Origin of endocrine-metabolic changes in the weanling rat ventromedial syndrome

Central vs. Peripheral Action of Thyroid Hormone in Adaptive Thermogenesis: A Burning Topic

Thyroid hormones (TH) contribute to the control of adaptive thermogenesis, which is associated with both higher energy expenditure and lower body mass index. While it was clearly established that TH act directly in the target tissues to fulfill its metabolic activities, some studies have rather suggested that TH act in the hypothalamus to control these processes. This paradigm shift has subjected the topic to intense debates. This review aims to recapitulate how TH control adaptive thermogenesis and to what extent the brain is involved in this process. This is of crucial importance for the design of new pharmacological agents that would take advantage of the TH metabolic properties.

AMPK in the Ventromedial Nucleus of the Hypothalamus: A Key Regulator for Thermogenesis

Obesity has become a global health issue, but effective therapies remain very limited. Adaptive thermogenesis promotes weight loss by dissipating energy in the form of heat, thereby representing a promising target to counteract obesity. Notably, the regulation of thermogenesis is tightly orchestrated by complex neuronal networks, especially those in the hypothalamus. Recent evidence highlights the importance of adenosine monophosphate-activated protein kinase (AMPK) within the ventromedial nucleus of the hypothalamus (VMH) in modulating thermogenesis. Various molecules, such as GLP-1, leptin, estradiol, and thyroid hormones, have been reported to act on the VMH to inhibit AMPK, which subsequently increases thermogenesis through the activation of the sympathetic nervous system (SNS). In this review, we summarize the critical role of AMPK within the VMH in the control of energy balance, focusing on its contribution to thermogenesis and the associated mechanisms.

Postnatal Stress in Mice: Effects on Body Fat, Plasma Lipids, Glucose and Insulin

Mice pups were exposed to stressful stimuli everyday during the first 3 weeks of life. Body weight, food intake and spontaneous locomotor activity, triglycerides, cholesterol, phospholipids, glucose and insulin basal levels, as well as epididymal fat pad weight and its cell volume were measured in stressed and control animals. Results indicated that postnatal stressful manipulations induced an increase in body weight, epididymal fat pad weight and its cell volume, as well as in insulin, glucose, cholesterol and triglycerides plasma levels, at 4 months of age. No significant changes in food consumption, locomotor activity and phospholipids plasma levels were found. Present data suggest that early stressful manipulations may induce residual effects on lipid and glucid metabolism.

Fat storage is partially dependent on vagal activity and insulin secretion of hypothalamic obese rat

Hypothalamic MSG-obese rats show hyperinsulinemia and tissue insulin resistance, and they display intense parasympathetic activity. Current analysis investigates whether early subdiaphragmatic vagotomy prevents tissue insulin sensitivity impairment in adult obese MSG-rats. Hypothalamic obesity was induced by MSG (4 mg/g BW), daily, from birth up to 5 days. Control animals receiving saline solution. On the 30th day rats underwent bilateral subdiaphragmatic vagotomy or sham surgery. An intravenous glucose tolerance test (i.v.GTT) was performed when rats turned 90 days old. Total white fat tissue (WAT) from rat carcass was extracted and isolated; the interscapular brown fat tissue (IBAT) was weighed. Rather than blocking obesity, vagotomy reduced WAT and IBAT in MSG-obese rats when the latter were compared to sham MSG-rats. High blood fasting insulin and normal glucose levels were also observed in MSG-obese rats. Although glucose intolerance, high insulin secretion, and significant insulin resistance were recorded, vagotomy improved fasting insulinemia, glucose tolerance and insulin tissue sensitivity in MSG-obese rats. Results suggest that increased fat accumulation is caused, at least in part, by high blood insulin concentration, and enhanced parasympathetic activity on MSG-obese rats.

Weanling ventromedial hypothalamic syndrome. bone geometry and biomechanics

The effect of growth-retarding, obesifying lesions in the ventromedial hypothalamic nucleus (VMN) on bone geometry and biomechanics was investigated in male weanling rats. The animals received bilateral, symmetrical, electrolytic lesions (VMNL rats) shortly after weanling (age 27 days); sham-operated rats served as controls (SCON). The rats were maintained for 42 postoperative days and then terminated. Body weight, nose-tail length, food intake, carcass water, and lean body mass were all significantly (p < 0.001) reduced in the VMNL group compared to SCON rats. Carcass fat, lipogenic efficiency (carcass fat % laid down/mean food intake) (both p < 0.001) and epididymal fat pad weight (p < 0.01) were significantly increased in VMNL versus SCON. Femur length, anteroposterior diameter (both p < 0.001), and mediolateral femur diameter (p < 0.01) were significantly reduced in VMNL versus SCON rats, but torque and angle of torque were comparable among the groups. VMNL rats femora also showed a significant greater maximum shear stress compared to the control animals. The reduced parameters in the VMNL rats are in good agreement with the previously demonstrated reduced plasma and pituitary growth hormone levels found in this hypothalamus preparation.

Reduced femoral geometry but normal biomechanics in the dorsomedial hypothalamic nucleus-lesioned rat

Bone geometry, structure, and biomechanical properties were investigated in a model of growth retardation, the dorsomedial hypothalamic nucleus-lesioned (DMNL) weanling rat. Male weanling Sprague-Dawley rats received bilateral electrolytic lesions in the dorsomedial hypothalamic nucleus (DMN) at age 27 days. Sham-operated rats served as controls. All rats were maintained postoperatively for 40 days. Upon sacrifice, DMNL rats weighed less (p < 0.01), were shorter (p < 0.01), and ate less (p < 0.01) when compared to controls, but their body composition was normal. The femora in DMNL were shorter (p < 0.01), had a smaller outer anteroposterior (AP) diameter (p < 0.04), polar moment of the area (p < 0.02), and maximal (p < 0.02) and minimal (p < 0.03) principal moment of the area when compared with sham-operated rats. Notably, mean torque at failure, torsional energy, stiffness, and maximal stress did not demonstrate statistically significant differences between the two groups. These data clearly show that despite the reduced size and bone growth, DMNL rats responded normally to the mechanical challenges applied to test bone biomechanical properties. The data, therefore, add to previous evidence and strengthen the hypothesis that DMNL rats are governed by an "organismic" set point.

