Cogeneration of retrogradely labeled corticocortical projection and GABA-immunoreactive local circuit neurons in cerebral cortex.

The times of origin of cortico-cortical projection neurons and local circuit neurons in rat visual cortex were determined. The birthdates of the projection neurons were assessed using a technique that combined retrograde labeling with lectin-bound horseradish peroxidase and tritiated thymidine autoradiography. The birthdates of some cortical local circuit neurons were determined by combining GABA immunocytochemistry with [3H]thymidine autoradiography. Double-labeled neurons (those with retrograde or immunoreactive label in their perikarya and autoradiographic silver grains over their nuclei) were born during the third week of gestation. Projection and local circuit neurons born on gestational day 14, 15, 17, 19 or 20 were located primarily in layer VIb, VIa, V, III or II, respectively. Thus, both populations of neurons are generated by parallel and concurrent inside-to-outside patterns.