Gender dimorphism in aspartame-induced impairment of spatial cognition and insulin sensitivity

Abstract

Previous studies have linked aspartame consumption to impaired retention of learned behavior in rodents. Prenatal exposure to aspartame has also been shown to impair odor-associative learning in guinea pigs; and recently, aspartame-fed hyperlipidemic zebrafish exhibited weight gain, hyperglycemia and acute swimming defects. We therefore investigated the effects of chronic lifetime exposure to aspartame, commencing in utero, on changes in blood glucose parameters, spatial learning and memory in C57BL/6J mice. Morris Water Maze (MWM) testing was used to assess learning and memory, and a random-fed insulin tolerance test was performed to assess glucose homeostasis. Pearson correlation analysis was used to investigate the associations between body characteristics and MWM performance outcome variables. At 17 weeks of age, male aspartame-fed mice exhibited weight gain, elevated fasting glucose levels and decreased insulin sensitivity compared to controls (P<0.05). Females were less affected, but had significantly raised fasting glucose levels. During spatial learning trials in the MWM (acquisition training), the escape latencies of male aspartame-fed mice were consistently higher than controls, indicative of learning impairment. Thigmotactic behavior and time spent floating directionless was increased in aspartame mice, who also spent less time searching in the target quadrant of the maze (P<0.05). Spatial learning of female aspartame-fed mice was not significantly different from controls. Reference memory during a probe test was affected in both genders, with the aspartame-fed mice spending significantly less time searching for the former location of the platform. Interestingly, the extent of visceral fat deposition correlated positively with non-spatial search strategies such as floating and thigmotaxis, and negatively with time spent in the target quadrant and swimming across the location of the escape platform. These data suggest that lifetime exposure to aspartame, commencing in utero, may affect spatial cognition and glucose homeostasis in C57BL/6J mice, particularly in males.

Figure 1. Aspartame consumption reduces insulin sensitivity of male C57BL/6J mice during a random-fed insulin tolerance test.

Values are mean±SEM of (A) male glucose levels and (B) female glucose levels, n = 18 per group, *** P<0.001 compared to controls.

Figure 2. Effect of aspartame consumption on spatial learning in C57BL/6J mice: gender-specific differences.

Acquisition curves of escape latency in male (A) and female (B) aspartame-fed and control diet mice. Percentage time spent in target quad in male (C) and female (D) mice. Mean Distance to goal in male (E) and female (F) mice. Each group consisted of 12 mice; * P<0.05, ** P<0.01, *** P<0.001.

Figure 3. Effects of aspartame consumption on non-cognitive behavior.

Percentage of time spent floating in male (A) and female (B) aspartame-fed and control diet mice. Thigmotaxis in male (C) and female (D) mice, n = 12 per group; * P<0.05, ** P<0.01, *** P<0.001.

Figure 4. Effects of diet and gender on spatial and non-spatial escape strategies during the MWM test.

(A) Orientation of entry point and escape platform on trial days 1–4. Arrows indicate location of entry point. (B) Distribution of search strategies in aspartame-fed mice compared to controls on trial days 1–4. As the acquisition training advanced, control C57BL/6J mice exhibited progressive behavioral changes from random chaining, thigmotaxis and floating into predominantly spatial strategies such as direct swim and direct search. Mice in the aspartame group exhibited less spatial strategies and more non-spatial behavior throughout the trials.

Figure 5. Results from a probe trial intended to measure spatial memory.

Percentage of time spent searching in the former location of the platform (Target Quad) in (A) males and (B) females, compared to time spent in the adjacent quads. Aspartame-fed males showed a reduction in the number of times they crossed over former location of the platform (C: Platform Crossings and (E: Annulus Crossing Index), n = 12 per group, * P<0.05, ** P<0.01, *** P<0.001. There was no significance in the reduction of platform crossings and Annulus Crossing Index in females. Actual overlapping swim paths of the control (G,I) and aspartame-fed mice (H,J). Swim paths illustrate less intense swimming in the location of the platform by aspartame-fed mice, compared to controls.