Metabolic hormones, dopamine circuits, and feeding

Does activation of midbrain dopamine neurons promote or reduce feeding?

BACKGROUND

Dopamine (DA) signalling in the brain is necessary for feeding behaviour, and alterations in the DA system have been linked to obesity. However, the precise role of DA in the control of food intake remains debated. On the one hand, food reward and motivation are associated with enhanced DA activity. On the other hand, psychostimulant drugs that increase DA signalling suppress food intake. This poses the questions of how endogenous DA neuronal activity regulates feeding, and whether enhancing DA neuronal activity would either promote or reduce food intake.

METHODS

Here, we used designer receptors exclusively activated by designer drugs (DREADD) technology to determine the effects of enhancing DA neuronal activity on feeding behaviour. We chemogenetically activated selective midbrain DA neuronal subpopulations and assessed the effects on feeding microstructure in rats.

RESULTS

Treatment with the psychostimulant drug amphetamine or the selective DA reuptake inhibitor GBR 12909 significantly suppressed food intake. Selective chemogenetic activation of DA neurons in the ventral tegmental area (VTA) was found to reduce meal size, but had less impact on total food intake. Targeting distinct VTA neuronal pathways revealed that specific activation of the mesolimbic pathway towards nucleus accumbens (NAc) resulted in smaller and shorter meals. In addition, the meal frequency was increased, rendering total food intake unaffected. The disrupted feeding patterns following activation of VTA DA neurons or VTA to NAc projection neurons were accompanied by locomotor hyperactivity. Activation of VTA neurons projecting towards prefrontal cortex or amygdala, or of DA neurons in the substantia nigra, did not affect feeding behaviour.

CONCLUSIONS

Chemogenetic activation of VTA DA neurons or VTA to NAc pathway disrupts feeding patterns. Increased activity of mesolimbic DA neurons appears to both promote and reduce food intake, by facilitating both the initiation and cessation of feeding behaviour.

From Belly to Brain: Targeting the Ghrelin Receptor in Appetite and Food Intake Regulation

Ghrelin is the only known peripherally-derived orexigenic hormone, increasing appetite and subsequent food intake. The ghrelinergic system has therefore received considerable attention as a therapeutic target to reduce appetite in obesity as well as to stimulate food intake in conditions of anorexia, malnutrition and cachexia. As the therapeutic potential of targeting this hormone becomes clearer, it is apparent that its pleiotropic actions span both the central nervous system and peripheral organs. Despite a wealth of research, a therapeutic compound specifically targeting the ghrelin system for appetite modulation remains elusive although some promising effects on metabolic function are emerging. This is due to many factors, ranging from the complexity of the ghrelin receptor (Growth Hormone Secretagogue Receptor, GHSR-1a) internalisation and heterodimerization, to biased ligand interactions and compensatory neuroendocrine outputs. Not least is the ubiquitous expression of the GHSR-1a, which makes it impossible to modulate centrally-mediated appetite regulation without encroaching on the various peripheral functions attributable to ghrelin. It is becoming clear that ghrelin’s central signalling is critical for its effects on appetite, body weight regulation and incentive salience of food. Improving the ability of ghrelin ligands to penetrate the blood brain barrier would enhance central delivery to GHSR-1a expressing brain regions, particularly within the mesolimbic reward circuitry.

Effect of Orexin A antagonist (SB-334867) infusion into the nucleus accumbens on consummatory behavior and alcohol preference in Wistar rats

Objective:

Nucleus accumbens (NAcc) has a role in addiction and ingestive behavior. In order to assess orexinergic system involved in this, we infused Orexin A antagonist and assessed the effect on food intake fluid intake and alcohol preference in Wistar rats.

Materials and Methods:

Inbred Wistar rats (n = 54) were divided into control and experimental groups (low dose and high dose). Using stereotaxic method, guide cannula was set in place bilaterally to reach NAcc. Low dose (3 ng) and high dose (6 ng) of Orexin A antagonist (SB-334867) was infused, and the food consumption, water intake and alcohol intake, and two bottle free choice preference test for alcohol were carried out in the experimental group. The control group received saline infusion and rest of the methods followed were same. The measurements were carried out immediately after the infusion, at 1 h, 2 h, 4 h, and for the whole day and represented in the figure and tables.

Results:

A decrease in water intake observed immediately after the infusion in 1^st h (P < 0.05) and 2^nd h (P < 0.01), which was more in high dose group compared to low dose and controls. Alcohol intake was also following the same pattern. In two bottle free choice, rats did not show any specific preference to alcohol.

Conclusion:

There was dose dependent reduction in intake of food and fluids in treated rats. This suggested a possible role for orexinergic system in ingestive behavior. However, Orexin A may not have a role in modulation of alcohol addiction by the rewarding center NAcc.

Homeostasis Meets Motivation in the Battle to Control Food Intake

Signals of energy homeostasis interact closely with neural circuits of motivation to control food intake. An emerging hypothesis is that the transition to maladaptive feeding behavior seen in eating disorders or obesity may arise from dysregulation of these interactions. Focusing on key brain regions involved in the control of food intake (ventral tegmental area, striatum, hypothalamus, and thalamus), we describe how activity of specific cell types embedded within these regions can influence distinct components of motivated feeding behavior. We review how signals of energy homeostasis interact with these regions to influence motivated behavioral output and present evidence that experience-dependent neural adaptations in key feeding circuits may represent cellular correlates of impaired food intake control. Future research into mechanisms that restore the balance of control between signals of homeostasis and motivated feeding behavior may inspire new treatment options for eating disorders and obesity.

FoxO1 in dopaminergic neurons regulates energy homeostasis and targets tyrosine hydroxylase

Dopaminergic (DA) neurons are involved in the integration of neuronal and hormonal signals to regulate food consumption and energy balance. Forkhead transcriptional factor O1 (FoxO1) in the hypothalamus plays a crucial role in mediation of leptin and insulin function. However, the homoeostatic role of FoxO1 in DA system has not been investigated. Here we report that FoxO1 is highly expressed in DA neurons and mice lacking FoxO1 specifically in the DA neurons (FoxO1 KO^DAT) show markedly increased energy expenditure and interscapular brown adipose tissue (iBAT) thermogenesis accompanied by reduced fat mass and improved glucose/insulin homoeostasis. Moreover, FoxO1 KO^DAT mice exhibit an increased sucrose preference in concomitance with higher dopamine and norepinephrine levels. Finally, we found that FoxO1 directly targets and negatively regulates tyrosine hydroxylase (TH) expression, the rate-limiting enzyme of the catecholamine synthesis, delineating a mechanism for the KO phenotypes. Collectively, these results suggest that FoxO1 in DA neurons is an important transcriptional factor that directs the coordinated control of energy balance, thermogenesis and glucose homoeostasis.

Dopaminergic neurons are important for regulating energy homeostasis. Here, the authors show the transcription factor FoxO1 negatively regulates tyrosine hydroxylase expression in midbrain dopaminergic neurons, and plays an important role in regulation of glucose homeostasis, energy expenditure, and resistance to diet-induced obesity.

Responses to drugs of abuse and non-drug rewards in leptin deficient ob/ob mice

RATIONALE

Although leptin receptors are found in hypothalamic nuclei classically associated with homeostatic feeding mechanisms, they are also present in brain regions known to regulate hedonic-based feeding, natural reward processing, and responses to drugs of abuse. The ob/ob mouse is deficient in leptin signaling, and previous work has found altered mesolimbic dopamine signaling and sensitivity to the locomotor activating effects of amphetamine in these mice.

