|Systematic (IUPAC) name|
|None (without an MAO-B inhibitor)
Moderate (with an MAO-B inhibitor)
|Metabolism||MAO-A, SSAOs, PNMT, AANAT, FMO3, and others|
|Biological half-life||Exogenous: 5–10 minutes
Endogenous: ~30 seconds
|Molecular mass||121.18 g/mol|
|Melting point||−60 °C (−76 °F) |
|Boiling point||197.5 °C (387.5 °F) |
Phenethylamine (PEA), also known as β-phenylethylamine (β-PEA) and 2-phenylethylamine is an organic compound and a natural monoamine alkaloid, a trace amine, and also the name of a class of chemicals with many members that are well known for their psychoactive and stimulant effects.
Phenylethylamine functions as a monoaminergic neuromodulator and, to a lesser extent, a neurotransmitter in the human central nervous system. It is biosynthesized from the amino acid L-phenylalanine by enzymatic decarboxylation via the enzyme aromatic L-amino acid decarboxylase. In addition to its presence in mammals, phenethylamine is found in many other organisms and foods, such as chocolate, especially after microbial fermentation. It is sold as a dietary supplement for purported mood and weight loss-related therapeutic benefits; however, orally ingested phenethylamine experiences extensive first-pass metabolism by monoamine oxidase B (MAO-B) and then aldehyde dehydrogenase (ALDH), which metabolize it into phenylacetic acid. This prevents significant concentrations from reaching the brain when taken in low doses.
The group of phenethylamine derivatives is referred to as the phenethylamines. Substituted phenethylamines, substituted amphetamines, and substituted methylenedioxyphenethylamines (MDxx) are a series of broad and diverse classes of compounds derived from phenethylamine that include empathogens, stimulants, psychedelics, anxiolytics (hypnotics) and entactogens, as well as anorectics, bronchodilators, decongestants, and antidepressants, among others.
Phenethylamine is widely distributed throughout the plant kingdom and also present in animals, such as humans; it is also produced by certain fungi and bacteria (genus: Lactobacillus, Clostridium, Pseudomonas, and Enterobacteriaceae) and acts as a potent anti-microbial against certain pathogenic strains of Escherichia coli (e.g., the O157:H7 strain) at sufficient concentrations.
Physical and chemical properties
Phenethylamine is a primary amine, the amino-group being attached to a benzene ring through a two-carbon, or ethyl group. It is a colourless liquid at room temperature that has a fishy odour and is soluble in water, ethanol and ether. Its density is 0.964 g/ml and its boiling point is 195 °C. Upon exposure to air, it forms a solid carbonate salt[clarification needed] with carbon dioxide. Phenethylamine is strongly basic, pKb = 4.17 (or pKa = 9.83), as measured using the HCl salt and forms a stable crystalline hydrochloride salt with a melting point of 217 °C.
One method for preparing β-phenethylamine, set forth in J. C. Robinson's and H. R. Snyder's Organic Syntheses (published 1955), involves the reduction of benzyl cyanide with hydrogen in liquid ammonia, in the presence of a Raney-Nickel catalyst, at a temperature of 130 °C and a pressure of 13.8 MPa. Alternative syntheses are outlined in the footnotes to this preparation.
A much more convenient method for the synthesis of β-phenethylamine is the reduction of ω-nitrostyrene by lithium aluminum hydride in ether, whose successful execution was first reported by R. F. Nystrom and W. G. Brown in 1948. Phenethylamine can also be produced via the cathodic reduction of benzyl cyanide in a divided cell.
Phenethylamine, being similar to amphetamine in its action at their common biomolecular targets, releases norepinephrine and dopamine. Phenethylamine also appears to induce acetylcholine release via a glutamate-mediated mechanism.
Reviews that cover attention deficit hyperactivity disorder (ADHD) and phenethylamine indicate that several studies have found abnormally low urinary phenethylamine content in ADHD individuals when compared with controls. In treatment responsive individuals, amphetamine and methylphenidate greatly increase urinary phenethylamine content. An ADHD biomarker review also indicated that urinary phenethylamine levels could be a diagnostic biomarker for ADHD.
Thirty minutes of moderate to high intensity physical exercise has been shown to induce an enormous increase in urinary phenylacetic acid, the primary metabolite of phenethylamine. Two reviews noted a study where the mean 24 hour urinary phenylacetic acid concentration following just 30 minutes of intense exercise rose 77% above its base level; the reviews suggest that phenethylamine synthesis sharply increases during physical exercise during which it is rapidly metabolized due to its short half-life of roughly 30 seconds. In a resting state, phenethylamine is synthesized in catecholamine neurons from L-phenylalanine by aromatic amino acid decarboxylase at approximately the same rate as dopamine is produced. Because of the pharmacological relationship between phenethylamine and amphetamine, the original paper and both reviews suggest that phenethylamine plays a prominent role in mediating the mood-enhancing euphoric effects of a runner's high, as both phenethylamine and amphetamine are potent euphoriants.
