Developmental changes in GABA transporter (GAT1 and GAT3) mRNA expressions in the rat olfactory bulb

Developmental expressions of GABA transporters 1 and 3 (GAT1 and GAT3) were investigated in the rat olfactory bulb by using in situ hybridization histochemistry. We found that the expression of GAT1 and GAT3 mRNAs was dramatically changed in the granule cell layer (GCL), external plexiform layer (EPL) and glomerular layer (GL) during postnatal development. Among bulbar neurons, granule cells and periglomerular cells are GABAergic and they are localized in the GCL and GL, respectively. In the EPL, granule cells make GABAergic synapses with mitral cell dendrites. Thus, the changes seen in the GCL, EPL and GL seemed related to the development of the GABAergic system in the olfactory bulb. On the whole, our results demonstrated that expression patterns of GAT1 and GAT3 mRNAs have become similar to adult patterns on postnatal day 14 (P14), suggesting that GABA transporters may play a pivotal role in GABAergic neurotransmission after P14. However, expression patterns of GAT1 and GAT3 mRNAs in early postnatal days were quite distinct from those in adulthood. For example, in the GCL, immature granule cells already exhibited strong levels of GAT1 mRNA on P1 and the expression level was higher than that of granule cells in adulthood. GAT3 mRNA was strongly expressed in presumable radial glial cells surrounding the subependymal layer. In the EPL, few signals for the two transcripts were detected on P1 but they were markedly increased by P14. Our results indicate that GAT1 and GAT3 may play important roles in the development of the GABAergic system in the olfactory bulb.