Structure and projections of white matter neurons in the postnatal rat visual cortex

Transient contributions of subplate neurons to the initial development of t he cortex are well-characterized, yet little data are available on a subpop ulation of subplate neurons that persist in the white matter (WM) of the ce rebral cortex across development. To characterize the WM neurons, different ial interference contrast and Nomarski optics were used to visualize indivi dual cells in the WM in slices of rat visual cortex at postnatal ages 9-23. Soma-dendritic morphology and local axonal projection patterns, including probable synaptic innervation sites of their axons, were identified by intr acellular filling with biocytin during electrophysiologic recordings. Dendr itic branches of all WM neurons, tripartitioned here into upper, middle, an d deep divisions, extend throughout the WM and frequently into the overlyin g cortex. Axonal arborizations from most WM neurons, including apparent bou tons, project into adjacent WM with many also innervating overlying cortica l layers, whereas some project into the stratum oriens/alveus of the hippoc ampal formation. Processes of a subset of WM neurons appear to be confined to the WM itself. By using antimicrotubule associated protein (MAPS) immuno staining to quantify the density of WM neurons in rat visual cortex, we fin d that their overall numbers decrease to approximately 30% of initial level s during postnatal development. During this same developmental period, an i ncreasing percentage of WM neurons contain the synthetic enzyme for nitric oxide, nitric oxide synthase (NOS), as evaluated by immunostaining. Thus, W M neurons that survive the initial perinatal period of cell death are posit ioned under the laminae of the maturing cortex to potentially modulate the integration of visual signals through either conventional synaptic or nonco nventional (diffusible NO signaling) mechanisms. (C) 2001 Wiley-Liss. Inc.