Feeding, activity, and body temperature following 6-hydroxydopamine lesions in diabetes (db/db) mice.

Reductions in central catecholamines produced by intraventricular injections of 6-hydroxydopamine (6-OHDA) cause weight loss and decreased plasma glucose in diabetes (db/db) mice. The present study examined the effects of this treatment in short-term (64-day) and long-term (120-day) survival groups of female diabetes (C57 BL/KsJ-db/db) and lean mice. Phenotypically heterozygotes (db/m) and homozygotes (m/m) were used as controls. Diabetes Ss treated with 6-OHDA decreased food intake, lost weight, and maintained a lower weight than vehicle-treated controls until vehicle-treated Ss began to enter the terminal stages of the syndrome, indicated by a loss of body weight. Diabetes Ss given 6-OHDA lost weight despite reduced body temperatures and activity levels. Blood glucose levels were always lower in 6-OHDA than in ad lib fed vehicle-treated db/db Ss. The 6-OHDA treatment also improved pancreatic islet granulation. Pair feeding vehicle-treated with 6-OHDA-treated db/db Ss did not halt weight gain in the vehicle-treated group. However, measurement of carcass fat indicated similar losses in db/db-6-OHDA Ss and vehicle-treated Ss when the vehicle group was pair-fed with lean controls. Treatment with 6-OHDA produced long-term improvement in the diabetes syndrome, but the decreased body weight of the 6-OHDA-treated diabetes Ss could not be completely accounted for by changes in food intake or energy expenditure. (43 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Specificity of leptin action on elevated blood glucose levels and hypothalamic neuropeptide Y gene expression in ob/ob mice

Correction of the obese state induced by genetic leptin deficiency reduces elevated levels of both blood glucose and hypothalamic neuropeptide Y (NPY) mRNA in ob/ob mice. To determine whether these responses are due to a specific action of leptin or to the reversal of the obese state, we investigated the specificity of the effect of systemic leptin administration to ob/ob mice (n = 8) on levels of plasma glucose and insulin and on hypothalamic expression of NPY mRNA. Saline-treated controls were either fed ad libitum (n = 8) or pair-fed to the intake of the leptin-treated group (n = 8) to control for changes of food intake induced by leptin. The specificity of the effect of leptin was further assessed by 1) measuring NPY gene expression in db/db mice (n = 6) that are resistant to leptin, 2) measuring NPY gene expression in brain areas outside the hypothalamus, and 3) measuring the effect of leptin administration on hypothalamic expression of corticotropin-releasing hormone (CRH) mRNA. Five daily intraperitoneal injections of recombinant mouse leptin (150 micrograms) in ob/ob mice lowered food intake by 56% (P < 0.05), body weight by 4.1% (P < 0.05), and levels of NPY mRNA in the hypothalamic arcuate nucleus by 42.3% (P < 0.05) as compared with saline-treated controls. Pair-feeding of ob/ob mice to the intake of leptin-treated animals produced equivalent weight loss, but did not alter expression of NPY mRNA in the arcuate nucleus. Leptin administration was also without effect on food intake, body weight, or NPY mRNA levels in the arcuate nucleus of db/db mice. In ob/ob mice, leptin did not alter NPY mRNA levels in cerebral cortex or hippocampus or the expression of CRH mRNA in the hypothalamic paraventricular nucleus (PVN). Leptin administration to ob/ob mice also markedly reduced serum glucose (8.3 +/- 1.2 vs. 24.5 +/- 3.8 mmol/l; P < 0.01) and insulin levels (7,263 +/- 1,309 vs. 3,150 +/- 780 pmol/l), but was ineffective in db/db mice. Pair-fed mice experienced reductions of glucose and insulin levels that were < 60% of the reduction induced by leptin. The results suggest that in ob/ob mice, systemic administration of leptin inhibits NPY gene overexpression through a specific action in the arcuate nucleus and exerts a hypoglycemic action that is partly independent of its weight-reducing effects. Furthermore, both effects occur before reversal of the obesity syndrome. Defective leptin signaling due to either leptin deficiency (in ob/ob mice) or leptin resistance (in db/db mice) therefore leads directly to hyperglycemia and the overexpression of hypothalamic NPY that is implicated in the pathogenesis of the obesity syndrome.     

