Abstracts

KCC2 is a potassium chloride cotransporter that accelerates the functional maturation of GABA(A) receptors, by decreasing intracellular chloride. In neurons with low KCC2, GABA(A) receptor activation causes depolarization, calcium entry through voltage sensitive calcium channels and increase in KCC2 mRNA. In neurons with high KCC2 expression, GABA(A) receptor activation induces hyperpolarization and decrease in KCC2 mRNA. We hypothesized that conditions that involve activation of GABA(A) receptors may also differentially regulate KCC2 depending on GABA(A) receptor function. We studied the effect of neonatal kainate (KA)-induced status epilepticus (SE) on KCC2 mRNA expression in the hippocampus. Gramicidin-perforated patch clamp was used to study GABA(A) receptor function of CA1 pyramidal neurons of PN4-6 male and female Sprague-Dawley rats. 3 episodes of SE were induced by injections of KA at postnatal days (PN) 4, 5, and 6. SE-induced hippocampal injury was assessed with fluorojade B staining and Nissl staining at PN7 and PN10. KCC2 specific in situ hybridization was done in hippocampal sections of PN10 rats. 1) Stimulation at the stratum radiatum in the presence of glutamate receptor antagonists evoked GABA(A) receptor mediated postsynaptic potentials that were excitatory in male and inhibitory in female PN4-6 CA1 pyramidal neurons. 2) No significant neuronal injury was observed in the hippocampus of rats subjected to the 3 episodes of KA-induced SE. 3) In PN10 control rats, the CA3 and CA1 pyramidal neurons expressed the highest levels of KCC2 mRNA expression, and the interneurons of the dentate hilus the lowest, in both sexes. 4) Females expressed more KCC2 mRNA in all studied hippocampal regions than males. 5) In PN10 males subjected to 3 episodes of neonatal KA-induced SE, KCC2 mRNA expression was higher than in male controls. 6) In PN10 females, neonatal KA-induced SE upregulated KCC2 mRNA only in the dentate hilus. The developmental increase in KCC2 and functional maturation of GABA(A) receptors occur faster in female than in male hippocampus, in a region-specific pattern. The mode of regulation of KCC2 expression by neonatal KA-induced SE correlates with the baseline level of KCC2 expression and GABA(A) receptor function. In neurons with depolarizing GABA(A) receptor responses or low baseline expression of KCC2, neonatal SE increases KCC2. This does not occur in neurons with hyperpolarizing GABA(A) receptor responses. As the distinct effects of neonatal seizures may also extend to other calcium-regulated genes involved in the differentiation of hippocampus or epileptogenesis, these results underline the importance of studying separately the translational effects of neonatal SE in the male and female developing brain. (Supported by NIH NINDS grant NS 045243 (ASG) and NS20253.)