Hypoinsulinemia may mediate the lowering of self-stimulation thresholds by food restriction and streptozotocin-induced diabetes

Impact of insulin and insulin resistance on brain dopamine signalling and reward processing- an underexplored mechanism in the pathophysiology of depression?

Type 2 diabetes and major depressive disorder (MDD) are the leading causes of disability worldwide and have a high comorbidity rate with fatal outcomes. Despite the long-established association between these conditions, the underlying molecular mechanisms remain unknown. Since the discovery of insulin receptors in the brain and the brain's reward system, evidence has accumulated indicating that insulin modulates dopaminergic (DA) signalling and reward behaviour. Here, we review the evidence from rodent and human studies, that insulin resistance directly alters central DA pathways, which may result in motivational deficits and depressive symptoms. Specifically, we first elaborate on the differential effects of insulin on DA signalling in the ventral tegmental area (VTA) - the primary DA source region in the midbrain - and the striatum as well as its effects on behaviour. We then focus on the alterations induced by insulin deficiency and resistance. Finally, we review the impact of insulin resistance in DA pathways in promoting depressive symptoms and anhedonia on a molecular and epidemiological level and discuss its relevance for stratified treatment strategies.

The Rise and Fall of Dopamine: A Two-Stage Model of the Development and Entrenchment of Anorexia Nervosa

Dopamine has long been implicated as a critical neural substrate mediating anorexia nervosa (AN). Despite nearly 50 years of research, the putative direction of change in dopamine function remains unclear and no consensus on the mechanistic role of dopamine in AN has been achieved. We hypothesize two stages in AN- corresponding to initial development and entrenchment- characterized by opposite changes in dopamine. First, caloric restriction, particularly when combined with exercise, triggers an escalating spiral of increasing dopamine that facilitates the behavioral plasticity necessary to establish and reinforce weight-loss behaviors. Second, chronic self-starvation reverses this escalation to reduce or impair dopamine which, in turn, confers behavioral inflexibility and entrenchment of now established AN behaviors. This pattern of enhanced, followed by impaired dopamine might be a common path to many behavioral disorders characterized by reinforcement learning and subsequent behavioral inflexibility. If correct, our hypothesis has significant clinical and research implications for AN and other disorders, such as addiction and obesity.

Characterization of a differential reinforcement of low rates of responding task in non-deprived male and female rats: Role of Sigma-1 receptors

Impulsive action can be defined as the inability to withhold a response and represents one of the dimensions of the broad construct impulsivity. Here, we characterized a modified differential reinforcement of low rates of responding (DRL) task developed in our laboratory, in which impulsive action is measured in ad libitum fed/watered subjects. Specifically, we first determined the effects of both sex and estrous cycle on impulsive action by systematically comparing male and estrous-synchronized female subjects. In addition, we evaluated the convergent validity of this modified DRL task by testing the effects of the D2R/5HT2AR antagonist, aripiprazole, and the noncompetitive NMDAR antagonist, MK-801. Finally, we tested the effects of the selective antagonist BD-1063 and agonist PRE-084 of Sigma-1 receptor (Sig-1R) on impulsive action using this modified DRL task. We found that female rats showed and increased inability to withhold a response when compared to males, and this effect was driven by the metestrus/diestrus phase of the estrous cycle. In addition, aripiprazole and MK-801 fully retained their capability to reduce and increase impulsive action, respectively. Finally, the selective Sig-1R antagonist, BD-1063 dose-dependently reduced the inability to withhold a response in both sexes, though more potently in female rats. In summary, we show that impulsive action, as measured in a modified DRL task which minimizes energy-homeostatic influences, is a function of both sex and estrous cycle. Furthermore, we validate the convergent validity of the task and provide evidence that Sig-1R antagonism may represent a novel pharmacological strategy to reduce impulsive action.

Impact of Gut and Metabolic Hormones on Feeding Reward

Ingestion of food activates a cascade of endocrine responses (thereby reflecting a contemporaneous feeding status) that include the release of hormones from the gastrointestinal (GI) tract, such as cholecystokinin (CCK), glucagonlike peptide YY (PYY), peptide PP, and oleoylethanolamide, as well as suppression of ghrelin secretion. The pancreas and adipose tissue, on the other hand, release hormones that serve as a measure of the current metabolic state or the long-term energy stores, that is, insulin, leptin, and adiponectin. It is well known and intuitively understandable that these hormones target either directly (by crossing the blood-brain barrier) or indirectly (e.g., via vagal input) the "homeostatic" brainstem-hypothalamic pathways involved in the regulation of appetite. The current article focuses on yet another target of the metabolic and GI hormones that is critical in inducing changes in food intake, namely, the reward system. We discuss the physiological basis of this functional interaction, its importance in the control of appetite, and the impact that disruption of this crosstalk has on energy intake in select physiological and pathophysiological states. We conclude that metabolic and GI hormones have a capacity to strengthen or weaken a response of the reward system to a given food, and thus, they are fundamental in ensuring that feeding reward is plastic and dependent on the energy status of the organism. © 2021 American Physiological Society. Compr Physiol 11:1425-1447, 2021.

Enhanced vulnerability to tobacco use in persons with diabetes: A behavioral and neurobiological framework

Tobacco use significantly magnifies the negative health complications associated with diabetes. Although tobacco use is strongly discouraged in persons with diabetes, clinical evidence suggests that they often continue to smoke and have more difficulty quitting despite serious contraindications. Here, we suggest that a potential reason for enhanced vulnerability to tobacco use in persons with diabetes is greater rewarding effects of nicotine. This review summarizes pre-clinical evidence indicating that the rewarding effects of nicotine are enhanced in rodent models of type 1 and type 2 diabetes. We also provide a framework of neurobiological mechanisms that are posited to promote tobacco use in persons with diabetes. This framework suggests that diabetes induces a disruption in insulin signaling that leads to a suppression of dopamine systems in the mesolimbic reward pathway. Lastly, we consider the clinical implications of enhanced rewarding effects of nicotine that may promote tobacco use in persons with diabetes. The clinical efficacy of smoking cessation medications that enhance dopamine are important to consider, given that persons with diabetes may display disrupted dopaminergic mechanisms. Future work is needed to better understand the complex interaction of dopamine and insulin in order to develop better smoking cessation medications for persons with diabetes.

