Addiction and the Brain: Our Reward Circuit

Addiction and the Brain: Our Reward Circuit
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Scientists have known for many years that the brain and substance misuse are closely linked. Thanks to advances in imaging and other technology, researchers are beginning to understand more about the brain’s role in substance use disorders than they ever have before.

Although researchers have only scratched the surface when it comes to understanding this amazingly complex organ, studies show that prolonged exposure to substances of misuse dramatically affects the circuitry of the brain.

With greater knowledge, it becomes clear that alcohol and substance use disorders are not a moral failing or a sign of laziness or lack of willpower. We also begin to understand why the disordered use of substances – what most people call addiction – is so difficult to overcome.

The Brain: A Complex Arrangement

The National Institute on Drug Abuse (NIDA), explains that the brain is a complex arrangement of billions of specialized cells, usually referred to as neurons. Neurons are the fundamental working units of the brain.

The neurons, which are organized into neural circuits, or networks, send signals – neurochemical messages – back and forth between other neurons in the circuits, which exist throughout the brain and extend to the spinal cord and peripheral nervous system.

Different brain circuits are involved in a range of functions, such as memory, motivation, reward, and executive functions such as planning and prioritizing, organization, flexible thinking, and impulse control.

Alcohol and Brain Circuitry

A recent study conducted at the Scripps Research Institute indicates that alcohol changes the way the brain sends signals. The study suggests that changes in brain circuitry may influence how some people shift from casual to heavy drinking. It may also shed a light on why many people are unable to scale back once heavy drinking begins.

During the study, which was conducted on rodents in a laboratory setting, scientists surgically implanted optic fibers in the brains of rats that allowed them to pinpoint a specific neural network with lasers. By using the laser, researchers could deactivate the neurons by flipping a switch.

First, researchers gave unlimited access to alcohol. It turns out that the rats didn’t drink much – about the equivalent of a glass of wine or a single beer for humans.

Once this baseline level of drinking was established, researchers gradually increased the alcohol intake for several months until the rats acquired self-administration, a.k.a. got addicted. When access to alcohol was removed, the rats showed signs of severe cravings and other withdrawal symptoms.

When alcohol was reintroduced, the addicted rats didn’t return to their baseline level of drinking. Instead, they drank more than when the group of neurons was active. However, when researchers turned on the lasers to deactivate the group of neurons, the rats immediately dropped back to their baseline drinking levels.

With the neurons deactivated, the rats no longer consumed as much alcohol, and physical symptoms such as cravings, trembling, and abnormal gait were markedly reduced. When the lasers were turned off and the neurons were re-activated, the rats once again returned to the higher level of alcohol consumption.

Scripps researchers believe the study indicates that the brain’s circuitry drives specific behaviors, including compulsive drinking. Research in humans is a long way down the road, and this study is just a piece of a very large, complicated puzzle. However, it’s possible that specific neurons could be inhibited to reverse cravings and reduce unwanted behavior in people with an alcohol use disorder.

Brain Circuitry and Compulsive Overeating

Researchers at the University of North Carolina School of Medicine conducted a similar study and found that overeating, like compulsive drinking, involves a specific network of neurons.

When that certain area of the brain was stimulated, the mice gorged on food, eating up to half a day’s calories in 20 minutes, even when they were well-fed and not hungry. Once the group of neurons was deactivated, the mice lost their cravings for food. Even mice that hadn’t been fed ignored food when available.

Researchers think this indicates that binge eating is driven by biological mechanisms, and isn’t a matter of willpower, choice, or other personal characteristics. Scientists hope to develop medications or brain stimulation techniques that attenuate these default brain mechanisms in order to treat overweight, obesity, and food-related disorders such as binge eating, bulimia, and anorexia.

Cigarettes and Cocaine

Other research suggests that the brain’s wiring is closely involved in various forms of substance use disorder. For instance, in one study, people with cocaine use disorder were asked to glance at a picture of cocaine. A fraction of a second was enough to trigger severe cravings.

Another study revealed that a particular area of the brain was activated when cigarette smokers lit a cigarette – and the same area was activated when smokers watched images of people smoking.

Implications Alcohol and Substance Use Disorder Treatment

Repeated, prolonged exposure to alcohol and other substances of misuse results in changes in brain physiology – the science is clear on that. In effect, the exposure and associated behavior rewires brain circuitry, making it extremely difficult to resist powerful urges, even when people understand that the consequences of their actions may be negative and/or counterproductive.

In the future, a deeper understanding of the underlying neural circuitry related to alcohol and substance use disorders may allow scientists and medical professionals to determine who is at risk, and potentially prevent them from developing alcohol or substance use disorders altogether. A more thorough understanding of how chronic exposure to substances of misuse affects the brain may lead to more effective treatment for a range of compulsive behaviors.

Research also explains why adolescents and young adults are more susceptible to the physiological changes associated with addictive behaviors: their brains aren’t fully developed. Therefore, any change in brain structure results in disproportionate behavioral consequences, as compared to the adult brain. This rapidly developing field of research may lead to discoveries that make it possible to prevent the development of alcohol and substance use disorders in adolescents and adults, and potentially reverse the physiological changes associated with long-term exposure to substances of misuse.

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