Random Assembly of GABA ρ1 and ρ2 Subunits in the Formation of Heteromeric GABA C Receptors

Abstract

1. Various combinations of the rho subunits (rho(1A), rho(1B), rho(2A), rho(2B)) of GABA(C) receptors cloned from white perch retina were expressed in Xenopus oocytes, and electrophysiological and pharmacological methods were used to test their ability to co-assemble into heteromeric receptors. Simultaneous injection of the two subunits, irrespective of their relative proportions, led invariably to the formation of a preponderance of heteromeric receptors. 2. The GABA deactivation responses elicited from these cells could be described by a single exponential decay, and their pharmacological responses deviated significantly from those expected of a simple mixture of two homomeric rho(1) and rho(2) receptors. In contrast, a double exponential function comprising fast and slow components was required to fit the GABA deactivation responses elicited from oocytes sequentially expressing rho(1) and rho(2) subunits, a condition that favors the formation of a mixture of homomeric rho(1) and rho(2) receptors. 3. Both the GABA-response kinetics and the sensitivity to picrotoxin of the heteromeric perch rho(1B)rho(2A) receptor varied with the proportion of the subunit RNA injected, indicating there is no fixed stoichiometry for their co-assembly into heteromeric rho(1)rho(2) receptors. 4. If native GABA(C) receptors in retinal neurons behave in a similar manner as in the oocyte expression system, these finding suggest that the properties of their GABA(C) receptors are likely to be influenced by the transcription/translation efficiency of GABA rho subunit genes.

Pharmacological Analysis of the Activation and Receptor Properties of the Tonic GABACR Current in Retinal Bipolar Cell Terminals

“…The inhibition of GABA C Rs by picrotoxin is dependent on subunit composition, with homomeric ρ1 receptors exhibiting approximately 10-fold higher IC 50 values than either ρ2 homomers or ρ1- ρ2 heteromers in the presence of high GABA concentrations [21]–[23], [41]. The picrotoxin-sensitivity of tonic and synaptic GABA C R currents in BCTs was therefore examined.…”

“…BCs express both ρ1 and ρ2 GABA C R subunits, which readily form heteromeric receptors, and it is likely that most BC GABA C Rs are ρ1-ρ2 heteromers [22], [23], [59]-[61]. However, the additional expression of homomeric receptors would extend the functional diversity of GABA C R-mediated inhibition, as receptor properties are dependent on ρ subunit composition.…”

Mechanisms of neurosteroid interactions with GABAA receptors

“…This channel underlies GABA-C responses in the retina. Although homomeric receptors containing the ρ1 subunit are thought to be most physiologically relevant, there is evidence for heteromeric combinations among ρ subunits 1-3 (Pan et al, 2006), and even among ρ subunits and GABA A subunits (Milligan et al, 2004). Because the GABA-C receptor can function as a homo-oligomer, this in principle could greatly simplify efforts to find steroid binding sites.…”

An Electrophysiological Study of Retinal Function in the Diabetic Female Rat

“…Although the precise mechanism for this difference is unknown, it is interesting to note that GABA C receptors on rat bipolar cells are heteromeric complexes of 1 and 2 subunits, 71 formed most likely by random assembly of the GABA subunits. 72,73 Clearly, the observed differences in GABA activity in diabetic rats may result from alterations in the transcription/translation efficacy of GABA subunit genes.…”