PYRAMIDAL NEURONS IN LAYER-5 OF THE RAT VISUAL-CORTEX .3. DIFFERENTIAL MATURATION OF AXON TARGETING, DENDRITIC MORPHOLOGY, AND ELECTROPHYSIOLOGICAL PROPERTIES

This paper describes the early morphological and physiological develop ment of pyramidal neurons in layer 5 of the rat visual cortex in relat ion to the targets chosen by their axons. Cells were prelabeled by ret rograde transport from the superior colliculus or the contralateral vi sual cortex and intracellularly injected either in fixed slices or aft er recording in living slices. In the adult, corticotectal cells have thick apical dendrites with an extensive terminal arborization extendi ng into layer 1, and fire characteristic bursts of action potentials w hen injected with a depolarizing current; interhemispheric cells have slender apical dendrites that terminate without a terminal tuft, usual ly in layer 2/3, and they display a more regular firing pattern (Kaspe r et al.: J Comp Neurol, this issue, 339:459-474). At embryonic day E1 8 (when axons of the two classes of cells are already taking different routes towards their targets) and E21, pyramidal-like cells throughou t the cortical plate all have similar soma-dendritic morphology, with spindle-shaped cell bodies and few, short basal dendrites but apical d endrites that all end in distinct tufts in the marginal zone. At postn atal day P3, after the axons of both cell classes have reached their t argets, all pyramidal neurons in layer 5 still have distinct terminal arborizations in layer 1, though they vary in complexity and extent. T he somata are now more mature (round to ovoid in shape), and the basal dendritic tree has extended. As early as P5, all cells studied could be clearly classified as tufted or untufted (considerably earlier than previously reported; Koester and O'Leary: J Neurosci 12:1382, '92), a nd these features correlated precisely with the projection target, as in the adult. Measurement showed that although interhemispheric cells lose their terminal tufts, in general the trunks of their apical dendr ites do not withdraw but continue to grow, at roughly the same rate as those of corticotectal cells. The two classes of layer 5 pyramidal ne urons differentiate from each other in three distinct phases: pathway selection by axons precedes the loss of the apical tuft by interhemisp heric cells, and these morphological characteristics are established 1 0 days before the onset of burst-firing in corticotectal cells. These three steps may be guided by different molecular signals.