Activation of the brain reward system produces a wide range of feelings, from slight mood elevation to intense pleasure and euphoria. For example, sexual intercourse causes a release of chemical substances. Different neurotransmitters that are activating the reward system of the brain that will create a feeling of pleasure and well being. This will determine that the individual to look forward to repeating this experience. In the similar way, other behaviors necessary for survival are rewarded: seeking food, drink, shelter, etc.
Some chemical substances, like those in drugs, activate the brain reward system directly, avoiding sensory receptors. These can be caffeine of coffee and tea, alcohol, nicotine, and others. Other drugs, such as cocaine and heroin, have much more intensive effects, resulting in the biological change of the brain's reward function.
The main structures of the brain reward system are located in the limbic system. The functions of the limbic system include monitoring internal homeostasis, mediating memory and learning, and experiencing emotions. It also influences sexual and feeding behaviors and motivation.
The primary parts of the limbic system include the hypothalamus, amygdala, hippocampus, septal nuclei, anterior cingulate gyrus, and others. The most involved structures in addiction development are the nucleus accumbens from the limbic striatum, the amygdala, and the tegmental area. There are different theories stipulated regarding the biological basis of addiction.
Scientists have proved that repeated use of drugs like cocaine and heroin can deplete neurotransmitters (in this case, dopamine) from the brain's reward system. This depletion makes normal rewards lose their motivational significance. At the same time, the system becomes more sensitive to the drug, producing addiction. The biological basis of this sensibility is not clear.
Scientists at the Yale School of Medicine sustain that the use of cocaine triggers a buildup of a protein that penetrates in the brain and stimulates genes that intensify the craving for the drug. This protein is normally not produced in our brain. Its buildup begins after using cocaine several times, or even for several years. This can partially explain the involvement of genetic predisposition in addiction creation.
For some people, addiction is developed rapidly after a few uses of drugs, other people can use drugs for a longer period of time without becoming addicted. And there is also a difference in treatment and abstinence. Some addicts can be cured and never come back to drugs, while others can never be treated. Though biological basis of addiction is still unclear to scientists, it is obvious that addiction is a serious disease that creates chemical and morphological changes in brain systems.