Dampened Mesolimbic Dopamine Function and Signaling by Saturated but not Monounsaturated Dietary Lipids

Overconsumption of dietary fat is increasingly linked with motivational and emotional impairments. Human and animal studies demonstrate associations between obesity and blunted reward function at the behavioral and neural level, but it is unclear to what degree such changes are a consequence of an obese state and whether they are contingent on dietary lipid class. We sought to determine the impact of prolonged ad libitum intake of diets rich in saturated or monounsaturated fat, separate from metabolic signals associated with increased adiposity, on dopamine (DA)-dependent behaviors and to identify pertinent signaling changes in the nucleus accumbens (NAc). Male rats fed a saturated (palm oil), but not an isocaloric monounsaturated (olive oil), high-fat diet exhibited decreased sensitivity to the rewarding (place preference) and locomotor-sensitizing effects of amphetamine as compared with low-fat diet controls. Blunted amphetamine action by saturated high-fat feeding was entirely independent of caloric intake, weight gain, and plasma levels of leptin, insulin, and glucose and was accompanied by biochemical and behavioral evidence of reduced D1R signaling in the NAc. Saturated high-fat feeding was also tied to protein markers of increased AMPA receptor-mediated plasticity and decreased DA transporter expression in the NAc but not to alterations in DA turnover and biosynthesis. Collectively, the results suggest that intake of saturated lipids can suppress DA signaling apart from increases in body weight and adiposity-related signals known to affect mesolimbic DA function, in part by diminishing D1 receptor signaling, and that equivalent intake of monounsaturated dietary fat protects against such changes.

Pituitary adenylate cyclase-activating polypeptide induces a depressive-like phenotype in rats

BACKGROUND

Major depressive disorder (MDD) is a chronic, life-threatening psychiatric condition characterized by depressed mood, psychomotor alterations, and a markedly diminished interest or pleasure in most activities known as anhedonia. Available pharmacotherapies have limited success and the need for new strategies is clear. Recent studies attribute a major role to the pituitary adenylate cyclase-activating polypeptide (PACAP) system in mediating the response to stress. PACAP knockout mice display profound alterations in depressive-like behaviors, and genetic association studies have demonstrated that genetic variants of the PACAP gene are associated with MDD. However, the effects of PACAP administration on depressive-like behaviors in rodents have not yet been systematically examined.

OBJECTIVES

The present study investigated the effects of central administration of PACAP in rats on depressive-like behaviors, using well-established animal models that represent some of the endophenotypes of depression.

METHODS

We used intracranial self-stimulation (ICSS) to assess the brain reward function, saccharin preference test to assess anhedonia, social interaction to assess social withdrawal, and forced swim test (FST) to assess behavioral despair.

RESULTS

PACAP raised the current threshold for ICSS, elevation blocked by the PACAP antagonist PACAP(6-38). PACAP reduced the preference for a sweet saccharin solution and reduced the time the rats spent interacting with a novel animal. Interestingly, PACAP administration did not affect immobility in the FST.

CONCLUSIONS

Our results demonstrate a role for the central PACAP/PAC1R system in the regulation of depressive-like behaviors and suggest that hyperactivity of the PACAP/PAC1R system may contribute to the pathophysiology of depression, particularly the associated anhedonic symptomatology and social dysfunction.

Insulin, Central Dopamine D2 Receptors, and Monetary Reward Discounting in Obesity

Animal research finds that insulin regulates dopamine signaling and reward behavior, but similar research in humans is lacking. We investigated whether individual differences in body mass index, percent body fat, pancreatic β-cell function, and dopamine D2 receptor binding were related to reward discounting in obese and non-obese adult men and women. Obese (n = 27; body mass index>30) and non-obese (n = 20; body mass index<30) adults were assessed for percent body fat with dual-energy X-ray absorptiometry and for β-cell function using disposition index. Choice of larger, but delayed or less certain, monetary rewards relative to immediate, certain smaller monetary rewards was measured using delayed and probabilistic reward discounting tasks. Positron emission tomography using a non-displaceable D2-specific radioligand, [¹¹C](N-methyl)benperidol quantified striatal D2 receptor binding. Groups differed in body mass index, percent body fat, and disposition index, but not in striatal D2 receptor specific binding or reward discounting. Higher percent body fat in non-obese women related to preference for a smaller, certain reward over a larger, less likely one (greater probabilistic discounting). Lower β-cell function in the total sample and lower insulin sensitivity in obese related to stronger preference for an immediate and smaller monetary reward over delayed receipt of a larger one (greater delay discounting). In obese adults, higher striatal D2 receptor binding related to greater delay discounting. Interestingly, striatal D2 receptor binding was not significantly related to body mass index, percent body fat, or β-cell function in either group. Our findings indicate that individual differences in percent body fat, β-cell function, and striatal D2 receptor binding may each contribute to altered reward discounting behavior in non-obese and obese individuals. These results raise interesting questions about whether and how striatal D2 receptor binding and metabolic factors, including β-cell function, interact to affect reward discounting in humans.

Modulation of Food Reward by Endocrine and Environmental Factors: Update and Perspective

OBJECTIVE

Palatable foods are frequently high in energy density. Chronic consumption of high-energy density foods can contribute to the development of cardiometabolic pathology including obesity, diabetes, and cardiovascular disease. This article reviews the contributions of extrinsic and intrinsic factors that influence the reward components of food intake.

METHODS

A narrative review was conducted to determine the behavioral and central nervous system (CNS) related processes involved in the reward components of high-energy density food intake.

RESULTS

The rewarding aspects of food, particularly palatable and preferred foods, are regulated by CNS circuitry. Overlaying this regulation is modulation by intrinsic endocrine systems and metabolic hormones relating to energy homeostasis, developmental stage, or gender. It is now recognized that extrinsic or environmental factors, including ambient diet composition and the provocation of stress or anxiety, also contribute substantially to the expression of food reward behaviors such as motivation for, and seeking of, preferred foods.

CONCLUSIONS

High-energy density food intake is influenced by both physiological and pathophysiological processes. Contextual, behavioral, and psychological factors and CNS-related processes represent potential targets for multiple types of therapeutic intervention.

