H3R is involved in the regulation of the levels of histamine, acetylcholine, serotonin, noradrenaline, glutamate, γ-aminobutyric acid, and neuropeptide Y; therefore, it is a valuable molecular target for potential drugs for Alzheimer’s and Parkinson’s diseases, schizophrenia, narcolepsy, obesity, and the Attention Deficit Hyperactivity Disorder (ADHD) [24,25,26,27,28,29]. This evidence concerns the gene HRH3 and attention deficit-hyperactivity disorder.