By oral route, phenylethylamine's half-life is 5–10 minutes; endogenously produced PEA in catecholamine neurons has a half-life of roughly 30 seconds. It is metabolized by phenylethanolamine N-methyltransferase, MAO-A, MAO-B, semicarbazide-sensitive amine oxidases (SSAOs), aldehyde dehydrogenase, and flavin-containing monooxygenase 3. N-methylphenethylamine, an isomer of amphetamine, is produced in humans via the metabolism of phenethylamine by phenylethanolamine N-methyltransferase. When the initial phenylethylamine brain concentration is low, brain levels can be increased 1000-fold when taking a monoamine oxidase inhibitor (MAOI), particularly a MAO-B inhibitor, and by 3–4 times when the initial concentration is high. β-Phenylacetic acid is the primary urinary metabolite of phenethylamine and is produced via monoamine oxidase metabolism and subsequent aldehyde dehydrogenase metabolism. In humans, phenylacetaldehyde is the intermediate product which is produced by monoamine oxidase and then further metabolized into β-phenylacetic acid by aldehyde dehydrogenase.
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The pharmacology of TAs might also contribute to a molecular understanding of the well-recognized antidepressant effect of physical exercise . In addition to the various beneficial effects for brain function mainly attributed to an upregulation of peptide growth factors [52,53], exercise induces a rapidly enhanced excretion of the main β-PEA metabolite β-phenylacetic acid (b-PAA) by on average 77%, compared with resting control subjects , which mirrors increased β-PEA synthesis in view of its limited endogenous pool half-life of ~30 s [18,55].
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While diagnosis of ADHD is usually done by analysis of the symptoms (American Psychiatric Association, 2000), PEA was recently described as a biomarker for ADHD (Scassellati et al., 2012). This novel discovery will improve the confidence of the diagnostic efforts, possibly leading to reduced misdiagnosis and overmedication. Specifically, the urinary output of PEA was lower in a population of children suffering from ADHD, as compared to the healthy control population, an observation that was paralleled by reduced PEA levels in ADHD individuals (Baker et al., 1991; Kusaga, 2002). In a consecutive study (Kusaga et al., 2002), those of the children suffering with ADHD were treated with methylphenidate, also known as Ritalin. Patients whose symptoms improved in response to treatment with methylphenidate had a significantly higher PEA level than patients who did not experience such an improvement in their condition (Kusaga et al., 2002).
- Lynnes T, Horne SM, Prüß BM (2014). "ß-Phenylethylamine as a novel nutrient treatment to reduce bacterial contamination due to Escherichia coli O157:H7 on beef meat". Meat Sci. 96 (1): 165–71. PMID 23896151. doi:10.1016/j.meatsci.2013.06.030.
Acetoacetic acid (AAA) and ß-phenylethylamine (PEA) performed best in this experiment. On beef meat pieces, PEA reduced the bacterial cell count by 90% after incubation of the PEA treated and E. coli contaminated meat pieces at 10°C for one week.
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Phenylethylamine (10), amphetamine [AMPH (11 & 12)], methylenedioxy methamphetamine [METH (13)] and N-methyl-4-phenylpyridinium (15) are all more potent inhibitors of VMAT2...
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TAAR1 activity appears to depress monoamine transport and limit dopaminergic and serotonergic neuronal firing rates via interactions with presynaptic D2 and 5-HT1A autoreceptors, respectively (Wolinsky et al., 2007; Lindemann et al., 2008; Xie and Miller, 2008; Xie et al., 2008; Bradaia et al., 2009; Revel et al., 2011; Leo et al., 2014). ... TAAR1 and TAAR4 labeling in all neurons appeared intracellular, consistent with previous reported results for TAAR1 (Miller, 2011). A cytoplasmic location of ligand and receptor (e.g., tyramine and TAAR1) supports intracellular activation of signal transduction pathways, as suggested previously (Miller, 2011). ... Additionally, once transported intracellularly, they could act on presynaptic TAARs to alter basal activity (Miller, 2011). ... As reported for TAAR1 in HEK cells (Bunzow et al., 2001; Miller, 2011), we observed cytoplasmic labeling for TAAR1 and TAAR4, both of which are activated by the TAs (Borowsky et al., 2001). A cytoplasmic location of the ligand and the receptor (e.g., tyramine and TAAR1) would support intracellular activation of signal transduction pathways (Miller, 2011). Such a co-localization would not require release from vesicles and could explain why the TAs do not appear to be found there (Berry, 2004; Burchett and Hicks, 2006).
- Deepak N, Sara T, Andrew H, Darrell DM, Glen BB (2011). "Trace amines and their relevance to psychiatry and neurology: a brief overview". Bulletin of Clinical Psychopharmacology. 21 (1): 73–79. doi:10.5350/KPB-BCP201121113.