Enhanced nicotine self-administration and suppressed dopaminergic systems in a rat model of diabetes

Patients with diabetes display a heightened propensity to use tobacco; however, it is unclear whether they experience enhanced rewarding effects of nicotine. Thus, this study examined the reinforcing effects of nicotine in a rodent model of diabetes involving administration of streptozotocin (STZ), a drug that is toxic to pancreatic insulin-producing cells. The first study compared STZ- and vehicle-treated rats that had 23-hour access to intravenous self-administration (IVSA) of nicotine or saline and concomitant access to food and water. In order to examine the contribution of dopamine to our behavioral effects, dopamine transporter (DAT), D1 and D2 receptor levels were compared in the nucleus accumbens (NAc) following 10 days of nicotine or saline IVSA. Dopamine levels in the NAc were also compared following nicotine administration. Lastly, nicotine metabolism and dose-dependent effects of nicotine IVSA were assessed. The results revealed that STZ-treated rats displayed enhanced nicotine intake and a robust increase in food and water intake relative to controls. Protein analysis revealed an increase in DAT and a decrease in D1 receptor levels in the NAc of STZ- versus vehicle-treated rats regardless of IVSA condition. STZ-treated rats also displayed suppressed NAc dopamine levels during baseline and in response to nicotine. STZ treatment did not alter our assessment of nicotine metabolism. Furthermore, STZ treatment increased nicotine IVSA in a dose-dependent manner. Our findings suggest that STZ-treatment increased the rewarding effects of nicotine. This suggests that strong reinforcing effects of nicotine may contribute to greater tobacco use in patients with diabetes.

Putting desire on a budget: dopamine and energy expenditure, reconciling reward and resources

Accumulating evidence indicates integration of dopamine function with metabolic signals, highlighting a potential role for dopamine in energy balance, frequently construed as modulating reward in response to homeostatic state. Though its precise role remains controversial, the reward perspective of dopamine has dominated investigation of motivational disorders, including obesity. In the hypothesis outlined here, we suggest instead that the primary role of dopamine in behavior is to modulate activity to adapt behavioral energy expenditure to the prevailing environmental energy conditions, with the role of dopamine in reward and motivated behaviors derived from its primary role in energy balance. Dopamine has long been known to modulate activity, exemplified by psychostimulants that act via dopamine. More recently, there has been nascent investigation into the role of dopamine in modulating voluntary activity, with some investigators suggesting that dopamine may serve as a final common pathway that couples energy sensing to regulated voluntary energy expenditure. We suggest that interposed between input from both the internal and external world, dopamine modulates behavioral energy expenditure along two axes: a conserve-expend axis that regulates generalized activity and an explore-exploit axes that regulates the degree to which reward value biases the distribution of activity. In this view, increased dopamine does not promote consumption of tasty food. Instead increased dopamine promotes energy expenditure and exploration while decreased dopamine favors energy conservation and exploitation. This hypothesis provides a mechanistic interpretation to an apparent paradox: the well-established role of dopamine in food seeking and the findings that low dopaminergic functions are associated with obesity. Our hypothesis provides an alternative perspective on the role of dopamine in obesity and reinterprets the "reward deficiency hypothesis" as a perceived energy deficit. We propose that dopamine, by facilitating energy expenditure, should be protective against obesity. We suggest the apparent failure of this protective mechanism in Western societies with high prevalence of obesity arises as a consequence of sedentary lifestyles that thwart energy expenditure.

Rescue of dopamine transporter function in hypoinsulinemic rats by a D2 receptor-ERK-dependent mechanism

The dopamine (DA) transporter (DAT) is a major target for abused drugs and a key regulator of extracellular DA. A rapidly growing literature implicates insulin as an important regulator of DAT function. We showed previously that amphetamine (AMPH)-evoked DA release is markedly impaired in rats depleted of insulin with the diabetogenic agent streptozotocin (STZ). Similarly, functional magnetic resonance imaging experiments revealed that the blood oxygenation level-dependent signal following acute AMPH administration in STZ-treated rats is reduced. Here, we report that these deficits are restored by repeated, systemic administration of AMPH (1.78 mg/kg, every other day for 8 d). AMPH stimulates DA D(2) receptors indirectly by increasing extracellular DA. Supporting a role for D(2) receptors in mediating this "rescue," the effect was completely blocked by pre-treatment of STZ-treated rats with the D(2) receptor antagonist raclopride before systemic AMPH. D(2) receptors regulate DAT cell surface expression through ERK1/2 signaling. In ex vivo striatal preparations, repeated AMPH injections increased immunoreactivity of phosphorylated ERK1/2 (p-ERK1/2) in STZ-treated but not control rats. These data suggest that repeated exposure to AMPH can rescue, by activating D(2) receptors and p-ERK signaling, deficits in DAT function that result from hypoinsulinemia. Our data confirm the idea that disorders influencing insulin levels and/or signaling, such as diabetes and anorexia, can degrade DAT function and that insulin-independent pathways are present that may be exploited as potential therapeutic targets to restore normal DAT function.

A modified adjusting delay task to assess impulsive choice between isocaloric reinforcers in non-deprived male rats: effects of 5-HT₂A/C and 5-HT₁A receptor agonists

RATIONALE

Existing animal models of impulsivity frequently use food restriction to increase subjects' motivation. In addition, behavioral tasks that assess impulsive choice typically involve the use of reinforcers with dissimilar caloric content. These factors represent energy-homeostasis limitations, which may confound the interpretation of results and limit the applicability of these models.

OBJECTIVES

This study was aimed at validating face and convergent validities of a modified adjusting delay task, which assesses impulsive choice between isocaloric reinforcers in ad libitum fed rats.

METHODS

Male Wistar rats (n = 18) were used to assess the preferredness and reinforcing efficacy of a "supersaccharin" solution (1.5% glucose/0.4% saccharin) over a 1.5% glucose solution. A separate group of rats (n = 24) was trained in a modified adjusting delay task, which involved repeated choice between the glucose solution delivered immediately and the supersaccharin solution delivered after a variable delay. To pharmacologically validate the task, the effects of the 5-HT(2A/C) receptor agonist (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane [(±)-DOI] and the 5-HT(1A) receptor agonist (±)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide [(±)-8-OH-DPAT] on impulsive choice were then evaluated.

