From the Guidelines
The receptor of an antagonist is the specific protein structure on a cell's surface or within a cell that the antagonist molecule attaches to, such as mu, kappa, and delta opioid receptors in the case of naloxone, as described in the study by 1. The receptor of an antagonist is typically the same one that would normally bind to neurotransmitters, hormones, or other signaling molecules in the body. When an antagonist occupies a receptor, it prevents the normal biological response that would occur when the receptor's natural ligand binds. This mechanism is commonly utilized in medications to block unwanted physiological responses.
Key Points
- Antagonists bind to specific receptors, blocking the normal function of the receptor and preventing the natural ligand or agonist from binding.
- The specificity of an antagonist for a particular receptor type determines both its therapeutic effects and potential side effects, as seen with naloxone's action on mu, kappa, and delta opioid receptors 1.
- Antagonists can be used to treat various conditions, such as opioid overdose, hypertension, and allergic responses, by blocking the action of specific receptors.
Receptor Types
- Mu, kappa, and delta opioid receptors: targeted by naloxone to reverse opioid effects 1.
- Beta-adrenergic receptors: targeted by beta-blockers like metoprolol to reduce heart rate and blood pressure.
- Histamine receptors: targeted by antihistamines to reduce allergic responses.
- a1-Adrenergic receptors: targeted by a1-AR antagonists to treat lower urinary tract symptoms in older men, as discussed in the study by 2.
From the FDA Drug Label
While the mechanism of action of naloxone is not fully understood, in vitro evidence suggests that naloxone antagonizes opioid effects by competing for the µ, κ, and σ opiate receptor sites in the CNS, with the greatest affinity for the µ receptor. The receptor of the antagonist naloxone is the:
- µ (mu) opiate receptor
- κ (kappa) opiate receptor
- σ (sigma) opiate receptor 3
From the Research
Definition of an Antagonist
- An antagonist is a type of drug that binds to a receptor, blocking the action of an endogenous agonist, such as epinephrine or norepinephrine, at the receptor site 4.
- The receptor of an antagonist can be specific to a particular type of receptor, such as beta-1-adrenergic receptors or beta-2-adrenergic receptors 4.
- Antagonists can also have varying levels of affinity for their respective receptors, which can affect their ability to block the action of agonists 5.
Types of Antagonists
- Full antagonists have zero intrinsic activity and only block the action of agonists at the receptor site 6.
- Partial agonists have some intrinsic activity and can activate the receptor to some extent, while also blocking the action of full agonists 6.
- Partial antagonists have some intrinsic activity, but also block the action of full agonists, and can exhibit varying levels of affinity for their respective receptors 6.
Receptor-Specific Antagonists
- Opioid antagonists block opioid receptors, which can be used to treat opioid use disorders, reverse opioid overdose, and manage opioid-induced constipation 7.
- Adrenergic antagonists, such as beta blockers, block adrenergic receptors, which can be used to treat conditions such as hypertension and asthma 4.
- The specific receptor that an antagonist binds to can affect its clinical utility, pharmacology, safety, and tolerability 7.