Astroglia inhibit the proliferation of neocortical cells and prevent the generation of small GABAergic neurons in vitro

We quantitatively studied the dynamics of rat neocortical precursor prolife ration in vitro, and additionally examined the effects of neuron-glia inter actions on the proliferation and differentiation of neurons, and particular ly of gamma-aminobutyric acid (GABA)-containing cells. In cultures grown on glia-free substrate, cellular proliferation was detected at least until th e end of the second week in vitro, but most neurons which expressed detecta ble amounts of microtubule-associated protein at 12 days in vitro were gene rated early during the first week. Further double-labelling experiments, co mbining 5'-bromo-2'-deoxyuridine with GABA or beta-tubulin III immunohistoc hemistry, provided direct evidence that neuronal proliferation continued th rough the second week in vitro, and that a population of small GABAergic ne urons was generated between 3 and 12 days in vitro. Culturing cells on a gl ial substrate significantly reduced the generation of small GABAergic cells and strongly inhibited the total cell proliferation. Inhibition also occur red if astrocytes were added to the culture after 6 days in vitro, but was significantly decreased if cells were grown on a fixed glial substrate, sug gesting that the effect might be at least partially mediated by active inte ractions between neurons and glia. In conclusion, our results show that the sustained proliferation of precursor cells in neocortical cultures is nece ssary for the differentiation of small GABAergic neurons, and that mature a stroglia effectively inhibit the proliferation of neocortical precursors th ereby affecting the appearance of a population of GABAergic cells.