GHRL and Parkinson disease: The development of mesodiencephalic dopaminergic (mdDA) neurons and mdDA progenitors, including the nigrostriatal subset that preferentially degenerates in PD, depends on Wnt/β-catenin signalling.162 Ghrelin, an endogenous ligand for growth hormone secretagogue receptor 1a, plays a fundamental role in regulating energy homeostasis in PD.