Obese gene expression: reduction by fasting and stimulation by insulin and glucose in lean mice, and persistent elevation in acquired (diet-induced) and genetic (yellow agouti) obesity

Mutations in the obese (ob) gene lead to obesity. This gene has been recently cloned, but the factors regulating its expression have not been elucidated. To address the regulation of the ob gene with regard to body weight and nutritional factors, Northern blot analysis was used to assess ob mRNA in adipose tissue from mice [lean, obese due to diet, or genetically (yellow agouti) obese] under different nutritional conditions. ob mRNA was elevated in both forms of obesity, compared to lean controls, correlated with elevations in plasma insulin and body weight, but not plasma glucose. In lean C57BL/6J mice, but not in mice with diet-induced obesity, ob mRNA decreased after a 48-hr fast. Similarly, in lean C57BL/6J controls, but not in obese yellow mice, i.p. glucose injection significantly increased ob mRNA. For up to 30 min after glucose injection, ob mRNA in lean mice significantly correlated with plasma glucose, but not with plasma insulin. In a separate study with only lean mice, ob mRNA was inhibited >90% by fasting, and elevated approximately 2-fold 30 min after i.p. injection of either glucose or insulin. These results suggest that in lean animals glucose and insulin enhance ob gene expression. In contrast to our results in lean mice, in obese animals ob mRNA is elevated and relatively insensitive to nutritional state, possibly due to chronic exposure to elevated plasma insulin and/or glucose.     

Development of obesity in diabetic mice pair-fed with lean siblings.

Investigated the role of hyperphagia in the obesity of the diabetic mouse, C57BL/6J/db/db. Ingestion patterns and the amount of food for 25 diabetic mice were controlled by yoking their food intake to that of 16 nonobese siblings obtaining their food by barpressing. Over a period of 6 wks, young (initial ages were 28 days) pair-fed diabetic Ss accumulated 42% more body weight and approximately 5 times more extractable carcass lipid than did their siblings. Weight gain and absolute levels of carcass fat were reduced in food-restricted diabetic Ss compared with db Ss on unrestricted food intake. However, carcass fat as percentage of wet carcass weight was virtually identical in the restricted and unrestricted db rats (47.6 vs 49.6%). From these results it is concluded that the heightened adiposity of the diabetic mouse does not require hyperphagia for its expression and thus represents a metabolic obesity. (34 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Lesions of the substantia nigra retard Pavlovian eye-blink but not heart rate conditioning in the rabbit.

Bilateral radio-frequency (RF) lesions of the substantia nigra retarded Pavlovian eye-blink (EB) conditioning without affecting concurrent heart rate (HR) conditioning. Dopamine (DA) depletion occurred only in the caudate nucleus, whereas norepinephrine (NE) depletion was limited to the hypothalamus. Bilateral 6-hydroxydopamine (6-OHDA) lesions of substantia nigra retarded acquisition of both EB and HR responses. Six-OHDA lesions produced significant NE depletion in the nucleus acumbens/septi, frontal cortices, and hypothalamus, as well as DA depletion in the caudate nucleus. Trials required to reach EB conditioning criterion were significantly correlated to the caudate DA levels. The magnitude of conditioned bradycardia was on the other hand significantly correlated with hypothalamic NE levels. Results suggest that interruption of the nigro-striatal dopaminergic pathway retards Pavlovian somatomotor learning without affecting concurrent autonomic learning, although the latter may depend on an intact ascending NE pathway to the hypothalamus, which passes through the tegmentum and thus is also destroyed in some cases by substantia nigra lesions. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Leptin inhibits glycogen synthesis in the isolated soleus muscle of obese (ob/ob) mice

The ob gene product, leptin, causes significant and dose-dependent inhibition of basal and insulin-stimulated glycogen synthesis in isolated soleus muscle from ob/ob mice, and a smaller, non-significant inhibition in muscle from wild-type mice. Leptin had no inhibitory effect on glycogen synthesis in soleus muscle from the diabetic (db/db) mice, which lack the functional leptin receptor. The full-length leptin receptor (Ob-Rb), is expressed in soleus muscle of both ob/ob and wild-type mice, however with no detectable differences in expression level. These results suggest that hyperleptinaemia may attenuate insulin action on glucose storage in skeletal muscle.     