OBJECTIVES

We directly assessed responses to three drugs of abuse and non-drug rewards in the leptin-deficient ob/ob mouse.

METHODS

Ob/ob mice were tested in assays of sweet preference, novelty seeking, and drug reward/reinforcement.

RESULTS

In assays of novelty seeking, novel open field activity and operant sensation seeking were reduced in ob/ob mice, although novel object interaction and novel environment preference were comparable to wild types. We also found that ob/ob mice had specific phenotypes in regard to cocaine: conditioned place preference for 2.5 mg/kg was increased, while the locomotor response to 10 mg/kg was reduced, and cocaine self-administration was the same as wild types. Ob/ob mice also acquired self-administration of the potent opioid remifentanil, but breakpoints for the drug were significantly reduced. Finally, we found significant differences in ethanol drinking in ob/ob mice that correlated negatively with body weight and positively with operant sensation seeking.

CONCLUSIONS

In conclusion, ob/ob mice displayed task-specific deficits in novelty seeking and dissociable differences in reward/reinforcement associated with cocaine, remifentanil, and ethanol.

Glucagon-Like Peptide-1 Receptor Activation in the Ventral Tegmental Area Decreases the Reinforcing Efficacy of Cocaine

Cocaine addiction continues to be a significant public health problem for which there are currently no effective FDA-approved treatments. Thus, there is a clear need to identify and develop novel pharmacotherapies for cocaine addiction. Recent evidence indicates that activation of glucagon-like peptide-1 (GLP-1) receptors in the ventral tegmental area (VTA) reduces intake of highly palatable food. As the neural circuits and neurobiological mechanisms underlying drug taking overlap to some degree with those regulating food intake, these findings suggest that activation of central GLP-1 receptors may also attenuate cocaine taking. Here, we show that intra-VTA administration of the GLP-1 receptor agonist exendin-4 (0.05 μg) significantly reduced cocaine, but not sucrose, self-administration in rats. We also demonstrate that cocaine taking is associated with elevated plasma corticosterone levels and that systemic infusion of cocaine activates GLP-1-expressing neurons in the nucleus tractus solitarius (NTS), a hindbrain nucleus that projects monosynaptically to the VTA. To determine the potential mechanisms by which cocaine activates NTS GLP-1-expressing neurons, we microinjected corticosterone (0.5 μg) directly into the hindbrain fourth ventricle. Intraventricular corticosterone attenuated cocaine self-administration and this effect was blocked in animals pretreated with the GLP-1 receptor antagonist exendin-(9-39) (10 μg) in the VTA. Finally, AAV-shRNA-mediated knockdown of VTA GLP-1 receptors was sufficient to augment cocaine self-administration. Taken together, these findings indicate that increased activation of NTS GLP-1-expressing neurons by corticosterone may represent a homeostatic response to cocaine taking, thereby reducing the reinforcing efficacy of cocaine. Therefore, central GLP-1 receptors may represent a novel target for cocaine addiction pharmacotherapies.

The DRD3 Ser9Gly Polymorphism Predicted Metabolic Change in Drug-Naive Patients With Bipolar II Disorder

Patients with bipolar II disorder (BDII) have a higher prevalence rate of metabolic disturbance. Whether BDII itself, in addition to its current standard treatment, is a risk factor for metabolic syndrome warrants additional study. The dopamine receptor D3 (DRD3) gene, one of the candidate genes for BDII, is also involved in the dopaminergic system. We investigated whether it is related to changes in the metabolic indices of patients with BDII given 12 weeks of standard treatment.Patients with a first diagnosis of BDII (n = 117) were recruited. Metabolic profiles (cholesterol, triglycerides, fasting serum glucose, body mass index) were measured at baseline and at 2, 8, and 12 weeks. The genotype of the DRD3 Ser9Gly polymorphism (rs6280) was determined. Multiple linear regressions with generalized estimating equation methods were used.Seventy-six (65.0%) patients completed the 12-week intervention. Significant differences in triglyceride change were associated with the DRD3 Ser9Gly genotype (P = 0.03). Patients with the Ser/Ser genotype had significantly smaller triglyceride increases and a lower risk of developing metabolic syndrome than did those with the Ser/Gly+Gly/Gly genotype. However, the associations between the DRD3 Ser9Gly polymorphism with changes in triglyceride level become nonsignificant after correcting for multiple comparisons.We conclude that the DRD3 Ser9Gly polymorphism is nominally associated with changes in triglycerides and metabolic syndrome after 12 weeks of standard BDII treatment.

Treadmill Intervention Attenuates the Cafeteria Diet-Induced Impairment of Stress-Coping Strategies in Young Adult Female Rats

The current prevalence of diet-induced overweight and obesity in adolescents and adults is continuously growing. Although the detrimental biochemical and metabolic consequences of obesity are widely studied, its impact on stress-coping behavior and its interaction with specific exercise doses (in terms of intensity, duration and frequency) need further investigation. To this aim, we fed adolescent rats either an obesogenic diet (cafeteria diet, CAF) or standard chow (ST). Each group was subdivided into four subgroups according to the type of treadmill intervention as follows: a sedentary group receiving no manipulation; a control group exposed to a stationary treadmill; a low-intensity treadmill group trained at 12 m/min; and a higher intensity treadmill group trained at 17 m/min. Both the diet and treadmill interventions started at weaning and lasted for 8 weeks. Subjects were tested for anxiety-like behavior in the open field test and for coping strategies in the two-way active avoidance paradigm at week 7 and were sacrificed at week 8 for biometric and metabolic characterization. CAF feeding increased the weight gain, relative retroperitoneal white adipose tissue (RWAT %), and plasma levels of glucose, insulin, triglycerides and leptin and decreased the insulin sensitivity. Treadmill intervention partially reversed the RWAT% and triglyceride alterations; at higher intensity, it decreased the leptin levels of CAF-fed animals. CAF feeding decreased the motor activity and impaired the performance in a two-way active avoidance assessment. Treadmill intervention reduced defecation in the shuttle box, suggesting diminished anxiety. CAF feeding combined with treadmill training at 17 m/min increased the time spent in the center of the open field and more importantly, partially reversed the two-way active avoidance deficit. In conclusion, this study demonstrates that at doses that decreased anxiety-like behavior, treadmill exercise partially improved the coping strategy in terms of active avoidance behavior in the CAF-fed animals. This effect was not observed at lower doses of treadmill training.

The Effects of Insulin-Induced Hypoglycaemia on Tyrosine Hydroxylase Phosphorylation in Rat Brain and Adrenal Gland

In this study we investigated the effects of insulin-induced hypoglycaemia on tyrosine hydroxylase (TH) protein and TH phosphorylation in the adrenal gland, C1 cell group, locus coeruleus (LC) and midbrain dopaminergic cell groups that are thought to play a role in response to hypoglycaemia and compared the effects of different concentrations of insulin in rats. Insulin (1 and 10 U/kg) treatment caused similar reductions in blood glucose concentration (from 7.5-9 to 2-3 mmol/L); however, plasma adrenaline concentration was increased 20-30 fold in response to 10 U/kg insulin and only 14 fold following 1 U/kg. Time course studies (at 10 U/kg insulin) revealed that in the adrenal gland, Ser31 phosphorylation was increased between 30 and 90 min (4-5 fold), implying that TH was activated to increase catecholamine synthesis in adrenal medulla to replenish the stores. In the brain, Ser19 phosphorylation was limited to certain dopaminergic groups in the midbrain, while Ser31 phosphorylation was increased in most catecholaminergic regions at 60 min (1.3-2 fold), suggesting that Ser31 phosphorylation may be an important mechanism to maintain catecholamine synthesis in the brain. Comparing the effects of 1 and 10 U/kg insulin revealed that Ser31 phosphorylation was increased to similar extent in the adrenal gland and C1 cell group in response to both doses whereas Ser31 and Ser19 phosphorylation were only increased in response to 1 U/kg insulin in LC and in response to 10 U/kg insulin in most midbrain regions. Thus, the adrenal gland and some catecholaminergic brain regions become activated in response to insulin administration and brain catecholamines may be important for initiation of physiological defences against insulin-induced hypoglycaemia.

