Feeding, blood glucose and plasma insulin of mice at dusk

Fasting of mice: a review

Fasting of mice is a common procedure performed in association with many different types of experiments mainly in order to reduce variability in investigatory parameters or to facilitate surgical procedures. However, the effects of fasting not directly related to the investigatory parameters are often ignored. The aim of this review is to present and summarize knowledge about the effects of fasting of mice to facilitate optimization of the fasting procedure for any given study and thereby maximize the scientific outcome and minimize the discomfort for the mice and hence ensure high animal welfare. The results are presented from a number of experimental studies, providing evidence for fasting-induced changes in hormone balance, body weight, metabolism, hepatic enzymes, cardiovascular parameters, body temperature and toxicological responses. A description of relevant normal behaviour and standard physiological parameters is given, concluding that mice are primarily nocturnal and consume two-thirds of their total food intake during the night. It is argued that overnight fasting of mice is not comparable with overnight fasting of humans because the mouse has a nocturnal circadian rhythm and a higher metabolic rate. It is suggested that because many physiological parameters are regulated by circadian rhythms, fasting initiated at different points in the circadian rhythm has different impacts and produces different results.

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.

Intrameal hepatic-portal infusion of glucose reduces spontaneous meal size in rats

To test whether glucose (GLC) or insulin (INS) acutely reduces spontaneous meal size, we tested the effects of remotely controlled, intrameal hepatic-portal vein infusions of GLC or INS on rats' spontaneous feeding patterns. Experiment 1 included four blocks of three test infusions and one control infusion. The test infusions in each block were 0.25, 0.5, 1.0 or 1.5 mmol GLC; 2, 4, 6, or 8 mU INS; or the four combinations with dose ratios of 1 mmol GLC/8 mU INS, respectively. Control infusions and the INS vehicle were saline infusions that were equiosmotic to the GLC infusion used in that block. Infusions (0.1 ml x 5 min) were done during the first spontaneous dark-phase meal. None of the test infusions affected meal size, meal duration or the duration of the subsequent intermeal interval. In Experiment 2, a similar design was used to test infusions of 1 mmol GLC, 2 mU INS and GLC/INS. Both GLC alone and GLC/INS reduced the size and duration of the first spontaneous dark-phase meal. The subsequent intermeal interval was unaffected, but GLC alone also increased the satiety ratio (min/g) of the meal. The size and duration of the second dark-phase meal were unaffected. INS alone did not affect any meal parameters. In Experiment 3, infusions of 1 mmol GLC and 2 mU INS were repeated during each of the first three meals of the dark phase. These infusions reduced the size and duration of each meal, as well as 6-h cumulative food intake, but did not affect any other meal parameter. These experiments demonstrate for the first time that intrameal hepatic-portal infusions of GLC or of GLC and INS is sufficient to acutely and selectively reduce spontaneous meal size in the rat. The findings are consistent with the idea that meal-contingent changes in hepatic-portal GLC concentration contribute to satiation.

Circadian feeding and drinking patterns of genetically obese mice fed solid chow diet

Feeding and drinking were recorded in male ob/ob mice and lean mice fed pelleted Purina Lab Chow No. 5001 with water to drink. The circadian patterns of eating and drinking of obese mice differed from those of lean mice, in both the proportional temporal distributions of feeding and of drinking behavior across the 24-hour day and in the absolute amounts consumed hourly. The pattern of increased food consumption by the obese mice was different than that underlying increased water consumption. When meal parameters were analyzed in terms of 'complete meals' of both feeding and drinking (the end of a meal defined as at least 12 consecutive minutes with no ingestion), obese and lean mice had the same number of meals and their periodicity was similar, but meal size was much greater in the obese mice. In the dark, both obese and lean mice showed strong postprandial correlations of meal size with time from the start of a meal to start of the next meal.

Food deprivation dissociates pancreatic glucagon's effects on satiety and hepatic glucose production at dark onset

We compared the effects of different durations of pretest food deprivation on pancreatic glucagon's (PG) satiating and glycogenolytic actions in order to test the hypothesis that stimulation of hepatic glucose production causes PG's satiety effect. Rats were maintained on a 12:12 LD cycle (lights off: 1015) and deprived of food 45 min or 8, 12, 18, or 24 hr before intraperitoneal injection of 400 micrograms/kg PG. Testing began at 1015, the beginning of the dark phase. Food intake was not inhibited after 45 min of pretest food deprivation (30 min change, 2.5 +/- 4.0%, p greater than 0.05), but was inhibited after 8 or more hr food deprivation. The largest inhibitory effect, 16.2 +/- 3.8%, p less than 0.01, occurred after 8 hr food deprivation. In separate experiments, rats were food deprived 45 min or 8 hr, similarly injected, and killed 10 min after refeeding for blood and liver samples. Hepatic glycogen content at meal onset was higher in rats deprived 45 min than in rats deprived 8 hr (3.2 +/- 0.3 vs. 1.7 +/- 0.3% liver weight, p less than 0.01), and PG injection produced a higher level of hepatic vein blood glucose in the less deprived rats (196 +/- 5 vs. 168 +/- 12 mg/dl, p less than 0.05). Thus, in rats tested at the beginning of the dark phase of the LD cycle after 45 min or 8 hr food deprivation, there is an inverse relation between PG's potencies to inhibit food intake and to stimulate hepatic glucose production.(ABSTRACT TRUNCATED AT 250 WORDS)

Is regulation of body weight elucidated

A new perspective on mechanisms involved in the regulation of a constant fat body mass and its relation to body energy balance is presented on the basis of a series of experiments. A study of the neuroendocrine conditions underlying the daily weight gain-weight loss cycle in rat and man and experimentally induced over and underweight, leads to the notion that lipogenesis and lipolysis above and below a range of physiological fluctuations of body fat develop a counter-regulatory tendency to correcting lipolysis and lipogenesis respectively. This development is attributed to a chronic central action of plasma insulin concentration on hypothalamic insulin receptors. This liporegulatory system which controls and regulates the filling and emptying of fat stores modulates the feeding system which controls and regulates the filling of a gastrointestinal store by eating and its emptying by metabolic food utilization.

