Background: Previous studies suggest that brain serotonin neurotransmission may mediate the actions of lithium carbonate. Acute tryptophan depletion (ATD) reduces brain serotonin and allows the study of this neurotransmitter in patient groups. Serotonin modulates electroencephalographic (EEG) activity, which is abnormal in bipolar disorder, and EEG abnormalities persist in euthymic bipolar patients. The EEG may therefore be a sensitive marker of 5-HT function in bipolar disorder.
Aims: This study examined the effects of ATD on mood, suicidal ideation and EEG activity in bipolar patients who were symptomatically stable on lithium.
Methods: 19 subjects satisfying DSM-IV criteria for bipolar I disorder participated in a within-subject, double-blind, placebo-controlled random-order crossover study. Following acute tryptophan depletion (induced by a 100g amino acid drink following an overnight fast) symptoms were evaluated, quantitative power spectrum brain mapping and measurement of auditory evoked potentials were carried out.
Results: ATD produced a significant fall in the amplitude of N1P2 and P300 components of the auditory evoked potential, but no significant changes in the power spectrum. There was an 83% reduction in plasma tryptophan (p<0.05, paired t-test) after the depleting but not the control drink. No significant changes in mood or suicidally scores were recorded after ATD.
Conclusions: ATD attenuates auditory evoked potentials in bipolar disorder but does not reverse lithium's effects on mood and suicidally in bipolar disorder.