Dopamine receptors (GPCR-A17) are widespread in the body of vertebrates, playing major roles in processes of the central nervous system, as well as in the periphery. In the CNS, dopaminergic neurons are critically involved in voluntary movement, memory, learning, sleep, attention, feeding, and rewarding, Well known examples of disorders as a result of malfunction of the central dopaminergic system are Parkinson’s disease (loss of striatal dopaminergic innervations in the brain), schizophrenia, depression, ADHD, and addiction (among many others). In the periphery, dopamine plays important physiological roles in the regulation of olfaction, retinal processes, hormonal regulation, cardiovascular functions, sympathetic regulation, immune system, renal functions, and more. Five major classes (D1-D5) have been identified thus far, which can be grouped into two sub classes. The group of D1-like receptors (members D1 and D5; all stimulating the second messenger system adenylate cyclase), and the group of D2-like receptors (members D2, D3 and D4; all inhibiting adenylate cyclase). As widespread and abundant as dopaminergic neurons are in the body of vertebrates, as comprehensive and diverse is the list of Axon LigandsTM interacting at all subtypes of dopaminergic receptors (selectively, or specific combinations).
 The Physiology, Signaling, and Pharmacology of Dopamine Receptors. J-M Beaulieu, R.R. Gainetdinov. Pharmacol. Rev. 2011, 63, 182-217.