Short-chain polyhydroxymonocarboxylic acids as physiological signals for food intake

In order to clarify the effects of endogenous organic acids on short and long-term feeding behavior, ingestive behavior was monitored for 2 hr before and after intra-third ventricular infusions of 3,4-dihydroxybutyric acid (2-deoxytetronic acid, 2-DTA), 2,4,5-trihydroxypentanoic acid (3-deoxypentonic acid, 3-DPA), and 3-hydroxybutyric acid (3-HBA). In addition, meal patterns were recorded for 2 days before and after the ventricular infusions. 2-DTA suppressed both short and long-term feeding by decreasing meal size (MS). 3-DPA elicited transient feeding behavior, but caused no change in long-term feeding. 3-HBA initially stimulated feeding, but subsequently suppressed long-term feeding by decreasing MS and prolonging postprandial intermeal interval (IMI). The suppressive effects of 3-HBA on feeding behavior lasted about 24 hr longer than those of 2-DTA. Based upon these observations as well as our previous reports, it appears that some of the processes affecting hunger and satiation are mediated by changes in central and peripheral concentrations of these organic acids.

Metabolic and feeding patterns: role of sympathetic and parasympathetic efferent pathways

The continuous transfer of fuel from the blood to tissues, and its adjustment to the rate of energy metabolism involve close relationships between the control of endogenous stores and that of the periodic intake of food. Neural and blood-borne signals to the brain and efferent sympathetic and parasympathetic pathways seem responsible for three different metabolic and feeding cycles. The matching of input and output of glucose to and from the blood, i.e. the regulation of the blood glucose level proper, is achieved by a short-term feedback mechanism which involves the action of insulin and glucagon on hepatic glucose production and peripheral glucose utilization. Oscillations in level of blood glucose, plasma insulin and glucagon in 8-11 min cycles, as observed in certain species, presumably reflect the play of this short-term regulation of the blood glucose level. The respective role of a pancreatic feedback and of a neural loop in this short-term regulation is discussed. Signals to brain targets and efferent pathways determine the periodic onset of meals after the energy ingested in the preceding meal has been exhausted in free-fed and briefly deprived rats. New evidence has been provided that the fall in blood glucose level exerts a decisive and direct action on the brain to determine meal onset and/or meal size. Superimposed on this prandial periodicity, a lipostatic mechanism modulates the feeding pattern in a dark-light periodicity by the alternation of fat synthesis and fat mobilization. The effects of VMH lesions and vagotomy, either combined or separately, on daily metabolic and feeding patterns suggest that the neural input to the pancreas and adipose tissues plays a primary role in this endogenous cycle and is also involved in the regulation of a constant fat body mass.

Meal patterns and glucoprivic feeding in the guanethidine-sympathectomized, adrenodemedullated rat

The feeding behavior of rats sympathectomized by neonatal administration of guanethidine (GUA) and/or adult adrenal demedullation (MDL) was investigated. GUA treatment tended to decrease body weight gain and food intake, chiefly by decreasing meal size and increasing satiety ratios. It also attenuated the increase in food intake caused by 2-deoxy-D-glucose (2DG; 150, 300, 450 mg/kg, IP) but not by insulin (3, 6, 9 U/kg, IP). MDL altered meal patterns in the same manner as GUA treatment but the effects were of smaller magnitude. It did not influence the response to either glucoprivic challenge. Combined GUA treatment and MDL generally produced additive effects. These results suggest that the major sympathetic influence on feeding is through adrenergic innervation and not circulating catecholamines. The hypothesis that the alteration in feeding patterns produced by ventromedial hypothalamic lesions is due to decreased sympathetic activity was not supported.

Diurnal variations in circulating hormone levels and brown adipose tissue activity in "cafeteria"-fed rats

Circulating hormone levels and interscapular brown adipose tissue (BAT) mitochondrial GDP-binding capacity were measured at 4-hr intervals over 24 hr in rats fed a stock diet or a palatable "cafeteria" diet. Plasma thyroxine, insulin and glucagon levels were unaffected by diet, but plasma triiodothyronine, BAT mass, mitochondrial protein content and GDP-binding were higher in cafeteria-fed rats. Blood glucose levels and BAT mitochondrial GDP-binding were higher at night in the cafeteria rats. The diurnal variations in these parameters correlated with changes in diet-induced thermogenesis previously observed in cafeteria-fed rats.

Food deprivation induced parallel changes in blood glucose, plasma free fatty acids and feeding during two parts of the diurnal cycle in rats

The changes in plasma glucose and free fatty acid levels and in subsequent feeding induced by 4 to 10 hr of food deprivation were investigated in rats and compared for the two parts of the diurnal cycle. It was found that increasing fast duration at night induced a more rapid fall of plasma glucose and elevation of plasma free fatty acids than in the day. However, a similar increment of the first post fast meal was elicited by an identical decrement of blood glucose level for the two periods except after a 10 hr fast during the day. The acute effect of darkness and light per se being experimentally excluded, it was concluded that the size of the first meal following short term food deprivation was dependent throughout the diurnal cycle on the fast induced glucoprivic condition.