Muscimol prevents NMDA antagonist neurotoxicity by activating GABA _A receptors in several brain regions

N-Methyl-D-aspartate (NMDA) glutamate receptor antagonists are being developed as therapeutic agents for several clinical conditions. However, the ability of these agents to produce neurotoxicity and psychosis can compromise their clinical usefulness. In addition, an NMDA receptor hypofunction (NRHypo) state may play a role in neurodegenerative and psychotic disorders. A better understanding of the mechanism underlying these adverse effects should allow for the safer use of these agents and might clarify mechanisms underlying certain clinical disorders. NRHypo neurotoxicity is mediated by a complex disinhibition mechanism in which NMDA antagonists abolish GABAergic inhibition, resulting in the simultaneous excessive release of acetylcholine and glutamate onto the vulnerable retrosplenial cortex (RSC) neurons. Systemically administered GABAergic agents are potent protectors against NRHypo neurotoxicity. To determine where in brain GABAergic agents could be acting to protect against NRHypo neurotoxicity, we injected the GABAergic agonist, muscimol, into different brain regions of rats treated systemically with a neurotoxic dose of the potent NMDA antagonist, MK-801. We report that muscimol injections into the anterior thalamus or diagonal band of Broca provide substantial protection, suggesting that disinhibition of neurons in these regions underlies NRHypo neurotoxicity. Muscimol injections into the RSC also provide substantial protection possibly by directly inhibiting the vulnerable RSC neuron. Injections of muscimol into other areas known to project to the RSC (ventral orbital cortex, anterior cingulate cortex and subiculum) provide only minimal protection. We conclude that GABAergic agents prevent NRHypo neurotoxicity mainly by activating GABA receptors in the anterior thalamus, diagonal band of Broca and RSC.