L-dopa repairs deficits in locomotor and investigatory exploration produced by denervation of catecholamine terminal fields in the forebrain of rats.

In a previous study by the authors (see record 1980-27512-001), rats were shown to have decreased locomotor and investigatory exploration after bilateral microinjections of 6-hydroxydopamine (6-OHDA) into the anterolateral hypothalamus. Theses deficits correlated with the loss of catecholamine (CA) terminals in neocortical, limbic, and anteromedioventral striatal brain sites. To test whether this correlation was causal, central CAs were increased in 2 experiments with male Sprague-Dawley rats by the ip injection of levodopa (10–40 mg/kg) after inhibition of extracerebral levo-amino-acid decarboxylase. Such treatment repaired the deficits in locomotor exploration and investigation in 6-OHDA Ss. Pretreatment with the CA antagonist chlorpromazine (1–2 mg/kg) blocked the increase in locomotor exploration and investigation produced by levodopa in 6-OHDA Ss. Results suggest, but do not prove, that levodopa produced these behavioral effects by increasing central CAs at the denervated CA receptor sites in the forebrain. Data are complementary evidence for the hypothesis that forebrain CA synaptic action is necessary for normal exploratory behavior. (22 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Genetically obese (ob/ob) mice are predisposed to gastric stress ulcers.

In Exp I, 20 adult male genetically obese (ob/ob) mice and 20 lean littermate controls were food deprived and subsequently physically restrained at normal room temperatures. Obese Ss became hypothermic and developed gastric stress ulcers. Lean Ss maintained normal body temperatures and did not form gastric ulcers. In Exp II, 5 male obese and 4 lean littermates were used to test the effects of noradrenaline (NA) during restraint, and 5 obese and 5 lean mice were used to test the effects of NA alone. It was expected that in lean, but not in obese, Ss that NA would induce an increase in O? consumption beyond that induced by initial restraint. O? consumption was measured during food deprivation and restraint. Obese and lean Ss had parallel metabolic responses, with obese Ss using significantly less O? at all times. The predisposition to formation of gastric ulcers is a new phenotypic expression of the ob/ob genotype. The pathogenesis of this susceptibility appears to be related to a genetic disturbance in heat production. (14 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Catecholamine depletion in mice upon reexposure to stress: Mediation of the escape deficits produced by inescapable shock.

Reports 9 experiments with 372 male Swiss-Webster mice in which, immediately following exposure to 60 inescapable shocks, Ss had significantly reduced hypothalamic norepinephrine (NE). Within 24 hrs NE levels returned to control values. Reexposure to as few as 10 shocks 24 hrs after initial stress exposure resulted in significant decline of NE. At this interval after shock, escape performance was severely disrupted, with a large proportion of Ss exhibiting numerous failures to escape shock. Increasing brain dopamine (DA) and NE by levodopa treatment prior to shock prevented the escape deficits. Conversely, pairing 5 inescapable shocks with NE depletion by FLA-63, or both DA and NE depletion by alpha-methylparatyrosine, disrupted escape performance 24 hrs later. Residual drug effects, state dependence, or sustained amine turnover could not account for the behavioral changes. Data are discussed in terms of catecholamine mediation of escape performance through variations in response maintenance abilities. It is suggested that long-term effects of inescapable shock may be due to sensitization effects or conditioned amine depletion. (28 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Hypothalamic pro-opiomelanocortin mRNA is reduced by fasting and [corrected] in ob/ob and db/db mice, but is stimulated by leptin

