Semiquantitative analysis of the expression of GABA-A receptor subunits in the developing embryonic chick brain stem.

The expression levels of seven types of gamma-aminobutyric acid-A (GABA-A) receptor subunits (alpha1, beta2, beta3, beta4, gamma1, gamma2, and gamma4) were quantified in the embryonic chick brain stem at 2 to 20 d of incubation (E2 to E20) and just after hatching. The expression level of mRNA was measured by using semiquantitative reverse transcription-polymerase chain reaction (RT-PCR). When property regions were amplified, two products were observed for each of the beta2, beta4, and gamma2 subunits because of alternative splicing. These were named beta2S and beta2L, beta4S and beta4L, and gamma2S and gamma2L for shorter and longer fragments, respectively. Transcripts of alpha1, beta2L, beta2S, beta3, beta4L, beta4S, gamma1, and gamma2S subunits were first detected from E2 to E5 brain stems. The expression level of each subunit increased gradually with development and reached a plateau at E9 to E12. In contrast, a delay occurred in the appearance of both the gamma4 and gamma2L subunits, which were not detected until E8 to E10. The absence of gamma4 and/or gamma2L subunits may explain differences in the pharmacological characteristics of GABA-A receptors at the early stages of development.