Modulation of neuronal and recombinant GABA_A receptors by redox reagents

1. The functional role played by the postulated disulphide bridge in ?-aminobutyric acid type A (GABA(A)) receptors and its susceptibility to oxidation and reduction were studied using recombinant (murine receptor subunits expressed in human embryonic kidney cells) and rat neuronal GABA(A) receptors
in conjunction with whole-cell and single channel patch-clamp
techniques. 2. The reducing agent dithiothreitol (DTT) reversibly
potentiated GABA-activated responses (I(GABA)) of a1ß1 or a1ß2 receptors while the oxidizing reagent 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB) caused inhibition. Redox modulation of I(GABA) was independent of GABA concentration, membrane potential and the receptor agonist and did not affect the GABA EC₅₀ or Hill coefficient. The endogenous antioxidant reduced glutathione (GSH) also potentiated I(GABA) in a1ß2 receptors, while both the oxidized form of DTT and glutathione (GSSG) caused small inhibitory effects. 3. Recombinant receptors composed of a1ß1?2S or a1ß2?2S were considerably less sensitive to DTT and DTNB. 4. For neuronal GABA(A) receptors, I(GABA) was enhanced by flurazepam and relatively unaffected by redox reagents.
However, in cultured sympathetic neurones, nicotinic
acetylcholine-activated responses were inhibited by DTT whilst in
cerebellar granule neurones, NMDA-activated currents were potentiated by
DTT and inhibited by DTNB. 5. Single GABA-activated ion channel
currents exhibited a conductance of 16 pS for a1ß1 constructs. DTT did
not affect the conductance or individual open time constants determined
from dwell time histograms, but increased the mean open time by
affecting the channel open probability without increasing the number of
cell surface receptors. 6. A kinetic model of the effects of DTT and DTNB suggested that the receptor existed in equilibrium between oxidized and reduced forms. DTT increased the rate of entry into reduced receptor forms and also into desensitized states. DTNB reversed these kinetic effects. 7. Our results indicate that GABA(A) receptors formed by a and ß subunits are susceptible to regulation by redox agents. Inclusion of the ?2 subunit in the receptor, or recording from some neuronal GABA(A) receptors,
resulted in reduced sensitivity to DTT and DTNB. Given the suggested
existence of aß subunit complexes in some areas of the central nervous
system together with the generation and release of endogenous redox compounds, native GABA(A) receptors may be subject to regulation by redox mechanisms.