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The effects of acute tryptophan depletion on affective behaviour and cognition in Brown Norway and Sprague Dawley rats

Previous studies in rats and humans have shown that the essential amino acid tryptophan (TRP) is depleted after consumption of a gelatin-based protein-carbohydrate mixture, which is lacking L-tryptophan (TRP−). In rats, TRP depletion caused impaired object recognition but only had a modest effect on affective behaviour. Because these studies were preformed with Wistar rats, the aim of the present experiment was to evaluate strain differences in behavioural responses to acute TRP depletion between Brown Norway (BN) and Sprague Dawley (SD) rats. The rats were repeatedly treated with TRP− or a balanced control (TRP+) and were tested in tests of anxiety- and depression-related behaviour (open-field test, home cage emergence test, social interaction test, forced swim test) and memory. SD rats, but not BNs, showed more anxiety- and depression-related behaviour and impaired object recognition after TRP− treatment. There was a dissociation between plasma TRP levels, central 5-HT concentrations and 5-HIAA/5-HT turnover. Both strains showed about 60% decrease in plasma TRP/ΣLNAA levels, whereas hippocampal 5-HT levels were lower after TRP− in BN but not SD rats. Conversely, 5-HIAA/5-HT turnover was lower after TRP− in SD but not BN rats, suggesting a dissociation between 5-HT storage and release in SDs. The present study suggests that acute tryptophan depletion effects are strain dependent on the behavioural and the neurochemical level.