| Description | GPCR family 3 receptors (also known as family C) are structurally similar to other GPCRs, but do not show any significant sequence similarity and thus represent a distinct group. Structurally they are composed of four elements; an N-terminal signal sequence; a large hydrophilic extracellular agonist-binding region containing several conserved cysteine residues which could be involved in disulphide bonds; a shorter region containing seven transmembrane domains; and a C-terminal cytoplasmic domain of variable length [ ]. Family 3 members include the metabotropic glutamate receptors, the extracellular calcium-sensing receptors, the gamma-amino-butyric acid (GABA) type B receptors, and the vomeronasal type-2 receptors [, , , ]. As these receptors regulate many important physiological processes they are potentially promising targets for drug development.GABA is the principal inhibitory neurotransmitter in the brain, and signals through ionotropic (type A and type C) and metabotropic (type B) receptor systems. The type B receptors have been cloned, and photoaffinity labelling experiments suggest that they correspond to two highly conserved receptor forms in the vertebrate nervous system [ ]. These receptors are involved in the fine tuning of inhibitory synaptic transmission []. Presynaptic receptors inhibit neurotransmitter release by down-regulating high-voltage activated calcium channels, while postsynaptic receptors decrease neuronal excitability by activating a prominent inwardly rectifying potassium (Kir) conductance that underlies the late inhibitory postsynaptic potentials [, ]. The type B receptors negatively couple to adenylyl cyclase and show sequence similarity to the metabotropic receptors for the excitatory neurotransmitter L-glutamate. The physiological form of the B receptor is a heterodimer of the B1 and B2 subtypes, in which B1 seems to bind agonists, while B2 mediates coupling to G proteins [, , ].This entry includes GPR156, a class C orphan GPCR that regulates hair cell orientation, which is important for capturing sound in the cochlea, balancing movements in vestibular organs and sensing water movements in the fish lateral line, and it also plays a role in lipid metabolism [ ]. Unlike GABR1/2, it lacks role the extracellular and exhibits constitutive activity [].Metabotropic glutamate receptor-like proteins (Grl) are similar in structure to GABA-B receptors and belong to the same receptor family [ , ]. | Name | GPCR family 3, GABA-B receptor |
| Short Name | GPCR3_GABA-B | Type | Family |
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