The controls of eating: a shift from nutritional homeostasis to behavioral neuroscience

Gut peptides in the control of food intake: 30 years of ideas

The demonstration of the ability of exogenous cholecystokinin (CCK) to inhibit food intake began a series of investigations into whether and how gut and brain peptides affected ingestive behavior. In that original demonstration, Gerry Smith and colleagues both established criteria for evaluating roles for gut peptides in food intake and shifted the focus of feeding controls to factors that contribute to limiting meal size. Although new gut peptides with novel mechanisms and durations of action have been identified in the past few years, Smith's criteria and his distinction between direct and indirect controls of meal size continue to provide a framework for understanding how such peptides may contribute to overall feeding control.

Motivation concepts in behavioral neuroscience

Concepts of motivation are vital to progress in behavioral neuroscience. Motivational concepts help us to understand what limbic brain systems are chiefly evolved to do, i.e., to mediate psychological processes that guide real behavior. This article evaluates some major motivation concepts that have historic importance or have influenced the interpretation of behavioral neuroscience research. These concepts include homeostasis, setpoints and settling points, intervening variables, hydraulic drives, drive reduction, appetitive and consummatory behavior, opponent processes, hedonic reactions, incentive motivation, drive centers, dedicated drive neurons (and drive neuropeptides and receptors), neural hierarchies, and new concepts from affective neuroscience such as allostasis, cognitive incentives, and reward 'liking' versus 'wanting'.

Behavioral models of binge-type eating

PURPOSE

To describe and evaluate behavioral models of binge-type eating.

DATA IDENTIFICATION

Studies were identified using Medline and hand searches of bibliographies of identified articles.

STUDY SELECTION

Isomorphic studies were selected that were judged to have some measure of construct validity.

DATA EXTRACTION

Face and construct validity were assessed, as well as simplicity and cost of use.

RESULTS OF DATA SYNTHESIS

Several different models of binge-type eating exist, each with different strengths of validity and use. These include models using sham feeding, restriction/refeeding cycles and/or stress, limited access (LA) to optional foods, and eating induced by operant schedules of behavior.

CONCLUSIONS

We concur with Harry Harlow, who was quoted by Gerry Smith as saying: "You'd be crazy to use animal models, but you'd also be crazy not to use them."

Lessons in the interactions of hormones and ingestive behavior

Research by Jim Gibbs and Gerry Smith from the Bourne Laboratory in the early 1970s demonstrated that peptide signals from the digestive system, especially cholecystokinin (CCK), have a profound effect on ingestive behavior. My laboratory consequently pursued a parallel course with the hormone insulin. Integrating the research on meal-generated signals, such as CCK, with adiposity-indicating signals, such as insulin, has progressed a long way, thanks in large part to suggestions and inputs from Gerry Smith along the way. This short article documents that progress.

The relationship between food reward and satiation revisited

The postingestive satiating action of food is often viewed as producing a positive affective state that rewards eating. However, in an early test of this idea, Van Vort and Smith [Physiol. Behav. 30 (1983) 279] reported that rats did not learn to prefer a food that was "real-fed" and satiating over a food that was "sham-fed" and not satiating. Subsequent investigators obtained similar findings with concentrated nutrient sources. With dilute nutrient sources, however, rats learned to prefer the real-fed to the sham-fed food. These and other findings demonstrate that nutrients have rewarding postingestive effects that enhance food preferences via a conditioning process. These reward effects appear separate from the satiating actions of nutrients, which may actually reduce food reward. Food intake and preference are controlled by a complex interaction of positive and negative signals generated by nutrients in the mouth and at postingestive sites.

Opioids as agents of reward-related feeding: a consideration of the evidence

Gerard Smith was one of the pioneers in the field of neuropeptidergic control of food intake. He established methodology and criteria used to determine whether a neuropeptide acts as an endogenous satiety factor. More recently, he theorized that there are direct and indirect controls of meal size. Direct controls include those that depend upon contact of food with preabsorptive receptors from the tip of the tongue to the end of the small intestine, and indirect controls include those that do not depend upon direct contact of mucosal receptors, such as learning and metabolism. In this review, we consider the evidence that opioids are mediators of reward-related feeding. We address these issues adopting Smith's approach to problem solving, including an evaluation of the opioids as controllers of the meal. We also present a novel concept of "hedonic restriction," resulting in a change in opioid gene expression. Overall, we believe the evidence supporting opioid participation in reward-driven and other types of ingestion is very strong, but much work remains before we understand how opioids contribute to the widely distributed neural network that controls ingestive behavior.

