Phenylethylamine and tyramine are mixed-acting sympathomimetic amines in the brain

Abstract

On the helical strip of a capacitance vessel, the pulmonary artery of the rabbit, phenylethylamine (PEA) and tyramine act solely via displacement of noradrenaline from their storage sites and this effect is inhibited by desmethylimipramine (DMI). In contrast, on a resistance vessel, the perfused central ear artery of the rabbit, PEA enhances stimulation induced contractions in 0.2–0.8 gmg/ml concentration [catecholaminergic activity enhancer (CAE) effect], and increases smooth muscle tone (noradrenaline displacing effect) in 4–6 μg/ml concentration. This latter effect only is blocked by DMI. Tyramine acts similarly and is more potent than PEA. On the isolated brain stem PEA, tyramine and (-)methamphetamine are, in the presence of cocaine and DMI, highly potent enhancers of stimulation induced release of ³H-noradrenaline, ³H-dopamine and ³H-serotonin. Compounds with specific CAE effect in the brain, (-)deprenyl and l-phenyl-2-propylaminopentane [(-)PPAP], antagonize tetrabenazine-induced depression of performance of rats in the shuttle box. PEA and tyramine, which are rapidly metabolized in vivo, are ineffective in this test up to 40 mg/kg, whereas (-)methamphetamine, the stable PEA derivative, is highly effective. Compounds with CAE effect enhance at low concentrations the slow inward Ca²⁺ current in the sino-auricular fibers of the frog heart and inhibit it in high concentration. PEA and tyramine enhance Ca²⁺ influx from 0.05 to 4 μg/ml and inhibit it in 8 μg/ml. In conclusion, PEA and tyramine stimulate primarily coupling of action potential to transmitter release in the catecholaminergic neurons in the brain and displace catecholamines in higher concentration only.