Modulation of GABAA receptor function by PKA and PKC protein phosphorylation
We studied the modulation of ¦Á4¦Â3¦Ã2L and ¦Á4¦Â3¦Ä GABAA receptor currents by two protein kinases, PKA and PKC. Although modulation of synaptic ¦Á1¦Â¦Ã2 GABAA receptor isoforms has been studied widely, study of the modulation of peri- and extrasynaptic ¦Á¦Â¦Ä and non-synaptic ¦Á¦Â¦Ã GABAA receptors by protein phosphorylation is lacking. Using patch-clamp recording, we compared the effects of protein phosphorylation of ¦Á4¦Â3¦Ã2L and ¦Á4¦Â3¦Ä GABAA receptors under different levels of activation that included spontaneous openings, tonic currents activated by low GABA concentrations and phasic currents activated by high GABA concentrations. We found that PKA-activation preferentially increased spontaneous ¦Á4¦Â3¦Ä receptor currents by increasing single channel open frequency and decreased GABA-activated steady-state, tonic ¦Á4¦Â3¦Ä currents, but only had small effects on spontaneous and GABA-activated tonic ¦Á4¦Â3¦Ã2L currents, indicating the differential modulation of tonic inhibition mediated by ¦Á4¦Â3¦Ã2L and ¦Á4¦Â3¦Ä GABAA receptors. In contrast, both PKA and PKC had similar effects on desensitization of phasic ¦Á4¦Â3¦Ã2L and ¦Á4¦Â3¦Ä currents, implying a common modulatory mechanism of protein phosphorylation in regulating synaptic current kinetics. Our study suggested that protein phosphorylation had profound effects on different GABAA receptor isoforms, which should also be determined by receptor sub-cellular (synaptic or non-synaptic) localization and by the level of ambient GABA (spontaneous or GABA-activated), thus ensuring precise modulation of specific GABAA receptor properties in specific brain areas.