A great many neuroscientists are interested in clinical depression and antidepressants. We're still a long way from understanding depression on a biological level - and if anyone tries to tell you otherwise, they're probably trying to sell you something. I've previously discussed the controversies surrounding the neurotransmitter serotonin - according to popular belief, the brain's "happy chemical". My conclusion was that although clinical depression is not caused by "low serotonin" alone, serotonin does play an important role in mood at least in some people.
A paper published recently in Molecular Psychiatry makes a number ofimportant contributions to the literature on depression and antidepressants; I haven't seen it discussed elsewhere, so here is make take on it. The paper is by a Portuguese research group, Bessa et. al., and it's titled The mood-improving actions of antidepressants do not depend on neurogenesis but are associated with neuronal remodeling. The findings are right there in the title, but a little history is required in order to appreciate their significance.
For a long time, the only biological theory which attempted to explain clinical depression and how antidepressants counteract it was the monoamine hypothesis. During the early 1960s, it was noticed that early antidepressant drugs, such as imipramine, all inhibited either the breakdown or the removal (reuptake) of chemicals in the brain called monoamines, including serotonin. This led many to conclude that antidepressants improve mood by raising monoamine levels, and that depression is probably caused by some kind of monoamine deficiency. For various reasons (not all of them good ones), it was later decided that serotonin was the crucial monoamine involved in mood, although for several years another, noradrenaline, was favored by most people.
This "monoamine hypothesis" was always a little shaky, and over the past decade or so, an alternative approach has become increasingly fashionable. If you were so inclined, you might even call it a new paradigm. This is the proposal that antidepressants work by promoting the survival and proliferation of new neurones in certain areas of the brain - the "neurogenesis hypothesis". Neurogenesis, the birth of new cells from stem cells, occurs in a couple of very specific regions of the adult brain, including the elaborately named subgranular zone (SGZ) of the dentate gyrus (DG) of the hippocampus. Many experiments on animals have shown that chronic stress, and injections of the "stress hormone" corticosterone, can suppress neurogenesis, while a wide range of antidepressants block this effect of stress and promote neurogenesis. (Other evidence shows that antidepressants probably do this by inducing the expression of neurotrophic signaling proteins, like BDNF.)
The literature on stress, neurogenesis, and antidepressants, is impressive and growing rapidly. For good reviews, see Duman (2004) and Duman & Monteggia (2006). However, the crucial question - do antidepressants work by boosting hippocampal neurogenesis? - remains a controversial one. The hippocampus is not an area generally thought of as being involved in mood or emotion, and damage to the human hippocampus causes amnesia, not depression. Given that the purpose (if any) of adult neurogenesis remains a mystery, it's entirely possible that neurogenesis has nothing to do with depression and mood.
To establish whether neurogenesis is involved in antidepressant action, you need to to manipulate it - for example, by blocking neurogenesis and seeing if this makes antidepressants ineffective. This is practically quite tricky, but Luca Santarelli et. al. (2003)managed to do itby irradiating the hippocampi of mice with x-rays. They found that this made two antidepressants (fluoxetine, aka Prozac, and imipramine) ineffective in protecting the animals against the detrimental effects of chronic stress. This was a landmark result, and raised a lot of interest in the neurogenesis theory.
This new paper, however, says differently. The authors gave lab rats a six-week Chronic Mild Stress treatment, a Guantanamo Bay-style program of intermittent food deprivation, sleep disruption, and confinement. Chronic stress has various effects on rats, including increased anxiety and decreased time spent grooming leading to fur deterioration. These behaviours and others can be quantified, and are treated as a rat analogue of human clinical depression - whether this is valid is obviously debatable, but I'm willing to accept it at least until a better animal model comes along.
