Deletion mouse shows resistance to depression, similarity to mice treated with antidepressants

Before, I talk about a mouse model that is resistant to depression, I think I had better talk about mouse models of depression so that everyone is on the same page. If you ask a nonscientist whether they think there can be a mouse model of depression, you would probably get a raised eyebrow if the person didn't totally laugh in your face. But mouse models of depression -- ridiculous as they may sound -- are actually important learning tools for understanding the disease...that is as long as you think of them in context.

How would we define a mouse model of depression? Well, since it is really difficult to just ask the mouse how he or she is feeling, we are forced to rely on two general criterion:

• 1) Does the mouse show the external symptoms of depression? We tend to call these negative symptoms. Negative symptoms are stuff like low activity, helplessness, and memory deficits. These are things that we can actually measure without having to ask the mouse anything.
• 2) Are those symptoms amenable to treatment with antidepressants? We would like whatever the model we pick to be amenable to treatment with antidepressants. This way not only can we better understand how the drugs work, but we can also test new drugs to find likely candidates as they become available.

Within these bounds, scientists have developed several different systems -- treatments performed on mice -- that mimic depression. For many of them, you repeat the treatment over and over until the animal gives up trying to fight it. This is what we call learned helplessness and is a negative symptom that we associate with depression. Here are some examples. (I list them because they are important to understanding the paper.)

• Forced swim test (FST) -- You place a mouse in a pool of water where they can't get out for about 6 min. At some point, the animal will give up swimming around and just float. The level of immobility in the last 4 min is a measure of how much the animal has given up.
• Tail suspension test (TST) -- You hold a mouse by its tail for 6 min. After a period of clawing around trying to get away, the animal will remain still. How still it remains during the 6 min is again a measure of helplessness.
• Conditioned suppression of motility test (CSMT) -- You place the animal in an illuminated box with squares on the bottom so that you can measure mobility. On day one, you deliver repeated mild footshocks to the animal. On day two, you measure how much the animals moves around after replacing it in the box. This is an experiment in conditioning. The unconditioned stimulus is the footshocks, and the conditioned stimulus is the box. The response is freezing. It turns out that by applying footshocks to the animal, they will rapidly associate the box with the footshocks and just freeze whenever you place them in it. The amount of freezing on subsequent days is what you measure.
• Learned helplessness test (LHT) -- This is similar to the CSMT only the box that administers the footshocks has two areas separated by a gate. During the training period, the gate is raised and the animal cannot escape the footshocks. During the testing period, you lower the gate and wait to see if the animal runs to the other side where footshocks are not being administered. The degree of learned helplessness is measured by how much longer it takes for the animal to run to the other side.
• Novelty-suppressed feeding test (NSFT) -- You make a mouse hungry by denying it food for about 24 hours. Then you place it in a box with a dark area and a light area with the food in the light area. The mouse is hungry, but mice don't like to run into brightly lit areas for food. You measure how long it takes for it to overcome this aversion to novelty and run out to get the food.

My suspicion is that I have just disgusted a lot of people, but I hate to say that this is how depression research is done. People who study this stuff justify it to themselves by thinking of how many people suffer horribly under depression and by trying to limit the number of animals used.

Good animal models are needed to test whether antidepressant are effective and to help understand the pathways necessary for the formation of depression. The experimental models listed above all pass the two criterion I described. If you treat a mouse this way, it will develop negative symptoms of depression. All of the models improve on antidepressant treatment, and thus are said to predict the effectiveness of antidepressants. (There is actually a bit more compexity because some tests are thought to be better associated with the effectiveness of acute treatment of antidepressants whereas others are associated with the effectiveness of chronic treatment. You get the picture though.)

Anyway, getting back to the paper, Heurteaux et al. in Nature Neuroscience show that if you delete a potassium channel in mice called TREK-1, the mice become resistant to the formation of depression in all of those models I listed above. Even more, they begin to resemble wild type mice that were treated with antidepressants, and antidepressants do not change the performance of the TREK-1 knockout mice in these tests.