Amylin-mediated control of glycemia, energy balance, and cognition

Amylin, a peptide hormone produced in the pancreas and in the brain, has well-established physiological roles in glycemic regulation and energy balance control. It improves postprandial blood glucose levels by suppressing gastric emptying and glucagon secretion; these beneficial effects have led to the FDA-approved use of the amylin analog pramlintide in the treatment of diabetes mellitus. Amylin also acts centrally as a satiation signal, reducing food intake and body weight. The ability of amylin to promote negative energy balance, along with its unique capacity to cooperatively facilitate or enhance the intake- and body weight-suppressive effects of other neuroendocrine signals like leptin, have made amylin a leading target for the development of novel pharmacotherapies for the treatment of obesity. In addition to these more widely studied effects, a growing body of literature suggests that amylin may play a role in processes related to cognition, including the neurodegeneration and cognitive deficits associated with Alzheimer's disease (AD). Although the function of amylin in AD is still unclear, intriguing recent reports indicate that amylin may improve cognitive ability and reduce hallmarks of neurodegeneration in the brain. The frequent comorbidity of diabetes mellitus and obesity, as well as the increased risk for and occurrence of AD associated with these metabolic diseases, suggests that amylin-based pharmaceutical strategies may provide multiple therapeutic benefits. This review will discuss the known effects of amylin on glycemic regulation, energy balance control, and cognitive/motivational processes. Particular focus will be devoted to the current and/or potential future clinical use of amylin pharmacotherapies for the treatment of diseases in each of these realms.

Hippocampus ghrelin signaling mediates appetite through lateral hypothalamic orexin pathways

Feeding behavior rarely occurs in direct response to metabolic deficit, yet the overwhelming majority of research on the biology of food intake control has focused on basic metabolic and homeostatic neurobiological substrates. Most animals, including humans, have habitual feeding patterns in which meals are consumed based on learned and/or environmental factors. Here we illuminate a novel neural system regulating higher-order aspects of feeding through which the gut-derived hormone ghrelin communicates with ventral hippocampus (vHP) neurons to stimulate meal-entrained conditioned appetite. Additional results show that the lateral hypothalamus (LHA) is a critical downstream substrate for vHP ghrelin-mediated hyperphagia and that vHP ghrelin activated neurons communicate directly with neurons in the LHA that express the neuropeptide, orexin. Furthermore, activation of downstream orexin-1 receptors is required for vHP ghrelin-mediated hyperphagia. These findings reveal novel neurobiological circuitry regulating appetite through which ghrelin signaling in hippocampal neurons engages LHA orexin signaling.

Palatability Can Drive Feeding Independent of AgRP Neurons

Feeding behavior is exquisitely regulated by homeostatic and hedonic neural substrates that integrate energy demand as well as the reinforcing and rewarding aspects of food. Understanding the net contribution of homeostatic and reward-driven feeding has become critical because of the ubiquitous source of energy-dense foods and the consequent obesity epidemic. Hypothalamic agouti-related peptide-secreting neurons (AgRP neurons) provide the primary orexigenic drive of homeostatic feeding. Using models of neuronal inhibition or ablation, we demonstrate that the feeding response to a fast ghrelin or serotonin receptor agonist relies on AgRP neurons. However, when palatable food is provided, AgRP neurons are dispensable for an appropriate feeding response. In addition, AgRP-ablated mice present exacerbated stress-induced anorexia and palatable food intake--a hallmark of comfort feeding. These results suggest that, when AgRP neuron activity is impaired, neural circuits sensitive to emotion and stress are engaged and modulated by food palatability and dopamine signaling.

The Vulnerable Ventral Tegmental Area in Parkinson's Disease

INTRODUCTION

The involvement of dopaminergic neurons in the ventral tegmental area (VTA) in Parkinson's disease (PD) has not been universally recognized by neuroscientists and neurologists. Here, we conduct a review of previous research documenting dopaminergic neuronal loss in both the substantia nigra pars compacta (SNpc) and VTA and add three new post-mortem PD cases to the literature.

METHODS

PD and control brains were sectioned, stained for tyrosine hydroxylase, and cells in the SNpc and VTA were counted.

RESULTS

Based on the review, we report two main results: 1) the VTA does degenerate in PD, and 2) the VTA degenerates less than the SNpc.

CONCLUSION

Inconsistent clinical information about these cases limits our ability to interpret how the VTA contributes to PD symptoms. However, our data in combination with prior PD neuropathological cases in the literature unequivocally establish that the VTA is involved in PD, and could be relevant for future investigation of non-motor symptoms in PD.

Modulation of cue-induced firing of ventral tegmental area dopamine neurons by leptin and ghrelin

Background/objectives:

The rewarding value of palatable foods contributes to overconsumption, even in satiated subjects. Midbrain dopaminergic activity in response to reward-predicting environmental stimuli drives reward-seeking and motivated behavior for food rewards. This mesolimbic dopamine (DA) system is sensitive to changes in energy balance, yet it has thus far not been established whether reward signaling of DA neurons in vivo is under control of hormones that signal appetite and energy balance such as ghrelin and leptin.

Subjects/methods:

We trained rats (n=11) on an operant task in which they could earn two different food rewards. We then implanted recording electrodes in the ventral tegmental area (VTA), and recorded from DA neurons during behavior. Subsequently, we assessed the effects of mild food restriction and pretreatment with the adipose tissue-derived anorexigenic hormone leptin or the orexigenic hormone ghrelin on VTA DA reward signaling.

Results:

Animals showed an increase in performance following mild food restriction (P=0.002). Importantly, food-cue induced DA firing increased when animals were food restricted (P=0.02), but was significantly attenuated after leptin pretreatment (P=0.00). While ghrelin did affect baseline DA activity (P=0.025), it did not affect cue-induced firing (P⩾0.353).

Conclusions:

Metabolic signals, such as leptin, affect food seeking, a process that is dependent on the formation of cue-reward outcomes and involves midbrain DA signaling. These data show that food restriction engages the encoding of food cues by VTA DA neurons at a millisecond level and leptin suppresses this activity. This suggests that leptin is a key in linking metabolic information to reward signaling.

Cooperative interaction between leptin and amylin signaling in the ventral tegmental area for the control of food intake

Peripheral coadministration of amylin and leptin produces enhanced suppression of food intake and body weight, but the central nuclei mediating these effects remain unclear. Because each of these peptides controls feeding via actions at the ventral tegmental area (VTA), we tested the hypothesis that the VTA is a site of action for the cooperative effects of leptin and amylin on energy balance control. First, we show that intra-VTA injection of amylin and leptin at doses of each peptide that are effective in reducing food intake and body weight when administered separately produces an enhanced suppression of feeding when administered in combination. We also demonstrate that subthreshold doses of both amylin and leptin cause significant hypophagia and body weight loss when coadministered into the VTA. Additionally, we provide evidence that VTA amylin receptor blockade significantly attenuates the ability of intra-VTA leptin to reduce feeding and body weight gain. Together, these data provide the first evidence that the VTA mediates the interaction of amylin and leptin to cooperatively promote negative energy balance.

