Anxiogenic and depressive-like effects, but no cognitive deficits, after repeated moderate tryptophan depletion in the rat

The tryptophan (TRP) depletion method has been used as a tool to investigate the effects of acute lowered serotonin levels in the brain. In the present study, the effects of this treatment were investigated in rat models of anxiety (open field test, home cage emergence test), depression (forced swimming test, sucrose preference test) and cognition (spatial discrimination learning, sustained attention). It was found that the repeated TRP depletion increased anxiety-related behaviour in the open field test and increased immobility in the forced swimming test. The other behavioural tests did not reveal effects of treatment. TRP levels were decreased in plasma (34%) and hippocampus (33%) but not in the cortex. Stress-induced corticosterone levels were not affected after TRP depletion. The present findings indicate that repeated moderate TRP depletion leads to anxiogenic and depressive-like behaviour in the rat and corroborates the notion of the involvement of serotonin in these behaviours.