Aging and the hypothalamus: research perspectives

There are several hypothalamic theories of aging, none of which has been validated. An approach to validation is to search for consequences of anatomic ablations of hypothalamic regions that are functional hallmarks of aging, or consequences of ablation that postpone the appearance of hallmarks of aging or extend longevity. Ablation of the hypothalamic ventromedial nucleus (VMN) in the weanling rat is associated with subsequent increased body fat, glucose intolerance, hyperlipidemia, and decreased renal function. Each of these consequences is characteristic of aging in humans and in several animal models of aging. Ablation of the hypothalamic dorsomedial nucleus (DMN) in the weanling rat leads to a symmetrically smaller animal with normal glucose and lipid metabolism, decreased body fat for size, and reduced risk of decreased renal function and circulating IGF-I levels. These are findings consistent with calorie restriction models in rodents that significantly extend life span. This review compares outcomes of lesions in the VMN, DMN, and lateral hypothalamic area (LHA) for relevance to aging. To establish a relationship between these anatomic areas of the hypothalamus and aging, it is concluded that the VMN, DMN, and LHA lesions should be examined for impact on longevity and compared with data obtained from simultaneously studied intact ad-lib-fed and 40% calorie-restricted animals. Lesioned animals also should be rigorously studied for neurotransmitters (e.g., neuropeptide Y, beta-endorphin, serotonin, corticotropin-releasing factor, and galanin), and for behavioral changes consistent with aging, for accumulation of specific tissue lipofuscin and amyloid that are associated with normal aging and for other age-dependent findings, such as incidence of tumors and cataract.

Somatic parameters, organ growth, and plasma substrates in weanling rats with lateral hypothalamic lesions one month postoperatively

Somatic and some metabolic aspects of the syndrome that follows bilateral destruction of the lateral hypothalamic area (LHA) have been studied primarily in mature rats. Fewer data are available for the weanling rat. Weanling Sprague-Dawley rats received small (10 mC) bilateral electrolytic lesions (LHAL). Sham-operated controls were pair-gained to LHAL rats (CON-PG) or fed ad lib (CON-ADLIB). All rats were killed 1 month after LHAL. Both LHAL and CON-PG weighed less, had less carcass fat, and were shorter than CON-ADLIB. Also, LHAL were somewhat, but significantly (SIGN), shorter than CON-PG. Rats with LHAL has less carcass protein than CON-ADLIB in percent but not in absolute terms. Liver, epididymal fat pads, diaphragm, kidneys adrenals, testes, spleen, and heart grew SIGN smaller in LHAL vs. CON-ADLIB, but in no instance was there a SIGN difference between LHAL and CON-PG. In body weight percentage, some of these differences (liver, kidneys, heart) were not SIGN. Both LHAL and CON-PG had larger adrenals than CON-ADLIB and both LHAL and CON-PG had SIGN less protein in their livers, epididymal fat pads, and diaphragm than CON-ADLIB. In organ weight percentage, however, LHAL rats had more protein in their livers and fat pads than CON-ADLIB and LHAL rats had less protein in fat pads than CON-PG in absolute but not in percent organ weight terms. Plasma glucose was similar in all groups, but LHAL had SIGN lower triglycerides and total cholesterol than CON-ADLIB.(ABSTRACT TRUNCATED AT 250 WORDS)

The lateral hypothalamic area revisited: neuroanatomy, body weight regulation, neuroendocrinology and metabolism

This article reviews findings that have accumulated since the original description of the syndrome that follows destruction of the lateral hypothalamic area (LHA). These data comprise the areas of neuroanatomy, body weight regulation, neuroendocrinology, neurochemistry, and intermediary metabolism. Neurons in the LHA are the largest in the hypothalamus, and are topographically well organized. The LHA belongs to the parasympathetic area of the hypothalamus, and connects with all major parts of the brain and the major hypothalamic nuclei. Rats with LHA lesions regulate their body weight set point in a primary manner and not because of destruction of a "feeding center". The lower body weight is not due to finickiness. In the early stages of the syndrome, catabolism and running activity are enhanced, and so is the activity of the sympathetic nervous system (SNS) as shown by increased norepinephrine excretion that normalizes one mo later. The LHA plays a role in the feedback control of body weight regulation different from ventromedial (VMN) and dorsomedial (DMN). Tissue preparations from the LHA promote glucose utilization and insulin release. Although it does not belong to the classical hypothysiotropic area of the hypothalamus, the LHA does affect neuroendocrine secretions. No plasma data on growth hormone are available following electrolytic lesions LHA but electrical stimulation fails to elicit GH secretion. Nevertheless, antiserum raised against the 1-37 fragment of human GHRF stains numerous perikarya in the dorsolateral LHA. The plasma circadian corticosterone rhythm is disrupted in LHA lesioned rats, but this is unlikely due to destruction of intrinsic oscillators. Stimulation studies show a profound role of the LHA in glucose metabolism (glycolysis, glycogenesis, gluconeogenesis), this mechanism being cholinergic. Its role in lipolysis appears not to be critical. In general, stimulation of the VMN elicits opposite effects. Lesion studies in rats show altered in vitro glucose carbon incorporation into several tissue fractions both a few days, and one mo after lesion production. Several of these changes may be due to the reduced food intake, others appear to be due to a "true" lesion effect.

