Shinjini Bandopadhyay, Amity University Kolkata
Addiction is defined as a chronic, relapsing disease of the brain. It is a biological process that alters the neurochemical pathways in the brain. Although addictions may be of varying types, the most frequently occurring and the most damaging ones include substance abuse (drugs, alcohol, tobacco).
Substance abuse physically affects the structure of the brain and exploits a mechanism known as the brain’s ‘reward system’ by manipulating it.
THE BRAIN REWARD CENTRE:
The core reward system of the brain is a circuit linking the ventral tegmental area, nucleus accumbens, and ventral pallidum by the medial forebrain bundle. The nucleus accumbens is a cluster of neurons located underneath the cerebral cortex; it is an essential component of the mesolimbic pathway. This is a major route for the release of the neurotransmitter– Dopamine and is hence called a dopaminergic pathway.
The experience of pleasure is distinctly identified by the release of dopamine in the nucleus accumbens. The mesolimbic pathway gets stimulated during these naturally rewarding experiences (intrinsic rewards such as satisfying food, social interaction, sexual activity, etc.)
This region of the brain is crucial for inducing motivation to perform rewarding activities, and also modulates the rewarding effect (as is, in the case of addictive drugs). Our brain is programmed to repeat rewarding behaviors as a survival instinct and to allow and re-experience the feelings of pleasure.
Since the reward circuit in the brain includes areas that regulate motivation and memory besides pleasure, addictive substances can stimulate this network and then overload it.
(Fig.1. Diagram depicting the brain reward center complex)
DRUGS & DOPAMINE:
Dopamine not only contributes to pleasure but also plays important role in learning and memory, which are two key factors necessary to transition from enjoying something to becoming addicted to it.
According to recent research, dopamine is believed to interact with a different neurotransmitter— Glutamate, to hijack the brain’s system of reward-related learning. This same system plays a role in sustaining life because it connects activities/behaviors required for human survival. Addictive drugs create a shortcut to the brain’s reward system by excessively flooding the nucleus accumbens with dopamine.
The natural reward pathways may be overshadowed by the effects derived from exposure to artificial rewards (such as drugs) as the latter release more substantial amounts of ‘feel-good’ chemicals such as dopamine. Addictive drugs are capable of releasing 2-10 times the amount of dopamine and the process is faster.
HOW DOES ONE GET ADDICTED?
Repeated exposure to an addictive substance causes nerve cells in the nucleus accumbens as well as the prefrontal cortex to co-relate simply liking something with the ability of craving something. This urges one to seek the source of pleasure one was previously exposed to. The more one repeats such specific behavior, the more reinforced it becomes, which creates a feedback loop. After some time, physical changes eventually occur in the brain structure, which alters the reward pathway permanently.
Dopamine signaling is involved in storing information about the environmental stimuli associated with different experiences that induce pleasure and/or euphoria. The hippocampus establishes this connection and retains these memories which are created as a part of the ‘learning process’. Whenever a person is surrounded by the same stimuli again, they are reminded of the relief and satisfaction previously experienced by the addictive behavior. This leads to the next step which is– Compulsion. It’s where the memory of the first exposure to a drug leads to compulsive searching for re-consumption.
DEVELOPMENT OF DOPAMINE TOLERANCE:
For some addictive drugs (e.g. opiates), tolerance to euphoric effects manifests with chronic usage. With the continued intake of the drug, progressively the brain develops an adaptation to the dopamine secreted.
Long-term drug usage affects the brain reward system and the neurotransmitter itself. The dopamine receptors begin to decrease gradually while they adjust to the excessive concentrations of dopamine in the brain. This decrease in dopamine leads to the impulsive behavior of increased self-administration of drugs to try and maintain the earlier dopamine levels. This is then mediated by the nucleus accumbens; the addicted person experiences a strong desire to take more of the substance, even if it no longer gives them equal pleasure or satisfaction, simply to obtain the same dopamine ‘high’ again. This is an attempt to override the adaptation of the brain known as tolerance.
THE RELAPSE:
All addictive drugs directly or indirectly induce common actions within the brain, such as enhancing the dopaminergic reward synaptic function in the nucleus accumbens. Once this pathway gets activated, it can lead to permanent changes in the brain which make withdrawal difficult and recovery may become a life-long process. Relapse is a major characteristic of addiction and even after long periods of abstinence, certain stimuli might trigger a renewed and persistent craving for the addictive substance.
RECOVERY:
From a neurobiological standpoint, significant progress is predicted in targeting the reward pathways of the brain to control the severity of the damage caused by addiction. Further understanding the mechanisms of signaling in the brain and the role of neurotransmitters (such as dopamine) may aid potential treatment approaches for faster, more effective recoveries from addiction.
Also read:Drug to treat African trypanosomiasis (sleeping sickness) disease discovered
REFERENCES:
- Chen W, Nong Z, Li Y, Huang J, Chen C, & Huang L (2017). Role of dopamine signaling in drug addiction. Current Topics in Medicinal Chemistry, 17(21). DOI: https://doi.org/10.2174/1568026617666170504100642
- Gardner E L (2011). Addiction and brain reward and antireward pathways. Chronic Pain and Addiction, 30, 22–60. DOI: https://dx.doi.org/10.1159%2F000324065
- Urban N B L & Martinez D (2012). Neurobiology of addiction. Psychiatric Clinics of North America, 35(2), 521–541. DOI: https://doi.org/10.1016/j.psc.2012.03.011
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