Enhanced food-anticipatory circadian rhythms in the genetically obese Zucker rat

Persistence of a behavioral food-anticipatory circadian rhythm following dorsomedial hypothalamic ablation in rats

Circadian rhythms of behavior in rodents are regulated by a system of circadian oscillators, including a master light-entrainable pacemaker in the suprachiasmatic nucleus that mediates synchrony to the day-night cycle, and food-entrainable oscillators located elsewhere that generate rhythms of food-anticipatory activity (FAA) synchronized to daily feeding schedules. Despite progress in elucidating neural and molecular mechanisms of circadian oscillators, localization of food-entrainable oscillators driving FAA remains an enduring problem. Recent evidence suggests that the dorsomedial hypothalamic nucleus (DMH) may function as a final common output for behavioral rhythms and may be critical for the expression of FAA (Gooley JJ, Schomer A, and Saper CB. Nat Neurosci 9: 398-407, 2006). To determine whether the reported loss of FAA by DMH lesions is specific to one behavioral measure or generalizes to other measures, rats received large radiofrequency lesions aimed at the DMH and were recorded in cages with movement sensors. Total and partial DMH ablation was associated with a significant attenuation of light-dark-entrained activity rhythms during ad libitum food access, because of a selective reduction in nocturnal activity. When food was restricted to a single 3-h daily meal in the middle of the lights-on period, all DMH and intact rats exhibited significant FAA. The rhythm of FAA persisted during a 48-h food deprivation test and reappeared during a 72-h deprivation test after ad libitum food access. The DMH is not the site of oscillators or entrainment pathways necessary for all manifestations of FAA, but may participate on the output side of this circadian function.

Staying awake for dinner: hypothalamic integration of sleep, feeding, and circadian rhythms

Daily patterns of sleep and wakefulness are inextricably linked to the regulation of feeding and energy metabolism. Both are affected by homeostatic as well as circadian drives, and both are tightly linked to thermoregulation. In this chapter, we review the basic drain circuitry that regulates sleep and wakefulness, including the flip-flop switch relationship of the arousal system and the ventrolateral preoptic sleep-promoting neurons. We then examine the role of the orexin/hypocretin neurons, which stabilize the switch while driving both wakefulness and foraging for food. We also review the role of the subparaventricular nucleus and the dorsomedial nucleus of the hypothalamus in circadian integration and modulation of both feeding and wake-sleep patterns.

Sensory experience of food and obesity: a positron emission tomography study of the brain regions affected by tasting a liquid meal after a prolonged fast

The sensory experience of food is a primary reinforcer of eating and overeating plays a major role in the development of human obesity. However, whether the sensory experience of a forthcoming meal and the associated physiological phenomena (cephalic phase response, expectation of reward), which prepare the organism for the ingestion of food play a role in the regulation of energy intake and contribute to the development of obesity remains largely unresolved. We used positron emission tomography (PET) and 15O-water to measure changes in regional cerebral blood flow (rCBF) and to assess the brain's response to the oral administration of 2 ml of a liquid meal (Ensure Plus, 1.5 kcal/ml) after a 36-h fast and shortly before consuming the same meal. Twenty-one obese (BMI > 35 kg/m2, 10M/11F, age 28 +/- 6 years, body fat 40 +/- 6%) and 20 lean individuals (BMI < 25 kg/m2, 10M/10F, age 33 +/- 9 years, body fat 21 +/- 7%) were studied. Compared to lean individuals, obese individuals had higher fasting plasma glucose (83.3 +/- 6.2 vs. 75.5 +/- 9.6 mg/dl; P = 0.0003) and insulin concentrations (6.1 +/- 3.5 vs. 2.5 +/- 1.7 microU/ml; P < 0.0001) and were characterized by a higher score of dietary disinhibition (i.e., the susceptibility of eating behavior to emotional factors and sensory cues, 5.7 +/- 3.6 vs. 3.5 +/- 2.7; P = 0.01) assessed by the Three Factor Eating Questionnaire. In response to the sensory experience of food, differences in rCBF were observed in several regions of the brain, including greater increases in the middle-dorsal insula and midbrain, and greater decreases in the posterior cingulate, temporal, and orbitofrontal cortices in obese compared to lean individuals (P < 0.05, after small volume correction). In a multiple regression model, percentage of body fat (P = 0.04), glycemia (P = 0.01), and disinhibition (P = 0.07) were independent correlates of the neural response to the sensory experience of the meal in the middle-dorsal insular cortex (R2 = 0.45). We conclude that obesity is associated with an abnormal brain response to the sensory aspects of a liquid meal after a prolonged fast especially in areas of the primary gustatory cortex. This is only partially explained by the elevated glycemia and high level of disinhibition which characterize individuals with increased adiposity. These results provide a new perspective on the understanding of the neuroanatomical correlates of abnormal eating behavior and their relationship with obesity in humans.

