Clonidine

Clonidine is an alpha-2 NE receptor agonist. Alpha-2 NE receptors are Gi coupled (and therefore serve as autoreceptors).

Clonidine will preferentially bind to the alpha-2 NE receptors which are autoreceptors. Therefore, Clonidine, though an agonist, will actually decrease NE release, and has an almost identical effect to antagonists.