GABA(B) receptors mediate motility signals for migrating embryonic cortical cells

During development, postmitotic neurons migrate from germinal regions into the cortical plate (cp), where lamination occurs. In rats, GABA is transiently expressed in the cp, near target destinations for migrating neurons. In vitro GABA stimulates neuronal motility, suggesting cp cells release GABA, which acts as a chemoattractant during corticogenesis. Pharmacological studies indicate GABA stimulates migration via GABA(B)-receptor (GABA(B)-R) activation. Using immunohistochemistry, RT-PCR and Western blotting, we examined embryonic cortical cell expression of GABA(B)-Rs in vivo. At E17, GABA(B)-R1(+) cells were identified in the ventricular zone (vz) and cp. RT-PCR and Western blotting demonstrated the presence of GABA(B)-R1a and GABA(B)-R1b mRNA and proteins. Using immuno- cytochemistry, GABA(B)-R expression was examined in vz and cp cell dissociates before and after migration to GABA in an in vitro chemotaxis assay. GABA-induced migration resulted in an increase of GABA(B)-R(+) cells in the migrated population. While <20% of each starting dissociate was GABA(B)-R(+), >70% of migrated cells were immunopositive. We used a microchemotaxis assay to analyze cp cell release of diffusible chemotropic factor(s). In vitro, cp dissociates induced vz cell migration in a cell density-dependent manner that was blocked by micromolar saclofen (a GABA(B)-R antagonist). HPLC demonstrated cp cells release micromolar levels of GABA and taurine in several hours. Micromolar levels of both molecules stimulated cell migration that was blocked by micromolar saclofen. Thus, migratory cortical cells express GABA(B)-Rs, cp cells release GABA and taurine, and both molecules stimulate cortical cell movement. Together these findings suggest GABA and/or taurine act as chemoattractants for neurons during rat cortical histogenesis via mechanisms involving GABA(B)-Rs.