An adrenergic alpha-agonist (or alpha-adrenergic agonist) is a drug that selectively stimulates alpha adrenergic receptor

Adrenergic receptor

The adrenergic receptors are a class of metabotropic G protein-coupled receptors that are targets of the catecholamines, especially noradrenaline and adrenaline ....

s. The alpha-adrenergic receptor has two subclasses α₁ and α₂.

Classes

Although complete selectivity between receptor agonism is rarely achieved, some agents have partial selectivity.

α₁ agonists

α₁ agonists: stimulates phospholipase C activity. (vasoconstriction and mydriasis; used as vasopressors, nasal decongestants and eye exams). Selected examples are:

• Methoxamine
  Methoxamine
  Methoxamine is an α1-adrenergic receptor agonist, similar in structure to phenylephrine and 2,5-DMA. It was Developed by Burroughs-Wellcome in the 1940s. The drug induces vasoconstriction of skin and splanchnic blood vessels, thereby increasing peripheral vascular resistance and raising mean...
  
  
• Methylnorepinephrine
• Midodrine
  Midodrine
  Midodrine is a vasopressor/antihypotensive agent. Midodrine was approved in the United States by the Food and Drug Administration in 1996 for the treatment of orthostatic hypotension...
  
  
• Oxymetazoline
  Oxymetazoline
  Oxymetazoline is a selective alpha-1 agonist and partial alpha-2 agonist topical decongestant, used in the form of Oxymetazoline hydrochloride, in products such as Afrin, Dristan, Nasivin, Logicin, Vicks Sinex, Visine L.R., Sudafed OM, and Zicam. It was developed from Xylometazoline at E.Merck...
  
  
• Phenylephrine
  Phenylephrine
  Phenylephrine is a selective α1-adrenergic receptor agonist used primarily as a decongestant, as an agent to dilate the pupil, and to increase blood pressure...
  
  

α₂ agonists

α₂ agonists: inhibits adenylyl cyclase activity. (reduce brainstem vasomotor center-mediated CNS activation; used as antihypertensives, sedatives & treatment of opiate and alcohol withdrawal symptoms). Selected examples are:

• Clonidine
  Clonidine
  Clonidine is a sympatholytic medication used to treat medical conditions, such as high blood pressure, some pain conditions, ADHD and anxiety/panic disorder...
  
   (mixed alpha2-adrenergic and imidazoline-I1 receptor agonist)
• Guanfacine
  Guanfacine
  Guanfacine is a sympatholytic. It is an agonist of the α2A subtype of norepinephrine receptors. These receptors are concentrated heavily in the prefrontal cortex and the locus coeruleus, with the potential to improve attention abilities via modulating post-synaptic α2A receptors in the prefrontal...
  
  , (preference for alpha2A-subtype of adrenoceptor)
• Guanabenz
  Guanabenz
  Guanabenz is an alpha agonist of the alpha-2 type that is used as an antihypertensive drug. It is used to treat high blood pressure ....
  
   (most selective agonist for alpha2-adrenergic as opposed to imidazoline-I1)
• Guanoxabenz
  Guanoxabenz
  Guanoxabenz is a metabolite of guanabenz....
  
   (metabolite of guanabenz)
• Guanethidine
  Guanethidine
  Guanethidine is an antihypertensive drug that reduces the release of catecholamines, such as norepinephrine. Guanethidine is transported across the sympathetic nerve membrane by the same mechanism that transports norepinephrine itself , and uptake is essential for the drug's action...
  
   (peripheral alpha2-receptor agonist)
• Xylazine
  Xylazine
  Xylazine is a drug that is used for sedation, anesthesia, muscle relaxation, and analgesia in animals such as horses, cattle and other non-human mammals. An analogue of clonidine, it is an agonist at the α2 class of adrenergic receptor....
  
  ,
• Tizanidine
  Tizanidine
  Tizanidine is a drug that is used as a muscle relaxant. It is a centrally acting α2 adrenergic agonist. It is used to treat the spasms, cramping, and tightness of muscles caused by medical problems such as multiple sclerosis, spastic diplegia, back pain, or certain other injuries to the spine or...
  
