← In the Realm of Hungry Ghosts Close Encounters with Addiction
In the Realm of Hungry Ghosts Close Encounters with Addiction Chapter 15. Cocaine, Dopamine, and Candy Bars The Incentive System in Addiction
Author: Gabor Mate Publisher: Berkeley, CA: North Atlantic Books Publish Date: 2010-1-5 Review Date: Status:📚
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Many addicts have told me that cocaine is a tougher taskmaster than heroin, harder to escape. Although it doesn’t cause physical withdrawal symptoms nearly as distressing, the psychological drive to use it seems more difficult to resist—even after it no longer gives much pleasure. Cocaine increases brain levels of the neurotransmitter dopamine by blocking it from being transported back into the nerve cells that release it. (Recall that all drugs work by locking into receptor sites on cell surfaces.) Cocaine’s effects wear off very quickly because it occupies its receptor sites for only a brief time. The urge to use, to get the next dopamine hit, then redoubles. Like other stimulant drugs—speed, nicotine, and caffeine—cocaine taps directly into a brain system that, in its own way, is just as powerful as the opioid attachment/reward system described in the previous chapter. It plays a key role in all substance addictions and also in behavioral addictions. There is an area in the midbrain that, when triggered, gives rise to intense feelings of elation or desire. It’s called the ventral tegmental apparatus, or VTA. When researchers insert electrodes into the VTA of lab rats and the animals are given a lever that allows them to stimulate this brain center, they’ll do so to the point of exhaustion. They ignore food and pain just so they can reach the lever. Human beings may also endanger themselves in order to continue self-triggering this brain area. One human subject stimulated himself fifteen hundred times in a three-hour period, “to a point that he was experiencing an almost overwhelming euphoria and elation, and had to be disconnected despite his vigorous protests.”1 Dopamine is the neurotransmitter chiefly responsible for the power of the VTA and its associated network of brain circuits. Nerve fibers from the VTA trigger dopamine release in a brain center that plays a central role in all addictions: the nucleus accumbens, or NA, located on the underside of the front of the brain. Sudden increases in dopamine levels in the NA set off the initial excitement and elation experienced by drug users, and this is also what rats and people are after when they keep pushing those levers. All abusable substances raise dopamine in the NA, stimulants like cocaine most dramatically. As in the case of the opioid apparatus, Nature did not design the VTA, the NA, or other parts of the brain’s dopamine system just so the addicts and drug users of the world could feel happier or more energized and focused. Indeed, the human brain’s dopamine circuits are no less important to survival than is its opioid system. If opioids help consummate our reward-seeking activities by giving us pleasure, dopamine initiates these activities in the first place. It also plays a major role in the learning of new behaviors and their incorporation into our lives.
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Along with its connections in the forebrain and the cortex, the VTA thus forms the neurological basis of another major brain system involved in the addiction process: the incentive-motivation apparatus. This system responds to reinforcement, and reinforcers all have the effect of increasing dopamine levels in the NA. Let’s take a hypothetical situation involving a hypothetical “you.” You see a chocolate bar in a Halloween bag, and you’re seized by a desire to munch on it: a classic example of a positively reinforced behavior. That is, you’ve tasted a similar chocolate bar before and liked the experience. Now, when this new bar appears in your sight, dopamine is released in the NA, inciting you to take a bite. Your four-year-old daughter, to whom the bar belonged, accuses you of thieving. “The dopamine made me do it,” you say in self-defense. Your daughter, nothing if not a reasonable preschooler, drops her resentment. “Of course, Daddy,” she says sweetly, “because a cue associated with a previously pleasurable experience triggers a surge of dopamine in the NA and incites consummatory behavior. Seeing my candy bar was your cue, and eating it was the consummatory behavior. You have such a silly, predictable reinforcement system.” “Wow,” you say. “That’s exactly right, honey. Will you share that last piece of chocolate with me?” “No way! Your dopamine circuits aren’t my problem.” Environmental cues associated with drug use—paraphernalia, people, places, and situations—are all powerful triggers for repeated use and for relapse, because they themselves trigger dopamine release. People trying to quit smoking, for example, are advised to avoid poker if they are used to having a cigarette while playing cards. Unless they move to a different area of town or to a recovery home, my Downtown Eastside patients find it virtually impossible to stop drug use, even when they form a strong intention to do so. Not only are drugs readily available, but everything and everyone in the environment reminds them of their habit.
Note: Brewer: “Avoiding cues (triggers) might help prevent people from being triggered, but didn’t directly target the core habit loop. For example, staying away from friends who smoke can be helpful. Yet if getting yelled at by the boss triggered someone to smoke, avoiding the boss might lead to other stressors, such as unemployment.” “Our data showed that mindfulness decoupled this link between craving and smoking.”
