Layer-specific expression of Cl− transporters and differential [Cl−]i in newborn rat cortex

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Abstract
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 V/VI, while the reverse was true for KCC2 mRNA. The [Cl−]i levels 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.