Morphological and electrophysiological properties of atypically oriented layer 2 pyramidal cells of the juvenile rat neocortex

We used whole-cell patch clamp recordings combined with intracellular dye-f illing to examine the morphological and electrophysiological properties of atypically oriented pyramidal cells located at the layer 1/2 border of the juvenile rat neocortex. Orientation of the apical dendrite varied from obli que (>20 degrees from vertical) to truly horizontal (90 degrees from vertic al). The length of the apical dendrite ranged from 150 to 400 mum. The tota l horizontal domain of the dendritic tree (including basal dendrites) of th e longest horizontal pyramids exceeded 500 mum, but we also found short hor izontal cells with horizontal dendritic domains of less than 300 mum. In ad dition, atypically oriented pyramids had long horizontal axon collaterals i n layer 1/2. Electrophysiologically, atypically oriented pyramidal cells ha d intrinsic membrane properties similar to regularly oriented pyramids that have been described in the superficial layers at this age in the rat. Cell s that fired repetitively were all regular spiking. In addition, we identif ied a subgroup of neurons (20%) in this sample, which were unable to fire m ore than a few spikes at the beginning of the current pulse. We suggest that the unique orientation and size of their dendritic trees an d the length and arrangement of their local axons collaterals make atypical ly oriented pyramids in layer 2 ideally suited to perform horizontal integr ation of synaptic inputs in the neocortex. (C) 2000 IBRO. Published by Else vier Science Ltd. All rights reserved.