Cognitive deficits and Alzheimer-like neuropathological impairments during adolescence in a rat model of type 2 diabetes mellitus

Abstract

Numerous studies have shown that many patients who suffer from type 2 diabetes mellitus exhibit cognitive dysfunction and neuronal synaptic impairments. Therefore, growing evidence suggests that type 2 diabetes mellitus has a close relationship with occurrence and progression of neurodegeneration and neural impairment in Alzheimer's disease. However, the relationship between metabolic disorders caused by type 2 diabetes mellitus and neurodegeneration and neural impairments in Alzheimer's disease is still not fully determined. Thus, in this study, we replicated a type 2 diabetic animal model by subcutaneous injection of newborn Sprague-Dawley rats with monosodium glutamate during the neonatal period. At 3 months old, the Barnes maze assay was performed to evaluate spatial memory function. Microelectrodes were used to measure electrophysiological function in the hippocampal CA1 region. Western blot assay was used to determine expression levels of glutamate ionotropic receptor NMDA type subunit 2A (GluN2A) and GluN2B in the hippocampus. Enzyme-linked immunosorbent assay was used to determine levels of interleukin-1β, tumor necrosis factor α, and interleukin-6 in the hippocampus and cerebral cortex, as well as hippocampal amyloid beta (Aβ)_1-40 and Aβ_1-42 levels. Our results showed that in the rat model of type 2 diabetes mellitus caused by monosodium glutamate exposure during the neonatal period, latency was prolonged and the number of errors increased in the Barnes maze. Further, latency was increased and time in the escape platform quadrant shortened. Number of times crossing the platform was also reduced in the Morris water maze. After high frequency stimulation of the hippocampus, synaptic transmission was inhibited, expression of GluN2A and GluN2B were decreased in the hippocampus, expression of interleukin 1β, interleukin 6, and tumor necrosis factor α was increased in the hippocampus and cortex, and levels of Aβ_1-40 and Aβ_1-42 were increased in the hippocampus. These findings confirm that type 2 diabetes mellitus induced by neonatal monosodium glutamate exposure results in Alzheimer-like neuropathological changes and further causes cognitive deficits and neurodegeneration in young adulthood.

Keywords: Alzheimer's disease; cognitive deficits; hyperglycemia; hyperinsulinemia; insulin resistance; monosodium glutamate; neonatal period; nerve regeneration; neural regeneration; type 2 diabetes mellitus.

Figure 1

Effect of MSG exposure during the neonatal period on levels of FBG, FINS, and ISI value in 3-month-old rats. (A) Experimental process. (B−D) Levels of FBG and FINS and ISI value in control and MSG rats, respectively. Data are expressed as the mean ± SD (n = 10 per group). ***P < 0.001, vs. control group (independent-sample t-test). BM: Barn maze assay; MWM: Morris water maze assay; FBG: fasting blood glucose; FINS: fasting insulin; ISI: insulin sensitivity index; Con: control; MSG: monosodium glutamate; P: postnatal day; M: months.

Figure 2

Effect of MSG exposure during the neonatal period on learning and spatial memory in 3-month-old rats in the Barnes maze. (A) Latency to enter the escape box during training. (B, C) Primary latency to the target hole (B) and number of errors (C) during probe trials. Data are expressed as the mean ± SD (n = 10 per group). **P < 0.01, ***P < 0.001, vs. control group (independent-sample t-test). Con: Control; MSG: monosodium glutamate; S: second(s).

Figure 3

Effect of MSG exposure during the neonatal period on learning and spatial memory in 3-month-old rats using the Morris water maze. (A) Time taken to reach the escape platform during training. (B) Swimming distance within the third quadrant. (C) Representative swim-paths in the probe trial on day 6 (when the escape platform was removed). (D) Number of times that rats crossed the target region within 1 minute on day 6. Data are expressed as the mean ± SD (n = 10 per group). **P < 0.01, ***P < 0.001, vs. control group (independent-sample t-test). Con: Control; MSG: monosodium glutamate.

Figure 4

Effect of MSG exposure during the neonatal period on neuronal synaptic plasticity of the hippocampus of 3-month-old rats. (A) Input-output curve of fEPSP in CA3–CA1 regions, normalized by fEPSP amplitude induced by minimum stimulation intensity. (B) Slope of fEPSP after HFS, normalized by baseline. The arrow indicates onset of HFS. Traces reflect average fEPSPs from five sweeps before (thin) and after (thick) LTP induction. (C) Quantitative analyses for fEPSPs measured 60–180 minutes after HFS relative to baseline. (D, E) Expression of GluN2A and GluN2B in hippocampal extracts of control and MSG rats by western blot assay (D) and quantitative analyses (E). Data are expressed as the mean ± SD (n = 5 per group). *P < 0.05, **P < 0.01, ***P < 0.001, vs. control group (independent-sample t-test). fEPSP: Field excitatory postsynaptic potentials; HFS: high frequency stimulation; LTP: long-term potentiation; GluN2A: glutamate ionotropic receptor NMDA type subunit 2A; GluN2B: glutamate ionotropic receptor NMDA type subunit 2B; DM1A: alpha-tubulin; Con: control; MSG: monosodium glutamate.

Figure 5

Effect of MSG exposure during the neonatal period on expression of inflammatory cytokines in the hippocampus and cortex of 3-month-old rats. (A−C) Expression levels of IL-1β (A), IL-6 (B), and TNF-α (C) in cortical extracts of contro of control and MSG rats by enzyme linked immunosorbent assay. (D−F) Expression levels of IL-1β (D), IL-6 (E), and TNF-α (F) in hippocampal extracts l and MSG rats by enzyme-linked immunosorbent assay. Data are expressed as the mean ± SD (n = 3 per group). *P < 0.05, **P < 0.01, vs. control group (independent-sample t-test). IL-1β: Interleukin-1β; IL-6: interleukin-6; TNF-α: tumor necrotic factor-α; Con: control; MSG: monosodium glutamate.

Figure 6

Effect of MSG exposure during the neonatal period on hippocampal concentration of Aβ in 3-month-old rats. (A, B) Hippocampal concentration of Aβ_1–40 and Aβ_1–42 in control and MSG rats by enzyme-linked immunosorbent assay, respectively. Data are expressed as the mean ± SD (n = 5 per group). *P < 0.05, vs. control group (independent-sample t-test). Aβ_1–40: Amyloid beta 1–40; Aβ_1–42: amyloid beta 1–42; Con: control; MSG: monosodium glutamate.