Prenatal and postnatal development of GABA-accumulating cells in the occipital neocortex of rat.

The development of the 3H-GABA-accumulating cells in the neocortex has been followed by light microscopical autoradiography, and after resectioning of the original autoradiograms, by electron microscopy. The validity of the methods used are discussed. The study has been limited to the primary visual cortex and its precursors of rat, from embryonic day (E) 15 to adult. GABA-accumulating cells were found from E 16 onwards in the occipital cortex, which is one to two days after cells arrive in the pallial anlage and one day before the first synapses have been found. Until E 18, the prevalent positions of labeled cells were in lamina I and below the cortical plate. Later, labeled cells also occurred as strands within the cortical plate. During the perinatal period, more and more GABA-accumulating neurons and glial cells began to differentiate and show a characteristic distribution at the periphery of unlabeled cell clusters. From postnatal day 11, no apparent change in density or position of labeled neurons took place. At prenatal stages, two main types of labeled cells were found: 1) Comparatively large cells with rounded nuclei and rough endoplasmic reticulum consisting of narrow, electron-lucent cisterns. These cells were tentatively identified as preneurons. 2) Smaller, polymorphous cells with irregular nuclei and rough endoplasmic reticulum with wide cisterns filled with a dense matrix. These cells are probably precursors of glial cells. Both labeled neurons and glial cells were identified at postnatal stages. In young and adult rats, only neurons to be characterized as nonpyramidal neurons were labeled. Synapses were not found on the perikarya of labeled cells until E 21. Also, in postnatal preparations, labeled neurons showed few axo-somatic synapses. These data were correlated with other events of the structural and functional development of the neocortex. The delay between the appearance of GABA accumulating cells and synaptogenesis indicates that apart from being an inhibitory neurotransmitter, GABA might play a specific morphogenetic role in synaptogenesis. This could even be its primary function during early developmental stages.