GABA(C) receptor rho subunits are heterogeneously expressed in the human CNS and form homo- and heterooligomers with distinct physical properties

In the central nervous system, receptors for gamma-aminobutyric acid (GABA) are responsible for inhibitory neurotransmission. Anatomical and electroph ysiological studies indicate that GABA(C) receptors are composed of rho sub units. While the rho 1 subunit of various species forms homooligomeric rece ptors with GABA(C)-like properties, molecular cloning has identified additi onal rho subunits whose functional role is unclear. By RT-PCR, we demonstra ted that rho 1 expression is primarily restricted to the retina, whereas th e rho 2 subunit was present in all brain regions tested. Transfection of HE K-293 cells with rho 2 cDNA resulted in GABA-gated whole-cell currents that differed from those mediated by the rho 1 subunit in two respects: maximal amplitude (rho 1:rho 2 approximate to 4:1) and inactivation time course (r ho 1:rho 2 approximate to 2:1). Cotransfection of rho 1 and rho 2 cDNA in a 1:1 ratio generated whole-cell currents with large amplitudes characterist ic of rho 1 but more rapid inactivation typical for rho 2. This observation suggested formation of heterooligomeric GABA(C) receptors with distinct fe atures. Therefore, we tested the assembly of rho 1 and rho 2 subunits by co transfecting rho 2 cDNA together with a chimeric rho 1 beta 1 subunit, know n to interfere with rho 1 assembly in a dominant-negative fashion. Reductio n of rho 2 generated currents correlated with the ratio of chimeric to rho 2 cDNA. Secondly, we determined that the picrotoxinin sensitivity of cells transfected with various ratios of rho 1 and rho 2 cDNA differed from that expected of a pure mixture of homooligomeric receptors. The latter two obse rvations support the idea that rho 1 and rho 2 subunits form heterooligomer ic GABA(C) receptors in mammalian cells. Together, our results indicate tha t the presence of both rho subunits enables the formation of heterooligomer ic receptors with physical properties distinct from homooligomers, thus inc reasing the diversity of GABA(C) receptors in the CNS.