Depression strikes some 35 million people worldwide, according to the World Health Organization, contributing to lowered quality of life as well as an increased risk of heart disease and suicide. Treatments typically include psychotherapy, support groups and education as well as psychiatric medications. SSRIs, or selective serotonin reuptake inhibitors, currently are the most commonly prescribed category of antidepressant drugs in the U.S., and have become a household name in treating depression.
The action of these compounds is fairly familiar. SSRIs increase available levels of serotonin, sometimes referred to as the feel-good neurotransmitter, in our brains. Neurons communicate via neurotransmitters, chemicals which pass from one nerve cell to another. A transporter molecule recycles unused transmitter and carries it back to the pre-synaptic cell. For serotonin, that shuttle is called SERT (short for “serotonin transporter”). An SSRI binds to SERT and blocks its activity, allowing more serotonin to remain in the spaces between neurons. Yet, exactly how this biochemistry then works against depression remains a scientific mystery.
In fact, SSRIs fail to work for mild cases of depression, suggesting that regulating serotonin might be an indirect treatment only. “There’s really no evidence that depression is a serotonin-deficiency syndrome,” says Alan Gelenberg, a depression and psychiatric researcher at The Pennsylvania State University. “It’s like saying that a headache is an aspirin-deficiency syndrome.” SSRIs work insofar as they reduce the symptoms of depression, but “they’re pretty nonspecific,” he adds.
Now, research headed up by neuroscientists David Gurwitz and Noam Shomron of Tel Aviv University in Israel supports recent thinking that rather than a shortage of serotonin, a lack of synaptogenesis (the growth of new synapses, or nerve contacts) and neurogenesis (the generation and migration of new neurons) could cause depression. In this model lower serotonin levels would merely result when cells stopped making new connections among neurons or the brain stopped making new neurons. So, directly treating the cause of this diminished neuronal activity could prove to be a more effective therapy for depression than simply relying on drugs to increase serotonin levels.
Evidence for this line of thought came when their team found that cells in culture exposed to a 21-day course of the common SSRI paroxetine (Paxil is one of the brand names) expressed significantly more of the gene for an integrin protein called ITGB3 (integrin beta-3). Integrins are known to play a role in cell adhesion and connectivity and therefore are essential for synaptogenesis. The scientists think SSRIs might promote synaptogenesis and neurogenesis by turning on genes that make ITGB3 as well as other proteins that are involved in these processes. A microarray, which can house an entire genome on one laboratory slide, was used to pinpoint the involved genes. Of the 14 genes that showed increased activity in the paroxetine-treated cells, the gene that expresses ITGB3 showed the greatest increase in activity. That gene,ITGB3, is also crucial for the activity of SERT. Intriguingly, none of the 14 genes are related to serotonin signaling or metabolism, and, ITGB3 has never before been implicated in depression or an SSRI mode of action.
These results, published October 15 2013 in Translational Psychiatry, suggest that SSRIs do indeed work by blocking SERT. But, the bigger picture lies in the fact that in order to make up for the lull in SERT, more ITGB3 is produced, which then goes to work in bolstering synaptogenesis and neurogenesis, the true culprits behind depression. “There are many studies proposing that antidepressants act by promoting synaptogenesis and neurogenesis,” Gurwitz says. “Our work takes one big step on the road for validating such suggestions.”
The research is weakened by its reliance on observations of cells in culture rather than in actual patients. The SSRI dose typically delivered to a patient’s brain is actually a fraction of what is swallowed in a pill. “Obvious next steps are showing that what we found here is indeed viewed in patients as well,” Shomron says.
The study turned up additional drug targets for treating depression—two microRNA molecules, miR-221 and miR-222. Essentially, microRNAs are small molecules that can turn a gene off by binding to it. The microarray results showed a significant decrease in the expression of miR-221 and miR-222, both of which are predicted to target ITGB3, when cells were exposed to paroxetine. So, a drug that could prevent those molecules from inhibiting the production of the ITGB3 protein would arguably enable the growth of more new neurons and synapses. And, if the neurogenesis and synaptogenesis hypothesis holds, a drug that specifically targeted miR-221 or miR-222 could bring sunnier days to those suffering from depression.
Physical activity reorganizes the brain so that its response to stress is reduced and anxiety is less likely to interfere with normal brain function, according to a research team based at Princeton University.
The researchers report in the Journal of Neuroscience that when mice allowed to exercise regularly experienced a stressor — exposure to cold water — their brains exhibited a spike in the activity of neurons that shut off excitement in the ventral hippocampus, a brain region shown to regulate anxiety.
These findings potentially resolve a discrepancy in research related to the effect of exercise on the brain — namely that exercise reduces anxiety while also promoting the growth of new neurons in the ventral hippocampus. Because these young neurons are typically more excitable than their more mature counterparts, exercise should result in more anxiety, not less. The Princeton-led researchers, however, found that exercise also strengthens the mechanisms that prevent these brain cells from firing.
Different brain areas are activated when we choose to suppress an emotion, compared to when we are instructed to inhibit an emotion, according a new study from the UCL Institute of Cognitive Neuroscience and Ghent University.
In this study, published in Brain Structure and Function, the researchers scanned the brains of healthy participants and found that key brain systems were activated when choosing for oneself to suppress an emotion. They had previously linked this brain area to deciding to inhibit movement.
“This result shows that emotional self-control involves a quite different brain system from simply being told how to respond emotionally,” said lead author Dr Simone Kuhn (Ghent University).
In most previous studies, participants were instructed to feel or inhibit an emotional response. However, in everyday life we are rarely told to suppress our emotions, and usually have to decide ourselves whether to feel or control our emotions.
In this new study the researchers showed fifteen healthy women unpleasant or frightening pictures. The participants were given a choice to feel the emotion elicited by the image, or alternatively to inhibit the emotion, by distancing themselves through an act of self-control.
The researchers used functional magnetic resonance imaging (fMRI) to scan the brains of the participants. They compared this brain activity to another experiment where the participants were instructed to feel or inhibit their emotions, rather than choose for themselves.
Different parts of the brain were activated in the two situations. When participants decided for themselves to inhibit negative emotions, the scientists found activation in the dorso-medial prefrontal area of the brain. They had previously linked this brain area to deciding to inhibit movement.
In contrast, when participants were instructed by the experimenter to inhibit the emotion, a second, more lateral area was activated.
“We think controlling one’s emotions and controlling one’s behaviour involve overlapping mechanisms,” said Dr Kuhn.
“We should distinguish between voluntary and instructed control of emotions, in the same way as we can distinguish between making up our own mind about what do, versus following instructions.”
Regulating emotions is part of our daily life, and is important for our mental health. For example, many people have to conquer fear of speaking in public, while some professionals such as health-care workers and firemen have to maintain an emotional distance from unpleasant or distressing scenes that occur in their jobs.
Professor Patrick Haggard (UCL Institute of Cognitive Neuroscience) co-author of the paper said the brain mechanism identified in this study could be a potential target for therapies.
“The ability to manage one’s own emotions is affected in many mental health conditions, so identifying this mechanism opens interesting possibilities for future research.
“Most studies of emotion processing in the brain simply assume that people passively receive emotional stimuli, and automatically feel the corresponding emotion. In contrast, the area we have identified may contribute to some individuals’ ability to rise above particular emotional situations.
“This kind of self-control mechanism may have positive aspects, for example making people less vulnerable to excessive emotion. But altered function of this brain area could also potentially lead to difficulties in responding appropriately to emotional situations.”