Anticipatory cues differentially provoke in vivo peptidergic and monoaminergic release at the medial prefrontal cortex

Like primary reinforcers, the anticipation of reward ought to affect neurochemical release in brain regions, such as the medial prefrontal cortex (mPFC), which are associated with appraisal processes. To assess the neurochemical changes associated with anticipation, rats were exposed to the pairing of auditory (60-dB white noise), visual, and olfactory cues with the daily presentation of a palatable snack (Cue Relevant group). Rats of a second group were similarly trained, but for a 2-week period, the snack was no longer provided following cue presentation (Extinction group). In the third condition, the presentation of the snack and cues was uncorrelated (Cue Irrelevant group). Analyses of dialysates collected in vivo from the mPFC revealed that release of corticotropin-releasing hormone (CRH), gastrin-releasing peptide (GRP), and the 5-HT catabolite, 5-hydroxyindole acetic acid (5-HIAA), had increased bilaterally in response to the anticipatory cues, whereas DA release increased only within the right mPFC. In the case of CRH and GRP, these increases were also apparent in the extinction condition, despite the fact that behavioral arousal to the anticipatory cues (increased exploration, rearing, grooming, and vigilance) was only evident in the Cue Relevant condition. In contrast, the elevated DA and 5-HIAA were apparent exclusively in the Cue Relevant condition. Thus, CRH and GRP systems may serve to allocate salience and/or incentive reward value to biologically significant stimuli or reflect the emotional response to the anticipatory stimulus. The activity of DA and 5-HT neurons, in contrast, is more closely aligned with the cognitive appraisal of predictor stimuli.

Persistence of abnormal neural responses to a meal in postobese individuals

OBJECTIVE

To determine whether abnormal obese-like neural responses to a meal persist in postobese individuals, who achieved and maintained a normal body weight despite a past history of severe obesity.

DESIGN AND SUBJECTS

Cross-sectional study of the brain's response to tasting and consuming a satiating meal in 11 postobese (age: 40+/-6 y, body mass index (BMI): 23.6+/-1.9 kg/m(2)), 23 obese (age: 29+/-6 y, BMI: 39.6+/-3.8 kg/m(2)) and 21 lean (age: 33+/-9 y, BMI: 22.8+/-2.1 kg/m(2)) subjects.

MEASUREMENTS

Regional cerebral blood flow (rCBF, a marker of neural activity) at baseline (after a 36-h fast), after tasting and after consuming a satiating liquid meal was assessed using positron emission tomography and state-dependent changes (taste-baseline; satiation-baseline), and compared across groups. Subjective ratings of hunger and fullness were measured by a visual analogue scale and body fatness by dual-energy X-ray absorptiometry.

RESULTS

In response to tasting the liquid meal, changes in rCBF were different in the obese as compared to the lean individuals (P<0.05, corrected for multiple comparisons) in the middle insula (peak voxel, x=-41, y=1, z=8; Montreal Neurological Institute coordinates) and posterior cingulate cortex (peak voxel, x=17, y=-47, z=40). The middle insular cortex exhibited a similar increase of neural activity in the obese and postobese subjects, whereas in the lean subjects the regional activity did not change. In the posterior cingulate cortex, the changes in rCBF in the postobese subjects were not different from those in the other groups. In response to a satiating amount of the same liquid meal, changes in rCBF were different in the obese as compared to the lean individuals (P<0.05, corrected for multiple comparisons) in the posterior hippocampus (peak voxel, x=21, y=-45, x=4), posterior cingulate cortex (peak voxel, x=17, y=-47, z=40), and amygdala (peak voxel, x=27, y=1, z=-24). The posterior hippocampus exhibited a similar decrease of neural activity in the obese and postobese subjects, whereas in the lean subjects the regional activity increased. In the posterior cingulate cortex and amygdala, the changes in rCBF were not different between the postobese and lean individuals. None of the changes in neural activity were correlated with the age of the individuals, the subjective ratings of hunger and fullness, or the meal induced-changes in plasma glucose, insulin, or serum free fatty acids.

CONCLUSION

Persistence of abnormal neural responses to a meal in the postobese individuals, a group at high risk for relapse, indicates that a predisposition to obesity may involve areas of the brain that control complex aspects of eating behavior including anticipation and reward, chemosensory perception, and autonomic control of digestion (insular cortex), as well as enteroception and learning/memory (hippocampus).

Restricted-feeding-induced anticipatory activity rhythm is associated with a phase-shift of the expression of mPer1 and mPer2 mRNA in the cerebral cortex and hippocampus but not in the suprachiasmatic nucleus of mice

