POSTNATAL-DEVELOPMENT OF PARVALBUMIN AND CALBINDIN D28K IMMUNOREACTIVITIES IN THE CEREBRAL-CORTEX OF THE RAT

Parvalbumin and calbindin D28k immunoreactivities were examined in the neocortex of the rat during postnatal development. Parvalbumin-immuno reactive nonpyramidal neurons first appear in layer V and later in lay ers VI and IV, and then in II and III. Immunoreactive terminals formin g baskets surrounding unlabelled somata appear about 2 days later. The first parvalbumin-immunoreactive neurons appear in the retrosplenial and cingulate cortices, and the rostral region of the primary somatose nsory cortex at postnatal days 8 or 9 (P8-P9). These regions are follo wed by the primary visual, primary auditory and motor cortices at P11. Parvalbumin immunoreactivity appears last in the secondary areas of t he sensory regions and association cortices. Adult patterns are reache d at the end of the 3rd week. Calbindin D28K-immunoreactive nonpyramid al neurons are found at birth in all cortical layers excepting the mol ecular layer. The intensity of the immunoreaction increases during the first 8 or 11 days of postnatal life, first in the inner and later in the upper cortical layers, following, therefore, an ''inside-out'' gr adient. Heavily-labelled calbindin D28K-immunoreactive nonpyramidal ce lls dramatically decrease in number from P11 to P15 due mainly to a de crease of the multipolar subtypes. This suggests that two populations of calbindin D28k-immunoreactive non-pyramidal neurons are produced in the neocortex during postnatal development: one population of neurons transitorily expresses calbindin D28k immunoreactivity; the other pop ulation is composed of neurons that are permanently calbindin D28k imm unoreactive. In addition to heavily labelled nonpyramidal cells, a ban d of weakly labelled pyramid-like neurons progressively appears in lay ers Il and Ill throughout the cerebral cortex, beginning in layer IV i n the somatosensory cortex by the end of the 2st week. Adult patterns are reached at the end of the 3rd week. These results indicate that pa rvalbumin and calbindin D28k immunoreactivities in the cerebral neocor tx follow different characteristic patterns during postnatal developme nt. The appearance of parvalbumin immunoreactivity correlates with the appearance of the related functional activity in the different cortic al regions, and, probably, with the appearance of inhibitory activity in the neocortex. On the other hand, the early appearance of calbindin D28k immunoreactivity in the neocortex may be related to the early ap pearance of calbindin immunoreactivity in many other brain regions, an d suggests another, as yet unknown, role for this calcium-binding prot ein during development of the cerebral cortex.