No abstract

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 a 2-receptor agonists or antagonists to individual receptor subtypes. However, some unexpected and novel functions of a 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 a 2-receptor subtypes, a 2A , a 2B , and a 2C. A similar feedback loop also controls the release of catecholamines from the adrenal gland. a 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.

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 ␣ 2 -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 ␣ 2 -adrenoceptors, we have generated mice with a targeted deletion of all 3 ␣ 2 -adrenoceptor subtypes (␣ 2ABC Ϫ/Ϫ ). ␣ 2ABC Ϫ/Ϫ mice were completely unresponsive to the analgesic and hypnotic effects of clonidine; however, clonidine significantly lowered heart rate in ␣ 2ABC Ϫ/Ϫ mice by up to 150 bpm. Clonidine-induced bradycardia in conscious ␣ 2ABC Ϫ/Ϫ mice was 32.3% (10 g/kg) and 26.6% (100 g/kg) of the effect in wild-type mice. A similar bradycardic effect of clonidine was observed in isolated spontaneously beating right atria from ␣ 2ABC -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 ␣ 2ABC -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.