RESULTS

Supersaccharin was highly reinforcing and uniformly preferred over the glucose solution by all subjects. Rats quickly learned the task, and impulsivity was a very stable and consistent trait. DOI and 8-OH-DPAT significantly and dose dependently increased impulsive choice in this modified adjusting delay task.

CONCLUSIONS

We validated a rodent task of impulsive choice, which eliminates typical energy-homeostasis limitations and, therefore, opens new avenues in the study of impulsivity in preclinical feeding and obesity research.

A role for neuropeptide Y Y5 but not the Y1-receptor subtype in food deprivation-induced reinstatement of heroin seeking in the rat

RATIONAL AND OBJECTIVES

Neuropeptide Y (NPY), an orexigenic peptide that is released during periods of food restriction, has been shown to have a significant modulatory impact on drug-related behaviors. We have previously reported that both acute food deprivation (FD) and NPY injections can reinstate extinguished drug-seeking behavior, a proposed animal model of relapse to drug abuse. However, it is not clear whether the FD effect on drug seeking is dependent on NPY transmission. Here, we used the reinstatement model to assess the role of NPY Y1 and Y5-receptor-mediated transmission in FD-induced reinstatement of heroin seeking.

METHODS

Rats were trained to self-administer heroin for 10-12 days (0.1 mg/kg/infusion/intravenous). Animals then underwent extinction training followed by drug-seeking reinstatement tests under 21 h of FD and sated conditions.

RESULTS

Injections of a novel NPY Y5-receptor antagonist, Lu AA33810 (0.0, 1.0, or 30.0 mg/kg/IP), resulted in a significant attenuation of FD-induced reinstatement of extinguished heroin seeking. However, no significant effects on reinstatement were found for the Y1-receptor antagonist, BIBO 3304 (0.0, 5.0, or 10.0 nmol/intracerebroventricular).

CONCLUSIONS

These results suggest that while signals mediated through NPY Y1 receptors play a modest role in reinstatement, activation of Y5 receptors has a critical function in FD-induced reinstatement of heroin-seeking behavior.

Effects of insulin and leptin in the ventral tegmental area and arcuate hypothalamic nucleus on food intake and brain reward function in female rats

There is evidence for a role of insulin and leptin in food intake, but the effects of these adiposity signals on the brain reward system are not well understood. Furthermore, the effects of insulin and leptin on food intake in females are underinvestigated. These studies investigated the role of insulin and leptin in the ventral tegmental area (VTA) and the arcuate hypothalamic nucleus (Arc) on food intake and brain reward function in female rats. The intracranial self-stimulation procedure was used to assess the effects of insulin and leptin on the reward system. Elevations in brain reward thresholds are indicative of a decrease in brain reward function. The bilateral administration of leptin into the VTA (15-500 ng/side) or Arc (15-150 ng/side) decreased food intake for 72 h. The infusion of leptin into the VTA or Arc resulted in weight loss during the first 48 (VTA) or 24 h (Arc) after the infusions. The administration of insulin (0.005-5 mU/side) into the VTA or Arc decreased food intake for 24 h but did not affect body weights. The bilateral administration of low, but not high, doses of leptin (15 ng/side) or insulin (0.005 mU/side) into the VTA elevated brain reward thresholds. Neither insulin nor leptin in the Arc affected brain reward thresholds. These studies suggest that a small increase in leptin or insulin levels in the VTA leads to a decrease in brain reward function. A relatively large increase in insulin or leptin levels in the VTA or Arc decreases food intake.

Energy regulatory signals and food reward

The hormones insulin, leptin, and ghrelin have been demonstrated to act in the central nervous system (CNS) as regulators of energy homeostasis, acting at medial hypothalamic sites. Here, we summarize research demonstrating that, in addition to direct homeostatic actions at the hypothalamus, CNS circuitry that subserves reward and is also a direct and indirect target for the action of these endocrine regulators of energy homeostasis. Specifically, insulin and leptin can decrease food reward behaviors and modulate the function of neurotransmitter systems and neural circuitry that mediate food reward, the midbrain dopamine (DA) and opioidergic pathways. Ghrelin can increase food reward behaviors, and support midbrain DA neuronal function. We summarize discussion of behavioral, systems, and cellular evidence in support of the contributions of reward circuitry to the homeostatic roles of these hormones in the CNS. The understanding of neuroendocrine modulation of food reward, as well as food reward modulation by diet and obesity, may point to new directions for therapeutic approaches to overeating or eating disorders.

Dopamine and binge eating behaviors

Central dopaminergic mechanisms are involved in the motivational aspects of eating and food choices. This review focuses on human and animal data investigating the importance of dopamine on binge eating behaviors. Early work examining dopamine metabolites in the cerebrospinal fluid and plasma of bulimic individuals suggested decreased dopamine turnover during the active phase of the illness. While neuroimaging studies of dopamine mechanisms in bulimia nervosa (BN) and binge eating disorder (BED) are limited, genetic studies in humans have implicated an increased frequency of dopamine transporter and associated D2 receptor polymorphisms with binge pathology. Recent studies in rodent models of dietary-induced binge eating (DIBE) have investigated plausible dopamine mechanisms involved in sustaining binge eating behaviors. In DIBE models, highly palatable foods (fats, sugars and their combination), as well as restricted access conditions appear to promote ingestive responses and result in sustained dopamine stimulation within the nucleus accumbens. Taken together with studies on the comorbidity of illicit drug use and eating disorders, the data reviewed here support a role for dopamine in perpetuating the compulsive feeding patterns of BN and BED. As such, we propose that sustained stimulation of the dopamine systems by bingeing promoted by preexisting conditions (e.g., genetic traits, dietary restraint, stress, etc.) results in progressive impairments of dopamine signaling. To disrupt this vicious cycle, novel research-based treatment options aiming at the neural substrates of compulsive eating patterns are necessary.

