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Alpha-2 adrenergic receptors (or adrenoceptors) are G-protein coupled receptors (GPCR) that are spread throughout the central and peripheral nervous systems. They are activated by catecholamines like norepinephrine and epinephrine, as well as drugs like clonidine and dexmedetomidine. Alpha-2 agonists are used in anaesthesia to affect sedation, muscle relaxation, and analgesia. They produce a calming effect, promote cardiovascular stability, and reduce anesthetic requirements. While the alpha-2 receptor has been linked to muscle relaxation, the specific effects depend on its location and interaction with other receptors.
| Characteristics | Values |
|---|---|
| Alpha-2 adrenergic receptor agonists | Clonidine, Dexmedetomidine, Lofexidine, TDIQ, Tizanidine, Xylazine |
| Alpha-2 adrenergic receptor antagonists | Prazosin, Yohimbine, BRL44408, ARC-239, Rauwolscine |
| Alpha-2 adrenergic receptor subtypes | α2A, α2B, α2C-adrenergic, α2D-adrenergic (in some species) |
| Alpha-2 adrenergic receptor functions | Sedation, analgesia, sympatholytic effects, vasodilatation, bradycardia, neuromodulation, muscle relaxation |
| Alpha-2 adrenergic receptor locations | Central nervous system, peripheral nervous system, vascular prejunctional terminals, vascular smooth muscle cells, pontine locus coeruleus, medullospinal tracts, rostral ventrolateral medulla, spinal cord |
| Alpha-2 adrenergic receptor applications | Anaesthesia, sedation, adjuncts for sedation, reduction of anesthetic requirements, treatment of hypertension, attention-deficit/hyperactivity disorder, pain and panic disorders, opioid withdrawal, cigarette craving |
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What You'll Learn
- Alpha-2 adrenergic receptor agonists are used in anaesthesia
- Alpha-2 receptors are spread throughout the central and peripheral nervous system
- Alpha-2 receptors are activated by norepinephrine and epinephrine
- Alpha-2 agonists produce effects within both the central and peripheral nervous systems
- Alpha-2 receptors are G-protein coupled receptors
Alpha-2 adrenergic receptor agonists are used in anaesthesia
Alpha-2 adrenergic receptor agonists have been used for decades to treat common medical conditions such as hypertension, attention-deficit/hyperactivity disorder, various pain and panic disorders, symptoms of opioid, benzodiazepine, and alcohol withdrawal, and cigarette craving. However, they are also used in anaesthesia.
Alpha-2 adrenoceptor agonists are not routinely used by the majority of anaesthetists, despite having many desirable effects, including anxiolysis, analgesia, sedation, anaesthetic-sparing, and peri-operative haemodynamic-stabilising effects. Their potential remains to be fully realised. This may be because there are no highly specific alpha-2 adrenoceptor agonists currently available for anaesthesia. For example, clonidine has some alpha-1 activity, and there is a possibility of undesirable haemodynamic effects at certain doses. Indeed, there is still debate as to whether these drugs offer real clinical benefits.
Clonidine, mivazerol, and, to a lesser extent, dexmedetomidine are not pure alpha-2 adrenoceptor agonists but are also able to combine with nonadrenergic imidazoline receptors. These receptors are binding sites that specifically recognise the imidazoline or oxazoline chemical structure and have been classified into I1, found in the brain, and I2, found in the brain, kidney, and pancreas. Clonidine has proved to be a clinically useful adjunct in clinical anaesthetic practice, as well as in chronic pain therapy, because it has both anaesthetic and analgesic-sparing activity.
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Alpha-2 receptors are spread throughout the central and peripheral nervous system
Alpha-2 adrenergic receptors (or adrenoceptors) are G protein-coupled receptors (GPCRs) associated with the Gi heterotrimeric G-protein. They are spread throughout the central and peripheral nervous systems, specifically in the pontine locus coeruleus, medullospinal tracts, rostral ventrolateral medulla, and the dorsal horn of the spinal cord.
The alpha-2 receptor consists of three homologous subtypes: alpha-2A, alpha-2B, and alpha-2C adrenergic. Some species, other than humans, express a fourth subtype, alpha-2D-adrenergic. Endogenous agonists, such as norepinephrine and epinephrine, have similar affinities for all three subtypes. However, norepinephrine has a higher affinity for the alpha-2 receptor than epinephrine, and thus relates less to the latter's functions.
The alpha-2A adrenergic receptor is localized in the following central nervous system (CNS) structures: the brainstem (especially the locus coeruleus as presynaptic and somatodendritic autoreceptor) and the prefrontal cortex. The alpha-2B receptors are found more frequently on vascular smooth muscle and have been shown to mediate vasopressor effects. The alpha-2C subtype is also found mainly in the central nervous system.
Alpha-2 adrenergic receptor agonists have been used for decades to treat common medical conditions such as hypertension, attention-deficit/hyperactivity disorder, various pain and panic disorders, and symptoms of opioid, benzodiazepine, and alcohol withdrawal. They are also used as adjuncts for sedation and to reduce anesthetic requirements. The two major drugs in this group are clonidine and dexmedetomidine.
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Alpha-2 receptors are activated by norepinephrine and epinephrine
Alpha-2 (α2) adrenergic receptors are activated by the catecholamines norepinephrine (noradrenaline) and epinephrine (adrenaline). They are a type of G protein-coupled receptor (GPCR) associated with the Gi heterotrimeric G-protein.
