Tryptophan tolerance and metabolism in endogenous depression

Area of Science:

• Biochemistry
• Neuroscience
• Psychiatry

Background:

• Tryptophan metabolism is crucial for neurotransmitter synthesis.
• Altered tryptophan-kynurenine pathways are implicated in depression.
• L-tryptophan is a potential therapeutic agent for depression.

Purpose of the Study:

• To investigate L-tryptophan and kynurenine plasma levels in depressed patients.
• To assess the relationship between L-tryptophan loading and kynurenine levels.
• To determine if these levels predict therapeutic response to L-tryptophan.

Main Methods:

• Oral L-tryptophan loading (100 mg/kg) in 49 female depressives and 26 controls.
• Measurement of free and total tryptophan and kynurenine plasma concentrations.
• Analysis of plasma concentration-time curves and correlation with treatment response.

Main Results:

• No significant differences in plasma tryptophan or kynurenine levels between depressives and controls post-loading.
• Kynurenine peak concentration at 4 hours correlated with its area under the curve.
• No significant correlation found between L-tryptophan dose and kynurenine levels, nor between plasma levels and treatment outcomes.

Conclusions:

• L-tryptophan absorption, clearance, and kynurenine degradation appear normal in depressed individuals.
• Plasma tryptophan and kynurenine levels at 4 hours are poor predictors of L-tryptophan treatment efficacy in depression.