Studies in mice suggest that directly blocking eukaryotic elongation factor 2 (eEF2) kinase (also called CaMKIII) was enough to generate antidepressant-like behavioral effects. They say it is a key molecule in the pathway by which ketamine and other N-methyl-D-aspartate receptor (NMDAR) antagonists trigger rapid synthesis of brain-derived neurotrophic factor (BDNF).
The work was done by scientists at the University of Texas Southwestern Medical Center’s departments of psychiatry and neuroscience. Their findings are described in a paper published in Nature titled “NMDA receptor blockade at rest triggers rapid behavioural antidepressant responses.”
Patients with major depressive disorder who are treated with a single dose of the ionotropic glutamatergic NMDAR (N-methyl-D-aspartate receptor) antagonist ketamine report significant antidepressant effects that kick in within just two hours after treatment and can last for up to two weeks, state Lisa M. Monteggia, Ph.D., and colleagues. However, the mechanism underlying these effects are unclear.
To investigate this unknown further, Dr. Monteggia’s team evaluated the biochemical effects of ketamine and other NMDAR antagonists on antidepressant-predictive tasks in mouse models. They first confirmed that administering ketamine or the NMDAR antagonists MK-801 and CPP to wild-type mice induced rapid behavioral responses in antidepressant-predictive tasks including the forced swim test (FST), novelty-suppressed feeding (NSF), and learned helplessness. Conversely, acute treatment with conventional antidepressants had no beneficial effects on the animals’ responses to the tasks.
The effects of ketamine and CPP but not of MK-801 persisted for 24 hours, and ketamine’s behavioral effect lasted for 1 week. The researchers note that all three drugs have short, 2–3 hour half lives, which suggests their sustained effects after a single treatment are due to synaptic plasticity rather than persistent blockade of receptors.
The researchers then investigated whether the antidepressant-like response to ketamine in mice is mediated through BDNF. When they administered ketamine to inducible Bdnf-knockout mice and their wild-type littermates, the wild-type animals subjected to the FST showed significant reductions in immobility, indicative of antidepressant-like responses, within 30 minutes. Conversely, the Bdnf-knockout mice demonstrated no antidepressant-like responses to ketamine.
Similarly, administration of MK-801 triggered fast-acting antidepressant-like responses in the wild-type mice but not in the knockouts. Interestingly, while the wild-type mice continued to display antidepressant-linked behavior 24 hours after ketamine administration, the benefits of MK-801 had stopped by 24 hours.
In wild-type animals administration of ketamine or MK-801 had no effects on Bdnf mRNA expression, either at 30 minutes post-treatment or after 24 hours. Conversely, BDNF protein levels were markedly increased at 30 minutes post-administration but not at 24 hours. The acute effects of ketamine also extended to the BDNF precursor proBDNF.
“These data indicate that rapid increases in BDNF protein translation, not transcription, are necessary for fast-onset antidepressant responses,” the authors note. “However, continued BDNF protein upregulation does not underlie ketamine’s long-term behavioral effects.”
They next went on to examine the effects of ketamine on mice that were pretreated with either the protein synthesis inhibitor anisomycin or the RNA polymerase inhibitor actinomycin D. Anisomycin prevented the ketamine-induced rapid behavioral responses seen at 30 minutes in the FST and NSF paradigms and also prevented keamine’s long-term effects on FST.
This indicated that ketamine-related modified behaviors depend on new protein synthesis and that this rapid protein translation was somehow involved in sustained antidepressant-like responses. Conversely, actinomycin D did not affect ketamine’s antidepressant-like effect on FST at either time point, indicating that it is independent of new gene expression.
To home in closer on the protein mechanisms underpinning the antidepressant effects of ketamine the team examined whether the fast-acting antidepressant response is mediated via eEF2, a critical catalytic factor for ribosomal translocation during protein synthesis. Administration of either ketamine or MK-801 to wild-type mice led to rapid decreases in the level of phosphorylated eEF2 in the hippocampus but not the cortex.
To further examine whether eEF2K inhibition alters BDNF protein expression in vivo, the eEF2K inhibitors rottlerin or NH125 were administered to wild-type mice. Thirty minutes after administration of either drug, significantly increased BDNF protein expression and significantly decreased phosphorylated eEF2 levels were found in the hippocampal regions of the animals’ brains.
In fact, administering either of these eEF2 inhibitors was sufficient to trigger antidepressant-like responses within 30 minutes in wild-type mice undertaking the FST activity. Moreover, the acute dose of rottlerin or NH125 did not affect locomotor activity but did generate long-lasting antidepressant-related behavioral effects.
To validate the supposition that antidepressant effects after eEF2K inhibition are mediated through BDNF, the researchers then administered rottlerin to Bdnf-knockout mice and tested FST behavior. Like the NMDAR antagonists, rottlerin was ineffective in Bdnf-knockouts, “showing that increased Bdnf expression upon eEF2K inhibition is required to produce antidepressant-like behavioral responses,” the authors write.
“Our data support the hypothesis that ketamine produces rapidly acting antidepressant-like behavioral effects through inhibition of spontaneous NMDA-receptor-mediated spontaneous miniature excitatory postsynaptic currents, leading to decreased eEF2K activity, thus permitting rapid increases in BDNF translation, which may, in turn, exert strong influences on presynaptic or postsynaptic efficacy,” they conclude.
“These data demonstrate that eEF2K inhibition, resulting in de-suppression of protein translation, is sufficient to produce antidepressant-like effects, implicating eEF2K inhibitors as potential novel major depressive disorder treatments with rapid onset. Moreover, our results show that synaptic translational machinery may serve as a viable therapeutic target for the development of faster-acting antidepressants.”