Glucose and insulin increase the transport of leptin through the blood-brain barrier in normal mice but not in streptozotocin-diabetic mice

The neurovascular unit in healthy and injured spinal cord

The neurovascular unit (NVU) reflects the close temporal and spatial link between neurons and blood vessels. However, the understanding of the NVU in the spinal cord is far from clear and largely based on generalized knowledge obtained from the brain. Herein, we review the present knowledge of the NVU and highlight candidate approaches to investigate the NVU, particularly focusing on the spinal cord. Several unique features maintain the highly regulated microenvironment in the NVU. Autoregulation and neurovascular coupling ensure regional blood flow meets the metabolic demand according to the blood supply or local neural activation. The blood-central nervous system barrier partitions the circulating blood from neural parenchyma and facilitates the selective exchange of substances. Furthermore, we discuss spinal cord injury (SCI) as a common injury from the perspective of NVU dysfunction. Hopefully, this review will help expand the understanding of the NVU in the spinal cord and inspire new insights into SCI.

Leptin and the Blood-Brain Barrier: Curiosities and Controversies

Leptin for over 25 years has been a central theme in the study of appetite, obesity, and starvation. As the major site of leptin production is peripheral, and the site of action of greatest interest is the hypothalamus, how leptin accesses the central nervous system (CNS) and crosses the blood-brain barrier (BBB) has been of great interest. We review here the ongoing research that addresses fundamental questions such as the sites of leptin resistances in obesity and other conditions, the causes of resistances and their relations to one another, the three barrier sites of entry into the CNS, why recent studies using suprapharmacological doses cannot address these questions but give insight into nonsaturable entry of leptin into the CNS, and how that might be useful in using leptin therapeutically. The current status of the controversy of whether the short form of the leptin receptor acts as the BBB leptin transporter and how obesity may transform leptin transport is reviewed. Review of these and other topics summarizes in a new appreciation of what leptin may have actually evolved to do and what physiological role leptin resistance may play. © 2021 American Physiological Society. Compr Physiol 11:1-19, 2021.

Hormone levels in preterm and donor human milk before and after Holder pasteurization

BACKGROUND

After birth, breastfeeding is the exclusive source of hormonal signaling between mother and infant. Hospitalized infants often receive donor milk when their own mother's milk is unavailable.

METHODS

The presence of insulin, leptin, cortisol, progesterone, and testosterone was examined in samples from milk bank donors and mothers of preterm infants. We further investigated the effect of Holder pasteurization (HoP) on hormone levels.

RESULTS

Comparing nonpasteurized samples, leptin levels were nearly threefold higher in milk from mothers of preterm infants versus donated milk, and regardless of milk source, leptin levels were significantly decreased by HoP. Insulin concentrations were also decreased by HoP, and among mothers of preterm infants, obesity was associated with significantly higher content of leptin and insulin. While combined use of donor milk and HoP was associated with cortisol levels nearly threefold higher than those in nonpasteurized own mother's milk, progesterone and testosterone content did not differ by source or pasteurization.

CONCLUSIONS

The hormonal composition of breast milk is impacted by HoP and maternal obesity. Compared to nonpasteurized maternal milk, use of pasteurized donor milk dramatically decreases the intake of leptin while increasing the intake of cortisol. Further research is necessary to define optimal breast milk processing practices.

The Promising Role of Probiotics in Managing the Altered Gut in Autism Spectrum Disorders

Gastrointestinal symptoms (GIS) have been reported repeatedly in people with autism spectrum disorder (ASD) and studies have reported interesting correlations between severity of behavioral and gastrointestinal symptoms. Growing evidence indicates that the gut microbiota in ASD is altered with various shifts described at different taxonomic levels, pointing to the importance of considering the gut-brain axis in treatment of these disorders. Probiotics are live beneficial bacteria that are ingested as food or customized pills. These beneficial bacteria, when added in sufficient amounts, can correct the dysbiosis. Because probiotics have shown success in treating irritable bowel syndrome (IBS), it is plausible to investigate whether they can induce alleviation of behavioral symptoms as well. Probiotics show, in some clinical studies, their potential benefits (1) in improving gastrointestinal dysfunction, (2) in correcting dysbiosis, (3) in consequently reducing the severity of ASD symptoms. This review compiles data from selected studies that investigate these benefits and the mechanisms that mediate these effects, which include the production of metabolites, hormones, and neurotransmitters and the regulation of pro-inflammatory and regulatory cytokines. Future research based on more randomized, controlled studies with a larger population size and standardized use of strains, concentration of probiotics, duration of treatments, and methods of DNA extraction is still needed in this area, which may lead to more robust results.

