Increased glutamate receptor gene expression in the cerebral cortex of insulin induced hypoglycemic and streptozotocin-induced diabetic rats.

Hypoglycemia causes brain fuel deprivation, resulting in functional brain failure and brain death. It is a serious complication of insulin therapy in diabetic patients. A single intrafemoral dose of streptozotocin was administered to induce diabetes. Hypoglycemia was induced by appropriate doses of insulin s.c. in control and diabetic rats. Glutamate content and glutamate receptor kinetics were studied using [3H]glutamate. [3H]MK 801 was used to study the NMDA receptor kinetics. NMDA2B and metabotropic glutamate receptor (mGluR) 5 subunits receptor gene expressions were done using real time PCR. There was a significant (P<0.001) increase in the glutamate content in the cerebral cortex of hypoglycemic and diabetic rats when compared with control with more glutamate content in the hypoglycemic group. Scatchard analysis using [3H]glutamate and [3H]MK 801 in the cerebral cortex showed a significant (P<0.001) increase in the maximal binding (Bmax) in both hypoglycemic and diabetic rats when compared with control with no significant change in equilibrium dissociation constant. The glutamate and NMDA receptor binding parameters were significantly (P<0.001) enhanced in the hypoglycemic rats compared with hyperglycemic rats. Real time PCR analysis also showed a significant increase (P<0.001) in the gene expression of NMDA2B and mGluR5 subunits of glutamate receptor. This increased gene expression of NMDA2B and mGluR5 glutamate receptor subunits confirmed the enhanced mRNA of receptor subunits and subsequently at the protein level from the receptor kinetic studies. The enhanced glutamate receptors were more prominent in hypoglycemic group which is of significance in this study. Up-regulation of glutamate leads to Ca2+ overload in cells, potentially leading to cell damage and death. This functional damage during hypoglycemia is suggested to contribute to cognitive and memory deficits which has immense clinical relevance in the therapeutic management of diabetes.