Effects of acute dopaminergic and serotonergic manipulations in the ACI paradigm depend on the basal valence of cognitive judgement bias in rats

Recent findings have revealed that pharmacological enhancement of dopaminergic (DA) function by the administration of a DA precursor (dihydroxy-l-phenylalanine; L-DOPA), but not the selective serotonin reuptake inhibitor (SSRI) citalopram, increases an optimism bias in humans. To test whether dopamine might play a similar role in non-human animals, in the present study, we evaluated the effects of acute injections of L-DOPA, the D2 receptor antagonist haloperidol, and the SSRI escitalopram on cognitive judgement bias of rats in the ambiguous-cue interpretation (ACI) paradigm. Three different doses of each drug were administered in a fully randomised Latin-square design, along with saline treatment as a control, 30 min before the ACI tests. Initial analysis revealed that only animals treated with L-DOPA were more 'pessimistic' than the saline-treated controls. Neither haloperidol nor escitalopram significantly affected the cognitive judgement bias of rats. However, further analysis revealed that the effects of the tested compounds might depend on the basal cognitive judgement bias of the tested animals. When we divided the rats into 'optimistic' and 'pessimistic' groups based on their cognitive judgement bias in the drug-free state, it turned out that acute administration of L-DOPA caused a 'pessimistic' shift in 'optimistic' animals while showing no significant effects on 'pessimists'. Acute administration of haloperidol caused a 'pessimistic' shift in 'optimistic' animals and an 'optimistic' shift in 'pessimists'. Acute administration of escitalopram caused a 'pessimistic' shift in 'optimistic' animals and had no significant effects on 'pessimists', except that the middle tested dose rendered the rats more 'optimistic'.