Aminergic and peptidergic amplification of intracellular cyclic AMP levels in a molluscan neural network.

5-HT, DA and SCP sub B can activate adenylate cyclase and increase the intracellular cAMP levels in the Limax PC lobe, with differing time courses and to differing extents. 5-HT and SCP sub b are potent stimulators of adenylate cyclase and when both were applied simultaneously, an additive effect was observed. In contrast, DA shows a great variability in the time course of cAMP synthesis and is a weak stimulator. Ergonovine, a DA antagonist, failed to inhibit cyclase activation, indicating that ergonovine-sensitive receptors are absent or ergonovine- sensitive DA receptors are not coupled to adenylate cyclase. 5-HT and SCP sub B cause a rapid synthesis of cAMP, reaching the maximum 20 to 30-fold increase within a minute. DA's effect is slow in onset and very prolonged, reaching a maximum of only 2 to 3-fold increase of cAMP level. The reason for variability in DA action is not clear.