GAD and GABA transporter (GAT-1) mRNA expression in the developing rat hippocampus

Synaptic inhibition in the mammalian central nervous system is mostly mediated by GABA (gamma-aminobutyric acid). Inhibitory interneurons can be identified by staining for glutamate decarboxylase (GAD), the key enzyme which produces the transmitter. After release, GABA is removed from the extracellular space by specific transporters which are localized at the presynaptic endings of interneurons, in adjacent glial processes and, possibly, also in the postsynaptic target cell membranes. The GABAergic system undergoes profound functional and structural changes during the first 2 weeks of postnatal development, including migration of interneurons and changes in the level of expression and subcellular distribution of GABA transporters. We therefore analyzed the distribution of mRNA coding for GAD and GAT-1 (the main neuronal GABA transporter) in the developing rat hippocampus. Our data show that both transcripts are present in putative interneurons from the first postnatal day and exhibit a largely similar distribution throughout postnatal ontogenesis, with some specific differences in certain hippocampal subfields. Quantification of stained somata confirmed the postnatal redistribution of putative interneurons in the area dentata from dendritic layers towards the hilus. We also found a general staining of principal cell layers for both probes, which differs with postnatal age and between GAD and GAT-1 mRNA. Together, our data reveal a profound reorganization of the GABAergic system in the rat hippocampus during the first weeks of postnatal development.