Diabetes-Related Changes in L-Tryptophan–Induced Release of Striatal Biogenic Amines

In vivo voltammetry was used to measure the synaptic release of rat striatal dopamine and serotonin after the administration of the amino acid L-tryptophan to streptozocin-induced diabetic rats. Dopamine and serotonin release from rat striatum was studied at a short-term or acute (3-day) interval and a long-term or chronic (3- to 7-wk) interval after the induction of diabetes. The study was also done in age-, sex-, and food-matched controls. The findings show that L-tryptophan decreased dopamine release from rat striatum in nondiabetic rats. The decreased striatal dopamine release, after L-tryptophan administration, was exacerbated in acutely diabetic rats and further exacerbated in chronically diabetic rats. By contrast, rat striatal serotonin release predictably increased after L-tryptophan injection in nondiabetic rats. A further increased striatal serotonin release was seen in acutely diabetic rats. Chronically diabetic rats, however, responded to L-tryptophan with a dramatic and significant decrease in striatal serotonin release. The results show that in acutely diabetic and normal rats, L-tryptophan administration reduced striatal dopamine and increased striatal serotonin release, whereas in chronically diabetic rats, the release of both biogenic amines was decreased. The findings indicate that the progression of diabetes is associated with an impaired ability to release primary neurotransmitter biogenic amines.