Edinburgh Napier University

  The pharmacological properties of homo-oligomeric channels formed by the GABA type A receptor-like ρl and ρ2 polypeptides are very reminiscent of those of the GABA type C receptors that have been extensively characterized in the retina. Similar receptors have been reported to occur in certain brain regions of a variety of vertebrate species. We have used in situ hybridization to investigate the expression patterns of the ρ1- and ρ -polypeptide genes in the brain of the 1-day-old chick (Gallus domesticus) and the adult rat (Rattus norvegicus). Our results show that in the chick both the ρ1- and ρ2-subunit transcripts are present in the cerebellum, the optic tectum, the epithalamus and the nucleus pretectalis. However, the two messenger RNAs are often found in different populations of cells. Thus, only the ρ1-subunit gene is expressed in the deep cerebellar nuclei, the dorsal thalamus, the ectostriatum and the tractus vestibulomesencephalicus, while only the ρ2-subunit gene is transcribed in the nucleus habenularis lateralis and the nucleus isthmo-opticus. In contrast, neither of the ρ-polypeptide messenger RNAs can be detected by in situ hybridization in the rat central nervous system. Reverse transcription-polymerase chain reaction amplification has been used to confirm the expression of the two ρ-subunit genes in the chicken brain. Surprisingly, this highly sensitive technique also revealed transcription of these genes in the rat brain. We conclude that the ρl- and ρ2-subunit genes are expressed at a much higher level in the avian brain than in the rat brain and that, at least in birds, subtypes of the GABC receptor exist.