DETERMINATION OF THE LUMPED CONSTANT FOR THE ALPHA-METHYLTRYPTOPHAN METHOD OF ESTIMATING THE RATE OF SEROTONIN SYNTHESIS

The lumped constant (LC) for the alpha-methyl-L-tryptophan method to c onvert the brain's uptake of labeled alpha-methyl-L-tryptophan into th e regional rate of serotonin synthesis was estimated. The method invol ved independently estimating the unidirectional uptake constant of the tracer (alpha-[C-14]methyl-L-tryptophan) to the tissue and the tracee (tryptophan) (with the addition of a radioactive compound) and calcul ating their ratio. The LC was estimated from logarithmically transform ed data. Similar experiments were performed using rats treated with th e drug probenecid, which blocks the efflux of 5-hydroxyindoleacetic ac id (a metabolite of serotonin) from the brain. The experiments using p robenecid, corrected for the difference in the levels of plasma free t ryptophan (increased in probenecid-treated rats) relative to control e xperiments, gave an average LC for the rat brain of 0.46 +/- 0.14 (mea n +/- SD). This value was not significantly different from the one obt ained in controls (0.43 +/- 0.13). In addition, the LC was also calcul ated using unidirectional uptake constants in the probenecid-treated r ats for alpha-methyl-L-tryptophan and L-tryptophan. This LC value was 0.39 +/- 0.10. There was no significant difference between these three LC values. Thus, an average +/- SD LC of 0.42 +/- 0.07 for 28 brain s tructures investigated in this study was obtained. Statistically the L C obtained in different structures had a variability that could be acc ounted for by errors in measurements alone. In other words, dispersion in the LC values could be fully accounted for by chance alone. Data c onfirmed that the LC value did not change when the rate of serotonin s ynthesis was increased by probenecid treatment. We also showed that th e rate of 5-hydroxyindoleacetic acid accumulation in probenecid-treate d rats was 58 pmol g(-1) min(-1) (rat brain), which is about twice as much as reported by others for a normal rat. This difference could als o be accounted for by the increase in the plasma level of free tryptop han in probenecid-treated rats.