Heterologous sensitization of adenylate cyclase activity by serotonin in the rat cerebral cortex

In vitro exposure of rat cerebrocortical slices to microM concentrations of serotonin (5HT) results in an increased response of adenylate cyclase to isoproterenol (ISO). No change in the affinity of the beta-adrenoceptor toward the agonist was found after 5HT exposure when measuring ISO displacement of [3H]CGP 12177 binding. A similar increase of adenylate cyclase response was also found when using VIP as a stimulatory agent. The dose-response curve of adenylate cyclase to the GTP analogue, GppNHp, was modified by 5HT, which promotes a significantly higher maximal response without altering the potency of GppNHp. Forskolin-stimulated adenylate cyclase activity was not affected by 5HT. Serotonergic 5HT2 receptors are involved in the sensitization of adenylate cyclase to GppNHp, since the selective 5HT2 antagonist ketanserin inhibits the effect of 5HT, whereas the 5HT2 agonist DOI mimics 5HT. The involvement of 5HT2 receptor-coupled activation of protein kinase C is also demonstrated: direct protein kinase C activators such as phorbol esters and s,n-dioctanoylglycerol behave in the same manner as 5HT, while the protein kinase C inhibitor CGP 41251 prevents 5HT from increasing adenylate cyclase responsiveness to GppNHp. Moreover, in vitro exposure of cortical slices to 5HT results in reduced inhibition of adenylate cyclase by somatostatin. Since no change was observed at the receptor level and in the direct stimulation of the catalytic subunit of the enzyme, we propose that 5HT might accomplish the sensitization of adenylate cyclase through protein kinase C by inactivating the inhibitory coupling protein Gi and facilitating the interaction of the exogenous GppNHp with the stimulatory coupling protein Gs.