Synergistic Effects of Obesity and Hyperglycemia on Hippocampal Neurodegenerative Decline Disrupt the Neural Circuitry Regulating Motivation in Zucker Diabetic Fatty Rats

TL;DR

This study shows that obesity and high blood sugar together cause early brain changes in rats, leading to memory and movement problems.

Contribution

The study reveals the synergistic impact of obesity and hyperglycemia on hippocampal neurodegeneration and cognitive decline in a rat model.

Findings

Obesity and hyperglycemia together cause significant memory loss and hippocampal neuron loss in rats.

Hyperglycemia worsens recognition memory and increases oxidative stress in the hippocampus.

Obesity alone reduces motor activity and hippocampal neuronal density.

Abstract

Background/Objectives: Type 2 diabetes (T2D) and obesity are chronic metabolic disorders associated with cognitive impairment and neuronal damage. The hippocampus, a region sensitive to nutrient excess, is critical for integrating sensory and metabolic signals. This study aimed to determine the early onset of cognitive and motor deficits induced by obesity and/or hyperglycemia and to characterize associated hippocampal alterations in Zucker Diabetic Fatty (ZDF) rats. Methods: Male ZDF rats (13 weeks old) were categorized into three groups: lean control, obese normoglycemic (ZDF-NG), and obese hyperglycemic (ZDF-HG). Assessments included zoometric parameters (weight and adiposity), biochemical assays (glucose tolerance, insulin response, and lipid profile), and behavioral tests (Open Field and Novel Object Recognition). Hippocampal health was evaluated through stereological neuronal density analysis and redox balance markers. Results: Both obese groups exhibited significant visceral adiposity and hyperlipidemia. The ZDF-HG group was further characterized by glucose intolerance, hepatic insulin resistance, and reduced β-cell function. Behavioral results showed that while obesity decreased motor activity, hyperglycemia significantly exacerbated the loss of both short- and long-term recognition memory. Histologically, obesity was associated with decreased neuronal density in the hippocampal DG and CA1 regions. Furthermore, hippocampal ROS was significantly elevated in the ZDF-HG group, and glutathione reductase activity was reduced in both obese phenotypes. Conclusions: The findings demonstrate that obesity initiates hippocampal neurodegeneration and motor decline, and that hyperglycemia severely impairs recognition memory. These results emphasize the critical interplay between metabolic dysfunction and cognitive decline, highlighting the necessity of managing both obesity and T2D to prevent early neurodegenerative changes.

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