Background-Elevated plasma norepinephrine levels are associated with increased mortality in patients and in animal models with chronic heart failure. To test which ␣ 2 -adrenoceptor subtypes operate as presynaptic inhibitory receptors to control norepinephrine release in heart failure, we investigated the response of gene-targeted mice lacking ␣ 2 -adrenoceptor subtypes (␣ 2 -KO) to chronic left ventricular pressure overload. In addition, we determined the functional consequences of genetic variants of ␣ 2 -adrenoceptors in human patients with chronic heart failure. Methods and Results-Cardiac pressure overload was induced by transverse aortic constriction. Three months after aortic banding, survival was dramatically reduced in ␣ 2A -KO (52%) and ␣ 2C -KO (47%) mice compared with wild-type and ␣ 2B -deficient (86%) animals. Excess mortality in ␣ 2A -and ␣ 2C -KO strains was attributable to heart failure with enhanced left ventricular hypertrophy and fibrosis and elevated circulating catecholamines. The clinical importance of this finding is emphasized by the fact that heart failure patients with a dysfunctional variant of the ␣ 2C -adrenoceptor had a worse clinical status and decreased cardiac function as determined by invasive catheterization and by echocardiography. Conclusions-Our results indicate an essential function of ␣ 2A -and ␣ 2C -adrenoceptors in the prevention of heart failure progression in mice and human patients. Identification of heart failure patients with genetic ␣ 2 -adrenoceptor variants as well as new ␣ 2 -receptor subtype-selective drugs may represent novel therapeutic strategies in chronic heart failure and other diseases with enhanced sympathetic activation.

alpha(2)-Adrenergic receptors play an essential role in regulating neurotransmitter release from sympathetic nerves and from adrenergic neurons in the CNS. However, the role of each of the three highly homologous alpha(2)-adrenergic receptor subtypes (alpha(2A), alpha(2B), alpha(2C)) in this process has not been determined unequivocally. To address this question, the regulation of norepinephrine and dopamine release was studied in mice carrying deletions in the genes encoding the three alpha(2)-adrenergic receptor subtypes. Autoradiography and radioligand binding studies showed that alpha(2)-receptor density in alpha(2A)-deficient brains was decreased to 9 +/- 1% of the respective wild-type value, whereas alpha(2)-receptor levels were reduced to 83 +/- 4% in alpha(2C)-deficient mice. These results indicate that approximately 90% of mouse brain alpha(2)-receptors belong to the alpha(2A) subtype and 10% are alpha(2C)-receptors. In isolated brain cortex slices from wild-type mice a non-subtype-selective alpha(2)-receptor agonist inhibited release of [(3)H]norepinephrine by maximally 96%. Similarly, release of [(3)H]dopamine from isolated basal ganglion slices was inhibited by 76% by an alpha(2)-receptor agonist. In alpha(2A)-receptor-deficient mice, the inhibitory effect of the alpha(2)-receptor agonist on norepinephrine and dopamine release was significantly reduced but not abolished. Only in tissues from mice lacking both alpha(2A)- and alpha(2C)-receptors was no alpha(2)-receptor agonist effect on transmitter release observed. The time course of onset of presynaptic inhibition of norepinephrine release was much faster for the alpha(2A)-receptor than for the alpha(2C)-subtype. After prolonged stimulation with norepinephrine, presynaptic alpha(2C)-adrenergic receptors were desensitized. From these data we suggest that two functionally distinct alpha(2)-adrenergic receptor subtypes, alpha(2A) and alpha(2C), operate as presynaptic inhibitory receptors regulating neurotransmitter release in the mouse CNS.

