Diabetic encephalopathy (DE), one of the most prevalent chronic complications of diabetes mellitus, is short of effective prevention and formidable therapeutic strategies. The aim of the present study is to reveal the imbalance of tryptophan (Trp) and its metabolites in streptozotocin (STZ)-induced experimental DE rats to underscore their critical values in clinical diagnosis of the disease.For this purpose, we first developed an accurate and appropriate simultaneous method for measuring Trp and its metabolites using liquid chromatography-tandem mass spectrometry, which was in accordance with the requirements of biological sample analysis. Secondly, a single STZ intraperitoneal injection was administered to male Sprague-Dawley rats, and their cognitive function was detected by Morris water maze tests. Cerebrospinal fluid (CSF), serum, and brain tissue were then collected for the determination of Trp and its metabolites. Compared with age-matched control rats, the levels of neuroprotective serotonin decreased significantly in the samples of cortices, hippocampi, striatum, CSF, and serums in the STZ-induced DE rats, while the levels of neurotoxic 3-hydroxykynurenine increased significantly. Moreover, analogous changes of both compounds were found in the central nervous system and peripheral blood of the STZ-induced DE rats. In conclusion, we established a quantitative method for the simultaneous detection of Trp and its metabolites, and we also present a critical elucidation of the nervous system dysfunction in DE.

Diabetic encephalopathy (DE) is one of the most prevalent chronic complications of diabetes mellitus (DM), with neither effective prevention nor proven therapeutic regimen. This study aims to uncover the potential dysregulation pattern of the neurotransmitters in a rat model of streptozotocin (STZ)-induced experimental DE. For that purpose, male Sprague-Dawley (SD) rats were treated with a single intraperitoneal injection of STZ. Cognitive performance was detected with the Morris water maze (MWM) test. Serum, cerebrospinal fluid (CSF), and brain tissues were collected to measure the levels of neurotransmitters. Compared with the control rats, the acetylcholine (ACh) levels in serum, CSF, hippocampus, and cortex were all significantly down-regulated as early as 6 weeks in the STZ treatment group. In contrast, the glutamate (Glu) levels were decreased in CSF and the hippocampus, but unaffected in the serum and cortex of STZ-treated rats. As for γ-aminobutyric acid (GABA), it was down-regulated in serum, but up-regulated in CSF, hippocampus, and the cortex in the STZ-treated group. The mRNA expressions of neurotransmitter-related rate limiting enzymes (including AChE, GAD1, and GAD2) and pro-inflammatory cytokines (including IL-1β and TNF-α) were all increased in the DE rats. Our data suggest that DM induces isoform-dependent and tissue-specific neurotransmitter abnormalities, and that neuroinflammation may underlay the nervous system dysfunction observed in the progression of DE.