Influx of calcium through L-type calcium channels in early postnatal regulation of chloride transporters in the rat hippocampus.

During the early postnatal period, GABA(B) receptor activation facilitates L-type calcium current in rat hippocampus. One developmental process that L-type current may regulate is the change in expression of the K(+)Cl(-) co-transporter (KCC2) and N(+)K(+)2Cl(-) co-transporter (NKCC1), which are involved in the maturation of the GABAergic system. The present study investigated the connection between L-type current, GABA(B) receptors, and expression of chloride transporters during development. The facilitation of L-type current by GABA(B) receptors is more prominent in the second week of development, with the highest percentage of cells exhibiting facilitation in cultures isolated from 7 day old rats (37.5%). The protein levels of KCC2 and NKCC1 were investigated to determine the developmental timecourse of expression as well as expression following treatment with an L-type channel antagonist and a GABA(B) receptor agonist. The time course of both chloride transporters in culture mimics that seen in hippocampal tissue isolated from various ages. KCC2 levels increased drastically in the first two postnatal weeks while NKCC1 remained relatively stable, suggesting that the ratio of the chloride transporters is important in mediating the developmental change in chloride reversal potential. Treatment of cultures with the L-type antagonist nimodipine did not affect protein levels of NKCC1, but significantly decreased the upregulation of KCC2 during the first postnatal week. In addition, calcium current facilitation occurs slightly before the large increase in KCC2 expression. These results suggest that the expression of KCC2 is regulated by calcium influx through L-type channels in the early postnatal period in hippocampal neurons.