Maturation of cultured embryonic CNS tissues during chronic exposure to agents which prevent bioelectric activity.

Summary Fetal rodent cerebral cortex and spinal cord tissues were explanted at stages prior to the onset of synaptic function. The culture media contained chemical agents which produced sustained depression of the characteristic bbioelectric excitability of the neurons. Addition of xylocaine (50 μg/ml) or a 10-fold increase in Mg 2+ concentration were effective in blocking all complex bioelectric activities—or, at least, in markedly raising the threshold for triggering such synaptically mediated phenomena. They did not, however, appear to interfere with the characteristic morphologic growth and development of the tissues in vitro , as observed by light microscopy. Representative CNS cultures tested at various intervals during chronic exposure (5–30 days) to these blocking agents showed no spontaneous, or evoked, complex bioelectric activity, although simple action potentials could be triggered, in some cases, at unusually high stimulus intensities. These CNS tissues have, therefore, been cultured under conditions which apparently prevent all spontaneous, as well as fortuitously evoked, bioelectric discharges during the critical period when synaptic networks normally form in control cultures. Nevertheless, within minutes after removing the blocking agent from the bathing fluid, the first electric stimulus applied to such a virginal explant often evoked a complex bioelectric response similar to those seen in mature control explants. The mimicry of these responses to patterns characteristic of organized multisynaptic networks, in situ , suggests that ontogenetic development of some types of complex interneuronal CNS functions may be programmed to occur independent of prior bioelectric excitation of the cellular elements composing the system. Moreover, after endogenous formation of such a neuronal cell assembly it can be maintained in a quiescent state for at least several weeks and, yet, remain organized with characteristic bioelectric excitability.