The effects of insulin and/or tryptophan (Try) administration on Try, serotonin (5-HT), 5-hydroxyindolacetic acid (5-HIAA) in raphe nuclei and hypothalamus and serum levels of large neutral amino acids [Try, valine, isoleucine, leucine, tyrosine and phenylalanine, (LNAA)] was investigated in diabetic rats. 2. Alloxan diabetic animals (15 days) were treated up to 45 days with L-Try (50 mg/kg i.p., daily) and/or insulin (4 I.U./rat, daily). 3. In saline treated diabetic rats a significant decrease in Try and significant increase in 5-HT concentrations was observed in the raphe nuclei and hypothalamus, while the values of 5-HIAA were significantly increased only in the raphe nuclei as compared to the nondiabetic, saline treated control rats. 4. The administration of Try in combination with insulin to diabetic rats altered the levels of brain Try, 5-HT, 5-HIAA and serum concentrations of valine, leucine and isoleucine towards the values in the control (nondiabetic) group. 5. Our results suggest that simultaneous use of Try and insulin has positive effect on the alterations in Try and 5-HT metabolism, that occurred in long-term alloxan diabetic rats despite insulin application.
