The Regional Rate of Serotonin Synthesis Estimated by the α-Methyl-Tryptophan Method in Rat Brain from a Single Time Point

A simplified approach to the measurement of the rate of serotonin (5-HT) synthesis in laboratory animals is presented using autoradiography and labeled α-methyl-l-tryptophan. The method is based on the assumption that the volume of distribution of an apparent precursor pool is the same in all brain structures. The apparent distribution volume (V_app) of the tracer was estimated from our previous data and found to be 0.45 ± 0.1 ml g⁻¹ (n = 147) as an average in all brain structures. When this value of V_app was used, the rates of 5-HT synthesis in the rat brain structures, calculated from the single time point method, were not significantly different from those calculated by the dual time point method, where both V_app and K* were estimated. The rates from the two approaches did not differ when compared by analysis of variance (p > 0.8), Mann–Whitney rank sum test (p > 0.11), Bonferroni-corrected two-tailed t test, or when the ratio between two groups was tested against the hypothesis of being 1. The agreement between rates of 5-HT synthesis estimated by the two methods was very good, suggesting that the simplified method proposed here is appropriate for these kind of measurements.