Reduction in the activity of the alpha-melanocyte-stimulating hormone (alpha-MSH) system causes obesity, and infusions of alpha-MSH can produce satiety, raising the possibility that alpha-MSH may mediate physiological satiety signals. Since alpha-MSH is coded for by the pro-opiomelanocortin (POMC) gene, we examined if POMC gene expression would be inhibited by fasting in normal mice or in models of obesity characterized by leptin insufficiency (ob/ob) or leptin insensitivity (db/db). In wild-type mice, hypothalamic POMC mRNA was decreased > 60% after a 2-day fast and was positively correlated with leptin mRNA. Similarly, compared with controls, POMC mRNA was decreased by at least 60% in both db/db and ob/ob mice. POMC mRNA was negatively correlated with both neuropeptide Y (NPY) and melanin-concentrating hormone (MCH) mRNA. Finally, treatment of both male and female ob/ob mice with leptin stimulated hypothalamic POMC mRNA by about threefold. These results suggest that impairment in production, processing, or responsiveness to alpha-MSH may be a common feature of obesity and that hypothalamic POMC neurons, stimulated by leptin, may constitute a link between leptin and the melanocortin system.     

Calcium induces a conformational change in the ligand binding domain of the low density lipoprotein receptor

TNF-alpha may play a role in mediating insulin resistance associated with obesity. This concept is based on studies of obese rodents and humans, and cell culture models. TNF elicits cellular responses via two receptors called p55 and p75. Our purpose was to test the involvement of TNF in glucose homeostasis using mice lacking one or both TNF receptors. C57BL/6 mice lacking p55 (p55(-)/-), p75, (p75(-)/-), or both receptors (p55(-)/-p75(-)/-) were fed a high-fat diet to induce obesity. Marked fasting hyperinsulinemia was seen for p55(-)/-p75(-)/- males between 12 and 16 wk of feeding the high-fat diet. Insulin levels were four times greater than wild-type mice. In contrast, p55(-)/- and p75(-)/- mice exhibited insulin levels that were similar or reduced, respectively, as compared with wild-type mice. In addition, high-fat diet-fed p75(-)/- mice had the lowest body weights and leptin levels, and improved insulin sensitivity. Obese (db/db) mice, which are not responsive to leptin, were used to study the role of p55 in severe obesity. Male p55(-)/-db/db mice exhibited threefold higher insulin levels and twofold lower glucose levels at 20 wk of age than control db/db expressing p55. All db/db mice remained severely insulin resistant based on fasting plasma glucose and insulin levels, and glucose and insulin tolerance tests. Our data do not support the concept that TNF, acting via its receptors, is a major contributor to obesity-associated insulin resistance. In fact, data suggest that the two TNF receptors work in concert to protect against diabetes.     

The fat mouse: a powerful genetic model to study elevated plasminogen activator inhibitor 1 in obesity/NIDDM

Plasminogen activator inhibitor-1 is elevated in obesity and may be a risk factor for obesity/NIDDM related cardiovascular disease. In spite of this, little is known about the tissue and cellular origin of elevated PAI-1 in obesity or of the mediators and molecular mechanisms that regulate it. We have begun to systematically address these issues using genetically obese (ob/ob, db/db) mice. Plasma PAI-1 levels were 5-fold higher in obese mice compared to their lean counterparts. Subsequent RT-PCR and in situ hybridization studies suggest that the increased plasma PAI-1 originates primarily from the adipocyte in response to chronically elevated levels of tumor necrosis factor-alpha (TNF-alpha), insulin, and transforming growth factor-beta (TGF-beta). Thus, the signals and mechanisms that lead to elevated plasma PAI-1 observed in obesity are complex, and appear to involve interactions between multiple mediators and the adipose tissue itself.     

Neonatal 6-hydroxydopamine alters the behavior of enriched-impoverished rats in a novel test environment.

