Major depressive disorder and bipolar depression are among the most prevalent and disabling mental disorders worldwide. Real-world effectiveness trials in major depressive disorder have underscored that most pharmacological options target monoamines, which are involved in a minority (15-20%) of synaptic contacts in the mammalian brain. Most synapses (∼50%) use the amino acid glutamate as their primary neurotransmitter, and preclinical models of depression have implicated aberrant glutamatergic neurotransmission for 25 years (1). More recently, the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist ketamine was shown to produce rapid and robust antidepressant effects in patients with treatment-resistant major depressive disorder and bipolar depression (2–7).