Association of a critical CSF tryptophan threshold level with depressive relapse.

This work studies association between relapse during acute tryptophan depletion (ATD) and CSF level of tryptophan (TRP) in remitted depressives treated with sertraline or bupropion. Eight medication-responding depressives ingested an ATD amino acid mixture during 48-h continuous CSF sampling before and after treatment. Mood rating scores were compared with nadir levels of TRP in CSF. CSF TRP nadirs averaged 8.7% of am baselines in remitted patients. Mood relapsed whenever the CSF nadir was below 40 nmol/l TRP in remitted patients, and never when above (Fisher's exact test, P=0.029). Relapsing medication responders also showed very low preantidepressant ATD-induced nadirs. ATD-induced relapses were associated with low CSF TRP levels. Individual susceptibility to depletion may be independent of antidepressant treatment, mood state, or treatment status. Resistance to relapse may invoke an undefined, protective CNS mechanism against extremely low CSF levels of TRP during ATD.