A mechanism underlying mature-onset obesity: evidence from the hyperphagic phenotype of brain-derived neurotrophic factor mutants

Mice deficient in brain-derived neurotrophic factor (BDNF) develop mature-onset obesity, primarily due to overeating. To gain insight into the mechanism of this hyperphagia, we characterized food intake, body weight, meal pattern, and meal microstructure in young and mature mice fed balanced or high-fat diets. Hyperphagia and obesity occurred in mature but not young BDNF mutants fed a balanced diet. This hyperphagia was mediated by increased meal number, which was associated with normal meal size, meal duration, and satiety ratio. In contrast, the high-fat diet induced premature development of hyperphagia and obesity in young BDNF mutants and a similar magnitude hyperphagia in mature mutants. This hyperphagia was supported by increased meal size and was accompanied by a reduced satiety ratio. Thus the mechanism underlying hyperphagia was present before significant weight gain, but whether it occurred, and whether meal frequency or meal size was altered to support it, was modulated by a process associated with aging and by diet properties. Meal pattern changes associated with the balanced diet suggested meal initiation, and the oropharyngeal positive feedback that drives feeding, were enhanced and might have contributed to overeating in BDNF mutants, whereas negative feedback was normal. Consistent with this hypothesis, meal microstructure revealed that all hyperphagic mutant groups exhibited increased intake rates at meal onset. Therefore, the central nervous system targets of BDNF actions may include orosensory brain stem neurons that process and transmit positive feedback or forebrain neurons that modulate its strength.

Independent influences of body mass and gastric volumes on satiation in humans

BACKGROUND & AIMS

We assessed the association of body mass and gastric volumes (fasting and postprandial) with satiation and postprandial symptoms.

METHODS

Healthy obese and nonobese subjects underwent measurement of caloric intake at maximum satiation; postprandial symptoms were measured with visual analogue scales 30 minutes after a meal. Gastric volume during fasting and after 300 mL of Ensure was measured with technetium-99m single-photon emission computed tomography imaging. We used multiple regression analysis to assess the associations among variables.

RESULTS

Among 134 participants (81 women and 53 men), the median age was 26 years (range, 12-58 years), and the median body mass index was 24 kg/m(2) (range, 17-48 kg/m(2)). Increased body mass index, but not height, was associated with delayed satiation (P < 0.003, adjusted for sex). Overweight and obese subjects ingested, on average, 225 +/- 57 more kilocalories (945 +/- 239 kJ) at maximum satiation compared with normal weight individuals. Increased fasting gastric volume was not associated with body mass index or height, but it was significantly associated with delayed satiation (P = 0.001, adjusted for body mass index and sex). An increase of 50 mL in the fasting gastric volume was associated with 114 +/- 32 kcal (479 +/- 134 kJ) more ingested at maximum satiation. Increased body mass index was associated with lower fullness scores 30 minutes after a meal (P = 0.0012, adjusted for sex and volume of Ensure ingested). In contrast, scores of postprandial bloating and pain were higher with increased body mass index (both P < 0.05, adjusted for sex and volume of Ensure ingested).

CONCLUSIONS

Greater body mass index and fasting gastric volume are associated with reduced satiation. Increased body mass index or height was not associated with greater gastric volumes.

Hindbrain noradrenergic lesions attenuate anorexia and alter central cFos expression in rats after gastric viscerosensory stimulation

Behavioral, autonomic, and endocrine outputs of the CNS are subject to important feedback modulation by viscerosensory signals that are conveyed initially to the hindbrain nucleus of the solitary tract (NST). In the present study, noradrenergic (NA) neurons [i.e., those that express the NA synthetic enzyme dopamine beta hydroxylase (DbH)] in the caudal NST were lesioned to determine their role in mediating anorexic responses to gastric stimulation and in conveying gastric sensory signals to the hypothalamus and amygdala. For this purpose, saporin toxin conjugated to an antibody against DbH was microinjected bilaterally into the caudal NST in adult rats. Control rats received similar microinjections of vehicle. Several weeks later, rats were tested for the ability of systemic cholecystokinin octapeptide (CCK) (0 or 10 microg/kg) to inhibit food intake. CCK-induced anorexia was significantly attenuated in toxin-treated rats. Rats subsequently were used in a terminal cFos study to determine central neural activation patterns after systemic CCK or vehicle and to evaluate lesion extent. Toxin-induced loss of DbH-positive NST neurons was positively correlated with loss of CCK-induced anorexia. Hypothalamic cFos expression was markedly attenuated in lesioned rats after CCK treatment, whereas CCK-induced neural activation in the parabrachial nucleus and amygdala appeared normal. These findings suggest that hindbrain NA neurons are an integral component of brainstem circuits that mediate CCK-induced anorexia and also are necessary for hypothalamic but not parabrachial or amygdala responses to gastric sensory stimulation.

