HTR4 and Parkinson disease: Shi reported decreased 5-HT4R expression in the hypothalamus and hippocampus of 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine-induced PD mice, whereas injection of the 5-HT4R antagonist GR125487 aggravated the neurodegenerative processes in the mouse nigrostriatal pathway and modulated the inflammation-related intestinal microbiota composition [43].