Distribution of alpha 1 and alpha 2 GABAA receptor subunits in developing chick optic tectum

In a previous work we described a differential ontogeny of type I and type II benzodiazepine (BZD) receptors. Molecular cloning has led to the characterization of different GABAA receptor subunits, with the alpha subunit reportedly containing the BZD binding site. The aim of this work was to study the developmental pattern of expression of α1 and α2 GABAA receptor subunit genes in chick optic tectum. In situ hybridization, using a 45-base antisense oligonucleotide complementary to each one of these subunits, was assayed. Expression levels of representative mRNAs were quantified in each lamina by measurement of X-ray film optical densities. Both subunits were expressed as early as embryonic day (ED) 10, exhibiting higher levels of mRNA in compartment i during all days analyzed. While α2 levels remained high, α1 levels increased during development. The superficial transient cell compartment 3 showed that α2 expression peaked on ED 12 and then decreased concomitantly with the segregation pattern of this lamina into four others, while α1 subunit levels exhibited relatively constant high levels in the superficial laminae. Nevertheless, α2 levels compared to α1 were greater for each one of the laminae studied throughout development. In conclusion, the developmental pattern of alpha subunits expression progresses through a characteristic temporospatial pattern related to the laminar organization in each layer and correlates with the temporal expression of BZDI and BZDII receptors.