A simplified autoradiographic method with alpha-[14C]methyl-tryptophan to measure serotonin synthesis rate in the rat brain.

Background: To minimize blood sampling necessary for the autoradiographic (ARG) method using alpha-[(14)C]methyl-tryptophan (alpha-MTrp), a simplified method using a standard exposure time (theta(S)) was proposed and the accuracy of the method was evaluated.
Methods: A total of 168 rats from three sets of experiments with different serotonergic drugs to evaluate changes in cerebral serotonin (5-hydroxytryptamine; 5-HT) synthesis rate were used. In the acute treatment study, rats received a drug injection 30 min before the tracer experiment. In the repeated treatment study, the same dose of drug was injected for 7 days. The two-time point method provided a parameter of brain trapping constant (K*) from the slope of linear regression of the Patlak plot, from which 5-HT synthesis rate is calculated. The measured exposure time (theta(M)) for 60 and 150 min after the tracer injection obtained from individual blood sampling in the original method and the theta(S) calculated from theta(M) were used for evaluation of differences in K* values.
Results: No significant difference in theta(M) was noted among different experiments, although plasma radioactivity at the end of experiment was significantly different between the acute and the repeated treatments for one of the three drugs. No difference in K* for each treatment was observed between the original method and the simplified single blood sampling method because there was no difference in theta(M) among three experiments and between theta(M) and theta(S).
Conclusion: The simplified alpha-MTrp ARG method, which uses theta(S) and a single arterial blood sample at the end of each experiment, can be used for the measurement of 5-HT synthesis rate in the rat brain.