Background. Serotonergic circuits have been proposed to mediate cognitive processes, particularly learning and memory. Cognitive impairment is often seen in bipolar disorders in relation to a possible lowered serotonergic turnover.
Methods. We investigated the effects of acute tryptophan depletion (ATD) on cognitive performance in healthy first-degree relatives of bipolar patients (FH) (N = 30) and matched controls (N = 15) in a placebo-controlled, double-blind cross-over design. Performance on planning, memory and attention tasks were assessed at baseline and 5 h after ATD.
Results. Following ATD, speed of information processing on the planning task was impaired in the FH group but not in the control group. FH subjects with a bipolar disorder type I relative (FH I) showed impairments in planning and memory, independent of ATD. In all subjects, ATD impaired long-term memory performance and speed of information processing. ATD did not affect short-term memory and focused and divided attention.
Conclusions. The results suggest serotonergic vulnerability affecting frontal lobe areas in FH subjects, indicated by impaired planning. Biological vulnerability in FH I subjects is reflected in impaired planning and memory performance. In conclusion, the cognitive dysfunctions in FH subjects indicate an endophenotype constituting a possible biological marker in bipolar psychopathology. Serotonin appears to be involved in speed of information processing, verbal and visual memory and learning processes.