Adrenaline or epinephrine is a molecule that functions as both a hormone and a neurotransmitter. Epinephrine is a type of monoamine (made of one amino acid, one base unit of protein) that functions in the autonomic nervous system (ANS), more specifically in the sympathetic branch of the ANS. The sympathetic nervous system (SNS) is responsible for the “fight-or-flight” response. In a sympathetic response, the nervous system directs blood away from the internal organs responsible for digestion and urination, and away from the skin. The heart, lungs, and skeletal muscles receive more blood, and the pupils are dilated (enlarged) so that the organism is better able to respond to a threat. Epinephrine is stimulatory on the SNS and promotes the fight-or-flight response. The adrenal medulla (a gland located on top of both kidneys) is what secretes most of the epinephrine found in the human body. However, epinephrine also acts as a neurotransmitter within the nervous system. Since epinephrine is nonspecific and acts as an agonist (because it is stimulatory on postsynaptic neurons), it can be used to treat a large number of severe allergies (anaphylactic) such as allergies to food, insect stings, animals, or drugs.
Synthesis and Mechanism of Action
Epinephrine is mainly synthesized and secreted by the adrenal medulla. Not only does it act as an agonist on postsynaptic neurons (and other effector cells such as muscles, blood vessels, etc.), but it also can function as a hormone to have a metabolic effect and as a bronchodilator on those organs that do not have direct SNS innervation. This helps to coordinate the effect of the SNS with other actions in the body to have a more efficient and synchronized response to threats.
The adrenals secrete epinephrine into the bloodstream (without the use of ducts) where it travels around the body and stimulates certain postganglionic fibers. Since epinephrine acts as both a hormone and a neurotransmitter, it functions on almost all tissues of the body. The receptor in each tissue type is part of what determines the result of the effect of epinephrine stimulation on a tissue. There are multiple types of adrenergic receptors (these are postsynaptic receptors that recognize and bind to both epinephrine and an oxidized variant, norepinephrine) to which epinephrine can bind. Epinephrine is nonspecific when binding to these receptors, thus it has multiple effects on the nervous system. It can stimulate pathways that inhibit the pancreas from secreting insulin and stimulates glycolysis (release of glycogen) in the liver and muscles. Epinephrine can also stimulate increased lipolysis by adipose tissue (release of fatty acids from fat storage). The increase in serum levels of glycogen and fat allows for a more ready supply of energy for the cells and muscles to use in response to a threat.
Adrenaline given intramuscularly through an injection can be used to treat multiple medical conditions including, most notably, anaphylaxis and cardiac arrest. The injection of epinephrine in cardiac arrest patients works to increase peripheral resistance, thus keeping most of the blood in and around the heart. This allows for better gas diffusion in the lungs, which in turn increases the amount of oxygen available for the heart and other tissues. Currently, the benefits of using epinephrine in the treatment of a patient experiencing cardiac arrest are being debated. Some think that the administration of epinephrine can actually have adverse effects on the patient. In anaphylaxis, epinephrine is the drug most commonly used to reduce the immune response to extreme allergy. Specifically, epinephrine is important for reducing the swelling (inflammatory edema) and inflammation that is associated with severe allergies. By reducing the swelling in anaphylaxis, the patient is able to overcome the allergic response.
It is important to note, however, that there may also be adverse effects of getting an injection of epinephrine. Some of these effects are heart palpitations, tachycardia, arrhythmia, anxiety, panic attack, pulmonary edema, tremor, hypertension, and headache. This is just a short list of the possible side effects of intramuscular or intravenous injections of epinephrine. It is important that any injections of epinephrine be carefully monitored by a health care professional to ensure that overdosage and adverse effects are avoided as much as possible.
Riannon C. Atwater
See also: Autonomic Nervous System; Noradrenaline
Clark, Josh. (2007). How can adrenaline help you lift a 3,500-pound car? HowStuffWorks.com. Retrieved from http://entertainment.howstuffworks.com/arts/circus-arts/adrenaline-strength.htm
Klabunde, Richard E. (2012). Norepinephrine, epinephrine and acetylcholine—synthesis, storage, release and metabolism. In Cardiovascular pharmacology concepts. Retrieved from http://www.cvpharmacology.com/norepinephrine.htm
MedlinePlus. (2012). Epinephrine injections. Retrieved from http://www.nlm.nih.gov/medlineplus/druginfo/meds/a603002.html