Anorectic brainstem peptides: more pieces to the puzzle

Eating a meal is a mechanical process involving autonomous pathways that relay sensory and motor information between the whole length of the digestive tract and the central nervous system. This circuitry is able to initiate and terminate the meal, primarily by gut-brainstem-gut reflex arcs, and is independent of the caloric content of a meal. However, as part of our ability to regulate body weight over time, we must be able to modulate the amount of energy that we take in as food and the amount of energy that we expend. Thus, the gut-brainstem axis must be coupled to other systems that take account of factors such as food availability and preference, changing energy requirements and our social habits. Here, we review the importance of the brainstem nucleus of the tractus solitarius as a site of integration and the routes by which it connects the gut-brainstem axis with regulatory neuronal and endocrine networks that allow for strict body weight management.

Reduced feeding during water deprivation depends on hydration of the gut

Removal of drinking water at the start of the dark period reduced food intake in freely feeding rats within 45 min. Both first and later meals were smaller during 7.5 h of water deprivation, but meal frequency did not change. Ingestion of a normal-sized meal (3 g) rapidly increased plasma tonicity when drinking water was withheld, but intravenous infusions of hypertonic NaCl causing similar increases in plasma tonicity did not reduce feeding. Feeding during 6 h of water deprivation was restored by slowly infusing the volume of water normally drunk into the stomach, jejunum, or cecum, but not in the vena cava or hepatic portal vein. The infusions did not alter water or electrolyte excretion or affect food intake in rats allowed to drink. We conclude that the inhibition of feeding seen during water deprivation is mediated by a sensor that is located in the gastrointestinal tract or perhaps in the mesenteric veins draining the gut, but not the hepatic portal vein or the liver. In the absence of drinking water, signals from this sensor provoke the early termination of a meal.

Expression of dopaminergic receptors in the hypothalamus of lean and obese Zucker rats and food intake

As revealed by previous microdialysis studies, basal and food intake-accompanied dopamine release significantly differs in the hypothalamus of obese vs. lean Zucker rats. In the present study, we determined whether dopaminergic receptors are also compromised in obesity. Dopaminergic D(1) and D(2) receptor mRNA expression was studied in the ventromedial hypothalamus (VMH), lateral hypothalamic area (LHA), and the adenohypophysis (AH) of obese and lean Zucker rats using RT-PCR technique. In obese Zucker rats, we found an upregulation of D(1) receptor mRNA in the VMH and AH and a downregulation in the LHA, whereas D(2) receptor mRNA was downregulated in both the VMH and LHA, but not changed in the AH, compared with lean rats. Also, an increase of D(1) receptor staining was seen in the paraventricular nucleus of obese rats by immunohistochemistry. We selected the VMH to test if the observed changes in the dopamine receptor expression of obese rats induce behavioral sensitization to dopamine as expressed by hyperphagia. The overnight food-deprived rats received a single VMH injection (10 nmol) of sulpiride (D(2) receptor antagonist) or saline as control, then food was provided and 1-h food intake was measured. Food intake after sulpiride vs. saline injection was greater in obese rats but was not different in lean rats. Our data suggest that downregulation of D(2) receptor in the hypothalamus at least in the VMH induces behavior sensitization for having large meals. Low D(2) receptor expression may be causal for an exaggerated dopamine release observed in obese rats during food ingestion and for reduced satiety feedback effect of dopamine. High level of D(1) receptor expression in the VMH and low in the LHA may also contribute to the specific feeding pattern in obese rats represented by large meal size and low meal number.

Opioid peptides and the control of human ingestive behaviour

A variety of evidence suggests that endogenous opioid peptides play a role in the short-term control of eating. More recently, opioid receptor antagonists like naltrexone have been approved as a treatment for alcohol dependence. Here we review the evidence for a role of opioid peptides in both normal and abnormal eating and drinking behaviours and in particular try to identify the nature of the role of opioids in these behaviours. Particular attention is paid to the idea that opioid reward processes may be involved both in the short-term control of eating and hedonic aspects of alcohol consumption, and parallels are drawn between the effects of opiate antagonists on food pleasantness and the experience of drinking alcohol. The review also explores the extent to which data from studies using opiate antagonists and agonists provide evidence for a direct role of endogenous opioids in the control of ingestive behaviour, or alternatively whether these data may be better explained through non-specific effects such as the nausea commonly reported following administration of opiate antagonists. The review concludes that the present data suggests a single opioid mechanism is unlikely to explain all aspects of ingestive behaviour, but also concludes that opioid-mediated reward mechanisms play an important control in hedonic aspects of ingestion. The review also highlights the need for further empirical work in order to elucidate further the role of opioid peptides in human ingestive behaviour.