␣ 2 -Adrenoceptors mediate diverse functions of the sympathetic system and are targets for the treatment of cardiovascular disease, depression, pain, glaucoma, and sympathetic activation during opioid withdrawal. To determine whether ␣ 2 -adrenoceptors on adrenergic neurons or ␣ 2 -adrenoceptors on nonadrenergic neurons mediate the physiological and pharmacological responses of ␣ 2 -agonists, we used the dopamine ␤-hydroxylase (Dbh) promoter to drive expression of ␣ 2A -adrenoceptors exclusively in noradrenergic and adrenergic cells of transgenic mice. Dbh-␣ 2A transgenic mice were crossed with double knockout mice lacking both ␣ 2A -and ␣ 2C -receptors to generate lines with selective expression of ␣ 2A -autoreceptors in adrenergic cells. These mice were subjected to a comprehensive phenotype analysis and compared with wild-type mice, which express ␣ 2A -and ␣ 2C -receptors in both adrenergic and nonadrenergic cells, and ␣ 2A /␣ 2C double-knockout mice, which do not express these receptors in any cell type. We were surprised to find that only a few functions previously ascribed to ␣ 2 -adrenoceptors were mediated by receptors on adrenergic neurons, including feedback inhibition of norepinephrine release from sympathetic nerves and spontaneous locomotor activity. Other agonist effects, including analgesia, hypothermia, sedation, and anesthetic-sparing, were mediated by ␣ 2 -receptors in nonadrenergic cells. In dopamine ␤-hydroxylase knockout mice lacking norepinephrine, the ␣ 2 -agonist medetomidine still induced a loss of the righting reflex, confirming that the sedative effect of ␣ 2 -adrenoceptor stimulation is not mediated via autoreceptor-mediated inhibition of norepinephrine release. The present study paves the way for a revision of the current view of the ␣ 2 -adrenergic receptors, and it provides important new considerations for future drug development.Adrenergic receptors are important targets for the treatment of human diseases and conditions including hypertension and heart failure, psychiatric and neurological diseases, asthma, and pain (Westfall and Westfall, 2006). To date, nine different adrenergic receptor subtypes have been cloned and grouped into three receptor groups, including ␣ 1A,B,D , ␣ 2A,B,C , and ␤ 1,2,3 (Bylund et al., 1994). However, the therapeutic potential of these subtypes has not been fully explored because of the lack of ligands with sufficient subtype-selectivity. At present, only four of the nine possible subtype distinctions (i.e., ␣ 1 , ␣ 2 , ␤ 1 , and ␤ 2 ) have achieved clinical relevance (Westfall and Westfall, 2006). Especially within the ␣ 1 -and ␣ 2 -receptor subgroups, the physiological significance of individual receptor subtypes has remained unclear until recently. For the ␣ 2 -adrenoceptors, mouse models with targeted deletions of the individual subtypes have greatly advanced our understanding of the physiological role and the therapeutic potential of these receptors (Gilsbach and Hein, 2008). Activation of ␣ 2A -receptors could be linked with bradycardia ...

A substantial percentage of human pregnancies are lost as spontaneous abortions after implantation. This is often caused by an inadequately developed placenta. Proper development of the placental vascular system is essential to nutrient and gas exchange between mother and developing embryo. Here we show that alpha(2)-adrenoceptors, which are activated by adrenaline and noradrenaline, are important regulators of placental structure and function. Mice with deletions in the genes encoding alpha(2A)-, alpha(2B)- and alpha(2C)-adrenoceptors died between embryonic days 9.5 and 11.5 from a severe defect in yolk-sac and placenta development. In wildtype placentae, alpha(2)-adrenoceptors are abundantly expressed in giant cells, which secrete angiogenic factors to initiate development of the placental vascular labyrinth. In placentae deficient in alpha(2A)-, alpha(2B)- and alpha(2C)-adrenoceptors, the density of fetal blood vessels in the labyrinth was markedly lower than normal, leading to death of the embryos as a result of reduced oxygen and nutrient supply. Basal phosphorylation of the extracellular signal regulated kinases ERK1 and ERK2 was also lower than normal, suggesting that activation of the mitogen-activated protein kinase (MAP kinase) pathway by alpha(2)-adrenoceptors is required for placenta and yolk-sac vascular development. Thus, alpha(2)-adrenoceptors are essential at the placental interface between mother and embryo to establish the circulatory system of the placenta and thus maintain pregnancy.