Inhibition of GABAA receptor function by tyrosine kinase inhibitors and their inactive analogues

The effects of tyrosine kinase inhibitors which target the ATP binding site or the substrate binding site of tyrosine kinases were assessed on murine recombinant type A gamma-aminobutyric acid (GABAA) receptors expressed in Xenopus oocytes or HEK cells using two-electrode voltage clamp or patch clamp recording. Genistein inhibited in a noncompetitive manner GABA-activated currents recorded from alpha 1 beta 1 gamma 2S receptor constructs by reducing the maximum normalized response from 1.83 +/- 0.04 to 0.71 +/- 0.04 and reducing the ECS, from 35.7 +/- 2.1 mu M to 15.1 +/- 3.9 mu M. After mutating the two "functionally active" substrate tyrosine (Y) residues in gamma 2S and expressing the mutant receptor alpha 1 beta 1 gamma 2S(Y365F, Y367F), genistein still noncompetitively inhibited the responses to GABA reducing the maximum current from 1.81 +/- 0.03 to 0.26 +/- 0.01 and the ECS, from 33.1 +/- 2.3 mu M to 5.8 +/- 2.2 mu M. The inactive compound, daidzein, also similarly inhibited responses to GABA on these two receptor constructs. Inhibitors targeting the substrate binding site of tyrosine kinases, the tyrphostins, also inhibited both the wild-type and the tyrosine mutant GABAA receptors. Tyrphostin A25 and the inactive tyrphostin Al reduced the maximum normalized responses for alpha 1 beta 1 gamma 2S and alpha 1 beta 1 gamma 2S(Y365F, Y367F) receptors by 73 and 64%, respectively. The tyrosine kinase inhibitors and their inactive controls did not display any significant voltage sensitivity to the antagonism of GABA-activated responses. Moreover, genistein or tyrphostin A25 did not affect the potentiation of responses to GABA by pentobarbitone or diazepam. Mutating the two "functionally silent" tyrosine residues, Y370 and Y372, known to be substrates for tyrosine kinases in the pi subunit and coexpression in the alpha 1 beta 1(Y370F, Y372F)gamma 2S(Y365F, Y367F) construct failed to affect the inhibitory action of genistein. The study concludes that tyrosine kinase inhibitors and their inactive controls can directly interact with GABAA receptors completely independent of any effects on tyrosine kinases.