Anyway, some of the rats were injected with antidepressants during the final two weeks of the stress procedure. As expected, these rats coped better with the stress at the end of six weeks. This graph
shows the effects of stress and antidepressants on the rat's behaviour in the Forced Swim (Porsolt) Test. Higher bars indicate more "depressed" behaviour. The second pair of bars, representing the stressed rats who got placebo injections, is a lot higher than the first pair of bars representing rats who were not subjected to any stress. In other words, stress made rats "depressed" - no surprise. The other four pairs of bars are pretty much the same height as the first pair; these are rats who got antidepressants, showing that they were resistant to the effects of stress.
The crucial finding is that the white and the black bars are all pretty much the same height. The black bars represent animals who were given injections of methylazoxymethanol (MAM), a cytostatic toxin which blocks cell division (rather like cancer chemotherapy). As you can see, MAM had no effect at all on b
ehaviour in the swim test. It had no effect on most other tests, although it did seem to make the rats more anxious in one experiment.
However, MAM powerfully inhibited neurogenesis. This second graph shows the number of hippocampal cells expressing KI-67, a protein which is a marker of neuroproliferation. As expected, stress reduced neurogenesis and antidepressants increased it. MAM (black bars again) reduced neurogenesis, and in particular, it completely blocked the ability of antidepressants to increase it.
But as we saw earlier, MAM did not stop antidepressants from protecting rats against stress. So, the authors concluded, neurogenesis is not necessary for antidepressants to work. This contradicts the landmark finding of Santarelli et. al. - why the discrepency? There are so many differences between the two experiments that there could be any number of explanations - the current study used rats, while Santarelli used mice, for one thing, and that could well be important. Whatever the reason, this result suggests at the least that neurogenesis is not the only mechanism by which antidepressants counteract the effects of stress in animals.
The most interesting aspect of this paper, to my mind, was an essentially unrelated new finding. Stress was found to reduce the volume of several areas of the rat's brain, including the hippocampus and also
the medial prefrontal cortex (mPFC). Unlike the hippocampus, this is an area known to be involved in motivation and emotion. Importantly, the authors found that following stress, the mPFC did not shrink because neurones were dying or because fewer neurones were being born, but rather because the existing neurones were changing shape - stress caused atrophy of the dendritic spines which branch out from neurones. Dendrites are essential for communication between neurones.
As you can see in the drawings above, stress (the middle column) caused shrinking and stunting of the dendrites in pyrimidal neurones from three areas relative to the unstressed rats (left), while those rats recieving antidepressants as well as stress showed no such effect (right). The cytostatic MAM had no effect whatsoever on dendrites. Further work found that antidepressants increase expression of NCAM1, a protein which is involved in dendritic growth.
So what does this mean? Well, for one thing, it doesn't prove that antidepressants work by increasing dendritic branching. Cheekily, the authors come close to implying this in their choice of title for the paper, but the published evidence shows no direct evidence for this. To find out, you would have to show that blocking the effects of antidepressants on dendrites also blocks their beneficial effects. I suspect this is what the authors are now working hard to try to do, but they haven't done so yet.
It also doesn't mean that taking Prozac will change the shape of your brain cells. It might well do, but this was a study in rats given huge doses of antidepressants (by human standards), so we really don't know whether the findings apply to humans. On the other hand, if Prozac changes the shape of your cells, this study suggests that stressful situations do too - and Prozac, if anything, will put your cells back to "normal".
Finally, I don't want to suggest that the neurogenesis theory of depression is now "dead". In neuroscience, theories never live or die on the basis of single experiments (unlike in physics). But it does suggest that the much-blogged-about neurogenesis hypothesis is not the whole story. Depression isn't just a case of too little serotonin, and it isn't just a case of too little neurogenesis or too little BDNF either.
J M Bessa, D Ferreira, I Melo, F Marques, J J Cerqueira, J A Palha, O F X Almeida, N Sousa (2008). The mood-improving actions of antidepressants do not depend on neurogenesis but are associated with neuronal remodeling Molecular Psychiatry DOI: 10.1038/mp.2008.119