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 GABA(C) receptors in radial migration we analyzed the effects of specific GABA(A) and GABA(C) receptor antagonists on the migration of BrdU-labeled neurons in vitro using organotypic neocortical slice cultures. These experiments revealed that the GABA(A) specific inhibitor bicuculline methiodide facilitated neuronal migration, while the GABA(C) 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 GABA(C) receptor mediated effects dominate. Addition of the specific GABA(C) 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 GABA(C) receptor mRNA and Western blot assays using an antibody directed against rho subunits revealed the expression of GABA(C) receptor mRNA and translated GABA(C) receptor protein in the immature cortex. Microfluorimetric Ca(2+) imaging in neurons of identified cortical layers using Calcium Green revealed the functional expression of GABA(A) and GABA(C) receptors in the intermediate zone, while only GABA(A) receptor mediated responses were observed in the upper cortical plate. In summary, these results demonstrate that activation of GABA(C) receptors is a prerequisite for accurate migration and that GABA(C) receptors are functionally expressed in the intermediate zone.