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The one property all addictive substances share is an ability to manipulate the brain's natural reward circuit in order to induce feelings of pleasure or relieve distress.1,2
The reward circuit is designed to reinforce life-sustaining activities, such as eating and sex.1 Engaging in these activities stimulates neurons just above the brainstem to release dopamine into an area of the brain called the nucleus accumbens (NA).3,4 The increased dopamine activity in the NA causes feelings of euphoria.3 Through this reward process, humans and other animals are taught to favor—and therefore prioritize—behaviors essential for their survival.2 Increased dopamine activity also prompts the creation of a lasting memory associating the pleasurable feelings with the circumstances and setting under which they occur. In effect, these memories (also called conditioned associations) supplement the brain's positive reinforcement of certain behaviors.3 From that point on, previously neutral stimuli will evoke—or "trigger"—memories of the reward experience.3 In the case of opioids, when the opioid molecule binds to mu receptors in the brain, it initiates the same biochemical sequence and dopamine release normally reserved for rewarding life-sustaining behaviors.3 Ironically, because the brain rewards opioid use in this same way, opioid use is positively reinforced just as though it were critical to survival. This "feeling"—that opioid use is essential for survival—is postulated as being responsible for many of the behavioral symptoms associated with opioid dependence.
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