Body weight gain after VMH lesions in adult female rats guanethidine-sympathectomized at birth

The role of the sympathetic nervous system in body weight gain produced by lesions of the ventromedial nucleus of the hypothalamus (VMH) was studied in adult female rats that had been sympathectomized from birth for 3 weeks with daily injections of guanethidine (0.01 ml/g body weight) starting the second day after birth. Female littermates injected with 0.15 M NaCl served as controls. Body weight gain during the dynamic phase after the VMH lesion was the same in the sympathectomized and control groups of rats, whereas the treated rats gained weight at a slower rate than the controls in the static phase. The increase in food intake stimulated by the VMH lesion peaked sooner and remained elevated longer in the controls than in the experimental animals despite the similar increases in body weight gain. These results indicate that the sympathetic nervous system may play an important role in body weight gain during the static phase following a VMH lesion in adulthood.

Metabolic-endocrine correlates of the lateral hypothalamic syndrome: the first 48 hours

Mature (224 g) male Sprague-Dawley rats received bilateral electrolytic (1 mA for 8 sec) lesions in the lateral hypothalamic area (LHAL) or sham operations (CON). One group of CON was allowed to eat ad lib (CON-ADLIB), a second CON group was pair-fed to the LHAL rats (CON-PF). Tap water was available ad lib. Two days after the operation/sham operation all rats were killed by decapitation. Body weight, body weight change, food intake, carcass fat, liver weight, epididymal fat pad weight, in vitro incorporation of U-C14-glucose into liver total lipid, glycogen and CO2 (oxidation) (DPM, DPM/mg protein) as well as oxidation in fat pad tissue, plasma glucose and insulin were significantly reduced in LHAL and CON-PF rats compared with CON-ADLIB. Glucose carbon incorporation into epididymal fat pad lipid and glycogen were normal in LHAL and CON-PF. Liver protein and plasma free fatty acids (FFA) were both higher in LHAL and CON-PF than in CON-ADLIB groups. Thus, many of the somatic and metabolic changes that appear in the first few days after lesion production are simply due to hypophagia. However, CON-PF rats also exhibited some significant differences from the LHAL group, i.e., their plasma glucose and incorporation of glucose carbon into liver glycogen (DPM) were significantly lower than in LHAL rats; alternatively, plasma FFA levels were higher in CON-PF than in LHAL rats. Also, liver weight/100 g body weight was lower and fat pad weight/100 g body weight was higher in CON-PF than in LHAL rats.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Further evidence for the existence of an "organismic" set point in rats with dorsomedial hypothalamic nucleus lesions (DMNL rats): normal catch-up growth

The present study was performed to assess the capacity of rats with dorsomedial hypothalamic nucleus lesions (DMNL rats) and sham-operated controls (CON) for catch-up growth following body weight (b.wt.) reduction prior to DMNL (and sham-lesion) production. Male SD rats (45 days, 157 +/- 1.3 g) were maintained for 11 days ad lib (ADLIB) after arrival and then divided into two groups. One group continued to feed ADLIB, the other group was fed half of the ration eaten by ADLIB rats for 32 days. At this point each group was divided into two subgroups. One subgroup received DMNL, the other subgroup consisted of CON. From then on all rats were fed ADLIB [except for one group of CON that was pair-fed to the ADLIB DMNL rats (PF-CON)] for 37 days (69th day of experiment) and then killed. DMNL rats lesioned at normal b.wt. (ADLIB DMNL) showed a precipitous drop in food intake, b.wt. and efficiency of food utilization (EFU). In striking contrast, rats that had received DMNL after b.wt. restriction (REST DMNL) and were then refed ADLIB showed a dramatic rise in food intake, b.wt., change in b.wt. and EFU, the latter being almost twice that of the ADLIB DMNL. Notably, the PF-CON weighed less than the ADLIB CON and utilized food poorer than ADLIB CON, REST CON and ADLIB DMNL. Liver weight (both absolute and relative (per kg 3/4 b.wt.) was reduced in DMNL irrespective of dietary treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropeptide Y: brain localization and central effects on plasma insulin levels in chicks

A rich network of NPY-like immunoreactive fibers was found in the paraventricular nucleus and the ventromedial region of the hypothalamus juxtapositioned to the third ventricle, including the median eminence. Brain regions, areas or nuclei found densely innervated by NPY-like immunoreactive fibers included the olfactory bulb region, septal area, organum vasculosum of the lamina terminalis, preoptic periventricular nucleus, hypothalamic periventricular nucleus, medial suprachiasmatic nucleus, subseptal (subfornical) organ, ventromedial hypothalamic nucleus, infundibular nucleus and nucleus tractus solitarius. NPY-like containing perikarya were localized within the hippocampus, bed nucleus of the stria terminalis and surrounding the nucleus rotundus and nucleus of the basal optic root. Since the immunocytochemical study showed that NPY was localized in brain structures known to alter food intake and the compound is a member of the pancreatic polypeptide family, a second study was designed to determine if the neuropeptide altered plasma concentrations of insulin, glucagon and glucose following intracerebroventricular administration. It was found that NPY significantly increased plasma concentration of insulin. It is proposed that two reasons why NPY is such a potent orexigenic agent is that the paraventricular nucleus and structures surrounding the third ventricle throughout the ventromedial hypothalamic region show high levels of NPY-like immunoreactivity. Secondly, NPY effects an increase in plasma insulin in the periphery.

