Effects of food restriction and adrenalectomy in rats with VMH or PVH lesions

The effects of adrenalectomy in rats with ventromedial or paraventricular hypothalamic lesions have been studied in two experiments. Rats with ventromedial hypothalamic lesions or lesions in the paraventricular nucleus were allowed to gain weight for fourteen days at which time they were adrenalectomized. Before adrenalectomy, animals with VMH lesions ate more, gained significantly more weight than animals with lesions in the paraventricular nucleus, and both were significantly heavier and consumed more food than sham-operated controls. Following adrenalectomy, food intake decreased and both groups of lesioned animals lost weight. The animals with VMH lesions stabilized at weights above the control animals. Implantation of corticosterone enhanced weight gain and food intake in animals with lesions in either the paraventricular nucleus or the ventromedial hypothalamus. In the second experiment, one subgroup of rats with VMH lesions was adrenalectomized, and allowed to eat ad lib. Two other groups of sham-operated rats with VMH lesions served as controls. One group ate ad lib and one group was pair fed to the food intake of the adrenalectomized VMH-lesioned rats. Weight gain in the adrenalectomized VMH-lesioned rats and the pair-fed VMH-lesioned controls was similar and less than the VMH-lesioned rats eating ad lib. GDP binding to interscapular brown adipose tissue was related to the degree of weight gain, not to the presence of the VMH lesion. These data show that corticosterone is essential for the expression of obesity in both PVH- and VMH-lesioned rats. They also argue that the reduction in the activity of the sympathetic nervous system of VMH-lesioned rats as estimated by the GDP binding to mitochondria from brown adipose tissue is associated with hyperphagia.     

Hyperinsulinemia in rats with ventromedial hypothalamic lesions: role of hyperphagia

The relation of hyperinsulinemia to hyperphagia was examined in rats with lesions of the ventromedial hypothalamus (VMH). Plasma insulin and glucose levels were assayed after a 4-hr fast and 17 min after the initiation of a meal (6 ml of sweetened milk in 7 min) in animals with sham lesions, VMH animals maintained at preoperative body weight by food restriction, and VMH animals fed ad lib. Both VMH groups displayed basal and postabsorptive hyperinsulinemia, compared with the sham-operated control group, but insulin levels were greatest under the ad lib feeding condition. It is suggested that VMH hyperinsulinemia is due both to a primary effect of the lesion and to hyperphagia and that marked obesity can result in the absence of basal hyperinsulinemia as a result of hyperphagia with consequent postabsorptive hyperinsulinemia.     

The effects of subdiaphragmatic vagotomy on circadian corticosterone rhythmicity in rats with continuous or restricted food access

Plasma corticosteroid circadian periodicity was determined in three groups of individual adult male rats. Categories were: postsubdiaphragmatic vagotomy, sham-operated, and controls. Blood was sampled every 4 hours over a 48-hour period with ad lib feeding and after a 13-day period on daytime (0930–1330) restriction of food and water availability. Plasma corticosterone circadian periodicity was normal under ad lib conditions in control, sham-operated, and vagotomized animals. Under food-restricted conditions, both the sham-operated and vagotomized animals exhibited the 12-hour shift in the circadian peak of plasma corticosterone levels that we have previously described in normal animals under such conditions. There were no differences between groups in the total amount of food and water consumed or the percentage of nocturnal food intake on an ad lib feeding schedule. Both vagotomized and sham-operated animals manifested reductions in food intake under conditions of food restriction, although vagotomized consumed less than sham-operated animals. The present data indicate that an intact vagus nerve is not necessary for the establishment of circadian periodicity of plasma corticosterone levels or the shift in the periodicity of corticosteroid secretion produced by a restricted feeding regimen.     

