Intrinsic circuitry and physiological properties of pyramidal neurons in rat barrel cortex

Pyramidal neurons in the rat posteromedial barrel subfield (PMBSF) were characterized physiologically and filled with biocytin in in vitro brain slices. Intrinsic axons belonging to supragranular neurons projected horizontally and vertically, arborizing in layers II/III and V, but had few or no projections to layers IV or VI. These axons projected horizontally for up to 2 mm, spanning two to seven barrel columns. Layer V neurons had more diffuse axon arbors that projected either vertically, arborizing in layers III to V, or horizontally, branching profusely in layers V and VI. The basal dendritic trees of neurons in layers II/III, V and VI spanned one or two barrel columns without being skewed toward particular barrel columns. Physiologically, regular-spiking neurons were classified as "RS1" or "RS2" according to their degree of late spike frequency adaptation. RS1 neurons were found in superficial and deep layers, whereas RS2 neurons were significantly more prevalent in the latter. Infragranular, but not supragranular neurons showed slow, inward rectification at hyperpolarized potentials. All neurons generated fast and medium afterhyperpolarizations following individual spikes; however, only infragranular pyramids had depolarizing afterpotentials interposed between the two afterhyperpolarizations. RS1 neurons had larger cell bodies, longer total basal dendritic lengths, and more densely branched proximal dendritic trees than RS2 neurons. These findings indicate that pyramidal neurons in the deep and superficial layers of the rat PMBSF have distinct patterns of intracortical axon arbors and distinct physiological properties. These features are probably involved in shaping and modulating the response properties of PMBSF neurons.