Expression of alpha 5 GABAA receptor subunit in developing rat hippocampus.

The GABAergic system plays an important role in the hippocampal development. Here we have studied the developmental expression of the alpha 5 subunit of the GABA(A) receptor (from rat hippocampus) by RT-competitive PCR, immunoblot and immunocytochemistry. Our results demonstrated an early induction of the alpha 5 subunit expression (at mRNA and protein levels) during the first postnatal week, peaking at P5 and decreasing after this age. The peak of alpha 5 subunit expression precedes the peak of expression for the synaptophysin, GAD65 and GAD67. Thus, the increase in the alpha 5 GABA(A) receptor subunit expression may precede the GABAergic synaptogenesis. Importantly, between P0 and P7, the expression of the alpha 5 subunit was concentrated at the cell somata of the pyramidal and granular cells. After P10, its localization shifted from the cell bodies to the dendritic layers. This developmental pattern is similar to that reported for the Na(+)-K(+)-2Cl(-) system and it might be correlated with the transition from excitatory to inhibitory GABAergic activity.