To determine the neurological changes associated with ELT-induced maladaptive eating, Shin et al. developed a strain of mice that allowed for the visualization of leptin receptor (Lepr)-positive neurons. To achieve this, they crossed a Lepr-Cre mouse strain with an Ai14 strain, then backcrossed these mice with wild-type C57BL/6J mice for multiple generations to refresh their genetic background (3). Once a suitably diverse strain had been confirmed, pregnant mice were placed in individual housing after 14-16 days of pregnancy (2).
To induce early life trauma, pups were separated from their mothers and littermates at postnatal days 3-4 for approximately 23 hours. They were then returned to the litter and weaned with control pups after postnatal day 21; control pups were never separated from their litter (2).
Prior to the start of this binge-like eating paradigm, mice were fed a standard diet of 18% fat (NC). Once these mice were at a suitable age (10-13 weeks), all were placed into individual housing and offered free access (ad libitum) to the high fat diet (HFD, 60% fat) for two days, and then switched to the standard diet for five days. On day 8, the mice were re-exposed to this HFD again, now with food intake being monitored. After 24 hours, the HFD was again removed and replaced with the standard diet, thus completing the first binge eating cycle. Subsequent cycles used a six day NC-only diet, which was followed by the HFD for 24 hours. Food intake as well as body weight were measured throughout, and O2 consumption, CO2 production, and respiratory exchange rate were measured after HFD reintroduction to determine energy expenditure (2).
After the binge eating paradigm was concluded, mice underwent a locomotion assessment, as well as an open field test and novel object recognition test to determine whether ELT had any long-lasting impact on stress responses. Glucose tolerance and blood pressure were monitored throughout the study, and, following the completion of the test protocol, immunohistochemistry, RNA in situ hybridization, and circuit mapping quantification were performed following euthanization (2).
Immediately following separation, ELT pups had lower leptin and higher corticosterone levels when compared with controls. As leptin and corticosterone are implicated in long-term regulation of energy balance and stress-induced HPA axis activity, respectively, this suggested that the early-life trauma paradigm was successful (3,4). Despite this, once the mice reached adulthood, no differences in anxiety-like behaviors [increased vigilance and heart rate, freezing/hypoactivity, and suppressed food consumption (5)], locomotion, glucose metabolism, and baseline leptin/corticosterone levels were observed between the two test groups (2).
Comparing feeding behaviors, the ELT group had a greater tendency towards binge-like eating. When the HFD was repeatedly reintroduced (re-HFD), the ELT mice showed binge eating tendencies that were particularly aggravated in the fourth cycle of re-HFD, but not in the first. These mice also displayed rapid weight gain when fed the high fat diet, but not when offered continual access to the normal food supply, suggesting that early life trauma primes these mice for the development of high fat diet-induced obesity, primarily from overconsumption (2).