Cyclic AMP Analogues Potentiate k-Opiate and Attenuate α2-Adrenoceptor Agonist Effects on Intrasynaptosomal Free Calcium

The intrasynaptosomal free calcium concentration (Ca2+]i) was measured in quin2-loaded synaptosomes prepared from rat cerebral cortex. Membrane-permeant cyclic adenosine-3',5'-monophosphate (cAMP) analogues 8-bromo-cyclic adenosine-3',5'-monophosphate (8-Br-cAMP) and dibutyryl-cyclic adenosine-3',5'-monophosphate (db-cAMP)] increased Ca2+]i in a dose-dependent manner; The maximal increases were approximately 50% for 8-Br-cAMP and 35% for db-cAMP and occurred at approximately 10 microM with both analogues. Clonidine (1 microM) alone reduced Ca2+]i by 26.5%; db-cAMP and 8-Br-cAMP attenuated this reduction to 14.2 and 8.2%, respectively. In contrast, the reduction (19.9%) in Ca2+]i induced by the preferential kappa-opiate agonist dynorphin A(1-13) was not attenuated by the cAMP analogues; in fact, db-cAMP and 8-Br-cAMP potentiated the effect of dynorphin A(1-13) (1 microM), producing decreases in Ca2+]i of 33.6 and 29.6%, respectively. We conclude that although alpha 2-adrenergic and kappa-opiate receptors both reduce Ca2+]i, the alpha 2-adrenoceptor-mediated response and the kappa-opiate receptor-mediated response involve different effector mechanisms. It appears that presynaptic alpha 2-adrenoceptor agonist effects are linked to reductions in adenylate cyclase activity and cAMP production and a resultant increase in Ca2+ sequestration, Ca2+-channel blockade, or both. On the other hand, the kappa-opiate-mediated effects possibly involve an increase in cAMP production and a blockade of Ca2+ entry.