Fructose:glucose ratios--a study of sugar self-administration and associated neural and physiological responses in the rat

This study explored whether different ratios of fructose (F) and glucose (G) in sugar can engender significant differences in self-administration and associated neurobiological and physiological responses in male Sprague-Dawley rats. In Experiment 1, animals self-administered pellets containing 55% F + 45% G or 30% F + 70% G, and Fos immunoreactivity was assessed in hypothalamic regions regulating food intake and reward. In Experiment 2, rats self-administered solutions of 55% F + 42% G (high fructose corn syrup (HFCS)), 50% F + 50% G (sucrose) or saccharin, and mRNA of the dopamine 2 (D2R) and mu-opioid (MOR) receptor genes were assessed in striatal regions involved in addictive behaviors. Finally, in Experiment 3, rats self-administered HFCS and sucrose in their home cages, and hepatic fatty acids were quantified. It was found that higher fructose ratios engendered lower self-administration, lower Fos expression in the lateral hypothalamus/arcuate nucleus, reduced D2R and increased MOR mRNA in the dorsal striatum and nucleus accumbens core, respectively, as well as elevated omega-6 polyunsaturated fatty acids in the liver. These data indicate that a higher ratio of fructose may enhance the reinforcing effects of sugar and possibly lead to neurobiological and physiological alterations associated with addictive and metabolic disorders.

New therapeutic strategies targeting D1-type dopamine receptors for neuropsychiatric disease

The neurotransmitter dopamine acts via two major classes of receptors, D1-type and D2-type. D1 receptors are highly expressed in the striatum and can also be found in the cerebral cortex. Here we review the role of D1 dopamine signaling in two major domains: L-DOPA-induced dyskinesias in Parkinson's disease and cognition in neuropsychiatric disorders. While there are many drugs targeting D2-type receptors, there are no drugs that specifically target D1 receptors. It has been difficult to use selective D1-receptor agonists for clinical applications due to issues with bioavailability, binding affinity, pharmacological kinetics, and side effects. We propose potential therapies that selectively modulate D1 dopamine signaling by targeting second messengers downstream of D1 receptors, allosteric modulators, or by making targeted modifications to D1-receptor machinery. The development of therapies specific to D1-receptor signaling could be a new frontier in the treatment of neurological and psychiatric disorders.

High on food: the interaction between the neural circuits for feeding and for reward

Hunger, mostly initiated by a deficiency in energy, induces food seeking and intake. However, the drive toward food is not only regulated by physiological needs, but is motivated by the pleasure derived from ingestion of food, in particular palatable foods. Therefore, feeding is viewed as an adaptive motivated behavior that involves integrated communication between homeostatic feeding circuits and reward circuits. The initiation and termination of a feeding episode are instructed by a variety of neuronal signals, and maladaptive plasticity in almost any component of the network may lead to the development of pathological eating disorders. In this review we will summarize the latest understanding of how the feeding circuits and reward circuits in the brain interact. We will emphasize communication between the hypothalamus and the mesolimbic dopamine system and highlight complexities, discrepancies, open questions and future directions for the field.

Amylin modulates the mesolimbic dopamine system to control energy balance

Amylin acts in the CNS to reduce feeding and body weight. Recently, the ventral tegmental area (VTA), a mesolimbic nucleus important for food intake and reward, was identified as a site-of-action mediating the anorectic effects of amylin. However, the long-term physiological relevance and mechanisms mediating the intake-suppressive effects of VTA amylin receptor (AmyR) activation are unknown. Data show that the core component of the AmyR, the calcitonin receptor (CTR), is expressed on VTA dopamine (DA) neurons and that activation of VTA AmyRs reduces phasic DA in the nucleus accumbens core (NAcC). Suppression in NAcC DA mediates VTA amylin-induced hypophagia, as combined NAcC D1/D2 receptor agonists block the intake-suppressive effects of VTA AmyR activation. Knockdown of VTA CTR via adeno-associated virus short hairpin RNA resulted in hyperphagia and exacerbated body weight gain in rats maintained on high-fat diet. Collectively, these findings show that VTA AmyR signaling controls energy balance by modulating mesolimbic DA signaling.

Body weight and food intake in Parkinson's disease. A review of the association to non-motor symptoms

Research on eating behaviours has extensively highlighted that cognitive systems interact with the metabolic system in driving food intake and in influencing body weight regulation. Parkinson's disease is a good model for studying these complex interactions since alterations in both body weight and cognitive domains have been frequently reported among these patients. Interestingly, even if different non-motor symptoms may characterize the course of the disease, their contribution to weight and food preference has been poorly investigated. This review describes body weight alterations and eating habits in patients with Parkinson's disease, including those who underwent deep brain stimulation surgery. In particular, the review considers the link between non-motor symptoms, affecting sensory perception, cognition, mood and motivation, and food intake and weight alterations. The take home message is twofold. First, we recommend a comprehensive approach in order to develop effective strategies in the management of patients' weight. Second, we also suggest that investigating this issue in patients with Parkinson's disease may provide some useful information about the mechanisms underlying food and weight regulation in healthy subjects.

Obesity-linked homologues TfAP-2 and Twz establish meal frequency in Drosophila melanogaster

In all animals managing the size of individual meals and frequency of feeding is crucial for metabolic homeostasis. In the current study we demonstrate that the noradrenalin analogue octopamine and the cholecystokinin (CCK) homologue Drosulfakinin (Dsk) function downstream of TfAP-2 and Tiwaz (Twz) to control the number of meals in adult flies. Loss of TfAP-2 or Twz in octopaminergic neurons increased the size of individual meals, while overexpression of TfAP-2 significantly decreased meal size and increased feeding frequency. Of note, our study reveals that TfAP-2 and Twz regulate octopamine signaling to initiate feeding; then octopamine, in a negative feedback loop, induces expression of Dsk to inhibit consummatory behavior. Intriguingly, we found that the mouse TfAP-2 and Twz homologues, AP-2β and Kctd15, co-localize in areas of the brain known to regulate feeding behavior and reward, and a proximity ligation assay (PLA) demonstrated that AP-2β and Kctd15 interact directly in a mouse hypothalamus-derived cell line. Finally, we show that in this mouse hypothalamic cell line AP-2β and Kctd15 directly interact with Ube2i, a mouse sumoylation enzyme, and that AP-2β may itself be sumoylated. Our study reveals how two obesity-linked homologues regulate metabolic homeostasis by modulating consummatory behavior.

The size of individual meals and feeding frequency are important for homeostatic control. Due to the complex neuroendocrine system regulating human food intake it is difficult to uncover the mechanisms underlying eating disorders. The genetically tractable model system Drosophila melanogaster has a comparatively simple brain; yet, similar to humans, its eating behavior can adapt to respond to nutritional needs. Our study describes how the obesity-linked homologues TfAP-2 (human TFAP2B) and Tiwaz (human KCTD15) regulate a unique feedback system involving noradrenalin-like octopamine and the CCK homolog Dsk, that exert positive and negative effects on Drosophila feeding behavior. Our findings provide insight into how two conserved obesity-linked genes regulate feeding behavior in order to maintain metabolic balance.