"Nonhedonic" food motivation in humans involves dopamine in the dorsal striatum and methylphenidate amplifies this effect

The drive for food is one of the most powerful of human and animal behaviors. Dopamine, a neurotransmitter involved with motivation and reward, its believed to regulate food intake in laboratory animals by modulating its rewarding effects through the nucleus accumbens (NA). Here we assess the involvement of dopamine in "nonhedonic" food motivation in humans. Changes in extracellular dopamine in striatum in response to nonhedonic food stimulation (display of food without consumption) were evaluated in 10 food-deprived subjects (16-20 h) using positron emission tomography (PET) and [11C]raclopride (a D2 receptor radioligand that competes with endogenous dopamine for binding to the receptor). To amplify the dopamine changes we pretreated subjects with methylphenidate (20 mg p.o.), a drug that blocks dopamine transporters (mechanism for removal of extracellular dopamine). Although the food stimulation when preceded by placebo did not increase dopamine or the desire for food, the food stimulation when preceded by methylphenidate (20 mg p.o.) did. The increases in extracellular dopamine were significant in dorsal (P < 0.005) but not in ventral striatum (area that included NA) and were significantly correlated with the increases in self-reports of hunger and desire for food (P < 0.01). These results provide the first evidence that dopamine in the dorsal striatum is involved in food motivation in humans that is distinct from its role in regulating reward through the NA. In addition it demonstrates the ability of methylphenidate to amplify weak dopamine signals.

The effects of amygdala lesions on conditioned stimulus-potentiated eating in rats

Both control rats and rats with neurotoxic lesions of the amygdala central nucleus ate more food during presentations of a conditioned stimulus (CS) previously paired with food than during an unpaired CS. This potentiation occurred regardless of whether the food was presented in its usual place or in a different location. By contrast, rats with neurotoxic lesions of basolateral amygdala showed no evidence for conditioned potentiation of eating. These results are considered in the context of anatomical projections from these amygdalar areas to other brain regions involved in feeding, and the role of amygdala subregions in the acquisition of motivational value in conditioning.

The evolution of an operant ecologist

George Collier has maintained a research program concerned with the controls of eating for forty-three years. In the first ten years, Collier approached the problem within the operant conditioning framework proposed by Skinner. This consisted of the intrameal analysis of the reflex strength of a food-rewarded operant in short sessions in food-deprived rats. At the end of the first decade, Collier shifted his attack on the problem by studying the ecological control of meal patterns by procurement and consummatory costs in free-feeding rats. This paper analyzes the reasons for this apparently abrupt evolution from operant conditioning to operant ecology and concludes that they accumulated over about ten years and were intellectual and personal, not technical or professional. This paper also summarizes the major achievements of the research program of operant ecology, notes some emerging problems with economic explanations of ecological controls, and argues that Collier's robust and quantitative behavioral results are ripe for physiological analysis.

Food deprivation-induced expression of minoxidil sulfotransferase in the hypothalamus uncovered by microarray analysis

We used oligonucleotide microarrays to analyze comprehensively hypothalamic gene expression changes that correlate with energy homeostasis. We compared the hypothalamic gene expression profiles of freely fed and 48-h fasted rats using 26,379 oligonucleotide probe sets. Expression of 96 genes was up-regulated and expression of 73 genes was down-regulated in a statistically significant manner with fasting. The gene encoding the enzyme minoxidil sulfotransferase, an enzyme that catalyzes the transfer of sulfonate groups to biogenic amines and other substrates, was foremost among a set of genes whose mRNAs were uniformly detectable and displaying the greatest transcriptional changes with fasting. Northern blot analysis indicated that minoxidil sulfotransferase mRNA is up-regulated in the fasted rat and mouse, ob/ob mouse, and fa/fa rat. Results of reverse transcription quantitative PCR indicated that minoxidil sulfotransferase mRNA is also up-regulated in the microdissected arcuate and paraventricular nuclei of the fasted rat. Several index genes known to be either up-regulated (neuropeptide Y) or down-regulated (amphetamine-regulated transcript and proopiomelanocortin) with fasting were also found to be present among our set of "differentially expressed" genes. This study identifies a novel gene induced by fasting and demonstrates the feasibility of using oligonucleotide microarrays for the study of complex neuronal processes.

