Hippocampal synaptic transmission and Ca²⁺ signaling are altered by the selective 5-HT_2A receptor agonist, 25CN-NBOH

Classical psychedelics exert therapeutic effects on affective disorders, with serotonin 5-HT_2A receptor activation thought to play a central role. The cellular mechanisms by which 5-HT_2A signaling modulates neural circuits remain incompletely understood and may differ across mood-regulating brain regions. While psychedelic actions have been extensively studied in the medial prefrontal cortex, the hippocampus which is critical for mood, and memory has received less attention, and effects of selective 5-HT_2A agonists on hippocampal neurons remain poorly characterized. Here, we examined the effects of the selective 5-HT_2A agonist 25CN-NBOH on synaptic transmission, intrinsic excitability, and intracellular calcium in mouse hippocampal CA1 pyramidal neurons using whole-cell patch-clamp electrophysiology and Fura-2 AM imaging. 25CN-NBOH (10 μM) increased both spontaneous excitatory and inhibitory synaptic transmission, as indicated by elevated sEPSC and sIPSC frequency and amplitude, without affecting miniature events, suggesting action potential-dependent mechanisms. These synaptic effects persisted in the presence of the 5-HT_2A antagonist MDL-100907, indicating 5-HT_2A -independent synaptic facilitation. Despite increased synaptic drive, neuronal firing and action potential properties were unchanged, consistent with balanced excitation and inhibition. In contrast, 25CN-NBOH induced robust intracellular calcium elevations in CA1 neurons that were significantly reduced by MDL-100907, as well as by TTX and AMPA/NMDA receptor blockade, indicating dependence on both 5-HT_2A receptor activation and glutamatergic presynaptic activity. Together, these findings reveal separable mechanisms of action: 5-HT_2A-independent, presynaptic facilitation, and 5-HT_2A-dependent calcium signaling. These results highlight distinct modes of hippocampal modulation by a selective 5-HT_2A receptor agonist and suggest mechanisms through which psychedelics may promote plasticity-related processes.