Physiologically, limbic system is the brain’s reward system. Mesolimbic and mesocortical pathways (Mid-brain) have our cognitive control and functions which would be both reward related and aversion related. Reward related functions are always stronger and more motivating than aversion, hence drug abuse behavior is stronger in addicts. The reward system connects certain structures in such a way that they generate the feeling of pleasure for certain behaviors or stimulus given to the body. In day-to-day life, it is activated by healthy, life-sustaining activities like eating and socializing. But ‘Drug Abuse’ is another behavior that generates euphoria, which is enjoyed by the mind. And human nature is to repeat those actions that give us some definite satisfaction. The limbic system does more than just giving a reward response. For any behavior to be reinforced, it has to be first registered as pleasant to the mind and body. Limbic system in the brain recognizes our positive and negative emotions, which very well explains, how our body and mind get dependent on any psychoactive mood altering substance.
However, with recent developments, neurobiology has been able to study how addiction is related to the brain. Researchers and scientists studied the brain’s reaction to different substances and discovered that drug abuse actually alters the chemical makeup of the brain, which is what causes addiction and they named it as a ‘Brain Disease.’
Since the brain is placed as the epicenter of human activity, it is the initiation point of every phenomenon that occurs. Whatever we feel, experience or think, all is first registered by the brain and within lesser than nanoseconds, it commands us to act. It communicates through neurons, neurotransmitters, receptors and transporters. Yes, it is very complicated and sounds mystical too (specially to people like me who have not studied biology after high school)
Psychoactive substances interrupt with the communication systems of the brain. They interfere with the way a message is transmitted and sent to its defined destination. No matter how much people say that cannabis would not be called a drug, it is a plant, but the reality remains unchanged, which is, that everything has certain chemical composition. For example, Heroin and Marijuana’s chemical structure are like copies of a natural neurotransmitter. Amphetamines mimic catecholamine neurotransmitters, causing general physiological changes which prepare the body for physical activity and fight-or-flight response. So, the moment the body is intoxicated with a drug containing amphetamines or its derivatives, it starts reacting in unnatural ways by raising blood pressure, increased alertness resulting in sleeplessness, blood glucose levels. They can easily activate neurons, giving various directions to the body to react in a different way, than would a natural neurotransmitter would! Cocaine can make a person delirious, because it releases natural neurotransmitters in bulk and blocks the natural recycling process of neurotransmitters by the brain. Amphetamine causes drug tolerance very quickly and rapidly too.
Chronic addictive drug use causes alterations in the process by which information from a gene is used to synthesize a gene product- RNA or Protein. Nigrostriatal Pathway is a dopaminergic pathway that plays an extensive and unavoidable role in addiction. It comprises of Transcription Factor, which is a protein that controls what all information has to go from the DNA to the messenger RNA. Psychoactive substances block, many of the neural and behavioral alterations that are to be taken to the RNA. Altered dopamine transmission is the first thing that comes to notice in cases of drug abuse.
These have been many studies extensively done by medical science researchers on addiction. There is a wide range of chemicals that people use to reach a euphoric state. But the gist of it all is, that they play with the normal functioning of brain by altering the natural chemicals that ought to be there.
Whenever a person happens to do something that takes him to a different state of mind for a while, like reaching the oomph point in a sexually pleasurable activity, treating the taste buds with a new and a very different flavor or winning an excessive amount of money or any other valuable resource that he could use to be really rich, the brain takes it all in the same way. It activates the same gland and releases the same chemical Dopamine, which functions as a neurotransmitter, in all scenarios that seem to be pleasurable or exciting. Similarly, whenever the body receives an antibody that becomes too exciting for it like any drug, chemical or alcohol, the body releases certain amount of dopamine or cuts down its level. It is a problematic scenario is there is too much or too little of dopamine released by the brain. Drugs are addictive because they release up-to 10 times more dopamine than a natural neurotransmitter can! But when the body gets tolerant to the same drug and its amount, it can even cut down its normal release of dopamine!