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Denis G Baskin

Publications and source records attributed to Denis G Baskin.

At least 19 recordsLinked to original sources
PubMed2006

Distribution of insulin receptor substrate-2 in brain areas involved in energy homeostasis.

Body weight regulation depends on neuronal signaling by adiposity-related hormones such as insulin and leptin. Activation of receptors for these hormones induces cell signaling via the insulin receptor substrate/phosphatidylinositol 3-kinase (IRS-PI3K) pathway, and growing evidence from knockout models implicates IRS-2 as a key component of this signal transduction mechanism. As a first step towards the identification of brain areas that utilize IRS-PI3K signaling in the control of energy homeostasis, we used immunohistochemical techniques to investigate the neuronal distribution of IRS-2 protein in rat brain. In the hypothalamus, strong IRS-2 staining was detected chiefly in the arcuate (ARC), ventromedial (VMN) nucleus and parvocellular paraventricular nucleus (PVN). Within the ARC, IRS-2 was co-localized with alpha melanocyte stimulating hormone (alpha-MSH) as well as neuropeptide Y (NPY). In the hindbrain, IRS-2 staining was detected in the area postrema (AP), medial nucleus of the solitary tract (mNTS), dorsal motor nucleus of the vagus nerve (DMV) and the hypoglossal nucleus (HN). Co-localization studies in the mNTS demonstrated the presence of IRS-2 in catecholamine neurons. IRS-2 protein was also found in the ventral tegmental area (VTA), an important area for reward perception, and was detected in dopamine neurons in this brain area. In summary, neurons containing IRS-2 immunoreactivity were identified in forebrain, midbrain and hindbrain areas and in cell types that are crucial for the control of food intake and autonomic function. An improved understanding of mechanisms underlying normal and abnormal energy homeostasis may be gained by analysis of the role played by signaling through IRS-2 in these brain areas.

Animals↗
PubMed2006

Leptin regulation of the anorexic response to glucagon-like peptide-1 receptor stimulation.

Leptin reduces food intake in part by enhancing satiety responses to gastrointestinal signals produced in response to food consumption. Glucagon-like peptide 1 (GLP-1), secreted by the intestine when nutrients enter the gut, is one such putative satiety signal. To investigate whether leptin enhances the anorexic effects of GLP-1, rats received either saline or a subthreshold dose of leptin before intraperitoneal injection of either GLP-1 or Exendin-4 (Ex4; a GLP-1 receptor agonist). Leptin pretreatment strongly enhanced anorexia and weight loss induced by GLP-1 or Ex4 over 24 h. Conversely, fasting attenuated the anorexic response to GLP-1 or Ex4 treatment via a leptin-dependent mechanism, as demonstrated by our finding that the effect of fasting was reversed by physiological leptin replacement. As expected, Ex4 induced expression of c-Fos protein, a marker of neuronal activation, in hindbrain areas that process afferent input from satiety signals, including the nucleus of the solitary tract and area postrema. Unexpectedly, leptin pretreatment blocked this response. These findings identify physiological variation of plasma leptin levels as a potent regulator of GLP-1 receptor-mediated food intake suppression and suggest that the underlying mechanism is distinct from that which mediates interactions between leptin and other satiety signals.

Animals↗
PubMed2005

Effects of hypothalamic neurodegeneration on energy balance.

Normal aging in humans and rodents is accompanied by a progressive increase in adiposity. To investigate the role of hypothalamic neuronal circuits in this process, we used a Cre-lox strategy to create mice with specific and progressive degeneration of hypothalamic neurons that express agouti-related protein (Agrp) or proopiomelanocortin (Pomc), neuropeptides that promote positive or negative energy balance, respectively, through their opposing effects on melanocortin receptor signaling. In previous studies, Pomc mutant mice became obese, but Agrp mutant mice were surprisingly normal, suggesting potential compensation by neuronal circuits or genetic redundancy. Here we find that Pomc-ablation mice develop obesity similar to that described for Pomc knockout mice, but also exhibit defects in compensatory hyperphagia similar to what occurs during normal aging. Agrp-ablation female mice exhibit reduced adiposity with normal compensatory hyperphagia, while animals ablated for both Pomc and Agrp neurons exhibit an additive interaction phenotype. These findings provide new insight into the roles of hypothalamic neurons in energy balance regulation, and provide a model for understanding defects in human energy balance associated with neurodegeneration and aging.

