Alpha2-adrenergic receptor

The α2 receptor is a type of adrenergic receptor.

Effect
The α2 receptor has several, general, functions in common with other α-receptors, but also has individual effects.

General
Common (or still unspecified) effects include:
 * Vasoconstriction of arteries to heart (coronary artery).
 * Vasoconstriction of veins
 * Decrease motility of smooth muscle in gastrointestinal tract
 * Contraction of male genitalia during ejaculation

Individual
Individual actions of the α2 receptor include:
 * Mediates synaptic transmission in pre- and postsynaptic nerve terminals.
 * Decrease release of acetylcholine
 * Decrease release of noradrenaline
 * Inhibit noradrenaline system in brain
 * Inhibition of lipolysis in adipose tissue.
 * inhibition of insulin release in pancreas.
 * induction of glucagon release from pancreas.
 * platelet aggregation
 * contraction of sphincters of the gastrointestinal tract
 * &darr; Secretion from salivary gland
 * relax gastrointestinal tract(presynaptic effect)

Mechanism
A G protein - Gi renders adenylate cyclase inactivated, resulting in decrease of cAMP.

The relaxation of gastrointestinal tract motility is by presynaptic inhibition, where transmitters inhibit further release by homotropic effects.

Agonists
adrenaline has higher affinity for the alpha-2 receptor than has noradrenaline, which, in turn, has much higher affinity than has isoprenaline. Other agonists include:
 * clonidine* (antihypertensive) -- see below for alternative mechanism --
 * dexmedetomidine
 * lofexidine (antihypertensive)
 * xylazine (veterinary; in non-human species this is an immobilizing and anesthetic drug, presumptively also mediated by alpha-2 adrenergic receptors because it is reversed by yohimbine)
 * tizanidine (in spasms, cramping)
 * guanfacine (antihypertensive)
 * clenbuterol* (decongestant and bronchodilator)

* denotes selective agonists to the receptor.


 * Clonidine is an Alpha 2 Agonist ; initially thought act via presynaptic Alpha 2 receptors --> reduced NE discharge. However! It binds to imidazoline receptors with a much greater affinity than Alpha 2s . Imidazoline Receptors occur in the Nucleus Tractus Solitarius & Ventrolateral Medulla.  Clonidine is now thought to decrease BP via this central mechanism. ***

Antagonists
(Alpha blockers)
 * yohimbine* (purported aphrodisiac)
 * mirtazapine (NaSSA)
 * mianserin (tetracyclic antidepressant)
 * idazoxan* (experimental)

* denotes selective agonists to the receptor.

Types
There are three types of α2 receptors: ADRA2A, ADRA2B, ADRA2C.