The symptoms of masticatory muscles in Parkinson’s disease rat model 

Objectives: Parkinson's disease (PD) is a major neurological disease characterized by a marked loss of dopaminergic neurons in the substantia nigra. Patients with PD often reveal chewing and swallowing dysfunctions. The purpose of this study is to examine the influence of PD on masticatory muscles. Methods: Ten 8-week-old Sprague-Dawley male rats were used. Five rats received injections of the neurotoxin, 6-hydroxydopamine, into the striatum, which induced an alternative striatal dopamine depletion in the experimental group. The remaining five rats were treated with a saline injection, and used as a control group. At the age of 14 weeks, a radio-telemetric device was implanted to record muscle activity continuously from the superficial masseter and anterior belly of digastric muscles. The data recorded for 1-week was evaluated by means of the total duration of muscle activity (duty time) determined for muscle activity exceeding 5, 20, and 50% of the day's activity. Rats were sacrificed at the age of 16 weeks, and their masseter and digastric muscles were dissected for immunohistochemistry and PCR analysis. Results: In the experimental PD model rats, the duty time of the digastric muscle at the 5% level was significantly (p<0.05) higher than that in the control group, whereas there were no differences in the masseter muscle at any levels. In the real-time PCR analysis, the expression of Myoshin heavy chain type I in digastric muscle was significantly (p<0.05) higher in the experimental rats than in the control. In the immunohistochemical analysis, no differences in fiber type composition of both muscles were observed between the experimental and control groups. Conclusions: PD affects a fast to slow transition of muscles in the digastric muscle, which may induce the chewing and swallowing dysfunction of PD's symptom.