Figure 1. Tryptophan metabolism in MS patients.

The figure highlights one of the plausible mechanisms causing decreased levels of serotonin in MS patients. The diagram on the left represents normal tryptophan metabolism. A synthetic pathway using tryptophan hydroxylase (TPH) allows serotonin formation, while enzymes IDO and TDO degrade tryptophan into various products. The diagram on the right highlights the diversion of tryptophan into this catabolic pathway through induction of the enzyme IDO by inflammatory cytokines like IL-1, TNF-α, and IFN-γ present in high levels during inflammatory periods in MS patients

MS: multiple sclerosis; IDO: indoleamine 2,3-dioxygenase; TDO: tryptophan dioxygenase; IL-1: interleukin-1; TNF-α: tumor necrosis factor alpha; IFN-γ: interferon gamma; TPH: tryptophan hydroxylase