Intrinsic connectivity of the rat subiculum: I. Dendritic morphology and patterns of axonal arborization by pyramidal neurons

The dendritic and axonal morphology of rat subicular neurons was studied in single cells labeled with Neurobiotin. Electrophysiological classification of cells as intrinsic burst firing or regular spiking neurons was correlat ed with morphologic patterns and cell locations. Every cell had dendritic b ranches that reached the outer molecular layer, with most cells having bran ches that reached the hippocampal fissure. All but two pyramidal cells had axon collaterals that entered the deep white matter (alveus). Branching pat terns of apical dendrites varied as a function of the cell's soma location along the fissure-alveus axis of the cell layer. The first major dendritic branch point for most cells occurred at the superficial edge of the cell la yer giving deep cells long primary apical dendrites and superficial cells s hort or absent primary apical dendrites. In contrast, basal dendritic arbor s were similar across cells regardless of cell position. Apical and basal d endrites of all cells had numerous spines. Superficial and deep cells also differed in axonal collateralization. Deep cells (mostly intrinsically burs ting [IB] class) had one or more ascending axon collaterals that typically remained within the region circumscribed by their apical dendrites. Superfi cial cells (mostly regular spiking [RS] class) tended to have axon collater als that reached longer distances in the cell layer. Numerous varicosities and axonal extensions were present on axon collaterals in the cell layer an d in the apical dendritic region, suggesting intrinsic connectivity. Axonal varicosities and extensions were found on axons that entered presubiculum, entorhinal cortex or CA1, supporting the notion that these were projection cells. Local collaterals were distinctly thinner than collaterals that wou ld leave the subiculum, suggesting little or no myelin on local collaterals and some myelin on efferent fibers. We conclude that both IB and RS classe s of subicular principal cells make synaptic contacts in and apical to the cell layer. Based on the patterns of axonal arborization, we suggest that s ubiculum has at least a crude columnar and laminar architecture, with ascen ding collaterals of deep cells forming columns and broader axonal arbors of superficial cells serving to distribute activity across multiple columns. (C) 2001 Wiley-Liss, Inc.