Developmental changes in GABA neurons of the rat dentate gyrus: an in situ hybridization and birthdating study.

The temporal and spatial distribution of glutamate decarboxylase 67 (GAD67) mRNA-containing neurons in the dentate gyrus was analyzed from embryonic day 20 (E20) to postnatal day 15 (PN15) with nonradioactive in situ hybridization methods. A major goal was to follow the development of an early-appearing population of gamma-aminobutyric acid (GABA) neurons within the developing molecular layer. At E20, GAD67 mRNA-containing neurons were highly concentrated slightly above the outer border of the developing granule cell layer. By PN3-PN5, many labeled cells were distributed within the developing granule cell layer; by PN15, labeled neurons occupied positions normally seen in the adult, such as along the inner border of the granule cell layer. This developmental pattern is unique and led to additional studies to determine the potential fate of the early-appearing GABA population. The possibility of apoptotic cell death was investigated with in situ end labeling techniques at developmental ages E21-PN15. Very few apoptotic cells were detected in the developing molecular layer at all ages examined. Birthdating studies of neurons labeled with bromodeoxyuridine revealed a changing pattern, similar to that of GAD67 mRNA, for cells with birthdates (E14) of many of the mature GAD-containing neurons in the dentate gyrus. The changes in the mRNA and birthdating patterns from E20-PN15 suggest that some of the early-appearing GABA neurons within the developing molecular layer of the dentate gyrus may alter their positions to eventually become the mature GABA population along the inner border of the granule cell layer.