Opioid receptors are a type of G protein–coupled receptor (GPCR). These receptors are distributed throughout the central nervous system and within the peripheral tissue of neural and non-neural origin. They are also located in high concentrations in the Periaqueductal gray, Locus coeruleus, and the Rostral ventromedial medulla. The receptors are responsible for analgesia, and consist of an extracellular amino acid N-terminus, seven trans-membrane helical loops, three extracellular loops, three intracellular loops, and an intracellular carboxyl C-terminus. The three extracellular loops of the GPCR form parts of the pocket in which signalling molecules can bind, to initiate a response. G proteins are specialised proteins whereby the nucleotides Guanosine diphosphate (GDP), and Guanosine triphosphate (GTP) bind to. They are classified as heterotrimeric, meaning they contain three different sub-units, which include an alpha (α) subunit, a beta (β) subunit, and a gamma (γ) sub-unit. The gamma and beta sub-units are permanently bound together, producing a single Gβγ sub-unit. Heterotrimeric G proteins act as ‘molecular switches’, which play a key role in signal transduction, because they relay information from activated receptors to appropriate effector proteins. All G protein α sub-units contain palmitate, which is a 16-carbon saturated fatty acid, that is attached near the N-terminus through a labile, reversible thioester linkage to a cysteine amino acid. It is this palmitoylation that allows the G protein to interact with membrane phospholipids due to the hydrophobic nature of the alpha sub-units. The gamma sub-unit is also lipid modified and can attach to the plasma membrane as well. These properties of the two sub-units, allow the opioid receptor's G protein to permanently interact with the membrane via lipid anchors.