Effect of high-fat diet on metabolic indices, cognition, and neuronal physiology in aging F344 rats

Abstract

The prevalence of obesity and type 2 diabetes increases with age. Despite this, few studies have examined these conditions simultaneously in aged animals, and fewer studies have measured the impact of these conditions on brain function. Using an established animal model of brain aging (F344 rats), we investigated whether a high-fat diet (HFD) exacerbates cognitive decline and the hippocampal calcium-dependent afterhyperpolarization (a marker of age-dependent calcium dysregulation). Young and mid-aged animals were maintained on control or HFD for 4.5 months, and peripheral metabolic variables, cognitive function, and electrophysiological responses to insulin in the hippocampus were measured. HFD increased lipid accumulation in the periphery, although overt diabetes did not develop, nor were spatial learning and memory altered. Hippocampal adiponectin levels were reduced in aging animals but were unaffected by HFD. For the first time, however, we show that the AHP is sensitive to insulin, and that this sensitivity is reduced by HFD. Interestingly, although peripheral glucose regulation was relatively insensitive to HFD, the brain appeared to show greater sensitivity to HFD in F344 rats.

Figure 1. Body weight and food consumption across 23 weeks of HFD

A shows mean body weights across weeks of treatment. B shows amounts of food consumed (averaged on a weekly basis). Data analysis includes the period from 4–23 weeks during which the HFD was in effect. Mean ± SEM are shown. Asterisks indicate significant difference @ p < 0.05 level.

Figure 2. Glucose and insulin tolerance tests over a 2 hour period using a single dose approach

A shows mean glucose levels during both GTT (A1) and ITT (A2) prior to initiation of the high fat diet (HFD). Similar results during GTT (B1) and ITT (B2) and taken after 19 weeks of the HFD are shown in B. Pound sign shows a significant effect of diet on area-under-the-curve (AUC) data (see Table 2). A main effect of age was noted at 30 min following insulin injection (B2; asterisk sign), indicating greater insulin sensitivity with age. Mean ± SEM are shown. Significant differences between age and diet groups was tested at the @ p < 0.05 level.

Figure 3. Correlations across peripheral lipid index (PLI), spatial learning and memory index (SLMI), and adiponectin levels

A for each animal, an index was created from cholesterol levels, HDL: LDL, and retroperitoneal fat weight (higher number is associated with greater dyslipidemia). Lower (better) scores on the PLI are seen in younger animals on control diet (CD). Main effects of both diet and age were seen and clearly separate the animal groups. Asterisks represent significant differences between treatment groups @ p < 0.05 level. B shows the correlation between the PLI and the SLMI for all animals in the study. Ninety-five percent confidence intervals are shown along with trendline and results of the analysis. C shows a similar analysis between ranked adiponectin staining levels and the SLMI for twenty-four animals from which both data sets were available. Ninety-five percent confidence intervals are shown along with trendline and results of the analysis.

Figure 4. Morris water maze learning and memory task

A day 1 was cue day. Days 4, 5, and 6 were training days, followed by a probe on day 7. Day 8 was reversal training, followed 72 h later by a reversal probe on day 11. B Based on the cumulative search index, a significant age-dependent decrease in the learning phase of the task (training days 1–6) was seen in control diet (CD) animals. C The cumulative index of the probe trial is shown. As shown in A and C, HFD had no impact on learning and memory performance. Means ± SEM are shown. * Significant aging differences @ p < 0.05.

Figure 5. Insulin’s acute effects on the afterhyperpolarization (AHP)

A shows a representative AHP recorded in a young animal on the control diet (CD) before (black) and after (red) insulin administration. Action potentials are truncated to emphasize the AHP. B shows a representative AHP from a mid-aged CD animal before (black) and after (red) insulin administration. C shows the average medium AHP (mAHP) and reveals a significant aging effect, but no diet effect. D and E show the degree of insulin-mediated AHP reduction, and hence, insulin sensitivity in CD and HFD animals (respectively). While insulin reduced the sAHP in both age groups, the degree of inhibition appeared greater in older animals on CD (D). HFD was able to reduce insulin sensitivity in older animals (E). Asterisks indicate significant difference @ p < 0.05 level.

Figure 6. Insulin receptor and insulin receptor substrate-1 expression and adiponectin immunostaining

A shows results of Western blot analyses on hippocampal insulin receptor substrate-1 (IRS-1; 175 kDa) and insulin receptor β subunit (IR-β; 95 kDa) separated into membrane and cytosolic fractions. Data represent means ± SEM. B shows semiquantitative mean density measures of immunostained brain sections for IR-α, IRS-1 and adiponectin in stratum pyramidale of area CA1. Asterisks indicate significant difference @ p < 0.05 level. C shows representative photomicrographs of two adiponectin immunostained sections with different magnifications of the hippocampus area, obtained from young-adult (left panel) and mid-aged animals (right panel). While no diet effect was seen, note the significant age-dependent reduction in immunostained neurons in stratum pyramidale.