Daily restricted feeding (RF) can produce food-entrainable oscillations in both intact and suprachiasmatic nucleus (SCN)-lesioned animals. Thus, there are two circadian rhythms, one of which is SCN-dependent and the other SCN-independent. Recently, it has been established that several mouse clock genes, such as mPer1, mPer2 and mPer3 are expressed in the SCN and other brain tissues. Although the role of mPer genes expressed in the SCN has recently been evaluated in the SCN-dependent rhythm, their function in the SCN-independent rhythm is still poorly understood. In order to understand the role of these genes in SCN-independent rhythm, we examined the expression pattern of mPer1 and mPer2 mRNA in each brain area of mice under RF. Mice were allowed access to food for 4 h during either the daytime under a light-dark cycle or the subjective daytime under constant dark. After 6 days of scheduled RF, the night-time or subjective night-time peak of mPer mRNA changed to a daytime peak in the cerebral cortex and hippocampus, with moderate expression in the striatum, pyriform cortex and paraventricular nucleus, and no expression in the SCN. The daytime peak in the cerebral cortex returned to a night-time peak after the release of RF to a free-feeding schedule. Although the basal rhythm of mPer expression disappeared in SCN-lesioned mice, RF produced mPer mRNA rhythm in the cerebral cortex of these mice. The present results provide evidence of an association between food-entrainable oscillations and the expression of mPer1 and mPer2 in the cerebral cortex and hippocampus.

Restricted feeding entrains liver clock without participation of the suprachiasmatic nucleus

BACKGROUND

There are two main stimuli that entrain the circadian rhythm, the light-dark cycle (LD) and restricted feeding (RF). Light-induced entrainment requires induction of the Per1 and Per2 genes in the suprachiasmatic nucleus (SCN), the locus of a main oscillator. In this experiment, we determined whether RF resets the expression of circadian clock genes in the mouse liver with or without participation of the SCN.

RESULTS

Mice were allowed access to food for 4 h during the daytime (7 h advance of feeding time) under LD or constant darkness (DD). The peaks of mPer1, mPer2, D-site-binding protein (Dbp) and cholesterol 7alpha-hydroxylase (Cyp7A) mRNA in the liver were advanced 6-12 h after 6 days of RF, whereas those in SCN were unaffected. The advance of mPer expression in the liver by RF was still observed in SCN-lesioned mice. A 7 h advance in the LD cycle advanced the peaks of clock gene expression in both the liver and SCN, whereas, a shift in the LD did not move the phase of the liver clock when the shift was carried out under a fixed RF schedule during the night-time.

CONCLUSIONS

These results suggest that restricted feeding strongly entrained the expression of circadian clock genes in the liver without the participation of an SCN clock function.

Diversity and development of circadian rhythms in the European rabbit

Three main concerns underlie this review: 1) The need to draw together the widely dispersed information available on the circadian biology of the rabbit. Although the rabbit is a classic laboratory mammal, this extensive body of information is often overlooked by chronobiologists, and despite several advantages of this species. In terms of its general biology the rabbit is the best studied laboratory mammal in the wild, it demonstrates a wide variety of robust circadian functions, and being a lagomorph, it provides a useful comparison with more commonly studied rodent species. 2) The need to more fully exploit a developmental approach to understanding circadian function, and the particular suitability of the rabbit for this. Female rabbits only visit their altricial young for a few minutes once every 24 h to nurse, and survival of the young depends on the tight circadian-controlled synchronization in behavior and physiology of the two parties. Patterns of circadian rhythmicity in neonatal pups associated with nursing do not form a smooth continuum into weaning and adult life, and may reflect the action of separate mechanisms operating in their own right. 3) Using information from the first two points, to emphasize the diversity and complexity of circadian rhythms underlying behavioral and physiological functions in adult and developing mammals. Information accruing on circadian functions in the rabbit makes it increasingly difficult to account for these in terms of one or two regulatory mechanisms or "oscillators." Thus, it is argued that in addition to the reductionist, molecular approaches currently dominating much of chronobiology, the study of circadian systems as emergent characteristics of whole organisms operating in complex environments merits special attention.

Neonatal monosodium glutamate alters circadian organization of feeding, food anticipatory activity and photic masking in the rat

In rodents, parenteral administration of monosodium glutamate (MSG) induces marked degeneration of the retina and arcuate nucleus (AN) and disrupts daily rhythms of food intake. We quantified the effects of neonatal MSG (2 mg/g SC, postnatal days 1, 3, 5, 7, 9) on the expression of feeding and activity rhythms in adult rats under schedules of light-dark (LD), constant dark (DD), restricted daily feeding and total food deprivation. AN lesions were confirmed by neuropeptide Y (NPY) immunocytochemistry and Nissl stain. Compared to age-matched control rats, the amplitude (quantified as LD ratios) of daily food intake and food-bin activity rhythms was significantly attenuated in MSG rats in LD 12:12 and on the first day of DD. Control rats, but not MSG rats, showed lower amplitude rhythms in DD compared to LD. The phase angle of feeding and activity rhythms did not differ between groups in either condition. In a short LD cycle (2:2), control rats, but not MSG rats, showed significant inhibition (masking) of activity during the 2 h light periods. When food access was restricted to a 4 h daily meal, MSG rats showed enhanced expression and persistence of food-entrained anticipatory activity rhythms by comparison with control rats. These results indicate that attenuation of daily feeding rhythms in MSG rats is due in part to loss of direct inhibitory effects of light on behavior, and that the AN likely modulates, but does not mediate entrainment of feeding-related rhythms to daily cycles of LD or food access.