Localization and functional expression of alternatively spliced forms of the GABA(A) receptor gamma(2) subunit

Recent studies have identified two alternatively spliced forms of the GABA(A) receptor gamma(2) subunit that differ by the presence (gamma(2L)) or absence (gamma(2S)) of an eight-amino acid segment. This insert in the gamma(2L) isoform exists in the proposed cytoplasmic loop region, between M3 and M4, and contains a consensus sequence for phosphorylation by protein kinase C. To examine the regional distribution of this novel receptor subunit in the brain, gamma(2L) subunit mRNA was detected using both in situ hybridization histochemistry and and PCR amplification methods. Hybridization histochemistry with a gamma(2L), subunit-specific oligonucleotide probe revealed that the gamma(2L), subunit mRNA is widely distributed throughout the mouse brain. The highest levels of expression are found in the cerebral cortex, hippocampus, olfactory lobe, and cerebellum. The presence of the gamma(2L), subunit in these regions was confirmed using PCR. Additionally, PCR experiments detected yes subunit mRNA in the cerebral cortex and hippocampus but not in the cerebellum. To examine the functional properties of the gamma(2) subunit isoforms, gamma(2S) and gamma(2L), subunit mRNAs were coexpressed with alpha(1)beta(1) subunit mRNAs in Xenopus oocytes. These experiments indicate that the gamma(2L) and gamma(2S) subunit variants exhibit similar pharmacological properties, including the ability of both isoforms to confer diazepam sensitivity to the receptor complex. In addition, potentiation of GABA responses by pentobarbital in oocytes expressing either subunit isoform is similar. These data indicate that the presence or absence of the additional eight amino acids in the gamma(2) subunit isoforms does not appear to alter the response of the GABA(A) receptor complex to either benzodiazepines and barbiturates at the level of protein phosphorylation present in the oocyte.