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5-Hydroxytryptophan
Chemical compound From Wikipedia, the free encyclopedia
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5-Hydroxytryptophan (5-HTP), used medically as oxitriptan, is a naturally occurring amino acid and chemical precursor as well as a metabolic intermediate in the biosynthesis of the neurotransmitter serotonin.
5-HTP can be manufactured and used as a drug and supplement with the INN oxitriptan. Brand names include Cincofarm, Levothym, Levotonine, Oxyfan, Telesol, Tript-OH, and Triptum. As a drug, it is used in the treatment of depression and for certain other indications.
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Production
5-HTP is produced from the amino acid tryptophan through the action of the enzyme tryptophan hydroxylase. Tryptophan hydroxylase is one of the biopterin-dependent aromatic amino acid hydroxylases. Production of 5-HTP is the rate-limiting step in 5-HT (serotonin) synthesis. 5-HTP is normally rapidly converted to 5-HT by amino acid decarboxylase.[1]
Metabolism
5-HTP is decarboxylated to serotonin (5-hydroxytryptamine or 5-HT) by the enzyme aromatic-L-amino-acid decarboxylase with the help of vitamin B6.[2] This reaction occurs both in nervous tissue and in the liver.[3] 5-HTP crosses the blood–brain barrier,[4] while 5-HT does not. Excess 5-HTP, especially when administered with vitamin B6, is thought to be metabolized and excreted.[5][6]
| 5-HTP | AAAD | Serotonin | |
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Dietary sources
Though 5-HTP is found in food only in insignificant quantities, it is a chemical involved intermediately in the metabolism of tryptophan, an amino acid found in all unfractionated foods, with lower total amino acid content correlating with increased tryptophan absorption.[7]
Use as a medication and supplement
5-HTP is used medically and as a supplement under the name oxitriptan in the treatment of depression and for certain other indications.
It can be potentiated in combination with a peripherally selective aromatic L-amino acid decarboxylase (AAAD) inhibitor such as carbidopa or benserazide. These agents increase the strength and duration of oxitriptan. An investigational combination formulation is oxitriptan/carbidopa.
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Psychedelic effects
5-HTP robustly produces the head-twitch response (HTR) in rodents when administered at relatively high doses.[8][9][10][11][12] It dose-dependently induces the HTR in mice across a dose range of 50 to 250 mg/kg via intraperitoneal administration, with an inverted U-shaped dose–response curve and maximal induction of the HTR at a dose of 200 mg/kg.[12][1] Similarly to the case of 5-HTP, intracerebroventricular injection of serotonin, but not peripheral administration of serotonin, produces the HTR.[9][8][11] The HTR is induced by serotonergic psychedelics like lysergic acid diethylamide (LSD) and psilocybin and is a behavioral proxy of psychedelic effects.[13][8]
The HTR of 5-HTP is blocked by serotonin 5-HT2A receptor antagonists, which block the hallucinogenic effects of serotonergic psychedelics in humans, is prevented by aromatic L-amino acid decarboxylase (AAAD) inhibitors, which block conversion of 5-HTP into serotonin, and is potentiated by monoamine oxidase A (MAO-A) inhibitors, which prevent the degradation of serotonin and other endogenous tryptamines.[9][8][10][11][12] It is also suppressed by the serotonin 5-HT1A receptor full agonist 8-OH-DPAT, is greatly augmented by the serotonin 5-HT2C receptor antagonist RS-102221, and is reduced by the trace amine-associated receptor 1 (TAAR1) antagonist EPPTB.[12] In addition, the HTR of 5-HTP is abolished by indolethylamine N-methyltransferase (INMT) inhibitors, which block conversion of serotonin and other endogenous tryptamines into N-methylated tryptamines, such as N-methylserotonin (NMS; norbufotenin), bufotenin (5-hydroxy-N,N-dimethyltryptamine; 5-HO-DMT), and N,N-dimethyltryptamine (DMT).[8][14][11] These N-methylated tryptamines are well-known for their psychedelic effects, whereas serotonin itself, without biotransformation, does not seem to produce psychedelic effects.[8][11] 5-HTP has not been found to produce psychedelic effects in humans, which has been attributed to the high doses required to produce such effects.[8][10] The 5-HTP doses that produce the HTR in rodents are orders of magnitude higher than the doses of 5-HTP that have been used safely and therapeutically in humans.[10][12] It remains unknown whether 5-HTP can produce psychedelic effects in humans.[15][12] The highest dosage of 5-HTP that is known to have been evaluated in humans is about 3,000 mg per day.[12][1] Serotonin syndrome and associated hallucinations have been reported with overdose of serotonin-elevating drugs, but psychedelic-like effects have not been reported.[12]
The lack of the HTR and psychedelic effects with serotonin itself has been attributed to the fact that these effects appear to be dependent on activation of a population of intracellular 5-HT2A receptors expressed in cortical neurons in the medial prefrontal cortex (mPFC) that lack the serotonin transporter (SERT) and are inaccessible to serotonin.[16][17] Serotonin itself is too hydrophilic to enter serotonergic neurons without the SERT, whereas serotonergic psychedelics and serotonin's N-methylated metabolites and analogues are lipophilic and readily enter these neurons.[16][17] These findings may also explain why selective serotonin reuptake inhibitors (SSRIs) and related serotonergic agents do not produce psychedelic effects.[16]
The properties of 5-HTP in animal drug discrimination tests have been studied.[18][19][20][21][22][23] 5-HTP generalizes with the serotonin releasing agent fenfluramine and its cue is markedly potentiated by the selective serotonin reuptake inhibitor (SSRI) fluoxetine.[18][19] However, numerous serotonin receptor antagonists, including methysergide, cyproheptadine, metergoline, methiothepin (metitepine), ketanserin, pirenperone, pizotifen, and mianserin, all failed to block the discriminative stimulus properties of 5-HTP.[18][19][20][21] Conflictingly however, in a subsequent study, pizotifen was able to fully block the discriminative stimulus properties of 5-HTP.[18][21] The inability of serotonin 5-HT2A receptor antagonists to block the discriminative stimulus properties of 5-HTP is in notable contrast to their ability to block the 5-HTP-induced HTR.[24] 5-HTP only partially substitutes for LSD in drug discrimination tests, whereas LSD and quipazine fully substitute for 5-HTP.[20] The full substitution of LSD and quipazine for 5-HTP can be blocked by the serotonin 5-HT2A receptor antagonist ketanserin.[20] The findings of drug discrimination tests suggest that 5-HTP has a more complex or compound discriminative stimulus compared to other agents like LSD and that its stimulus properties may not be readily explained by either the serotonin 5-HT1 or 5-HT2 receptors alone.[18][20][23] Instead, a combination of actions at these and/or other receptors may be involved in its stimulus effects.[18][20][23]
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