Coupling of neuronal 5‐HT7 receptors to activation of extracellular‐regulated kinase through a protein kinase A‐independent pathway that can utilize Epac

The roles of 3′,5′‐cyclic adenosine monophosphate (cAMP) and protein kinase A in 5‐hydroxytryptamine (5‐HT)7 receptor‐mediated activation of extracellular‐regulated kinase (ERK) were studied in cultured hippocampal neurons and transfected PC12 cells. Activation of ERK by neuronal Gs‐coupled receptors has been thought to proceed through a protein kinase A‐dependent pathway. In fact we identified coupling of 5‐HT7 receptors to activation of adenylyl cyclase and protein kinase A. However, no inhibition of agonist‐stimulated ERK activation was found when cells were treated with H‐89 and KT5720 at concentrations sufficient to completely inhibit activation of protein kinase A. However, activation of ERK was found to be sensitive to the adenylyl cyclase inhibitor 9‐(tetrahydrofuryl)‐adenine, suggesting a possible role for a cAMP‐guanine nucleotide exchange factor (cAMP‐GEF). Co‐treatment of cells with 8‐(4‐chlorophenylthio)‐2′‐O‐methyladenosine 3′,5′‐cyclic monophosphate, a direct activator of the cAMP‐GEFs Epac1 and 2, reversed the inhibition of agonist‐stimulated ERK activation induced by adenylyl cyclase inhibition. Additionally, over‐expression of Epac1 enhanced 5‐HT7 receptor‐mediated activation of ERK. These results demonstrate that the activation of ERK mediated by neuronal Gs‐coupled receptors can proceed through cAMP‐dependent pathways that utilize cAMP‐GEFs rather than protein kinase A.