Incorporation of the pi subunit into functional gamma-aminobutyric acid(A)receptors

mRNA encoding the recently cloned gamma-aminobuytyric acid, receptor (GABAR ) pi subunit is expressed in the hippocampus and in several non-neuronal ti ssues including the uterus and ovaries. Whereas native GABARs are pentamers composed primarily of alpha beta gamma, alpha beta delta, or alpha beta ep silon subunits, it has not been demonstrated clearly that the pi subunit in corporates into functional GABARs to form cup rr receptors and, if so, with what properties. We provide electrophysiological evidence that the rr subu nit can coassemble with either alpha 5 beta 3 or alpha 5 beta 3 gamma 3 sub units to produce recombinant GABARs with distinct pharmacological and bioph ysical properties. Compared with alpha 5 beta 3 receptors, GABARs produced by coexpression of alpha 5 beta 3 pi 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 benzodiazep ines. Incorporation of both pi and gamma 3 subunits into an alpha 5 beta 3 gamma 3 pi isoform was suggested by reduced enhancement by diazepam and a h igh zinc IC50 value. Current-voltage relations for the alpha 5 beta 3 pi su bunit combination outwardly rectified more than currents from alpha 5 beta 3 gamma 3 but less than alpha 5 beta 3 combination GABARs. Single-channel a lpha 5 beta 3 GABAR currents had a main conductance state of 15.2 picoSeime ns (pS). Coexpression of the pi subunit with alpha 5 beta 3 subtypes increa sed the conductance level to 23.8 pS, similar to the conductance level of a lpha 5 beta 3 gamma 3 GABARs (26.9 pS). We conclude that the rr subunit coa ssembles with alpha, beta, and gamma subunits to form functional alpha beta pi or alpha beta gamma pi GABARs and, thus, could have a significant impac t on the function of native GABARs expressed in the brain or non-neuronal t issue.