GABAergic neurons of different morphological classes are cogenerated in the mouse barrel cortex

In the barrel cortex of the mouse, GAD-immunoreactive cells are positioned following an 'inside-out' gradient. Each cortical layer is populated by a variety of classes of GABAergic neurons. The period of neuronal generation for each cortical layer lasts for several days, but it is not known whether the different classes of GABAergic neurons are generated synchronously. In order to correlate the time of generation with cell morphology, we combined [3H]thymidine autoradiography with glutamic acid decarboxylase immunocytochemistry, and used a recently developed method to morphologically classify GAD-positive neurons. We found that all classes of GAD-positive neurons destined to populate each cortical layer are generated concurrently. Therefore, the forms shown by the GAD-positive neurons in the adult animal are not related to the times of their neurogenesis.