GABA stimulates chemotaxis and chemokinesis of embryonic cortical neurons via calcium-dependent mechanisms

During rat cortical development, when neurons migrate from the ventricular zone to the cortical plate, GABA localizes within the target destinations of migratory neurons. At this time, cells in germinal zones and along migratory pathways express GABA receptor subunit transcripts, implying that in vivo, GABA may be a chemoattractant. We used an in vitro strategy to study putative chemotropic effects of GABA on embryonic rat cortical cells. GABA stimulated neuronal migration in vitro at embryonic day 15 (E15). From E16 onward, two concentration ranges (fM and microM) induced motility. Femtomolar GABA primarily stimulated chemotaxis (migration along a chemical gradient), whereas micromolar GABA predominantly initiated chemokinesis (increased random movement). These effects were mimicked by structural analogs of GABA with relative specificity at GABAA (muscimol), GABAB (R-baclofen), and GABAC (trans- or cis-4- aminocrotonic acid) receptors. Antagonists of GABAB (saclofen) and GABAC (picrotoxin) receptors partially inhibited responses to both femto- and micromolar GABA; however, only responses to femtomolar GABA were partially blocked by bicuculline, a well established antagonist of GABA at GABAA receptors. Hence, chemotactic responses to femtomolar GABA seem to involve all three classes of GABA receptor proteins, whereas chemokinetic responses to micromolar GABA involve GABAB and GABAC receptor proteins. GABA-induced motility was blocked by loading the cells with the Ca(2+)-chelating molecule bis(2-aminophenoxy)ethane- N,N,N′,N′-tetra-acetic acid, suggesting that intracellular Ca2+ mediates GABA-induced cell movement. Optical recordings of cells loaded with Ca2+ indicator dye revealed that both femto- and micromolar GABA evoked increases in intracellular Ca2+. Thus, GABA-stimulated increases in intracellular Ca2+ may mediate both chemotactic and chemokinetic responses in embryonic cortical cells.