GABAergic cells and signals in CNS development.
GABA is formed primarily from decarboxylation of glutamate by a family of cytosolic and membrane-bound GAD enzymes. In the adult, GAD-derived GABA sustains the vitality of the central nervous system (CNS), since blockage of GAD rapidly leads to convulsions and death. In plants, cytosolic GAD synthesizes GABA in response to hormones and environmental stress. Since decarboxylation involves protonation, secretion of GABA serves to buffer cytosolic pH in plant cells. Families of GAD and GABAA receptor/Cl- channel transcripts and encoded proteins emerge early and seemingly everywhere during CNS development, with their abundance closely paralleling neurogenesis and peaking before birth. Micromolar GABA acts at receptor/Cl-channels to depolarize progenitor cells in the cortical neuroepithelium; it also elevates their cytosolic Ca2+ (Cac2+) levels. In some way, these effects decrease proliferation. GABA directs the migration of postmitotic neuroblasts at femtomolar concentrations and stimulates their random motility at micromolar concentrations via Ca2+ signaling mechanisms. Activation of GABAA receptors by micromolar GABA may limit motility via membrane depolarization and elevated Cac2+. These results indicate that in vitro GABA can affect embryogenesis of the CNS through effects on cell proliferation and migration. As neurons differentiate postnatally, Cl(-)-dependent depolarization disappears together with GABAergic Cac2+ signals. Physiologically occurring GABAergic signals at Cl-channels exist in tonic and transient forms. Since the former are found on progenitor cells while both are present in postmitotic neurons, mechanisms to generate transients differentiate in the latter. Surprisingly, tonic and transient forms of GABAergic signaling at Cl-channels are rapidly and smoothly interconvertible and seem to be derived from online GABA synthesis in a surface-accessible compartment of the membrane.