Biochemical and autoradiographic measurements of brain serotonin synthesisrate in the freely moving rat: A reexamination of the alpha-methyl-L-tryptophan method

Biochemical approaches were used in freely moving rats to determine, under steady-state conditions, the brain/arterial plasma partition coefficients o f L-tryptophan and alpha-[H-3]methyl-L-tryptophan, from which the lumped co nstant for the alpha-methyl-L-tryptophan method of estimating the rate of b rain serotonin synthesis is calculated. The lumped constants were significa ntly different in the various structures examined: 0.149 +/- 0.003 in the r aphe dorsalis, 0.103 +/- 0.002 in the raphe centralis, 0.087 +/- 0.003 in t he reticular formation, and 0.62 +/- 0.08 in the pineal gland. From these d ata we proposed a two-compartment model to calculate the rate of serotonin synthesis by quantitative autoradiography using a three-time point experime nt. Rates of synthesis for the raphe dorsalis and the reticular formation ( 620 +/- 57 and 80 +/- 35 pmol/g of tissue/min, respectively) were similar t o those measured simultaneously by biochemical means, but rates were 50% hi gher for the raphe centralis (568 +/- 90 vs. 381 +/- 31 pmol/g of tissue/mi n). The lack of dynamic equilibrium of the tracer between plasma and tissue pools may explain the discrepancy between the two methods. Our findings di d not confirm previous data, indicating that the application of the autorad iographic method to measure the rate of brain serotonin synthesis using alp ha-methyl-L-tryptophan as tracer has limitations.