However, this study has limitations, as we did not evaluate the long-term effects of the peripheral GLP-1R antagonist exendin (9–39) on motor behavior and the firing activity of dopaminergic neurons in MPTP-induced chronic PD mice, nor did it compare the excitatory effects of peripheral exendin-4 on normal mice vs. MPTP-induced chronic PD mice. The gene discussed is GLP1R; the disease is Parkinson disease.