Serotonin Engages Divergent 5-HT Receptor Pathways for Cell Type-Resolved Modulation of Prefrontal Layer 5 Microcircuits

Background and PurposeSerotonin (5-HT) is a key neuromodulator in the prefrontal cortex (PFC), yet its cell type-specific effects across excitatory and inhibitory microcircuits remain incompletely understood. We aimed to determine how 5-HT shapes intrinsic excitability and synaptic transmission in defined neuronal populations of the medial PFC (mPFC). Experimental ApproachWe performed whole-cell recordings from morphologically and electrophysiologically characterised layer 5 (L5) neurons in rat mPFC. Pyramidal neurons were classified into adaptive-spiking high input resistance (ASH), adaptive-spiking low input resistance (ASL), and regular-spiking (RS) types; interneurons were categorised as non-fast-spiking (nFS), regular-fast-spiking (rFS), and stuttering-fast-spiking (sFS) interneurons. These cell types exhibited distinct membrane properties, firing patterns, and axonal projections. We assessed serotonergic modulation using bath-applied 5-HT and subtype-specific receptor mechanisms. Key Results5-HT produced divergent postsynaptic responses across pyramidal neuron subtypes: ASH and RS neurons depolarised via 5-HT2A receptor (5-HT2AR) activation, whereas ASL neurons exhibited 5-HT1AR-mediated hyperpolarisation. Among interneurons, rFS and sFS cells depolarised through ionotropic 5-HT3ARs, while nFS interneurons were largely unaffected. At the synaptic level, 5-HT suppressed excitatory synaptic transmission between pyramidal neurons via presynaptic 5-HT1BRs, and conversely enhanced GABAergic transmission from FS interneurons via presynaptic 5-HT3ARs. Conclusions & ImplicationsSerotonin exerts bidirectional, cell type-specific serotonergic modulation of local microcircuits that acts to suppress excitation while facilitating inhibition. This balanced and targeted regulation, arising from distinct distributions of somatodendritic and presynaptic 5-HT receptors, provides a mechanistic basis for serotonergic influence on attention, cognitive flexibility, and emotional regulation.