The neuroanatomical axis for control of energy balance

The hypothalamic feeding-center model, articulated in the 1950s, held that the hypothalamus contains the interoceptors sensitive to blood-borne correlates of available or stored fuels as well as the integrative substrates that process metabolic and visceral afferent signals and issue commands to brainstem mechanisms for the production of ingestive behavior. A number of findings reviewed here, however, indicate that sensory and integrative functions are distributed across a central control axis that includes critical substrates in the basal forebrain as well as in the caudal brainstem. First, the interoceptors relevant to energy balance are distributed more widely than had been previously thought, with a prominent brainstem complement of leptin and insulin receptors, glucose-sensing mechanisms, and neuropeptide mediators. The physiological relevance of this multiple representation is suggested by the demonstration that similar behavioral effects can be obtained independently by stimulation of respective forebrain and brainstem subpopulations of the same receptor types (e.g., leptin, CRH, and melanocortin). The classical hypothalamic model is also challenged by the integrative achievements of the chronically maintained, supracollicular decerebrate rat. Decerebrate and neurologically intact rats show similar discriminative responses to taste stimuli and are similarly sensitive to intake-inhibitory feedback from the gut. Thus, the caudal brainstem, in neural isolation from forebrain influence, is sufficient to mediate ingestive responses to a range of visceral afferent signals. The decerebrate rat, however, does not show a hyperphagic response to food deprivation, suggesting that interactions between forebrain and brainstem are necessary for the behavioral response to systemic/ metabolic correlates of deprivation in the neurologically intact rat. At the same time, however, there is evidence suggesting that hypothalamic-neuroendocrine responses to fasting depend on pathways ascending from brainstem. Results reviewed are consistent with a distributionist (as opposed to hierarchical) model for the control of energy balance that emphasizes: (i) control mechanisms endemic to hypothalamus and brainstem that drive their unique effector systems on the basis of local interoceptive, and in the brainstem case, visceral, afferent inputs and (ii) a set of uni- and bidirectional interactions that coordinate adaptive neuroendocrine, autonomic, and behavioral responses to changes in metabolic status.

Taste, food intake and obesity

Research in human eating behaviour prior to 1990 has shown that taste impacts the palatability and selection of food for intake; sensory-specific satiety; satiation; and thermic effect of food. Research in the last decade has added information to these areas; expanded the field to comparisons of the impact of 'wanting' vs. 'liking' food on intake, and provided insight into the relationship of food intake and brain functioning through new imaging techniques. This article will review literature from the last decade on research in the area of taste and its impact on food intake. Emphasis will be placed on differences seen between lean and obese humans and how these may contribute to the development of human obesity. Suggestions for future research directions will also be discussed.

Estradiol, CCK and satiation

Estradiol has long been known to inhibit feeding in animals, but the mechanism(s) mediating its effects have not been clear. Demonstrations that estradiol's feeding effects are expressed as decreases in meal size coupled with the emerging consensus that cholecystokinin (CCK) released from the small intestines during meals is a physiological negative-feedback signal controlling meal size (i.e. satiation) suggested a new approach to the problem of the mechanisms of estradiol's inhibitory effect on feeding. Progress on this approach is reviewed here. Experimental manipulations of exogenous and endogenous CCK and estradiol have produced converging evidence that estradiol cyclically increases the activity of the CCK satiation-signaling pathway so that meal size and food intake decrease during the ovulatory or estrus phase of the ovarian cycle. This is a striking example of the modulation of the operation of a control of meal size by the physiological context in which the meal occurs. Estradiol also produces a tonic decrease in meal size, but this apparently does not involve the CCK satiation-signaling pathway. Where and how estradiol acts to increase the potency of the CCK satiating-signaling pathway are not known. Several possible sites are suggested by the observations that estradiol treatment increases feeding- and CCK-induced expression of c-Fos in ovariectomized animals in brain areas including the nucleus tractus solitarius, paraventricular nucleus of the hypothalamus, and central nucleus of the amygdala. Tests with null mutation mice indicate that estrogen receptor-alpha is necessary for estradiol's feeding effects. Finally, the possibilities that estradiol exerts important influences on normal or disordered eating in women are discussed. It is concluded that estradiol exerts a biologically significant action on CCK satiation in animals. Further research to determine whether this action of estradiol has a role in the pathogenesis, course, or treatment of disordered eating in women is indicated.