Failure to demonstrate disruption of ultradian growth hormone rhythm and insulin secretion by dorsomedial hypothalamic nucleus lesions that cause reduced body weight, linear growth and food intake

Weanling male rats received bilateral electrolytic lesions in the dorsomedial hypothalamic nuclei (DMNL rats); sham-operated animals served as controls. At the end of a 39-day postoperative period DMNL rats were lighter and shorter than controls and also exhibited significant hypophagia. Their efficiency of food utilization (weight gained for the amount of food eaten) was normal, however. Subsequent determination of plasma growth hormone (GH) and insulin (IRI) levels every 15 min for 6-h periods from freely moving chronically cannulated rats showed no differences in pulsatile patterns and peaks of GH nor in plasma IRI levels between DMNL rats and controls. There was also no significant difference between mean 6-H GH and IRI concentrations between the two groups. The reduced body weight, length and food intake are apparently unrelated to the normal GH and IRI secretory patterns. In conjunction with previous data indicating normal somatomedin activity and normal responses to various homeostatic challenges, the data make a strong case for the argument that DMNL rats are not "growth-retarded". Rather, they are normal animals that are "scaled-down" to a smaller size with maintenance of normal homeostatic capacity. This has been hypothesized to be due to the existence in these animals of an "organismic" set point.

Differential effect of common snack foods on caloric intake, growth and obesity in weanling male and female rats with hypothalamic obesity

In order to see whether weanling normophagic rats with hypothalamic obesity (VMNL rats) become hyperphagic and more obese than when fed lab chow, and to see in addition whether there is a possible sex difference in whatever response is found, male and female VMNL rats were fed lab chow for 14 days after lesion production and then, for the following 42 days, they received Hostess HoHos, potato chips, marshmallows and french fries in addition to lab chow. At termination, body weights were similar among the groups, but VMNL rats were fatter and shorter than controls. Also, female VMNL rats were fatter and shorter than male VMNL rats and layed down more fat per unit of food energy. Total caloric intake was greater in controls than in VMNL rats and in males than in females. However, the females showed a decrease in intake and the males an increase over time. Males ate more lab chow and french fries than females. Lab chow was the only food that VMNL rats ate more of than controls. The VMNL rats ate less french fries, HoHos and potato chips than controls, but similar amounts of marshmallows. There was no sex difference in macronutrient intake and per cent macronutrient intake. Moreover, controls ate as much carbohydrate and less per cent carbohydrate than VMNL rats, more fat than VMNL rats, but normal per cent fat and normal protein and per cent protein. Sex X lesion interactions indicated that female VMNL rats ate more per cent carbohydrate than female controls and male VMNL rats, ate more protein than male controls. The greater degree of obesity in the females despite the lower caloric intake may be due to enhanced food energy utilization. The data also show that weanling VMNL rats do not exhibit the great preference for palatable diets that have been reported for mature rats with ventromedial hypothalamic area lesions and that the only sex difference in intake is with french fries, which have neither the highest fat nor the highest carbohydrate content of the foods tested.

Effect of palatable diet on growth, caloric intake and endocrine-metabolic profile in weanling rats with dorsomedial hypothalamic lesions

Weanling male Sprague-Dawley rats received bilateral electrolytic lesions in the dorsomedial hypothalamic nuclei (DMNL rats); sham-operated rats served as controls. All animals were fed lab chow for 15 postoperative days. At that time they were subdivided into two groups each. One DMNL and one control group continued to be fed lab chow until the termination of the experiment on postoperative day 116. A second DMNL and control group were fed a high-fat diet and 32% sucrose solution (HF/SS diet). All DMNL rats showed reduced body weight and linear growth, but the HF/SS diet depressed these parameters further below the levels of the chow-fed groups. Both DMNL and control rats fed HF/SS had more carcass fat, heavier epididymal fat pads, more carcass fat per calories eaten, higher plasma levels of glucose, glycerol and free fatty acids but lower insulin levels than chow-fed DMNL rats and controls. This occurred in the face of lower body weights and caloric intake. Neither growth hormone nor insulin showed lesion effects. Rats with DMNL exhibited the same inverse relationship between plasma insulin and free fatty acids as controls. The data indicate that DMNL rats respond to the HF/SS diet essentially like sham-operated controls, i.e., they develop dietary obesity. Although they do show some small deficits, their lipogenic capacity is actually significantly greater than that of HF/SS-diet fed controls.

Failure to demonstrate early metabolic changes in weanling growth-retarded hypophagic rats with lesions in the dorsomedial hypothalamic nuclei

Experiment 1: Weanling male rats received bilateral electrolytic lesions in the dorsomedial hypothalamic nuclei (DMNL rats); sham-operated animals served as controls. Rats were killed four hours and three and seven days postoperatively (post-op). Plasma was obtained and epididymal fat pads, diaphragm and liver aliquots were harvested and the in vitro incorporation of U-14C-glucose into CO2, glycogen, lipid and saponifiable fatty acids (FAs) were measured. Body weight, carcass lipid and food intake were significantly lower in DMNL rats than in controls. The only significant lesion-induced metabolic changes were hypoglycemia and greater tracer incorporation into epididymal fat pad lipid and diaphragm glycogen. Both DMNL rats and controls showed similar time courses of tracer incorporation into epididymal CO2 and FAs, diaphragm lipid and liver CO2, glycogen, lipid and FAs. Lesioned rats also showed more pronounced decreases of tracer incorporation from day 0 to day 3 in epididymal glycogen and lipid and diaphragm CO2 and glycogen. These data make it appear unlikely that very early deficits in glucose metabolism are the cause of the growth retardation seen in long-term studies with DMNL rats. The data also demonstrate considerable locus specificity, since weanling rats with ventromedial hypothalamic lesions (VMNL rats) in similar short-term studies have shown dramatic alterations in the above parameters. Experiment 2: Weanling DMNL rats and sham-operated rats were injected via tail vein with tritiated water one hour post-op. One hour after the injection they were decapitated. There were no significant differences between DMNL rats and controls in mumoles tritiated water incorporated into total liver, grams liver tissue, mg liver glycogen and ml or mg plasma glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

Ventromedial and dorsomedial hypothalamic syndromes in the weanling rat: is the "center" concept really outmoded?

