Targeting the glutamatergic system to treat major depressive disorder: rationale and progress to date.

Major depressive disorder (MDD) is a severe, debilitating medical illness that affects millions of individuals worldwide. The young age of onset and chronicity of the disorder has a significant impact on the long-term disability that affected individuals face. Most existing treatments have focused on the 'monoamine hypothesis' for rational design of compounds. However, patients continue to experience low remission rates, residual subsyndromal symptoms, relapses and overall functional impairment. In this context, growing evidence suggests that the glutamatergic system is uniquely central to the neurobiology and treatment of MDD. Here, we review data supporting the involvement of the glutamatergic system in the pathophysiology of MDD, and discuss the efficacy of glutamatergic agents as novel therapeutics. Preliminary clinical evidence has been promising, particularly with regard to the N-methyl-D-aspartate (NMDA) antagonist ketamine as a 'proof-of-concept' agent. The review also highlights potential molecular and inflammatory mechanisms that may contribute to the rapid antidepressant response seen with ketamine. Because existing pharmacological treatments for MDD are often insufficient for many patients, the next generation of treatments needs to be more effective, rapid acting and better tolerated than currently available medications. There is extant evidence that the glutamatergic system holds considerable promise for developing the next generation of novel and mechanistically distinct agents for the treatment of MDD.