An Examination of the Involvement of Phospholipases A2 and C in the α‐Adrenergic and γ‐Aminobutyric Acid Receptor Modulation of Cyclic AMP Accumulation in Rat Brain Slices

: Experiments were undertaken to define the role of two calcium-associated enzyme systems in modulating transmitter-stimulated production of cylic nucleotides in rat brain. Cyclic AMP (cAMP) accumulation was examined in cerebral cortical slices using a prelabeling technique. The enhancement of isoproterenol-stimulated cAMP production by α-adrenergic and γ-aminobutyric acid-B (GABAB) agonists was reduced by exposing the tissue to EGTA, a chelator of divalent cations, or quinacrine, a nonselective inhibitor of phospholipase A2. Likewise, chronic (2 weeks) administration of corticosterone decreased the α-adrenergic and GABAB receptor modulation of second messenger production. Neither cyclooxygenase nor iipoxygenase inhibitors selectively influenced the facilitating response of α-adrenergic and GABAB agonists. Other experiments revealed that although norepinephrine and 6-fluoronorepinephrine stimulated inositol phosphate (IP) production in cerebral cortical slices with potencies equal to those displayed in the cyclic nucleotide assay, selective α1,-adrenergic agonists were less efficacious on IP formation and were without effect in the cAMP assay. Conversely, a selective α2-adrenergic receptor agonist facilitated the cAMP response to a β-adrenergic agonist without affecting IP formation. The rank orders of potency of a series of α-adrenergic antagonists suggest that IP accumulation is mediated solely by α1-adrenergic receptors, whereas the augmentation of cAMP accumulation is regulated by a mixed population of α-adrenergic sites. The results suggest that the α-adrenergic and GABAB receptor-mediated enhancement of isoproterenol-stimulated cAMP formation appears to be more closely associated with phospholipase A2 than phospholipase C and may be mediated by arachidonate or some other fatty acid.