ABSTRACT
Slow transmission is mediated by G-protein-linked receptors. G proteins, trimers consisting of α-, β -, and γ-subunits are so-called because the α-subunit binds guanine nucleotides. Guanosine 5'-diphosphate (GDP) leaves the α-subunit in exchange for guanosine 5'-triphosphate (GTP). The α-subunit has an intrinsic GTPase activity that cleaves the terminal phosphodiester bond in the GTP converting it to GDP. Adenylyl cyclase is activated by a specific family of G proteins, the GSproteins, so called because their action on adenylyl cyclase is stimulatory. Some neurotransmitter receptors are negatively coupled to adenylyl cyclase. These receptors associate with Gi proteins that inhibit the activity of the enzyme. Diacylglycerol, a hydrophobic molecule, diffuses within the lipid where it activates protein kinase C. The binding of inositol-1,4,5-trisphosphate to its receptors causes the calcium channels to open and Ca2+ flows out of the smooth endoplasmic reticulum into the cytosol. Neurons contain a calcium binding protein called calmodulin which shares considerable homology with troponin.
