In society, drug abuse and addiction is often perceived as solely the result of poor life choices. As a consequence of this view, addicts-- who are certainly suffering enough already-- are stigmatized by society.
Such a perspective flies in the face of neuroscience. A plethora of research has shown that neural changes underlie the process of addiction; in turn, differences in neural circuitry can underlie differing susceptibilities to addiction. Thus, while poor individual choices certainly play a role, differences in our brains can lead to altered risk for addiction.
How is neural circuitry involved in addiction? How do different drugs affect the brain in different ways? What is the role of neurotransmitters in all of this? We answer these questions and more in our fifth edition of Basics of Neuroscience.
An addiction is a condition of dependency that often manifests in response to chronic overuse of narcotic substances or activities. Drug addictions affect approximately 23 million people in the United States alone , and many more worldwide, especially in developing countries. Individuals suffering from addictions must endure a long and tenuous path to escape this burden. Approximately 50% of people find themselves experiencing relapses, recurrences of use of the addictive entity, after recovering from an addiction .
As described in our article on types of addiction, addictions can form in response to nearly all neuromodulating substances or activities that result in positive emotional response. Development of addictions to both of these categories, both substance and activity, operates within similar neural structures and processes.
To fully comprehend addiction, one must first understand how pleasure is perceived by the brain. Throughout all neural function, information is carried, and simultaneously processed, through neural circuits, or complex chains of neural pathways that regulate themselves. A neural circuit that is especially involved in the regulation of pleasure is the anterior cingulate circuit, which runs parallel to the corpus callosum (a bundle of neurons that joins the two cerebellar hemispheres), and is heavily implicated in motivation . The anterior cingulate circuit runs through a chunk of the brain known as the ventral striatum, which contains a part of the limbic system called the nucleus accumbens. The ventral striatum receives some input from the amygdala and hippocampus, which are involved with memory and learning (see Basics of Neuroscience IV for more information on this), potentially indicating some entanglement of memory processing and pleasure. In addition to this, another structure called the ventral tegmentum, sends dopaminergic neurons, or those that release the neurotransmitter dopamine, into the ventral striatum. Finally, parts of the prefrontal cortex, such as the anterior cingulate cortex, the orbitofrontal cortex, and the medial frontal cortex, complete the circuit and finalize the sensation of pleasure. It is the prefrontal cortex that will inevitably make executive decisions concerning drug use. In short, substances and activities result in pleasure, whether directly or indirectly, through “flooding the nucleus accumbens with dopamine. The hippocampus lays down memories of this rapid sense of satisfaction, and the amygdala creates a conditioned response to certain stimuli.” .
As this process is repeated multiple times, associations form between the addictive entity and the pleasure causing release of dopamine solidify and strengthen through LTP (discussed in Basics of Neuroscience IV), the brain starts to replace the feeling of amiability towards an entity with that of desire, resulting in a motivational drive to pursue said entity. Ultimately, the prefrontal cortex is then influenced by this motivational drive to pursue the addictive entity, making it difficult to resist this urge.
An additional effect known as tolerance compounds the addiction. Because addictive entities provide an extremely accelerated shortcut to a feeling of reward, they flood the brain with a level of dopamine higher than that obtained through natural causes. This overwhelms dopamine receptors, resulting in a lowered rate of dopamine production, creating a dependence on the addictive entity. In addition, the brain becomes less sensitive to the addictive entity, requiring a higher quantity to produce the same effect.