Agouti-Related Protein↗
PubMed2005

Leptin action in the forebrain regulates the hindbrain response to satiety signals.

The capacity to adjust energy intake in response to changing energy requirements is a defining feature of energy homeostasis. Despite the identification of leptin as a key mediator of this process, the mechanism whereby changes of body adiposity are coupled to adaptive, short-term adjustments of energy intake remains poorly understood. To investigate the physiological role of leptin in the control of meal size and the response to satiety signals, and to identify brain areas mediating this effect, we studied Koletsky (fa(k)/fa(k)) rats, which develop severe obesity due to the genetic absence of leptin receptors. Our finding of markedly increased meal size and reduced satiety in response to the gut peptide cholecystokinin (CCK) in these leptin receptor-deficient animals suggests a critical role for leptin signaling in the response to endogenous signals that promote meal termination. To determine if the hypothalamic arcuate nucleus (ARC) (a key forebrain site of leptin action) mediates this leptin effect, we used adenoviral gene therapy to express either functional leptin receptors or a reporter gene in the area of the ARC of fa(k)/fa(k) rats. Restoration of leptin signaling to this brain area normalized the effect of CCK on the activation of neurons in the nucleus of the solitary tract and area postrema, key hindbrain areas for processing satiety-related inputs. This intervention also reduced meal size and enhanced CCK-induced satiety in fa(k)/fa(k) rats. These findings demonstrate that forebrain signaling by leptin, a long-term regulator of body adiposity, limits food intake on a meal-to-meal basis by regulating the hindbrain response to short-acting satiety signals.

Animals↗
PubMed2005

Insulin signaling in the central nervous system: a critical role in metabolic homeostasis and disease from C. elegans to humans.

Insulin and its signaling systems are implicated in both central and peripheral mechanisms governing the ingestion, distribution, metabolism, and storage of nutrients in organisms ranging from worms to humans. Input from the environment regarding the availability and type of nutrients is sensed and integrated with humoral information (provided in part by insulin) regarding the sufficiency of body fat stores. In response to these afferent inputs, neuronal pathways are activated that influence energy flux and nutrient metabolism in the body and ensure reproductive competency. Growing evidence supports the hypothesis that reduced central nervous system insulin signaling from either defective secretion or action contributes to the pathogenesis of common metabolic disorders, including diabetes and obesity, and may therefore help to explain the close association between these two disorders. These considerations implicate insulin action in the brain, an organ previously considered to be insulin independent, as a key determinant of both glucose and energy homeostasis.

Animals↗
PubMed2004

Evidence that paraventricular nucleus oxytocin neurons link hypothalamic leptin action to caudal brain stem nuclei controlling meal size.

