Serotonin-immune interactions in elderly volunteers and in patients with Alzheimer's disease (DAT): lower plasma tryptophan availability to the brain in the elderly and increased serum interleukin-6 in DAT

Aging (Milano). 1998 Aug;10(4):316-23. doi: 10.1007/BF03339794.

Abstract

The aims of this study were to examine the plasma availability of tryptophan, the precursor of 5-hydroxytryptamine (5-HT), and serum cytokines, such as interleukin-6 (IL-6) and IL-8, in normal elderly volunteers and in patients with Alzheimer's disease (DAT). Elderly normal volunteers (mean age = 78.3 +/- 5.7 years) had a significantly lower tryptophan/competing amino acids (valine + leucine + isoleucine + phenylalanine + tyrosine) ratio than younger subjects (mean age = 32.9 +/- 8.1 years). In normal volunteers, there were significant and inverse relationships between age and either plasma tryptophan or the tryptophan/competing amino acids ratio, and between the availability of tryptophan to the brain and serum IL-6 or IL-8. DAT patients had significantly higher serum IL-6, but not IL-8, than age-matched normal volunteers. There were no significant differences in the availability of tryptophan to the brain between DAT patients and age-matched normal volunteers. The results suggest that: 1) in normal humans, the availability of plasma tryptophan to the brain decreases with age, and with activation of the immune system; and 2) increased production of IL-6 may play a role in the pathogenesis of DAT.

MeSH terms

  • Adult
  • Aged
  • Aged, 80 and over
  • Aging / blood
  • Aging / immunology*
  • Aging / metabolism*
  • Alzheimer Disease / blood
  • Alzheimer Disease / immunology*
  • Alzheimer Disease / metabolism*
  • Brain / metabolism*
  • Case-Control Studies
  • Female
  • Humans
  • Interleukin-6 / blood*
  • Interleukin-8 / blood
  • Male
  • Middle Aged
  • Serotonin / metabolism*
  • Tryptophan / blood*

Substances

  • Interleukin-6
  • Interleukin-8
  • Serotonin
  • Tryptophan