The consumption of some substances produces such an intense effect on the brain's reward circuits that it can make you forget about hunger or thirst. This is intuited by observing a person suffering from an addiction, but this week, a team of scientists from several American institutions publishes an article in the magazine Science in which he describes part of the mechanism that explains this behavior and raises the possibility of new solutions for addicts.
Drugs like cocaine or heroin are addictive because they produce changes in the brain, hijacking the systems that make us crave water or food, essential for our survival. These substances intensify the desire for them and concentrate it, reducing the pleasure felt when receiving other rewards that help lead a healthy life. Several previous studies have analyzed these mechanisms, but the authors of the work published this Thursday wanted to combine the latest technologies in animal models to understand the biological bases of this ability of narcotics to change the priorities of those who take them.
To identify the place in the brain that is activated by drugs, they used mice that were given cocaine and morphine. Afterwards, they observed them with techniques to measure the activity of the whole brain and saw that with both drugs the activity in the nucleus accumbens increased, a group of neurons related to basic activities for survival such as sexual desire or hunger. Cocaine prevents the body from reabsorbing dopamine and this intensifies the activation of reward circuits. Morphine binds to opioid receptors, which can also release dopamine in the nucleus accumbens. In both cases, the more times the drug was administered, the greater the neuronal activity in this region.
Using techniques such as optogenetics, to activate the neurons of the nucleus accumbens with light so that they react as if the mouse had received a drug, they observed that they lost their appetite as happened with addictive substances. Using other technologies to follow the activity of individual neurons, they were able to verify that, in most cases, there was an overlap when they responded to the pleasure of drinking or eating and that of receiving narcotics.
The scientists observed that some circuits were activated by the consumption of large amounts of food and that this activation increased food consumption, in an example of a vicious circle. However, the researchers saw that this hunger mechanism was self-limited in the field of natural rewards and did not achieve the amplification of desire that accompanied drug use.
Eric Nestler, co-author of the study, explains that identifying the biochemical pathways used by drugs to hijack reward circuits allows us to know that, “based on these studies in mice, manipulation of these new pathways blocks the harmful effects of drugs and simultaneously repairs responses to natural rewards.” “This offers tangible paths toward the development of new treatments for addiction,” concludes Nestler, director of the Friedman Brain Institute at Mount Sinai Hospital in New York (USA).
However, Nestler recognizes that the same overlap shows the difficulty of finding new ways to treat addictions, because the goal of these treatments is to counteract the effect of drugs “without affecting the person's response to natural rewards.”
Elena Martin, researcher at the Pompeu Fabra University of Barcelona, specialist in addictions, considers that the study affects things that were known, but uses a large number of new techniques that allow us to obtain much more precise knowledge. In her opinion, “these results are important for understanding other addictions, such as food addiction.” “There are researchers who doubt that food can cause addiction, because it is a natural reinforcer, but this overlap in the activation of neurons that we see between cocaine, morphine and food would lead us to think that food addiction is possible,” Martin points out.
Addiction is possible, in part, because of the plasticity of the brain, its ability to adapt to new circumstances and reorganize our priorities if necessary. These changes begin by intensely increasing dopamine levels in the nucleus accumbens, but end up producing more lasting changes in the prefrontal cortex, the part of the brain that determines personality and the ability to control oneself. Until not long ago, it was thought that the most serious effects of brain changes caused by drugs were irreversible, but work such as that of researcher Nora Volkow has changed that perspective. Now, treatments such as cognitive behavioral therapy, which offers tools to regain control, are used to fight food or drug addiction. The study by Nestler and his colleagues shows the biological basis why this common treatment may make sense.
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