Dermorphin is a selective mu-opioid receptor agonist. It was isolated from isolated from skin extracts of frogs belonging to the genus Phyllomedusa and was shown to relife pain in patients with postoperative pain syndrome.

Morphiceptin (Tyr-Pro-Phe-Pro-NH2), an amide of a fragment of the milk protein beta-casein, is a selective ligand of the mu opioid receptor, an important target in pain regulation. Now is used the analogs of morphiceptin, which are studied for the treatment of different diseases.

Laudanosine, a potentially epileptogenic metabolite of the neuromuscular relaxant atracurium besylate with potentially toxic effects. Laudanosine is a non-competitive and voltage-dependent inhibitor of alpha7, alpha4beta2 or alpha4beta4 nicotinic acetylcholine receptors, opioid mu 1 type receptors and possesses a low-affinity to the GABA receptors, but didn’t interact with the benzodiazepine or muscarinic receptors.

Salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, SAL) is a tetrahydroisoquinoline derivative, detected in humans. Salsolinol can be formed in the mammalian brain by three different mechanisms: (1) via the nonenzymatic Pictet–Spengler condensation of dopamine and aldehydes producing salsolinol as two racemic isomers (R or S). (2) By the nonenzymatic condensation of dopamine and pyruvate yielding 1-carboxyl-tetrahydroisoquinoline, followed by decarboxylation and reduction, which produces (R)-salsolinol. (3) By selective synthesis of (R)-salsolinol from dopamine and acetaldehyde, the level of which is increased after ethanol intake. Apart from that salsolinol has also been detected in certain beverages and foodstuff, including soy sauce, cheese. Only the (R) enantiomer of Salsolinol and occur in the human brain, cerebrospinal fluid (CSF) and intraventricular fluid (IVF), and the (S) enantiomer was not detected. Salsolinol was also found to be involved in neurotoxicity processes altering the normal function and survival of dopamine neurons. It has been proposed, that salsolinol participated in the etiopathogenesis of Parkinson’s disease. Salsolinol could inhibit various enzymes, for example, monoamine oxidase and tyrosine hydroxylase and recently was revealed, that (R)-SAL and (S)-SAL were agonists of the μ-opioid receptor. (S)-SAL is a more potent agonist than the (R)-SAL stereoisomer, it was suggested that an opioid action of SAL or its enantiomers is involved in the rewarding effects of ethanol.

Levallorphan (brand name Lorfan), is an opiate antagonist of morphine family. Levallorphan was formerly used in general anesthesia, mainly to reverse the respiratory depression produced by opioid analgesics and barbiturates used for induction of surgical anaesthesia whilst maintaining a degree of analgesia. Levallorphan was also used in combination with opioid analgesics to reduce their side effects, mainly in obstetrics. The combination of levallorphan with pethidine was used so frequently, a standardized formulation was made available, known as Pethilorfan, by Roche Products Ltd in later 1950s. Is known to be used for narcotic overdose. Levallorphan is similar to naloxone but differs from naloxone in that it also possesses some agonist properties. It acts as an antagonist and partial agonist of the mu opioid receptor (MOR). Levallorphan can produce severe mental reactions at sufficient doses including hallucinations, dissociation, and other psychotomimetic effects, dysphoria, anxiety, confusion, dizziness, disorientation, derealization, feelings of drunkenness, and bizarre, unusual, or disturbing dreams.

TIANEPTINE, a tricyclic antidepressant, is a drug used for the treatment of the major depressive disorder. It was discovered by The French Society of Medical Research in the 1980s. Unlike other tricyclic antidepressants, TIANEPTINE is a selective serotonin reuptake enhancer with minimal effects on norepinephrine and dopamine uptake. Also, it is a full agonist at the mu-opioid and delta-opioid receptors with no effect at the kappa-opioid receptors. Selective mu-opioid agonists typically induce euphoria, which may contribute to TIANEPTINE's antidepressant effect. It is marketed as Coaxil/Stablon in many European countries, but it is not available in the US.