Activation of alpha-2 adrenergic receptors augments neurotransmitter-stimulated cyclic AMP accumulation in rat brain cerebral cortical slices.

Norepinephrine-stimulated cyclic AMP (cAMP) accumulation in brain is mediated by both alpha and beta adrenergic receptors. The functional activity of these receptors can be differentiated by examining the response to isoproterenol alone and isoproterenol + 6-fluoronorepinephrine, and alpha adrenergic agonist. The present study was undertaken to define the pharmacological characteristics of the alpha adrenergic component associated with cAMP accumulation in brain. Using a prelabeling technique it was found that norepinephrine- or isoproterenol plus 6-fluoronorepinephrine-stimulated cAMP accumulation was only inhibited partially by an alpha-1 adrenergic receptor antagonist. In contrast, alpha-2 adrenergic receptor antagonists inhibited completely that portion of the response exceeding that produced by isoproterenol alone (alpha adrenergic augmentation). Furthermore, selective alpha-1 adrenergic agonists were incapable of enhancing the cAMP response to isoproterenol, whereas alpha-2 adrenergic agonists were active in this regard. Partial agonists for the alpha-2 adrenergic receptor were ineffective as augmentors. The data suggest that the alpha adrenergic component of the norepinephrine response in rat brain slices has characteristics of both alpha-1 and alpha-2 receptors, with the alpha-2 adrenergic component being a major contributor in this regard.