How drugs affect addicts’ brains so that they can even forget to eat and drink
A study on mice suggests that a large part of neurons that regulate thirst and hunger are impacted by the impulses generated by cocaine and morphine
The consumption of certain substances produces such an intense effect on the brain’s reward circuits that it can make a person forget hunger and thirst. That’s a conclusion easily drawn after witnessing a person suffering from addiction, but this week, a team of scientists from several U.S. institutions published an article in the journal Science that describes part of the mechanism responsible for such behavior, and suggests the possibility of new solutions for addicts.
Drugs like cocaine and heroin can hook people because they generate changes in the brain, taking control of systems that make us crave water and food, survival basics. These substances intensify desire to consume more of them, and reduce the pleasure felt from other rewards that help us to lead a healthy life. Several previous studies have analyzed these mechanisms, but the authors of the report published on Thursday sought to combine the latest technology using animal subjects in order to understand the biological basis for narcotics’ power to affect the priorities of those who consume such substances.
To identify the place in the brain that is activated by drugs, researchers used mice, who were given cocaine and morphine. Later, the rodents were observed with techniques that measured their brains and saw how both drugs increased the activity in their nucleus accumbens, a group of neurons related to basic survival activities like sexual desire and hunger. Cocaine prevents bodies from reabsorbing dopamine, which intensifies the activation of reward circuits. Morphine becomes attached to opioid receptors, which can also free up dopamine in the nucleus accumbens. In both cases, the more times the drugs were administered, the greater the neuronal activity in the region.
When they used techniques like optogenetics, which employ light to activate the neurons of the nucleus accumbens so that they would react as though the mouse had received a drug, the scientists observed that the rodents lost their appetite, as if they had ingested addictive substances. Employing other tools that followed the activity of individual neurons, researchers found that, in most cases, there was an overlap between the pleasure response to eating and drinking and that of consuming narcotic drugs.
Scientists observed that some circuits were activated with the consumption of large quantities of food, and that this activation increased consumption, a vicious cycle. Still, researchers saw that the hunger mechanism self-limited when it came to natural rewards, and did not reach the same levels as the amplification of desire that accompanied the consumption of drugs.
Eric Nestler, the study’s co-author, explains that identifying the biochemical methods that are used when drugs take control of reward circuits teaches us that, “based on these studies on mice, the manipulation of these new paths block the damaging effects of the drugs and simultaneously repair responses to natural rewards.” “This offers tangible paths towards developing new treatments for addiction,” continues Nestler, who is the director of The Friedman Brain Institute at New York’s Mount Sinai Hospital.
Nonetheless, Nestler acknowledges that the same overlap shows the difficulty of finding new ways to treat addiction, because the goal of these treatments is to counteract the effect of the drugs, “without affecting the person’s response to natural rewards.”
Elena Martín, researcher at Barcelona’s Pompeu Fabra University and addictions specialist, thinks that the study touches on factors that were previously understood, but that it does use many new techniques for a much greater precision in that knowledge. In her opinion, “These results are important for understanding other addictions, such as food addiction.” She continues, “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 leads us to think that food addiction is possible.”
Addiction is possible, in part, because of the brain’s plasticity, its ability to adapt to new circumstances and even reorganize our priorities when necessary. These changes begin by intensely increasing dopamine levels in the nucleus accumbens, but end up producing longer-lasting changes in the prefrontal cortex, the part of the brain that determines personality and the ability to control oneself. Until recently, the most serious effects of drug-induced brain changes were thought to be irreversible, but work like that of researcher Nora Volkow has changed that perspective. Now, treatments such as cognitive behavioral therapy, which offer tools to regain control, are used to combat food and drug addiction. The study by Nestler and his colleagues illustrates the biological basis that give such common treatments validity.
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