Common Neural Mechanisms of Palatable Food Intake and Drug Abuse: Knowledge Obtained with Animal Models

Co-administration of bromocriptine and corticosterone produces short- and long-lasting reduction in intake of high-fat food in male rats

Dopaminergic and glucocorticoid activity has been associated with reduced food consumption; however, their possible synergic action has not yet been studied. With the aim of examining the effect of the co-administration of the dopamine receptor D2 agonist bromocriptine and corticosterone on palatable food intake, male Wistar rats were administered either bromocriptine (1 mg/kg), corticosterone (2 mg/kg), bromocriptine + corticosterone (1 mg + 2 mg/kg) or a vehicle, with a fifth group used as a control. In all cases, substances were administered 30 min before exposure to standard food or palatable food, the latter high in carbohydrates [high carbohydrate food (HCF), 75%] or high-fat food (HFF, 67%). Food consumption and body weight were recorded daily. Results showed higher consumption of standard food but lower consumption of HCF and HFF in the groups that received bromocriptine, alone or in combination. In general, lower total kcal intake was observed in the bromocriptine and bromocriptine + corticosterone groups during the period of pharmacological treatment and following re-exposure to palatable food. The low HFF intake in the bromocriptine + corticosterone group persisted 10 days after the pharmacological treatment was interrupted. This effect suggests plastic changes in either the mechanisms involved in the incentive value of palatable food - particularly foods with high-fat content - or those that regulate lipid metabolism. Our findings suggest that homeostatic and reward mechanisms could be influenced by the co-participation of the dopaminergic and hypothalamic-pituitary-adrenal systems, and the macronutrient content of food.

Neurobiological Mechanisms Modulating Emotionality, Cognition and Reward-Related Behaviour in High-Fat Diet-Fed Rodents

Affective and substance-use disorders are associated with overweight and obesity-related complications, which are often due to the overconsumption of palatable food. Both high-fat diets (HFDs) and psychostimulant drugs modulate the neuro-circuitry regulating emotional processing and metabolic functions. However, it is not known how they interact at the behavioural level, and whether they lead to overlapping changes in neurobiological endpoints. In this literature review, we describe the impact of HFDs on emotionality, cognition, and reward-related behaviour in rodents. We also outline the effects of HFD on brain metabolism and plasticity involving mitochondria. Moreover, the possible overlap of the neurobiological mechanisms produced by HFDs and psychostimulants is discussed. Our in-depth analysis of published results revealed that HFDs have a clear impact on behaviour and underlying brain processes, which are largely dependent on the developmental period. However, apart from the studies investigating maternal exposure to HFDs, most of the published results involve only male rodents. Future research should also examine the biological impact of HFDs in female rodents. Further knowledge about the molecular mechanisms linking stress and obesity is a crucial requirement of translational research and using rodent models can significantly advance the important search for risk-related biomarkers and the development of clinical intervention strategies.

Imbalanced sensitivities to primary and secondary rewards in internet gaming disorder

BACKGROUND

Internet gaming disorder (IGD) is a type of behavioral addiction characterized by poorly controlled and interfering patterns of game playing. Studies have suggested that the IGD is usually accompanied by increased desire or craving for gaming, suggesting that secondary rewards related to gaming may become more salient than those for primary rewards like food. However, this hypothesis has not been formally tested and potential neural mechanisms remain unclear.

METHODS

This is a functional magnetic resonance imaging (fMRI) study. Twenty-one IGD subjects and 23 matched individuals with recreational game use (RGU) were scanned when exposed to gaming (secondary rewards), food (primary rewards) and neutral cues. Group-by-cue-type interaction analyses and subsequent within-group analyses for fMRI data were performed and seed-based functional connectivity (FC) analyses explored further potential neural features.

RESULTS

IGD subjects' subjective craving responses to gaming cues were higher than to food cues, while the opposite was observed in RGU subjects. Group-by-cue interaction effects implicated the precuneus and precuneus-caudate FC. Simple effect analysis showed that for IGD subjects, gaming-related cues elicited higher FC in precuneus-caudate relationships than did food-related cues. In the RGU subjects, the opposite was observed. Significant correlations were found between brain features and craving scores.

CONCLUSIONS

These results support the hypothesis regarding imbalances in sensitivities to different types of reward in IGD, and suggest neural mechanisms by which craving for gaming may make secondary rewards more salient than primary ones, thus promoting participation in addictive patterns of gaming.

Bidirectional role of dopamine in learning and memory-active forgetting

Dopaminergic neurons projecting from the Substantia Nigra to the Striatum play a critical role in motor functions while dopaminergic neurons originating in the Ventral Tegmental Area (VTA) and projecting to the Nucleus Accumbens, Hippocampus and other cortical structures regulate rewarding learning. While VTA mainly consists of dopaminergic neurons, excitatory (glutamate) and inhibitory (GABA) VTA-neurons have also been described: these neurons may also modulate and contribute to shape the final dopaminergic response, which is critical for memory formation. However, given the large amount of information that is handled daily by our brain, it is essential that irrelevant information be deleted. Recently, apart from the well-established role of dopamine (DA) in learning, it has been shown that DA plays a critical role in the intrinsic active forgetting mechanisms that control storage information, contributing to the deletion of a consolidated memory. These new insights may be instrumental to identify therapies for those disorders that involve memory alterations.