This report juxtaposes findings from weanling rats with precise lesions in the ventromedial (VMNL rats) to data of weanling rats with lesions in the dorsomedial (DMNL) hypothalamic nuclei. Despite the proximity of the two nuclei their destruction produces opposite effects in most cases but similar responses in other parameters. Absolute and relative food intake are normal in VMNL rats yet they become obese in the face of normal body weight gains. DMNL rats show both reduced absolute food intake and body weight but normal relative food intake and body composition. Both VMNL and DMNL cause reduced linear growth and running wheel activity. DMNL rats defend their lower body weight set point against various challenges and maintain normal body composition. Organ growth in both absolute and relative terms is reduced in VMNL rats. In DMNL rats relative organ growth is normal. Pancreatic growth, protein/pancreas and content and concentrations of several pancreatic enzymes are normal in DMNL but reduced in VMNL rats. Mean 24-hour plasma growth hormone (GH) and corticosterone (B) levels are reduced and insulin levels are greatly elevated in VMNL rats; prolactin (PRL) levels are normal. In DMNL rats, GH, B, insulin and somatomedin activity are normal but PRL is elevated. Circadian rhythms of GH, insulin and triiodothyronine are normal in DMNL rats but B levels are disrupted, as they are in VMNL rats. Glucose incorporation and oxidation in adipose tissue of VMNL rats are enhanced in VMNL rats but normal in DMNL rats. Gluconeogenesis in VMNL rats is enhanced as early as 4 hours post-operatively; in DMNL rats it is normal at this time and several weeks thereafter. Basal lipolysis in epididymal fat pads is elevated in both VMNL and DMNL rats but epinephrine-stimulated lipolysis is elevated in VMNL and decreased in DMNL rats. Both VMNL and DMNL rats show normal basal and epinephrine-stimulated lipolysis in interscapular brown adipose tissue. Several hepatic enzymes are normal in DMNL and depressed in VMNL rats. The above data suggest that the DMN and its circuitry are part of an "organismic" set point system with a "true" body weight and no fat set point, as seems to be the case in the VMNL rat.

Hypothalamic obesity, brown adipose tissue, and sympathoadrenal activity in rats

Obesity-producing, hypothalamic knife cuts and ventromedial hypothalamic (VMH) lesions in ad libitum-fed adult rats increased intake of a high-fat diet (123 and 130%) and energy retention (880 and 1,099%) during the 4-wk period postsurgery; even when pair fed to control rats, energy retention of the knife-cut and lesioned rats was still elevated (105 and 155%). Thermogenic capacity of brown adipose tissue (BAT), estimated from guanosine diphosphate (GDP) binding to BAT mitochondria, was unchanged in hyperphagic knife-cut and VMH-lesioned rats and was reduced approximately 50% when these rats were pair fed to controls. Urinary excretion of norepinephrine (NE) was approximately twofold higher in ad libitum-fed, knife-cut, and lesioned rats than in control rats; restriction of energy intake decreased NE excretion to control values. Rates of NE turnover in heart paralleled urinary NE excretion, whereas NE turnover in BAT was generally not increased in the hyperphagic rats. Urinary epinephrine excretion, an index of adrenal medullary activity, was depressed in all knife-cut and VMH-lesioned rats. Hyperphagia coupled with a lack of increased heat production in BAT causes gross obesity in ad libitum-fed, knife-cut, and VMH-lesioned rats, whereas obesity in pair-fed rats develops in part at least as a result of reduced heat production by BAT.

Pancreatic and intestinal digestive enzymes in post-weanling rats with hypothalamic obesity

Male Sprague-Dawley rats received bilateral electrolytic lesions in the ventromedial hypothalamic nuclei (VMNL rats) at the age of 31 days; sham-lesioned rats served as controls. For 28 post-operative days all animals self-selected from three synthetic diets, each high in carbohydrate, fat and protein, respectively. Following this, half of the VMNL rats and half of the controls were switched to lab chow for 14 days. Body weights were comparable among the groups, but linear growth was greatly reduced and body fat (Lee Index) was elevated in VMNL rats, irrespective of diet. In the sham-lesioned controls, the synthetic diets reduced most parameters of exocrine pancreatic activity. In VMNL rats, in contrast, pancreatic parameters were unaffected by the synthetic diet. The data suggest that VMN lesions disinhibit the exocrine pancreas. In contrast, most parameters of intestinal activity were not influenced by VMN lesions.

Postnatal development of sensory influences on neurons in the ventromedial hypothalamic nucleus of the rat

Extracellular unitary records were obtained from neurons in the ventromedial hypothalamic nucleus (VMH) of very young (1-25 days of postnatal age) and adult rats. Spontaneous unitary activity and evoked responses to both external (somatic, gustatory, and olfactory) and internal sensory (systemic administration of hypertonic saline and glucose solutions) stimulation were determined in order to assess the functional development of VMH neurons and their afferents. The basic electrophysiological characteristics of VMH neurons were established prenatally. From the date of birth, many VMH neurons had: spontaneous action potential generation; evoked responses to external or internal sensory stimulation; and convergent sensory inputs. In contrast, the major developmental change in the neurophysiological properties of VMH neurons was the diminution with increasing age of the convergence of external and internal sensory influences. This developmental 'fine-tuning' of a complex functional feature of VMH neurons is important because the maturation of convergence coincides with a 'critical period' of VMH ontogenesis demonstrated in behavioral and experimental brain damage reports.

