alpha2-adrenergic receptor subtypes - novel functions uncovered in gene-targeted mouse models.

The family of adrenergic receptors contains nine different subtypes of G protein-coupled receptors which mediate the biological effects of adrenaline and noradrenaline. With few exceptions, the full therapeutic potential of subtype-selective therapy has not yet been explored for the group of adrenergic receptors. In the absence of sufficiently subtype-selective ligands which can distinguish between individual receptor subtypes of the adrenergic family, gene-targeted mouse models with deletions in these receptor genes have recently been generated and characterized. These genetic mouse models have helped to assign specific pharmacological effects of alpha(2)-receptor agonists or antagonists to individual receptor subtypes. However, some unexpected and novel functions of alpha(2)-adrenergic receptors were also uncovered in these mouse models: Presynaptic control of catecholamine release from adrenergic nerves in the central and sympathetic nervous system may be regulated by three different alpha(2)-receptor subtypes, alpha(2A), alpha(2B), and alpha(2C). A similar feedback loop also controls the release of catecholamines from the adrenal gland. alpha(2B)-receptors are not only involved in regulating vascular tone in the adult organism, but they are essential for the development of the vascular system of the placenta during prenatal development. The challenge will now be to generate strategies to identify whether the findings obtained in gene-targeted mice may predict the action of receptor subtype-selective drugs in humans.