A single, low dose of the NMDA receptor antagonist ketamine produces rapid antidepressant actions in treatment-resistant depressed patients. Understanding the cellular mechanisms underlying this will lead to new therapies for treating major depression. NMDARs are heteromultimeric complexes formed through association of two GluN1 and two GluN2 subunits. We show that in vivo deletion of GluN2B, only from principal cortical neurons, mimics and occludes ketamine's actions on depression-like behavior and excitatory synaptic transmission. Furthermore, ketamine-induced increases in mTOR activation and synaptic protein synthesis were mimicked and occluded in 2BΔCtx mice. We show here that cortical GluN2B-containing NMDARs are uniquely activated by ambient glutamate to regulate levels of excitatory synaptic transmission. Together these data predict a novel cellular mechanism that explains ketamine's rapid antidepressant actions. In this model, basal glutamatergic neurotransmission sensed by cortical GluN2B-containing NMDARs regulates excitatory synaptic strength in PFC determining basal levels of depression-like behavior.
Depression is the leading cause of disability worldwide, with hundreds of millions of people living with the condition. The ‘gold standard’ for depression treatment involves a combination of psychotherapy and medication. Unfortunately, current antidepressant medications do not help everyone, waiting lists for psychotherapy are often long, and both normally take a number of weeks of regular treatment before they begin to have an effect. As patients are often at a high risk of suicide, it is crucial that treatments that act more quickly, and that are safe and effective, are developed.
One substance that may fulfill these requirements is a drug called ketamine. Studies have shown that depression symptoms can be reduced within hours by a single low dose of ketamine, and this effect on mood can last for more than a week. However, progress has been hindered by a lack of knowledge about what ketamine actually does inside the brain.
Neurons communicate with one another by releasing chemicals known as neurotransmitters, which transfer information by binding to receptor proteins on the surface of other neurons. Drugs such as ketamine also bind to these receptors. Ketamine works by blocking a specific receptor called the n-methyl d-aspartate (NMDA) receptor, but how this produces antidepressant effects is not fully understood.
The NMDA receptor is actually formed from a combination of individual protein subunits, including one called GluN2B. Now Miller, Yang et al. have created mice that lack receptors containing these GluN2B subunits in neurons in their neocortex, including the prefrontal cortex, a brain region involved in complex mental processes such as decision-making. This allowed Miller, Yang et al. to discover that when the neurotransmitter glutamate binds to GluN2B-containing NMDA receptors, it limits the production of certain proteins that make it easier for signals to be transmitted between neurons. Suppressing the synthesis of these proteins too much may cause depressive effects by reducing communication between the neurons in the prefrontal cortex.
Both mice lacking GluN2B-containing receptors in their cortical neurons and normal mice treated with ketamine showed a reduced amount of depressive-like behavior. This evidence supports Miller, Yang et al.'s theory that by blocking these NMDA receptors, ketamine restricts their activation. This restores normal levels of protein synthesis, improves communication between neurons in the cortex, and reduces depression.
Understanding how ketamine works to alleviate depression is an important step towards developing it into a safe and effective treatment. Further research is also required to determine the conditions that cause overactivation of the GluN2B-containing NMDA receptors.