Molecular model of baclofen (red, top left) binding to gamma-aminobutyric acid receptor B (GABA-B receptor, blue-purple), an example of a G-coupled protein receptor (GPCR), and triggering a signal cascade. GABA-B receptors are found in the central nervous system and parts of the peripheral nervous system. They are crucial for opening potassium ion channels and reducing adenyl cyclase activity. Binding of the drug baclofen to GABA-B causes the alpha subunit (light blue, bottom left) of the G-protein to exchange its bound molecule of GDP (guanosine diphosphate) for a molecule of GTP (guanosine triphosphate). This releases the alpha subunit from the GABA-B receptor allowing it to bind to the enzyme adenylyl cyclase (blue) and activate it. Adenylyl cyclase (dark blue, right) catalyses the conversion of ATP (adenosine triphosphate) to cyclic adenosine monophosphate (cAMP). cAMP regulates numerous cell functions. For a version of this image with labels see F033/4087.