There is some confusion on what the term "addiction" means. Most people understand addiction as a state of physical dependence on a substance that creates an intensive withdrawal syndrome if it is not taken for a certain period of time. Scientists prefer to define addiction as a behavioral syndrome where drug use seems to dominate over other individual's motivations and where the normal restrictions on behavior are ineffective.
This definition is more proper because not every addiction creates physical dependence. However, the most popular drugs such as cannabis, cocaine, heroine, nicotine and others create biochemical and morphological changes in brain, especially in the mesolimbie dopamine system. Dopamine is a chemical substance that acts like a neurotransmitter in the central nervous system. Besides other functions dopamine apparently has a certain role in the regulation of mood and affect motivations and reward processes.
There are several dopamine systems in the brain. One of them is mesolimbie dopamine system that responds for motivational processes. So, the addictive mechanism of some drugs consists in their potent effects on behavior by enhancing mesolimbie dopamine system activity. For example, the drug can increase the neuronal firing rate of dopamine cells, producing an increase in dopamine release.
The enhanced dopamine activity is experienced by the drug user as mood elevation and euphoria. This is the supposed mechanism of heroin. Cocaine inhibits the reuptake of dopamine which also increases the quantity of dopamine in the synapse and also enhances dopamine activity with the same results: mood elevation and euphoria. Cocaine's effect is shorter than the effect of heroin. The influence of heroin and cocaine on different parts of mesolimbie dopamine system is proved by fact that the combined use of these drugs produces very intensive dopamine activation. This combination is known by users as "speed-ball" and is extremely dangerous being associated with a high fatality rate.
Besides the momentary effects, repeated use of drugs like cocaine (psychomotor stimulants) and heroine (opiates) produces permanent changes in the mesolimbic dopamine system, depleting dopamine from this system. This makes normal rewards lose their motivational significance. Also the mesolimbie dopamine system becomes more sensitive to these substances which may be the main point in addiction development. There have been made different studies in the addiction domain, in order to better understand this phenomenon and to find treatment for addicted people.
Scientists found that certain brain regions respond for different manifestations of addiction. Recently it has been discovered that the hippocompus is involved in addiction. Hippocompus is a formation of the limbic system that is responsible for the creation of new memories. Experiments show that the use of drugs activates this region. This can partially explain the fact that former drug users that have been abstinent for a long period of time, sometimes even for years, are carving for drugs when seeing something that in their memories associates with drugs (for example, a 100$ banknote or a familiar street where they used to procure drugs).
The involvement of amygdala is also being studied by neuroscientists. Amygdala is also a formation of limbic system that is responsible for the manifestation of negative emotions. Its role in addiction development is still unclear.