Identifying novel phenotypes of vulnerability and resistance to activity-based anorexia in adolescent female rats

Abstract

Objective: Activity-based anorexia is a translational rodent model that results in severe weight loss, hyperactivity, and voluntary self-starvation. The goal of our investigation was to identify vulnerable and resistant phenotypes of activity-based anorexia in adolescent female rats.

Method: Sprague-Dawley rats were maintained under conditions of restricted access to food (N = 64; or unlimited access, N = 16) until experimental exit, predefined as a target weight loss of 30-35% or meeting predefined criteria for animal health. Nonlinear mixed effects statistical modeling was used to describe wheel running behavior, time to event analysis was used to assess experimental exit, and a regressive partitioning algorithm was used to classify phenotypes.

Results: Objective criteria were identified for distinguishing novel phenotypes of activity-based anorexia, including a vulnerable phenotype that conferred maximal hyperactivity, minimal food intake, and the shortest time to experimental exit, and a resistant phenotype that conferred minimal activity and the longest time to experimental exit.

Discussion: The identification of objective criteria for defining vulnerable and resistant phenotypes of activity-based anorexia in adolescent female rats provides an important framework for studying the neural mechanisms that promote vulnerability to or protection against the development of self-starvation and hyperactivity during adolescence. Ultimately, future studies using these novel phenotypes may provide important translational insights into the mechanisms that promote these maladaptive behaviors characteristic of anorexia nervosa.

Keywords: activity-based anorexia; adolescence; animal model; anorexia nervosa; exercise; food restriction; hyperactivity; rat; resistance; vulnerability.

FIGURE 1

A comparison of features of anorexia nervosa and activity-based anorexia. Figure adapted from Ref. to reflect proposed changes in terminology for diagnosing anorexia nervosa in DSM-5.

FIGURE 2

Initial observations of the activity-based anorexia (ABA) model in adolescent female Sprague-Dawley rats indicated variability in the response to restricted food access. The sample included 16 control animals (24 h/day access to food and a running wheel) and 64 ABA animals (1 h/day access to food and 24 h/day access to a running wheel). Both groups received unlimited access to food and the wheel for three baseline days. Restricted food access at the onset of the dark cycle began on experimental day 1 for ABA animals. Overall, control animals increased their body mass (A), maintained food intake (B), and increased wheel running activity over time (C). ABA animals lost body mass (A), reduced food intake (B), and increased wheel running (C). As a group, the ABA animals had a rapid increase in wheel running during the first 3 days of restricted food access, which then plateaued and shifted down to converge with control animals after approximately 7–8 days. This effect was the result of early dropout by high wheel runners during days 2–4.

FIGURE 3

Day of experimental exit from the activity-based anorexia model indicated multiple sub-populations. (A) The probability density graph (95% confidence interval indicated by the shaded area) indicated a double peak with two sub-populations—the first peak occurred at day 4 and the second peak at day 6. (B) The survival probability graph (95% confidence interval indicated by the dotted line; 95% Hall-Wellner band indicated by the shaded area) indicated that 40% of the animals exited the experiment by day 4, representing a vulnerable phenotype. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIGURE 4

A targeted regression analysis was used to generate classifiers of activity-based anorexia. The R package “party” implementation of recursive partitioning is shown as a regression tree graph. The different splits were ultimately used to define the classification of the activity-based anorexia data into groups with sample size (n) and day of experimental exit (y) represented on the nodes of the figure. Wheel running and food intake drove the classification of smaller subgroups. Main subpopulations of interest include the animals at the extremes – the early exit, vulnerable phenotype and the late exit, resistant phenotype; the majority of animals were identified as one of two intermediate phenotypes (fast exit or slow exit).