Simply described as a neurotransmitter, dopamine is an amine produced inside the human body that is responsible for carrying the nerve impulse across the neurons. It is not only synthesized in humans but also in plants and other organisms. It is made from two chemicals, Catecholamine and Phenethylamine, and its precursor include L-DOPA which is mainly produced in the brain and kidneys.
Not only important for the brain and nervous system, but it also plays a major role in many pathways outside the CNS (Central Nervous System). Which has been a part of 1100 research papers in the last 60 years but is still a controversy among neuroscientists.
There are very few neurons in the human body that actually synthesize it. Which is mainly produced in the brain and adrenal glands found above the kidneys. The basic component is the protein, mainly obtained from food we eat. Although many types of food contain dopamine, but the brain is unable to use this and so it must be synthesized by the brain itself for it to perform its normal function.
Itself it is a precursor for other types of neurotransmitters such as Epinephrine and Norepinephrine. The degradation of dopamine causes the production of Homovanilic Acid (HVA) which has no known activity in the body. HVA is filtered by the kidneys from the blood and is excreted in the urine.
Dopamine functions by binding and activating the receptors called Dopamine Receptors present on the cell surface. In animals, especially mammals, five types of dopamine receptors have been identified.
These are further classified into two families: D-1 like and D-2 like receptors. The D-1 like receptors may be responsible for the activation or inhibition of the neurons, while the D-2 like are responsible for the inhibition of the target neurons.
So the function of dopamine depends on the type of receptors present on the cell surface. The human nervous system contain a large number of D-1 like receptors.
Inside brain, dopamine functions as neurotransmitter as well as a neuromodulator (a process by which neuron uses chemicals to control the function of other neurons). After its production, dopamine is packed inside the vesicles and these vesicles remain inside the cells until a signal arrives. Dopamine release is in turn regulated by action potentials and by other receptors.
Once inside the synapse (gap between two consecutive neurons), dopamine performs its task by activating the next neuron and after that it is absorbed by the releasing cells, packed into the vesicles again or maybe broken down.
The correct release of dopamine is important for the body movement. Inside our brain there is a structure called Basal Ganglia that is responsible for all our movements. This ganglia, on the other hand, is activated by dopamine. So if the proper amount of dopamine reaches ganglia, all the movements of our body will be coordinated. If there is a disturbance in the production of dopamine, several diseases result.
For example, in case of Parkinson’s disease, not enough dopamine is produced to activate the basal ganglia and this results in delayed body movements. In another syndrome, too much dopamine is produced and this causes unnecessary movements of certain parts of body.
Dopamine secretions have also been found to regulate your memory and thinking. The levels of dopamine are really important in this case and slight increase or decrease may cause the disruption of the normal process.
If it is present during a certain event, you will remember the happening. If it’s absent you will forget what actually happened. The levels of dopamine also depends on the interest of the person. If a person is attentive and wants to learn a subject or a skill, the levels of dopamine will rise and will help the brain to build up a memory.
It plays a role in keeping a person interested and focused on a task. It works by responding to your vision, through your optic nerve, and causes your brain to pay attention.
Dopamine inactivates the prolactin-producing cells. Prolactin is a hormone that stimulates the production of milk from the mammary glands. In its absence, prolactin is produced by the cells while in its absence, prolactin is not produced.
Gonadotrophin releasing hormone is especially important for the development of male and female reproductive organs. The release of this hormone is controlled by dopamine.
The sensation of colors by the eye is also enhanced by the dopamine. There are special neurons found in the retina of eye, which release dopamine and these neurons are active during the day while become silent at night time. This retinal dopamine activates the cone cells and suppress the function of rod cells, thus helping the eye to perceive colors and bright light.
Also named as “The Molecule of Happiness”, dopamine is also involved in the expression of happiness and pleasure. Researchers also show that it can also be involved in depression when acted by other molecules such as serotonin and norepinephrine.
Have you ever wondered why we feel awake and energized during the day? Well, the explanation involves dopamine. Usually, the brain secretes more dopamine during the day and less at night. As the night approaches, less dopamine and more melatonin is produced causing you to feel sleepy.
Dopamine also has some important function outside the nervous system. In blood, almost 95% of dopamine is found conjugated with sulfur, named as dopamine sulphate. Dopamine sulphate has no known functions in the body, so it is filtered by the kidneys and excreted in urine. Some arteries may have dopamine receptors where it may act as vasodilator.
It also has effects on the immune cells mainly lymphocytes. The immune cells themselves can produce dopamine. It binds to the receptors of lymphocytes and reduce their activity. Its function in this regard is still unclear but it is thought to play a role in auto-immune diseases.
The receptors of this are also present in the renal system where it stimulates the increase in blood flow to kidneys, glomerular filtration rate and excretion of sodium ions in urine. Thus decreased renal dopamine functions may result in the buildup of sodium in the body. Which in turn causes high blood pressure.
The exocrine part of pancreas (the one which releases gastric enzymes) releases dopamine in the small intestine but its function in the digestive system are still unclear.
Being an organic molecule, it is also synthesized and sold by many pharmaceutical companies and it is included in the list of Essential Medicines by WHO. It is used for the treatment of low blood pressure, slow heart rate and cardiac arrest, as a stimulant drug. While the positive effects are many, its side effects include the negative effects on kidneys and heart function.