Final answer:
Norepinephrine binding to α2-adrenergic receptors causes a decrease in cyclic AMP levels within the target cell, as these receptors inhibit adenylyl cyclase through the Gi protein.
Step-by-step explanation:
A decrease in cyclic AMP (cAMP) within the target cell occurs when norepinephrine binds to α2-adrenergic receptors. The mechanism involves the α2-adrenergic receptors coupling to a G-protein known as Gi, which inhibits the action of adenylyl cyclase and thus reduces the synthesis of cAMP. Conversely, binding to β-adrenergic receptors is associated with an increase in cAMP as these receptors are coupled to Gs proteins, which stimulate adenylyl cyclase activity. Norepinephrine mainly acts through the α-adrenergic receptors, leading to a decrease in cAMP levels within the cell, which in turn can cause effects such as smooth muscle contraction.