Energy regulatory signals and food reward

The hormones insulin, leptin, and ghrelin have been demonstrated to act in the central nervous system (CNS) as regulators of energy homeostasis, acting at medial hypothalamic sites. Here, we summarize research demonstrating that, in addition to direct homeostatic actions at the hypothalamus, CNS circuitry that subserves reward and is also a direct and indirect target for the action of these endocrine regulators of energy homeostasis. Specifically, insulin and leptin can decrease food reward behaviors and modulate the function of neurotransmitter systems and neural circuitry that mediate food reward, the midbrain dopamine (DA) and opioidergic pathways. Ghrelin can increase food reward behaviors, and support midbrain DA neuronal function. We summarize discussion of behavioral, systems, and cellular evidence in support of the contributions of reward circuitry to the homeostatic roles of these hormones in the CNS. The understanding of neuroendocrine modulation of food reward, as well as food reward modulation by diet and obesity, may point to new directions for therapeutic approaches to overeating or eating disorders.

Insulin, leptin and reward

Feeding for pleasure, or "non-homeostatic feeding", potentially contributes to the rapid development of obesity worldwide. Obesity is associated with an imbalance of regulatory hormones which normally act to maintain stable energy balance and body weight. The adiposity hormones insulin and leptin are two such signals elevated in obesity with the capacity to dampen feeding behavior through their action on hypothalamic circuits which regulate appetite and metabolism. Recent evidence suggests that both hormones achieve this degree of regulation by inhibiting the rewarding aspects of feeding behavior, perhaps by signaling within midbrain reward circuits. This review describes the capacity of both insulin and leptin to regulate reward-related behavior.

Appetite and reward

The tendency to engage in or maintain feeding behaviour is potently influenced by the rewarding properties of food. Affective and goal-directed behavioural responses for food have been assessed in response to various physiological, pharmacological and genetic manipulations to provide much insight into the neural mechanisms regulating motivation for food. In addition, several lines of evidence tie the actions of metabolic signals, neuropeptides and neurotransmitters to the modulation of the reward-relevant circuitry including midbrain dopamine neurons and corticolimbic nuclei that encode emotional and cognitive aspects of feeding. Along these lines, this review pulls together research describing the peripheral and central signalling molecules that modulate the rewarding effects of food and the underlying neural pathways.

Deficit in brain reward function and acute and protracted anxiety-like behavior after discontinuation of a chronic alcohol liquid diet in rats

RATIONALE

Discontinuation of chronic and excessive alcohol consumption leads to a dysphoric state in humans. It is not known if there are changes in brain reward function after the discontinuation of an alcohol liquid in rats.

OBJECTIVES

The aim of these studies was to investigate the effect of withdrawal from an alcohol liquid diet on brain reward function and acute and protracted anxiety-like behavior.

MATERIALS AND METHODS

The intracranial self-stimulation procedure was used to assess brain reward function, and the elevated plus maze test was used to assess anxiety-like behavior.

RESULTS

Discontinuation of chronic, 12 weeks, exposure to a 6.2% v/v alcohol liquid diet lead to a minor deficit in brain reward function and did not increase anxiety-like behavior. Discontinuation of chronic, 12 weeks, exposure to a 10% v/v alcohol liquid diet lead to a pronounced deficit in brain reward function and increased anxiety-like behavior. Two weeks after discontinuation of the 10% v/v alcohol liquid diet, the rats with a history of alcohol dependence did not display increased anxiety-like behavior. Restraint stress increased anxiety-like behavior in the rats with a history of alcohol dependence, but not in the control rats. Brain reward thresholds were assessed during the chronic 10% v/v alcohol exposure period. During this period, there were no differences between the brain rewards thresholds of the alcohol and control rats.

CONCLUSION

These findings indicate that withdrawal from a 10% v/v alcohol liquid diet leads to a pronounced deficit in brain reward function and acute and protracted anxiety-like behavior in rats.

Endocrine links between food reward and caloric homeostasis

Both intrinsic and extrinsic (endocrine) inputs to the central nervous system (CNS) modulate motivation for feeding. Endocrine inputs such as insulin and leptin can have very rapid effects, but also the potential for chronic actions to decrease rewarding attributes of food. Future studies should elucidate the neural and cellular mechanisms which underlie these endocrine actions in the CNS.

Insulin, leptin, and food reward: update 2008

The hormones insulin and leptin have been demonstrated to act in the central nervous system (CNS) as regulators of energy homeostasis at medial hypothalamic sites. In a previous review, we described new research demonstrating that, in addition to these direct homeostatic actions at the hypothalamus, CNS circuitry that subserves reward and motivation is also a direct and an indirect target for insulin and leptin action. Specifically, insulin and leptin can decrease food reward behaviors and modulate the function of neurotransmitter systems and neural circuitry that mediate food reward, i.e., midbrain dopamine and opioidergic pathways. Here we summarize new behavioral, systems, and cellular evidence in support of this hypothesis and in the context of research into the homeostatic roles of both hormones in the CNS. We discuss some current issues in the field that should provide additional insight into this hypothetical model. The understanding of neuroendocrine modulation of food reward, as well as food reward modulation by diet and obesity, may point to new directions for therapeutic approaches to overeating or eating disorders.

Food restriction and streptozotocin treatment decrease 5-HT1A and 5-HT2A receptor-mediated behavioral effects in rats

Food restriction and hypoinsulinemia can affect the synthesis, turnover, and receptor function of serotonin (5-HT) in brain. This study explored the effects of food restriction and streptozotocin treatment on behavioral effects related to 5-HT1A (+)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT) and 5-HT2A [(+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI)] receptor activation. Lower lip retraction and flat body posture (8-OH-DPAT) and head twitching (DOI) were measured in rats during free feeding, food restriction, after treatment with streptozotocin, and finally after insulin replacement. 8-OH-DPAT induced lower lip retraction and flat body posture whereas DOI induced head twitching. One week of food restriction (10 g/day) decreased 8-OH-DPAT-induced lower lip retraction, 8-OH-DPAT-induced flat body posture, and DOI-induced head twitching. Subsequently, 1 week of free access to food restored sensitivity to 8-OH-DPAT and DOI-induced behavioral effects. Finally, 1 week after streptozotocin, 8-OH-DPAT-induced flat body posture and DOI-induced head twitching were markedly reduced whereas 8-OH-DPAT-induced lower lip retraction was unchanged. One week of insulin replacement restored sensitivity to 8-OH-DPAT and DOI-induced behavioral effects. These results show that modest food restriction or experimentally induced diabetes can profoundly affect sensitivity to drugs acting at 5-HT1A or 5-HT2A receptors; these results could be relevant to understanding the comorbidity of depression and diabetes.

