Experiencing a traumatic event such as violence or disaster can cause recurring flashbacks, anxiety, fear, and impaired cognition, collectively diagnosed as post-traumatic stress disorder (PTSD). Although a variety of antidepressants and cognitive behavioral therapy such as EMDR (eye movement desensitization and reprocessing) are used to treat PTSD, SSRIs (selective serotonin reuptake inhibitors) are the only antidepressants approved for PTSD.

However, SSRIs are only effective in some patients and exhibit delayed action. Even when a specific treatment is effective, relapses are notoriously frequent and have acquired the label “spontaneous recovery.”

Earlier studies have pointed out the significance of glutamatergic neuron activity, and particularly the role of the excitatory neuroreceptor NMDAR (N-methyl-D-aspartate receptor) in the pathophysiology of PTSD. NMDAR controls changes in neuronal circuitry and function (synaptic plasticity) in learning and memory.

Boyoung Lee, PhD, a scientist at the Center for Cognition and Sociality, in the Institute for Basic Science (IBS) in Daejeon, South Korea, is first author of this paper.

Boyoung Lee, PhD, a scientist at the Center for Cognition and Sociality, in the Institute for Basic Science (IBS) in Daejeon, South Korea, has been trying to understand the mechanisms of PTSD and depression through studies in animal models since 2010 as a postdoctoral fellow at the Korea Institute of Science and Technology and later at Yale University. In her earlier work, Lee had focused on the mechanism of action of a rapid-acting antidepressant called ketamine.

“NMDA receptors play a pivotal role in regulating these disorders. So, I wanted to investigate the novel NMDA modulator, NYX-783, from Aptinyx, for its effectiveness and molecular mechanisms in animal models of PTSD,” said Lee. NYX-783 is a newly discovered positive NMDAR modulator that is currently in Phase IIb trials for PTSD.

These findings were published in the journal Molecular Psychiatry, in an article titled: “Positive modulation of N-methyl-D-aspartate receptors in the mPFC reduces the spontaneous recovery of fear.” The study examined the molecular mechanism of NYX-783 action in mice.

Auditory fear conditioning (AFC) and single-prolonged stress (SPS) are two paradigms commonly used to model PTSD in animals. In AFC, mice are habituated to an environment and subjected to a combination of a tone and electric shock to generate fear conditioning and induce PTSD, whereas in SPS, mice are exposed to multiple stressors in addition to fear conditioning. Stress prior to fear conditioning exacerbates PTSD and further challenges its treatment.

The study led by senior author of the paper Ronald Duman, PhD, and his team subjected these mouse models of PTSD to a series of memory extinction procedures in a new environment to abolish their traumatic memories. In conjunction with this cognitive behavioral therapy, the researchers injected the mice with NYX-783, alongside ketamine.

The team used two distinct injections protocols for the drug to inhibit spontaneous recovery of fear, with different results. “We found that the protocol for drug injection one hour before the first extinction was effective, whereas the protocol for drug injection 24 h before the first extinction was insignificant. We also tested the one hour protocol for ketamine and found that it is insignificant, suggesting NYX-783 is more effective than ketamine, at least on this one hour protocol,” said Lee.

The researchers found injecting the model mice with the drug one hour before fear extinction therapy resulted in the best outcome. The team monitored freezing behavior in the treated mice upon exposure to the conditioning tone, to measure their level of fear and found mice injected NYX-783 fared much better than those injected with ketamine or saline. The drug was particularly effective in suppressing spontaneous recovery and was more effective in female than male mice.

“We found that NYX-783 is effective in reducing spontaneous recovery of fear in animal models of PTSD via GluN2B containing NMDA receptors on pyramidal neurons of medial prefrontal cortex (mPFC). In addition, we found that the specific expression of BDNF (brain derived neurotrophic factor) in the infralimbic mPFC is required for the inhibitory effect of NYX-783 on spontaneous recovery,” said Lee.

“These findings point to the potential utility of NYX-783 in combination with cognitive behavioral therapies commonly used to treat PTSD, such as progressive exposure, cognitive processing therapy, and EMDR, to reduce the burden of traumatic memories,” said John Krystal, MD, professor of translational research, psychiatry, and psychology at Yale School of Medicine, who was not involved in the current study.

(A) In PTSD model mice, NYX-783 acts on the GluN2B subunit of NMDA receptors in glutamatergic neurons, which results in upregulation of BDNF and inhibition of PTSD spontaneous recovery. (B) When GluN2B is knocked down in glutamatergic neurons, it eliminated the effectiveness of NYX-783, since the drug no longer has a valid target. (C) When GluN2B is knocked down in GABAergic neurons, it lessens the degree of inhibition on the glutamatergic neuron. More glutamate is released, which activates the postsynaptic pyramidal neuron, resulting in a baseline reduction in spontaneous recovery. [Institute for Basic Science]

In mechanistic studies on genetically manipulated mice, the team discovered NYX-783 inhibits fear and suppresses spontaneous recovery of fearful memories by modulating NMDA receptors, specifically by acting on its GluN2B subunit. When the researchers knocked down the GluN2B subunit of NMDARs in excitatory glutamatergic neurons in the mPFC, the efficacy of the drug diminished. Mutant mice that expressed a low level of GluN2B exhibited spontaneous recovery, even when injected with NYX-783. On the other hand, the performance of the drug was not affected when the same receptors were knocked down in inhibitory GABAergic interneurons. Knocking down NMDA receptors in interneurons alone could reduce spontaneous recovery.

“This suggests that the beneficial effects of NYX-783 depend in part upon the presence of GluN2B-containing NMDA receptors and most likely their enhanced direct stimulation by NYX-783,” said Krystal. “This paper suggests that facilitation of GluN2B, in this case by NYX-783, might play a role in the treatment of anxiety disorders like PTSD.”

In future studies Lee and her team at IBS, intend to explore the use of other NMDA modulators in treating PTSD and depression. “These findings suggest that NYX-783 may be an effective medication for PTSD and the development of NMDAR modulators may be a viable strategy to treat PTSD.”

Ronald Duman, PhD, was a pioneering neuroscientist known for his work on the role of neurotrophins and neurogenesis in antidepressant action as well as the identification of mechanisms contributing to the rapid antidepressant effects of ketamine and psychedelic drugs. [Source: John Krystal]

Krystal said, “This study highlights the scope of the impact of the late Ronald Duman, PhD, who died suddenly and unexpectedly on February 4, 2020. Dr. Duman was a neuroscience pioneer known for many discoveries, including the role of neurotrophins and neurogenesis in antidepressant action as well as the identification of mechanisms contributing to the rapid antidepressant effects of ketamine and psychedelic drugs. In a study led by his then postdoctoral fellow, Matthew Girgenti, PhD, Duman also showed that prior exposure to ketamine increased the rapidity of fear extinction.”

Krystal added, “This paper seems like a tribute to Ron’s memory by his students and colleagues in that this project was conceived and initiated prior to his death and then seen through to completion and publication after his death.”