The α2-adrenergic receptor binds norepinephrine, which is released by sympathetic postganglionic fibres, and epinephrine, which is released by the adrenal medulla. Norepinephrine has a slightly higher affinity for the α2 receptor than epinephrine.
Alpha-2 receptors are spread throughout the central and peripheral nervous systems, particularly in areas such as the locus coeruleus and spinal cord. They have several functions in common with α1-adrenergic receptors but also have specific effects of their own. Agonists of the α2-adrenergic receptor are frequently used in anaesthesia to induce sedation, muscle relaxation and analgesia through effects on the central nervous system (CNS).
The α2-adrenergic receptor is classically located on vascular prejunctional terminals where it inhibits the release of norepinephrine in a form of negative feedback. It is also located on the vascular smooth muscle cells of certain blood vessels, such as those found in skin arterioles or veins.
Alpha-2 adrenergic receptor agonists have been used for decades to treat common medical conditions such as hypertension, attention-deficit/hyperactivity disorder, various pain and panic disorders, symptoms of opioid, benzodiazepine, and alcohol withdrawal, and cigarette craving.
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Alpha-2 agonists produce effects within both the central and peripheral nervous systems
The α2-adrenergic receptor binds both norepinephrine released by sympathetic postganglionic fibres and epinephrine (adrenaline) released by the adrenal medulla, binding norepinephrine with slightly higher affinity. It has several functions in common with the α1-adrenergic receptor, but also has specific effects of its own. Agonists (activators) of the α2-adrenergic receptor are frequently used in anaesthesia. In the brain, α2-adrenergic receptors can be localized either pre- or post-synaptically, and the majority of receptors appear to be post-synaptic.
The α2A adrenergic receptor is localised in the following central nervous system (CNS) structures: brainstem (especially the locus coeruleus as presynaptic and somatodendritic autoreceptor). The α2-adrenergic receptor is classically located on vascular prejunctional terminals where it inhibits the release of norepinephrine (noradrenaline) in a form of negative feedback. It is also located on the vascular smooth muscle cells of certain blood vessels, such as those found in skin arterioles or on veins, where it sits alongside the more plentiful α1-adrenergic receptor.
Alpha-2 agonists have been used for decades to treat common medical conditions such as hypertension, attention-deficit/hyperactivity disorder (ADHD), various pain and panic disorders, symptoms of opioid, benzodiazepine, and alcohol withdrawal, and cigarette craving. They have also been used as adjuncts for sedation and to reduce anesthetic requirements. Alpha-2 agonists cause neuromodulation in these centres, leading to sedation, analgesia, vasodilation, and bradycardia with little effect on the respiratory drive, which accounts for their good safety profile.
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Alpha-2 receptors are G-protein coupled receptors
The alpha-2 adrenergic receptor (or adrenoceptor) consists of three homologous subtypes: α2A- α2B, and α2C-adrenergic. Some non-human species also express a fourth subtype, α2D-adrenergic. Catecholamines like norepinephrine (noradrenaline) and epinephrine (adrenaline) signal through the α2-adrenergic receptor in the central and peripheral nervous systems.
The α2A adrenergic receptor is localised in the following central nervous system (CNS) structures: brainstem (especially the locus coeruleus as presynaptic & somatodendritic autoreceptor). The α2-adrenergic receptor is classically located on vascular prejunctional terminals where it inhibits the release of norepinephrine (noradrenaline) in a form of negative feedback. It is also located on the vascular smooth muscle cells of certain blood vessels, such as those found in skin arterioles or on veins, where it sits alongside the α1-adrenergic receptor.
Α2 agonists are drugs that activate the α2-adrenergic receptor and are frequently used in anaesthesia to affect sedation, muscle relaxation, and analgesia through effects on the central nervous system (CNS). They have also been used to treat common medical conditions such as hypertension, attention-deficit/hyperactivity disorder, various pain and panic disorders, and symptoms of opioid, benzodiazepine, and alcohol withdrawal.
Nonselective α2 agonists include the antihypertensive drug clonidine, which can be used to lower blood pressure and reduce hot flashes associated with menopause. It has also been used successfully in children with ADHD who experience tics resulting from treatment with a CNS stimulant drug.
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Frequently asked questions
Alpha-2 adrenergic receptors are spread throughout the central and peripheral nervous systems. They are G-protein coupled receptors (GPCR) that have specific effects on muscle relaxation, sedation, analgesia, and sympatholytic effects.
Alpha-2 receptors cause muscle relaxation by acting as an allosteric inhibitor through Gi function, leading to a decrease in neurotransmitter release and central vasodilation. This results in a reduction of calcium in the cytoplasm, which causes smooth muscle contraction and relaxation.
Some examples of alpha-2 receptor agonist drugs include clonidine, tizanidine, and dexmedetomidine. These drugs are used in modern anaesthesia due to their calming effects, ability to reduce anesthetic requirements, and promotion of cardiovascular stability.
Alpha-2 receptor agonists have been used to treat common medical conditions such as hypertension, attention-deficit/hyperactivity disorder, various pain and panic disorders, and symptoms of opioid, benzodiazepine, and alcohol withdrawal. They are also used in anaesthesia as adjuncts for sedation.
