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 γ-aminobutyric acid (GABA) receptor affects amyloid β protein (25-35) (Aβ (25-35)), a synthetic 25-35 amyloid peptide, -induced neurotoxicity using cultured rat cortical neurons. Aβ (25-35) produced a concentration-dependent reduction of cell viability, which was significantly reduced by (5R,10S)-(+)-5-methyl-10,11-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 NG-nitro-l-arginine methyl ester (l-NAME), a nitric oxide synthase inhibitor. Pretreatment with muscimol, a GABAA receptor agonist, over a concentration range of 0.1-10 μM 24 h before the treatment with 10 μM Aβ (25-35) showed concentration-dependent inhibition on the Aβ (25-35)-induced neuronal apoptotic death. However, baclofen (1 and 10 μM), a GABAB receptor agonist, failed to inhibit the Aβ (25-35)-induced neuronal death. In addition, pretreatment with muscimol (1 μM) for 24 h inhibited the Aβ (25-35) (10 μM)-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 μM) were completely blocked by the simultaneous treatment with 10 μM bicuculline, a GABAA receptor antagonist, indicating that the protective effects of muscimol were due to GABAA receptor stimulation. When, however, treated just 15 min before the treatment with Aβ (25-35), muscimol (1 μM) did not show any protective effect against Aβ (25-35) (10 μM)-induced neurotoxicity in cultured neurons. These results suggest that the chronic activation of GABAA receptor may ameliorate Aβ-induced neurotoxicity by interfering with the increase of [Ca2+]c, and then by inhibiting glutamate release, generation of ROS and caspase-3 activity.