Dopamine is a neurotransmitter which plays an important role in many functions of the body. It regulates movement, mood, appetite, sexual drive and motivation. Dopamine acts as a chemical messenger between nerve cells and affects various physiological processes such as growth hormone release or muscle contraction.
The human brain contains approximately 100 billion neurons connected by over 10 trillion synapses. These connections are made with dopamine receptors located throughout the central nervous system (CNS). Dopamine is involved in all aspects of life including memory, learning, behavior, emotion and motivation.
In humans, dopamine levels decrease during periods of stress or fatigue. Low levels of dopamine may cause problems such as depression or obesity. Dopamine is also involved in drug addiction.
Dopamine is produced by the pituitary gland and secreted into the blood stream through the ductus arteriosus. The amount of dopamine released depends upon several factors such as age, gender, physical condition, diet and other medications taken. Dopamine levels increase after eating food containing tyramine or nicotine. Tyramine causes increased production of dopamine in the brain.
Consumption of alcohol, nicotine and various drugs can result in high levels of dopamine.
However excess levels of dopamine may also lead to schizophrenia. Dopamine is a naturally occurring neurotransmitter involved in the body’s reward and pleasure centers. Low dopamine levels are linked with Parkinson’s disease. This condition involves a shortage of the brain chemical (neurotransmitter) called dopamine causing symptoms such as tremors or slow movements.
Excess levels of dopamine may also lead to schizophrenia. Dopamine can either be destroyed or removed from the synaptic space by an enzyme called COMT (catechol-O-methyltransferase). COMT has a higher affinity for dopamine than other catecholamines such as norepinephrine and epinephrine. This means that the enzyme will remove dopamine from the synaptic space faster than it will remove norepinephrine.
When the COMT enzyme is genetically deficient, less dopamine is removed from the synaptic space, leading to an excess of dopamine in the brain.
Too much Dopamine
Most of what people think of as “reward” in the brain is really dopamine-related. Dopamine is the main “pleasure chemical” of the brain — it is released whenever we experience anything we find pleasurable, be that food, alcohol, drugs, sexual activity, or just good conversation. Studies have shown that dopamine-related circuitry can become “desensitized” with repeated exposure to pleasurable stimuli. For example, a person who compulsively eats ice cream every day will eventually find that the pleasure they derive from it decreases, and they need to eat more and more to get the same effect.
Eventually this may progress to the point at which they experience depression and withdrawal symptoms if they don’t get their “fix.”
How does this happen? Do we simply “wear out” the pleasure centers with excessive exposure to the things we like?
Not quite. The reality is much more complex, and involves genetics, developmental factors, and environmental influences. It appears that for some individuals, certain aspects of their personality make them more likely to become addicted. These traits often include a high sensitivity to reward (ie. they need less reward to feel satisfaction) and/or a tendency towards impulsive behavior (focusing on the short term rather than the long term).
As with all diseases, there is a strong genetic factor involved in addiction. If there wasn’t, there would be little point in calling it an “addiction” — anyone could become addicted to anything if their brain was wired differently. That said, the genetics of addiction is quite complex. While there is a genetic factor in some cases of alcohol or drug abuse, it accounts for only a small portion of the risk for addiction overall (usually 25% or less).
Sources & references used in this article:
Molecular diversity of the dopamine receptors by O Civelli, JR Bunzow, DK Grandy – Annual review of …, 1993 – annualreviews.org
Dopamine beta-hydroxylase of adrenal chromaffin granules: structure and function by LC Stewart, JP Klinman – Annual review of biochemistry, 1988 – annualreviews.org
Basal regulation of HPA and dopamine systems is altered differentially in males and females by prenatal alcohol exposure and chronic variable stress by KA Uban, WL Comeau, LA Ellis, LAM Galea… – …, 2013 – Elsevier
Stress responses and the mesolimbic dopamine system: social contexts and sex differences by BC Trainor – Hormones and behavior, 2011 – Elsevier
Environmental regulation of the development of mesolimbic dopamine systems: a neurobiological mechanism for vulnerability to drug abuse? by MJ Meaney, W Brake, A Gratton – Psychoneuroendocrinology, 2002 – Elsevier
Adrenal medulla grafts enhance functional activity of the striatal dopamine system following substantia nigra lesions by JB Becker, WJ Freed – 1988 – deepblue.lib.umich.edu