Anorexia nervosa (AN) is a complex psychiatric disorder characterized by severe caloric restriction and distorted body image, leading to significant psychological and physiological complications. Brain-derived neurotrophic factor (BDNF) plays a critical role in cognitive function and metabolic regulation. A mutation in the BDNF gene is associated with anorexia nervosa. This study examines the effects of food restriction, refeeding and short-term refeeding on the expression of Bdnf and its receptor (tropomyosin receptor kinase B TrkB/Ntrk2) in key brain regions involved in reward and cognitive function. We assessed BDNF mRNA levels in the dorsal striatum (DS), nucleus accumbens, ventral tegmental area, and prefrontal cortex (PFC) of AN-like mice subjected to different feeding regimes combined with or without physical activity. Cognitive flexibility was assessed using the Y-maze test. Whole RNA sequencing was also performed to analyse gene expression changes. Food restriction induced a transient decrease in cognitive flexibility and significantly decreased Bdnf expression in the DS and PFC. Progressive refeeding restored Bdnf in the DS but not the PFC. Short refeeding restored Bdnf levels to baseline. TrkB expression is increased by restriction only in the PFC. The presence of a running wheel cancelled these effects, suggesting an interaction between physical activity and diet. Pathway analysis of dysregulated genes revealed enrichment in immune regulation and cell-cell communication pathways. These findings highlight the complex relationship between diet, exercise, and brain function in AN-like mouse model and suggest avenues for further research into the clinical relevance of BDNF and TrkB as biomarkers of eating disorders.
Anorexia nervosa (AN) is a complex and potentially life-threatening eating disorder characterized by self-imposed dietary restriction and usually excessive physical exercise [1]. Individuals with AN experience severe weight loss due to caloric restriction, leading to a number of somatic complications such as hormonal and metabolic changes, and loss of bone density [2, 3]. Metabolic changes are often associated with psychiatric symptoms across various mental diseases and more specifically in AN where recent genomic analysis encourage for a reconceptualization of AN as to be a metabo-psychiatric disorder [4, 5]. These include high levels of anxiety and depression associated with intense fear of weight gain, distorted body image, obsessive behaviors related to food and body shape and impaired cognitive flexibility and decision-making, which further complicate the clinical presentation and treatment approach [6,7,8]. The disorder predominantly affects adolescents and young adults, with a higher prevalence in women, accounting for up to 90% of cases [9]. Epidemiological studies estimate that the prevalence of AN to be around 1% in young adult females [10]. Despite various therapeutic approaches, there is currently no cure for AN, and many individuals experience recurrent relapses. It is estimated that 9-52% of patients relapse after treatment [11], underlining the urgent need to understand the factors that contribute to relapse. Indeed, the impact of different refeeding protocols on brain function remains poorly understood, which is an important gap, as these neural changes may play a critical secondary role in influencing relapse vulnerability. Furthermore, given that AN has the highest mortality rate of any psychiatric disorder [12,13,14], there is an urgent need to improve treatment outcomes and reduce the high risk of mortality associated with this disorder [13]. Genome-wide association studies (GWAS) and case-controlled studies have identified several genetic risk factors associated with the disorder [4, 15,16,17]. Among these, a specific allele of the brain-derived neurotrophic factor (BDNF) gene, specifically the Val66Met polymorphism (also known as rs6265), has received considerable attention [18], and is now under intense scrutiny [19, 20].
Known for its role in neuronal development, neurogenesis, and synaptic plasticity [21, 22], BDNF has recently emerged as a key player in metabolic regulation, influencing processes both centrally in the brain and peripherally. Centrally, BDNF influences hypothalamic circuits that regulate energy balance, appetite, and satiety, contributing to its anorexic effects [23]. Peripherally, BDNF can directly affect metabolic processes, including increasing lipid oxidation and energy expenditure, ultimately leading to weight loss and increased physical activity levels [24], highlighting a comprehensive effect of BDNF on metabolic health and physical activity. The Val66Met mutation in the BDNF gene, has been implicated in several metabolic diseases and psychiatric disorders, particularly in AN, affecting both neuroplasticity and metabolic regulation [19, 20]. In individuals with AN, Met variant carriers show altered reward function, as evidenced by increased reward circuit activity in response to images of thinness in the ventral striatum, a key region for reward processing [25, 26].
Finding animal models that mimic the full spectrum of AN symptoms is challenging due to their specificity to humans. A widely used AN-like rodent model is the activity-based anorexia (ABA) model, which combines time-limited access to food with free and continuous access to a running wheel [27]. This paradigm leads to a paradoxical pattern of behavior in which animals voluntarily increase their physical activity levels while simultaneously reducing food intake, resulting in severe weight loss and physiological, behavioral and cognition changes reminiscent of human AN [27].
The expression of BDNF in brain regions associated with the neurobiological basis of AN has been studied, partly using the ABA paradigm. For instance, it has been observed that rodents exposed to the ABA typically exhibit reduced BDNF expression in the medial prefrontal cortex (PFC) and amygdala [28, 29]. The medial PFC is involved in decision-making and executive function, both of which may be impaired in individuals with AN. Meanwhile, the amygdala plays a key role in emotional responses and fear, both of which are heightened in individuals with AN [30]. Conversely, increased BDNF levels in the hippocampus, a region involved in memory and stress responses, suggest adaptive or maladaptive responses to food restriction and stress [28]. Levels remain relatively unchanged in the nucleus accumbens, a region central to reward processing. This suggests that some reward-related behaviors in anorexia nervosa may not be directly related to changes in BDNF in this region [31].
However, the relatively short duration of the ABA protocol may not fully capture the chronicity and complexity of human AN, limiting its translational relevance. This highlights the need for further refinement of this model to better understand the long-term neurobiological effects of AN and to develop more effective treatments. In this study, we used a modified version of the ABA protocol, known as the Food Restriction and Wheel (FRW) model, which incorporates the chronic aspect of AN, a critical feature that closely aligns with the prolonged course of the disorder [32, 33]. The FRW model has been well validated, particularly in terms of its metabolic relevance, and effectively mimics the metabolic and endocrine changes observed in AN [32,33,34].
This study aimed to assess the levels of gene expression of brain-derived neurotrophic factor (BDNF), its high-affinity receptor tropomyosin receptor kinase B (TrkB) [35] and the BDNF regulatory enzyme N-acetyltransferase 8-like (Nat8l), in key brain regions implicated in AN, the dorsal striatum, the nucleus accumbens, the ventral tegmental area and the prefrontal cortex in the FRW model of anorexia nervosa. The primary hypothesis was that alterations in the expression of Nat8l, a regulator of BDNF in the dorsal striatum (DS) [36], modulate BDNF/TrkB signaling in a region-specific manner, thereby contributing to the behavioral and neurophysiological abnormalities characteristic of anorexia nervosa. Gene expression profiles were assessed in three nutritional states: chronic food restriction, progressive refeeding, and short-term refeeding. Progressive refeeding models the clinical phase of structured nutritional rehabilitation during inpatient treatment for AN, characterized by a controlled and gradual caloric increase aimed at physiological recovery and body mass index normalization. In contrast, short-term refeeding simulates the spontaneous, unstructured refeeding episodes observed in some patients, involving abrupt and excessive calorie intake, which may affect neural recovery processes. The final aim of this study was thus to determine whether changes in Nat8l, BDNF, and TrkB expression across these conditions provide mechanistic insight into the neural adaptations associated with the onset, maintenance, and treatment response of anorexia nervosa.