GABAC receptors are functionally expressed in the intermediate zone and regulate radial migration in the embryonic mouse neocortex

Radial neuronal migration in the cerebral cortex depends on trophic factors and the activation of different voltage- and ligand-gated channels. To examine the functional role of GABAC receptors in radial migration we analyzed the effects of specific GABAA and GABAC receptor antagonists on the migration of BrdU-labeled neurons in vitro using organotypic neocortical slice cultures. These experiments revealed that the GABAA specific inhibitor bicuculline methiodide facilitated neuronal migration, while the GABAC specific inhibitor (1,2,5,6-tetrahydropyridine-4-yl) methylphosphinic-acid (TPMPA) impeded migration. Co-application of TPMPA and bicuculline methiodide or the unspecific ionotropic GABA receptor antagonist picrotoxin both impeded migration, suggesting that the GABAC receptor mediated effects dominate. Addition of the specific GABAC receptor agonist cis-4-aminocrotonic acid (CACA) also hampered migration, indicating that a physiological GABAergic stimulation is required for appropriate function. RT-PCR experiments using specific probes for GABAC receptor mRNA and Western blot assays using an antibody directed against rho subunits revealed the expression of GABAC receptor mRNA and translated GABAC receptor protein in the immature cortex. Microfluorimetric Ca2+ imaging in neurons of identified cortical layers using Calcium Green revealed the functional expression of GABAA and GABAC receptors in the intermediate zone, while only GABAA receptor mediated responses were observed in the upper cortical plate. In summary, these results demonstrate that activation of GABAC receptors is a prerequisite for accurate migration and that GABAC receptors are functionally expressed in the intermediate zone.