GABAergic transcallosal neurons in developing rat neocortex

In the mature cerebral cortex the interhemispheric connections across the corpus callosum appear to be essentially completely excitatory on the basis of both immunocytochemical and electrophysiological studies. During late embryonic development, however, immunocytochemical staining reveals numerous GABA-positive fibres in the callosum, which later largely disappear. The origin of these fibres and whether they represent functional GABAergic neurons has not been established. In the present study we used a combination of retrograde labelling in vivo with electrophysiology and immunocytochemistry in cell culture to show that transiently at birth in rat pups a substantial number of transcallosal cortical cells are functional GABAergic neurons. Possible roles and fates for these neurons are discussed.