Hypothalamic hyperphagia despite imposed diurnal or nocturnal feeding and drinking rhythms

The metabolic basis of dual periodicity of feeding in rats

This article examines how the depletion and replenishment of various energy stores give rise to periodic eating and how constant body-energy levels are maintained over time.

Measures of the energy expended throughout the 24-hour feeding pattern in rats indicate that two different energy stores (one of small capacity and one of large) determine two superimposed feeding periodicities: one from meal to meal (prandial), the other from day to night (nycthemeral). The article reviews how experimental overrepletion or overdepletion of gastrointestinal content, blood glucose, or body fats affect food intake. These data suggest that gastrointestinal content determines both meal size and meal-to-meal periodicity. Other evidence indicates that glucose uptake rate in tissues, which is modulated by fat synthesis and fat mobilization, affects the periodic onset of feeding and the difference between nocturnal and diurnal postprandial satiety.

There follows an examination of the neuroendocrine bases for the interacting mechanisms governing energy input and output balance and of the role of the ventromedial hypothalamus in body-fat regulation and the lateral hypothalamus in feeding.

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.

Eating rates in normal and hypothalamic hyperphagic rats

Two experiments were conducted to determine if hypothalamic hyperphagic rats eat at a faster than normal rate and if slowing the rate of eating would reduce their overeating. Experiment 1 used a 45 mg pellet eatometer to measure 24-h meal patterns and eating rates in control rats and rats made hyperphagic with medial hypothalamic (MH) knife cuts. The MH hyperphagic rats consumed larger and more frequent meals but ate at a slower rate than did control animals during both the dynamic and static phases of the syndrome. In Experiment 2, rats were given 30-min test meals with the same pellets presented in food cups (rather than one pellet at a time as with the eatometer). The MH rats consumed the pellets at the same rate as the controls, although their meals were twice as large. Experiment 1 also determined if slowing the rate of eating would reduce MH hyperphagia; eating rates were constrained by increasing the delay between successive pellet deliveries (normally < 1 s). An interpellet delay of 20 s reduced meal size and increased meal length and frequency but did not reliably reduce 24-h food intake in normal rats and dynamic or static phase MH rats. Interpellet delays of 40 or 60 s reduced daily food intake of static phase MH rats but only to control levels. These results indicate that MH hyperphagia is not associated with elevated feeding rates and is relatively unaffected by constraints on the rat's rate of eating.

Circadian regulation of feeding in rats: suprachiasmatic versus ventromedial hypothalamic nuclei

The role of the suprachiasmatic nuclei as a major component of a specific circadian system controlling feeding periodicity is reviewed. Evidence is presented supporting the assumption that the ventromedial hypothalamus and the suprachiasmatic nucleus may act as a constant (tonic) regulator and a circadian modulator respectively of feeding in rats. It is concluded that a specific circadian system differing from the metabolic control mechanism superimposes the circadian periodicity of feeding. A model is put forward for the possible functional relationships between circadian and metabolic (homeostatic) control mechanisms of feeding in rats.

Adipose tissue mobilization is unaffected by obesifying hypothalamic knife cuts

Unilateral electrolytic lesions of the medial hypothalamus have previously been reported to spare starvation-induced lipid mobilization from the ipsilateral retroperitoneal fat pad. This suggests that deficient lipid mobilization contributes to the enhanced lipid deposition which results when such lesions are bilateral. In contrast, unilateral parasagittal hypothalamic knife cuts, which also obesify when bilateral, failed to prevent starvation-induced ipsilateral lipid mobilization. This finding indicates that impairment of neurally mediated lipid mobilization is probably neither a necessary nor a sufficient feature of hypothalamic obesity.

Persistence of rat nocturnal feeding and drinking during diurnal presentation of palatable diet

Male laboratory rats were fed a highly palatable diet during the light phase of a 12:12 LD cycle. Nocturnality of feeding was reduced from approximately 77% of the total daily intake to 56%. Increase in diurnal feeding was attributable to the ingestion of a large meal at the beginning of the L phase. Reduction of nocturnal feeding was attributable to decreased ingestion in the first 3 hr of the D phase. Outside these times the daily feeding pattern was unmodified. These findings do not represent a reversal of the feeding rhythm as previously reported, but a modification which is not consistent with interpretation as a phase-shift. Throughout the testing period total daily food intake remained unchanged and body weight was defended despite the increase in calories ingested. Daily drinking patterns and total daily water intake were not significantly altered by the experimental manipulation.

A chronometric approach to the study of feeding behavior

In many mammalian genera, the stimulus to feed is intimately associated with circadian rhythms. This stimulus arises from within the brain from biological time-keeping systems. Such a chronometric approach to feeding behavior follows from a consideration of the terrestrial mammal's space-time pattern within the ecological niche. The ecological niche is a division of time as well as space. The restriction of certain behaviors to certain times of day and the concomitant evolution of nocturnality or diurnality represent strong advantages for survival in the wild. Experimental data, primarily from studies on the rat, in support of the chronometric approach, include: the reinstatement of cyclic feeding patterns after food deprivation; the continuation of circadian pattern of wheel running and nocturnal drinking during food deprivation; consideration of the ontogeny of the feeding pattern; the phenomenon of anticipatory appetite--the experimental demonstration that time of day can act as a conditioned stimulus for feeding; the evaluation of rhythms in digestion, absorption and assimilatory biochemical processes; the realization that many of these rhythms are not simply a consequence of the presence of food in the gut; the realization that the brain exerts considerable control over the peripheral rhythmic nutritional processes via ANS and endocrinological systems; and the fact that within the brain the SCN and structures well known to be involved in nutritional regulation, such as the VMH, LHA and monoamine systems, may all be involved in the circadian pattern of feeding. Further, the function of these neurological structures may be understood better by consideration of data from temporal changes in feeding patterns.

Hyperphagia and obesity following serotonin depletion by intraventricular p-chlorophenylalanine

Loss of brain serotonin was associated with overeating and increased body weight. Rats injected with p-chlorophenylalanine intraventricularly began overeating after 3 days and continued to display marked hyperphagia, primarily in the daytime, accompanied by increased body weight for 1 to 2 weeks. The effect was related to drug dose and to the degree and duration of serotonin depletion. Norepinephrine and dopamine levels were not significantly affected. It is concluded that p-chlorophenylalanine disinhibits feeding, as it does a number of other behaviors, by depleting serotonin. This suggests that hypothalamic lesions or dietary deficiencies which selectively and sufficiently deplete serotonin would lead to overeating.