agonists
An agonist is a chemical that activates a receptor to produce a biological response. Receptors are cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an antagonist blocks the action of the agon ...
are a class of sympathomimetic agents that selectively stimulates alpha
adrenergic receptor
The adrenergic receptors or adrenoceptors are a class of G protein-coupled receptors that are targets of many catecholamines like norepinephrine (noradrenaline) and epinephrine (adrenaline) produced by the body, but also many medications like b ...
s. The alpha-adrenergic receptor has two subclasses α1 and α2. Alpha 2 receptors are associated with sympatholytic properties. Alpha-adrenergic agonists have the opposite function of
alpha blocker
Alpha blockers, also known as α-blockers or α-adrenoreceptor antagonists, are a class of pharmacological agents that act as antagonists on α-adrenergic receptors ( α-adrenoceptors).
Historically, alpha-blockers were used as a tool for p ...
s. Alpha adrenoreceptor ligands mimic the action of
epinephrine
Adrenaline, also known as epinephrine, is a hormone and medication which is involved in regulating visceral functions (e.g., respiration). It appears as a white microcrystalline granule. Adrenaline is normally produced by the adrenal glands a ...
and
norepinephrine
Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic compound, organic chemical in the catecholamine family that functions in the brain and human body, body as a hormone, neurotransmitter and neuromodulator. The ...
signaling in the heart, smooth muscle and central nervous system, with norepinephrine being the highest affinity. The activation of α1 stimulates the membrane bound enzyme
phospholipase C
Phospholipase C (PLC) is a class of membrane-associated enzymes that cleave phospholipids just before the phosphate group (see figure). It is most commonly taken to be synonymous with the human forms of this enzyme, which play an important role i ...
, and activation of α2 inhibits the enzyme
adenylate cyclase
Adenylate cyclase (EC 4.6.1.1, also commonly known as adenyl cyclase and adenylyl cyclase, abbreviated AC) is an enzyme with systematic name ATP diphosphate-lyase (cyclizing; 3′,5′-cyclic-AMP-forming). It catalyzes the following reaction:
:A ...
. Inactivation of adenylate cyclase in turn leads to the inactivation of the secondary messenger
cyclic adenosine monophosphate
Cyclic adenosine monophosphate (cAMP, cyclic AMP, or 3',5'-cyclic adenosine monophosphate) is a second messenger, or cellular signal occurring within cells, that is important in many biological processes. cAMP is a derivative of adenosine tri ...
and induces smooth muscle and blood vessel constriction.
Classes
Although complete selectivity between receptor agonism is rarely achieved, some agents have partial selectivity. NB: the inclusion of a drug in each category just indicates the activity of the drug at that receptor, not necessarily the selectivity of the drug (unless otherwise noted).
phospholipase C
Phospholipase C (PLC) is a class of membrane-associated enzymes that cleave phospholipids just before the phosphate group (see figure). It is most commonly taken to be synonymous with the human forms of this enzyme, which play an important role i ...
activity. (vasoconstriction and mydriasis; used as vasopressors, nasal decongestants and during eye exams). Selected examples are:
* Adrenoswitch-1 (photoswitchable partial α1 agonist and light-controlled mydriatic)
* Methoxamine
*
Midodrine
Midodrine, sold under the brand names ProAmatine and Orvaten among others, is an antihypotensive medication used to treat orthostatic hypotension (low blood pressure when standing) and urinary incontinence. It is taken oral administration, by m ...
*
Metaraminol
Metaraminol, also known as metaradrine and sold under the brand name Aramine among others, is a sympathomimetic medication which is used in the prevention and treatment of hypotension (low blood pressure), particularly as a complication of an ...
*
Phenylephrine
Phenylephrine, sold under the brand names Neosynephrine and Sudafed PE among others, is a medication used as a decongestant for uncomplicated nasal congestion in the form of a nasal spray or oral tablet, to dilate the pupil, to increase bloo ...