Ketogenic Diet Decreases Alcohol Intake in Adult Male Mice

The classic ketogenic diet is a diet high in fat, low in carbohydrates, and well-adjusted proteins. The reduction in glucose levels induces changes in the body’s metabolism, since the main energy source happens to be ketone bodies. Recent studies have suggested that nutritional interventions may modulate drug addiction. The present work aimed to study the potential effects of a classic ketogenic diet in modulating alcohol consumption and its rewarding effects. Two groups of adult male mice were employed in this study, one exposed to a standard diet (SD, n = 15) and the other to a ketogenic diet (KD, n = 16). When a ketotic state was stable for 7 days, animals were exposed to the oral self-administration paradigm to evaluate the reinforcing and motivating effects of ethanol. Rt-PCR analyses were performed evaluating dopamine, adenosine, CB1, and Oprm gene expression. Our results showed that animals in a ketotic state displayed an overall decrease in ethanol consumption without changes in their motivation to drink. Gene expression analyses point to several alterations in the dopamine, adenosine, and cannabinoid systems. Our results suggest that nutritional interventions may be a useful complementary tool in treating alcohol-use disorders.

Neuroendocrine Response to Exogenous Ghrelin Administration, Combined With Alcohol, in Heavy-Drinking Individuals: Findings From a Randomized, Double-Blind, Placebo-Controlled Human Laboratory Study

BACKGROUND

Accumulating evidence has established a role for the orexigenic hormone ghrelin in alcohol-seeking behaviors. Accordingly, the ghrelin system may represent a potential pharmacotherapeutic target for alcohol use disorder. Ghrelin modulates several neuroendocrine pathways, such as appetitive, metabolic, and stress-related hormones, which are particularly relevant in the context of alcohol use. The goal of the present study was to provide a comprehensive assessment of neuroendocrine response to exogenous ghrelin administration, combined with alcohol, in heavy-drinking individuals.

METHODS

This was a randomized, crossover, double-blind, placebo-controlled human laboratory study, which included 2 experimental alcohol administration paradigms: i.v. alcohol self-administration and i.v. alcohol clamp. Each paradigm consisted of 2 counterbalanced sessions of i.v. ghrelin or placebo administration. Repeated blood samples were collected during each session, and peripheral concentrations of the following hormones were measured: leptin, glucagon-like peptide-1, pancreatic polypeptide, gastric inhibitory peptide, insulin, insulin-like growth factor-1, cortisol, prolactin, and aldosterone.

RESULTS

Despite some statistical differences, findings were consistent across the 2 alcohol administration paradigms: i.v. ghrelin, compared to placebo, increased blood concentrations of glucagon-like peptide-1, pancreatic polypeptide, cortisol, and prolactin, both acutely and during the whole session. Lower levels of leptin and higher levels of aldosterone were also found during the ghrelin vs placebo session.

CONCLUSION

These findings, gathered from a clinically relevant sample of heavy-drinking individuals with alcohol use disorder, provide a deeper insight into the complex interplay between ghrelin and appetitive, metabolic, and stress-related neuroendocrine pathways in the context of alcohol use.

Activation of the amylin pathway modulates cocaine-induced activation of the mesolimbic dopamine system in male mice

Besides food intake reduction, activation of the amylin pathway by salmon calcitonin (sCT), an amylin and calcitonin receptor agonist, inhibits alcohol-mediated behaviors in rodents. This involves brain areas processing reward, i.e. the laterodorsal (LDTg), ventral tegmental area (VTA) and nucleus accumbens (NAc). However, the effects of stimulation of the amylin pathway on behaviors caused by cocaine and the brain areas involved in these processes have not yet been investigated. We therefore explored in male mice, the effects of systemic administration of sCT on cocaine-induced locomotor stimulation, dopamine release in the NAc and cocaine reward, as well as reward-dependent memory of cocaine, in the conditioned place preference (CPP) paradigm. Moreover, the outcome of systemic sCT and cocaine co-administration for five days on locomotor activity was investigated. Lastly, the impact of sCT infusions into the LDTg, VTA, NAc shell or core on cocaine-evoked locomotor stimulation was explored. We found that sCT attenuated cocaine-induced locomotor stimulation and accumbal dopamine release, without altering cocaine's rewarding properties or reward-dependent memory retrieval in the CPP paradigm. Five days of cocaine administration caused locomotor stimulation in mice pre-treated with vehicle, but not with sCT. In mice infused with vehicle into the aforementioned reward-related areas, cocaine caused locomotor stimulation, a response that was not evident following sCT infusions. The current findings suggest a novel role for the amylinergic pathway as regulator of cocaine-evoked activation of the mesolimbic dopamine system, opening the way for the investigation of the amylin signalling in the modulation of other drugs of abuse.