Maturation of projection neurons in the visual cortex of the rat

A variety of in vitro brain slice preparations were employed to study the development of electrophysiological and morphological characteristics of pyramidal neurons from layer 5 of the rat visual cortex. By combining conventional intracellular recording techniques with intracellular dye injection into neurons which were back-labelled from their respective target, it was possible to demonstrate a correlation between the intrinsic electrophysiological properties and the morphology of projection neurons. Superior colliculus projecting neurons fired action potentials in a burst-firing pattern whereas interhemispheric projecting neurons fired action potentials in a regular spiking pattern. Both groups differed also in the morphology of their dendritic arborizations. A developmental study of the physiological properties of morphologically identified layer 5 neurons demonstrated that subthreshold properties and characteristic action potential parameters of these neurons change progressively during the early postnatal period. Membrane time constants and input resistances changed to lower values and action potentials became higher, faster and thus briefer. No burst-firing patterns were found in early developmental stages. Using intracellular injection of Lucifer Yellow in fixed slices, it was then shown that superior colliculus (SC) projecting neurons and interhemispheric (CLH) projecting neurons differ significantly in their morphology already early in development. SC-nneurons have more basal dendrites which give rise to more branches and total tips than neurons projecting to the CLH.