Normal pancreatic and intestinal enzymes in hypophagic growth-retarded rats that received dorsomedial hypothalamic lesions shortly after weaning

Male weanling Sprague-Dawley rats received bilateral electrolytic lesions in the dorsomedial hypothalamic nuclei (DMNL rats). Sham-operated rats served as controls. After being fed lab chow for two postoperative weeks, the animals were divided into four groups. One group of DMNL rats and controls received a high-caloric diet (high-fat diet, chocolate chip cookies, 32% sucrose solution, potato chips and marshmallows), whereas another group of DMNL rats and controls continued to receive lab chow. The experiment was terminated on the 185th postoperative day. In accordance with previous findings, DMNL rats, irrespective of diet, were lighter and shorter than controls. In addition, DMNL rats fed junk food were lighter than DMNL rats fed lab chow, and junk-fed controls weighed as much as chow-fed controls. Both DMNL rats and controls fed junk food were also shorter and showed higher carcass fat than their chow-fed counterparts. Also, DMNL rats fed junk food had less carcass fat than junk-fed sham-operated controls, whereas in accordance with previous findings, there was no difference between chow-fed DMNL rats and chow-fed sham-operated controls. Irrespective of diet, DMNL rats ate less calories than their respective sham-operated controls. Both absolute and percent pancreas weight and protein/pancreas were unaffected in DMNL rats but were reduced in both junk-fed groups in comparison with their chow-fed counterparts. Both concentrations and contents of pancreatic trypsinogen, amylase and lipase were unaffected in DMNL rats but total activities of all three enzymes were dramatically reduced in the junk-fed compared with the chow-fed DMNL rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of oral and parenteral quinine on rats with ventromedial hypothalamic knife-cut obesity

The addition of quinine to the food reversed the obesity in rats with hypothalamic hyperphagia induced by knife cuts. Similarly, the injection of quinine into rats with hypothalamic knife cuts reduced food intake and body weight but the effects were smaller than those observed when quinine was added to the diet. Urinary quinine excretion was similar by the oral and parenteral routes. The food intake of the knife-cut animals receiving quinine gradually fell to the same level as in the sham-operated animals receiving quinine by either route. The weights of retroperitoneal fat pads were related to the weights of the animals and were reduced in the quinine-treated groups. Plasma insulin concentrations were significantly higher in the knife-cut animals and were reduced toward control levels by quinine treatment. Gluconeogenesis, measured by incorporation of radioactivity from labeled bicarbonate into glucose, was unaffected by treatment with quinine or by knife cuts. Lipogenesis from tritiated water in vivo was not different between treatment groups in the liver or retroperitoneal fat pads. However, in vivo lipogenesis was reduced in knife-cut rats fed ad libitum compared with quinine-treated rats. The response of lipogenesis to insulin in vitro was also not different between treatment groups. These data suggest that a major part of the reduction in food intake in hyperphagic rats eating a quinine-adulterated diet is due to postingestional events.

Paraventricular nucleus lesions in weanling female rats result in normophagia, normal body weight and composition, linear growth and normal levels of several plasma substrates

Female weanling rats received small (1 mAmp for 5 sec) electrolytic lesions in the paraventricular nuclei. Sham-operated rats served as controls. The rats were maintained for 42 days and body weight, linear growth, Lee Index, food intake and efficiency of food utilization were determined throughout the study. Plasma glucose, glycerol, free fatty acids, total protein and carcass fat and protein were determined at sacrifice. There was no significant difference between the lesioned and the sham-operated rats in any of the parameters measured. The findings are interpreted to mean that the PVN of the weanling rat is not functionally mature or alternatively, that there exists a sex difference in weanling rats in response to PVN lesions.

Brown adipose tissue metabolism in hypothalamic-obese rats

Studies were performed to evaluate the metabolic changes of brown adipose tissue (BAT) in rats with hypothalamic obesity (VMNL). In vitro 14C-palmitate oxidation and incorporation into triglycerides were similar in VMNL and control rats. However, protein and fatty acid content and incorporation of 14C-palmitate into phospholipid were significantly less in both hyperphagic and normophagic VMNL rats. In order to assess in vivo BAT lipogenesis, rats were injected with 3H2O. Plasma H2O incorporation into BAT lipids was significantly greater in VMNL rats. Likewise, BAT lipid content was higher in obese rats. In another experiment BAT was incubated with U-14C-glucose to evaluate glucose utilization by BAT. 14C-glucose was oxidized and incorporated into both lipids and glycogen more rapidly by obese than by normal rat BAT. Glycogen content was greater in VMNL rats. Tissues were also incubated with 1-14C-pyruvate and 2-14C pyruvate. Pyruvate incorporation into glyceride glycerol and oxidation of 2-14C pyruvate through the Krebs cycle were similar in both obese and control rats. However, the incorporation of pyruvate into glyceride fatty acids was increased in VMNL rats. The results indicate that both fatty acid and lipid synthesis are increased in BAT of obese rats whereas lactate production is decreased and Krebs cycle activity is normal. Some of these changes appear to be independent of the level of food intake.

