Alpha2-adrenoceptor subtypes--unexpected functions for receptors and ligands derived from gene-targeted mouse models.

Alpha2-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 alpha2A-, alpha2B- or alpha2C-adrenoceptors have provided new insight into adrenergic receptor biology: (1) In principle, all three alpha2-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 alpha2-ligands could be assigned to specific receptor subtypes, e.g. hypotension, sedation and analgesia are mediated via alpha2A-receptors. (3) Alpha2-adrenoceptor deficient mice have helped to uncover novel and unexpected functions of these receptor, e.g. feedback control of catecholamine release via alpha2C-receptors in adrenal chromaffin cells and control of angiogenesis during embryonic development. (4) Additional pharmacological targets for alpha2-adrenoceptor ligands were identified, e.g. inhibition of cardiac HCN2 and HCN4 pacemaker channels by clonidine.

Direct inhibition of cardiac hyperpolarization-activated cyclic nucleotide-gated pacemaker channels by clonidine.

BACKGROUND: Inhibition of cardiac sympathetic tone represents an important strategy for treatment of cardiovascular disease, including arrhythmia, coronary heart disease, and chronic heart failure. Activation of presynaptic alpha2-adrenoceptors is the most widely accepted mechanism of action of the antisympathetic drug clonidine; however, other target proteins have been postulated to contribute to the in vivo actions of clonidine. METHODS AND RESULTS: To test whether clonidine elicits pharmacological effects independent of alpha2-adrenoceptors, we have generated mice with a targeted deletion of all 3 alpha2-adrenoceptor subtypes (alpha2ABC-/-). Alpha2ABC-/- mice were completely unresponsive to the analgesic and hypnotic effects of clonidine; however, clonidine significantly lowered heart rate in alpha2ABC-/- mice by up to 150 bpm. Clonidine-induced bradycardia in conscious alpha2ABC-/- mice was 32.3% (10 microg/kg) and 26.6% (100 microg/kg) of the effect in wild-type mice. A similar bradycardic effect of clonidine was observed in isolated spontaneously beating right atria from alpha2ABC-knockout and wild-type mice. Clonidine inhibited the native pacemaker current (I(f)) in isolated sinoatrial node pacemaker cells and the I(f)-generating hyperpolarization-activated cyclic nucleotide-gated (HCN) 2 and HCN4 channels in transfected HEK293 cells. As a consequence of blocking I(f), clonidine reduced the slope of the diastolic depolarization and the frequency of pacemaker potentials in sinoatrial node cells from wild-type and alpha2ABC-knockout mice. CONCLUSIONS: Direct inhibition of cardiac HCN pacemaker channels contributes to the bradycardic effects of clonidine gene-targeted mice in vivo, and thus, clonidine-like drugs represent novel structures for future HCN channel inhibitors.

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.