Analysis of the Effect of Central LEAP-2 on Food Intake and Its Impact on Nucleus Accumbens Dopamine Release

Background Introduction

Gut-brain peptides such as ghrelin and their receptors have been established as important factors in regulating hunger and reward processing. However, the liver-expressed antimicrobial peptide 2 (Leap2), an inverse agonist of the ghrelin receptor, has only recently begun to attract attention. Previous studies have shown that ghrelin can enhance reward-related behaviors, while synthetic ghrelin receptor antagonists can reduce such behaviors. The central action and mechanism of Leap2 are still not fully understood. The objective of this study is to further understand the role of Leap2 in reward-related behaviors by studying its central effects in mice, aiming to provide a theoretical basis for the treatment of obesity and other reward-related mental and neurological disorders.

Source of Paper

This research paper was written by Maximilian Tufvesson-Alm, Qian Zhang, Cajsa Aranäs, Sebastian Blid Sköldheden, Christian E. Edvardsson, and Elisabet Jerlhag. It was published in the journal Progress in Neurobiology, article number 236 (2024) 102615, and went online on April 17, 2024.

Experimental Procedure

Subjects and Environment

The study used male NMRI mice, which were acclimated in the laboratory for one week before starting the experiments. During the experiments, mice were provided with high-calorie foods such as peanut butter, Nutella spread, and chocolate, and continuously supplied with standard mouse chow and water. The experimental environment was maintained at a constant temperature of 20°C, with 50% humidity, under a 12-hour light-dark cycle.

Drugs and Administration

Leap2 (Leap-2 (38-77) (human)/Leap-2 (37-76) (mouse), provided by Phoenix Pharmaceuticals Inc.) was dissolved in Ringer’s solution for intracerebroventricular (ICV) injection or localized injection into the laterodorsal tegmental nucleus (LDTg). The dosage was determined based on previous experiments, ensuring it did not affect the general behavior of the mice.

Food Intake Experiment

To test the effect of Leap2 on food intake, the research measured the intake of peanut butter, Nutella spread, and chocolate at different time points, and compared the differences between the Leap2 treatment group and the control group.

Conditioned Place Preference (CPP) Experiment

The CPP experiment was used to evaluate the impact of Leap2 on reward-related memory. By associating food with one side of the experimental box during the training phase, the preference of mice for food-related locations before and after Leap2 administration was assessed.

Microdialysis and Dopamine Analysis

To investigate the impact of Leap2 on dopamine signaling in the nucleus accumbens, microdialysis technology was used to measure changes in dopamine in the nucleus accumbens of mice exposed to and consuming peanut butter. The effects of Leap2 on dopamine signaling were evaluated by comparing different treatment groups.

qPCR Analysis

qPCR technology was used to detect the expression of Leap2 in the brains of untreated mice before and after consuming peanut butter, focusing on brain regions related to reward.

Data Analysis

All statistical analyses were performed using GraphPad Prism® 9.5.1 software. Data were processed using two-way ANOVA, Wilcoxon signed-rank test, and other statistical methods.

Main Results

Leap2’s Impact on Food Intake

The research found that Leap2 significantly reduced the intake of highly preferred foods (e.g., peanut butter) in mice, while having a relatively small impact on the intake of regular mouse chow. Specifically, the data showed that the effect on peanut butter intake was significant within two hours, and this effect was positively correlated with the mice’s preference for high-preference foods.

Leap2’s Impact on Reward Memory

In the CPP experiment, mice showed a strong preference for peanut butter during the training phase, which was significantly reduced after Leap2 treatment. Additionally, Leap2 was able to completely eliminate the CPP effect for Nutella spread, but had no significant impact on chocolate. This suggests that Leap2 may primarily affect the intake and memory of high-preference, high-reward foods.

Impact on Dopamine Signaling

Microdialysis experiment results showed that Leap2 significantly inhibited dopamine release in the nucleus accumbens during peanut butter exposure and consumption. In the control group, dopamine levels significantly increased after peanut butter exposure, whereas there was no such phenomenon in the Leap2 treatment group. This indicates that Leap2 may influence food reward behaviors by regulating central dopamine signaling.

Leap2 Expression in the Brain

qPCR results indicated that Leap2 is expressed in multiple brain regions, particularly those related to reward, such as the nucleus accumbens and LDTg. After providing high-preference peanut butter, the expression of Leap2 in these brain regions significantly decreased, which may imply Leap2’s important role in regulating reward-related behaviors.

Leap2’s Role in the LDTg

By locally injecting Leap2 into the LDTg, the study found that mice’s intake of high-preference food under acute binge-eating conditions significantly decreased. This further confirms Leap2’s role in regulating reward-related eating behaviors in specific brain regions.

Research Significance and Value

This study comprehensively revealed the impact of Leap2 on food intake and reward-related behaviors through various experimental methods. The research indicated that Leap2 can not only reduce the intake of highly preferred foods but also regulate reward-related memory and dopamine signaling. The expression of Leap2 in multiple reward-related brain regions and its dietary regulation highlight its potential as a treatment target for obesity and other reward-related neuropsychiatric disorders.

Research Highlights

1. Significant Inhibitory Effect of Leap2 on Highly Preferred Food Intake: Particularly for peanut butter, suggesting Leap2 as a potential target for regulating high-calorie diets.
2. Impact on Reward Memory and Dopamine Signaling: Leap2 significantly inhibits reward memory and dopamine release induced by highly preferred foods, highlighting its regulatory role in the neural reward system.
3. Brain Region-Specific Expression and Regulation: High expression and dietary variation of Leap2 in reward-related brain regions suggest its positive feedback role in dietary reward behaviors.

Conclusion

This study fully reveals the regulatory role of Leap2 in the central control of highly preferred food intake and reward-related behaviors, establishing its potential application basis in the treatment of obesity and related mental disorders. Future research needs to further explore the central concentration, distribution, and long-term effects of Leap2 under different dietary conditions to fully understand its role in the neural reward system.