Brown adipose tissue of rats with obesity-inducing ventromedial hypothalamic lesions

Male and female Holtzman rats were made hyperphagic and obese with bilateral radiofrequency heat lesions of the ventromedial hypothalamic (VMH) area. When VMH rats were maintained at 28 degrees C, their brown adipose tissue (BAT) DNA, protein, and cytochrome oxidase contents were normal although more stored lipid was present, as judged from a threefold increase in wet weight. Thermogenic activity of BAT mitochondria was normal in male VMH rats, as judged from the unchanged level of guanosine diphosphate (GDP) binding (known to be a sensitive index of the functional activity of the thermogenic proton conductance pathway), and reduced in female VMH rats. When rats with VMH lesions were exposed to cold (4 degrees C for 24 h), the visible hyperemia of their BAT and normal large increase in mitochondrial GDP binding indicated normal thermogenic responsiveness. We conclude that the medial nuclei of the hypothalamus and associated afferent or efferent nerve tracts do not represent an essential central nervous system link for cold-induced, sympathetic-mediated activation of BAT thermogenesis. It is possible, however, that diet-induced, sympathetic-mediated activation of BAT function and growth might require an intact VMH region because no enhancement of BAT mitochondrial function normally associated with hyperphagia was detected in these hyperphagic VMH-lesioned animals.

Hypothalamic regulation of lipid metabolism in the rat: effect of hypothalamic stimulation on lipogenesis

The rates of fatty acid synthesis and syntheses of other lipids were measured in vivo by determining 3H-incorporation from tritiated water or 14C-incorporation from [U-14C]glucose into white adipose tissues (WAT), brown adipose tissue (BAT) and liver, during electrical stimulation of the ventromedial hypothalamic nucleus (VMH) and the lateral hypothalamic nucleus (LH) of female rats. Electrical stimulation of the VMH markedly increased the rate of fatty acid synthesis and the conversion of glucose to total lipids, glyceride glycerol and phospholipids in interscapular BAT, but not in parametrial or retroperitoneal WAT. Conversely, on VMH stimulation, the syntheses of total lipids, glyceride glycerol and phospholipids from glucose in the liver decreased slightly, though not significantly. Electrical stimulation of the LH had no such effect. Administration of insulin increased the rate of fatty acid synthesis in both brown and white adipose tissues and in the liver. The rate of disappearance of [14C]glucose and the concentration of insulin in the blood were not changed significantly by stimulation of the VMH. These data indicate that electrical stimulation of the VMH, but not the LH, enhances lipogenesis in BAT preferentially through a mechanism not involving insulin, but probably through activation of sympathetic innervation of BAT. The physiological significance of the findings is discussed in relation to hypothalamic function in controlling non-shivering thermogenesis and obesity.

Alterations of tryptophan metabolism in a rat strain (Osborne-Mendel) predisposed to obesity

1. Osborne-Mendel (O-M) rats displayed differences in brain and systemic tryptophan metabolism. O-M rats had decreased brainstem tryptophan-5-hydroxylase activity and decreased serotonin (5-HT) levels as compared to Sprague-Dawley rats. However, brain tryptophan levels were actually increased in O-M rats. Norepinephrine, dopamine and 5-hydroxyindole-3-acetic acid levels were not different between strains. 2. Pineal serotonin levels were increased in O-M rats. 3. Liver tryptophan 2,3-dioxygenase activity was increased in O-M rats while tyrosine aminotransferase activity was not different between strains. 4. Total blood cholesterol was decreased in O-M rats while triglycerides, free fatty acids and albumin was not different between strains. Total serum tryptophan was not different between strains while O-M rats had an increased level of free (unbound) tryptophan.

A role for insulin in the diet-induced thermogenesis of cafeteria-fed rats

Rats fed a varied and palatable "cafeteria" diet exhibited hyperphagia, increases in resting metabolic rate (VO2) and the thermogenic response to noradrenaline as well as hypertrophy of brown adipose tissue (BAT). In streptozotocin-diabetic rats, cafeteria feeding failed to produce increases in VO2 or the response to noradrenaline, although BAT mass was greater than in their respective stockfed controls. Replacement doses of insulin (protamine-zinc-insulin, PZI) at two levels (2 and 4 units/rat every alternate day) failed to restore the thermogenic response of diabetic rats to the cafeteria diet. Acute replacement (8 units PZI) 12hr before the measurements resulted in resting and noradrenaline-stimulated values for VO2 that were similar to those of non-diabetic cafeteria rats. These findings suggest an insulin requirement for diet-induced thermogenesis and the failure of diabetic rats to maintain body temperature when exposed to cold (5 degrees C) suggests a further insulin requirement for cold-induced thermogenesis. In non-diabetic cafeteria rats, plasma insulin levels were significantly lower than those of stock fed controls in spite of a high carbohydrate intake and normal blood glucose.

Central nervous system regulation of liver and adipose tissue metabolism

Hypothalamic and autonomic nervous regulation of carbohydrate and amino acid metabolism in the liver and of lipid metabolism in adipose tissues is described. The direct neural mechanism underlying this regulation has been evaluated. Electrical stimulation of the ventromedial hypothalamic nucleus (VMH)-splanchnic nerve system causes glycogenolysis in the liver by rapid activation of glycogen phosphorylase, whereas electrical stimulation of the lateral hypothalamic nucleus (LH)-vagus nerve system promotes glycogenesis in the liver by activation of glycogen synthetase, through direct neural and neural-hormonal mechanisms. Studies on chemical coding of the hypothalamic neurones have revealed that norepinephrine-sensitive neurones in the VMH and acetylcholine-sensitive neurones in the LH are specifically involved in the regulation of liver phosphorylase and glycogen synthetase, respectively. Acetylcholine-sensitive neurones of the LH were also found to be concerned in regulation of hepatic tyrosine aminotransferase activity, through intermediation of the cholinergic system in the LH-vagal pathway. Finally, it has been shown that the VMH acts as a regulatory centre for lipolysis in adipose tissues by modulating activation of the sympathetic nervous system. In addition, stimulation of the VMH enhanced lipogenesis in brown adipose tissue preferentially, probably through a mechanism mediated by sympathetic innervation of this tissue. The latter finding suggests that both the breakdown and resynthesis of triglycerides in brown adipose tissue, but not in white adipose tissue, are accelerated by stimulation of the VMH.

