Preferential coassembly of α4 and δ subunits of the γ-aminobutyric acidA receptor in rat thalamus

Pharmacological study of rat thalamic gamma-aminobutyric acid, (GABA(A)) receptors revealed the presence of two distinct populations, namely, diazepam-sensitive and diazepam-insensitive [H-3]Ro15-4513 binding sites accounting for 94 +/- 2% (1339 +/- 253 fmol/mg protein) and 6 +/- 2% (90 +/- 44 fmol/mg protein) of total sites, respectively. Thalamic diazepam-insensitive sites exhibited a pharmacology that was distinct from diazepam-sensitive sites but comparable to that of the alpha 4 beta 3 gamma 2 subtype of the GABA(A) receptor stably expressed in L(tk(-)) cells. Immunoprecipitation experiments with a specific anti-alpha 4-antiserum immunoprecipitated 20 and 7% of total thalamic [H-3]muscimol and [H-3]Ro15-4513 sites, respectively. Combinatorial immunoprecipitation using antisera against the alpha 4,gamma 2, and delta subunit revealed that alpha 4 delta- and alpha 4 gamma 2-containing receptors account for 13 +/- 2 and 8 +/- 3% of [H-3]muscimol sites from thalamus, respectively. It also indicated that all delta subunits coexist with an alpha 4 subunit in this brain region. In conclusion, our results show that: in rat thalamus both alpha 4 beta gamma 2 and alpha 4 beta delta subtypes are expressed but alpha 4 beta delta is the major alpha 4-containing GABA(A) receptor population.