What plays a role in drug addiction?
The Neurobiology of Substance Use, Misuse, and Addiction
Substance use disorders result from changes in the brain that can occur with repeated use of alcohol or drugs. The most severe expression of the disorder, addiction, is associated with changes in the function of brain circuits involved in pleasure (the reward system), learning, stress, decision making, and self-control.
Every substance has slightly different effects on the brain, but all addictive drugs, including alcohol, opioids, and cocaine, produce a pleasurable surge of the neurotransmitter dopamine in a region of the brain called the basal ganglia; neurotransmitters are chemicals that transmit messages between nerve cells. This area is responsible for controlling reward and our ability to learn based on rewards. As substance use increases, these circuits adapt. They scale back their sensitivity to dopamine, leading to a reduction in a substance’s ability to produce euphoria or the “high” that comes from using it. This is known as tolerance, and it reflects the way that the brain maintains balance and adjusts to a “new normal”—the frequent presence of the substance. However, as a result, users often increase the amount of the substance they take so that they can reach the level of high they are used to. These same circuits control our ability to take pleasure from ordinary rewards like food, sex, and social interaction, and when they are disrupted by substance use, the rest of life can feel less and less enjoyable to the user when they are not using the substance.
Repeated use of a substance “trains” the brain to associate the rewarding high with other cues in the person’s life, such as friends they drink or do drugs with, places where they use substances, and paraphernalia that accompany substance-taking. As these cues become increasingly associated with the substance, the person may find it more and more difficult not to think about using, because so many things in life are reminders of the substance.
Changes to two other brain areas, the extended amygdala and the prefrontal cortex, help explain why stopping use can be so difficult for someone with a severe substance use disorder. The extended amygdala controls our responses to stress. If dopamine bursts in the reward circuitry in the basal ganglia are like a carrot that lures the brain toward rewards, bursts of stress neurotransmitters in the extended amygdala are like a painful stick that pushes the brain to escape unpleasant situations. Together, they control the spontaneous drives to seek pleasure and avoid pain and compel a person to action. In substance use disorders, however, the balance between these drives shifts over time. Increasingly, people feel emotional or physical distress whenever they are not taking the substance. This distress, known as withdrawal, can become hard to bear, motivating users to escape it at all costs. As a substance use disorder deepens in intensity, substance use is the only thing that produces relief from the bad feelings associated with withdrawal. And like a vicious cycle, relief is purchased at the cost of a deepening disorder and increased distress when not using. The person no longer takes the substance to “get high” but instead to avoid feeling low. Other priorities, including job, family, and hobbies that once produced pleasure have trouble competing with this cycle.
Healthy adults are usually able to control their impulses when necessary, because these impulses are balanced by the judgment and decision-making circuits of the prefrontal cortex. Unfortunately, these prefrontal circuits are also disrupted in substance use disorders. The result is a reduced ability to control the powerful impulses toward alcohol or drug use despite awareness that stopping is in the person’s best long-term interest.
This explains why substance use disorders are said to involve compromised self-control. It is not a complete loss of autonomy—addicted individuals are still accountable for their actions—but they are much less able to override the powerful drive to seek relief from withdrawal provided by alcohol or drugs. At every turn, people with addictions who try to quit find their resolve challenged. Even if they can resist drug or alcohol use for a while, at some point the constant craving triggered by the many cues in their life may erode their resolve, resulting in a return to substance use, or relapse.
Drug Abuse and Addiction Research at Johns Hopkins Institute of Basic Biomedical Sciences
IBBS researchers are studying how chronic drug use causes lasting changes in the brain that can lead to addiction. Their findings may aid in the development of more effective treatments for addiction.
Current addiction treatments use a combination of counseling and complete abstinence, slow weaning, or drug replacement that either substitutes for the drug or blocks withdrawal symptoms. Although these therapies control physical cravings, they don’t seem to reverse the lasting changes in the brain caused by drug abuse, and therefore may only provide a temporary fix.
During learning and memory formation, the brain’s neurons create new connections to strengthen or weaken communication routes between neighboring neurons. Similarly, chronic drug use modifies neuron connections, leading to permanent alterations in the brain’s circuitry. Taking drugs creates memories of objects, places or people that users associate with doing drugs, which triggers cravings and drug-seeking behavior when the user re-encounters those situations. Several IBBS neuroscientists study these molecular changes as they occur during learning, memory and chronic drug use.
Jay Baraban of the Solomon H. Snyder Department of Neuroscience studies how exposure to drugs such as cocaine or morphine triggers long-term adaptations in the brain that underlie addiction. Persistent changes in the strength of nerve connections encode memory and drug cravings. These adaptations are mediated by rapid synthesis of plasticity proteins that modify the strength of nerve connections. Baraban and colleagues have identified a pair of proteins that play a key role in driving rapid synthesis of synaptic proteins that change the efficacy of neuronal contacts and encode long-term memory. These researchers have engineered mice that lack these proteins in selected neuronal populations and are using these valuable tools to learn more about how this novel signaling pathway contributes to drug addiction.
Paul Worley, also from the neuroscience department, studies the molecular basis of specific forms of long-term learning and memory. His laboratory focuses on a class of proteins found at the interface between connecting neurons—synapses—that ramp up as the neurons engage in information processing and storage. These proteins directly modify the strength of the signals sent between neurons and are essential for information storage. Recent work reveals how molecules that regulate neuronal responses that signal reward, such as dopamine, can selectively strengthen communication across synapses, and implicates this process in addiction.
Mollie Meffert, a faculty member in the Department of Biological Chemistry and in the neuroscience department, investigates the formation of lasting memories. She focuses on growth factors in the hippocampus that turn on or off the particular genes involved in the growth of neurons and in establishing memories. Levels of these growth factors elevate during activity in the normal brain, and mice with lower-than-usual levels perform poorly on spatial memory tests such as navigating mazes. In addiction studies, researchers showed chronic drug use causes the release of brain-derived growth factors in rat brain areas involved in sensing the drug-associated “reward.” Meffert’s group studies how the brain-derived growth factors turn genes on or off to control long-lasting brain responses, such as those occurring in learning and memory, or addiction. By investigating the regulation of these genes in healthy and diseased neurons, the Meffert lab uncovered the mechanism by which brain-derived growth factors rapidly and specifically alter these genes. These findings may one day help us understand and develop therapeutic targets for failures in memory and brain processing as they pertain to addiction.