Here's a little background on TREK-1:

TREK-1 is one of the 17 members of the family of two-pore-domain potassium channels, which form a distinct class of K+ channels. They are open at membrane potentials in physiological conditions and are therefore likely to contribute to the background or leak currents that set the resting potential and oppose depolarizing influences. These channels are key components in shaping the overall excitability of individual neurons. TREK-1 is regulated by neurotransmitters that induce a fluctuation of cAMP via Gs- or Gi-coupled receptors and by those that activate the Gq protein pathway. These would include of course several different types of 5-HT receptor. TREK-1 is the probable mammalian homolog of the Aplysia S-type channel, which is highly controlled by 5-HT, is involved in simple forms of learning and memory and responds to stressful stimuli. The TREK-1 channel is particularly well expressed in the prefrontal cortex and the hippocampus--regions that seem to mediate cognitive aspects of depression, such as memory impairments and feelings of worthlessness, hopelessness, guilt and suicidality. TREK-1 channels are also particularly abundant in areas involved in emotional memory such as the striatum (particularly the nucleus accumbens), the amygdala and the hypothalamus and, as a result, they could be associated with anhedonia and the reduced motivation observed in depression. For all these reasons, we deemed it interesting to analyze the phenotypes of TREK-1-deficient mice in relation to depression and aspects of 5-HT neurotransmission known to be involved in the effects of antidepressant treatments. (Citations were removed.)

As you can see the gene they deleted called TREK-1 (also called Kcnk2) is a potassium channel. Potassium channels are responsible for setting the resting polarization of the cell. By opening they allow potassium to escape, hyperpolarizing it, and making it more resistant to the initiation of an action potential. Many of these receptors are activated by neurotransmitters like serotonin (5-HT). (I don't really want to go into this right now, so that rather cursory explanation is going to have to be enough.)

Anyway, it doesn't really matter for the purposes our discussion what the gene does. Rather it matters that its deletion prevents depression formation according to our models and that the deletion mimics treatment with an antidepressant on both acute and chronic time scales. There is a lot of data in this paper so here is just a sample (click to enlarge):

This is the data for the Forced Swim Test (FST) described above. You can see that for the wild type mice (Kcnk2 +/+) treatment with three different antidepressants (fluoxetine = Flu, paroxetine = Par, and amitriptyline = Ami) lowers the latency -- the amount of time where the mouse has given up and is not swimming. Not only does the deletion mouse (Kcnk2 -/-) have a lower latency to begin with, but treatment has no effect on the latency. It is like the animal is already treated with an antidepressant. This is also true when we look at longer durations of treatment with the antidepressant.

Most of the data is like that. In addition the paper also shows that Kcnk2 -/- mice have increased neurogenesis in the hippocampus (this is another model for how antidepressants work) and produce less stress hormones when placed in stressful situations.

So why would we care about this paper?

We care because It illuminates the possible mechanisms of action for antidepressants. At least traditionally, antidepressants were thought to function by enhancing transmission through the serotonin system by preventing reuptake of serotonin into the cell after synaptic release. In fact, an entire class of antidepressants including Prozac, Paxil, Zoloft, etc. are named SSRIs or Selective Serotonin Reuptake Inhibitors for this effect. Because this mouse showed an obviation of the effects of these drugs, we have to assume that TREK-1 is involved in their mechanism of action.

There are two ways to think about how deleting the TREK-1 channel might affect the action of these drugs. First, it could be that the TREK-1 channel modulates the postsynaptic effects of serotonin and thereby enhances the effect of the drugs. There is evidence for that -- this paper also shows that the Kcnk2 -/- mice show enhanced serotonin transmission.

Second, the drugs might work by directly blocking the channel, skirting the issue of serotonin reuptake entirely. The suggest that there is evidence for this as well:

Fluoxetine but also antidepressants such as paroxetine and sertraline inhibit the TREK-1 channel in the approx 3 to 10 muM IC50 range, whereas they do not affect TRAAK channels. Fluoxetine accumulates in brain tissue, where it reaches concentrations that are similar among the mouse strain used in this work, rats and humans. Therefore, some inhibition of the TREK-1 channel might be expected to occur during treatment of patients as brain concentrations of these drugs, particularly fluoxetine, reach the 1-10 muM range. The behavioral effects of SSRIs may be linked in part to this inhibitory effect, which mimics a TREK-1 deletion, in addition to being linked to their effect on 5-HT transporters. (Citations deleted.)

This may seem like really nitpicking but you need to understand that the why antidepressants work is still largely a mystery. We don't really understand their mechanism of action, so every little bit helps.