Regulation of native GABA_A receptors by PKC and protein phosphatase activity

Rationale and objective: Protein kinase C (PKC) modulation of ionotropic receptors is a common mechanism for regulation of channel function. The effects of PKC and phosphatase activation on native gamma-aminobutyric acid (GABA _A) receptors in adult brain are unknown. Previous studies of recombinant GABA_A receptors have provided evidence that PKC activation inhibits receptor function, whereas other studies suggest that PKC either increases or does not alter GABA_A receptor function. The present study explored (a) the effects of PKC and phosphatase activity on GABA-mediated ³⁶Cl^- uptake in cerebral cortical synaptoneurosomes and (b) the effect of PKC activity on muscimol-induced loss of righting reflex (LORR) in adult rats. Methods: GABA_A receptor function in vitro was measured by muscimol-induced ³⁶Cl^- uptake into cerebral cortical synaptoneurosomes. The in vivo effect of PKC on GABA_A-mediated function was measured by intracerebroventricular (i.c.v.) injection of 4-beta-phorbol-12,13-dibutyrate (PDBu) or calphostin C followed by determination of muscimol-induced LORR. Results: Adenosine triphosphate (ATP) and PDBu produced a concentration-dependent and specific reduction in muscimol-stimulated ³⁶Cl^- uptake that was blocked by the PKC inhibitor calphostin C. Both adenosine diphosphate and 4αPDBu were ineffective. Phosphatase inhibition produced similar inhibition of muscimol responses. Furthermore, i.c.v. administration of PDBu and calphostin C produced opposing effects on both the onset and the duration of muscimol-induced LORR in rats. Conclusions: The present study provides evidence that PKC activation reduces GABA_A receptor function in native receptors both in vitro and in vivo. Phosphatase inhibitors decrease muscimol-mediated Cl^- uptake in GABA_A receptors demonstrating coordinated regulation of native receptors by PKC and phosphatases.