Neural integration of satiation and food reward: role of GLP-1 and orexin pathways

Central nervous system control of food intake involves detecting, integrating and responding to diverse internal and external signals. For maintenance of energy homeostasis, the brain uses long-term signals of metabolic status and short-term signals related to the nutrient content of individual meals. Feeding is also clearly influenced by hedonic, reward-related factors: palatability, motivation, and learned associations and cues that predict the availability of food. Different neural circuits have been proposed to mediate these homeostatic and hedonic aspects of eating. This review describes research on neural pathways that appear to be involved in both, integrating gastrointestinal satiation signaling with food reward. First, the glucagon-like peptide 1 projections from the nucleus of the solitary tract to the nucleus accumbens and ventral tegmental area are discussed as a mechanism through which meal-related gut signals may influence palatability, motivation for food, and meal size. Second, the orexin projection from lateral hypothalamus to the nucleus of the solitary tract and area postrema is discussed as a mechanism through which cues that predict rewarding food may act to increase motivation for food and also to suppress satiation. Additional potential integrative sites and pathways are also briefly discussed. Based on these findings, it is suggested that the brain circuitry involved in energy homeostasis and the circuitry mediating food reward are, in fact, overlapping and far less distinct than previously considered.

Alterations in brain structure in adults with anorexia nervosa and the impact of illness duration

BACKGROUND

Brain structure alterations have been reported in anorexia nervosa, but findings have been inconsistent. This may be due to inadequate sample size, sample heterogeneity or differences in methodology.

METHOD

High resolution magnetic resonance images were acquired of 33 adult participants with anorexia nervosa and 33 healthy participants, the largest study sample to date, in order to assess whole-brain volume, ventricular cerebrospinal fluid, white matter and grey matter volume. Voxel-based morphometry was conducted to assess regional grey matter volume. Levels of depression, anxiety, obsessionality and eating disorder-related symptoms were measured and used to explore correlations with brain structure.

RESULTS

Participants with anorexia nervosa had smaller brain volumes as well as a global decrease in grey matter volume with ventricular enlargement. Voxel-based morphometry revealed a decrease in grey matter volume spanning across the cerebellum, temporal, frontal and occipital lobes. A correlation was found between grey matter volume loss and duration of illness in the cerebellum and mesencephalon. No correlations were found with clinical measures.

CONCLUSIONS

Findings are in accordance with several previous studies on brain structure and match functional studies that have assessed the symptomatology of anorexia nervosa, such as body image distortion and cognitive bias to food. The correlation with duration of illness supports the implication of cerebellar atrophy in the maintenance of low weight and disrupted eating behaviour and illustrates its role in the chronic phase of anorexia nervosa. The lack of other correlations suggests that these findings are not related to the presence of co-morbid disorders.

Deficits of brainstem and spinal cord functions after neonatal hypoxia-ischemia in mice

BACKGROUND

Perinatal cerebral hypoxia-ischemia (HI) can lead to severe neurodevelopmental disorders. Studies in humans and animal models mainly focused on cerebral outcomes, and little is known about the mechanisms that may affect the brainstem and the spinal cord. Dysfunctions of neuromodulatory systems, such as the serotonergic (5-HT) projections, critical for the development of neural networks, have been postulated to underlie behavioral and motor deficits, as well as metabolic changes.

METHODS

The aim of this study was to investigate brainstem and spinal cord functions by means of plethysmography and sensorimotor tests in a neonatal Rice-Vanucci model of HI in mice. We also evaluated bioaminergic contents in central regions dedicated to the motor control of autonomic functions.

RESULTS

Mice with cerebral infarct expressed motor disturbances and had a lower body weight and a decreased respiratory frequency than SHAM, suggesting defects of brainstem neural network involved in the motor control of feeding, suckling, swallowing, and respiration. Moreover, our study revealed changes of monoamine and amino acid contents in the brainstem and the spinal cord of HI mice.

CONCLUSION

Our results suggest that monoaminergic neuromodulation plays an important role in the physiopathology of HI brain injury that may represent a good therapeutic target.

Obesity attenuates D2 autoreceptor-mediated inhibition of putative ventral tegmental area dopaminergic neurons

The ventral tegmental area (VTA) in the midbrain is important for food reward. High‐fat containing palatable foods have reinforcing effects and accelerate obesity. We have previously reported that diet‐induced obesity selectively decreased the spontaneous activity of VTA GABA neurons, but not dopamine neurons. The spontaneous activity of VTA dopamine neurons is regulated by D₂ autoreceptors. In this study, we hypothesized that obesity would affect the excitability of VTA dopamine neurons via D₂ autoreceptors. To examine this hypothesis, we compared D₂ receptor‐mediated responses of VTA dopamine neurons between lean and obese mice. Mice fed on a high‐fat (45%) diet and mice fed on a standard diet were used as obese and lean models, respectively. Brain slice preparations were made from these two groups. Spontaneous activity of VTA neurons was recorded by extracellular recording. Putative VTA dopamine neurons were identified by firing inhibition with a D₂ receptor agonist quinpirole, and electrophysiological criteria (firing frequency <5 Hz and action potential current duration >1.2 msec). Single‐dose application of quinpirole (3−100 nmol/L) exhibited similar firing inhibition of putative VTA dopamine neurons between lean and obese mice. In stepwise application by increasing quinpirole concentrations of 3, 10, 30, and 100 nmol/L subsequently, quinpirole‐induced inhibition of firing decreased in putative VTA dopamine neurons of obese mice compared with those of lean mice. In conclusion, high‐fat diet‐induced obesity attenuated D₂ receptor‐mediated inhibition of putative VTA dopamine neurons due to the acceleration of D₂ receptor desensitization.

High‐fat diet‐induced obesity did not affect firing properties of putative ventral tegmental area (VTA) dopamine neurons. However, obesity attenuated dopamine D₂ receptor‐mediated inhibition of putative VTA dopamine neurons due to the acceleration of receptor desensitization. In obesity, decrease in D₂ receptor‐mediated autoinhibition may contribute to pathophysiology of reinforcing process in feeding.

Incretins and amylin: neuroendocrine communication between the gut, pancreas, and brain in control of food intake and blood glucose

Arguably the most fundamental physiological systems for all eukaryotic life are those governing energy balance. Without sufficient energy, an individual is unable to survive and reproduce. Thus, an ever-growing appreciation is that mammalian physiology developed a redundant set of neuroendocrine signals that regulate energy intake and expenditure, which maintains sufficient circulating energy, predominantly in the form of glucose, to ensure that energy needs are met throughout the body. This orchestrated control requires cross talk between the gastrointestinal tract, which senses the incoming meal; the pancreas, which produces glycemic counterregulatory hormones; and the brain, which controls autonomic and behavioral processes regulating energy balance. Therefore, this review highlights the physiological, pharmacological, and pathophysiological effects of the incretin hormones glucagon-like peptide-1 and gastric inhibitory polypeptide, as well as the pancreatic hormone amylin, on energy balance and glycemic control.

