Adolescent female C57BL/6 mice with vulnerability to activity-based anorexia exhibit weak inhibitory input onto hippocampal CA1 pyramidal cells

• Designed an animal model of anorexia nervosa, ABA, that suits C57BL/6 mice.
• Food restriction (FR) elicits hyperactivity reliably in adolescent female C57BL/6s.
• ABA vulnerability can be quantified by measuring hyperactivity following FR.
• Hippocampal CA1 pyramidal cells of ABA-resilient mice receive enhanced GAD contacts.

Anorexia nervosa (AN) is an eating disorder characterized by self-imposed severe starvation and often linked with excessive exercise. Activity-based anorexia (ABA) is an animal model that reproduces some of the behavioral phenotypes of AN, including the paradoxical increase in voluntary exercise following food restriction (FR). Although certain rodents have been used successfully in this animal model, C57BL/6 mice are reported to be less susceptible to ABA. We re-examined the possibility that female C57BL/6 mice might exhibit ABA vulnerability during adolescence, the developmental stage/sex among the human population with particularly high AN vulnerability. After introducing the running wheel to the cage for 3 days, ABA was induced by restricting food access to 1 h per day (ABA1, N = 13) or 2 h per day (ABA2, N = 10). All 23 exhibited increased voluntary wheel running (p < 0.005) and perturbed circadian rhythm within 2 days. Only one out of five survived ABA1 for 3 days, while 10 out of 10 survived ABA2 for 3 days and could subsequently restore their body weight and circadian rhythm. Exposure of recovered animals to a second ABA2 induction revealed a large range of vulnerability, even within littermates. To look for the cellular substrate of differences in vulnerability, we began by examining synaptic patterns in the hippocampus, a brain region that regulates anxiety as well as plasticity throughout life. Quantitative EM analysis revealed that CA1 pyramidal cells of animals vulnerable to the second ABA2 exhibit less GABAergic innervation on cell bodies and dendrites, relative to the animals resilient to the second ABA (p < 0.001) or controls (p < 0.05). These findings reveal that C57BL/6J adolescent females can be used to capture brain changes underlying ABA vulnerability, and that GABAergic innervation of hippocampal pyramidal neurons is one important cellular substrate to consider for understanding the progression of and resilience to AN.