Adrenergic receptors

The Adrenoceptors are GPCRs activated through both adrenaline (epinephrine) and noradrenaline (norepinephrine). α₁ receptors are usually postsynaptic and coupled via G_q to phospholipase C therefore are excitatory by increasing the concentrations of the second messengers inositol trisphosphate and diacylglycerol concentrations. All β adrenoceptors are attached to Gs proteins and raise the cAMP levels. In the CNS, α₂ receptors are presynaptic autoreceptors, decreasing NA release by lowering the cAMP-mediated phosphorylation of Ca²⁺ channels. The Presynaptic β receptors are also found on the noradrenergic terminals in the brain. These make possible NA release by increasing the cAMP-mediated phosphorylation and opening of Ca²⁺ channels. Both the inhibitory and excitatory effects of NA release are seen postsynaptically in the CNS neurons.

Typically clonidine, phenylephrine, and isoproterenol act as quite selective agonists on α₁, α₂, and β receptors correspondingly.  The Prazosin blocks α response. β-blockers (example, propranolol) are selective β receptor antagonists.