Protective roles of hepatic GABA signaling in acute liver injury of rats.

gamma-Aminobutyric acid (GABA) is produced by various cells through the catalytic activity of glutamic acid decarboxylase (GAD). Activation of type-A GABA receptor (GABA(A)R) inhibits stem cell proliferation but protects differentiated cells from injures. The present study investigated hepatic GABA signaling system and the role of this system in liver physiology and pathophysiology. RT-PCR and immunoblot assays identified GAD and GABA(A)R subunits in rat livers and in HepG2 and Clone 9 hepatocytes. Patch-clamp recording detected GABA-induced currents in Clone 9 hepatocytes and depolarization in WITT cholangiocytes. The function of hepatic GABA signaling system in rats was examined using models of D-galactosamine (GalN)-induced acute hepatocytic injury in vivo and in vitro. The expression of GAD increased whereas GABAAR subunits decreased in the liver of GalN-treated rats. Remarkably, treating rats with GABA or the GABAAR agonist muscimol, but not the GABA(B)R agonist baclofen, protected hepatocytes against GalN toxicity and improved liver function. In addition, muscimol treatment decreased the formation of pseudobile ductules and the enlargement of hepatocytic canaliculi in GalN-treated rats. Our results revealed that a complex GABA signaling system exists in the rat liver. Activation of this intrahepatic GABAergic system protected the liver against toxic injury.