Brain regional alpha-[11C]methyl-L-tryptophan trapping, used as an index of 5-HT synthesis, in healthy adults: absence of an age effect

Eur J Nucl Med Mol Imaging. 2007 Aug;34(8):1254-64. doi: 10.1007/s00259-007-0365-x. Epub 2007 Feb 21.


Purpose: Previous functional neuroimaging studies suggest that selective aspects of the brain serotonin (5-HT) system change during the aging process. Here, we assessed the effects of aging on the brain regional alpha-[(11)C]methyl-L: -tryptophan (alpha-[(11)C]MTrp) trapping rate constant (K*; microl.g(-1).min(-1)), which, with certain assumptions, could be taken as a proxy of 5-HT synthesis.

Methods: Thirty-six healthy right-handed subjects had positron emission tomography (PET) scans following injection with alpha-[(11)C]MTrp [18 males aged 46.6 +/- 22.2 years (range 20-80 years) and 18 females aged 33.0 +/- 15.5 years (range 20-80 years)]. The trapping rate constant, K*, was calculated with the graphical method for irreversible ligands using the sinus-venous input function. A priori selected volumes of interest (VOIs) were defined using an automatic algorithm.

Results: VOI analysis showed no correlation between age and brain regional K* values. As reported by others, significant age-related reductions of gray matter were observed in the thalamus and frontal and cingulate cortices; even with partial volume correction there was still no significant relationship between K* and age. Further exploratory SPM voxelwise correlation between age and alpha-[(11)C]MTrp trapping, using p = 0.05 (uncorrected), as well as voxel-based morphometry, was in agreement with the VOI analysis.

Conclusion: The dissociation between age-related changes in brain anatomy and this index of serotonin synthesis suggests independent mechanisms underlying the normal aging process.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Age Factors
  • Aged
  • Aged, 80 and over
  • Brain / diagnostic imaging*
  • Brain / metabolism*
  • Brain / pathology
  • Carbon Radioisotopes*
  • Female
  • Humans
  • Magnetic Resonance Imaging / methods
  • Male
  • Middle Aged
  • Models, Statistical
  • Positron-Emission Tomography / instrumentation
  • Positron-Emission Tomography / methods*
  • Serotonin / biosynthesis*
  • Tryptophan / analogs & derivatives*


  • Carbon Radioisotopes
  • Serotonin
  • tryptophan methyl ester
  • Tryptophan