Effects of food restriction on glucose tolerance, insulin secretion, and islet-cell proliferation in pregnant rats

Pregnancy is associated with increased glucose-stimulated insulin secretion and increased pancreatic islet-cell proliferation. In the present study it was investigated whether increased food intake, as occurs during pregnancy, is involved in the regulation of these phenomena. From Day 0 of pregnancy, rats received each day the mean amount of food they consumed daily during the estrous cycle prior to conception. This food restriction regime resulted in lower maternal body weight, and in lower fetal weight on Day 20 of gestation, but did not affect fetal survival. Food-restricted rats showed decreased insulin responses to an i.v. glucose challenge on Day 13, and lower islet-cell replication rates on Day 14 of pregnancy than pregnant rats fed ad lib. Plasma lactogenic activity in food-restricted animals was increased on Days 11 and 13; plasma progesterone levels were unchanged, but plasma leptin concentrations declined progressively during food restriction. Glucose tolerance was normal, suggesting that food restriction improved insulin action. On Day 20 of pregnancy, insulin responses were similar in food restricted and ad lib-fed rats; glucose tolerance was still unchanged. It thus seems that the improved insulin action as present on Day 13 had disappeared on Day 20. Also on Day 20, lactogenic activity as well as progesterone concentrations were similar in food-restricted and ad lib-fed rats. It was concluded that increased food intake plays an important role in the stimulation of islet-cell proliferation and insulin secretion, as well as in the diminished insulin action during the second week of rat pregnancy.     

Effect of insulin in rats with lesions of the dorsomedial hypothalamic nucleus.

Four hours after insulin injection Dorsomedial Hypothalamic Nuclei (DMN) lesioned rats consumed an amount of food that was comparable to that eaten by injected sham-operated animals. However, the DMN lesioned rats are not as initially responsive to the food intake stimulating properties of insulin as are the controls. A second study showed ad lib fed and fasted lesioned animals displayed a lower plasma glucose concentration after insulin challenge than did respectively treated controls. This suggests the initial insulin-induced feeding of the lesioned rats was blunted when compared to the controls even in the face of lower plasma glucose levels. Although a previous investigation revealed that DMN lesions destroy glucoreceptor tissue, the present data shows that DMN lesioned rats will increase their food intake in the face of insulin challenge, albeit their initial feeding response to insulin challenge is somewhat blunted. Finally, the present study confirms a previous report in that DMN lesioned rats can competently meter their 24 hour calorie intake.     

Lipid peroxidation and antioxidant status in rat: effect of food restriction and wheel running

Using the activity-based anorexia model, the aim of this investigation was to explore antioxidant enzyme activity (catalase, superoxide dismutase), total antioxidant status (TAS), and alpha-tocopherol in blood, liver, and gastrocnemius muscle associated with the food restriction and voluntary wheel running during 8 days. In addition, lipid peroxidation was measured by measurements of malondialdehyde (MDA). Wistars rats (n = 56) were randomly assigned to one of four groups: an ad lib sedentary group, a control wheel activity group, a food restriction-induced hyperactivity group (1 h/day ad lib food, 23 h/day ad lib wheel access), and a food-restricted sedentary group. The animals were killed when the rats in the food-restricted group had lost 25% of their free feeding weight. Antioxidant enzyme activities and TAS in blood, liver, and gastrocnemius muscle were unaffected by voluntary wheel running. A wheel activity effect (P < 0.05) was obtained for the MDA concentrations in plasma, with lower concentrations in trained animals. Food restriction effects were obtained for antioxidant capacity in liver, as well as for CAT activity in the gastrocnemius muscle and plasma MDA concentrations with lower values in the restricted animals. On the other hand, the food-restricted rats showed higher plasma TAS concentrations (P < 0.05) and higher alpha-tocopherol concentrations in the liver (P < 0.05) when compared to animals fed ad libitum. Our results also showed that food restriction coupled to wheel running decreased antioxidant parameters in liver, and plasmatic MDA concentrations and increased TAS plasma concentrations when compared to the ad libitum sedentary situation.     

