Beyond the Activity-Based Anorexia Model: Reinforcing Values of Exercise and Feeding Examined in Stressed Adolescent Male and Female Mice

Anorexia nervosa (AN), mostly observed in female adolescents, is the most fatal mental illness. Its core is a motivational imbalance between exercise and feeding in favor of the former. The most privileged animal model of AN is the “activity-based anorexia” (ABA) model wherein partly starved rodents housed with running wheels exercise at the expense of feeding. However, the ABA model bears face and construct validity limits, including its inability to specifically assess running motivation and feeding motivation. As infant/adolescent trauma is a precipitating factor in AN, this study first analyzed post-weaning isolation rearing (PWIR) impacts on body weights and wheel-running performances in female mice exposed to an ABA protocol. Next, we studied through operant conditioning protocols i) whether food restriction affects in a sex-dependent manner running motivation before ii) investigating how PWIR and sex affect running and feeding drives under ad libitum fed conditions and food restriction. Besides amplifying ABA-elicited body weight reductions, PWIR stimulated wheel-running activities in anticipation of feeding in female mice, suggesting increased running motivation. To confirm this hypothesis, we used a cued-reward motivated instrumental task wherein wheel-running was conditioned by prior nose poke responses. It was first observed that food restriction increased running motivation in male, but not female, mice. When fed grouped and PWIR mice were tested for their running and palatable feeding drives, all mice, excepted PWIR males, displayed increased nose poke responses for running over feeding. This was true when rewards were proposed alone or within a concurrent test. The increased preference for running over feeding in fed females did not extend to running performances (time, distance) during each rewarded sequence, confirming that motivation for, and performance during, running are independent entities. With food restriction, mice displayed a sex-independent increase in their preference for feeding over running in both group-housed and PWIR conditions. This study shows that the ABA model does not specifically capture running and feeding drives, i.e. components known to be affected in AN

Rôle du système endocannabinoïde dans le choix entre la prise de nourriture et l’activité physique chez la souris

