Molecular pharmacology of alpha 2-adrenoceptor subtypes

alpha 2-adrenergic receptors mediate many of the physiological actions of the endogenous catecholamines adrenaline and noradrenaline, and are targets of several therapeutic agents. alpha 2-adrenoceptor agonists are currently used as antihypertensives and as veterinary sedative anaesthetics. They are also used in humans as adjuncts to anaesthesia, as spinal analgesics, and to treat opioid, nicotine and alcohol dependence and withdrawal. Three human alpha 2-adrenoceptor subtype genes have been cloned and designated alpha 2-C10, alpha 2-C4, and alpha 2-C2, according to their location on human chromosomes 10, 4 and 2. They correspond to the previously identified pharmacological receptor subtypes alpha 2A, alpha 2C and alpha 2B. The receptor proteins share only about 50% identity in their amino acid sequence, but some structurally and functionally important domains are very well conserved. The most obvious functionally important differences between the receptor subtypes are based on their different tissue distributions; e.g. the alpha 2A subtype appears to be an important modulator of noradrenergic neurotransmission in the brain. The three receptors bind most alpha 2-adrenergic drugs with similar affinities, but some compounds (e.g. oxymetazoline) are capable of discriminating between the subtypes. Clinically useful subtype selectivity cannot be achieved with currently available pharmaceutical agents. The second messenger pathways of the three receptors show many similarities, but small functional differences between the subtypes may turn out to have important pharmacological and clinical consequences. All alpha 2-adrenoceptors couple to the pertussis-toxin sensitive inhibitory G proteins Gi and G(o), but recent evidence indicates that also other G proteins may interact with alpha 2-adrenoceptors, including Gs and Gq/11. Inhibition of adenylyl cyclase activity, which results in decreased formation of cAMP, is an important consequence of alpha 2-adrenoceptor activation. Many of the physiological effects of alpha 2-adrenoceptor activation cannot, however, be explained by decreases in cAMP formation. Therefore, alternative mechanisms have been sought to account for the various effects of alpha 2-adrenoceptor activation on electrophysiologic, secretory and contractile cellular responses. Recent results obtained from studies on ion channel regulation point to the importance of calcium and potassium channels in the molecular pharmacology of alpha 2-adrenoceptors.