Multiple neurotransmitter receptors in the brain: amines, adenosine, and cholecystokinin

Radioligand binding studies of neurotransmitter receptors have provided discrimination at the molecular level, permitting the differentiation of multiple receptor subtypes for several biogenic amines. Using this paradigm we have labeled two distinct receptors each for cholecystokinin (CCK) and for adenosine. Adenosine receptors were labeled in brain with [³H]N⁶-cyclohexyladenosine (³H-CHA) and [³H]1,3-diethyl-8-phenylxanthine (³H-DP). The adenosine receptor labeled by ³H-CHA appears to be an A₁ site, associated with reduction of adenylate cyclase activity, while ³H-DP sites resemple A₂ receptors linked to adenylate cyclase enhancement. Cholecystokinin-33 labeled by the Bolton-Hunter procedure with ¹²⁵I(¹²⁵I-BH-CCK) labels different receptors in brain and pancreas. The pancreatic receptor does not react with CCK derivatives of fewer than eight amino acids, while the brain receptor does recognize pentagastrin, the carboxyl-terminal five amino acids of CCK. The 'brain type' CCK receptor may normally interact with CCK-4, the carboxyl-terminal tetrapeptide of CCK, recently identified as a unique neuropeptide highly concentrated in the brain. CCK-8, the other major molecular form of CCK, may be the endogenous ligand for the 'pancreatic type' receptor.