physiopathological role for insulin within the brain.
Published: 2004
Language: eng
Number of pages: 316
Description: One central mechanism underlying cell death following cerebral ischemia is a loss of the balance between excitation and inhibition leading to excitotoxicity. We began by investigating whether oxygen-glucose deprivation (OGD), an ischemia-like challenge, altered cell-surface expression of type A gamma-aminobutyric acid receptors (GABAARs; which mediate fast synaptic inhibition) in cultured neurons. Cell-based ELISAs revealed that OGD produced a marked decrease within GABAAR surface expression, which was subsequently shown to result from increased clathrin-mediated endocytosis. Importantly, treating neurons with 0.5 muM insulin, which facilitated GABAAR insertion, counteracted the OGD-induced reduction in cell-surface GABA ARs. In parallel, insulin significantly protected cultured neurons against both glutamate toxicity and OGD; however, protection was absent when newly inserted receptors were rendered functionally irrelevant by the presence of the GABAAR antagonist bicuculline. Additional experiments revealed that inhibiting insulin receptor (IR) function markedly worsened cell viability following OGD, while the opposite effect was seen with an insulin sensitiser, suggesting that endogenous neural insulin signaling may protect neurons. To further examine this possibility, we characterised an  in vivo model wherein neuronal insulin signaling was compromised. Feeding Syrian golden hamsters a diet high in fructose lead to the development of a previously unrecognised state of brain insulin resistance, wherein the ability of insulin to stimulate phosphorylation of the IR and many of its downstream targets was significantly reduced. In addition to biochemical changes, fructose feeding also diminished the ability of insulin to induce long-term depression. Subsequently, acutely prepared hippocampal slices from normal diet and fructose fed (FF) animals, were challenged with hypoxia-aglycemia (an additional ischemia-like challenge), and slice viability assayed after a recovery period. Unexpectedly, the insult did not affect viability of slices prepared from FF hamsters, however, the severity of the challenge may not have been sufficient to allow a phenotype to emerge, for hamster slices were shown to be somewhat resistant to aglycemia-hypoxia. Overall, our results provide important information about the relevance of GABAAR trafficking to ischemia, and suggest the presence of a modifiable neural insulin system that is affected by insulin resistance and participates in the brain's response to ischemia.
Notes: Adviser:  Yu-Tian Wang.

Thesis (Ph.D.)--University of Toronto, 2004.

Electronic version licensed for access by U. of T. users.

Source: Dissertation Abstracts International, Volume: 65-05, Section: B, page: 2277.