Daily Exposure to Sucrose Impairs Subsequent Learning About Food Cues: A Role for Alterations in Ghrelin Signaling and Dopamine D2 Receptors

Cognitive representations of intracranial self-stimulation of midbrain dopamine neurons depend on stimulation frequency

Dopamine neurons in the ventral tegmental area support intracranial self-stimulation (ICSS), yet the cognitive representations underlying this phenomenon remain unclear. Here, 20-Hz stimulation of dopamine neurons, which approximates a physiologically relevant prediction error, was not sufficient to support ICSS beyond a continuously reinforced schedule and did not endow cues with a general or specific value. However, 50-Hz stimulation of dopamine neurons was sufficient to drive robust ICSS and was represented as a specific reward to motivate behavior. The frequency dependence of this effect is due to the rate (not the number) of action potentials produced by dopamine neurons, which differently modulates dopamine release downstream.

Free-choice high-fat diet consumption reduces lateral hypothalamic GABAergic activity, without disturbing neural response to sucrose drinking in mice

Nutrition can influence the brain and affect its regulation of food intake, especially that of high-palatable foods. We hypothesize that fat and sugar have interacting effects on the brain, and the lateral hypothalamus (LH) is a prime candidate to be involved in this interaction. The LH is a heterogeneous area, crucial for regulating consummatory behaviors, and integrating homeostatic and hedonic needs. GABAergic LH neurons stimulate feeding when activated, and are responsive to consummatory behavior while encoding sucrose palatability. Previously, we have shown that glutamatergic LH neurons reduce their activity in response to sugar drinking and that this response is disturbed by a free-choice high-fat diet (fcHFD). Whether GABAergic LH neurons, and their response to sugar, is affected by a fcHFD is yet unknown. Using head-fixed two-photon microscopy, we analyzed activity changes in LHVgat neuronal activity in chow or fcHFD-fed mice in response to water or sucrose drinking. A fcHFD decreased overall LHVgat neuronal activity, without disrupting the sucrose-induced increase. When focusing on the response per unique neuron, a vast majority of neurons respond inconsistently over time. Thus, a fcHFD dampens overall LH GABAergic activity, while it does not disturb the response to sucrose. The inconsistent responding over time suggests that it is not one specific subpopulation of LH GABAergic neurons that is driving these behaviors, but rather a result of the integrative properties of a complex neural network. Further research should focus on determining how this dampening of LH GABAergic activity contributes to hyperphagia and the development of obesity.

Agency rescues competition for credit assignment among predictive cues from adverse learning conditions

A fundamental assumption of learning theories is that the credit assigned to predictive cues is not simply determined by their probability of reinforcement, but by their ability to compete with other cues present during learning. This assumption has guided behavioral and neural science research for decades, and tremendous empirical and theoretical advances have been made identifying the mechanisms of cue competition. However, when learning conditions are not optimal (e.g., when training is massed), cue competition is attenuated. This failure of the learning system exposes the individual’s vulnerability to form spurious associations in the real world. Here, we uncover that cue competition in rats can be rescued when conditions are suboptimal provided that the individual has agency over the learning experience. Our findings reveal a new effect of agency over learning on credit assignment among predictive cues, and open new avenues of investigation into the underlying mechanisms.

Pattern of access to cafeteria-style diet determines fat mass and degree of spatial memory impairments in rats

Repeated ‘cycling’ between healthy and unhealthy eating is increasingly common but the effects of such cycling on cognitive function are unknown. Here we tested the effects of cycling between chow and a cafeteria diet (CAF) rich in saturated fat and refined carbohydrates on fat mass and place recognition memory in rats. Rats fed the chow diet (control group) were compared with groups fed CAF for either: 3 consecutive days per week followed by 4 days of chow, (3CAF:4CHOW group); 5 consecutive days per week followed by 2 days of chow (5CAF:2CHOW group); or 7 days per week (7CAF group). Total days of exposure to CAF were matched between the latter groups by staggering the introduction of CAF diet. After 16–18 days of CAF, spatial recognition memory was significantly worse in the 7CAF group relative to controls. After 23–25 days of CAF, both the 7CAF and 5CAF:2CHOW groups, but not the 3CAF:4CHOW group, were impaired relative to controls, mirroring changes in fat mass measured by EchoMRI. CAF feeding did not affect object recognition memory or total exploration time. These results indicate that even when matching total exposure, the pattern of access to unhealthy diets impairs spatial memory in a graded fashion.