  
• Methyldopa
  Methyldopa
  Methyldopa is an alpha-adrenergic agonist psychoactive drug used as a sympatholytic or antihypertensive. Its use is now mostly deprecated following the introduction of alternative safer classes of agents...
  
  
• Fadolmidine

Undetermined/unsorted

The following agents are also listed as agonists by MeSH

Medical Subject Headings

Medical Subject Headings is a comprehensive controlled vocabulary for the purpose of indexing journal articles and books in the life sciences; it can also serve as a thesaurus that facilitates searching...

.

• amidephrine
  Amidephrine
  Amidephrine is a alpha-adrenergic agonist....
  
  
• amitraz
  Amitraz
  Amitraz is an antiparasitic drug. Product names include Ridd, Aazdieno, Acarac, Aludex, Amitraze, Avartin, Baam, Edrizan, Maitac, Mitac, Mitaban, Triatox, Triatix, Vapcozin Taktic, Triazid, Topline, Tudy, Ectodex, Garial, Danicut, Ovidrex, Acadrex, Bumetran, and Ovasyn.It is an alpha-adrenergic...
  
  
• anisodamine
  Anisodamine
  Anisodamine, also known as 7β-hydroxyhyoscyamine, is an anticholinergic and α1-adrenergic receptor antagonist used in the treatment of acute circulatory shock in China. It is also a naturally occurring tropane alkaloid found in some plants of the Solanaceae family....
  
  
• apraclonidine
  Apraclonidine
  Apraclonidine , also known as Iopidine, is a sympathomimetic used in glaucoma therapy. It is an α2-adrenergic agonist and a weak alpha-1 adrenergic receptor agonist....
  
  
• brimonidine
  Brimonidine
  Brimonidine is a drug used to treat open-angle glaucoma or ocular hypertension.It acts via decreasing synthesis of aqueous humor, and increasing the amount that drains from the eye through uveoscleral outflow.As a treatment for glaucoma, it is usually given in eyedrop form.Brimonidine is an...
  
  
• cirazoline
  Cirazoline
  Cirazoline is a nonselective agonist of the alpha-1 adrenergic receptor , and an antagonist to the alpha-2 adrenergic receptor. It is believed that this combination of properties could make cirazoline an effective vasoconstricting agent....
  
  
• detomidine
  Detomidine
  Detomidine is an imidazole derivative and α2-adrenergic agonist, used as a large animal sedative, primarily used in horses. It is usually available as the salt detomidine hydrochloride...
  
  
• dexmedetomidine
  Dexmedetomidine
  Dexmedetomidine is a sedative medication used by intensive care units and anesthetists. It is relatively unique in its ability to provide sedation without causing respiratory depression...
  
  
• epinephrine
  Epinephrine
  Epinephrine is a hormone and a neurotransmitter. It increases heart rate, constricts blood vessels, dilates air passages and participates in the fight-or-flight response of the sympathetic nervous system. In chemical terms, adrenaline is one of a group of monoamines called the catecholamines...
  
  
• ergotamine
• etilefrine
  Etilefrine
  Etilefrine is a cardiac stimulant used as an antihypotensive. It is a sympathomimetic amine of the 3-hydroxy-phenylethanolamine series used in treating orthostatic hypotension of neurological, cardiovascular, endocrine or metabolic origin...
  
  
• indanidine
  Indanidine
  Indanidine is an alpha-adrenergic agonist....
  
  
• lofexidine
  Lofexidine
  Lofexidine is an alpha2-adrenergic receptor agonist, historically used as a short-acting anti-hypertensive, but more commonly used to alleviate physical symptoms of heroin and opiate withdrawal.-Indication:...
  
  
• medetomidine
  Medetomidine
  Medetomidine is a synthetic drug used as both a surgical anesthetic and analgesic often used as the hydrochloride salt medetomidine hydrochloride. It is a crystalline white alpha-two adrenergic agonist that can be administered as an intravenous drug solution with sterile water...
  
