When an agonistic ligand binds to the opioid receptor, a conformational change occurs, and the GDP molecule is released from the Gα sub-unit. This mechanism is complex, and is a major stage of the signal transduction pathway. When the GDP molecule is attached, the Gα sub-unit is in its inactive state, and the nucleotide-binding pocket is closed off inside the protein complex. However, upon ligand binding, the receptor switches to an active conformation, and this is driven by intermolecular rearrangement between the trans-membrane helices. The receptor activation releases an ‘ionic lock’ which holds together the cytoplasmic sides of transmembrane helices three and six, causing them to rotate. This conformational change exposes the intracellular receptor domains at the cytosolic side, which further leads to the activation of the G protein. When the GDP molecule dissociates from the Gα sub-unit, a GTP molecule binds to the free nucleotide-binding pocket, and the G protein becomes active. A Gα(GTP) complex is formed, which has a weaker affinity for the Gβγ sub-unit than the Gα(GDP) complex, causing the Gα sub-unit to separate from the Gβγ sub-unit, forming two sections of the G protein. The sub-units are now free to interact with effector proteins; however, they are still attached to the plasma membrane by lipid anchors. After binding, the active G protein sub-units diffuses within the membrane and acts on various intracellular effector pathways. This includes inhibiting neuronal adenylate cyclase activity, as well as increasing membrane hyper-polarisation. When the adenylyl cyclase enzyme complex is stimulated, it results in the formation of Cyclic Adenosine 3', 5'-Monophosphate (cAMP), from Adenosine 5' Triphosphate (ATP). cAMP acts as a secondary messenger, as it moves from the plasma membrane into the cell and relays the signal.