Long-Term Effects of Adolescent Chronic Stress and Binge Eating: Exhaustive Study Notes

The study, developed in collaboration with Dr. Sean Bates, investigates the long-term effects of chronic stress and binge eating specifically during the adolescent developmental period.
Significance of Adolescence: Adolescence is defined as a highly sensitive developmental window, particularly for brain systems responsible for emotional regulation.
The Stress-Eating Cycle: - Stress occurring during this window can lead to persistent long-term effects on anxiety and depression. - High-fat and high-sugar "highly palatable" foods are known to reduce stress-related negative affect (negative emotions) in the short term, acting as a maladaptive coping mechanism. - This creates a feedback loop: stress increases negative emotions, which drives binge eating for temporary relief, eventually increasing long-term vulnerability to stress.
Clinical Relevance: In human clinical patterns, stress and maladaptive coping behaviors (like binge eating) often develop concurrently or sequentially during adolescence.
Primary Research Goal: To model whether the combination of both stress and binge-like eating in adolescence leads to more severe long-term outcomes than experiencing either factor in isolation.

The study is built upon two primary hypotheses: 1. Prediction One: Adolescent stress will increase the intake of highly palatable food. Specifically, stressed mice are expected to consume significantly more high-fat diet (HFD) than stress-naive control mice. 2. Prediction Two: The combination of adolescent stress and exposure to a high-fat diet will produce significantly greater anxiety-like and depression-like behaviors in adulthood compared to either condition alone.
Sex Multiplier: This combined effect is predicted to be much more prevalent and pronounced in female mice.

Subjects: Male and female adolescent $C57BL/6$ (Black 6) mice.
Sample Size: The target for the complete study is a total of $64$ mice, divided into groups of $8$ per group per sex.
Experimental Groups: Mice are randomly assigned to one of four specific conditions: - Standard chow/Stress-naive. - Standard chow/Stressed. - High-Fat Diet (HFD)/Stress-naive. - High-Fat Diet (HFD)/Stressed.
Binge Eating Protocol: - The protocol is a modified version of the ORAR model. - Intermittent Access: Mice are given access to highly palatable food for $2$ hours per day, every other day. - Palatable Food Type: Reese's peanut butter chips are used as the high-fat/high-sugar stimulus. - Duration: The cycle consists of $12$ total sessions, spanning approximately $3$ weeks.
Stress Exposure: - Stress was incorporated at the midway point of the binge cycle (between session $6$ and session $7$ ). - The procedure uses the Single Prolonged Stress model.
Behavioral Testing Batteries: Following the stress and binge protocols, mice undergo the following tests: - Elevated Plus Maze (EPM): A measure of anxiety-like behavior. - Novelty Suppressed Feeding (NSF): A measure of anxiety and motivation. - Splash Test: A measure of grooming behavior used to assess depression-like states. - Conditioned Place Preference (CPP): Used to assess reward sensitivity, specifically using cocaine as the stimulus in this study.

General Intake: As hypothesized, high-fat diet groups consumed more overall than the standard chow groups across the $12$ sessions.
The Mid-Point Shift: - Prior to session $6$ , the stress-naive HFD group (represented in red) was consuming the highest amount of food. - Around session $6$ (the point of stress administration), the stressed HFD mice began to increase their intake, while the non-stressed groups began to level off.
Incubation Period: The Single Prolonged Stress procedure has a known incubation period of approximately $7$ days before behavioral manifestations occur. - In the current study timeline, this means effects would likely not be evident until sessions $9$ or $10$ . - Currently, the data shows a trend rather than a definitive conclusion, but it suggests stress may shift how animals engage with palatable food.

Elevated Plus Maze (Anxiety): - The stressed HFD group showed a trend toward increased time in the open arms. - While open-arm time usually indicates lower anxiety, in this context it may signify higher impulsivity or altered inhibitory control, likely caused by the combination of stress and reward stimuli.
Conditioned Place Preference (Reward): - The stress-naive chow group showed a slight negative trend, which is expected for a control group. - Other groups trended positive toward the cocaine-paired environment, suggesting potential differences in reward sensitivity.
Splash Test (Depression): - Latency to Groom: Tended to be higher in the HFD groups. - Total Grooming: Tended to be lower in the HFD groups. - These directional patterns suggest a shift toward depressive-like behavior driven primarily by the high-fat diet exposure, though the interaction with stress remains under clarification.

Temporal Stress Manipulation: A planned next step involves introducing stress before the binge cycle (rather than during) to see how stress history affects intake behavior.
Sex Differences: Comparative analysis will be performed once the female mouse dataset is finalized.
Molecular Analysis: The researchers intend to examine the expression of specific receptors in the Prefrontal Cortex (PFC) and the Striatum: - Adrenergic Receptors. - Opioid Receptors.
Pharmacological Intervention: The study will investigate the effects of Naltrexone (an opioid receptor antagonist). - Naltrexone is a clinical treatment for obesity and binge eating; the researchers want to determine if a history of stress alters its long-term effectiveness.

Question: Is there a specific reason for the hypothesis that there will be sex differences? - Response: Historically, females show higher vulnerability to anxiety and depression. Additionally, in humans, binge eating is more prevalent in females, and they may be more susceptible to the adverse effects of stress.
Question: Have these sex differences been reflected in the mice so far? - Response: Yes. During the $2$ -hour binge cycle where water is removed, most mice lose weight. However, the female mice in the binging groups actually started gaining weight because their intake was so high. This indicates sex-specific effects are already occurring, though the specific interaction with stress is still being analyzed.
Question: Is the binging behavior different between age groups? Could the female binging be related to an evolutionary/biological function like reproductive safety or nursing preparation? - Response: Most existing literature focuses on adult mice, and they show the same pattern—females are significantly more prone to binging. In this study, while females didn't necessarily eat more in absolute volume, they consumed a much higher proportion of their body weight compared to males. The evolutionary perspective regarding reproduction is a "really cool" perspective that has not been heavily discussed in the current literature.