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In pharmacology the term agonist-antagonist or mixed agonist/antagonist is used to refer to a drug which under some conditions behaves as an agonist (a substance that fully activates the receptor that it binds to) while under other conditions, behaves as an antagonist (a substance that binds to a receptor but does not activate and can block the activity of other agonists).
Types of mixed agonist/antagonist include receptor ligands that act as agonist for some receptor types and antagonist for others or agonist in some tissues while antagonist in others (also known as
In pharmacology the term agonist-antagonist or mixed agonist/antagonist is used to refer to a drug which under some conditions behaves as an agonist (a substance that fully activates the receptor that it binds to) while under other conditions, behaves as an antagonist (a substance that binds to a receptor but does not activate and can block the activity of other agonists).
Types of mixed agonist/antagonist include receptor ligands that act as agonist for some receptor types and antagonist for others or agonist in some tissues while antagonist in others (also known as selective receptor modulator
In the field of pharmacology, a selective receptor modulator or SRM is a type of drug that has different effects in different tissues. A SRM may behave as an agonist in some tissues while as an antagonist in others. Hence selective receptor mod ...
s).
Synaptic receptors
For synaptic receptors, an agonist is a compound that increases the activation of the receptor by binding directly to it or by increasing the amount of time neurotransmitters are in the synaptic cleft. An antagonist is a compound that has the opposite effect of an agonist. It decreases the activation of a synaptic receptor by binding and blocking neurotransmitters from binding or by decreasing the amount of time neurotransmitters are in the synaptic cleft. These actions can be achieved via multiple mechanisms. A common mechanism for agonists isreuptake inhibition
Reuptake is the reabsorption of a neurotransmitter by a neurotransmitter transporter located along the plasma membrane of an axon terminal (i.e., the pre-synaptic neuron at a synapse) or glial cell after it has performed its function of trans ...
, where the agonist blocks neurotransmitters from reentering the pre-synaptic axon terminal. This gives the neurotransmitter more time in the synaptic cleft to act on the synaptic receptors. Conversely, antagonists often bind directly to receptors in the synaptic cleft, effectively blocking neurotransmitters from binding.
At the alpha adrenoceptors, (''R'')-3-nitrobiphenyline is an α2C selective agonist as well as being a weak antagonist at the α and α subtypes.
Agonist-antagonist opioids
The best known agonist-antagonists are opioids. Examples of such opioids are: *pentazocine
Pentazocine, sold under the brand name Talwin among others, is a painkiller used to treat moderate to severe pain. It is believed to work by activating (agonizing) κ-opioid receptors (KOR) and μ-opioid receptors (MOR). As such it is called an ...
, agonist at the kappa (κ) and sigma (σ) and has a weak antagonist action at the mu opioid receptor (μ)
* butorphanol
Butorphanol is a morphinan-type synthetic agonist–antagonist opioid analgesic developed by Bristol-Myers. Butorphanol is most closely structurally related to levorphanol. Butorphanol is available as the tartrate salt in injectable, tablet, and ...
, partial agonist at μ- and a pure agonist at κ-opioid receptor and antagonist activity at the delta opioid receptor
Delta commonly refers to:
* Delta (letter) (Δ or δ), a letter of the Greek alphabet
* River delta, at a river mouth
* D (NATO phonetic alphabet: "Delta")
* Delta Air Lines, US
* Delta variant of SARS-CoV-2 that causes COVID-19
Delta may also r ...
(δ)
* nalbuphine
Nalbuphine, sold under the brand names Nubain among others, is an opioid analgesic which is used in the treatment of pain. It is given by injection into a vein, muscle, or fat.
Side effects of nalbuphine include sedation, sweatiness, clamm ...
, κ-agonist/μ-antagonist analgesic
*
Agonist–antagonist opioids usually have a ceiling effect – over particular dose they don't increase their potency. Hence agonist–antagonist opioids have a lower addiction potential but also lower analgesic efficacy and are more likely to produce psychotomimetic A drug with psychotomimetic (also known as psychotogenic) actions mimics the symptoms of psychosis, including delusions and/or delirium, as opposed to only hallucinations. Psychotomimesis is the onset of psychotic symptoms following the administrati ...
effects.
Agonist–antagonist opioids that block delta
Delta commonly refers to:
* Delta (letter) (Δ or δ), a letter of the Greek alphabet
* River delta, at a river mouth
* D ( NATO phonetic alphabet: "Delta")
* Delta Air Lines, US
* Delta variant of SARS-CoV-2 that causes COVID-19
Delta may also ...
while activating mu opioid receptors produce analgesia without the development of tolerance.
See also
*Competitive antagonist
A receptor antagonist is a type of receptor ligand or drug that blocks or dampens a biological response by binding to and blocking a receptor rather than activating it like an agonist. Antagonist drugs interfere in the natural operation of r ...
* Inverse agonist
* Partial agonist
In pharmacology, partial agonists are drugs that bind to and activate a given receptor, but have only partial efficacy at the receptor relative to a full agonist. They may also be considered ligands which display both agonistic and antagonis ...
References
{{DEFAULTSORT:Agonist-antagonist Pharmacodynamics