*
Amidephrine
Amidephrine (), or amidefrine, sold under the brand name Fentrinol among others, is a selective α1-adrenergic receptor agonist which is described as an adrenergic or sympathomimetic, vasoconstrictor, and topical nasal decongestant used to ...
adenylyl cyclase
Adenylate cyclase (EC 4.6.1.1, also commonly known as adenyl cyclase and adenylyl cyclase, abbreviated AC) is an enzyme with systematic name ATP diphosphate-lyase (cyclizing; 3′,5′-cyclic-AMP-forming). It catalyzes the following reaction:
:A ...
activity, reduces brainstem vasomotor center-mediated CNS activation; used as
antihypertensive
Antihypertensives are a class of drugs that are used to treat hypertension (high blood pressure). Antihypertensive therapy seeks to prevent the complications of high blood pressure, such as stroke, heart failure, kidney failure and myocardial infa ...
,
sedative
A sedative or tranquilliser is a substance that induces sedation by reducing irritability or Psychomotor agitation, excitement. They are central nervous system (CNS) Depressant, depressants and interact with brain activity, causing its decelera ...
& treatment of
opiate
An opiate is an alkaloid substance derived from opium (or poppy straw). It differs from the similar term ''opioid'' in that the latter is used to designate all substances, both natural and synthetic, that bind to opioid receptors in the brain ( ...
dependence and
alcohol withdrawal
Alcohol withdrawal syndrome (AWS) is a set of symptoms that can occur following a reduction in or cessation of alcohol use after a period of excessive use. Symptoms typically include anxiety, shakiness, sweating, vomiting, fast heart rate, a ...
Clonidine
Clonidine, sold under the brand name Catapres among others, is an α2A-adrenergic receptor agonist medication used to treat high blood pressure, attention deficit hyperactivity disorder (ADHD), drug withdrawal (e.g., alcohol, opioids, or nic ...
Guanfacine
Guanfacine, sold under the brand name Tenex ( immediate-release) and Intuniv ( extended-release) among others, is an oral alpha-2a agonist medication used to treat attention deficit hyperactivity disorder (ADHD) and high blood pressure.
Co ...
, (preference for alpha2A-subtype of adrenoceptor)
* Guanabenz (most selective agonist for alpha2-adrenergic as opposed to imidazoline-I1)
* Guanoxabenz (metabolite of guanabenz)
*
Xylazine
Xylazine is a structural analog of clonidine and an α2-adrenergic receptor, α2-adrenergic receptor agonist, sold under many trade names worldwide, most notably the Bayer brand name Rompun, as well as Anased, Sedazine and Chanazine.
Xylazine ...
(not approved for human use)
*
Tizanidine
Tizanidine, sold under the brand name Zanaflex among others, is an alpha-2 (α2) adrenergic receptor agonist, similar to clonidine, that is used to treat muscle spasticity due to spinal cord injury, multiple sclerosis, and spastic cerebral ...
*
Methyldopa
Methyldopa, also known as α-methyl-L-DOPA and sold under the brand name Aldomet among others, is a medication used for high blood pressure. It is one of the preferred treatments for high blood pressure in pregnancy. For other types of high blo ...
Norepinephrine
Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic compound, organic chemical in the catecholamine family that functions in the brain and human body, body as a hormone, neurotransmitter and neuromodulator. The ...
antagonist
An antagonist is a character in a story who is presented as the main enemy or rival of the protagonist and is often depicted as a villain.
*
Amitraz
Amitraz (development code BTS27419) is a non-systemic acaricide and insecticideCorta, E., Bakkali, A., Berrueta, L. A., Gallo, B., & Vicente, F. (1999). Kinetics and mechanism of amitraz hydrolysis in aqueous media by HPLC and GC-MS. Talanta, 48(1 ...
*
Detomidine
Detomidine is an imidazole derivative and Alpha-2 adrenergic receptor, α2-adrenergic receptor agonist, used as a large animal sedative, primarily used in horses. It is usually available as the salt (chemistry), salt detomidine hydrochloride. It ...
Nonspecific agonists act as agonists at both alpha-1 and alpha-2 receptors.
*
Xylometazoline
Xylometazoline, also spelled xylomethazoline, is a medication used to reduce symptoms of nasal congestion, allergic rhinitis, and sinusitis. It is used directly in the nose as a spray or drops.
Side effects include trouble sleeping, irritati ...
