How does ketamine lift depression?

The incredibly rapid improvement in depression (hours) produced by ketamine is unprecedented and surely is telling us something vitally important about depression.  If only we could figure out what it is.  Clinicians were used to waiting weeks for antidepressants of all sorts to work.  As a neurologist, I’d see it work in a week or so in my MS patients depressed due a relapse.

Two recent papers show just how hard it is going to be [Neuron  vol. 104 pp. 182 – 182, 338 – 352 ’19 ]. First off you have to accept the idea that even though animals (usually mice) can’t tell us how they feel, we still have reasonable animal models of depression (tail suspension test, forced swim test).  We can at least get a handle on anhedonia using the sucrose preference test.

Throw ketamine at an animal and measure the biochemical or the neurophysiologic effect of your choice. There are zillions of them.  Throw just about anything at the brain, and all sorts of things change.  The problem is showing that the change is relevant.  Is the known blockade of NMDA receptors by ketamine how it helps depression.  Give enough and you get out of body experiences and all sorts of craziness, not an antidepressant effect.

Homer1a is a protein found at the synapse, and like all scaffold proteins, it interacts with a bunch of different proteins. It links another type of glutamic acid receptor (mGluR1 and mGluR5) to inositol 1, 4, 5 trisphosphate receptors (IP3Rs) on the endoplasmic reticulum.  It also links mGluR1 and mGluR5 to NMDARs and other ion channels.

So what?

Other work by the authors showed that knockdown of Homer1a (using small interfering RNA – siRNA) in the medial prefrontal cortex (mPFC) abolished the antidepressant effects (in animal models) to ketamine.  Well that’s good, but even better is that knockdown also abolished the antidepressant effects of a tricyclic antidepressant (imipramine).

The present work showed that increasing the expression of Homer1a (the protein comes in various isoforms) in the frontal cortex reduced depression in the various models.

Pretty good — all we have to do is increase Homer1a expression to have a treatment of depression.

Don’t get your hopes up, and this is why depression research is so — well depressing.

Increasing Homer1a expression in another brain region (the hippocampus) has exactly the opposite effects.

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Comments

  • he  On October 30, 2019 at 6:21 pm

    I think saying ketamine is just an NMDA receptor antagonist says more about the scientists engaged in post hoc analyses trying to find an adequate explanation. It completely ignores the polypharmacology of the molecule in question (I have been guilty of this). I see this in the same category as electric shock and lithium and I know which one I would rather have if I was in trouble.

    Adding a synaptic scaffold protein to the story, while useful, doesn’t help much explain why it seems wholesale electrical insult to the brain is the best we have to make people better again. Need a better explanation to understand what is going on here.

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