Hypothalamic obesity: comparison of radio-frequency and electrolytic lesions in weanling rats

Female rats were subjected to radio-frequency or anodal electrolytic lesions of the ventromedial hypothalamus (VMH) when 28 days old. Blood samples for determination of basal plasma insulin and glucose levels were taken on postoperative day 30 (Experiment 1) and on day 10 (Experiment 2). Body weight and daily food intake of rats with either type of lesion did not differ from unoperated animals during the first 10 days, but rats with electrolytic lesions, unlike radio-frequency lesioned animals, displayed excess food intake and weight gain starting in the third postoperative week. Both types of lesions produced stunted linear growth and a higher than normal Lee Obesity Index. Only the rats with electrolytic VMH lesions were significantly hyperinsulinemic on postoperative day 30, with a mean plasma insulin level that was at least double that observed in unoperated or radio-frequency lesioned animals. On day 10, however, the animals with electrolytic lesions had markedly lower plasma insulin and glucose levels compared to the other two groups, which did not differ from one another. There was no apparent difference in the size of the lesions produced by the two techniques, and it is therefore concluded that some of the endocrine dysfunctions resulting from electrolytic VMH lesions are due to metallic ion deposits (stimulating adjacent tissue) rather than to tissue ablation.     

Food-restriction interacts with vasopressin modulation of activity

The effects of peripheral injection of various doses of lysine-vasopressin (LVP), administered 30 min before a 5-min session in a hole-board apparatus, were compared as a function of food restriction. Comparison of performance for various indices of general activity clearly showed that the food-restricted rats were more active and exhibited less photophobia than normally fed ones. The differences between the two groups were maintained in a second session 24 hours later. There was no sign of behavioral habituation to the apparatus among restricted animals. Different doses (0.2, 1, 2 micrograms of LVP) affected food-restricted animals differently from the rats fed ad lib. Only the highest dose reduced activity in both groups. A posttest injection of the smallest dose (0.2 micrograms) had an opposite effect on the activity in the hole-board, measured 24 hours after the injection. A second experiment showed that plasma and adrenal corticosterone were higher in deprived rats. The administration of 0.2 micrograms of LVP was followed by an increase in corticosterone. In food-restricted rats this increase was bigger and was still observed 24 hours after the injection. There is an interaction between feeding conditions and LVP injections which affects both the internal hormonal state and spontaneous reactivity to environment. These findings are of relevance to the effect of vasopressin on behavior.     

Effects on growth and endocrine status of maintaining obese and lean Zucker rats at 22 degrees C and 30 degrees C from weaning

Lean and genetically obese Zucker rats were maintained from 21 to 34 days of age at an ambient temperature of 22 degrees C or 30 degrees C. Maintenance at 30 degrees C normalised the lower rectal temperature of obese rats. At 30 degrees C obese rats reduced their food intake compared with obese animals at 22 degrees C but they were still hyperphagic compared with non-obese animals at 30 degrees C. At 30 degrees C obese rats accumulated as much lipid in their carcase, but deposited less protein than obese animals at 22 degrees C. Obese rats were hyperinsulinemic compared with non-obese animals at either temperature. Plasma T3 and T4 levels did not differ between phenotypes and T4 levels were lower in both phenotypes at 30 degrees C. Plasma corticosterone levels were higher in obese rats at both temperatures and this phenotypic difference was larger at 30 degrees C. In a second experiment rats were maintained at 22 degrees C from 21-34 days of age and obese rats pair-fed to the ad lib intake of non-obese animals. Obese rats still became more obese than non-obese animals and deposited less protein than either non-obese or ad lib fed obese animals. Pair-fed obese rats were hyperinsulinemic and had higher plasma corticosterone concentrations than non-obese animals. These findings suggest that the raised plasma corticosterone concentrations of obese rats kept at 30 degrees C, or not allowed to express their greater appetite, may cause reduced protein deposition.     

