3,4,5-trimethoxycinnamic acid methyl ester isolated from Polygala tenuifolia enhances hippocampal LTP through PKA and calcium-permeable AMPA receptor

Alzheimer's disease (AD) is a degenerative brain disorder characterized by progressive cognitive decline and neuronal death due to extracellular deposition of amyloid β (Aβ) and intracellular deposition of tau proteins. Recently approved antibody drugs targeting Aβ have been shown to slow the progression of the disease, but they have minimal effects on cognitive improvement. Therefore, there is a need to develop drugs with cognitive-enhancing effects that can be used in conjunction with these antibody treatments. In this study, we investigated whether Polygala tenuifolia (PT), traditionally known for its cognitive-enhancing effects, can improve synaptic plasticity and identified its active components and mechanisms. PT demonstrated a dose-dependent effect in enhancing long-term potentiation (LTP), and among its components, 3,4,5-trimethoxycinnamic acid methyl ester (TMCA) showed a similar LTP-enhancing effect. TMCA increased the phosphorylation of the GluA1 subunit of the α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and increased the amount of GluA1 on the synapse without affecting the amount of GluA2. Additionally, the increase in GluA1 induced by TMCA was inhibited by a PKA inhibitor. Consistent with these results, the enhancement of LTP by TMCA was inhibited by a GluA1 antagonist and a PKA inhibitor. In silico molecular docking experiments confirmed that TMCA binds to PKA. Finally, we confirmed the LTP-enhancing effect of TMCA in hippocampal slices from 5XFAD mice. These results suggest that PT and its active component, TMCA, can interact with PKA to enhance LTP, indicating the potential for improving cognitive function in AD patients.