Carbonic anhydrase-II messenger RNA in neurons and glia of chick brain: mapping by in situ hybridization

The enzyme carbonic anhydrase is widespread in brain tissue. In rodent brains it has been reported to be exclusively in oligodendroglia but there has been some debate about the generality of this finding. To investigate the cellular distribution of carbonic anhydrase by an independent technique, we have examined the chick brain by in situ hybridization to detect mRNA from the carbonic anhydrase-II gene, using as controls the actin and vimentin genes. The most intense carbonic anhydrase-II hybridization is to the choroid plexus, to the Bergmann glia of the cerebellum, and to the Müller cells in the retina. Elsewhere, some brain regions are negative while others show many individual strongly positive cells; carbonic anhydrase-II mRNA is particularly abundant in some parts of the hyperstriatum, tectum and thalamus. Some of the larger labelled cells are identifiable as neurons. By histochemistry, we confirm the presence of the carbonic anhydrase enzyme in choroid plexus and Bergmann glia, but the enzyme is also present in blood vessel walls where there is no carbonic anhydrase-II mRNA; this may be a different isozyme. During embryogenesis, carbonic anhydrase-II mRNA appears in the retina as early as two days of incubation, but does not appear in the brain until much later.