Insulin Protects Cortical Neurons Against Glutamate Excitotoxicity

Abstract

Glutamate excitotoxicity is implicated in the pathogenesis of numerous diseases, such as stroke, traumatic brain injury, and Alzheimer’s disease, for which insulin resistance is a concomitant condition, and intranasal insulin treatment is believed to be a promising therapy. Excitotoxicity is initiated primarily by the sustained stimulation of ionotropic glutamate receptors and leads to a rise in intracellular Ca2+ ([Ca2+]i), followed by a cascade of intracellular events, such as delayed calcium deregulation (DCD), mitochondrial depolarization, adenosine triphosphate (ATP) depletion that collectively end in cell death. Therefore, cross-talk between insulin and glutamate signaling in excitotoxicity is of particular interest for research. In the present study, we investigated the effects of short-term insulin exposure on the dynamics of [Ca2+]i and mitochondrial potential in cultured rat cortical neurons during glutamate excitotoxicity. We found that insulin ameliorated the glutamate-evoked rise of [Ca2+]i and prevented the onset of DCD, the postulated point-of-no-return in excitotoxicity. Additionally, insulin significantly improved the glutamate-induced drop in mitochondrial potential, ATP depletion, and depletion of brain-derived neurotrophic factor (BDNF), which is a critical neuroprotector in excitotoxicity. Also, insulin improved oxygen consumption rates, maximal respiration, and spare respiratory capacity in neurons exposed to glutamate, as well as the viability of cells in the MTT assay. In conclusion, the short-term insulin exposure in our experiments was evidently a protective treatment against excitotoxicity, in a sharp contrast to chronic insulin exposure causal to neuronal insulin resistance, the adverse factor in excitotoxicity.

Excitotoxic glutamate causes neuronal insulin resistance by inhibiting insulin receptor/Akt/mTOR pathway

“…These results are in complete agreement with our previous findings that mitochondria control IR autophosphorylation in neurons and that mitochondrial depolarisation causes the loss of insulin sensitivity during the IR autophosphorylation stage [9, 14]. Recently we showed that pre-treatment with insulin prevents the glutamate-evoked increases in [Ca 2+ ] i and decreases in ΔΨ m , protecting rat cortical neurons against excitotoxicity [15]. The glutamate effect and the protective effects of insulin were both completely abrogated by MK 801, an inhibitor of Ca 2+ influx, via the N-methyl-D-aspartate (NMDA) receptor and the plasmalemmal Na + /Ca 2+ exchanger operating in reverse mode [15].…”

Role of Insulin in Neurotrauma and Neurodegeneration: A Review

“…Using primary rat cortical neurons, researchers demonstrated that pretreatment with insulin was able to prevent oxidative stress-induced impairment of glucose accumulation and metabolism, which was inflicted by the addition of ascorbic acid and FeSO 4 (Duarte et al, 2006). Further, using the glutamate excitotoxicity model described above, cortical neurons pretreated with 100 nM of insulin for 5 min before glutamate stimulation showed a reversal of glutamate induced increases in mitochondrial membrane depolarization, increased intracellular calcium and decreased the number of cells displaying damage (Krasil'nikova et al, 2019).…”

Clinical Evidence of Antidepressant Effects of Insulin and Anti-Hyperglycemic Agents and Implications for the Pathophysiology of Depression—A Literature Review

“…The discrepancy among these results may be due to differences in experimental conditions. Moreover, as suggested by the result from Datusalia et al [89] which showed that long-lasting hyper-insulinemia increases glutamate excitotoxicity in cortical neurons of insulin-resistant rats, the development of insulin resistance, caused by prolonged insulin exposure, may mask insulin effects [85]. Concurrently, a recent study showed that the short-term insulin exposure can improve neuroprotection against excitotoxicity and prevent glutamate mediated excitotoxicity whereas chronic insulin exposure results in neuronal insulin resistance and worsen excitotoxicity [85].…”

“…Moreover, as suggested by the result from Datusalia et al [89] which showed that long-lasting hyper-insulinemia increases glutamate excitotoxicity in cortical neurons of insulin-resistant rats, the development of insulin resistance, caused by prolonged insulin exposure, may mask insulin effects [85]. Concurrently, a recent study showed that the short-term insulin exposure can improve neuroprotection against excitotoxicity and prevent glutamate mediated excitotoxicity whereas chronic insulin exposure results in neuronal insulin resistance and worsen excitotoxicity [85]. Although the exact molecular link between insulin resistance and glutamate excitotoxicity remains elusive, it has been suggested that the excitotoxic glutamate may induce the development of neuronal insulin resistance via inhibiting the IR/Akt/mTOR pathways [81].…”

“…Furthermore, recent evidences suggested that cross-talk between insulin and glutamate signaling in excitotoxicity may play important roles in the brain insulin resistance [81]. Insulin modulates currents of the N-methyl-d-aspartate (NMDA) receptor in a dose-, time-, and NMDA subunit-specific manner through the IGF-1 receptor signaling pathway [82,83] and increases the number of functional NMDA receptors in the cell membrane [84,85]. It has been reported that insulin increases the vulnerability of rat cortical neurons to the excitotoxic effects of glutamate, thereby contributing to cell death [86,87].…”