a1-Adrenoceptors are widely distributed within the CNS, where their activation generally results in depolarization and increased neuronal firing. Most of the peripheral actions of a1-adrenoceptors are mediated through Gq/11 coupling to the inositol phosphate pathway, while there is evidence for activation of adenylyl cyclase within the CNS. Perhaps the most extensively characterized action of a2-adrenoceptors is the pre-junctionally mediated inhibition of neurotransmitter release from many peripheral and central neurons. In line with other receptors with similar actions a2-adrenoceptor mediated effects are mediated through the inhibition of adenylyl cyclase as a consequence of interaction of the agonist-receptor complex with G;/o, although other second messengers remain to be characterized. Most (-adrenoceptor mediated actions involve stimulation of adenylyl cyclase through the interaction of the agonist-receptor complex with Gs.
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