Ketamine's antidepressant actions depend on the drug's ability to affect mRNA translation, researchers reported in the December 17, 2020, issue of Nature.

The work narrows down the molecular underpinnings of ketamine's actions, which could improve ketamine's utility as a medication.

Ketamine is an NMDA antagonist that was first synthesized in the 1960s, and initially approved and used as an anesthetic. Decades later, the discovery that a single dose of the drug could alleviate treatment-resistant depression within hours set off research that ultimately led to the approval of Spravato (esketamine, Johnson & Johnson) for treatment-resistant depression.

Ketamine's anesthetic effects are due to its action as a glutamate receptor antagonist, but how it is able to rapidly bring people out of deep depression is less well understood.

Previous studies have shown that ketamine exerts its antidepressant effects through mTOR, a protein complex that integrates nutrient sensing with growth control.

"There is literature that mTOR is a target of ketamine," senior author Nahum Sonenberg, a professor of biochemistry at McGill University, told BioWorld Science.

But its seat at the intersection of nutrient sensing and growth regulation means that mTOR itself affects dozens of proteins.

As far as depression is concerned, Sonenberg said, "we showed that all the effect is through these proteins."

"These proteins" are eukaryotic initiation factor 4E-binding protein (4E-BP) 1 and 2, two proteins that help mRNAs get translated by the ribosomal machinery.

Sonenberg, first author Argel Aguilar-Valles, and their colleagues focused on 4E-BPs because of their role in translation. Previous work has shown that neurons in animals treated with ketamine can sprout new synapses, neuronal connections that are important for synaptic plasticity.

The investigators showed that knockout animals lacking 4E-BPs "grow fairly normally. But then we give them ketamine, and nothing happens to this animal," Aguilar-Valles told BioWorld Science.

The team deleted 4E-BP 1 and 4E-BP2 in either excitatory neurons or inhibitory neurons, and to their surprise, found that for ketamine's antidepressant effects, the actions of 4E-BPs in inhibitory interneurons were more important than those in excitatory neurons.

The findings, Aguilar-Valles said, ran counter to the team's predictions. Inhibitory interneurons make up only 10% of neurons in the cortex, and neurons themselves, in turn, only account for half of the cellular population of the brain, which also boasts glial support cells, endothelial cells that line blood vessels, and many other cell types.

"We were expecting to see that if we knock it out in the more abundant cells, there will be more of an effect," he said.

Neurons and in particular excitatory neurons have historically garnered an outsize share of the attention in brain research, because electrophysiological recordings are a long-standing technical method to get a glimpse of what's going on inside the skull -- "we have this idea, because we are monitoring this activity, that maybe the effect is in the neurons," said Aguilar-Valles, who is an assistant professor at Carleton University. But as research methods evolve, there has been a growing realization that so-called support cells, such as astrocytes, also play major roles in information processing in addition to general housekeeping.

In one sense, identifying 4E-BPs as the key mTOR target does not narrow ketamine's effects down so much as punt to the next signaling node.

"This is the next frontier," Sonenberg said. "To find out which mRNAs are affected that make the proteins that give rise to depression."

Identifying those key proteins, he added, is "straightforward in principle," but technically highly challenging.

Another goal is to look at whether and how ketamine's effects on neural circuitry differ between males and females.

In the current work, "we didn't tackle a lot about sex differences," Aguilar-Valles said, even though "in human populations, there are major effects of sex and gender in psychiatric diagnoses."

Not all of those differences are due to biology, he stressed, but there is likely a biological contribution to them. "There might be some psychosocial effects, but also some biologicals," he said. "Sex is a biological variable."