5‐HT 1A receptor imaging in the human brain: Effect of tryptophan depletion and infusion on [ 18 F]MPPF binding

The 5‐HT 1A receptor has been implicated in a variety of physiological processes, psychiatric disorders, and neurodegenerative disorders. [ 18 F]MPPF is a useful radioligand for quantitative imaging of 5‐HT 1A receptors in human subjects. Previous studies have shown that the binding of some radioligands is sensitive to changes in neurotransmitter concentration, whereas in other cases, binding is not affected. In the present study we investigated if [ 18 F]MPPF binding to the 5‐HT 1A receptor is sensitive to changes in 5‐HT. Changes in 5‐HT levels were achieved by influencing its synthesis through tryptophan depletion, including a tryptophan‐free amino acid drink 4.5 h prior to the PET scan and tryptophan infusion (10 mg/ml, 50 mg/kg, 30 min, starting 60 min prior to the PET scan). Binding of [ 18 F]MPPF in the brain of six healthy, male volunteers was compared in these two conditions. Mean binding potentials in the medial temporal cortex, cortical regions, and raphe nucleus did not significantly differ between the two conditions. The results of the study show that, under the experimental conditions used, [ 18 F]MPPF binding was not affected. It is hypothesized that the increases in 5‐HT levels needed to produce a measurable effect on [ 18 F]MPPF binding would be significantly greater than that possible with tryptophan manipulation. Therefore, in pathological conditions, where such large increases in 5‐HT levels are not expected, [ 18 F]MPPF seems a useful ligand to measure 5‐HT 1A receptor distribution without the interference of endogenous 5‐HT. Synapse 46:108–115, 2002. © 2002 Wiley‐Liss, Inc.