The Y2 receptor agonist PYY(3-36) increases the behavioural response to novelty and acute dopaminergic drug challenge in mice

The gastrointestinal hormone PYY(3-36) is a preferential Y2 neuropeptide Y (NPY) receptor agonist. Recent evidence indicates that PYY(3-36) acts on central dopaminergic pathways, but its influence on dopamine-dependent behaviours remains largely unknown. We therefore explored the effects of peripheral PYY(3-36) treatment on the behavioural responses to novelty and to dopamine-activating drugs in mice. In addition, we examined whether PYY(3-36) administration may activate distinct dopamine and γ-aminobutyric acid (GABA) cell populations in the mesoaccumbal and nigrostriatal pathways. We found that i.p. PYY(3-36) injection led to a dose-dependent increase in novel object exploration. The effective dose of PYY(3-36) (1 μg/100 g body weight) also potentiated the locomotor reaction to the indirect dopamine receptor agonist amphetamine and increased stereotyped climbing/leaning responses following administration of the direct dopamine receptor agonist apomorphine. PYY(3-36) administration did not affect activity of midbrain dopaminergic cells as evaluated by double immuno-enzyme staining of the neuronal early gene product c-Fos with tyrosine hydroxylase. PYY(3-36) did, however, lead to a marked increase in the number of cells co-expressing c-Fos with glutamic acid decarboxylase in the nucleus accumbens and caudate putamen, indicating activation of GABAergic cells in dorsal and ventral striatal areas. Our results support the hypothesis that acute administration of the preferential Y2 receptor agonist PYY(3-36) modulates dopamine-dependent behaviours. These effects do not seem to involve direct activation of midbrain dopamine cells but instead are associated with neuronal activation in the major input areas of the mesoaccumbal and nigrostriatal pathways.

Peripheral interleukin-2 level is associated with negative symptoms and cognitive performance in schizophrenia

Although several studies have pointed to a possible role of interleukin 2 (IL-2) in schizophrenia (SZ), association between IL-2 and the different groups of symptoms has not been explored. The objective of this study was to investigate a possible correlation of peripheral IL-2 levels with symptoms and cognitive performance in patients with SZ. In addition, we compared the plasma levels of IL-2 between patients with SZ and healthy controls. Twenty-nine chronically medicated outpatients with SZ according to DSM-IV were compared with twenty-six healthy controls. The patients were evaluated with the Positive and Negative Syndrome Scale (PANSS), the Calgary Depression Scale for Schizophrenia (CDSS), the Clinical Global Impression (CGI) and the Global Assessment of Functioning (GAF). All the participants had blood collected into EDTA tubes by venipuncture between 9:00 and 10:00AM. Plasma concentrations of IL-2 were determined by cytometric bead array. A computerized neuropsychological battery assessed verbal learning, verbal fluency, working memory, set shifting, executive function, inhibition and intelligence. Patients with SZ had lower levels of IL-2 than healthy controls (p<0.001). In the SZ group, IL-2 levels were positively correlated with scores in the digit span test (rho=0.416, P=0.025) and intelligence (rho=0.464, P=0.011). We also found a negative correlation between IL-2 and total score in the negative subscale of PANSS (rho=-0.447, p=0.015). Our findings suggest that IL-2 may be involved in the mechanisms related to cognitive deterioration and negative symptomatology in schizophrenia.

Amylin activates distributed CNS nuclei to control energy balance

Amylin is a pancreas-derived neuropeptide that acts in the central nervous system (CNS) to reduce food intake. Much of the literature describing the anorectic effects of amylin are focused on amylin's actions in the area postrema, a hindbrain circumventricular structure. Although the area postrema is certainly an important site that mediates the intake-suppressive effects of amylin, several pieces of evidence indicate that amylin may also promote negative energy balance through action in additional CNS nuclei, including hypothalamic and mesolimbic structures. Therefore, this review highlights the distributed neural network mediating the feeding effects of amylin signaling with special attention being devoted to the recent discovery that the ventral tegmental area is physiologically relevant for amylin-mediated control of feeding. The production of amylin by alternative, extra-pancreatic sources and its potential relevance to food intake regulation is also considered. Finally, the utility of amylin and amylin-like compounds as a component of combination pharmacotherapies for the treatment of obesity is discussed.

Medial prefrontal D1 dopamine neurons control food intake

Although the prefrontal cortex influences motivated behavior, its role in food intake remains unclear. Here, we demonstrate a role for D1-type dopamine receptor-expressing neurons in the medial prefrontal cortex (mPFC) in the regulation of feeding. Food intake increases activity in D1 neurons of the mPFC in mice, and optogenetic photostimulation of D1 neurons increases feeding. Conversely, inhibition of D1 neurons decreases intake. Stimulation-based mapping of prefrontal D1 neuron projections implicates the medial basolateral amygdala (mBLA) as a downstream target of these afferents. mBLA neurons activated by prefrontal D1 stimulation are CaMKII positive and closely juxtaposed to prefrontal D1 axon terminals. Finally, photostimulating these axons in the mBLA is sufficient to increase feeding, recapitulating the effects of mPFC D1 stimulation. These data describe a new circuit for top-down control of food intake.

Insulin secretion, body composition and pig performance are altered by feeding pattern

Three studies investigated the effect of feeding strategy on production performance and endocrine status of growing pigs. For Experiment 1, 20 entire male pigs (70.0 ± 4.6 kg) were allocated randomly to individual pens in one of four climate-controlled rooms. Pigs were fed for 23 days either ad libitum or entrained to feed bi-phasically for two 90-min periods. For Experiment 2, 20 entire male pigs (41.2 ± 3.5 kg) were housed as per Experiment 1. Pigs were fed for 49 days either ad libitum or fed bi-phasically for two 60-min periods. For Experiment 3, 100 female pigs (66.1 ± 3.5 kg) were randomly allocated to individual pens within a commercial piggery and fed for 42 days either ad libitum or bi-phasically for two 60-min periods. Ear vein catheters were inserted into 10 pigs from each group and hourly blood samples were collected for 24 h in Experiments 1 and 2 and for 11 h in Experiment 3. Plasma insulin, non-esterified fatty acid and glucose concentrations were determined in Experiments 1 and 2, and glucose and insulin concentrations in Experiment 3. Feed intake and performance were recorded in all experiments and carcass composition was assessed by computed tomography for Experiment 2. There were no differences in final liveweight between the two treatment groups for all experiments. Pigs fed for two 90-min periods (Experiment 1) showed no difference in feed intake when compared with feeding ad libitum. Pigs in Experiment 2 fed for two 60-min intervals consumed 2.49 kg/pig.day compared with those fed ad libitum that consumed 2.68 kg/day (P = 0.057). In Experiment 3, pigs fed twice daily consumed 2.82 kg/pig.day compared with 2.91 kg/pig.day in ad libitum-fed pigs (P = 0.051). Bi-phasic fed pigs in Experiment 2 had improved (P < 0.05) feed conversion efficiency compared with pigs fed ad libitum. For all experiments, there was no difference in plasma glucose concentrations between the two treatments. In all three experiments, the circulating insulin concentrations for pigs fed ad libitum remained at a constant level throughout the sampling period. However, plasma insulin concentrations for the bi-phasic fed pigs significantly increased ~1 h after both feeding periods during all three experiments. Insulin secretion of pigs fed for two 90-min periods differed from that of pigs fed for two 60-min periods. Plasma insulin concentration increased five-fold following feeding for 60 min, compared with that in pigs fed for 90 min, which increased two-fold. Bi-phasic-fed pigs from Experiment 2 had reduced (P < 0.05) total carcass fat and significantly increased muscle when compared with pigs fed ad libitum. The data showed that feeding pigs at two succinct periods aligned insulin secretion to the time of feeding. Pigs fed for 60 min, unlike those fed for 90-min intervals, had reduced feed intake in comparison to those fed ad libitum. This may suggest that the duration of the feeding bout is important for this response and this may in turn influence both energy balance and the way energy is partitioned.

