GLP1R and Parkinson disease: The involvement of GLP-1 in PD can be simplified to the activation of the MAPK/extracellular signal-regulated kinase 1/2 (ERK) pathway, which has an important role in synaptic plasticity, and the PI3K/Akt pathway signaled by GLP-1 receptor stimulation and the subsequent increase in cAMP and PKA and PI3K activation [73].