POSTNATAL-DEVELOPMENT OF CALBINDIN-D28K IMMUNOREACTIVITY IN THE CEREBRAL-CORTEX OF THE CAT

To learn about maturational patterns of nonpyramidal neurons in the ce rebral cortex, calbindin-D28k immunoreactivity was studied in the kitt en cortex. Immunoreactive neurons first appear in the cortical and sub cortical areas related to the limbic system, including the cingulate a nd retrosplenial cortices, and in the secondary motor areas. These are followed by the primary motor and sensory association areas and, fina lly, by the primary sensory areas. In all cortical areas, calbindin-D2 8k immunoreactivity first develops in layer V pyramidal neurons and la ter in nonpyramidal neurons, except in the primary sensory areas, wher e immunoreactive pyramidal neurons are not found at any age. Transient calbindin-D28k immunoreactivity occurs in pyramidal neurons that are mainly localized in the cingulate and retrosplenial cortices and in th e secondary motor area, as well as in nonpyramidal neurons localized i n the subplate and layer I, and in a subset of large multipolar and bi tufted neurons in layer VI. Nonpyramidal neurons localized in layers I I to IV, and some neurons in layer VI, develop permanent calbindin-D28 k immunoreactivity. Calbindin-D28k immunoreactivity labels subsets of GABAergic interneurons that form vertical axonal tufts, so that tempor al and regional patterns of calbindin-D28k immunoreactivity during dev elopment may be implicated in the maturation of columnar (vertical) in hibition in the cerebral cortex. In addition to neurons, corticofugal and afferent fibres of subcortical origin exhibit calbindin-D28k immun oreactivity. Transient calbindin-D28k immunoreactivity occurs in corti cofugal fibres arising from the cingulate and prefrontal cortices, whi ch are probably corticostriatal projection fibres. In contrast, perman ent immunoreactivity occurs in what are probably thalamocortical fibre s ending in layer IV, and in punctate terminals located in the upper t hird of layer I.