The food intake-suppressive effects of glucagon-like peptide-1 receptor signaling in the ventral tegmental area are mediated by AMPA/kainate receptors

Glucagon-like peptide-1 receptor (GLP-1R) activation in the ventral tegmental area (VTA) is physiologically relevant for the control of palatable food intake. Here, we tested whether the food intake-suppressive effects of VTA GLP-1R activation are mediated by glutamatergic signaling within the VTA. Intra-VTA injections of the GLP-1R agonist exendin-4 (Ex-4) reduced palatable high-fat food intake in rats primarily by reducing meal size; these effects were mediated in part via glutamatergic AMPA/kainate but not NMDA receptor signaling. Additional behavioral data indicated that GLP-1R expressed specifically within the VTA can partially mediate the intake- and body weight-suppressive effects of systemically administered Ex-4, offering the intriguing possibility that this receptor population may be clinically relevant for food intake control. Intra-VTA Ex-4 rapidly increased tyrosine hydroxylase levels within the VTA, suggesting that GLP-1R activation modulates VTA dopaminergic signaling. Further evidence for this hypothesis was provided by electrophysiological data showing that Ex-4 increased the frequency of AMPA-mediated currents and reduced the paired/pulse ratio in VTA dopamine neurons. Together, these data provide novel mechanisms by which GLP-1R agonists in the mesolimbic reward system control for palatable food intake.

Obesity decreases excitability of putative ventral tegmental area GABAergic neurons

Palatable food has reinforcing effects on feeding and accelerates obesity. Alteration of food-related behavior in obesity may promote maintenance of obesity. The ventral tegmental area (VTA) of the midbrain is important for food reward. However, it is unknown whether activity of VTA neurons is altered in diet-induced obesity. In this study, we examined VTA neuronal activity using an electrophysiological technique in diet-induced obese mice. Male 4-week-old mice were fed a high-fat diet or a standard diet for 5–6 weeks. Mice fed a high-fat diet gained greater body weight with heavier visceral fat compared with those fed a standard diet. Brain slice preparations were obtained from the lean and obese mice. Spontaneous activity of VTA neurons was recorded extracellularly. We found a negative correlation between firing frequency (FF) and action potential (AP) current duration in lean and obese mice VTA neurons. VTA neurons were classified as group-1 neurons (FF <5.0 Hz and AP current duration >1.2 msec) or group-2 neurons (FF ≧5.0 Hz and AP current duration ≦1.2 msec). FF, AP current duration, and firing regularity of VTA group-1 neurons were similar between lean and obese mice. Obese mice VTA group-2 neurons had a lower FF and shorter AP current duration compared with lean mice. In conclusion, obesity minimally affects VTA group-1 neurons, which are presumed to be dopaminergic, but decreases excitability of VTA group-2 neurons, which are presumed to be GABAergic. This differential effect may contribute to the pathophysiology of reward-related feeding in obesity.

Prefrontal D1 dopamine signaling is necessary for temporal expectation during reaction time performance

Responses during a simple reaction time task are influenced by temporal expectation, or the ability to anticipate when a stimulus occurs in time. Here, we test the hypothesis that prefrontal D1 dopamine signaling is necessary for temporal expectation during simple reaction time task performance. We depleted dopamine projections to the medial prefrontal circuits by infusing 6-hydroxidopamine, a selective neurotoxin, into the ventral tegmental area (VTA) of rats, and studied their performance on a simple reaction time task with two delays. VTA dopamine depletion did not change movements or learning of the reaction time task. However, VTA dopamine-depleted animals did not develop delay-dependent speeding of reaction times, suggesting that mesocortical dopamine signaling is required for temporal expectation. Next, we manipulated dopamine signaling within the medial prefrontal cortex using local pharmacology. We found that SCH23390, a D1-type dopamine receptor antagonist, specifically attenuated delay-dependent speeding, while sulpiride, a D2-type receptor antagonist, did not. These data suggest that prefrontal D1 dopamine signaling is necessary for temporal expectation during performance of a simple reaction time task. Our findings provide insight into temporal processing of the prefrontal cortex, and how dopamine signaling influences prefrontal circuits that guide goal-directed behavior.

Central manipulation of dopamine receptors attenuates the orexigenic action of ghrelin

OBJECTIVE

Recent evidence suggests that ghrelin, a peptidic hormone stimulating food intake, interacts with the dopamine signaling. This interaction has been demonstrated to modulate several effects of ghrelin, such as locomotor activity, memory, and food intake. Ghrelin increases dopamine levels in the shell of the nucleus accumbens stimulating food intake, while ablation of the ghrelin receptor attenuates the hypophagia caused by the activation of dopamine receptor 2. However, it is not known whether the orexigenic action of ghrelin is due to changes in central dopamine receptors.

MATERIALS AND METHODS

We used Sprague-Dawley rats injected with different dopamine receptor agonists, antagonists, and ghrelin.

RESULTS

We demonstrate that the specific central blockade of dopamine receptor 1, 2, and 3 (D1, D2, and D3, respectively) reduces the orexigenic action of ghrelin. Similarly, specific central stimulation, either singly of dopamine receptor 1 or dopamine receptors 2 and 3 simultaneously, causes a significant decrease in ghrelin-induced food intake. Co-stimulation of all three receptors (D1, D2, and D3) also led to a marked attenuation in ghrelin-induced food intake. Importantly, the reduction in ghrelin-induced feeding was not caused by malaise or any type of behavioral alteration.

CONCLUSION

Taken together, these data indicate that dopamine receptors play an important role in acute stimulation of feeding behavior induced by central injection of ghrelin.

The brain reward circuitry in mood disorders

Mood disorders are common and debilitating conditions characterized in part by profound deficits in reward-related behavioural domains. A recent literature has identified important structural and functional alterations within the brain's reward circuitry--particularly in the ventral tegmental area-nucleus accumbens pathway--that are associated with symptoms such as anhedonia and aberrant reward-associated perception and memory. This Review synthesizes recent data from human and rodent studies from which emerges a circuit-level framework for understanding reward deficits in depression. We also discuss some of the molecular and cellular underpinnings of this framework, ranging from adaptations in glutamatergic synapses and neurotrophic factors to transcriptional and epigenetic mechanisms.

Circannual changes in stress and feeding hormones and their effect on food-seeking behaviors

Seasonal fluctuations in food availability show a tight association with seasonal variations in body weight and food intake. Seasonal variations in food intake, energy storage, and expenditure appear to be a widespread phenomenon suggesting they may have evolved in anticipation for changing environmental demands. These cycles appear to be driven by changes in external daylength acting on neuroendocrine pathways. A number of neuroendocrine pathways, two of which are the endocrine mechanisms underlying feeding and stress, appear to show seasonal changes in both their circulating levels and reactivity. As such, variation in the level or reactivity to these hormones may be crucial factors in the control of seasonal variations in food-seeking behaviors. The present review examines the relationship between feeding behavior and seasonal changes in circulating hormones. We hypothesize that seasonal changes in circulating levels of glucocorticoids and the feeding-related hormones ghrelin and leptin contribute to seasonal fluctuations in feeding-related behaviors. This review will focus on the seasonal circulating levels of these hormones as well as sensitivity to these hormones in the modulation of food-seeking behaviors.

