July 14, 2011 | 6
Most people have heard of ketamine. Originally invented in 1962 to be used as an anesthetic, it is still used for children and in some topical anesthetics, but mostly when you hear of ketamine used clinically now, it’s actually used in combination with xylazine as a veterinary anesthetic (side note: SciCat coming to after a visit to the vet from a Ketamine/Xylazine combo is…hilarious. Hilarious and full of ANGER).
But of course, the medical uses of ketamine are not what people have heard about. Instead, people hear about the recreational uses of ketamine (aka Special K), where street users describe hallucinations and a sense of dissociation from the world. It’s achieved widespread fame as a drug of abuse, and that’s how most people know it nowadays.
Of course, you can do clinical trials in humans and get subjective reports from the patients. But if you want to see what’s REALLY going down with how ketamine is WORKING, you need a brain. And for brains, you need rats.
Li et al. “Glutamate N-methyl-D-aspartate receptor antagonists rapidly reverse behavioral and synaptic deficits caused by chronic stress exposure” Biological Psychiatry, 2011.
Ketamine does not behave like many other drugs that are known to be drugs of abuse. Unlike the benzodiazepines, it doesn’t act on GABA. It doesn’t act directly on dopamine like the stimulants, or on serotonin like the hallucinogens. Instead, ketamine acts on the neurotransmitter glutamate, the main excitatory neurotransmitter in the brain. But not DIRECTLY. Instead, it acts on a specific type of glutamate RECEPTOR, the N-methyl-D-aspartate (NMDA) receptor type. Ketamine acts at NMDA receptors as an antagonist, blocking the ability of glutamate to bind to the receptor and do its job.
And this is an aspect of the brain that has been relatively ignored in studies of depression. Often, scientists who study depression focus on serotonin, and on the birth of new neurons in the brain. Many believe that the increases in serotonin produced by traditional antidepressants such as Prozac lead to the increases in neurogenesis in the hippocampus which may help symptoms of depression. But serotonin doesn’t have to be the ONLY neurotransmitter involved. There could be other mechanisms that mediate how depression occurs, and thus other potential drug targets.
And we NEED some other drug targets. Major Depressive Disorder is widespread, and in a large number of cases, the available antidepressants never work. So recently scientists have begun to look at glutamate, and at ketamine. Clinical trials have shown that low doses of ketamine (which still can produce some dissociative symptoms, but no hallucinations) can produce a rapid antidepressant response in severely depressed or bipolar patients. The studies are still very small and limited. And so far, it’s unknown HOW ketamine is acting to relieve depression in these patients.
Bring on the rats.
For this study, the authors took rats, and induced a depression-like state (we say “depression-like” because you can’t ever ASK a rat how it feels about life) using a method called Chronic Unpredictable Stress. Kind of like exposing a rat to the equivalent of grad school. The stresses could be anything, and the rats get two stressors per day. But instead of experimental equipment breaking or their advisor yelling at them or running out of beer money, the rats get stressors like being placed in a chilly room, leaving the lights on overnight, bad smells, being put on a shaker plate or having their cages tilted at weird angles, or leaving the radio on loud. After 21 days of this, you get some stressed out unhappy rats. You can tell by giving them a test to see how much sugar water they want to drink. Happy rats LOVE sugar water, but unhappy rats will drink less of the sugar water.
Here you can see the results for the sucrose drinking rats exposed to chronic stress. The rats showed a decrease in sucrose preference, as well an increase in how long it took them to eat food in a novel environment (called Novelty Suppressed Feeding). BUT, when they gave the rats a single dose of either ketamine or a similar NMDA antagonist RO-256981, 24 hours before they began testing, the rats didn’t have these symptoms. They drank as much sucrose water and ate as much food as animals that had never been exposed to stress at all. And in the bottom two panels of the figure you can see that this effect of a single dose of ketamine or RU-256981 lasted for up to 7 days after the drug was given to the stressed animals.
