The reward pathway and addiction

CEBU, Philippines - Humans, as well as other organisms engage in behaviors that are rewarding; the pleasurable feelings provide positive reinforcement so that the behavior is repeated. There are natural rewards as well as artificial rewards, such as drugs.

Natural rewards such as food, water, sex, and nurturing allow the organism to feel pleasure when eating, drinking, procreating, and being nurtured. Such pleasurable feelings reinforce the behavior so that it will be repeated. Each of these behaviors is required for the survival of the species. Remind your audience that there is a pathway in the brain that is responsible for rewarding behaviors. This can be viewed in more detail in the next image.

An important part of the reward pathway is shown and the major structures are highlighted: the ventral tegmental area (VTA), the nucleus accumbens, and the prefrontal cortex. The neurons of the VTA contain the neurotransmitter dopamine, which is released in the nucleus accumbens and in the prefrontal cortex. This pathway is activated by a rewarding stimulus.

The discovery of the reward pathway was achieved with the help of animals such as rats. Rats were trained to press a lever for a tiny electrical jolt to certain parts of the brain. When an electrode is placed in the nucleus accumbens, the rat keeps pressing the lever to receive the small electrical stimulus because it feels pleasurable. This rewarding feeling is also called positive reinforcement. The importance of the neurotransmitter dopamine has been determined in these experiments because scientists can measure an increased release of dopamine in the reward pathway after the rat receives the reward. And, if the dopamine release is prevented (either with a drug or by destroying the pathway); the rat won't press the bar for the electrical jolt. So with the help of the rats, scientists figured out the specific brain areas as well as the neurochemicals involved in the reward pathway.

The Brain and Addiction

Now that we have defined the concept of reward, we can define addiction. Addiction is a state in which an organism engages in a compulsive behavior, even when faced with negative consequences. This behavior is reinforcing, or rewarding, as you have just discussed. A major feature of addiction is the loss of control in limiting intake of the addictive substance. The most recent research indicates that the reward pathway may be even more important in the craving associated with addiction, compared to the reward itself. Scientists have learned a great deal about the biochemical, cellular, and molecular bases of addiction; it is clear that addiction is a disease of the brain.

Definition of tolerance

When drugs such as methamphetamine are used repeatedly over time, tolerance may develop. Tolerance occurs when the person no longer responds to the drug in the way that person initially responded. Stated another way, it takes a higher dose of the drug to achieve the same level of response achieved initially. For example, in the case of methamphetamines, tolerance develops rapidly. The development of tolerance is not addiction, although many drugs that produce tolerance also have addictive potential. Tolerance to drugs can be produced by several different mechanisms, but in the case of methamphetamines, tolerance develops at the level of the cellular targets. For example, when drug binds to the receptors, it triggers the inhibition of an enzyme that orchestrates several chemicals in the cell to maintain the firing of impulses. After repeated activation of the receptor, the enzyme adapts so that the drug can no longer cause changes in cell firing. Thus, the effect of a given dose of the drug is diminished.

Definition of dependence

With repeated use of methamphetamines, dependence also occurs. Dependence develops when the neurons adapt to the repeated drug exposure and only function normally in the presence of the drug. When the drug is withdrawn, several physiologic reactions occur. These can be mild (e.g., for caffeine) or even life threatening (e.g., for alcohol). This is known as the withdrawal syndrome. In the case of methamphetamine, withdrawal can be very serious and the abuser will use the drug again to avoid the withdrawal syndrome. This may then lead to addiction wherein the whole day's awake moments of the drug dependent are obsessive to find the drug and once the use has started the compulsion to use more and more kicks in until the body shuts down. For more information, please feel free to contact us anytime at 2389143 or 2315229. You may also visit our website at [email protected]