α2-Adrenoceptor subtypes—Unexpected functions for receptors and ligands derived from gene-targeted mouse models

α2-Adrenoceptors belong to the group of nine adrenoceptors which mediate the biological actions of the endogenous catecholamines adrenaline and noradrenaline. Studies with gene-targeted mice carrying deletions in the genes encoding α2A-, α2B- or α2C-adrenoceptors have provided new insight into adrenergic receptor biology: (1) In principle, all three α2-receptor subtypes may operate as presynaptic inhibitory feedback receptors to control the release of noradrenaline and adrenaline or other transmitters from neurons. (2) Pharmacological effects of non-selective α2-ligands could be assigned to specific receptor subtypes, e.g. hypotension, sedation and analgesia are mediated via α2A-receptors. (3) α2-Adrenoceptor deficient mice have helped to uncover novel and unexpected functions of these receptor, e.g. feedback control of catecholamine release via α2C-receptors in adrenal chromaffin cells and control of angiogenesis during embryonic development. (4) Additional pharmacological targets for α2-adrenoceptor ligands were identified, e.g. inhibition of cardiac HCN2 and HCN4 pacemaker channels by clonidine.