Neurons immunoreactive for vasoactive intestinal polypeptide in the rat primary somatosensory cortex: Morphology and spatial relationship to barrel-related columns

Vasoactive intestinal polypeptide (VIP) in neocortex affects neuronal excit ability as well as cortical blood flow and metabolism. Interneurons immunor eactive for VIP (VLP-IR neurons) are characterized by their predominantly b ipolar appearance and the radial orientation of their main dendrites. In or der to determine whether the morphology of VIP-IR neurons is related to the functional organization of the cortex into vertical columns, we combined b oth immunostaining of neurons containing VIP and cytochrome oxidase histoch emistry for visualizing barrels, morphological layer TV correlates of funct ional columns, in the primary somatosensory (barrel) cortex of rats. VIP-IR neurons were localized in supragranular (48%), granular (16%), and infragr anular layers (36%) as well as in the white matter. In the granular layer, a clear trend that more neurons were located in interbarrel septa rather th an in barrels could be observed, resulting in a neuronal density which was about one-third higher in the septal area. VIP-IR neurons from the differen t cortical layers were three-dimensionally reconstructed from serial sectio ns by using a computer microscope system. The neurons were mostly bipolar. Striking morphological differences in both axonal and dendritic trees were found between neurons whose cell bodies were located in supragranular, gran ular, and the upper part of infragranular layers, and those whose cell bodi es were located in the area below. The former had dendrites which often rea ched layer I, where they bifurcated several times, and axonal trees which w ere particularly oriented vertically, with a tangential extent smaller than the width of barrels. Therefore, these neurons were mostly confined to eit her a barrel- or septum-related column. By contrast, the dendrites of neuro ns of the latter group did not reach the granular layer. Furthermore, these neurons had axons with sometimes very long horizontal collaterals, which o ften spanned two, in one case three, barrel columns. It is proposed that th e differential morphology of neurons with different locations as stated abo ve parallels to some extent the divergence of input streaming into the corr esponding layer-defined areas. As a possible consequence of this, VIP-IR ne urons may be capable of adapting the excitability and metabolism of cortica l compartments either in a spatially limited or more extensive way. J. Comp . Neurol. 420:291-304, 2000. (C) 2000 Wiley-Liss, Inc.