ApoE and cerebral insulin: Trafficking, receptors, and resistance

Central nervous system (CNS) insulin resistance is associated with Alzheimer's disease (AD). In addition, the apolipoprotein E4 (apoE4) isoform is a risk factor for AD. The connection between these two factors in relation to AD is being actively explored. We summarize this literature with a focus on the transport of insulin and apoE across the blood-brain barrier (BBB) and into the CNS, the impact of apoE and insulin on the BBB, and the interactions between apoE, insulin, and the insulin receptor once present in the CNS. We highlight how CNS insulin resistance is apparent in AD and potential ways to overcome this resistance by repurposing currently approved drugs, with apoE genotype taken into consideration as the treatment response following most interventions is apoE isoform-dependent. This review is part of a special issue focusing on apoE in AD and neurodegeneration.

Emerging Roles of Vascular Endothelium in Metabolic Homeostasis

Our understanding of the role of the vascular endothelium has evolved over the past 2 decades, with the recognition that it is a dynamically regulated organ and that it plays a nodal role in a variety of physiological and pathological processes. Endothelial cells (ECs) are not only a barrier between the circulation and peripheral tissues, but also actively regulate vascular tone, blood flow, and platelet function. Dysregulation of ECs contributes to pathological conditions such as vascular inflammation, atherosclerosis, hypertension, cardiomyopathy, retinopathy, neuropathy, and cancer. The close anatomic relationship between vascular endothelium and highly vascularized metabolic organs/tissues suggests that the crosstalk between ECs and these organs is vital for both vascular and metabolic homeostasis. Numerous reports support that hyperlipidemia, hyperglycemia, and other metabolic stresses result in endothelial dysfunction and vascular complications. However, how ECs may regulate metabolic homeostasis remains poorly understood. Emerging data suggest that the vascular endothelium plays an unexpected role in the regulation of metabolic homeostasis and that endothelial dysregulation directly contributes to the development of metabolic disorders. Here, we review recent studies about the pivotal role of ECs in glucose and lipid homeostasis. In particular, we introduce the concept that the endothelium adjusts its barrier function to control the transendothelial transport of fatty acids, lipoproteins, LPLs (lipoprotein lipases), glucose, and insulin. In addition, we summarize reports that ECs communicate with metabolic cells through EC-secreted factors and we discuss how endothelial dysregulation contributes directly to the development of obesity, insulin resistance, dyslipidemia, diabetes mellitus, cognitive defects, and fatty liver disease.

The blood-brain barrier as an endocrine tissue

The blood-brain barrier (BBB) was first noted for its ability to prevent the unregulated exchange of substances between the blood and the central nervous system (CNS). Over time, its characterization as an interface that enables regulated exchanges between the CNS and substances that are carried in the blood in a hormone-like fashion have emerged. Therefore, communication between the CNS, BBB and peripheral tissues has many endocrine-like properties. In this Review, I examine the various ways in which the BBB exhibits endocrine-related properties. The BBB is a target for hormones, such as leptin and insulin, that affect many of its functions. The BBB is also a secretory body, releasing substances either into the blood or the interstitial fluid of the brain. The BBB selectively allows classical and non-classical hormones entry to and exit from the CNS, thus allowing the CNS to be both an endocrine target and a secretory tissue. The BBB is affected by endocrine diseases such as diabetes mellitus and can cause or participate in endocrine diseases, including those related to thyroid hormones and obesity. The endocrine-like mechanisms of the BBB can extend the definition of endocrine disease to include neurodegenerative conditions, including Alzheimer disease, and of hormones to include cytokines, triglycerides and fatty acids.

Routes for the delivery of insulin to the central nervous system: A comparative review

Central nervous system (CNS) insulin resistance is a condition in which the cells within the CNS do not respond to insulin appropriately and is often linked to aberrant CNS insulin levels. CNS insulin is primarily derived from the periphery. Aberrant CNS insulin levels can arise due to various factors including i) decreased endogenous insulin transport into the brain, across the blood-brain barrier (BBB), ii) reduced CNS sequestration of insulin, and iii) increased CNS degradation. While the sole route of endogenous insulin transport into the brain is via the BBB, there are multiple therapeutic routes of administration that have been investigated to deliver exogenous insulin to the CNS. These alternative administrative routes can be utilized to increase the amount of CNS insulin and aid in overcoming CNS insulin resistance. This review focuses on the intravenous, intracerebroventricular, intranasal, ocular, and intrathecal routes of administration and compares the impact of insulin delivery.