*
Oxymetazoline
Oxymetazoline, sold under the brand name Afrin among others, is a topical decongestant and vasoconstrictor medication. It is available over-the-counter as a nasal spray to treat nasal congestion and nosebleeds, as eye drops to treat eye rednes ...
Epinephrine
Adrenaline, also known as epinephrine, is a hormone and medication which is involved in regulating visceral functions (e.g., respiration). It appears as a white microcrystalline granule. Adrenaline is normally produced by the adrenal glands a ...
Undetermined/unsorted
The following agents are also listed as agonists by
MeSH
Medical Subject Headings (MeSH) is a comprehensive controlled vocabulary for the purpose of indexing journal articles and books in the life sciences. It serves as a thesaurus of index terms that facilitates searching. Created and updated by th ...
.
*
Ergotamine
Ergotamine, sold under the brand name Ergomar among others, is an ergopeptine and part of the ergot family of alkaloids; it is structurally and biochemically closely related to ergoline. It is structurally similar to several neurotransmitter ...
*
Etilefrine
Etilefrine, sold under the brand name Effortil among others, is a sympathomimetic medication used as an antihypotensive agent to treat orthostatic hypotension. It is usually used oral administration, by mouth, but is also available as an injecti ...
Metaraminol
Metaraminol, also known as metaradrine and sold under the brand name Aramine among others, is a sympathomimetic medication which is used in the prevention and treatment of hypotension (low blood pressure), particularly as a complication of an ...
Naphazoline
Naphazoline is a medicine used as a decongestant, and a vasoconstrictor added to eye drops to relieve red eye. It has a rapid action in reducing swelling when applied to mucous membranes. It is a sympathomimetic agent with marked alpha adrener ...
Octopamine
Octopamine (OA), also known as ''para''-octopamine and norsynephrine among synonyms, is an organic chemical closely related to norepinephrine, and synthesized biologically by a homologous pathway. Octopamine is often considered the major "fig ...
*
Phenylpropanolamine
Phenylpropanolamine (PPA), sold under many brand names, is a sympathomimetic agent used as a decongestant and appetite suppressant. It was once common in prescription and over-the-counter cough and cold preparations. The medication is taken ...
*
Propylhexedrine
Propylhexedrine, sold under the brand name Benzedrex among others, is an alkylamine primarily utilized as a topical nasal decongestant. Its main indications are relief of congestion due to colds, allergies, and allergic rhinitis. Propylhexedr ...
Synephrine
Synephrine, or, more specifically, ''p''-synephrine, is an alkaloid, occurring naturally in some plants and animals, and also in approved drugs products as its ''m''-substituted analog known as neo-synephrine. ''p''-Synephrine (or formerly Sympat ...
Alpha-adrenergic agonists, more specifically the auto receptors of alpha 2 neurons, are used in the treatment of
glaucoma
Glaucoma is a group of eye diseases that can lead to damage of the optic nerve. The optic nerve transmits visual information from the eye to the brain. Glaucoma may cause vision loss if left untreated. It has been called the "silent thief of ...
by decreasing the production of aqueous fluid by the ciliary bodies of the eye and also by increasing uveoscleral outflow. Medications such as
clonidine
Clonidine, sold under the brand name Catapres among others, is an α2A-adrenergic receptor agonist medication used to treat high blood pressure, attention deficit hyperactivity disorder (ADHD), drug withdrawal (e.g., alcohol, opioids, or nic ...
and dexmedetomidine target pre-synaptic auto receptors, therefore leading to an overall decrease in norepinephrine which clinically can cause effects such as sedation, analgesia, lowering of blood pressure and bradycardia. There is also low quality evidence that they can reduce shivering post operatively.
The reduction of the stress response caused by alpha 2 agonists were theorised to be beneficial peri operatively by reducing cardiac complications, however this has shown not to be clinically effective as there was no reduction in cardiac events or mortality but there was an increased incidence of hypotension and bradycardia.
Alpha-2 adrenergic agonists are sometimes prescribed alone or in combination with stimulants to treat ADHD.
See also
*
Alpha blocker
Alpha blockers, also known as α-blockers or α-adrenoreceptor antagonists, are a class of pharmacological agents that act as antagonists on α-adrenergic receptors ( α-adrenoceptors).
Historically, alpha-blockers were used as a tool for p ...