 These are drugs that block the effects of noradrenergic alpha receptors while having little or no effect on beta receptors . Drugs belonging to this group can have very different effects , however , depending on whether they primarily block alpha @-@ 1 receptors , alpha @-@ 2 receptors , or both . Alpha @-@ 2 receptors , as described elsewhere in this article , are frequently located on norepinephrine @-@ releasing neurons themselves and have inhibitory effects on them ; consequently blockage of alpha @-@ 2 receptors usually results in an increase in norepinephrine release . Alpha @-@ 1 receptors are usually located on target cells and have excitatory effects on them ; consequently blockage of alpha @-@ 1 receptors usually results in blocking some of the effects of norepinephrine . Drugs such as phentolamine that act on both types of receptors can produce a complex combination of both effects . In most cases when the term " alpha blocker " is used without qualification , it refers to a selective alpha @-@ 1 antagonist . 
