The brain has evolved over time in a way that ensures our survival. Our brain's reward system is part of that survival system. We experience an urgent need for food when we are hungry and generally have a powerful desire for sex. The brain's reward system rewards food and sex because they ensure our survival. Unfortunately, drugs of abuse operate within these reward systems. This leads people to experience an urgent need or powerful desire for drugs or addictive activities.
The brain's reward system has ensured our species survival. Food, water, and sex activate the reward system. When the brain's reward center is activated, it releases dopamine. Dopamine creates a pleasing, enjoyable sensation. Thus, we are likely to repeat these behaviors that are necessary for survival. This is because dopamine rewarded us with a pleasurable feeling.
From an evolutionary standpoint, it is very helpful to have a reward system that works like this. For example, imagine that food is scarce and I am wandering about looking for food. When I eventually find food and eat it, this triggers my reward system. This pleasing feeling (dopamine "reward") will become associated with whatever behavior led me to that food. This causes me to want to repeat that behavior. Furthermore, the reward system is closely tied to emotional and subjective memories. If I was successful and found food in a particular place, in the future I might want to look for food in the same location. This reward system increases the likelihood I will be successful finding food. This is because my brain chemicals are rewarding me with a pleasing sensation. It also helps me to remember how and where this pleasant feeling occurred.
Unfortunately, the very same reward system that ensures our survival also rewards drug use. Addictive substances and activities trigger the release of dopamine. Dopamine rewards us with a pleasant sensation. This serves to motivate us to repeat these harmful behaviors. We know that people with addictions will go to any lengths to obtain their drug of choice. Similarly, they continue with their addiction despite the harm they cause to themselves or those they love. These characteristic addictive behaviors arise from the brain's pleasure and rewards centers.
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We now have a basic understanding of the purpose and functioning of the brain's reward system. Let's examine it a little more closely. The circuit most associated with pleasure and reward is the mesolimbic pathway. The mesolimbic pathway is located in the brainstem. This area of the brain is primarily concerned with basic survival. Within the mesolimbic pathway is an area called the ventral tegmental area (VTA). The VTA projects to the nucleus accumbens (thought to be the reward center). The neurotransmitter most commonly linked with the mesolimbic system is dopamine. Many people consider dopamine to be the driving force behind the human pursuit of pleasure. The release of dopamine is a pleasurable sensation. The release of dopamine motivates us to repeat behaviors or activities that prompted this release. This system's purpose was to promote survival by rewarding life sustaining behaviors such eating and procreation.
All addictive drugs and activities release varying amounts dopamine into the nucleus accumbens. However, stimulant drugs release the most. Stimulant drugs include drugs such as cocaine and methamphetamine. With drugs such as alcohol or heroin, the brain's own opiate system (endorphins) also gets involved. Although different addictions have different effects in the nucleus accumbens, they all activate the reward system. This is turn motivates us to repeat those behaviors, even though they may be harmful. For simplicity purposes, we have discussed the concepts of reward, pleasure, and craving together. However, there is a difference between "pleasure-seeking and "drug seeking." Pleasure-seeking refers to the pleasurable, rewarding aspect of addiction. Drug-seeking refer to the craving aspect of addiction. Dopamine may be more involved in drug-seeking (craving) component of addiction. The opiate (endorphin), GABA, or glutamatergic systems may be more involved in pleasure-seeking aspect of addiction.
Pleasure-seeking and drug-seeking (cravings) are inter-related, yet distinct. Research has indicated that natural rewards (food, water, sex) typically lessen their influence on the reward system over time. As a behavior occurs more often, dopamine levels tend to decrease as a result. Psychologists call this habituation. This makes sense. Once I've eaten enough food, I don't need to be rewarded for eating more food. Then I would be eating too much, or too much of one type of food. Another way of putting this is that novelty influences the dopamine release. As the novelty wears off, the dopamine release declines; i.e., habituation occurs.
Habituation and the resultant decrease of dopamine explain why drug users usually increase their "dose" over time. As the body gets used a certain amount of a drug, the dopamine decreases. Therefore, to achieve the same pleasurable effect, the dose needs to increase. Activity addicts increase the frequency or intensity of the activity. Addiction professional call this tolerance. Tolerance can subsequently lead to increased cravings for a drug or activity. This drives the addictive process forward. Once tolerance begins, powerful cravings gradually replace pleasure-seeking. This prompts the characteristic drug-seeking associated with addiction. In contrast to pleasure-seeking, cravings represent an attempt to avoid, or relieve, unpleasant symptoms. Stated more simply, the pursuit of pleasurable sensations characterizes the initial stages of addiction. In contrast, efforts to avoid uncomfortable sensations characterize the later stages of addiction. We discuss craving more thoroughly in the next section.