Chronic stimulation of GABAA receptor with muscimol reduces amyloid β protein (25-35)-induced neurotoxicity in cultured rat cortical cells

The present study was performed to examine how the stimulation of gamma-animobutyric acid (GABA) receptor affects amyloid P protein (2535) (A beta (25-35)), a synthetic 25-35 amyloid peptide, -induced neurotoxicity using cultured rat cortical neurons. A beta (25-35) produced a concentration-dependent reduction of cell viability, which was significantly reduced by (5R,10S)-(+)-5-methyl-10,l 1-dihydro-5H-dibenzo[a,d] cyclohepten-5,10-imine (MK-801), an N-methyl-D-aspartate (NMDA) receptor antagonist, verapamil, an L-type Ca2+ channel blocker, and N-G-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase inhibitor. Pretreatment with muscimol, a GABA(A) receptor agonist, over a concentration range of 0.1 - 10 mu M 24 h before the treatment with 10 mu M A beta (25-35) showed concentration-dependent inhibition on the Ap (25-35)-induced neuronal apoptotic death. However, baclofen (1 and 10 mu M)), a GABA(B) receptor agonist, failed to inhibit the A beta (25-35)-induced neuronal death. In addition, pretreatment with muscimol (1 mu M) for 24 h inhibited the A beta (25-35) (10 mu M)induced elevation of cytosolic Ca2+ concentration ([Ca2+](c)) and glutamate release, generation of reactive oxygen species (ROS), and caspase-3 activity in cultured neurons. These neuroprotective effects of muscimol (1 mu M) were completely blocked by the simultaneous treatment with 10 mu M bicuculline, a GABA(A) receptor antagonist, indicating that the protective effects of muscimol were due to GABA(A) receptor stimulation. When, however, treated just 15 min before the treatment with A beta (25-35), muscimol (1 mu M) did not show any protective effect against A beta (25-35) (10 mu M)-induced neurotoxicity in cultured neurons. These results suggest that the chronic activation of GABA(A) receptor may ameliorate Ap-induced neurotoxicity by interfering with the increase of [Ca2+], and then by inhibiting glutamate release, generation of ROS and caspase-3 activity. (c) 2005 Elsevier Ireland Ltd and the Japan Neuroscience Society. All rights reserved.