Redefining neuroendocrinology: Epigenetics of brain-body communication over the life course

The brain is the central organ of stress and adaptation to stress that perceives and determines what is threatening, as well as the behavioral and physiological responses to the stressor, and it does so somewhat differently in males and females. The expression of steroid hormone receptors throughout the brain has broadened the definition of 'neuroendocrinology' to include the reciprocal communication between the entire brain and body via hormonal and neural pathways. Mediated in part via systemic hormonal influences, the adult and developing brain possess remarkable structural and functional plasticity in response to stress, including neuronal replacement, dendritic remodeling, and synapse turnover. This article is both an account of an emerging field elucidating brain-body interactions at multiple levels, from molecules to social organization, as well as a personal account of my laboratory's role and, most importantly, the roles of trainees and colleagues, along with my involvement in interdisciplinary groups working on this topic.

Blood-Brain Barriers in Obesity

After decades of rapid increase, the rate of obesity in adults in the USA is beginning to slow and the rate of childhood obesity is stabilizing. Despite these improvements, the obesity epidemic continues to be a major health and financial burden. Obesity is associated with serious negative health outcomes such as cardiovascular disease, type II diabetes, and, more recently, cognitive decline and various neurodegenerative dementias such as Alzheimer's disease. In the past decade, major advancements have contributed to the understanding of the role of the central nervous system (CNS) in the development of obesity and how peripheral hormonal signals modulate CNS regulation of energy homeostasis. In this article, we address how obesity affects the structure and function of the blood-brain barrier (BBB), the impact of obesity on Alzheimer's disease, the effects of obesity on circulating proteins and their transport into the brain, and how these changes can potentially be reversed by weight loss.

Non-Neuronal Cells in the Hypothalamic Adaptation to Metabolic Signals

Although the brain is composed of numerous cell types, neurons have received the vast majority of attention in the attempt to understand how this organ functions. Neurons are indeed fundamental but, in order for them to function correctly, they rely on the surrounding "non-neuronal" cells. These different cell types, which include glia, epithelial cells, pericytes, and endothelia, supply essential substances to neurons, in addition to protecting them from dangerous substances and situations. Moreover, it is now clear that non-neuronal cells can also actively participate in determining neuronal signaling outcomes. Due to the increasing problem of obesity in industrialized countries, investigation of the central control of energy balance has greatly increased in attempts to identify new therapeutic targets. This has led to interesting advances in our understanding of how appetite and systemic metabolism are modulated by non-neuronal cells. For example, not only are nutrients and hormones transported into the brain by non-neuronal cells, but these cells can also metabolize these metabolic factors, thus modifying the signals reaching the neurons. The hypothalamus is the main integrating center of incoming metabolic and hormonal signals and interprets this information in order to control appetite and systemic metabolism. Hence, the factors transported and released from surrounding non-neuronal cells will undoubtedly influence metabolic homeostasis. This review focuses on what is known to date regarding the involvement of different cell types in the transport and metabolism of nutrients and hormones in the hypothalamus. The possible involvement of non-neuronal cells, in particular glial cells, in physiopathological outcomes of poor dietary habits and excess weight gain are also discussed.

Degeneration of Dopaminergic Neurons Due to Metabolic Alterations and Parkinson's Disease

The rates of metabolic diseases, such as type 2 diabetes mellitus (T2DM), obesity, and cardiovascular disease (CVD), markedly increase with age. In recent years, studies have reported an association between metabolic changes and various pathophysiological mechanisms in the central nervous system (CNS) in patients with metabolic diseases. Oxidative stress and hyperglycemia in metabolic diseases lead to adverse neurophysiological phenomena, including neuronal loss, synaptic dysfunction, and improper insulin signaling, resulting in Parkinson’s disease (PD). In addition, several lines of evidence suggest that alterations of CNS environments by metabolic changes influence the dopamine neuronal loss, eventually affecting the pathogenesis of PD. Thus, we reviewed recent findings relating to degeneration of dopaminergic neurons during metabolic diseases. We highlight the fact that using a metabolic approach to manipulate degeneration of dopaminergic neurons can serve as a therapeutic strategy to attenuate pathology of PD.

