Complementary Expressions of Transcripts Encoding GAD,, and GABA, Receptor a4, pl, and yl 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 (GAD,,), and specific GABA, 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 El7 and E20 in the dorsomedial sector of the neocot-tex, 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 GABA, receptor (~3, p3, and y2 subunits were found exclusively in the mantle zone, while those for a4, pl, and yl subunits were predominantly detected in the inner half of the germinal matrix (i.e., VZ). Furthermore, in the El5 germinal matrix of the lumbar spinal cord, cells exhibiting cw4 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 ceil division expressed the subunit mRNAs. In clear contrast, GAD,, mRNA was abundant in the outer half of the germinal matrix (i.e., TZ or SV), and in the intermediate zone as well. lmmunocytochemical staining of El7 neocottex with antiGABA 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