Alpha-2 adrenergic receptor

The alpha-2 (α2) adrenergic receptor (or adrenoceptor) is a G protein-coupled receptor (GPCR) associated with the Gi heterotrimeric G-protein. It consists of three highly homologous subtypes, including α2A-, α2B-, and α2C-adrenergic. Some species other than humans express a fourth α2D-adrenergic receptor as well. Catecholamines like norepinephrine (noradrenaline) and epinephrine (adrenaline) signal through the α2-adrenergic receptor in the central and peripheral nervous systems. The α2A adrenergic receptor is localised in the following central nervous system (CNS) structures: Brainstem (especially the locus coeruleus) Midbrain Hypothalamus Hippocampus Spinal cord Cerebral cortex Cerebellum Septum Whereas the α2B adrenergic receptor is localised in the following CNS structures: Olfactory system Thalamus Pyramidal layer of the hippocampus Cerebellar Purkinje layer and the α2C adrenergic receptor is localised in the CNS structures: Midbrain Thalamus Amygdala Dorsal root ganglia Olfactory system Hippocampus Cerebral cortex Basal ganglia Substantia nigra Ventral tegmentum The α2-adrenergic receptor is classically located on vascular prejunctional terminals where it inhibits the release of norepinephrine (noradrenaline) in a form of negative feedback. It is also located on the vascular smooth muscle cells of certain blood vessels, such as those found in skin arterioles or on veins, where it sits alongside the more plentiful α1-adrenergic receptor. The α2-adrenergic receptor binds both norepinephrine released by sympathetic postganglionic fibers and epinephrine (adrenaline) released by the adrenal medulla, binding norepinephrine with slightly higher affinity. It has several general functions in common with the α1-adrenergic receptor, but also has specific effects of its own. Agonists (activators) of the α2-adrenergic receptor are frequently used in anaesthesia where they affect sedation, muscle relaxation and analgesia through effects on the central nervous system (CNS).