Complementary expressions of transcripts encoding GAD67 and GABAA receptor alpha 4, beta 1, and gamma 1 subunits in the proliferative zone of the embryonic rat central nervous system.
The developmental stage at which nerve cells initially express specific neurotransmitters and their corresponding receptors remains elusive. In the present study, the distribution patterns of transcripts for the GABA-synthesizing enzyme, glutamate decarboxylase (GAD67), and specific GABAA receptor subunits were examined in the proliferative zone of the rat central nervous system using in situ hybridization. In order to define the DNA synthetic zone of the germinal matrix, tissue sections were taken from embryos whose mothers had been injected with 5-bromo-2'-deoxyuridine (BrdU) and had survived for 1 hr. BrdU immunocytochemistry was used to locate the relative position of BrdU-immunoreactive nuclei within the ventricular zone (VZ). At embryonic day (E) 15 in the alar plate of the lumbar spinal cord, and at E17 and E20 in the dorsomedial sector of the neocortex, densely packed BrdU-immunoreactive nuclei were consistently detected in lateral portions of the inner half of the germinal matrix, indicating that the inner half of the germinal matrix corresponded to the VZ, while the outer half corresponded to the transitional (TZ) or subventricular zone (SV). In situ hybridization in tissue sections adjacent to BrdU-immunoreacted ones showed that the transcripts for GABAA receptor alpha 3, beta 3, and gamma 2 subunits were found exclusively in the mantle zone, while those for alpha 4, beta 1, and gamma 1 subunits were predominantly detected in the inner half of the germinal matrix (i.e., VZ). Furthermore, in the E15 germinal matrix of the lumbar spinal cord, cells exhibiting alpha 4 subunit mRNA were much more abundant in the receding intermediate plate, which contains mostly postmitotic cells, than in the alar plate comprised of many DNA-synthesizing cells, strongly suggesting that only those cells completing final cell division expressed the subunit mRNAs. In clear contrast, GAD67 mRNA was abundant in the outer half of the germinal matrix (i.e., TZ or SV), and in the intermediate zone as well. Immunocytochemical staining of E17 neocortex with anti-GABA antibody revealed a well defined band of GABA-immunoreactive cells and processes in the SV and occasional positive cells in the VZ. It appears that cells in the proliferative zone may express GABA at the migratory stage, whereas cells in the VZ may express mRNAs for GABAA receptor alpha 4, beta 1, and gamma 1 subunits at the premigratory stage, just after completing cell division.(ABSTRACT TRUNCATED AT 400 WORDS)