Dopamine D2 -141C Ins/Del and Taq1A polymorphisms, body mass index, and prediction error brain response

The prediction error model is a widely used paradigm that is conceptually based on neuronal dopamine function. However, whether dopamine receptor gene alleles contribute to human neuroimaging prediction error results is uncertain. Recent research implicated the dopamine D2 receptor in behavior response during a prediction error paradigm and we expected that polymorphisms of that receptor would contribute to prediction error brain response. In this study, healthy female participants in the early follicular phase of the menstrual cycle underwent a taste prediction error paradigm during functional magnetic resonance imaging. Participants were also genotyped for dopamine receptor polymorphisms. Our data suggest that the dopamine D2 receptor -141C Ins/Del and Taq1A polymorphisms together with body mass index selectively explain putamen prediction error response. This was true using a region of interest analysis as well as for a whole-brain analysis (FWE corrected). Polymorphisms for dopamine D1 or D4 receptors, dopamine transporter, or COMT did not significantly contribute to prediction error activation. The prediction error model is a computational reward-learning paradigm that is important in psychiatric research and has been associated with dopamine. The results from this study indicate that dopamine D2 receptor polymorphisms together with body mass index are important determinants to include in research that tests prediction error response of the brain. Psychiatric disorders are frequently associated with elevated or reduced body weight. Adding BMI to genetic information in brain-imaging studies that use reward and the prediction error paradigm may be important to increase validity and reliability of results.

Locomotor sensitization is expressed by ghrelin and D1 dopamine receptor agonist in the nucleus accumbens core in amphetamine pre-exposed rat

Ghrelin modulates mesolimbic dopaminergic pathways in the brain in addition to its role in feeding. We investigated what roles ghrelin in the nucleus accumbens (NAcc) core may play in mediating locomotor activating effects of amphetamine (AMPH). First, when rats were administered with AMPH (1 mg/kg, i.p.) following a bilateral microinjection of ghrelin (0.1 or 0.5 μg/side) into the NAcc core, their locomotor activity was significantly enhanced, while these effects were blocked by co-microinjection of ghrelin receptor antagonist (0.5 μg/side) into this site. Second, we pre-exposed rats to saline or amphetamine (1 mg/kg, i.p.) every 2 to 3 days for a total of four times. After 2 weeks of drug-free withdrawal period, we examined the effect of saline, ghrelin (0.5 μg/side), D1 dopamine receptor agonist, SKF81297 (0.5 μg/side) or ghrelin (0.5 μg/side) + SKF81297 (0.5 μg/side) directly microinjected into the NAcc core on locomotor activity. When we measured rats' locomotor activity for 1 hour immediately following microinjections, only ghrelin + SKF81297 produces sensitized locomotor activity, while all others have no effects. These results suggest that ghrelin may have a distinct role in the NAcc core to provoke the sensitized locomotor activity induced by psychomotor stimulants, and further, it may produce these effects by interaction with D1 dopamine receptors.

Sugar sensor genes in the murine gastrointestinal tract display a cephalocaudal axis of expression and a diurnal rhythm

