Role of DOR in neuronal plasticity changes promoted by food-seeking behaviour.

Several lines of evidence support that food overconsumption may be related to the role of the endogenous opioid system in the control of food palatability. The opioid system, and particularly the delta opioid receptor (DOR), plays a crucial role in the regulation of food rewarding properties. In our study, we used operant conditioning maintained by chocolate-flavoured pellets to investigate the role of DOR in the motivation for palatable food and the structural plasticity changes promoted by this behaviour. For this purpose, we evaluated the specific role of this receptor in the behavioural and neuroplastic changes induced by palatable food in the prefrontal cortex (PFC), hippocampus (HCP) and nucleus accumbens (NAc) in constitutive knockout (KO) mice deficient in DOR. Mutant mice and their wild-type littermates were trained to obtain chocolate-flavoured pellets on fixed ratio 1 (FR1), FR5 and progressive ratio (PR) schedule of reinforcement. No significant differences between genotypes were revealed on operant behaviour acquisition in FR1. DOR knockout mice displayed lower number of active lever-presses than wild-type mice on FR5, and a similar decrease was revealed in DOR KO mice in the breaking point during the PR. This operant training to obtain palatable food increased dendritic spine density in the PFC, HCP and NAc shell of wild-type, but these plasticity changes were abolished in DOR KO mice. Our results support the hypothesis that DOR regulates the reinforcing effects and motivation for palatable food through neuroplastic changes in specific brain reward areas.

Imprinting fidelity in mouse iPSCs depends on sex of donor cell and medium formulation.

Reprogramming of somatic cells into induced Pluripotent Stem Cells (iPSCs) is a major leap towards personalised approaches to disease modelling and cell-replacement therapies. However, we still lack the ability to fully control the epigenetic status of iPSCs, which is a major hurdle for their downstream applications. Epigenetic fidelity can be tracked by genomic imprinting, a phenomenon dependent on DNA methylation, which is frequently perturbed in iPSCs by yet unknown reasons. To try to understand the causes underlying these defects, we conducted a thorough imprinting analysis using IMPLICON, a high-throughput method measuring DNA methylation levels, in multiple female and male murine iPSC lines generated under different experimental conditions. Our results show that imprinting defects are remarkably common in iPSCs, but their nature depends on the sex of donor cells and their response to culture conditions. Imprints in female iPSCs resist the initial genome-wide DNA demethylation wave during reprogramming, but ultimately cells accumulate hypomethylation defects irrespective of culture medium formulations. In contrast, imprinting defects on male iPSCs depends on the experimental conditions and arise during reprogramming, being mitigated by the addition of vitamin C (VitC). Our findings are fundamental to further optimise reprogramming strategies and generate iPSCs with a stable epigenome.

A behavioral and pharmacological characterization of palatable diet alternation in mice.

Obesity and eating disorders are widespread in Western societies. Both the increased availability of highly palatable foods and dieting are major risk factors contributing to the epidemic of disorders of feeding. The purpose of this study was to characterize an animal model of maladaptive feeding induced by intermittent access to a palatable diet alternation in mice. In this study, mice were either continuously provided with standard chow food (Chow/Chow), or provided with standard chow for 2days and a high-sucrose, palatable food for 1day (Chow/Palatable). Following stability of intake within the cycling paradigm, we then investigated the effects of several pharmacological treatments on excessive eating of palatable food: naltrexone, an opioid receptor antagonist, SR141716A, a cannabinoid-1 receptor antagonist/inverse agonist, and BD-1063, a sigma-1 receptor antagonist. Over successive cycles, Chow/Palatable mice showed an escalation of palatable food intake within the first hour of renewed access to palatable diet and displayed hypophagia upon its removal. Naltrexone, SR141716A, and BD-1063 all reduced overconsumption of palatable food during this first hour. Here we provide evidence of strong face and convergent validity in a palatable diet alternation model in mice, confirming multiple shared underlying mechanisms of pathological eating across species, and thus making it a useful therapeutic development tool.

Epigenetic and Proteomic Expression Changes Promoted by Eating Addictive-Like Behavior.

An increasing perspective conceptualizes obesity and overeating as disorders related to addictive-like processes that could share common neurobiological mechanisms. In the present study, we aimed at validating an animal model of eating addictive-like behavior in mice, based on the DSM-5 substance use disorder criteria, using operant conditioning maintained by highly palatable chocolate-flavored pellets. For this purpose, we evaluated persistence of food-seeking during a period of non-availability of food, motivation for food, and perseverance of responding when the reward was associated with a punishment. This model has allowed identifying extreme subpopulations of mice related to addictive-like behavior. We investigated in these subpopulations the epigenetic and proteomic changes. A significant decrease in DNA methylation of CNR1 gene promoter was revealed in the prefrontal cortex of addict-like mice, which was associated with an upregulation of CB1 protein expression in the same brain area. The pharmacological blockade (rimonabant 3 mg/kg; i.p.) of CB1 receptor during the late training period reduced the percentage of mice that accomplished addiction criteria, which is in agreement with the reduced performance of CB1 knockout mice in this operant training. Proteomic studies have identified proteins differentially expressed in mice vulnerable or not to addictive-like behavior in the hippocampus, striatum, and prefrontal cortex. These changes included proteins involved in impulsivity-like behavior, synaptic plasticity, and cannabinoid signaling modulation, such as alpha-synuclein, phosphatase 1-alpha, doublecortin-like kinase 2, and diacylglycerol kinase zeta, and were validated by immunoblotting. This model provides an excellent tool to investigate the neurobiological substrate underlying the vulnerability to develop eating addictive-like behavior.

Effects of genetic deletion of endogenous opioid system components on the reinstatement of cocaine-seeking behavior in mice.

The repeated cycles of cessation of consumption and relapse remain the major clinical concern in treating drug addiction. The endogenous opioid system is a crucial component of the reward circuit that participates in the adaptive changes leading to relapse in the addictive processes. We have used genetically modified mice to evaluate the involvement of µ-opioid receptor (MOR) and δ-opioid receptor (DOR) and their main endogenous ligands, the enkephalins derived from proenkephalin (PENK) and prodynorphin (PDYN), in the reinstatement of cocaine-seeking behavior. Constitutive knockout mice of MOR, DOR, PENK, and PDYN, and their wild-type littermates were trained to self-administer cocaine or to seek for palatable food, followed by a period of extinction and finally tested on a cue-induced reinstatement of seeking behavior. The four lines of knockout mice acquired operant cocaine self-administration behavior, although DOR and PENK knockout mice showed less motivation for cocaine than wild-type littermates. Moreover, cue-induced relapse was significantly decreased in MOR and DOR knockout mice. In contrast, PDYN knockout mice showed a slower extinction and increased relapse than wild-type littermates. C-Fos expression analysis revealed differential activation in brain areas related with memory and reward in these knockout mice. No differences were found in any of the four genotypes in operant responding to obtain palatable food, indicating that the changes revealed in knockout mice were not due to unspecific deficit in operant performance. Our results indicate that MOR, DOR, and PDYN have a differential role in cue-induced reinstatement of cocaine-seeking behavior.