Bidirectional modulation of substantia nigra activity by motivational state

A major output nucleus of the basal ganglia is the substantia nigra pars reticulata, which sends GABAergic projections to brainstem and thalamic nuclei. The GABAergic (GABA) neurons are reciprocally connected with nearby dopaminergic neurons, which project mainly to the basal ganglia, a set of subcortical nuclei critical for goal-directed behaviors. Here we examined the impact of motivational states on the activity of GABA neurons in the substantia nigra pars reticulata and the neighboring dopaminergic (DA) neurons in the pars compacta. Both types of neurons show short-latency bursts to a cue predicting a food reward. As mice became sated by repeated consumption of food pellets, one class of neurons reduced cue-elicited firing, whereas another class of neurons progressively increased firing. Extinction or pre-feeding just before the test session dramatically reduced the phasic responses and their motivational modulation. These results suggest that signals related to the current motivational state bidirectionally modulate behavior and the magnitude of phasic response of both DA and GABA neurons in the substantia nigra.

Amylin receptor signaling in the ventral tegmental area is physiologically relevant for the control of food intake

The ability of amylin, a pancreatic β-cell-derived neuropeptide, to promote negative energy balance has been ascribed to neural activation at the area postrema. However, despite amylin binding throughout the brain, the possible role of amylin signaling at other nuclei in the control of food intake has been largely neglected. We show that mRNA for all components of the amylin receptor complex is expressed in the ventral tegmental area (VTA), a mesolimbic structure mediating food intake and reward. Direct activation of VTA amylin receptors reduces the intake of chow and palatable sucrose solution in rats. This effect is mediated by reductions in meal size and is not due to nausea/malaise or prolonged suppression of locomotor activity. VTA amylin receptor activation also reduces sucrose self-administration on a progressive ratio schedule. Finally, antagonist studies provide novel evidence that VTA amylin receptor blockade increases food intake and attenuates the intake-suppressive effects of a peripherally administered amylin analog, suggesting that amylin receptor signaling in the VTA is physiologically relevant for food intake control and potentially clinically relevant for the treatment of obesity.

Dopamine and food addiction: lexicon badly needed

Over the last few years, the concept of food addiction has become a common feature in the scientific literature, as well as the popular press. Nevertheless, the use of the term addiction to describe pathological aspects of food intake in humans remains controversial, and even among those who affirm the validity of the concept, there is considerable disagreement about its utility for explaining the increasing prevalence of obesity throughout much of the world. An examination of the literature on food addiction indicates that mesolimbic and nigrostriatal dopamine systems often are cited as mechanisms that contribute to the establishment of food addiction. However, in reviewing this literature, it is important to have a detailed consideration of the complex nature of dopaminergic involvement in motivational processes. For example, although it is often stated that mesolimbic dopamine mediates reward, there is no standard or consistent technical meaning of this term. Moreover, there is a persistent tendency to link dopamine transmission with pleasure or hedonia, as opposed to other aspects of motivation or learning. The present article provides a critical discussion of some aspects of the food addiction literature, viewed through the lens of recent findings and current theoretical views of dopaminergic involvement in food motivation. Furthermore, compulsive food intake and binge eating will be considered from an evolutionary perspective, in terms of the motivational subsystems that are involved in adaptive patterns of food consumption and seeking behaviors and a consideration of how these could be altered in pathological conditions.

Presynaptic leptin action suppresses excitatory synaptic transmission onto ventral tegmental area dopamine neurons

BACKGROUND

Leptin is an adipocyte-derived cytokine that can act in the brain to suppress feeding and maintain energy homeostasis. Additionally, leptin activates its receptors in the ventral tegmental area (VTA), a critical site for neuroadaptations to rewarding stimuli, to modulate reward-seeking behaviors. Although leptin can decrease intrinsic excitability of dopamine neurons in the VTA, it is unknown whether leptin can modulate excitatory synaptic transmission in this brain region. Because plasticity of glutamatergic synapses onto VTA neurons can encode predictive information about reward, we hypothesized that leptin can decrease excitatory synaptic transmission onto dopamine neurons.

METHODS

Using whole-cell patch clamp electrophysiology in mouse midbrain slices, we tested the effects of leptin on evoked α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) or N-methyl-D-aspartate receptor (NMDAR)-mediated excitatory postsynaptic currents (EPSCs) onto VTA dopamine neurons.

RESULTS

Leptin depressed both AMPAR and NMDAR EPSCs in VTA dopamine neurons and reduced frequency but not amplitude of mini EPSCs. Bath application of the MEK1/2 inhibitor U0126 did not alter leptin-induced suppression of AMPAR EPSCs. However, external, but not internal, application of the phosphoinositol 3-kinase (PI3K) or Janus kinase 2 (Jak2) tyrosine kinase inhibitors abolished leptin-induced synaptic depression.

CONCLUSIONS

This study demonstrates that leptin causes a presynaptic inhibition of the probability of glutamate release onto VTA dopamine neurons. This synaptic inhibition requires Jak2 and PI3K activation. Leptin-induced weakening of synaptic strength onto dopamine cells may underlie its inhibitory effects on appetitive behavior for rewarding stimuli.

Effect of long-term treatment with pramipexole or levodopa on presynaptic markers assessed by longitudinal [123I]FP-CIT SPECT and histochemistry

A previous clinical trial studied the effect of long-term treatment with levodopa (LD) or the dopamine agonist pramipexole (PPX) on disease progression in Parkinson disease using SPECT with the dopamine transporter (DAT)-radioligand [(123)I]β-CIT as surrogate marker. [(123)I]β-CIT binding declined to significantly lower levels in patients receiving LD compared to PPX. However, the interpretation of this difference as LD-induced neurotoxicity, PPX-induced neuroprotection/-regeneration, or only drug-induced regulatory changes of DAT-availability remained controversial. To address this question experimentally, we induced a subtotal lesion of the substantia nigra in mice by bilateral injection of the neurotoxin 6-hydroxydopamine. After 4 weeks, mice were treated for 20 weeks orally with LD (100mg/kg/day) or PPX (3mg/kg/day), or water (vehicle) only. The integrity of nigrostriatal projections was assessed by repeated [(123)I]FP-CIT SPECT in vivo and by immunostaining for DAT and the dopamine-synthesizing enzyme tyrosine hydroxylase (TH) after sacrifice. In sham-lesioned mice, we found that both LD and PPX treatment significantly decreased the striatal FP-CIT binding (LD: -21%; PPX: -14%) and TH-immunoreactivity (LD: -42%; PPX: -45%), but increased DAT-immunoreactivity (LD: +42%; PPX: +33%) compared to controls without dopaminergic treatment. In 6-hydroxydopamine-lesioned mice, however, neither LD nor PPX significantly influenced the stably reduced FP-CIT SPECT signal (LD: -66%; PPX: -66%; controls -66%), TH-immunoreactivity (LD: -70%; PPX: -72%; controls: -77%) and DAT-immunoreactivity (LD: -70%; PPX: -75%; controls: -75%) in the striatum or the number of TH-positive cells in the substantia nigra (LD: -88%; PPX: -88%; controls: -86%), compared to lesioned mice without dopaminergic treatment. In conclusion, chronic dopaminergic stimulation with LD or PPX induced similar adaptive presynaptic changes in healthy mice, but no discernible changes in severely lesioned mice. These findings allow to more reliably interpret the results from clinical trials using neuroimaging of DAT as surrogate parameter.