Serotonergic regulation of neuronal excitability in the prefrontal cortex

The cerebral cortex receives a dense serotonergic innervation originating predominantly from the dorsal raphe nucleus. This innervation regulates cortical functioning by activating multiple serotonin receptors that are differentially expressed by pyramidal cells and interneurons. Electrophysiological studies in the prefrontal cortex indicate that receptors of the 5-HT_1A and 5-HT_2A subtypes are the main serotonin receptors regulating membrane excitability in pyramidal cells. Most pyramidal cells in layer V coexpress 5-HT_1A and 5-HT_2A receptors that together regulate how these neurons encode excitatory input into neuronal firing. In contrast, a subset of large pyramidal cells of deep layer V appears to express exclusively 5-HT_2A receptors that depolarize and excite these cells. Serotonin also depolarizes and excites at least two classes of GABAergic interneurons by acting on 5-HT₃ and 5-HT_2A receptors. The differential expression of serotonin receptors in different pyramidal cells and interneurons is consistent with a growing appreciation of the anatomical, molecular and functional heterogeneity of pyramidal cells and interneurons of the cerebral cortex. These findings begin to lay the ground for a cellular-level understanding of the serotonergic regulation of the prefrontal cortex.