Interestingly, PEA can also stimulate acetylcholine release through activation of glutamatergic signaling pathways (21), and PEA and p-TA have been reported to depress GABAB receptor-mediated responses in dopaminergic neurons (22,23). Although PEA, T and p-TA have been reported to be present in synaptosomes (nerve ending preparations isolated during homogenization and centrifugation of brain tissue) (24), research with reserpine and neurotoxins suggests that m- and p-TA may be stored in vesicles while PEA and T are not (25-27). ... the antidepressant effects of exercise have been suggested to be due to an elevation of PEA (57). l-Deprenyl (selegiline), a selective inhibitor of MAO-B, is used in the treatment of Parkinson’s disease and produces a marked increase in brain levels of PEA relative to other amines (20,58). ... Interestingly, the gene for aromatic amino acid decarboxylase (AADC), the major enzyme involved in the synthesis of the trace amines, is located in the same region of chromosome 7 that has been proposed as a susceptibility locus for ADHD (50)
- Scassellati C, Bonvicini C, Faraone SV, Gennarelli M; Bonvicini; Faraone; Gennarelli (October 2012). "Biomarkers and attention-deficit/hyperactivity disorder: a systematic review and meta-analyses" (PDF). J. Am. Acad. Child Adolesc. Psychiatry. 51 (10): 1003–1019.e20. PMID 23021477. doi:10.1016/j.jaac.2012.08.015. Retrieved 30 June 2014.
Although we did not find a sufficient number of studies suitable for a meta-analysis of PEA and ADHD, three studies20,57,58 confirmed that urinary levels of PEA were significantly lower in patients with ADHD compared with controls. ... Administration of D-amphetamine and methylphenidate resulted in a markedly increased urinary excretion of PEA,20,60 suggesting that ADHD treatments normalize PEA levels. ... Similarly, urinary biogenic trace amine PEA levels could be a biomarker for the diagnosis of ADHD,20,57,58 for treatment efficacy,20,60 and associated with symptoms of inattentivenesss.59 ... With regard to zinc supplementation, a placebo controlled trial reported that doses up to 30 mg/day of zinc were safe for at least 8 weeks, but the clinical effect was equivocal except for the finding of a 37% reduction in amphetamine optimal dose with 30 mg per day of zinc.110
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The 24 hour mean urinary concentration of phenylacetic acid was increased by 77% after exercise. ... These results show substantial increases in urinary phenylacetic acid levels 24 hours after moderate to high intensity aerobic exercise. As phenylacetic acid reflects phenylethylamine levels3 , and the latter has antidepressant effects, the antidepressant effects of exercise appear to be linked to increased phenylethylamine concentrations. Furthermore, considering the structural and pharmacological analogy between amphetamines and phenylethylamine, it is conceivable that phenylethylamine plays a role in the commonly reported "runners high" thought to be linked to cerebral β-endorphin activity. The substantial increase in phenylacetic acid excretion in this study implies that phenylethylamine levels are affected by exercise. ... A 30 minute bout of moderate to high intensity aerobic exercise increases phenylacetic acid levels in healthy regularly exercising men. The findings may be linked to the antidepressant effects of exercise.
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It has also been suggested that the antidepressant effects of exercise are due to an exercise-induced elevation of PE .
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Trace amines are metabolized in the mammalian body via monoamine oxidase (MAO; EC 18.104.22.168) (Berry, 2004) (Fig. 2) ... It deaminates primary and secondary amines that are free in the neuronal cytoplasm but not those bound in storage vesicles of the sympathetic neurone ... Similarly, β-PEA would not be deaminated in the gut as it is a selective substrate for MAO-B which is not found in the gut ...
Brain levels of endogenous trace amines are several hundred-fold below those for the classical neurotransmitters noradrenaline, dopamine and serotonin but their rates of synthesis are equivalent to those of noradrenaline and dopamine and they have a very rapid turnover rate (Berry, 2004). Endogenous extracellular tissue levels of trace amines measured in the brain are in the low nanomolar range. These low concentrations arise because of their very short half-life ...
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The preferred in vitro substrates of AOC2 were found to be 2-phenylethylamine, tryptamine and p-tyramine instead of methylamine and benzylamine, the favored substrates of AOC3.
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The biogenic amines, phenethylamine and tyramine, are N-oxygenated by FMO to produce the N-hydroxy metabolite, followed by a rapid second oxygenation to produce the trans-oximes (Lin & Cashman, 1997a, 1997b). This stereoselective N-oxygenation to the trans-oxime is also seen in the FMO-dependent N-oxygenation of amphetamine (Cashman et al., 1999) ... Interestingly, FMO2, which very efficiently N-oxygenates N-dodecylamine, is a poor catalyst of phenethylamine N-oxygenation. The most efficient human FMO in phenethylamine N-oxygenation is FMO3, the major FMO present in adult human liver; the Km is between 90 and 200 μM (Lin & Cashman, 1997b).
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