Morphiceptin
Encyclopedia
Morphiceptin is a tetrapeptide
Tetrapeptide
A tetrapeptide is a peptide consisting of four amino acids joined by peptide bonds. Many tetrapeptides are pharmacologically active, often showing affinity and specificity for a variety of receptors in protein-protein signaling...

 (Tyr-Pro-Phe-Pro-NH2) that is aselective
Binding selectivity
Binding selectivity refers to the differing affinities with which different ligands bind to a substrate forming a complex. A selectivity coefficient is the equilibrium constant for the reaction of displacement by one ligand of another ligand in a complex with the substrate...

 μ-opioid receptor
Mu Opioid receptor
The μ-opioid receptors are a class of opioid receptors with high affinity for enkephalins and beta-endorphin but low affinity for dynorphins. They are also referred to as μ opioid peptide receptors. The prototypical μ receptor agonist is the opium alkaloid morphine; μ refers to morphine...

 agonist
Agonist
An agonist is a chemical that binds to a receptor of a cell and triggers a response by that cell. Agonists often mimic the action of a naturally occurring substance...

. It is derived from β-casomorphin
Casomorphin
Casomorphins are peptides, i.e., protein fragments, derived from the digestion of milk protein casein. The distinguishing characteristic of casomorphins is that they have an opioid effect....

 and has over 1,000 times selectivity for μ- over δ-opioid receptors
Delta Opioid receptor
The δ-opioid receptors, also known as delta opioid receptor or simply delta receptor, abbreviated DOR, is an opioid receptor that has enkephalins as its endogenous ligands.-Function:...

. When administered intracerebroventricularly, morphiceptin had an analgesic
Analgesic
An analgesic is any member of the group of drugs used to relieve pain . The word analgesic derives from Greek an- and algos ....

 ED50 of 1.7 nmol per animal. The analgesic effects of morphiceptin were reversed by naloxone
Naloxone
Naloxone is an opioid antagonist drug developed by Sankyo in the 1960s. Naloxone is a drug used to counter the effects of opiate overdose, for example heroin or morphine overdose. Naloxone is specifically used to counteract life-threatening depression of the central nervous system and respiratory...

, meaning that the analgesic effect is mediated by the μ-opioid receptor.
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