Hindbrain projections of oxytocin neurons in the parvocellular paraventricular nucleus (pPVN) are hypothesized to transmit leptin signaling from the hypothalamus to the nucleus of the solitary tract (NTS), where satiety signals from the gastrointestinal tract are received. Using immunocytochemistry, we found that an anorectic dose of leptin administered into the third ventricle (3V) increased twofold the number of pPVN oxytocin neurons that expressed Fos. Injections of fluorescent cholera toxin B into the NTS labeled a subset of pPVN oxytocin neurons that expressed Fos in response to 3V leptin. Moreover, 3V administration of an oxytocin receptor antagonist, [d-(CH2)5,Tyr(Me)2,Orn8]-vasotocin (OVT), attenuated the effect of leptin on food intake over a 0.5- to 4-h period (P < 0.05). Furthermore, to determine whether oxytocin contributes to leptin's potentiation of Fos activation within NTS neurons in response to CCK, we counted the number of Fos-positive neurons in the medial NTS (mNTS) after 3V administration of OVT before 3V leptin and intraperitoneal CCK-8 administration. OVT resulted in a significant 37% decrease (P < 0.05) in the potentiating effect of leptin on CCK activation of mNTS neuronal Fos expression. Furthermore, 4V OVT stimulated 2-h food intake by 43% (P < 0.01), whereas 3V OVT at the same dose was ineffective. These findings suggest that release of oxytocin from a descending pPVN-to-NTS pathway contributes to leptin's attenuation of food intake by a mechanism that involves the activation of pPVN oxytocin neurons by leptin, resulting in increased sensitivity of NTS neurons to satiety signals.

Animals↗
PubMed2004

Insulin and its evolving partnership with leptin in the hypothalamic control of energy homeostasis.

Despite an alarming increase in the burden of obesity worldwide, body adiposity seems to be a regulated physiological variable. Regulation of adiposity occurs through a classical endocrine feedback loop, in which the pancreatic beta-cell-derived hormone insulin and the adipocyte-derived hormone leptin signal the status of body energy stores to the hypothalamus. Recent advances in our understanding of the signal transduction mechanisms used by insulin and leptin in the hypothalamus to modulate neuronal firing suggest that intracellular cross-talk occurs at several levels and is a potentially important determinant of regulated body weight. These pathways are thus an attractive target for pharmacological intervention in the treatment of obesity.

Animals↗
PubMed2003

Oxytocin innervation of caudal brainstem nuclei activated by cholecystokinin.

The integration of 'long-term' adiposity signaling with the 'short-term' meal-related signal cholecystokinin (CCK) is proposed to involve descending hypothalamic projections to areas of the caudal brainstem (CBS) that regulate the amount of food consumed during a single meal. One such projection extends from cell bodies in the hypothalamic paraventricular nucleus (PVN) to the nucleus tractus solitarius (NTS), where cells that respond to peripheral CCK are concentrated. Candidate neuronal cell types that may comprise this PVN-NTS projection includes those expressing corticotropin-releasing hormone (CRH) or oxytocin. We therefore sought to determine whether oxytocin or CRH axons are preferentially located in close anatomical proximity to neurons of the NTS that are activated by peripheral administration of CCK, as determined by immunocytochemical staining for Fos protein. Rats received injections of either an anorexic dose of CCK (8 nmol/kg, i.p.) or vehicle and were perfused 2 h later with 4% paraformaldehyde. Immunocytochemistry was performed on cryostat sections (14 microm) of caudal brainstem, using a polyclonal antibody to Fos protein and either a monoclonal antibody to oxytocin or a polyclonal antibody to CRH. As expected, CCK administration significantly increased the numbers of Fos-positive neurons by 489% (p<0.01) and 400% (p<0.01), respectively, in the medial and gelatinosus subdivisions of the NTS. These same regions received dense oxytocin axon innervation, whereas CRH immunoreactivity was not as prevalent in these areas. In areas outside the NTS, such as the dorsal motor nucleus of the vagus (DMV), Fos activation was absent despite a dense oxytocin and CRH innervation. To investigate whether CCK-induced reductions of food intake require intact oxytocin signaling, we performed a separate study in which CCK injection was preceded by injection into the fourth ventricle of an oxytocin receptor antagonist [d(CH(2))(5), Tyr (Me)(2), Orn(8)]-vasotocin (OVT). This study showed CCK was 23% and 22% less effective at inhibiting food intake at 30 min (p<0.05) and 1 h (p<0.05) food intake, respectively, in the presence of OVT. Taken together, the data indicate that oxytocin axons within the descending pathway from the PVN to the NTS are anatomically positioned to interact with NTS neurons that respond to vagally mediated peripheral CCK signals such as those that occur following ingestion of a meal. These findings support the hypothesis that oxytocin exerts a tonic stimulatory effect on the response of key neurons within the NTS to CCK and further reduce meal size.