But of course, if you want to determine a mechanism of how something is acting in the body, you have to BLOCK it. In this case, the authors gave a drug called rapamycin, which is a bacterial produce that inhibits…”the mammalian target of rapamycin”, otherwise called mTOR (you know that you know NOTHING about a protein when you call it “oh, you know, that target of that one drug we have…”). Luckily, we do NOW know a good bit about mTOR, which is a kinase that regulates things like cell growth and proliferation, as well as transcription of DNA. IT also lies downstream of NMDA receptor signaling, so is probably stimulated by drugs which hit the NMDA receptor. So IF ketamine is relieving anhedonia in these rats via mTOR, blocking mTOR will block the effects of ketamine.
So they gave rapamycin right before giving ketamine in the stressed out rats, and rapamycin blocked the effects of ketamine on sucrose preference and suppressed feeding. The rats looked as stressed as ever when mTOR was blocked, which suggests that ketamine was producing the behavioral effects via mTOR.
The authors then looked for various proteins that could be involved. They found that proteins that are associated with synapses, like glutamate receptors and proteins like synapsin 1 are reduced during stress in the rats, and that ketamine can increase these proteins again.
But what are these proteins DOING? It looks like they may be involved in difference spine densities in the stressed rat brains. They authors looked at neurons in the prefrontal cortex, looking specifically at an area called the apical tuft, which is where the tuft of dendrites comes out at the end of the axon (more on basic neuron anatomy here). This is because depressive symptoms in animals are associated with something called dendritic atrophy, where you get a decrease in the numbers of dendritic spines in areas like the apical tuft.
You can see here the photos of the dendritic spines from the rat prefrontal cortex (yup, we can take pictures of tiny parts of tiny neurons. Sometimes, that STILL blows my mind). The stressed rats have decreases in the number of little spines coming off the dendrites, and this can be reversed with ketamine.
But finally, we want to know how stress, and then ketamine, changes the way the neurons BEHAVE. To figure that out, we have to do electrophysiology, which is a technique where you take a REEEAAAALLY TEEEENY end of a glass tip, and suck a REAAAALLLLLY TEEEENY bit of cell membrane into it. If you do this in a live brain slice (which you can keep alive for a few hours outside of the rat’s head), you can get a live cell, and you’ve got something patched into it. You can then get recordings of what the cell is doing electrically (how it is firing, action potentials, etc), kind of like using peephole into a room.
In this case, they were interested two specific TYPES of neurons, those receiving the neurotransmitter serotonin, and those received the relatively new transmitter hypocretin/orexin (so new they are still arguing about the name). Those receiving serotonin are involved in signaling within the cortex, while those receiving hypocretin are involved in signaling which goes outside the cortex to the thalamus.
So they put serotonin and hypocretin on their slices and looked at how the neurons behaved.
You can see the little traces there, which show the neurons experiencing little postsynaptic currents. In the stressed rats (center) the currents were reduced in both cases, but when you gave ketamine, they were increased again. This may mean that the lack of dendritic spines seen in the stressed animals has functional effects on how well the neurons can make their little postsynaptic currents, which is a big effect on functionality.
What I find to be most interesting about this study is that there was only ONE DOSE of ketamine given here. ONE. The effects lasted up to a week. We don’t know if we’d get similar effects in humans (or whether the rats were experiencing hallucinations, for that matter, tough to ask them about that), but if we did, it’s possible that the mechanism through which ketamine works could be used to find new and effective antidepressant drugs. Or, if the effects of ketamine are mostly temporary and it’s not feasible to give it as a long term drug (and it may not be due to legal issues and the potential for abuse and thus possibly the selling of it to other people), we may be able to give it in the clinic, and use it to “kickstart” the effects of more traditional antidepressants like Prozac, where the ketamine may be able to bridge the gap while the Prozac is working (though no results yet on whether the ketamine increases neurogenesis like other antidepressants do, but knowing the work of this laboratory, I bet they’re on it). Or maybe we’ll get both. I don’t know if it’s a magic bullet (I doubt it), but I think it’s got potential as a new mechanism to pursue when looking for antidepressant drugs.
Li N, Liu RJ, Dwyer JM, Banasr M, Lee B, Son H, Li XY, Aghajanian G, & Duman RS (2011). Glutamate N-methyl-D-aspartate receptor antagonists rapidly reverse behavioral and synaptic deficits caused by chronic stress exposure. Biological psychiatry, 69 (8), 754-61 PMID: 21292242