We describe here an autoradiographic method to measure the in vivo rate of serotonin synthesis in rat brain. The method is based on the use of the l-tryptophan analogue a-methyl-l-tryptophan (a-MTrp), which is converted in vivo into a-methylserotonin (a-M5HT). Since a-M5HT is not a substrate for monoamine oxidase, it is accumulated in the brain tissue. Data are presented to confirm time-dependent conversion of a-MTrp into a-M5HT in the dorsal raphe nucleus and also in the pineal body, an organ outside the blood–brain barrier. It has also been shown that washing brain slices in 10% trichloroacetic acid results in <3% incorporation of a-MTrp into brain proteins. The rates of synthesis are calculated in several grossly dissected brain structures by using tracer kinetics and a three-compartment biological model. The half-life of the precursor pool is estimated to be ∼20 min. The rate of serotonin synthesis is highest in the pineal body.
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