 Like many other biologically active substances , norepinephrine exerts its effects by binding to and activating receptors located on the surface of cells . Two broad families of norepinephrine receptors have been identified , known as alpha and beta adrenergic receptors . Alpha receptors are divided into subtypes α1 and α2 ; beta receptors into subtypes β1 , β2 , and β3 . All of these function as G protein @-@ coupled receptors , meaning that they exert their effects via a complex second messenger system . Alpha @-@ 2 receptors usually have inhibitory effects , but many are located pre @-@ synaptically ( i.e. , on the surface of the cells that release norepinephrine ) , so the net effect of alpha @-@ 2 activation is often a decrease in the amount of norepinephrine released . Alpha @-@ 1 receptors and all three types of beta receptors usually have excitatory effects . 
