α2 receptors are located on the presynaptic terminal. eg Clonidine ligand is an α2 receptor agonist. http://en.wikipedia.org/wiki/Alpha-2_adrenergic_receptor
ALPHAGAN® P (brimonidine tartrate ophthalmic solution) 0.1% or 0.15% is an alpha adrenergic receptor agonist indicated for the reduction of elevated intraocular pressure (IOP) in patients with open-angle glaucoma or ocular hypertension.
(eg metoprolol is a blocker) & (eg albuterol is a agonist) receptors work by Gs (AC -> cAMP -> PKA), while (eg terazosin is an blocker) work by Gq (PLC -> -> DAG -> PKC. Besides the heart, receptors are used for aqueous humor production.