Modulation of Spontaneous and GABA-Evoked Tonic α4β3δ and α4β3γ2L GABAA Receptor Currents by Protein Kinase A

Tang X, Hernandez CC, Macdonald RL. Modulation of spontaneous and GABA-evoked tonic alpha 4 beta 3 delta and alpha 4 beta 3 gamma 2L GABA(A) receptor currents by protein kinase A. J Neurophysiol 103: 1007-1019, 2010. First published November 25, 2009; doi:10.1152/jn.00801.2009. Protein kinase A (PKA) has been reported to regulate synaptic alpha beta gamma-aminobutyric acid type A (GABA(A)) receptor currents, but whether PKA regulates GABA(A) receptor peri-and extrasynaptic tonic currents is unknown. GABA(A) receptors containing alpha 4 subunits are important in mediating tonic inhibition and exist as both alpha 4 beta delta and alpha 4 beta gamma receptors in the brain. To mimic GABA-independent and GABA-dependent tonic currents, we transfected HEK 293T cells with alpha 4 beta 3 delta or alpha 4 beta 3 gamma 2L subunits and recorded spontaneous currents in the absence of applied GABA and steady-state currents in the presence of 1 mu M GABA. Both alpha 4 beta 3 delta and alpha 4 beta 3 gamma 2L receptors displayed spontaneous currents, but PKA activation increased spontaneous alpha 4 beta 3 delta currents substantially more than spontaneous alpha 4 beta 3 gamma 2L currents. The increase in spontaneous alpha 4 beta 3 delta currents was due to an increase in single-channel open frequency. In contrast, PKA activation did not alter steady-state tonic currents recorded in the presence of 1 mu M GABA. We concluded that PKA had a GABA concentration-dependent effect on alpha 4 beta 3 delta and alpha 4 beta 3 gamma 2L currents. In the absence of GABA, spontaneous alpha 4 beta 3 delta and, to a lesser extent, alpha 4 beta 3 gamma 2L currents could provide a basal, tonic current that could be regulated by PKA. With increasing concentrations of extracellular GABA, however, tonic alpha 4 beta 3 delta and alpha 4 beta 3 gamma 2L currents would become more GABA dependent and less PKA sensitive. Thus in brain regions with fluctuating extracellular GABA levels, the dynamic range of GABA-activated tonic currents would be set by PKA and the increase in tonic current produced by increasing GABA would be reduced by PKA-mediated phosphorylation. When ambient GABA reaches micromolar concentrations, PKA would have no effect on steady-state tonic currents.