Serotonin engages divergent 5-HT receptor pathways for cell type-resolved modulation of prefrontal layer 5 microcircuits.

BACKGROUND AND PURPOSE: Serotonin (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 APPROACH: We 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

RESULTS

5-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, whereas nFS interneurons were largely unaffected. At the synaptic level, 5-HT suppressed excitatory synaptic transmission between pyramidal neurons via presynaptic 5-HT1BRs. Selective activation of presynaptic 5-HT3ARs enhanced inhibitory transmission from FS interneurons, whereas the overall effect of 5-HT on inhibitory synapses was suppressive, likely reflecting dominant presynaptic 5-HT1BR-mediated inhibition.

CONCLUSIONS AND IMPLICATIONS: Serotonin exerts complex, cell type-specific modulation of mPFC microcircuits through distinct somatodendritic and presynaptic receptor pathways. This balanced and targeted regulation provides a mechanistic basis for serotonergic influence on attention, cognitive flexibility, and emotional regulation.