Streptozocin-induced diabetic (STZ-D) mice have reduced brain concentrations of tryptophan, a precursor substance for 5-hydroxytryptamine, and show lengthened immobility in Porsolt's swim test, a putative animal model of depression. This study investigated whether tryptophan affects behavior in Porsolt's swim test in STZ-administered male National Institutes of Health Swiss mice. In addition, the effect of tryptophan on behavior in the resident-intruder test of aggression was studied. Tryptophan is effective in the treatment of mild depression and may reduce aggressive behavior. Diabetes was induced with injection of 200 mg/kg body wt i.p. STZ. Two weeks after STZ treatment, the mice received 0, 50, and 100 mg/kg i.p. tryptophan 60 min before the swim test. The STZ-administered mice exhibited lengthened immobility in the swim test, and tryptophan caused a dose-related shortening in their immobility times. The control and STZ mice, which were isolated for 1 wk before the resident-intruder test, did not show any difference in the time spent in social investigation or aggressive or defensive behaviors. However, 100 mg/kg i.p. tryptophan 60 min before the test reduced the social interaction and aggressive behavior of the STZ-D mice but increased these behaviors in controls. Results indicate that tryptophan shortens the increased immobility time and reduces social and aggressive behavior in STZ-D mice. Therefore, the reported reductions in the brain-tryptophan concentrations in STZ-D mice may participate in regulating their behavior.

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