Phenylethylamine (PEA) is an aromatic amine, which is a colorless liquid at room temperature. It is soluble in water, ethanol, and ether. Similar to other low-molecular-weight amines, it has a fishy odor. Upon exposure to air, it forms a solid carbonate salt with carbon dioxide. Phenethylamine is strongly basic and forms a stable crystalline hydrochloride salt with a melting point of 217 °C. Phenethylamine is also a skin irritant and possible sensitizer. Phenethylamine also has a constitutional isomer (+)-phenylethylamine (1-phenylethylamine), which has two stereoisomers: (R)-(+)-1-phenylethylamine and (S)-(-)-1-phenylethylamine. In the human brain, 2-phenethylamine is believed to function as a neuromodulator or neurotransmitter (a trace amine). Phenethylamine can be biosynthesized from the amino acid phenylalanine by enzymatic decarboxylation. It is also found in many foods such as chocolate, especially after microbial fermentation. However trace amounts from food are quickly metabolized by the enzyme MAO-B (into phenylacetic acid), preventing significant concentrations from reaching the brain. Phenylethylamine is a precursor to the neurotransmitter phenylethanolamine. High levels of PEA have been found in the urine of schizophrenics but it is not significantly elevated in the serum or CSF of schizophrenics (PMID:7906896, PMID:7360842).¬† Urinary levels of PEA are significantly lower in children with attention deficit hyperactivity disorder (ADHD) (PMID:12205654).¬† It has been found that PEA is the primary compound found in carnivore (especially cat) urine that leads to rodent (mouse and rat) avoidance. In other words, phenylethylamine is useful for scaring off rodent pests.¬† Quantitative HPLC analysis across 38 mammalian species has shown that PEA production in urine is especially enhanced in carnivores, with some producing >3,000-fold more than herbivores (PMID:21690383). Phenethylamine has been found to be a metabolite of Bacillus, Enterococcus and Lactobacillus (PMID:22953951; PMID:17307265; PMID:16630269).