The Effect of Intrahippocampal Insulin Infusion on Spatial Cognitive Function and Markers of Neuroinflammation in Diet-induced Obesity

Abstract

Obesity and high fat diet consumption contribute to the development of metabolic disorders, insulin resistance, neuroinflammation, and cognitive impairments. CNS administration of insulin into the brain can attenuate these cognitive impairments. The present study investigated whether hippocampal-dependent spatial memory impairments in a dietary induced mouse model of obesity could be improved by the direct administration of insulin into the hippocampus and whether this was associated with markers of hippocampal inflammation. C57Bl/6J mice consumed a low fat or high fat diet for 16 weeks and continuous intrahippocampal saline or insulin infusion for the final 4 weeks, during a period of behavioral testing, before gene expression analysis was performed. The high fat diet group demonstrated poorer spatial memory performance in the Morris water maze and Y-maze, supporting the hypothesis that high fat diet leads to hippocampal dependent cognitive impairment. Insulin infusion into the hippocampus reversed the deficit of high fat diet consumption on both of the tasks. Increased expression of inflammatory markers was detected in the hippocampus in the high fat diet group and expression of these markers was ameliorated in insulin infused mice. This demonstrates that CNS insulin can improve hippocampal-dependent memory and that hippocampal inflammation may be a factor in the development of cognitive deficits associated with diet-induced obesity. Furthermore, these data suggest that insulin may act to attenuate high fat diet induced cognitive deficits by reducing neuroinflammation.

Keywords: diet induced obesity; high fat diet; hippocampus; insulin; spatial cognition.

Figure 1

(A) Representation of the Morris Water Maze (MWM) apparatus, which was divided virtually into four quadrants. (a), fixed distal cues; (b), virtual maze quadrants; (c), platform. N = north (B) Representation of the Y-maze apparatus which consists of three equal arms and three separate fixed distal cues.

Figure 2

Body weight and energy intake. (A) After 1 week of high fat diet (HFD) feeding, mice had elevated body weight relative to low fat diet (LFD) controls, this was maintained throughout the study period. (B) Energy intake was elevated in HFD relative to LFD controls prior to surgery. (C) Body weights remained lower in LFD than HFD fed animals following surgery regardless of insulin infusion (+Ins). (D) Energy intake following surgery was greater in HFD than LFD fed animals. Values are expressed as mean ± SEM. *p < 0.05.

Figure 3

Peripheral tissue weights. (A) High fat diet (HFD) was associated with greater epididymal fat and (B) liver mass, but not (C) extensor digitorum longus muscle and (D) kidney mass compared with low fat diet (LFD) controls. Intrahippocampal insulin infusion (+Ins) did not affect any tissue weights. Values are expressed as mean + SEM. *p < 0.05.

Figure 4

Blood glucose, insulin and leptin. (A) High fat diet (HFD) produced glucose intolerance compared to low fat diet (LFD) controls. (B) However, plasma insulin levels were not significantly different between HFD and LFD at baseline or 15 min after glucose injection. (C) HFD caused insulin insensitivity compared to LFD controls, with a smaller reduction in blood glucose after insulin injection. (D) HFD mice had higher plasma leptin levels than LFD controls. Glucose tolerance, insulin sensitivity, and plasma leptin were not influenced by intrahippocampal insulin infusion (+Ins). Values are expressed as mean ± SEM. *p < 0.05.

Figure 5

MWM performance. Mean escape latencies were collected for each trial day to assess performance over time. Time spent in the target quadrant was measured to assess hippocampal-dependent memory. (A) Mean escape latencies did not differ between groups on Day 1 of visible platform training. (B) All groups reached the platform faster across the hidden platform days. (C) All groups swam similar path lengths during the probe trial. (D) High fat diet (HFD) mice spent less time in the target quadrant compared with low fat diet (LFD) controls. Insulin infusion (+Ins) improved performance in both dietary groups compared to saline infused controls, with insulin infused mice spending more time in the target quadrant. Values are expressed as mean ± SEM. * = interaction effect, p < 0.05. ⁺ = main effect of time, p < 0.05.

Figure 6

Y-Maze performance. Total arm entries were recorded to compare physical abilities. Novel arm entries, time spent in the novel arm, and latency to reach the novel arm were recorded as measures of hippocampal-dependent memory. (A) High fat diet (HFD) mice spent less time exploring the novel compared to low fat diet (LFD) controls. HFD intrahippocampal insulin infused (HFD+Ins) mice spent more time in the novel arm than HFD saline mice. (B) HFD mice took longer to initially enter the novel arm compared to LFD controls. HFD+Ins mice reached the novel arm faster than HFD saline mice. (C) Total novel arm and (D) Total arm entries did not differ between groups. Values are expressed as mean + SEM. *p < 0.05.

Figure 7

Gene expression of cytokines. Interleukin (IL)-1β, IL-6, IL-10, tumor necrosis factor (TNF)-α, nuclear factor (NF)κ-B1, and monocyte chemoattractant protein-1 (MCP-1) mRNA were measured in the hippocampus. (A) Expression of IL-1β mRNA was higher in high fat diet (HFD) mice compared to low fat diet (LFD) controls. HFD intrahippocampal insulin infused (HFD+Ins) mice expressed less IL-1β mRNA compared with HFD saline infused mice. Both LFD and HFD mice expressed similar levels of IL-6 (B) and IL-10 (C) mRNA; these levels were not affected by insulin infusion. (D) Expression of TNF-α mRNA was higher in HFD mice compared to LFD controls and HFD+Ins mice expressed less TNF-α compared to HFD saline infused mice. (E) Expression of NF-κB1 mRNA was comparable among all groups as was (F) MCP-1 expression. Values are expressed as mean + SEM. *p < 0.05.

Figure 8

Expression of growth factors. Hippocampal brain-derived neurotrophic factor (BDNF) and insulin-like growth factor-1 (IGF-1) mRNA were measured in animals after sacrifice. Expression of both BDNF (A) and IGF-1 (B) were comparable among all groups. Values are expressed as mean + SEM.