Cyclic Adenosine 3’,5’-Monophosphate in Rat Cerebral Cortical Slices: Effects of Methoxamine and Clonidine

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Abstract
In incubated slices of cerebral cortex from Sprague-Dawley rats, methoxamine and clonidine have no effect on basal levels of cyclic AMP. Methoxamine effectively inhibits the noradrenaline-stimulated formation of cyclic AMP. The inhibitory constant for methoxamine was 12.6 μmol/l. In the presence of 100 μmol/l adenosine, methoxamine does not inhibit the activity of noradrenaline, but is capable to activate α-adrenergic receptors leading to enhanced formation of cyclic AMP. The mechanism by which adenosine alters adrenergic receptors to become methoxamine-sensitive is not known. Clonidine inhibits the effect of noradrenaline alone or in combination with adenosine on the cyclic-AMP-generating system. It does not, as reported earlier, enhance the activity of submaximal concentrations of the β.-adrenergic agonist isoproterenol. These data do not support the concept of adrenergic receptors which require both, α- and β-stimulation for maximal activation of adenylate cyclase.