Physical inactivity, a major public health problem, is mainly caused by a lack of motivation for physical activity which can be associated, as in obesity, with hyper-motivation for palatable food. Conversely, an exacerbated motivation for physical exercise can lead to addiction, whether intrinsic or associated with e.g. voluntary dietary restriction as in restrictive anorexia (RA) in adolescents. These data illustrate the need to identify the neurobiological mechanisms responsible for the motivational balance between physical activity and food intake, before attempting to characterise the processes deregulated in these pathologies. Taking into account the fact that studies aiming to identify these bases use models of forced (treadmill) or volitional (wheel) exercise that do not allow for a selective measurement of motivation for physical activity, this Thesis relied on the development of paradigms using operant conditioning to confront them with classical models of physical exercise before then dissecting the role of the endocannabinoid system in the motivational balance between physical activity and food intake in the mouse.The first experimental part of my Thesis aimed to dissect the different dimensions (motivation, performance) associated with physical activity in the most widely used rodent model of RA, i.e. the "activity-based anorexia" (ABA) model. The latter is based on limited access to food (by the experimenter) combined with free and permanent access to an activity wheel. The use of an operant conditioning protocol to measure motivation for each reward (i.e. food and exercise), including in a choice protocol, revealed a dichotomy between physical activity performance - as measured in ABA - and motivation for this activity in adolescent females subjected to early stress (an etiological factor in RA).In view of the addictive power of exercise, the second objective of my Thesis was to analyse the hypothesis that exercise shares with drugs of abuse the ability to enhance the excitatory tone on ventral tegmental area (VTA) dopaminergic neurons. Using ex vivo electrophysiological approaches combined with a reinstatement/recovery conditioning protocol for the wheel, we showed that (i) physical activity, as do cocaine or amphetamine, increases the excitatory tone on VTA dopaminergic neurons, and (ii) this increase correlates with the degree of motivation of the animal.Using constitutive and conditional mutants for the CB1 receptor, we recently revealed that CB1 receptors on GABAergic neurons positively control motivation to run without affecting that for palatable food. However, these findings were based on temporary conditioning sessions, raising the question of the importance of this GABAergic control when choosing between these rewards on a permanent basis. The third objective of my Thesis was therefore to develop a "closed economy" choice protocol, in which animals, housed individually in operating chambers, have a permanent choice between standard food and exercise in a wheel. In addition, in order to estimate the so-called "essential values" of each of the two rewards, the effort demand (i.e. the price to be paid) to access each reward was progressively increased. The combined use of this new paradigm with viral approaches allowed us to reveal that CB1 receptors on GABAergic neurons in the ventromedial part of the striatum have a necessary and sufficient role in controlling the "essential value" of physical activity.L’inactivité physique, un problème majeur de santé publique, a pour cause principale une absence de motivation pour l'activité physique qui peut être associée, comme dans l'obésité, à une hyper-motivation pour l'alimentation palatable Inversement, une motivation exacerbée pour l'exercice physique peut engendrer une addiction, qu'elle soit intrinsèque ou associée à e.g. une restriction alimentaire volontaire comme dans l'anorexie restrictive (AR) chez l'adolescente. Ces données illustrent la nécessité d'identifier les mécanismes neurobiologiques responsables de l'équilibre motivationnel entre activité physique et prise alimentaire, pour ensuite tenter de caractériser les processus dérégulés dans ces pathologies. Prenant en compte le fait que les études visant à identifier ces bases utilisent des modèles d'exercice physique forcé (tapis roulant) ou volontaire en accès libre (roue) qui ne permettent pas une mesure sélective de la motivation pour l'activité physique, cette Thèse a reposé sur le développement de paradigmes utilisant le conditionnement opérant pour les confronter aux modèles classiques d'exercice physique avant d'ensuite disséquer le rôle du système endocannabinoïde dans l'équilibre motivationnel entre activité physique et prise alimentaire chez la souris.Le premier volet expérimental de ma Thèse a eu pour objectif de disséquer les différentes dimensions (motivation, performance) associées à l'activité physique dans le modèle d'étude de l'AR le plus utilisé chez le rongeur, i.e. l’"activity-based anorexia" (ABA). Ce modèle repose sur un accès à la nourriture limité (par l'expérimentateur) associé à un accès libre et permanent à une roue d’activité. L'utilisation d'un protocole de conditionnement opérant permettant de mesurer la motivation pour chaque récompense (i.e. nourriture et exercice), y compris dans un protocole de choix, a mis en évidence une dichotomie entre performance d'activité physique – telle qu'elle est mesurée dans l'ABA – et motivation pour cette activité chez des femelles adolescentes soumises à un stress précoce (un facteur étiologique dans l'AR).Eu égard au pouvoir addictif de l'exercice physique, le second objectif de ma Thèse était d’analyser l’hypothèse que l’exercice physique partage avec les drogues d’abus la capacité de renforcer le tonus excitateur sur les neurones dopaminergiques de l’aire tegmentale ventrale (ATV). A l’aide d’approches électrophysiologiques ex vivo associées à un protocole de conditionnement opérant d’extinction et de rechute ("reinstatement/craving") pour la roue, nous avons montré (i) que l’activité physique, comme la cocaïne ou l’amphétamine, augmente le tonus excitateur sur les neurones dopaminergiques de l’ATV, et (ii) que cette augmentation est corrélée au degré de motivation de l’animal.L'utilisation de mutants constitutifs et conditionnels pour le récepteur CB1 nous avait permis de révéler que les récepteurs CB1 des neurones GABAergiques exerçent un contrôle positif sur la motivation pour courir, mais pas pour la prise de nourriture palatable. Néanmoins, ces conclusions étaient basées sur des sessions de conditionnement temporaires, posant la question de ce contrôle lors d'un choix permanent entre ces récompenses. Le troisième objectif de ma Thèse a donc été de développer un protocole de choix en "économie fermée", dans lequel les animaux, hébergés individuellement dans les chambres opérantes, ont un choix permanent entre de la nourriture standard et l'exercice dans une roue. De plus, afin d'estimer les "valeurs essentielles" de chacune des deux récompenses, la demande d'efforts (i.e. le prix à payer) pour accéder à chaque récompense a été progressivement accrue. L'utilisation combinée de ce nouveau paradigme avec des approches virales nous a permis de révéler que les récepteurs CB1 des neurones GABAergiques de la partie ventro-mediale du striatum ont un rôle nécessaire et suffisant dans le contrôle de la "valeur essentielle" de l'activité physique

Cannabis and exercise: Effects of Δ9-tetrahydrocannabinol on preference and motivation for wheel-running in mice

