Interplay between 5-HT4 Receptors and GABAergic System within CA1 Hippocampal Synaptic Plasticity

The type 4 serotonin receptor (5-HT4R) is highly involved in cognitive processes such as learning and memory. Behavioral studies have shown a beneficial effect of its activation and conversely reported memory impairments by its blockade. However, how modulation of 5HT4R enables modifications of hippocampal synaptic plasticity remains elusive. To shed light on the mechanisms at work, we investigated the effects of the 5-HT4R agonist RS67333 on long-term potentiation (LTP) within the hippocampal CA1 area. Although high-frequency stimulation-induced LTP remained unaffected by RS67333, the magnitude of LTP induced by theta-burst stimulation was significantly decreased. This effect was blocked by the selective 5-HT4R antagonist RS39604. Further, 5-HT4R-induced decrease in LTP magnitude was fully abolished in the presence of bicuculline, a GABAAR antagonist; hence, demonstrating involvement of GABA neurotransmission. In addition, we showed that the application of a GABABR antagonist, CGP55845, mimicked the effect of 5-HT4R activation, whereas concurrent application of CGP55845 and RS67333 did not elicit an additive inhibition effect on LTP. To conclude, through investigation of theta burst induced functional plasticity, we demonstrated an interplay between 5-HT4R activation and GABAergic neurotransmission within the hippocampal CA1 area.