Analysis of Variance↗
PubMed2003

Insulin secretory deficiency and glucose intolerance in Rab3A null mice.

Insulin secretory dysfunction of the pancreatic beta-cell in type-2 diabetes is thought to be due to defective nutrient sensing and/or deficiencies in the mechanism of insulin exocytosis. Previous studies have indicated that the GTP-binding protein, Rab3A, plays a mechanistic role in insulin exocytosis. Here, we report that Rab3A(-/-) mice develop fasting hyperglycemia and upon a glucose challenge show significant glucose intolerance coupled to ablated first-phase insulin release and consequential insufficient insulin secretion in vivo, without insulin resistance. The in vivo insulin secretory response to arginine was similar in Rab3A(-/-) mice as Rab3A(+/+) control animals, indicating a phenotype reminiscent of insulin secretory dysfunction found in type-2 diabetes. However, when a second arginine dose was given 10 min after, there was a negligible insulin secretory response in Rab3A(-/-) mice, compared with that in Rab3A(+/+) animals, that was markedly increased above that to the first arginine stimulus. There was no difference in beta-cell mass or insulin production between Rab3A(-/-) and Rab3A(+/+) mice. However, in isolated islets, secretagogue-induced insulin release (by glucose, GLP-1, glyburide, or fatty acid) was approximately 60-70% lower in Rab3A(-/-) islets compared with Rab3A(+/+) controls. Nonetheless, there was a similar rate of glucose oxidation and glucose-induced rise in cytosolic [Ca(2+)](i) flux between Rab3A(-/-) and Rab3A(+/+) islet beta-cells, indicating the mechanistic role of Rab3A lies downstream of generating secondary signals that trigger insulin release, at the level of secretory granule transport and/or exocytosis. Thus, Rab3A plays an important in vivo role facilitating the efficiency of insulin exocytosis, most likely at the level of replenishing the ready releasable pool of beta-granules. Also, this study indicates, for the first time, that the in vivo insulin secretory dysfunction found in type-2 diabetes can lie solely at the level of defective insulin exocytosis.

Animals↗
PubMed2003

Binding of interleukin-8 to heparan sulfate and chondroitin sulfate in lung tissue.

Interleukin (IL)-8, a member of the CXC chemokine family, is a potent neutrophil chemotactic factor. Mechanisms that regulate the activity of chemokines in tissue are not clear. The goal of this study was to determine whether IL-8-glycosaminoglycan interactions are responsible for the binding of IL-8 in lung tissue. Experiments were performed with a quantitative tissue-binding assay to measure the amount of 125I-IL-8 binding and an in situ tissue-binding assay to characterize the location of IL-8 binding in lung tissue. Confocal microscopy demonstrated IL-8 binding to specific anatomic locations such as cell surfaces and extracellular matrix that were enriched with heparan sulfate and chondroitin sulfate. Removal of heparan sulfate or chondroitin sulfate from lung tissue significantly decreased the binding of 125I-IL-8. Two forms of IL-8 with single amino acid mutations in the glycosaminoglycan-binding domain showed decreased binding. In addition, studies with normal and monomeric IL-8 showed that dimerization increased the binding of 125I-IL-8 in lung tissue. These findings suggest that IL-8-glycosaminoglycan interactions determine the location where IL-8 binds in lung tissue and provides a site for the dimerization of IL-8, which increases the local concentration of IL-8 in the lungs.

Chondroitin Sulfates↗
PubMed2003

Immunocytochemical detection of phosphatidylinositol 3-kinase activation by insulin and leptin.

