Electrophysiological and morphological properties of layers V-VI principal pyramidal cells in rat prefrontal cortex in vitro

This study examined the electrophysiological and morphological characteristics of layers V-VI pyramidal prefrontal cortex (PFC) neurons. In vitro intracellular recordings coupled with biocytin injections that preserved some of the PFC efferents to the nucleus accumbens (NAc) were made in brain slices. Four principal pyramidal cell types were identified and classified as regular spiking (RS) (19%), intrinsic bursting (IB) (64%), repetitive oscillatory bursting (ROB) (13%), and intermediate (IM) (4%) types. All PFC cells exhibited either subthreshold oscillation in membrane voltage or pacemaker-like rhythmic firing. IB neurons were demonstrated electrophysiologically and cytochemically to be PFC-->NAc neurons. In all IB and some RS neurons, a tetrodotoxin-sensitive, slowly inactivating Na+ current and a transient Ni(2+)-sensitive, low-threshold Ca2+ current mediated subthreshold inward rectification. During sustained membrane depolarization, the Na+ current was opposed by a 4-aminopyridine- sensitive, outwardly rectifying, slowly inactivating K+ current. Together, these three currents controlled the firing threshold of the PFC neurons. All IB and ROB cells also had postspike Ca(2+)-mediated depolarizing afterpotentials, postburst Ca(2+)-dependent after hyperpolarizations, and low- and high-threshold Ca2+ spikes. In addition, ROB cells had a hyperpolarizing “sag” mediated by the cationic conductance, Ih. IB and ROB neurons had extensive dendritic trees and radially ascending or tangentially projecting axon collaterals. RS and IM cells had comparatively simpler morphological profiles. These electrophysiological and morphological properties of the four principal pyramidal PFC cell types have provided valuable details for understanding further how PFC processes input and transmit outputs to regions such as the NAc.