The hypothesis that neonatal norepinephrine (NE) depletion lessens the behavioral consequences of differential housing was tested. Male Wistar rats were injected with 6-hydroxydopamine (6-OHDA) or vehicle twice within 24 hr of birth, weaned at 25 days, and reared under either impoverished (IC) or enriched conditions (EC) for 30 days. In 3 experiments, rats were tested in the Morris water maze, the colony-intruder test, and 2 tests of dominance. 6-OHDA treatment reduced cortical and hypothalamic NE concentrations and increased brainstem NE concentrations. EC housing increased cortical dopamine (DA). Behavioral differences caused by postweaning enrichment–isolation were reduced by neonatal NE depletion, primarily in early test trials. The authors conclude that forebrain NE afferents from the locus coeruleus are important for housing-related behavioral changes and responsivity to novel testing environments. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Leptin rapidly suppresses insulin release from insulinoma cells, rat and human islets and, in vivo, in mice

Obesity is associated with diabetes, and leptin is known to be elevated in obesity. To investigate whether leptin has a direct effect on insulin secretion, isolated rat and human islets and cultured insulinoma cells were studied. In all cases, mouse leptin inhibited insulin secretion at concentrations within the plasma range reported in humans. Insulin mRNA expression was also suppressed in the cultured cells and rat islets. The long form of the leptin receptor (OB-Rb) mRNA was present in the islets and insulinoma cell lines. To determine the significance of these findings in vivo, normal fed mice were injected with two doses of leptin. A significant decrease in plasma insulin and associated rise in glucose concentration were observed. Fasted normal and leptin receptor-deficient db/db mice showed no response to leptin. A dose of leptin, which mimicked that found in normal mice, was administered to leptin-deficient, hyperinsulinemic ob/ob mice. This caused a marked lowering of plasma insulin concentration and a doubling of plasma glucose. Thus, leptin has a powerful acute inhibitory effect on insulin secretion. These results suggest that the action of leptin may be one mechanism by which excess adipose tissue could acutely impair carbohydrate metabolism.     

Effects of intracisternal 6-hydroxydopamine treatment on acquisition and performance of rats in a double T-maze.

136 male Sprague-Dawley rats in 4 experiments were subjected to various treatments with 6-hydroxydopamine (6-OHDA) to produce decrements in brain catecholamine content either before or after learning to respond in an appetitively motivated double –T maze task. Intracisternal injections of 6-OHDA not only impaired acquisition of the required behavioral response but also decreased performance of Ss which had previously acquired the task. Although reduced food consumption found in 6-OHDA-treated Ss may contribute to the observed deficits in –T maze responding, the behavioral deficit produced by 6-OHDA injection did not seem to be due only to a simple decrease in food intake. The decrements in acquisition and performance were clearly related to amount of central catecholamine depletion produced by 6-OHDA treatment. Further analysis suggested that the behavioral deficits were more related to the reductions in dopamine than they were to the depletion of brain norepinephrine. (33 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Dopamine in the lateral caudate-putamen of the rat is essential for somatosensory orientation.

Two studies examined the possible localization of somatosensory orientation in the caudate putamen (CP) of the rat. In Exp I 6-hydroxydopamine (6-OHDA [6 μg]) was injected into either the anterodorsal (AD), anteroventral (AV), posterodorsal (PD), or posteroventral (PV) CP of 19 Long-Evans and 39 Sprague-Dawley rats. Only Ss with PV-CP 6-OHDA injections showed impaired orientation scores. However, these PV injections often caused widespread CP dopamine (DA) depletions, and no specific CP region appeared to be particularly associated with somatosensory orientation. In Exp II, with 32 male Sprague-Dawley rats, multiple injections of 6-OHDA were directed toward the medial or lateral CP to assess their relative contributions directly. DA depletions confined to the lateral (but not medial) CP resulted in orientation deficits; these deficits were greater than would be predicted from the volume of CP/DA loss. The magnitude of the DA fluorescence loss in the lateral CP was more highly correlated with the orientation impairment than was the medial CP fluorescence loss. Thus, the lateral CP contributes to sensorimotor functions to a greater extent than does the medial CP, but the volume of CP/DA depletion also appears to be important. (31 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Insulin depletion leads to adipose-specific cell death in obese but not lean mice

