An adrenergic alpha-agonist (or alpha-adrenergic agonists) are a class sympathomimetic agents that selectively stimulates alpha adrenergic receptors. The alpha-adrenergic receptor has two subclasses α1 and α2. Alpha 2 receptors are associated with sympatholytic properties. α Adrenergic agonists have the opposite function of alpha blockers. Alpha adrenoreceptor ligands mimic the action of epinephrine and norepinephrine signaling in the heart, smooth muscle and central nervous system, with norepinephrine being the highest affinity. The activation of α1 stimulates the membrane bound enzyme phospholipase C, and α2 inhibits the enzyme Adenylate cyclase. Inactivation of adenylate cyclase in turn leads to the inactivation of the secondary messenger Cyclic adenosine monophosphate and induces smooth muscle and blood vessel constriction.
Alpha-adrenergic agonists, more specifically the auto receptors of alpha 2 neurons, are used in the treatment of glaucoma by decreasing the production of aqueous fluid by the ciliary bodies of the eye and also by increasing uveoscleral outflow. Medications such as clonidine target pre-synaptic auto receptors, therefore leading to an overall of decrease in norepinephrine.