Insulin acts at different CNS sites to decrease acute sucrose intake and sucrose self-administration in rats

Findings from our laboratory and others have demonstrated that the hormone insulin has chronic effects within the CNS to regulate energy homeostasis and to decrease brain reward function. In this study, we compared the acute action of insulin to decrease intake of a palatable food in two different behavioral tasks-progressive ratios sucrose self-administration and micro opioid-stimulated sucrose feeding-when administered into several insulin-receptive sites of the CNS. We tested insulin efficacy within the medial hypothalamic arcuate (ARC) and paraventricular (PVN) nuclei, the nucleus accumbens, and the ventral tegmental area. Administration of insulin at a dose that has no chronic effect on body weight (5 mU) into the ARC significantly suppressed sucrose self-administration (75+/-5% of paired control). However, although the mu opioid DAMGO, [D-Ala2,N-MePhe4,Gly5-ol]-enkephalin acetate salt, stimulated sucrose intake at all four CNS sites, the ventral tegmental area was the only sensitive site for a direct effect of insulin to antagonize acute (60 min) micro opioid-stimulated sucrose feeding: sucrose intake was 53+/-8% of DAMGO-induced feeding, when insulin was coadministered with DAMGO. These findings demonstrate that free feeding of sucrose, and motivated work for sucrose, can be modulated within unique sites of the CNS reward circuitry. Further, they support the interpretation that adiposity signals, such as insulin, can decrease different aspects of ingestion of a palatable food, such as sucrose, in an anatomically specific manner.

Modulation of food reward by adiposity signals

Extensive historical evidence from the drug abuse literature has provided support for the concept that there is functional communication between central nervous system (CNS) circuitries which subserve reward/motivation, and the regulation of energy homeostasis. This concept is substantiated by recent studies that map anatomical pathways, or which demonstrate that hormones and neurotransmitters associated with energy homeostasis regulation can directly modulate reward and motivation behaviors. Studies from our laboratory have focused specifically on the candidate adiposity hormones, insulin and leptin, and show that these hormones can decrease performance in behavioral paradigms that assess the rewarding or motivating properties of food. Additionally we and others have provided evidence that the ventral tegmental area may be one direct target for these effects, and we are currently exploring other potential anatomical targets. Finally, we are beginning to explore the interaction between adiposity signals, chronic maintenance diet of rats, and different types of food rewards to more closely simulate the current food environments of Westernized societies including the U.S. We propose that future studies of food reward should include a more complex environment in the experimental design that takes into account abundance and variety of rewarding foods, psychological stressors, and choices of reward modalities.

Chronic food restriction: enhancing effects on drug reward and striatal cell signaling

Chronic food restriction (FR) increases behavioral sensitivity to drugs of abuse in animal models and is associated with binge eating, which shares comorbidity with drug abuse, in clinical populations. Behavioral, biochemical and molecular studies conducted in this laboratory to elucidate the functional and mechanistic bases of these phenomena are briefly reviewed. Results obtained to date indicate that FR increases the reward magnitude and locomotor-activating effects of abused drugs, and direct dopamine (DA) receptor agonists, as a result of neuroadaptations rather than changes in drug disposition. Changes in striatal DA dynamics, and postsynaptic cell signaling and gene expression in response to D-1 DA receptor stimulation have been observed. Of particular interest is an upregulation of NMDA receptor-dependent MAP kinase and CaM Kinase II signaling, CREB phosphorylation, and immediate-early and neuropeptide gene expression in nucleus accumbens (NAc) which may facilitate reward-related learning, but also play a role in the genesis of maladaptive goal-directed behaviors. Covariation of altered drug reward sensitivity with body weight loss and recovery suggests a triggering role for one of the endocrine adiposity hormones. However, neither acute nor chronic central infusions of leptin or the melanocortin 3/4 receptor agonist, MTII, have attenuated d-amphetamine reward or locomotor activation in FR rats. Interestingly, chronic intracerebroventricular leptin infusion in ad libitum fed (AL) rats produced a sustained decrease in food intake and body weight that was accompanied by a reversible potentiation of rewarding and locomotor-activating effects of d-amphetamine. This raises the interesting possibility that rapid progressive weight loss is sufficient to increase behavioral sensitivity to drugs of abuse. Whether weight loss produced by leptin infusion produces the same neuroadaptations as experimenter-imposed FR, and whether any of the observed neuroadaptations are necessary for expression of increased behavioral responsiveness to acute drug challenge remain to be investigated.

Intraventricular insulin and leptin decrease sucrose self-administration in rats

Data from our laboratory and others have demonstrated an effect of the candidate adiposity signals insulin and leptin to decrease brain reward function, as assessed by lateral hypothalamic self-stimulation and food-conditioned place preference. In this study, we evaluated the effect of centrally administrated insulin or leptin to acutely decrease motivated performance for 5% sucrose, i.e., progressive ratio (PR) sucrose self-administration. Consistent with findings using other behavioral assays, both insulin and leptin significantly decreased the number of bar presses (62+/-7 and 76+/-8% of paired controls respectively), and the number of sucrose rewards obtained (87+/-4 and 91+/-4% of paired controls respectively), relative to within-subjects' control day performance on PR sucrose self-administration, whereas acute intraventricular cerebrospinal fluid had no effect. Rats fed a higher fat diet for 5 weeks were resistant to the effects of the intraventricular insulin or leptin, suggesting a central resistance to their action. Thus the findings of this study extend and support previous observations which suggest that neuroendocrine signals which regulate energy homeostasis in the CNS may also play a role in modulating reward circuitry, and specifically, food reward.