Regulation of food intake and body weight by insulin

A feedback system for the regulation of food intake and body weight, consisting of two elements is proposed. One is related to the quantitiy and quality of the food ingested. It consists of neural afferents, psychosocial conditioning factors, and peptide signals from the gastrointestinal tract released by specific nutrient intake. The other is also sensitive to nutrient intake, but importantly modulated by relative adiposity. We present evidence to suggest that insulin serves as the key feedback signal to the central nervous system to serve this second function (body adiposity signal). Insulin has been found in cerebrospinal fluid where its concentration is increased by systemic infusions of glucose or insulin and is proportional to its concentration in plasma. When insulin (10 and 100 μU/kg/day) is infused into the lateral cerebral ventricles of free feeding baboons a dose dependent suppression of food intake and body weight is found. Intravenous infusion of 25% and 50% of total calories as glucose elevates endogenous insulin concentrations and suppresses food intake. These findings suggest that the amount of insulin secreted per day and more modulates food intake to maintain a constant body weight.

Activation of brown adipose tissue thermogenesis by the ventromedial hypothalamus

It is well established that electrical stimulation of the ventromedial hypothalamus (VMH) causes a reduction in food intake, whereas electrolytic or chemical lesions in this area result in hyperphagia and obesity in the rat. This has led to the suggestion that either the ventromedial nucleus itself, or nerve fibres passing close by, are important in the control of food intake. However, obesity due to VMH lesions occurs in weanling rats in the absence of hyperphagia and can develop in adult rats pair-fed with controls, indicating that destruction of this area also causes an increased metabolic efficiency (that is, a reduced energy expenditure). In normal rats, hyperphagia induced by feeding a highly palatable cafeteria diet is often accompanied by a large increase in heat production (diet-induced thermogenesis) which tends to prevent excessive weight gain and obesity. This diet-induced thermogenesis is due to sympathetic activation of brown adipose tissue (BAT) and we have now investigated the possibility that the VMH is involved in the activation of this process. We found that electrical stimulation of this area produced increased BAT thermogenesis, which suggests that the VMH exerts a dual influence in the regulation of energy balance--an inhibitory effect on energy intake and a stimulatory effect on thermogenesis and energy output.

Rats as models for the study of obesity

Gerontologists are concerned about obesity for two reasons. First, adiposity increases in humans with increasing age. Secondly, there is a strong possibility that the interaction between obesity and aging is deleterious. Since rats are a widely used animal model for the study of aging, it is likely that rats will be utilized in experiments designed to study in interactions between adiposity and aging. The object of this review is to provide the basic information needed for utilizing rat in such studies. To this end, changes in adipose tissue mass that occur during the life span of normal rats are described as are the types of rat models that have been used in the study of obesity. In addition, the characteristics of each of these obesity models are detailed.

Relative normophagia and organ growth in growth-retarded weanling rats with dorsomedial hypothalamic lesions

Weanling rats with ventromedial (VMNL) and dorsomedial (DMNL) hypothalamic lesions and sham-operated controls were maintained up to 198 days after operation. Food intake was measured throughout the experiment and organ weights were recorded at various periods of sacrifice. Comparisons were made between controls and VMNL and DMNL rats, respectively. Food intake and organ weights were expressed in absolute terms and relative to body mass and Kleiber's "metabolic size." VMNL rats were always normophagic and showed lower organ weights, regardless of manner of computation. Rats with DMNL, on the other hand, were absolutely hypophagic but relatively normophagic for considerable periods of time when food intake was referred to body mass. A similar relationship obtained for organ weights. The data fit well with previous results and with a hypothesis that holds that DMNL bring about a "resetting" of some central nervous control system that not only allows the rat so operated to subsist on lower substrate levels but also regulates normal growth in relation to body mass.

Transplantation of pancreatic beta-cells prevents development of hypothalamic obesity in rats

The present experiments have tested the hypothesis that ventromedial hypothalamic (VMH) lesions enhance insulin secretion by neural mechanisms. Rats were made diabetic by injecting streptozotocin to destroy their own pancreatic beta-cells. Subsequently, transplants of fetal pancreatic tissue were placed under the renal capsule. VMH lesions were placed in rats whose diabetes was cured with transplants as well as sham-transplanted animals. The animals were followed for 4 wk. The lesioned rats with pancreatic transplants gained no more weight than the sham-operated controls. There was no significant rise in insulin in the transplanted rats after VMH lesioning, but the VMH lesioned rats with intact pancreatic tissue showed the expected rise in insulin. Food intake rose 71% in the VMH lesioned rats with intact beta-cells, but only 23% in the VMH lesioned rats with transplants. Hypertrophy of the pancreatic islets was also observed in the VMH lesioned rats with an intact pancreas, but was not found in the VMH lesioned rats with a transplanted pancreas. Thus, transplantation of pancreatic tissue beneath the renal capsule of diabetic rats prevented the characteristic hyperphagia, hyperinsulinemia, and obesity in VMH lesioned rats whose pancreas was free from intact innervation. The results support the hypothesis that neural mediation of the rise in insulin is the primary factor in the development of hypothalamic obesity.