Alpha-adrenergic

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 A sympatholytic (sympathoplegic) drug is a medication that opposes the downstream effects of postganglionic nerve firing in effector organs innervated by the sympathetic nervous system (SNS). They are indicated for various functions; for example, t ...

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).

α₁ agonist

α₁ agonist: stimulates

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 Methoxamine, sold under the brand names Vasoxine, Vasoxyl, and Vasylox among others, is a sympathomimetic medication used as an antihypotensive agent. It has mostly or entirely been discontinued. The drug is an α1- adrenergic receptor agoni ...

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 ...

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α₂ agonist

α₂ agonist: inhibits

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 ...

symptoms). Selected examples are: *

Brimonidine Brimonidine is an α2 agonist medication used to treat open-angle glaucoma, ocular hypertension, and rosacea. In rosacea it improves the redness. It is used as eye drops or applied to the skin. It is also available in the fixed-dose combinatio ...

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 ...

(mixed alpha2-adrenergic and imidazoline-I1 receptor agonist) *

Dexmedetomidine Dexmedetomidine, sold under the brand name Precedex among others, is a medication used for sedation. Veterinarians use dexmedetomidine for similar purposes in treating cats, dogs, and horses. It is also used in humans to treat acute agitation as ...

* Fadolmidine *

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 Guanabenz (pronounced GWAHN-a-benz, sold under the trade name Wytensin) is an Alpha-adrenergic agonist, alpha agonist that is selective to the alpha-2 adrenergic receptor. Guanabenz is used as an antihypertensive drug used in the treatment of hig ...

(most selective agonist for alpha2-adrenergic as opposed to imidazoline-I1) *

Guanoxabenz Guanoxabenz is a metabolite of guanabenz. References

Alpha-2 adrenergic receptor agonists Chloroarenes Guanidines {{antihypertensive-stub ...

(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 ...

Methylnorepinephrine Corbadrine, sold under the brand name Neo-Cobefrine and also known as levonordefrin and α-methylnorepinephrine, is a catecholamine sympathomimetic used as a topical nasal decongestant and vasoconstrictor in dentistry in the United States. It is ...

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 ...

* (''R'')-3-nitrobiphenyline, an α_2C selective agonist as well as a weak

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 ...

Lofexidine Lofexidine, sold under the brand name Lucemyra among others, is a medication historically used to treat high blood pressure; today, it is more commonly used to help with the physical symptoms of opioid withdrawal. It is taken by mouth. It is an ...

, an α_2A adrenergic receptor agonist. *

Medetomidine Medetomidine is a veterinary anesthetic drug with potent sedative effects and emerging illicit drug adulterant. It is a racemic mixture of two stereoisomers, levomedetomidine and dexmedetomidine, the latter being the isomer with the pharmac ...

, an α2

adrenergic agonist An adrenergic agonist is a drug that stimulates a response from the adrenergic receptors. The five main categories of adrenergic receptors are: α1, α2, β1, β2, and β3, although there are more subtypes, and agonists vary in specificity between ...

Nonspecific agonist

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 ...

Apraclonidine Apraclonidine (INN), also known under the brand name Iopidine, is a sympathomimetic used in glaucoma therapy. It is an α2 adrenergic receptor agonist and a weak α1 adrenergic receptor agonist. Topical apraclonidine is administered at a conc ...

Cirazoline Cirazoline is a full agonist at the α1A adrenergic receptor, a partial agonist at both the α1B and α1D adrenergic receptors, and a nonselective antagonist to the α2 adrenergic receptor. It is believed that this combination of properties co ...

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 ...

Indanidine Indanidine is an alpha-adrenergic agonist. References Alpha-1 adrenergic receptor agonists Imidazolines Indazoles {{heterocyclic-stub ...

Mephentermine Mephentermine, sold under the brand name Wyamine among others, is a sympathomimetic medication which was previously used in the treatment of low blood pressure but is mostly no longer marketed. It is used by injection into a vein or muscle, by ...

* Mivazerol *

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 ...

Norfenefrine Norfenefrine, also known as ''meta''-octopamine or norphenylephrine and sold under the brand name Novadral among others, is a sympathomimetic medication which is used in the treatment of hypotension (low blood pressure). Along with its structura ...

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 ...

Rilmenidine Rilmenidine is a prescription medication for the treatment of hypertension. It is taken orally and marketed under the brand names Albarel, Hyperium, Iterium and Tenaxum. Mechanism of action Rilmenidine, an oxazoline compound with antihyperten ...

Romifidine Romifidine is a drug that is used in veterinary medicine as a sedative mainly in large animals such as horses, although it may be used in a wide variety of species. It is not used in humans, but is closely related in structure to the commonly use ...

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 ...

Talipexole Talipexole (B-HT920, Domnin) is a dopamine agonist that is marketed as a treatment for Parkinson's Disease in Japan by Boehringer Ingelheim; it was introduced in 1996. As of December 2014 it was not approved for marketing in the US nor in Europ ...

Clinical significance

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 Dexmedetomidine, sold under the brand name Precedex among others, is a medication used for sedation. Veterinarians use dexmedetomidine for similar purposes in treating cats, dogs, and horses. It is also used in humans to treat acute agitation as ...

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.

References

External links

* * {{Hypnotics and sedatives