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The search for safe and efficient chronic pain treatments is dampened by the lack of reliable models that faithfully reproduce current pharmacological treatments for chronic spontaneous pain in humans. Preclinical models often assess the antinociceptive efficacy of non-contingent pharmacological treatments evaluated in the short-term. Here, we provide a protocol of contingent operant self-medication in mice, which allows the estimation of spontaneous pain relief and drug abuse liability in models of persistent pain. This paradigm requires preliminary habituation and animal handling, followed by training of mice in operant conditioning boxes, to allow subsequent analgesic drug self-administration. After the initial acquisition of food-maintained operant behavior, a chronic pain sensitization is induced. Posterior intravenous jugular catheterization and coupling of operant conditioning boxes to perfusion pumps allow quantification of operant responding for intravenous drug self-administration. All mice show an initial operant drug self-administration behavior associated with the previous food-maintained operant training. This initial operant responding is extinguished after administration of ineffective treatments, but continues when the compounds have analgesic efficacy or intrinsic reinforcing properties. The identification of a significant drug self-administration selectively expressed in mice exposed to the chronic pain condition is indicative of analgesic drug effects, whereas persistent self-administration in control mice is indicative of abuse liability. The present protocol provides the behavioral and surgical procedures needed to assess spontaneous pain relief and potential for abuse of pharmacological treatments, through contingent analgesic self-medication in mice.Graphic abstract: Experimental design. Animals are subjected to a 5-day food self-administration protocol with a fixed ratio of reinforcement of 1 (FR1, 1 interaction with the active nose-poke causes the release of 1 reinforcer/infusion), to acquire the operant behavior. After this training, mice are subjected to the chronic pain or sham procedure, and four days later an intravenous (i.v.) catheterization is performed, to allow self-administration with the selected compound or its vehicle. Three days after the catheterization, animals start the drug/vehicle self-administration protocol at FR1. The patency of the catheter is evaluated with the thiopental test after the last self-administration session. Adapted from Bura et al. (2018).

The study of food addiction comprises 3 hallmarks that include the persistence to response without an outcome, the strong motivation for palatable food, and the loss of inhibitory control over food intake that leads to compulsive behavior in addicted individuals. The complex multifactorial nature of this disorder and the unknown neurobiological mechanistic correlation explains the lack of effective treatments. Our operant conditioning model allows deciphering why some individuals are vulnerable and develop food addiction while others are resilient and do not. It is a translational approach since it is based on the Diagnostic and Statistical Manual of Mental Disorders 5th edition (DSM-5) and the Yale Food Addiction Scale (YFAS 2.0). This model allows to evaluate the addiction criteria in 2 time-points at an early and a late period by grouping them into 1) persistence to response during a period of non-availability of food, 2) motivation for food with a progressive ratio, and 3) compulsivity when the reward is associated with a punishment such as an electric foot-shock. The advantage of this model is that it allows us to measure 4 phenotypic traits suggested as predisposing factors related to vulnerability to addiction. Also, it is possible to evaluate the long food addiction mouse model with mice genetically modified. Importantly, the novelty of this protocol is the adaptation of this food addiction model to a short protocol to evaluate genetic manipulations targeting specific brain circuitries by using a chemogenetic approach that could promote the rapid development of this addictive behavior. These adaptations lead to a short food addiction mouse protocol, in which mice follow the same behavioral procedure of the early period in the long food addiction protocol with some variations due to the surgical viral vector injection. To our knowledge, there is no paradigm in mice allowing us to study the combination of such a robust behavioral approach that allows uncovering the neurobiology of food addiction at the brain circuit level. We can study using this protocol if modifying the excitability of a specific brain network confers resilience or vulnerability to developing food addiction. Understanding these neurobiological mechanisms is expected to help to find novel and efficient interventions to battle food addiction.

Adipose tissue has emerged as an important regulator of whole-body metabolism, and its capacity to dissipate energy in the form of heat has acquired a special relevance in recent years as potential treatment for obesity. In this context, the p38MAPK pathway has arisen as a key player in the thermogenic program because it is required for the activation of brown adipose tissue (BAT) thermogenesis and participates also in the transformation of white adipose tissue (WAT) into BAT-like depot called beige/brite tissue. Here, using mice that are deficient in p38α specifically in adipose tissue (p38αFab-KO), we unexpectedly found that lack of p38α protected against high-fat diet (HFD)-induced obesity. We also showed that p38αFab-KO mice presented higher energy expenditure due to increased BAT thermogenesis. Mechanistically, we found that lack of p38α resulted in the activation of the related protein kinase family member p38δ. Our results showed that p38δ is activated in BAT by cold exposure, and lack of this kinase specifically in adipose tissue (p38δ Fab-KO) resulted in overweight together with reduced energy expenditure and lower body and skin surface temperature in the BAT region. These observations indicate that p38α probably blocks BAT thermogenesis through p38δ inhibition. Consistent with the results obtained in animals, p38α was reduced in visceral and subcutaneous adipose tissue of subjects with obesity and was inversely correlated with body mass index (BMI). Altogether, we have elucidated a mechanism implicated in physiological BAT activation that has potential clinical implications for the treatment of obesity and related diseases such as diabetes.

In this study, we assessed the feasibility of using positron emission tomography (PET) and the tracer [ 11 C]OMAR ([ 11 C]JHU75528), an analogue of rimonabant, to study the brain cannabinoid type 1 (CB1) receptor system. Wild-type (WT) andCB1 knockout (KO) animals were imaged at baseline and after pretreatment with blocking doses of rimonabant. Brain uptake in WT animals was higher (50%) than in KO animals in baseline conditions. After pretreatment with rimonabant, WT uptake lowered to the level of KO animals. The results of this study support the feasibility of using PET with the radiotracer [ 11 C]JHU75528 to image the brain CB1 receptor system in mice. In addition, this methodology can be used to assess the effect of new drugs in preclinical studies using genetically manipulated animals.