the motor nucleus,whether UAC can cause changes of the periodontal proprioception,causing excitability changes of trigeminal mesencephalic nucleus and motor nucleus,and ultimately lead to masseter hypercontraction;Second,based on the unilateral anterior crossbite model,to detect the change of the neurons which have synaptic contacts of trigeminal mesencephalic nucleus by tract-tracing,immunofluorescence and Western blot to provide the basis of neurobiology of the "syndrome".Finally,we establish unilateral anterior crossbite rat model,using morphology,molecular neurobiology,electrophysiology,behavior testing and other methods to observe whether UAC can cause the anxiety-like behavior of animals,leading to excitability of the lateral habenular nucleus and after the inhibition of the excitability of lateral habenula,whether having changes in excitability of trigeminal mesencephalic nucleus.The main results: 1.Proprioceptive mechanisms in occlusion-stimulated masseter-hypercontractionIn rats in which CTb was injected into the inferior alveolar nerve,many cell bodies were found to be labeled retrogradely with CTb in the Vme on the ipsilateral side,whereas only a few labeled cell bodies were detected on the contralateral side.The CTb labeled neuronal cell bodies in the Vme were distributed densely at the caudal levels.Some of the CTb-labeled axonal profiles in Vmo were VGLUT1 immunopositive.2.Proprioceptive mechanisms in occlusion-stimulated maxillofacial muscles hypercontractionThe results showed that the central axons of the trigeminal mesencephalic nucleus can directly project to motor nuclear area including the Vmo,facial nerve nucleus and hypoglossal nerve nucleus.The expression of VGLUT1 protein in the facial nerve nucleus,the accessory nerve nucleus,the ambiguus nucleus,and the hypoglossal nucleus,suggesting that UAC enhances the excitability of Vme,resulting in the excitability of the motor nucleus neurons.The AChE expression of the stapedius muscle,the tongue muscle,the sternocleidomastoid was also increased.3.UAC causes the excitatory of lateral habreta and anxiety-like behavior of rats(1)There is the presence of anxiety in UAC rats,as open field experiments and elevated cross tests showed that there was a significant reduction in activity at 4 weeks after UAC stimulation and continued until the end of the 12-week observation period;(2)The excitability of the lateral habenular nucleus(LHb)neurons in UAC rats was significantly enhanced,and the expression of VGLUT2 mRNA in the LHb neurons was significantly enhanced.4.The effect of occlusion-stimulated lateral habenula activation to the trigeminal mesencephalic nucleus(1)At the plane of Vme,BDA and VGLUT2 double axons were terminated around parvalbumin(PV)-positive neurons by means of BDA injection into the LHb.(2)The VGLUT2 protein expression level of Vme neuronal region was significantly increased in UAC rats.(3)Injection of VGLUT2 shRNA lentivirus into the LHb significantly improved the activity of the UAC rat in open field experiments and elevated cross tests,indicating that the anxiety of the animals was alleviated;injection of VGLUT2 shRNA lentivirus into the LHb significantly reduced the VGLUT2 mRNA expression level of LHb neurons,and reduce VGLUT2 protein expression levels of Vme;(4)The electrophysiological experiments of trigeminal mesencephalic nucleus were tested after injection of VGLUT2 shRNA lentivirus into the LHb.The results revealed that the resting membrane potential and action potential release threshold of UAC group were significantly lower than those of sh RNA + UAC group,and the action potential amplitude,action potential decay time and action potential half-wide time were significantly longer than shRNA + UAC group.The amplitude of the potential was significantly lower than that of the shRNA + UAC group.The frequency and amplitude of the action potential of the Vme neurons in the shRNA + UAC group were significantly lower than those in the UAC group.These data suggests that shRNA leads to a significant reduction in the excitability of Vme neurons.Conclusions:1.Unilateral anterior crossbite(UAC)mode can cause masseter hyperactivity via periodontal-trigeminal mesencephalic nucleus(Vme)-trigeminal motor nucleus(Vmo)circuit.2.UAC can activate the trigeminal motor nucleus,facial nerve nucleus,the accessory nerve nucleus,the ambiguus nucleus,and the hypoglossal nucleus by activating the Vme,enhancing the corresponding skeletal muscle excitability.3.UAC can activate the lateral habenular nucleus(LHb),leading to the anxiety of rats;the activation of lateral habenular nucleus excited Vme through direct synaptic contact.Inhibition of the lateral habenular nucleus activity will not only help to reduce the anxiety caused by UAC,but also help to inhibit the excitement of Vme neurons.