Effects of forced exercise on the food intake of rats maintained on a 23-hr deprivation schedule

Adult male rats of the Sprague-Dawley strain were given daily forced exercise of progressively higher levels on motor-driven treadmills while maintained on a 23-hr food deprivation schedule. Food intake measures were taken for 68 days, spanning a range of 2–8 hr exercise-sessions per day (1–4 hr actual running time) at 1.46 km/hr. The food intake of both exercising and sedentary food-restricted animals was lower as compared to that of ad lib feeding animals. Exercise did not further suppress food intake. Shifts to higher exercise levels did not result in any transitory or permanent depression in food intake. It is concluded that high levels of exercise in trained male rats maintained on a 1-hr/day feeding schedule has no suppressive effects on food intake.     

Effects of oxytocin on the IGF-axis and some gastrointestinal hormones in ad libitum fed and food-restricted female rats

The aims of this study were to investigate if administration of oxytocin to ad libitum fed and food-restricted female rats affects weight gain, body fatness, the IGF-axis, and some vagally mediated gastrointestinal hormones, such as gastrin, cholecystokinin (CCK) and somatostatin. Ad libitum fed and food-restricted (receiving 70% of the food intake of the ad libitum fed group) female rats were injected subcutaneously, once a day, for 10 days, with saline (control) or oxytocin (1 mg kg^–1 bodyweight). The animals were killed 5 days after the last injection. Oxytocin-treated food-restricted females had more body fat and lower plasma levels of IGF-I, IGFBP-1 and IGFBP-3 compared with saline-treated counterparts. Oxytocin-treated ad libitum fed rats also had lower plasma levels of IGFBP-1 but contained less body fat, compared with saline-treated counterparts. There was no effect of oxytocin treatment on body weight or weight gain in either of the feeding groups. Except for gastrin, which was lower, there was no effect of oxytocin on the gastrointestinal hormones studied. The results indicate that oxytocin treatment influences fat deposition and the IGF-axis in female rats, but that the results are dependent on the nutritional status of the animal.     

Maternal malnutrition in the rat: Effects on food intake and body weight

The effects of dietary protein level on food intake and body weight were examined in adult female rats during a 35-day pre-mating period and during gestation and lactation. During the pre-mating period, no differences in daily food intake were observed among rats fed a 6% casein, 8% casein or 25% casein diet. However, during this period, rats fed the 6% casein diet gained significantly less weight than those with ad lib access to the 8% or 25% casein diets or than rats pair-fed the 25% casein diet in amounts equivalent to that consumed by rats in the 6% or 8% casein groups. Additionally, rats fed the 6% casein diet displayed decreased efficiency of energy utilization, calculated as weight gain per 100 kilocalories consumed, relative to rats fed the 8% or 25% casein diets. No differences in food intake were observed among the groups during gestation. However, rats fed the 6% casein diet gained less weight than rats fed the 8% or 25% casein diets. During lactation rats fed either the 6% or 8% casein diet consumed significantly less food than animals given the 25% casein diet ad lib. During the second week of lactation, rats receiving ad lib access to the 25% casein diet gained weight while those receiving the 6% or 8% casein diets continued to lose weight. At parturition, body weights of pups did not differ as a function of dietary condition. However, by day 12 of life, pups whose dams had ad lib access to the 25% casein diet weighed significantly more than pups whose dams consumed the 6% or 8% casein diet or whose dams were pair-fed the 25% casein diet in amounts equivalent to those consumed by rats fed the 6% or 8% casein diet.     

Effects of oxytocin on the IGF-axis and some gastrointestinal hormones in ad libitum fed and food-restricted female rats.