Intracellular signaling mediated by phosphatidylinositol 3-kinase (PI3K) is important for a number of cellular processes and is stimulated by a variety of hormones, including insulin and leptin. A histochemical method for assessment of PI3K signaling would be an important advance in identifying specific cells in histologically complex organs that are regulated by growth factors and peptide hormones. However, current methods for detecting PI3K activity require either homogenization of the tissue or cells or the ability to transfect probes that bind to phosphatidylinositol 3,4,5 trisphosphate (PIP3), the reaction product of PI3K catalysis. Here we report the validation of an immunocytochemical method to detect changes in PI3K activity, using a recently developed monoclonal antibody to PIP3, in paraformaldehyde-fixed bovine aortic endothelial cells (BAECs) in culture and in hepatocytes of intact rat liver. Treatment with either insulin or leptin increased BAEC PIP3 immunoreactivity, and these effects were blocked by pretreatment with PI3K inhibitors. Furthermore, infusion of insulin into the hepatic portal vein of fasted rats caused an increase of PIP3 immunostaining in hepatocytes that was associated with increased serine phosphorylation of the downstream signaling molecule protein kinase B/Akt (PKB/Akt). We conclude that immunocytochemical PIP3 staining can detect changes in PI3K activation induced by insulin and leptin in cell culture and intact liver.

Animals↗
PubMed2003

Arcuate nucleus-specific leptin receptor gene therapy attenuates the obesity phenotype of Koletsky (fa(k)/fa(k)) rats.

Leptin signaling in the hypothalamic arcuate nucleus (ARC) is hypothesized to play an important role in energy homeostasis. To investigate whether leptin signaling limited to this brain area is sufficient to reduce food intake and body weight, we used adenoviral gene therapy to express the signaling isoform of the leptin receptor, lepr(b), in the ARC of leptin receptor-deficient Koletsky (fa(k)/fa(k)) rats. Successful expression of adenovirus containing lepr(b) (Ad-lepr(b)) selectively in the ARC was documented by in situ hybridization. Using real-time PCR, we further demonstrated that bilateral microinjection of Ad-lepr(b) into the ARC restored low hypothalamic levels of lepr(b) mRNA to values approximating those of wild-type (Fa(k)/Fa(k)) controls. Restored leptin receptor expression in the ARC reduced both mean daily food intake (by 13%) and body weight gain (by 33%) and increased hypothalamic proopiomelanocortin mRNA by 65% while decreasing neuropeptide Y mRNA levels by 30%, relative to fa(k)/fa(k) rats injected with a control adenovirus (Ad-lacZ) (P < 0.05 for each comparison). In contrast, Ad-lepr(b) delivery to either the lateral hypothalamic area of fa(k)/fa(k) rats or to the ARC of wild-type Fa(k)/Fa(k) rats had no effect on any of these parameters. These findings collectively support the hypothesis that leptin receptor signaling in the ARC is sufficient to mediate major effects of leptin on long-term energy homeostasis. Adenoviral gene therapy is thus a viable strategy with which to study the physiological importance of specific molecules acting in discrete brain areas.

Adenoviridae↗
PubMed2003

Insulin and the blood-brain barrier.

Although several possible mechanisms exist by which the pancreatic hormone, insulin, could enter the brain from the blood, most evidence suggests that the majority of it enters primarily by a receptor-mediated transport process. Many factors influence the rate of entry, including fasting and refeeding and several pathological conditions. Within the brain insulin acts on specific receptors to influence a number of behaviors, and especially caloric homeostasis and cognition.

Animals↗
PubMed2003

Insulin activation of phosphatidylinositol 3-kinase in the hypothalamic arcuate nucleus: a key mediator of insulin-induced anorexia.

