Ketamine works, at the explanatory level (the actual mechanism is still debatable), by blocking one of the targets for the neurotransmitter glutamate in the brain, the N-methyl-D-aspartate (NMDA) glutamate receptor. A new study in Biological Psychiatry reports that enhancing, instead of blocking, that same target - the NMDA glutamate receptor - also causes antidepressant-like effects.

Would an 'Anti-Ketamine' Also Treat Depression?

Nov. 18, 2013 — Thirteen years ago, an article in this journal first reported that the anesthetic medication, ketamine, showed evidence of producing rapid antidepressant effects in depressed patients who had not responded to prior treatments. Ketamine works by blocking one of the targets for the neurotransmitter glutamate in the brain, the N-methyl-D-aspartate (NMDA) glutamate receptor.

Now, a new study in Biological Psychiatry reports that enhancing, instead of blocking, that same target -- the NMDA glutamate receptor -- also causes antidepressant-like effects.

Scientists theorize that NMDA receptor activity plays an important role in the pathophysiology of depression, and that normalizing its functioning can, potentially, restore mood to normal levels.

Prior studies have already shown that the underlying biology is quite complex, indicating that both hyperfunction and hypofunction of the NMDA receptor is somehow involved. But, most studies have focused on antagonizing, or blocking, the receptor, and until now, studies investigating NMDA enhancement have been in the early phases.

Sarcosine is one such compound that acts by enhancing NMDA function. Collaborators from China Medical University Hospital in Taiwan and the University of California in Los Angeles studied sarcosine in an animal model of depression and, separately, in a clinical trial of depressed patients.

"We found that enhancing NMDA function can improve depression-like behaviors in rodent models and in human depression," said Dr. Hsien-Yuan Lane, the corresponding author on the article.

In the clinical portion of the study, they conducted a 6-week trial where 40 depressed patients were randomly assigned to receive sarcosine or citalopram (Celexa), an antidepressant already on the market that was used as a comparison drug. Neither the patients nor their doctors knew which one they were receiving.

Compared to citalopram, patients receiving sarcosine reported significantly improved mood scores, were more likely to experience relief of their depression symptoms, and were more likely to continue in the study. There were no major side effects in either group, but patients receiving citalopram reported more relatively minor side effects than the patients being treated with sarcosine.

"It will be important to understand how sarcosine, which enhances NMDA receptor function, produces the interesting effects reported in this study. There are ways that its effects, paradoxically, might converge with those of ketamine, a drug that blocks NMDA receptors," commented Dr. John Krystal, Editor of Biological Psychiatry. "For example, both compounds may enhance neuroplasticity, the capacity to remodel brain networks through experience. Also, both potentially attenuate signaling through NMDA receptors, ketamine with single doses and sarcosine, with long-term administration, by evoking an adaptive down regulation of NMDA receptors."

Better understanding the reported findings may help to advance the development of medication treatments for patients who do not respond to available treatments. This is an important goal, with estimates indicating that as many as half of all patients do not experience complete relief of their depression.

Full Citation:
Chih-Chia Huang, I-Hua Wei, Chieh-Liang Huang, Kuang-Ti Chen, Mang-Hung Tsai, Priscilla Tsai, Rene Tun, Kuo-Hao Huang, Yue-Cune Chang, Hsien-Yuan Lane, Guochuan Emil Tsai. (2013, Nov 15). Inhibition of Glycine Transporter-I as a Novel Mechanism for the Treatment of Depression. Biological Psychiatry, 74(10): 734-741. DOI: 10.1016/j.biopsych.2013.02.020

Abstract

Background

Antidepressants, aiming at monoaminergic neurotransmission, exhibit delayed onset of action, limited efficacy, and poor compliance. Glutamatergic neurotransmission is involved in depression. However, it is unclear whether enhancement of the N-methyl-D-aspartate (NMDA) subtype glutamate receptor can be a treatment for depression.
 
Methods

We studied sarcosine, a glycine transporter-I inhibitor that potentiates NMDA function, in animal models and in depressed patients. We investigated its effects in forced swim test, tail suspension test, elevated plus maze test, novelty-suppressed feeding test, and chronic unpredictable stress test in rats and conducted a 6-week randomized, double-blinded, citalopram-controlled trial in 40 patients with major depressive disorder. Clinical efficacy and side effects were assessed biweekly, with the main outcomes of Hamilton Depression Rating Scale, Global Assessment of Function, and remission rate. The time course of response and dropout rates was also compared.
 
Results

Sarcosine decreased immobility in the forced swim test and tail suspension test, reduced the latency to feed in the novelty-suppressed feeding test, and reversed behavioral deficits caused by chronic unpredictable stress test, which are characteristics for an antidepressant. In the clinical study, sarcosine substantially improved scores of Hamilton Depression Rating Scale, Clinical Global Impression, and Global Assessment of Function more than citalopram treatment. Sarcosine-treated patients were much more likely and quicker to remit and less likely to drop out. Sarcosine was well tolerated without significant side effects.
 
Conclusions

Our preliminary findings suggest that enhancing NMDA function can improve depression-like behaviors in rodent models and in human depression. Establishment of glycine transporter-I inhibition as a novel treatment for depression waits for confirmation by further proof-of-principle studies.