Recent surveys have revealed close links between cannabis and exercise. Specifically, cannabis usage before and/or after exercise is an increasingly common habit primarily aimed at boosting exercise pleasure, motivation, and performance whilst facilitating post-exercise recovery. However, whether these beliefs reflect the true impact of cannabis on these aspects of exercise is unknown. This study has thus examined the effects of cannabis' main psychoactive ingredient, namely ?9-tetrahydrocannabinol (THC), on (i) mouse wheel-running preference and performance and (ii) running motivation and seeking behaviour. Wheel-running preference and performance were investigated using a T-maze with free and locked wheels located at the extremity of either arm. Running motivation and seeking were assessed by a cued-running operant task wherein wheel-running was conditioned by nose poking. Moreover, because THC targets cannabinoid type 1 (CB1) receptors, i.e. receptors previously documented to control running motivation, this study also assessed the role of these receptors in running preference, performance, and craving-like behaviour. Whilst acute blockade or genetic deletion of CB1 receptors decreased running preference and performance in the T-maze, THC proved ineffective on either variable. The failure of THC to affect running variables in the T-maze extended to running motivation, as assessed by cued-running under a progressive ratio (PR) reinforcement schedule. This ineffectiveness of THC was not related to the treatment protocol because it successfully increased motivation for palatable food. Although craving-like behaviour, as indexed by a cue-induced reinstatement of running seeking, was found to depend on CB1 receptors, THC again proved ineffective. Neither running motivation nor running seeking were affected when CB1 receptors were further stimulated by increasing the levels of the endocannabinoid 2-arachidonoylglycerol. These results, which suggest that the drive for running is insensitive to the acute stimulation of CB1 receptors, raise the hypothesis that cannabis is devoid of effect on exercise motivation. Future investigation using chronic administration of THC, with and without other cannabis ingredients (e.g. cannabidiol), is however required before conclusions can be drawn

Exercise craving potentiates excitatory inputs to ventral tegmental area dopaminergic neurons

International audiencePhysical exercise, which can be addictogenic on its own, is considered a therapeutic alternative for drug craving. Exercise might thus share with drugs the ability to strengthen excitatory synapses onto ventral tegmental area (VTA) dopaminergic neurones, as assessed by the ratio of AMPA receptor (AMPAR)‐mediated excitatory postsynaptic currents (EPSCs) to NMDA receptor (NMDAR)‐mediated EPSCs. As did acute cocaine, amphetamine, or Δ9‐tetrahydrocannabinol (THC) pretreatments, an acute 1‐h wheel‐running session increased the AMPAR/NMDAR ratio in VTA dopaminergic neurones. To dissect the respective influences of wheel‐running seeking and performance, mice went through an operant protocol wherein wheel‐running was conditioned by nose poking under fixed ratio schedules of reinforcement. Conditioned wheel‐running increased the AMPAR/NMDAR ratio to a higher extent than free wheel‐running, doing so although running performance was lower in the former paradigm than in the latter. Thus, the cue‐reward association, rather than reward consumption, played a major role in this increase. The AMPAR/NMDAR ratio returned to baseline levels in mice that had extinguished the cued‐running motivated task, but it increased after a cue‐induced reinstatement session. The amplitude of this increase correlated with the intensity of exercise craving, as assessed by individual nose poke scores. Finally, cue‐induced reinstatement of running seeking proved insensitive to acute cocaine or THC pretreatments. Our study reveals for the first time that the drive for exercise bears synaptic influences on VTA dopaminergic neurones which are reminiscent of drug actions. Whether these influences play a role in the therapeutic effects of exercise in human drug craving remains to be established

The motivation for exercise over palatable food is dictated by cannabinoid type-1 receptors

International audienceThe lack of intrinsic motivation to engage in, and adhere to, physical exercise has major health consequences. However, the neurobiological bases of exercise motivation are still unknown. This study aimed at examining whether the endocannabinoid system (ECS) is involved in this process. To do so, we developed an operant conditioning paradigm wherein mice unlocked a running wheel with nose pokes. Using pharmacological tools and conditional mutants for cannabinoid type-1 (CB1) receptors, we provide evidence that CB1 receptors located on GABAergic neurons are both necessary and sufficient to positively control running motivation. Conversely, this receptor population proved dispensable for the modulation of running duration per rewarded sequence. Although the ECS mediated the motivation for another reward, namely palatable food, such a regulation was independent from CB1 receptors on GABAergic neurons. In addition, we report that the lack of CB1 receptors on GABAergic neurons decreases the preference for running over palatable food when mice were proposed an exclusive choice between the two rewards. Beyond providing a paradigm that enables motivation processes for exercise to be dissected either singly or in concurrence, this study is the first to our knowledge to identify a neurobiological mechanism that might contribute to sedentary behavio