GAMMA-AMINOBUTYRIC ACID(A) RECEPTOR REGULATION BY A CHLORIDE-DEPENDENT KINASE AND A SODIUM-DEPENDENT PHOSPHATASE

Gamma-Aminobutyric acid(A) (GABA(A)) receptors are linked to ion channels which mediate many aspects of neural inhibition. Although the effects of phosphorylation on GABA(A) receptor function have been widely studied, the actual role of phosphorylation in the regulation of these receptors still remains controversial. In recent reports, we have described the effects of phosphorylating/dephosphorylating enzymes on the regulation of GABA(A) receptors in a rat cortical slice preparation (Shaw et al., Mol. Neuropharmacol, 2 (1992) 297-302; Shaw and Lanius, Dev. Brain Res., 70 (1992) 153-161; Pasqualotto et al., Neuroreport, 4 (1993) 447-450) and predicted that ionic co-factors are involved in mediating the regulation of GABA(A) receptors by kinases and phosphatases. In the present report, the effects of chloride, sodium, potassium, and calcium were examined alone and in the presence of cAMP-dependent protein kinase (protein kinase A) or alkaline phosphatase. The results showed a decrease in [H-3]SR 95531 (GABA(A) receptor antagonist) binding after incubation with chloride alone; this decrease was fur-ther enhanced in the presence of protein kinase A. Both effects could be blocked by a protein kinase A inhibitor. Conversely, an increase in [H-3]SR 95531 binding was observed after incubation with sodium alone; this increase was further enhanced in the presence of alkaline phosphatase. In both cases these increases in binding could be blocked by sodium orthovanadate, a phosphatase inhibitor. Potassium was ineffective under all conditions; calcium showed enzyme-independent effects at low concentrations only. These results suggest the existence of a novel chloride-dependent protein kinase which may have significant sequence homology to protein kinase A, and a novel sodium-dependent phosphatase. We speculate that together these enzymes are responsible for the majority of the rapid regulation of cortical GABA(A) receptors in response to various stimuli. These data provide a link between ionic currents generated by ionotropic receptor activation and the resulting regulation of these receptors via activation of regulatory enzymes.