Figure 6.
A schematic of the steps involved in induction of serotonergic (cAMP-dependent) enhancement of transmission, tetanic activation of LTF, and synaptic tagging. cAMP enhances transmission by activating Epac and HCNCs, the latter acting through an actin-dependent step. LTF induction involves tetanic elevation of [Na+]i, hyperpolarization of Na+-K+ exchange, and HCNC activation, the latter again probably acting through an actin-dependent step, as well as a rise in presynaptic [Ca2+]i, protein synthesis, and a number of enzymatic phosphorylation and dephosphorylation pathways. Tagging represents the previous activation of HCNCs by conditioning stimulation with [Ca2+]i elevation that renders subsequent effects of cAMP independent of HCN activation and actin, so that only activation of Epac is required for cAMP to further enhance transmission. 5-HT-R, 5-HT (serotonin) receptor; LTG, lamotrigine.