Which one of the following Norepinephrine acts by decreasing cAMP concentration?

The a2-adrenergic receptor is classically located on vascular prejunctional terminals where it inhibits the release of norepinephrine (noradrenaline) in a form of negative feedback. It is also located on the vascular smooth muscle cells of ceain blood vessels, such as those found in skin aerioles or on veins, where it sits alongside the more plentiful a1-adrenergic receptor. The a2-adrenergic receptor binds both norepinephrine released by sympathetic postganglionic fibers and epinephrine (adrenaline) released by the adrenal medulla, binding norepinephrine with slightly higher affinity. It has several general functions in common with the a1-adrenergic receptor, but also has specific effects of its own. Agonists (activators) of the a2-adrenergic receptor are frequently used in veterinary anaesthesia where they effect sedation, muscle relaxation and analgesia through effects on the central nervous system Ref: guyton and hall textbook of medical physiology 12 edition page number:605,606,607,608