Mutation of the obese gene produces obesity, hyperinsulinemia, and compensatory "overexpression" of the defective gene. As insulin activates obese gene expression, it seemed possible that hyperinsulinemia might be responsible for overexpression of the gene. To address this question we rapidly neutralized circulating insulin by injection of an insulin antibody. Unexpectedly, insulin depletion in obese (ob/ob or db/db) mice caused massive adipose RNA degradation confirmed by histological analysis to result from adipocyte cell death by a largely necrotic mechanism. This effect was not observed in lean littermates and was completely corrected by coadministration of insulin. Comparison of multiple tissues demonstrated that the effect was restricted to adipose tissue. Insulin depletion in obese mice by administration of streptozotocin also led to cell death, but this death was less extensive and appeared to be apoptotic in mechanism. Thus insulin may promote the survival side of the physiological balance between adipocyte survival and death.     

Effects of aging and a high fat diet on body weight and glucose tolerance in glucagon-like peptide-1 receptor -/- mice

Disruption of glucagon-like peptide-1 (GLP-1) receptor signaling in mice results in mild glucose intolerance, principally due to elimination of the incretin effect of GLP-1. Despite the inhibitory effects of GLP-1 on food intake, 6- to 8-week-old GLP-1 receptor -/-(GLP-1R-/-) mice were not obese and did not exhibit disturbances of feeding behavior. As both diabetes and obesity frequently become more phenotypically evident in older rodents, we studied the consequences of aging and a high fat diet on glucose control and body weight in GLP-1R-/- mice. No evidence of obesity or deterioration in glucose control was detected in 11- and 16-month-old GLP-1R-/- mice (mean weight, 34.7 +/- 2.0, 30.5 +/- 1.5, and 34.6 +/- 2.8 g in male and 25.3 +/-1.6, 28.4 +/-1.2, and 31.9 +/- 2.9 g in female GLP-1R+/+, GLP-1R+/-, and GLP-1R-/- mice, respectively; P = NS). After 18 weeks of high fat feeding, GLP-1R-/- mice gained similar (males) or less (females) weight than age- and sex-matched CD1 controls. No significant deterioration in glucose tolerance was observed after high fat feeding in GLP-1R-/- mice. These observations demonstrate that long term disruption of GLP-1 signaling in the central nervous system and peripheral tissues of older mice is not associated with the development of obesity or deterioration in glucose homeostasis.     

Hyperphagia and weight gain after gold-thioglucose: relation to hypothalamic neuropeptide Y and proopiomelanocortin

Genetic obesity is associated with increased neuropeptide Y (NPY) messenger RNA (mRNA) and decreased POMC mRNA in the hypothalamus of ob/ob and db/db mice, or impaired sensitivity to alphaMSH (derived from POMC) in the yellow agouti mouse. Acquired obesity can be produced by chemically lesioning the hypothalamus with either monosodium glutamate (MSG) in neonates or gold thioglucose (GTG) in adult mice. The present study examined whether elevated NPY mRNA and/or decreased POMC mRNA in the hypothalamus are associated with obesity due to hypothalamic lesions. GTG injection into adult mice produced a profound obese phenotype, including hyperphagia, increased body weight, and increased leptin mRNA and peptide, in association with reduced hypothalamic NPY mRNA and POMC mRNA. MSG treatment produced virtual elimination of NPY mRNA in the arcuate nucleus and a reduction of hypothalamic POMC mRNA, and led to elevated leptin. MSG pretreatment did not attenuate GTG-induced hyperphagia and obese phenotype. These results do not support a role for NPY-synthesizing neurons in the arcuate nucleus in mediating hypothalamic acquired obesity, but are consistent with the hypothesis that decreased activity of hypothalamic neurons synthesizing POMC play a role in mediating hypothalamic obesity.