Adrenergic Receptor

Adrenergic receptors are the target of many autonomic drugs that act on the sympathetic nervous system. Adrenergic receptors can mainly be classified into 5 types, known as ⍺1, ⍺2, β1, β2, β3. These adrenergic receptors are located in different tissues and stimulate different physiological responses. ⍺1 receptors are mainly located in smooth muscles, activating them result in vasoconstriction and the constriction of smooth muscles. ⍺2 receptors are mainly located in nerve endings of presynaptic nerve, activating them result in decreased presynaptic noradrenaline release, hence serving as a negative feedback mechanism. β1 receptors are mainly located in the heart, activating them result in increased contractility of the heart, and an increased heart rate. β2 receptors are mainly located in smooth muscles, activating them result in vasodilation and relaxation of smooth muscles, which produces opposing effect with ⍺1 receptor. β3 receptors are mainly located in adipocytes and detrusor muscles of the bladder, activating it result in relaxation of detrusor muscle and also the stimulation of lipolysis, which is a process to breakdown fat molecules into fatty acids as a source of energy.

Drugs affecting the sympathetic nervous system can be divided into 2 main categories: sympathomimetic drugs which mimics the action of sympathetic nervous system activation; and sympatholytic drugs which reduce or interrupt the action of sympathetic nervous system activation.