Irritability rather than depression during interferon treatment is linked to increased tryptophan catabolism

Objective: Treatment with recombinant interferon is associated with high rates of psychiatric comorbidity. We investigated the relation between catabolism of the essential amino acid tryptophan, being rate-limiting of peripheral and cerebral serotonin formation, and psychiatric symptoms in patients undergoing combination treatment with interferon-a and ribavirin. Patients and Methods: Eighteen patients with viral hepatitis C who received interferon were included. A psychiatrist screened patients before and while on interferon-a treatment for 2 months, using a structured diagnostic interview. Fasting plasma tryptophan and platelet serotonin levels were measured at each visit. Results: At baseline no evident psychopathology was observed. After 2 months of interferon treatment, 10 patients experienced increased irritability. No other structural psychopathology was observed. Decreased plasma tryptophan level correlated with the presence of irritability (p = .047). Platelet serotonin levels were found to be decreased during treatment (p = .002). Conclusions: Aggressive impulse dysregulation is highly prevalent in patients receiving interferon treatment. This is associated with decreased plasma tryptophan levels which may lead to attenuated peripheral and central serotonergic neurotransmission. Key words: interferon, tryptophan, inflammation, serotonin, irritability.