Depression and addiction often go together. While about 10 percent of the general population will develop a substance use disorder, in people diagnosed with depression that number is around 30 percent. The connection between substance use and depression is complicated. Depression can lead to addiction as a form of self-medicating and addiction can also lead to depression. Or both can be caused, or aggravated, by some third factor. Recent research has discovered another possible link between addiction and depression and this one has to do with how the brain is wired. A team lead by Dr. Scott Thompson at the University of Maryland School of Medicine in Baltimore recently discovered a neural pathway in mice that is related to both addiction and depression.The pathway connects two regions, the hippocampus and nucleus accumbens. The hippocampus is primarily involved in two processes, encoding short-term memories into long-term memories, and spatial learning. The nucleus accumbens is involved with a range of cognitive functions including motivation, aversion, reward, pleasure, and reinforcement learning. The connection between these two regions is what allowed our ancestors to remember where they found food or shelter and it’s what allows us to remember and repeat rewarding behaviors. Researchers wanted to know whether this circuit had any effect on addiction and depression. To that end, they introduced light-sensitive neurons into the region connecting the hippocampus and nucleus accumbens of mice. These light-sensitive neurons allow the researchers to boost or inhibit the signals between those two parts of the brain using light. They first created an artificial reward for the mice by activating the neural pathway in a specific location. The mice would then return to this location, looking for the reward. However, the next day, the researchers reversed the process, dampening the activity in the neurons connecting the nucleus accumbens and the hippocampus. When they did this, the mice lost interest in the area they previously found rewarding. The behavior of the mice, then, resembled addiction when the pathway was stimulated and resembled depression when the pathway was inhibited. The results of this experiment fit nicely with what we currently know about addiction and depression. For example, it has long been known the nucleus accumbens played a role in addictive behavior and the hippocampus plays a role in triggering behavior. This is why someone recovering from addiction should avoid certain people and places. Depression is often characterized by anhedonia, or inability to feel pleasure. Things people used to enjoy become uninteresting, much like the mice losing interest in the artificially reinforced location. However, the study does raise some interesting questions. For example, if depression and addiction are caused by opposite behavior in the same brain circuit, why to they so frequently occur together? Does it account for other symptoms of depression, such as sleep disturbance and irritability? Perhaps most importantly, will the findings apply to humans and can we manipulate the function of the circuit, perhaps through deep brain stimulation? Whatever the answers, this study has interesting implications for anyone struggling with depression or addiction.
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