Agonist-antagonist

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 is reuptake inhibition, 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 α_2C selective agonist as well as being a weak antagonist at the α_2A and α_2B subtypes.[2][3]

The best known agonist-antagonists are opioids. Examples of such opioids are:

• pentazocine, agonist at the kappa (κ) and sigma (σ) and has a weak antagonist action at the mu opioid receptor (μ)[4]
• butorphanol, partial agonist at μ- and a pure agonist at κ-opioid receptor and antagonist activity at the delta opioid receptor (δ)[5]
• nalbuphine, κ-agonist/μ-antagonist analgesic[6]

Agonist–antagonist opioids usually have a ceiling effect – over particular dose they don't increase their potency.[7] Hence agonist–antagonist opioids have a lower addiction potential but also lower analgesic efficacy and are more likely to produce psychotomimetic effects.[8]

Agonist–antagonist opioids that block delta while activating mu opioid receptors produce analgesia without the development of tolerance.[9]

• Competitive antagonist
• Inverse agonist
• Partial agonist