  
• mephentermine
  Mephentermine
  Mephentermine is a cardiac stimulant. It was formerly used in Wyamine nasal decongestant inhalers and before that as a stimulant in psychiatry.It has been used as a treatment for hypotension.-References:...
  
  
• metaraminol
  Metaraminol
  Metaraminol is a potent sympathomimetic amine used in the prevention and treatment of hypotension, particularly as a complication of anesthesia.It is an α1-adrenergic receptor agonist with some β effect....
  
  
• methoxamine
  Methoxamine
  Methoxamine is an α1-adrenergic receptor agonist, similar in structure to phenylephrine and 2,5-DMA. It was Developed by Burroughs-Wellcome in the 1940s. The drug induces vasoconstriction of skin and splanchnic blood vessels, thereby increasing peripheral vascular resistance and raising mean...
  
  
• mivazerol
  Mivazerol
  Mivazerol is a alpha-adrenergic agonist....
  
  
• naphazoline
  Naphazoline
  Naphazoline is the common name for 2--2-imidazoline hydrochloride. It is a sympathomimetic agent with marked alpha adrenergic activity. It is a vasoconstrictor with a rapid action in reducing swelling when applied to mucous membrane...
  
  
• norepinephrine
  Norepinephrine
  Norepinephrine is the US name for noradrenaline , a catecholamine with multiple roles including as a hormone and a neurotransmitter...
  
  
• norfenefrine
  Norfenefrine
  Norfenefrine is a cardiac stimulant....
  
  
• octopamine
  Octopamine
  Octopamine is an endogenous biogenic amine that is closely related to norepinephrine, and has effects on the adrenergic and dopaminergic systems. It is also found naturally in numerous plants, including bitter orange. Biosynthesis of the D--enantiomer of octopamine is by β-hydroxylation of...
  
  
• oxymetazoline
  Oxymetazoline
  Oxymetazoline is a selective alpha-1 agonist and partial alpha-2 agonist topical decongestant, used in the form of Oxymetazoline hydrochloride, in products such as Afrin, Dristan, Nasivin, Logicin, Vicks Sinex, Visine L.R., Sudafed OM, and Zicam. It was developed from Xylometazoline at E.Merck...
  
  
• phenylpropanolamine
  Phenylpropanolamine
  Phenylpropanolamine , also known as the stereoisomers norephedrine and norpseudoephedrine, is a psychoactive drug of the phenethylamine and amphetamine chemical classes which is used as a stimulant, decongestant, and anorectic agent. It is commonly used in prescription and over-the-counter cough...
  
  
• rilmenidine
  Rilmenidine
  Rilmenidine is a prescription medication for the treatment of hypertension. It is marketed under the brand names HYPERIUM and Tenaxum.- Form and composition :...
  
  
• romifidine
  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 used drug clonidine.Romifidine acts as an agonist at the...
  
  
• synephrine
  Synephrine
  Synephrine is a drug commonly used for weight loss. While its effectiveness is widely debated, synephrine has gained significant popularity as an alternative to ephedrine, a related substance which has been made illegal or restricted in many countries due to health risks and its use as a precursor...
  
  
• talipexole
  Talipexole
  Talipexole is a dopamine agonist that has been proposed as an antiparkinsonian agent. It also has α1-adrenergic and α2-adrenergic agonist activity....
  
  
• tizanidine
  Tizanidine
  Tizanidine is a drug that is used as a muscle relaxant. It is a centrally acting α2 adrenergic agonist. It is used to treat the spasms, cramping, and tightness of muscles caused by medical problems such as multiple sclerosis, spastic diplegia, back pain, or certain other injuries to the spine or...
  
  

Clinical significance

Alpha-adrenergic agonists are used in the treatment of glaucoma

Glaucoma

Glaucoma is an eye disorder in which the optic nerve suffers damage, permanently damaging vision in the affected eye and progressing to complete blindness if untreated. It is often, but not always, associated with increased pressure of the fluid in the eye...

by decreasing the production of aqueous fluid by the ciliary bodies of the eye and also by increasing uveoscleral outflow.

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