Chronic food restriction and dopamine transporter function in rat striatum

The present communication reports on DA uptake in rat striatum in a model of chronic food restriction. The K(m) for DA uptake was unaltered, but the V(max) was reduced by 32%, not supporting the idea that the enhanced behavioral sensitivity to cocaine or d-amphetamine upon chronic food restriction is due to a greater density of DAT at the plasma membrane for drug interaction. Chronic food restriction did not alter the potency of cocaine or D-amphetamine in inhibiting DA uptake in the striatum, suggesting that the enhanced behavioral sensitivity to these drugs upon chronic food restriction is not due to their enhanced affinity for DAT. These results point to factors other than DAT density or affinity underlying the sensitized response to psychostimulants in food restriction.

Deficits in dopamine clearance and locomotion in hypoinsulinemic rats unmask novel modulation of dopamine transporters by amphetamine

Insulin affects brain reward pathways and there is converging evidence that this occurs through insulin regulation of the dopamine (DA) transporter (DAT). In rats made hypoinsulinemic by fasting, synaptosomal DA uptake is reduced. Interestingly, [3H]DA uptake is increased in hypoinsulinemic rats with a history of amphetamine self-administration. The possibility that amphetamine and insulin act in concert to regulate DAT activity prompted this study. Here we show that [3H]DA uptake, measured in vitro and clearance of exogenously applied DA in vivo, is significantly reduced in rats made hypoinsulinemic by a single injection of streptozotocin. Strikingly, amphetamine (1.78 mg/kg, given every other day for 8 days) restored DA clearance in streptozotocin-treated rats but was without effect on DA clearance in saline-treated rats. Basal locomotor activity of streptozotocin-treated rats was lower compared to control rats; however, in streptozotocin-treated rats, hyperlocomotion induced by amphetamine increased over successive amphetamine injections. In saline-treated rats the locomotor stimulant effect of amphetamine remained stable across the four amphetamine injections. These results provide exciting new evidence that actions of amphetamine on DA neurotransmission are insulin-dependent and further suggest that exposure to amphetamine may cause long-lasting changes in DAT function.

Food restriction and leptin impact brain reward circuitry in lean and obese Zucker rats

The rewarding effect produced by electrically stimulating certain sites in the lateral hypothalamus (LH) can be potentiated by food restriction and body weight loss in lean rats. Central leptin and insulin administration can suppress the rewarding impact of the stimulation. To determine whether there are additional peripheral signals that mediate the effect of weight loss on brain reward circuitry, we assessed changes in LH-self-stimulation following food restriction in the obese Zucker rat which develops resistance to circulating leptin and insulin. In addition, we examined the impact of acute food deprivation and leptin administration on LH self-stimulation in lean and obese Zucker rats. The number of brain stimulation rewards earned was measured over a range of LH stimulation frequencies that drove reward rates from zero to asymptotic levels. Restriction reduced frequency thresholds in a subset of lean and obese rats, whereas BSR was unaltered by acute food deprivation. Despite impairment in leptin signaling, intraventricular leptin (4 microg) increased thresholds in most lean and obese rats in which the rewarding effect was sensitive to restriction. These results show that brain reward circuitry in the obese Zucker rat is sensitive to weight loss and leptin.

Intraventricular insulin and leptin reverse place preference conditioned with high-fat diet in rats

The authors hypothesized that insulin and leptin, hormones that convey metabolic and energy balance status to the central nervous system (CNS), decrease the reward value of food, as assessed by conditioned place preference (CPP). CPP to high-fat diet was blocked in ad-lib fed rats given intraventricular insulin or leptin throughout training and test or acutely before the test. Insulin or leptin given only during the training period did not block CPP. Thus, elevated insulin and leptin do not prevent learning a food's reward value, but instead block its retrieval. Food-restricted rats receiving cerebrospinal fluid, insulin, or leptin had comparable CPPs. Results indicate that the CNS roles of insulin and leptin may include processes involving memory and reward.

Restricted feeding with scheduled sucrose access results in an upregulation of the rat dopamine transporter

Recent studies suggest that the mesoaccumbens dopamine system undergoes neurochemical alterations as a result of restricted feeding conditions with access to sugars. This effect appears to be similar to the neuroadaptation resulting from drugs of abuse and may underlay some pathological feeding behaviors. To further investigate the cellular mechanisms of these alterations, the present study used quantitative autoradiography and in situ hybridization to assess dopamine membrane transporter (DAT) protein density and mRNA expression in restricted-fed and free-fed adult male rats. The restricted feeding regimen consisted of daily limited access to either a normally preferred sucrose solution (0.3 M) or a less preferred chow in a scheduled (i.e., contingent) fashion for 7 days. Restricted-fed rats with the contingent sucrose access lost less body weight, ate more total food, and drank more fluid than free-fed, contingent food, or noncontingent controls. In addition, these animals had selectively higher DAT binding in the nucleus accumbens and ventral tegmental area. This increase in protein binding also was accompanied by an increase in DAT mRNA levels in the ventral tegmental area. In contrast to the restricted-fed groups, no differential effect in DAT regulation was observed across free-fed groups. The observed alteration in behavior and DAT regulation suggest that neuroadaptation in the mesoaccumbens dopamine system develops in response to repeated feeding on palatable foods under dietary constraints. This supports the notion that similar cellular changes may be involved in restrictive eating disorders and bingeing.

Adiposity signals and food reward: expanding the CNS roles of insulin and leptin

The hormones insulin and leptin have been proposed to act in the central nervous system (CNS) as adiposity signals as part of a theoretical negative feedback loop that senses the caloric stores of an animal and orchestrates adjustments in energy balance and food intake. Much research has provided support for both the existence of such a feedback loop and the specific roles that insulin and leptin may play. Most studies have focused on hypothalamic sites, which historically are implicated in the regulation of energy balance, and on the brain stem, which is a target for neural and humoral signals relating to ingestive acts. More recent lines of research, including studies from our lab, suggest that in addition to these CNS sites, brain reward circuitry may be a target for insulin and leptin action. These studies are reviewed together here with the goals of providing a historical overview of the findings that have substantiated the originally hypothesized negative feedback model and of opening up new lines of investigation that will build on these findings and allow further refinement of the model of adiposity signal/CNS feedback loop. The understanding of how motivational circuitry and its endocrine or neuroendocrine modulation contributes to normal energy balance regulation should expand possibilities for future therapeutic approaches to obesity and may lead to important insights into mental illnesses such as substance abuse or eating disorders.

