Abstract

mRNA encoding the recently cloned γ-aminobuytyric acid_Areceptor (GABAR) π subunit is expressed in the hippocampus and in several non-neuronal tissues including the uterus and ovaries. Whereas native GABARs are pentamers composed primarily of αβγ, αβδ, or αβε subunits, it has not been demonstrated clearly that the π subunit incorporates into functional GABARs to form αβπ receptors and, if so, with what properties. We provide electrophysiological evidence that the π subunit can coassemble with either α5β3 or α5β3γ3 subunits to produce recombinant GABARs with distinct pharmacological and biophysical properties. Compared with α5β3 receptors, GABARs produced by coexpression of α5β3π subunits had a lower GABA EC₅₀ value, were enhanced to a lesser extent by loreclezole, had different IC₅₀ values for pregnenolone sulfate and lanthanum, and were insensitive to benzodiazepines. Incorporation of both π and γ3 subunits into an α5β3γ3π isoform was suggested by reduced enhancement by diazepam and a high zinc IC₅₀ value. Current-voltage relations for the α5β3π subunit combination outwardly rectified more than currents from α5β3γ3 but less than α5β3 combination GABARs. Single-channel α5β3 GABAR currents had a main conductance state of 15.2 picoSeimens (pS). Coexpression of the π subunit with α5β3 subtypes increased the conductance level to 23.8 pS, similar to the conductance level of α5β3γ3 GABARs (26.9 pS). We conclude that the π subunit coassembles with α, β, and γ subunits to form functional αβπ or αβγπ GABARs and, thus, could have a significant impact on the function of native GABARs expressed in the brain or non-neuronal tissue.