Distributed along the length of the gastrointestinal (GI) tract are nutrient sensing cells that release numerous signaling peptides influencing GI function, nutrient homeostasis and energy balance. Recent studies have shown a diurnal rhythm in GI nutrient sensing, but the mechanisms responsible for rhythmicity are poorly understood. In this report we studied murine GI sugar sensor gene and protein expression levels in the morning (7 AM) and evening (7 PM). Sweet taste receptor ( tas1r2/tas1r3/gnat3/gnat1) sugar transporter ( slc5a1, slc2a2, slc2a5) and putative sugar sensor ( slc5a4a and slc5a4b) gene expression levels were highest in tongue and proximal and distal small intestine, respectively. Clock gene ( cry2/arntl) activity was detected in all regions studied. Slc5a4a and slc5a4b gene expression showed clear diurnal rhythmicity in the small intestine and stomach, respectively, although no rhythmicity was detected in SGLT3 protein expression. Tas1r2, tas1r3, gnat1, and gcg displayed a limited rhythm in gene expression in proximal small intestine. Microarray analysis revealed a diurnal rhythm in gut peptide gene expression in tongue (7 AM vs. 7 PM) and in silico promoter analysis indicated intestinal sugar sensors and transporters possessed the canonical E box elements necessary for clock gene control over gene transcription. In this report we present evidence of a diurnal rhythm in genes that are responsible for intestinal nutrient sensing that is most likely controlled by clock gene activity. Disturbances in clock gene/nutrient sensing interactions may be important in the development of diet-related diseases, such as obesity and diabetes.

The neurological effects of ghrelin in brain diseases: Beyond metabolic functions

Ghrelin, a peptide released by the stomach that plays a major role in regulating energy metabolism, has recently been shown to have effects on neurobiological behaviors. Ghrelin enhances neuronal survival by reducing apoptosis, alleviating inflammation and oxidative stress, and accordingly improving mitochondrial function. Ghrelin also stimulates the proliferation, differentiation and migration of neural stem/progenitor cells (NS/PCs). Additionally, the ghrelin is benefit for the recovery of memory, mood and cognitive dysfunction after stroke or traumatic brain injury. Because of its neuroprotective and neurogenic roles, ghrelin may be used as a therapeutic agent in the brain to combat neurodegenerative disease. In this review, we highlight the pre-clinical evidence and the proposed mechanisms underlying the role of ghrelin in physiological and pathological brain function.

Adolescent Maturational Transitions in the Prefrontal Cortex and Dopamine Signaling as a Risk Factor for the Development of Obesity and High Fat/High Sugar Diet Induced Cognitive Deficits

Adolescence poses as both a transitional period in neurodevelopment and lifestyle practices. In particular, the developmental trajectory of the prefrontal cortex (PFC), a critical region for behavioral control and self-regulation, is enduring, not reaching functional maturity until the early 20 s in humans. Furthermore, the neurotransmitter dopamine is particularly abundant during adolescence, tuning the brain to rapidly learn about rewards and regulating aspects of neuroplasticity. Thus, adolescence is proposed to represent a period of vulnerability towards reward-driven behaviors such as the consumption of palatable high fat and high sugar diets. This is reflected in the increasing prevalence of obesity in children and adolescents as they are the greatest consumers of “junk foods”. Excessive consumption of diets laden in saturated fat and refined sugars not only leads to weight gain and the development of obesity, but experimental studies with rodents indicate they evoke cognitive deficits in learning and memory process by disrupting neuroplasticity and altering reward processing neurocircuitry. Consumption of these high fat and high sugar diets have been reported to have a particularly pronounced impact on cognition when consumed during adolescence, demonstrating a susceptibility of the adolescent brain to enduring cognitive deficits. The adolescent brain, with heightened reward sensitivity and diminished behavioral control compared to the mature adult brain, appears to be a risk for aberrant eating behaviors that may underpin the development of obesity. This review explores the neurodevelopmental changes in the PFC and mesocortical dopamine signaling that occur during adolescence, and how these potentially underpin the overconsumption of palatable food and development of obesogenic diet-induced cognitive deficits.

New Insights in Anorexia Nervosa

Anorexia nervosa (AN) is classically defined as a condition in which an abnormally low body weight is associated with an intense fear of gaining weight and distorted cognitions regarding weight, shape, and drive for thinness. This article reviews recent evidences from physiology, genetics, epigenetics, and brain imaging which allow to consider AN as an abnormality of reward pathways or an attempt to preserve mental homeostasis. Special emphasis is put on ghrelino-resistance and the importance of orexigenic peptides of the lateral hypothalamus, the gut microbiota and a dysimmune disorder of neuropeptide signaling. Physiological processes, secondary to underlying, and premorbid vulnerability factors-the "pondero-nutritional-feeding basements"- are also discussed.