Intraventricular insulin decreases kappa opioid-mediated sucrose intake in rats

The hormone insulin acts in the central nervous system (CNS) as a regulator of body adiposity and food intake. Recent work from our laboratory has provided evidence that one way by which insulin may decrease food intake is by decreasing the rewarding properties of food. Evidence from others suggests that endogenous opioids may mediate the palatable properties of foods, and insulin may decrease nonfood-related reward via interaction with some CNS kappa opioid systems. In the present study we examined the ability of insulin to interact with exogenous or endogenous kappa opioids to modulate feeding of palatable sucrose pellets by nondeprived rats. Insulin (5 mU intracerebroventricular (i.c.v.), t=-3h) completely reversed the ability of the exogenous kappa agonist U50,488 (26 microg, i.c.v., t=-15 min) to stimulate 90-min sucrose feeding (211+/-32% reduced to 125+/-23% of 90-min baseline intake). Further, i.c.v. insulin (5 mU, t=-3h) interacted with a subthreshold dose of the kappa receptor antagonist norbinaltorphimine (5 microg, i.c.v., t=-15 min) to decrease the 90-min sucrose intake baseline (77+/-11% versus 109+/-10% of 90 min baseline intake, insulin/norbinaltorphimine versus norbinaltorphimine). Together these studies provide new evidence that insulin in the CNS may decrease the action of CNS kappa opioid system(s) that mediate palatable feeding.

Food restriction modulates amphetamine-conditioned place preference and nucleus accumbens dopamine release in the rat

Food restriction has been shown to increase self-administration of psychostimulants, including cocaine and amphetamine (AMPH). Consistent with this, food-restricted rats are more sensitized to the rewarding effects of cocaine as measured by conditioned place preference (CPP). This study investigated whether moderate food restriction in rats (15 g/day) results in an increased CPP, relative to ad libitum-fed controls, to a second psychostimulant, AMPH. Conditioning trials consisted of six alternating injections of i.p. AMPH (0.425-6.8 mg/kg) and i.p. saline, paired with distinct environments. On Day 7, a drug-free 20-min choice test for environment was carried out to assess CPP. 0.85 mg/kg AMPH significantly increased CPP in food-restricted vs. ad libitum-fed rats. At 1.7 and 3.4 mg/kg AMPH, food-restricted rats showed decreased CPP, but increased locomotor activity, relative to ad libitum fed controls. To evaluate whether an alteration in extracellular fluid DA levels in the nucleus accumbens (NAc) core could account for these behavioral alterations, DA release was measured by microdialysis. DA release to a single acute i.p. injection of either 0.85 or 1.7 mg/kg AMPH was comparable in food-restricted and ad libitum fed rats. However, ad libitum fed rats demonstrated conditioned DA release after an AMPH conditioning paradigm analogous to the CPP paradigm, whereas food-restricted rats demonstrated no conditioned DA release. In conclusion, altered DA release in the nucleus accumbens core is not a primary effect of moderate food restriction and cannot completely account for either the altered CPP behavior or enhanced locomotor activity observed in this study.

Does neuropeptide Y contribute to the modulation of brain stimulation reward by chronic food restriction?

The rewarding effect produced by electrically stimulating particular sites in the lateral hypothalamus (LH) can be enhanced by chronic food restriction and body weight loss. The impact on brain stimulation reward (BSR) of certain hormones involved in the regulation of energy balance, such as leptin and corticotropin-releasing hormone, depends upon the sensitivity of BSR to food restriction. The present investigation assessed the influence of neuropeptide Y (NPY), a potent orexigenic peptide, on BSR generated by stimulating restriction-sensitive and -insensitive sites in the LH. Twelve male Long Evans rats were trained to press a lever for a rewarding train of stimulation. Rate-frequency curves, reflecting the number of rewards earned as a function of the stimulation frequency, were collected during free-feeding and then again following a period of food restriction and 20-25% body weight loss. NPY (4 microg) was administered intraventriculary during the food restriction condition. Alterations in the rewarding effect of the stimulation were assessed by measuring changes in the frequency required to maintain half-maximal rewards earned (M-50). In half of the subjects, food restriction produced significant decreases in M-50 values, indicating that the reward effectiveness of the stimulation was potentiated. In contrast, M-50 values were unaltered by food restriction in the remaining six animals. In most of the subjects in which M-50 values decreased following chronic food restriction, NPY failed to alter BSR. Similarly, BSR was unchanged by NPY administration in most of the rats with restriction-insensitive stimulation sites. These findings suggest that NPY does not take part in the process whereby food restriction and leptin modulate reward circuitry activated by stimulating restriction-sensitive sites.

PI 3-kinase regulation of dopamine uptake

The magnitude and duration of dopamine (DA) signaling is defined by the amount of vesicular release, DA receptor sensitivity, and the efficiency of DA clearance, which is largely determined by the DA transporter (DAT). DAT uptake capacity is determined by the number of functional transporters on the cell surface as well as by their turnover rate. Here we show that inhibition of phosphatidylinositol (PI) 3-kinase with LY294002 induces internalization of the human DAT (hDAT), thereby reducing transport capacity. Acute treatment with LY294002 reduced the maximal rate of [(3) H]DA uptake in rat striatal synaptosomes and in human embryonic kidney (HEK) 293 cells stably expressing the hDAT (hDAT cells). In addition, LY294002 caused a significant redistribution of the hDAT from the plasma membrane to the cytosol. Conversely, insulin, which activates PI 3-kinase, increased [(3)H]DA uptake and blocked the amphetamine-induced hDAT intracellular accumulation, as did transient expression of constitutively active PI 3-kinase. The LY294002-induced reduction in [(3)H]DA uptake and hDAT cell surface expression was inhibited by expression of a dominant negative mutant of dynamin I, indicating that dynamin-dependent trafficking can modulate transport capacity. These data implicate DAT trafficking in the hormonal regulation of dopaminergic signaling, and suggest that a state of chronic hypoinsulinemia, such as in diabetes, may alter synaptic DA signaling by reducing the available cell surface DATs.