HRH3 and Parkinson disease: Consequently, H3R ligands, either antagonist or inverse agonists, as they also demonstrate constitutive activity, contribute to one of the largest groups of contemporary GPCR ligand research, with implications to treat a number of CNS-based diseases, including Alzheimer’s (AD) and Parkinson’s disease (PD), schizophrenia, ADHD, obesity, and narcolepsy [6,7].