Reinforcement is important in all addictions, drug-related or not. In my own case, it doesn’t help matters that the Portland Hotel is located within a few blocks of those unscrupulous compact disc pushers at Sikora’s, my favorite music haunt, and that I drive by there most days on my way to or from work. As I described earlier, I can feel excitement rising as I approach the store, even when I have no plan to go there, along with an urge to park the car and walk in. In my NA, the dopamine is flowing. The incentive is powerful. Needless to say, life-essential reinforcers such as food and sex trigger VTA activation and dopamine release in the NA, since the performance of survival-related behaviors is the very purpose of the incentive-motivation system. Accordingly, this system is decisive in initiating activities such as foraging for food and other life-sustaining necessities, such as seeking sexual partners and exploring the environment. The VTA and NA and their connections with other brain circuits are also active when we explore novel objects and situations and evaluate them in light of previous reinforcing experiences. In other words, nerve fibers in the VTA are triggering dopamine release in the NA when a person needs to know, “Is this new whatever-it-is going to help me or hurt me? Will I like it or not?” The role of the dopamine system in novelty-seeking helps explain why some people are driven to risky behaviors such as street racing. It’s one way to experience the excitement of dopamine release.
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It is fascinating to look at some of the evidence linking the dopamine system to addictions. Animal experiments, distressing as they sometimes are to read about, can be stunning for their scientific ingenuity and technical expertise. Just how important dopamine receptors are to substance use was illustrated by a study of mice that had previously been trained to drink alcohol. They were given an “infusion” of dopamine receptors right into the NA. Before the infusion these rodents had fewer than normal dopamine receptors. The receptors were incorporated into a harmless virus that entered the animals’ brain cells so that, temporarily, a normal range of receptor activity was achieved. As long as this artificial supply of dopamine receptors was available, the mice reduced their alcohol intake considerably—but they gradually became boozers again as the implanted receptors were lost to natural attrition.2 Why is this relevant? First, as I’ve already explained, chronic cocaine use reduces the number of dopamine receptors and thereby keeps driving the addict to use the drug simply to make up for the loss of dopamine activity.
- N. D. Volkow et al., “Role of Dopamine in Drug Reinforcement and Addiction in Humans: Results from Imaging Studies,” Behavioral Pharmacology 13 (2002): 355–66.
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Dopamine receptor availability is also reduced in alcoholics, as well as in heroin and crystal meth addicts.3
- N. D. Volkow et al., “Low Level of Brain Dopamine D2 Receptors in Methamphetamine Abusers: Association with Metabolism in the Orbitofrontal Cortex,” American Journal of Psychiatry 158(12) (December 2001): 2015–21.
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More importantly, research now strongly suggests that the existence of relatively few dopamine receptors to begin with may be one of the biological bases of addictive behaviors.4
- Eliot L. Gardner, “Brain-Reward Mechanisms,” chap. 5, section II, in Substance Abuse: A Comprehensive Textbook, by Joyce H. Lowinson et al. (Philadelphia: Lippincott, Williams, & Wilkins, 2005), 71.
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As we have now seen, addiction inevitably involves both opioid and dopamine circuitry. The dopamine system is most active during the initiation and establishment of drug intake and other addictive behaviors. It is key to the reinforcing patterns of all drugs of abuse—alcohol, stimulants, opioids, nicotine, and cannabis.5 Desire, wanting, and craving are all incentive feelings, so it is easy to see why dopamine is central to non-drug-related addictions, too. On the other hand, opioids—innate or external—are more responsible for the pleasure-reward aspects of addiction.6
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Opioid circuits and dopamine pathways are important components of what has been called the limbic system, or the emotional brain. The circuits of the limbic system process emotions like love, joy, pleasure, pain, anger, and fear. For all their complexities, emotions exist for a very basic purpose: to initiate and maintain activities necessary for survival. In a nutshell, they modulate two drives that are absolutely essential to animal life, including human life: attachment and aversion. We always want to move toward something that is positive, inviting, and nurturing, and to repel or withdraw from something threatening, distasteful, or toxic. These attachment and aversion emotions are evoked by both physical and psychological stimuli, and when properly developed, our emotional brain is an unerring, reliable guide to life. It facilitates self-protection and also makes possible love, compassion, and healthy social interaction. When impaired or confused, as it often is in the complex and stressed circumstances prevailing in our “civilized” society, the emotional brain leads us to nothing but trouble. Addiction is one of its chief dysfunctions.