Derived from the amino acid tyrosine, dopamine is a phenethylamine neurotransmitter prominent in midbrain neurons. It is the metabolic precursor to the "fight-or-flight" neurohormones norepinephrine and epinephrine--better known as adrenaline. This family of neuromodulators are called catecholamines, which are generally responsible for increased heart rate, sweaty palms, dry mouth, enlarged pupils and increased muscle strength. In the case of dopamine, it also plays an important role in cognition, emotional responses, movement, balance, motivation and pleasure sensation.
Production
Within the midbrain, the substantia nigra and the ventral tegmental area synthesize the most dopamine, which is then transmitted along four major neuronal pathways. This dopamine serves as a neurotransmitter, acting on dopamine-specific receptors in various parts of the brain.
Dopaminergic pathways consist of bundles of neuron fibers that dispatch the neurohormone, sending signals from one part of the brain to the next. In each of the four tracts, dopamine elicits a different function.
The adrenal glands and kidneys produce the remainder of the neurotransmitter for participation in the sympathetic activation of the alpha and beta receptors within the body. The stimulation of these receptors is what invokes the "fight-or-flight" response. This aspect of dopamine function has great clinical significance in the treatment of shock and heart failure.
Nigrostriatal Pathway
Transmitting dopamine from the substantia nigra to the striatum, the nigrostriatal pathway is involved motor function and sensation. According to the Lundbeck Institute, damage to this tract is associated with Parkinson's disease and other Parkinson's-related disorders.
Any lesions within this tract reduce the transmission of dopamine to the striatum. This reduction produces tremors, cog-wheel rigidity, uncontrollable spasms, decreased purposeful movement, blunted affect, and restless leg syndrome.
Tuberoinfundibular Pathway
The tuberoinfundibular pathway governs neuronal control of the release of dopamine from the hypothalamus into the anterior pituitary gland. The dopamine transversing here is referred to as Prolactin-inhibiting factor, because it acts on the pituitary gland to suppress the secretion of prolactin.
Disturbances of this tract cause hyperprolactinemia, which is an abnormal level of prolactin in the bloodstream. Aberrant increases of prolactin can result in amenorrhea, painful intercourse, problems with ovulation and decreased libido in women. In men, milky discharge from nipples, erectile dysfunction and enlargement of the breasts can occur.
Mesocortical Pathway
The mesocortical pathway, which controls motivation and emotion, sends dopamine to the frontal cortex. Reduction of the neurotransmitter along these fibers may result in the "negative symptoms" of schizophrenia, or those manifestations that are known to be "withdrawn," such as flat expression, lack of emotion, apathy and poor social functioning.
Mesolimbic Pathway
The mesolimbic pathway extends to the nucleus accumbens--the pleasure center of the brain. Dopamine functions here by stimulating reward and emotional behavior. Yet, dopaminergic increases in this region may result in hallucinations and delusions.
For instance, the dopamine receptors in this area are targeted by antipsychotic drugs, such as haloperidol. These drugs act by blocking the receptors in order to reduce the "positive symptoms," or the outward manifestations, of schizophrenia, which are hearing voices and seeing hallucinations.
According to the University of Texas, addictive behaviors are also associated with this pathway. Cocaine, nicotine and amphetamines stimulate a spastic production of dopamine along these neurons. For instance, cocaine increases dopamine in this region by promoting its reuptake at the neurosynapse. On the other hand, amphetamines promote the release of the neurotransmitter in the presynpatic neurons.
Clinical Role
Functioning as catecholamine, dopamine takes an active role in the treatment of low blood pressure in situations of shock and acute heart failure. Proper doses of dopamine stimulate the alpha and beta receptors within the heart and blood vessels.
Thus, the narrowing of blood vessels, increased cardiac output and increased heart rate occur in response. This dopaminergic action protects the heart wall when blood volume resuscitation is achieved. According to "Harrison's Principles of Internal Medicine," dopamine is considered most useful in the treatment of patients who have lower cardiac output and poor tissue perfusion.
References
- "Harrison's Principles of Internal Medicine," 17th ed.; Anthony Fauci, MD, et al.; 2008
- "National Medical Series: Biochemistry," 4th ed.; Victor Davidson, Ph.D. & Donald Sittman, Ph.D.; 1999
- "Download of Psychiatry"; Saxby Pridmore, MD; 2006
- "British Journal of Psychiatry"; Dopamine and Antipsychotic Drug Action Revisited; H.M. Jones & L.S. Pilowsky; October 2002.
- "Hardcore Neuroscience"; Kevin Wang, MD, et al.; 2006
