Layer-specific expression of Cl−transporters and differential Cl−iin newborn rat cortex

GABA is the main inhibitory neurotransmitter in the adult brain. However, GABAergic transmission is depolarizing during early postnatal development, suggesting that changes in the expression of cation–Cl−co-transporters regulating neuronal Cl−homeostasis underlie the ontogeny of GABAergic functions. The developmental changes in the expressions of Cl−co-transporter mRNAs in the neocortex were in opposite directions for NKCC1 (Cl−uptake) and KCC2 (Cl−extrusion). In the newborn, NKCC1 mRNA expression was highest in ventricular zone followed by cortical plate, and then by Layer VVI, while the reverse was true for KCC2 mRNA. The Cl−ilevels were in the same rank order as for NKCC1 mRNA. Thus, the ontogeny of Cl−homeostasis in neocortical neurons could be regulated via the differential expression of NKCC1 and KCC2.