The aims of this study were to investigate if administration of oxytocin to ad libitum fed and food-restricted female rats affects weight gain, body fatness, the IGF-axis, and some vagally mediated gastrointestinal hormones, such as gastrin, cholecystokinin (CCK) and somatostatin. Ad libitum fed and food-restricted (receiving 70% of the food intake of the ad libitum fed group) female rats were injected subcutaneously, once a day, for 10 days, with saline (control) or oxytocin (1 mg kg-1 bodyweight). The animals were killed 5 days after the last injection. Oxytocin-treated food-restricted females had more body fat and lower plasma levels of IGF-I, IGFBP-1 and IGFBP-3 compared with saline-treated counterparts. Oxytocin-treated ad libitum fed rats also had lower plasma levels of IGFBP-1 but contained less body fat, compared with saline-treated counterparts. There was no effect of oxytocin treatment on body weight or weight gain in either of the feeding groups. Except for gastrin, which was lower, there was no effect of oxytocin on the gastrointestinal hormones studied. The results indicate that oxytocin treatment influences fat deposition and the IGF-axis in female rats, but that the results are dependent on the nutritional status of the animal.     

Effects of VMH lesions on schedule induced and schedule dependent behaviors

Ten male hooded rats were exposed to an FI 1 min food reinforcement generator schedule at 80% body weight and schedule dependent lever pressing and schedule induced licking and drinking were recorded. When lever pressing, licking, and drinking stabilized the 10 rats were divided into two groups. One group was composed of 4 animals subjected to sham lesion procedures plus one animal with asymmetrical ventromedial hypothalamic (VMH) lesions. The other group was composed of 5 animals with bilateral symmetrical VMH destruction. Results demonstrate that VMH destruction produces a slight transient decrease only in water intake when on schedule at 80% body weight. When animals are returned to ad lib eating and body weight increased and they are returned to the test chamber, the VMH lesion animals display increased licking and drinking. Although VMH lesion animals ate and drank more than controls, they did not eat more in response to food deprivation and did not drink more in response to water deprivation and the intraperitoneal administration of hypertonic saline. The presence or absence of food or water was the determining factor in the overeating or excessive drinking of the VMH lesion animals. Results are discussed in terms of gastrointestinal influences on the hypothalamic mechanisms involved in the production of schedule induced behaviors.     

Comparison of metabolic alterations in hypothalamic and high fat diet-induced obesity.

The heterogeneous nature of the experimental obesities induced by ventromedial hypothalamic (VMH) lesion and high fat diet (HFD) have been demonstrated by comparing VMH-lesioned and sham-operated rats fed a HFD or low fat diet (LFD). VMH rats had increased fat mass serum insulin and serum triglycerides but lower serum glucagon and smaller salivary glands than sham-operated animals. The body weight of HFD obese rats was intermediate between VMH and sham-operated animals on the LDF. Liver and fat pad weights showed effects of lesions and diet. Diet did not affect plasma glucagon or insulin. Pair-feeding VMH rats with sham-operated rats prevented weight gain but did not prevent the increase in insulin and fall in glucagon. Studies of insulin secretion from isolated perifused islets showed that basal and both phases of stimulated secretion were significantly increased in VMH groups. The changes in plasma insulin, plasma glucagon, and salivary gland weight in VMH groups are interpreted as showing decreased activity of the sympathetic nervous system following VMH lesions.     

The nature of the metabolic signal that triggers onset of puberty in female rats

These experiments examined the effects of different refeeding stimuli on reproductive activity as measured by the onset of first estrus in prepubertal, food-restricted female rats. Such rats were placed on a restricted food intake until day 50 of age to maintain a weight of 80-90 g, and to suppress onset of puberty (normal time of puberty: 37 +/- 1.4 days of age). In Experiment 1, rats were refed at different daily caloric intakes from day 50 through day 62. First estrus was observed in all rats, with highest caloric intake after 5.7 +/- 0.6 days of refeeding. Progressively fewer rats achieved first estrus, and the time to first estrus increased as the caloric intake per day decreased. These results suggest that the highest caloric intake most closely resembles an ad lib diet in such realimented rats. The second experiment determined the duration of an ad lib food stimulus needed to initiate first estrus. Similarly growth-restricted rats were refed (on Day 50 of age) ad lib meals of 67.2 +/- 0.1 kcal, presented for periods of 12, 24, 48, or 72 h. The majority of rats (6 of 7) achieved first estrus when the ad lib meals were presented for 72 h, but progressively fewer rats achieved first estrus when such meals were presented for less time. These data indicate that an extended ad lib food stimulus (72 h) is necessary to cause onset of cycling in the majority of food-restricted, prepubertal female rats.     

