GABA is the principal inhibitory neurotransmitter in the mature brain, but during early postnatal development the elevated [Cl⁻]_i in immature neocortical neurones causes GABA_A receptor activation to be depolarizing. The molecular mechanisms underlying this intracellular Cl⁻ accumulation remain controversial. Therefore, the GABA reversal potential (E_GABA) or [Cl⁻]_i in early postnatal rat neocortical neurones was measured by the gramicidin‐perforated patch‐clamp method, and the relative expression levels of the cation−Cl⁻ cotransporter mRNAs (in the same cells) were examined by semiquantitative single‐cell multiplex RT‐PCR to look for statistical correlations with [Cl⁻]_i. The mRNA expression levels were positively (the Cl⁻ accumulating Na⁺,K⁺−2Cl⁻ cotransporter NKCC1) or negatively (the Cl⁻ extruding K⁺−Cl⁻ cotransporter KCC2) correlated with [Cl⁻]_i. NKCC1 mRNA expression was high in early postnatal days, but decreased during postnatal development, whereas KCC2 mRNA expression displayed the opposite pattern. [Cl⁻]_i and NKCC1 mRNA expression were each higher in cortical plate (CP) neurones than in the presumably older layer V/VI pyramidal neurones in a given slice. The pharmacological effects of bumetanide on E_GABA were consistent with the different expression levels of NKCC1 mRNA. These data suggest that NKCC1 may play a pivotal role in the generation of GABA‐mediated depolarization in immature CP cells, while KCC2 promotes the later maturation of GABAergic inhibition in the rat neocortex.

GABA (γ-aminobutyric acid) is the main inhibitory neurotransmitter of the mature central nervous system. The developmental switch of GABAergic transmission from excitation to inhibition is induced by changes in Cl- gradients, which are generated by cation-Cl- co-transporters. An accumulation of Cl- by the Na+-K+-2Cl- co-transporter (NKCC1) increases the intracellular Cl- concentration ([Cl-]i) such that GABA depolarizes neuronal precursors and immature neurons. The subsequent ontogenetic switch, i.e., upregulation of the Cl--extruder KCC2, which is a neuron-specific K+-Cl- co-transporter, with or without downregulation of NKCC1, results in low [Cl-]i levels and the hyperpolarizing action of GABA in mature neurons. Development of Cl- homeostasis depends on developmental changes in NKCC1 and KCC2 expression. Generally, developmental shifts (decreases) in [Cl-]i parallel the maturation of the nervous system, e.g., early in the spinal cord, hypothalamus and thalamus, followed by the limbic system, and last in the neocortex. There are several regulators of KCC2 and/or NKCC1 expression, including BDNF, insulin-like growth factor, and CFTR. Therefore, regionally different expression of these regulators may also contribute to the regional developmental shifts of Cl- homeostasis. KCC2 and NKCC1 functions are also regulated by phosphorylation by enzymes such as PKC, Src-family tyrosine kinases, and WNK1-4 and their downstream effectors SPAK-OSR1. In addition, activation of these kinases is modulated by humoral factors such as estrogen and taurine. Because these transporters use the electrochemical driving force of Na+ and K+ ions, topographical interaction with the Na+-K+ ATPase and its modulators such as creatine kinase should modulate functions of Cl- transporters. Therefore, regional developmental regulation of these regulators and modulators of Cl- transporters may also play a pivotal role in the development of Cl- homeostasis.

The mechanism by which sex steroids rapidly modulate the excitability of neurons was investigated by intracellular recording of neurons in rat medial amygdala brain slices. Brief hyperpolarization and increased potassium conductance were produced by 17β-estradiol. This effect persisted after elimination of synaptic input and after suppression of protein synthesis. Thus, 17β-estradiol directly changes the ionic conductance of the postsynaptic membrane of medial amygdala neurons. In addition, a greater proportion of the neurons from females than from males responded to 17β-estradiol.