In peripheral tissues, insulin signaling involves activation of the insulin receptor substrate (IRS)-phosphatidylinositol 3-kinase (PI3K) enzyme system. In the hypothalamus, insulin functions with leptin as an afferent adiposity signal important for the regulation of body fat stores and hepatic glucose metabolism. To test the hypothesis that hypothalamic insulin action involves intracellular PI3K signaling, we used histochemical and biochemical methods to determine the effect of insulin on hypothalamic IRS-PI3K activity. Here, we report that insulin induces tyrosine phosphorylation of the insulin receptor and IRS-1 and -2, increases binding of activated IRS-1 and -2 to the regulatory subunit of PI3K, and activates protein kinase B/Akt, a downstream target of PI3K. Using an immunohistochemical technique to detect PI 3,4,5-triphosphate, the main product of PI3K activity, we further demonstrate that in the arcuate nucleus, insulin-induced PI3K activity occurs preferentially within cells that contain IRS-2. Finally, we show that the food intake- lowering effects of insulin are reversed by intracerebroventricular infusion of either of two PI3K inhibitors at doses that have no independent feeding effects. These findings support the hypothesis that the IRS-PI3K pathway is a mediator of insulin action in the arcuate nucleus and, combined with recent evidence that leptin activates PI3K signaling in the hypothalamus, provide a plausible mechanism for neuronal cross-talk between insulin and leptin signaling.

Animals↗
PubMed2003

Is the energy homeostasis system inherently biased toward weight gain?

We describe a model of energy homeostasis to better understand neuronal pathways that control energy balance and their regulation by hormonal signals such as insulin and leptin. Catabolic neuronal pathways are those that both reduce food intake and increase energy expenditure (e.g., melanocortin neurons in the hypothalamic arcuate nucleus) and are stimulated by input from insulin and leptin. We propose that in the basal state, catabolic effectors are activated in response to physiological concentrations of leptin and insulin, and that this activation is essential to prevent excessive weight gain. In contrast, anabolic pathways (e.g., neurons containing neuropeptide Y) are those that stimulate food intake and decrease energy expenditure and are strongly inhibited by these same basal concentrations of insulin and leptin. In the basal state, therefore, catabolic effector pathways are activated while anabolic effector pathways are largely inhibited. The response to weight loss includes both activation of anabolic and inhibition of catabolic pathways and is, thus, inherently more vigorous than the response to weight gain (stimulation of already-activated catabolic pathways and inhibition of already-suppressed anabolic pathways). Teleological, molecular, physiological, and clinical aspects of this hypothesis are presented, along with a discussion of currently available supporting evidence.

Adipose Tissue↗
PubMed2002

Apolipoprotein E mediates the retention of high-density lipoproteins by mouse carotid arteries and cultured arterial smooth muscle cell extracellular matrices.

Lipoprotein retention in the vascular extracellular matrix (ECM) plays a critical role in atherogenesis. Previous studies demonstrated the presence of apo A-I and E in atherosclerotic lesions, suggesting that HDL may be trapped by the artery wall. We sought to determine mechanisms by which HDL could be bound and retained by the arterial wall, and whether apo E was a principal determinant of this binding. We evaluated in situ accumulation of fluorescently labeled DiI-human HDL+/-apo E in perfused carotid arteries from apo E-null mice. Apo E was important in mediating HDL binding to the vascular wall, with a 48+/-16% increase in accumulation of DiI-labeled apo E-containing HDL (HDL3+E) compared with DiI-apo E-free HDL (HDL3-E) (P=0.003). To investigate possible mechanisms responsible for retention, we assessed binding of unlabeled HDL3-E and HDL3+E to ECM generated by cultured arterial smooth muscle cells. Similar to the in situ carotid artery data, HDL3+E bound better to the ECM than did HDL3-E (3-fold lower K(a) and 3.5-fold higher B(max) for HDL3+E versus HDL3-E). These differences were eliminated after either neutralization of arginine residues on apo E or digestion of matrix with chondroitin ABC lyase, suggesting that chondroitin and/or dermatan sulfate proteoglycans were responsible for apo E-mediated increased binding. These findings demonstrate that HDL can bind to both intact murine carotid arteries and smooth muscle cell-derived ECM, and that apo E is a principal determinant in mediating the ability of HDL to be trapped and retained via its interaction with ECM proteoglycans.

Animals↗