Role of gamma-aminobutyric acid in early neuronal development: studies with an embryonic neuroectodermal stem cell clone

gamma-Aminobutyric acid (GABA) has been known to function as an autocrine/paracrine signal molecule in addition to its well-known inhibitory neurotransmitter function. Studies on the developing brain and on primary brain cell cultures provided evidence for a variety of GABA functions in periods preceding the formation of synapses. The exact role of GABA in the early neural development, however, is still not well understood. In this study, one-cell-derived NE-4C neuroectodermal stem cells were induced to form neurons and astrocytes in vitro, and the role of GABA was investigated in defined phases of neurogenesis. Noninduced NE-4C cells contained GABA, expressed GABA(A)R alpha subunits, and carried functional GABA(A) ion channels. A moderate cytoplasmic GABA content was detected during the entire period of differentiation. By the time of the formation of differentiated neurons, neuron-like cells with both high and low GABA content were clearly distinguishable. HPLC analysis indicated that NE-4C cells released GABA into their fluid environment during all stages of neuronal development. By using the patch-clamp technique, GABA-evoked currents were recorded during the entire proliferation/differentiation period, whereas a GABA-evoked increase in intracellular Ca(2+) was detected only during the maturation of postmitotic neuronal precursors. Bicuculline blocked both the ion currents and the [Ca(2+)](i) increase in response to GABA. Neuron formation was facilitated by GABA through GABA(A) ion channels during postmitotic differentiation, but not earlier during the phases of cell fate commitment. Although the data clearly demonstrate an early responsiveness to GABA, understanding the significance of GABA influence in early neural cell fate decisions will require further investigation.