The receptors for these hormones are typical of serpentine seven transmembrane peptides. Each hormone acts as a ligand in a receptor pocket, causing conformational changes receptor activation. Conformational changes in intracellular receptor third loop and carboxyl terminal tail activate G-protein to intracellular side. G14 is the protein associated with receptors for GnRH and TRH, Gi with the dopamine receptor and the G protein with the receptors for other hormones listed above. GHRH, CRH, GnRH, TSH, ACTH, FSH, LH, and dopamine Protein G-GTP complex related to receptors for these hormones activate adenylyl cyclase, which synthesizes the second messenger cAMP. Cyclic AMP activates protein kinases, which phosphorylate specific intracellular proteins, thus producing the hormonal effect. Conversely, dopamine receptor binding lactotrophs causes conformational changes in the Gi protein that reduce the activity of adenylyl cyclase and inhibit prolactin secretion.