The Plasticity Of Masticatory Motor Center In Unilateral Mastication Model Rats

Objective: To established the simultaneous recording techniques of electrical activities in bilateral trigeminal motor nucleus(Mo5) and masseter muscles during H-reflex in rats,to analyze the neural pathways and their underlying mechanisms of H-reflex in masseter muscles, to investigate the plasticity of masticatory motor center and spatial learning and memory abilities in unilateral mastication model rats.Methods: 56 of 8-week-old male Wistar rats were chosen as research subjects in present study. Simultaneous recording techniques of H-reflex in masseter muscles were established:Adult Wistar rats were anesthetized(20% urethane) and bilateral Mo5 were chosen to conduct extracellular recordings. Bilateral electromyography(EMG) of masseter muscle and other physiological functions were simultaneously monitored by a polygraph. The responses evoked by electrical stimulation of masseter nerve were preliminarily analyzed. Plasticity of unilateral masticatory motor center:There were three groups in the this part, including Group1 for one mouth(20),Group2 for three mouthes(20) and Group 3 for sixteen mouthes(14). Rats were randomly divided into experimental and control groups, while unilateral mastication models were established in experimental groups. Spatial learning and memory abilities were tested in Morris water maze after one mouth, three mouthes and sixteen mothes. Subsequently,bilateral electromyography(EMG) of masseter muscle and other physiological functions were simultaneously monitored as it described before.Results:Simultaneous recording techniques of H-reflex: During the baseline recording for 30 minutes, a low-frequency discharge pattern was observed in bilateral Mo5(Left:1.25±0.09 Hz, Right:1.20±0.39 Hz, P>0.05), but masseter EMG did not reveal any measurable activity. Not only ipsilateral masseter H-reflex but also contralateral masseter H-reflex were induced simultaneously by giving unilateral masseter nerve electrical stimulation. Moreover, responses of ipsilateral and contralateral Mo5 were evoked during the masseter H-reflex. The response amptitude of ipsilateral masseter EMG was higher than that of contralateral one(P<0.05), but no significant difference between ipsilateral and contralateral Mo5 was observed(P>0.05). P lasticity of unilateral masticatory motor center: Statistical analyses in Morris water maze part: Escape latency were measured longer in experimental group than control group in Group3(P<0.05). Swimming speed in experimental group showed slower than control group in Group3(P<0.05). Number of crossing the plate was significantly different between experimental and control groups in Group3 and Group16(P<0.05). Statistical analyses of discharge frequency(DF) in nuclei of Mo5: The DF of Mo5 in modeling sides were lower than non-modeling sides in experimental groups in Group1, Group3(P<0.01) and Group16(P<0.05). The DF of Mo5 in modeling sides in experimental groups were lower than control groups in Group1(Left P<0.01,Right P<0.01) and Group3,Group16(Left P<0.01,Right P<0.01). The DF of Mo5 in non-modeling sides in Group16 was lower than control group(Left P<0.01, Right P<0.05). The DF of Mo5 in non-modeling sides in experimental groups in Group16 was lower than in experimental groups of Group1 and Group3(Group1 P<0.05,Group3 P<0.01). Comparation of the latency in the nuclei Mo5 reaction through electrical stimulation in bilateral masseter nerves(12m A, 200μs, 0.3Hz) : The latency in modeling sides in experimental groups were longer than in control groups in Group1(Left P<0.01,Right P<0.05) and Group3(Left P<0.05,Right P<0.05). The latency in non-modeling sides in experimental group in Group16 were longer than that of Group1. The latency in control groups in Group16 were longer than in Group1(Left P<0.01,Right P<0.05)and Group3(Right P<0.05). Comparation of Hmax in masseters through electrical stimulation in bilateral masseter nerves: The Hmax in modeling sides were lower than non-modeling sides in experimental groups of Group16(P < 0.05). The Hmax in modeling sides in experimental groups were lower than in control groups in Group3(P<0.05) and Group16(Left P<0.01,Right P<0.05).Conclusion: It is feasible to analyze the electrical activities in bilateral Mo5 and masseter muscle during masseter H-reflex. Unilateral mastication decreases the spatial learning and memory abilities in rats. Unilateral mastication decreases the Mo5 neurons excitability and may results in plasticity changes of the central control of the mastication.