| Parkinson’s disease(PD)is one of the most common neurodegenerative diseases in the world.The main pathological properties of PD are the progressive loss of dopaminergic(DA)neurons in the substantia nigra(SN)of the midbrain and the formation of Lewy bodies(LBs)by misfolded α-synuclein(α-Syn)in the remaining neurons.Besides the typical motor symptoms(static tremor,muscle rigidity,bradykinesia,posture disorders,etc.),PD patients also show non-motor symptoms,such as sleep disorders,emotional disorders,gastrointestinal dysfunction,etc.,of which the incidence of gastrointestinal dysfunction is the highest.The constipation or gastroparesis might last more than ten years.Some studies have shown that LBs firstly appeared in the enteric nervous system(ENS)in the body,and then spread to the dorsal motor nucleus of vagus(DMV)in the medulla oblongata through the vagus nerve,and further spread to SN.But the efferent fibers emitted by DMV only account for 20% of the total fibers in the vagus nerve,while the remaining 80% are afferent fibers.Therefore,whether these afferent fibers play a role in the progression of PD is investigated.In this study,electrophysiological,behavioral,immunofluorescence and Western blotting experiments were used to investigate whether the vagal afferent fibers projected from the gastrointestinal tract to the SN,whether α-Syn could be transmitted to the brain along vagal afferent fibers and whether the impairment to vagal afferent by capsaicin could prevent the development of PD.The results were as follows:1.Among the 55 neurons recorded in the SN of 25 rats,7 neurons were excited by gastric distention(GD),with a significant increase in discharge frequency(P<0.001),while 4neurons were inhibited,with a significant decrease in discharge frequency(P<0.001).DAergic neurons accounted for 63.6% of the total responding neurons.2.Among the 11 neurons recorded in the SN of 5 rats,5 neurons were excited by stomach hyperthermia stimulation,with a significant increase in discharge frequency(P<0.05),while 4 neurons were inhibited,with a significant decrease in discharge frequency(P<0.01).DAergic neurons accounted for 75.0% of the total responding neurons.3.Among the 36 neurons recorded in the SN of 15 rats,6 neurons were excited by gut hyperthermia stimulation,with a significant increase in discharge frequency(P<0.05),while 3 neurons were inhibited,with a significant decrease in discharge frequency(P<0.01).DAergic neurons accounted for 66.7% of the total responding neurons.4.After vagotomy,the response of neurons in the SN triggered by GD and hyperthermia stimulation disappeared.5.Electron microscopy showed α-Syn PFF was prepared as hollow filamentous fiber and then α-Syn PFF fibers became shorter after 1 min ultrasound treatment.6.To explore the role of vagus afferent fibers in the PD development,the vagus afferent fibers were damaged by subcutaneous injection of capsaicin(25 mg/kg,50 mg/kg,50mg/kg for three days),to observe whether it affects central α-Syn aggregation caused by α-Syn PFF(0.6 μg/g)gastrointestinal injection.The changes of behavior,α-Syn or TH levels in DMV or SN were detected in 1 month and 3 month of capsaicin+α-Syn PFF-treatment groups.The behavioral tests showed that the turnaround and arrival time in the pole climbing test,the retention time in the rotating test and the distance travelled in the open field test did not change significantly in the α-Syn PFF-treated 1 month group and capsaicin+α-Syn PFF treated 1 month group compared with the control group.Compared with the control group,only the turnaround and arrival time in the pole climbing test were prolonged by 22.1%(P<0.05)and 173.9%(P<0.05)in the α-Syn PFF-treated 3 month group,respectively.Compared with the α-Syn PFF-treated 3month group,the turnaround time in the pole climbing test decreased by 48.3%(P<0.05),in the capsaicin+α-Syn PFF-treated 3 month group.7.Immunofluorescence staining and Western blotting experiments showed that the expression of α-Syn was slightly increased in the DMV of the α-Syn PFF-treated 1month group compared with the control group,and the expression of α-Syn was slightly decreased in the DMV of the capsaicin+α-Syn PFF-treated 1 month group,while the difference was not statistically significant.The expression of α-Syn in the DMV was significantly increased by 70.8%(P<0.05)in the α-Syn PFF-treated 3 month mice.While expression of α-Syn in the DMV was significantly decreased by 42.3%(P<0.05)in the capsaicin+α-Syn PFF-treated 3 month group compared with the α-Syn PFF-treated 3 month group,and the difference was statistically significant.8.Immunofluorescence staining and Western blotting experiments showed that the expression of tyrosine hydroxylase(TH)and α-Syn in the SN were not significantly changed in the α-Syn PFF-treated 1 month and capsaicin+α-Syn PFF-treated 1 month group.The expression of TH in the SN was slightly decreased in the α-Syn PFF-treated3 month group and the expression of α-Syn was slightly increased in the SN of the α-Syn PFF-treated 3 month group,with no statistical significance.The expression of TH was slightly increased and α-Syn level was slightly decreased in the SN of the capsaicin+α-Syn PFF-treated 3 month group compared with α-Syn PFF-treated 3month group,but with no statistical significance.In summary,there is a direct or indirect sensory afferent projection communication mediated by the vagal afferents between the gastrointestinal tract and SN.The development of PD can be prevented by blocking the vagal afferents,indicating that α-Syn could spread along the vagal afferents to the central nervous system.This study might provide experimental basis for the prevention and treatment of PD. |
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