Comparison of radio-frequency and electrolytic lesions of the paraventricular hypothalamic nucleus

Radio-frequency and anodal electrolytic lesions of the paraventricular nucleus (PVN) were found to produce equal and dramatic increases in body weight in female rats. Neither of the groups with lesions had significantly elevated plasma insulin levels during a period of food restriction, but individual values varied greatly. Both groups displayed marked basal hyperinsulinemia after 30 days of food ad lib. It is concluded that radio-frequency and electrolytic PVN lesions produce similar obesity syndromes.     

Effects of fasting on plasma catecholamine, corticosterone and glucose concentrations under basal and stress conditions in individual rats

Plasma noradrenaline (NA), adrenaline (A), corticosterone (CS) and glucose concentrations were determined in blood sampled via a cardiac catheter from freely moving male rats under ad lib fed and 24 hr food deprived conditions using a repeated measures within-subject design. Resting plasma NA and glucose levels were lower in the fasted condition as compared to the fed situation. One day of refeeding reinstated the decreased NA and glucose values to prefasting level. Exposing 24 hr food deprived animals to the psychological stress of a novel environment resulted in larger NA and glucose increases, an unchanged A release and a lower CS rise in comparison with the responses during the free feeding state. The results show the usefulness of a within-subject design to detect subtle changes in plasma NA contents. Additionally, the data support the concept of selective control of the sympathetic nervous system and adrenal medullae. It is argued that hypothalamic noradrenergic mechanisms may be linked to the fasting-induced effects on the sympatho-adrenal system.     

Marked enhancement of noradrenaline turnover in extensive brain regions after activity-stress in rats

Male Wistar rats were exposed to a 5-day activity-stress procedure wherein animals were housed in running-wheel activity cages and fed for only 1 hr each day (wheel-housed/food-restricted rats). This activity-stress procedure produced marked elevation in levels of the major metabolite of noradrenaline (NA), 3-methoxy-4-hydroxyphenylethyleneglycol sulfate (MHPG-SO₄), in eight brain regions, while a reduction of NA level occurred in several of these brain regions. These rats also exhibited excessive running activity and developed severe gastric glandular ulcers. Rats fed ad lib and housed in activity cages (wheel-housed/ad lib-fed) and rats housed in standard individual cages and which received either 1-hr daily feeding (control cage-housed/food-restricted) or ad lib feeding (control cage-housed/ad lib-fed) showed neither significant changes in brain NA metabolism nor gastric ulcers. These results suggest that the interaction of a restricted feeding regimen and an increase running wheel activity caused marked enhancement of NA turnover in several brain regions, which is one of the neurochemical mechanisms underlying the physiological and behavioral changes produced by the activity-stress paradigm.     

2-deoxy-D-glucose inhibits food intake in rats after ventromedial hypothalamic lesions

Changes in food induced by 2 deoxy-D-glucose (2 DG) have been investigated in newly lesioned rats during the two parts of the diurnal cycle. In a first experiment rats were injected with saline or 2 DG (250, 500, 750 mg/kg) at the beginning of the light or the dark period. In a second experiment rats were injected with saline or 2 DG (250 mg/kg) in the middle of the two periods after 0, 2, 4 or 6 hr of fasting. Results show that 2 DG exerts an inhibitory effect on food intake at night, as it does in intact rats, under ad lib conditions or after a short deprivation time. The stimulating effects of 2 DG on diurnal food intake observed in intact rats is not replicated in VMH rats. On the contrary an inhibition of intake follows a short food deprivation. The data suggest that 2 DG stimulates food intake only under metabolic and feeding conditions characteristic of the diurnal phase in intact rats. Since lipogenesis and hyperphagia are observed 24 hr a day in VMH animals, only inhibition could follow 2 DG.