The role of astrocytes in the hypothalamic response and adaptation to metabolic signals

The hypothalamus is crucial in the regulation of homeostatic functions in mammals, with the disruption of hypothalamic circuits contributing to chronic conditions such as obesity, diabetes mellitus, hypertension, and infertility. Metabolic signals and hormonal inputs drive functional and morphological changes in the hypothalamus in attempt to maintain metabolic homeostasis. However, the dramatic increase in the incidence of obesity and its secondary complications, such as type 2 diabetes, have evidenced the need to better understand how this system functions and how it can go awry. Growing evidence points to a critical role of astrocytes in orchestrating the hypothalamic response to metabolic cues by participating in processes of synaptic transmission, synaptic plasticity and nutrient sensing. These glial cells express receptors for important metabolic signals, such as the anorexigenic hormone leptin, and determine the type and quantity of nutrients reaching their neighboring neurons. Understanding the mechanisms by which astrocytes participate in hypothalamic adaptations to changes in dietary and metabolic signals is fundamental for understanding the neuroendocrine control of metabolism and key in the search for adequate treatments of metabolic diseases.

Pharmacologic manipulation of lysosomal enzyme transport across the blood-brain barrier

The adult blood-brain barrier, unlike the neonatal blood-brain barrier, does not transport lysosomal enzymes into brain, making enzyme replacement therapy ineffective in treating the central nervous system symptoms of lysosomal storage diseases. However, enzyme transport can be re-induced with alpha-adrenergics. Here, we examined agents that are known to alter the blood-brain barrier transport of large molecules or to induce lysosomal enzyme transport across the blood-brain barrier ((±)epinephrine, insulin, retinoic acid, and lipopolysaccharide) in 2-week-old and adult mice. In 2-week-old adolescent mice, all these pharmacologic agents increased brain and heart uptake of phosphorylated human β-glucuronidase. In 8-week-old adult mice, manipulations with (±)epinephrine, insulin, and retinoic acid were significantly effective on uptake by brain and heart. The increased uptake of phosphorylated human β-glucuronidase was inhibited by mannose 6-phosphate for the agents (±)epinephrine and retinoic acid and by L-NG-nitroarginine methyl ester for the agent lipopolysaccharide in neonatal and adult mice. An in situ brain perfusion study revealed that retinoic acid directly modulated the transport of phosphorylated human β-glucuronidase across the blood-brain barrier. The present study indicates that there are multiple opportunities to at least transiently induce phosphorylated human β-glucuronidase transport at the adult blood-brain barrier.

From blood-brain barrier to blood-brain interface: new opportunities for CNS drug delivery

One of the biggest challenges in the development of therapeutics for central nervous system (CNS) disorders is achieving sufficient blood-brain barrier (BBB) penetration. Research in the past few decades has revealed that the BBB is not only a substantial barrier for drug delivery to the CNS but also a complex, dynamic interface that adapts to the needs of the CNS, responds to physiological changes, and is affected by and can even promote disease. This complexity confounds simple strategies for drug delivery to the CNS, but provides a wealth of opportunities and approaches for drug development. Here, I review some of the most important areas that have recently redefined the BBB and discuss how they can be applied to the development of CNS therapeutics.

The role of insulin in the vascular contributions to age-related dementia

In addition to its well-known role in energy metabolism in the body, insulin is a vasoactive hormone that regulates peripheral and cerebral blood flow and neuronal function. Vascular and metabolic dysfunctions are emerging risk factors for Alzheimer's disease (AD) and age-related dementias, and recent evidence suggests that the two pathways are constitutive and interrelated. As a result, an emphasis on correcting metabolic disorders is emerging as an important strategy in the treatment and prevention of age-related cognitive impairment and AD. We review the evidence regarding the unique and interactive effects of vascular and metabolic disorders in pathological brain aging, with special consideration of the role of insulin dysregulation in promoting AD pathologic processes and vascular brain injury. This article is part of a Special Issue entitled: Vascular Contributions to Cognitive Impairment and Dementia edited by M. Paul Murphy, Roderick A. Corriveau and Donna M. Wilcock.

Can insulin signaling pathways be targeted to transport Aβ out of the brain?

Although the causal role of Amyloid-β (Aβ) in Alzheimer’s disease (AD) is unclear, it is still reasonable to expect that lowering concentrations of Aβ in the brain may decrease the risk of developing the neurocognitive symptoms of the disease. Brain capillary endothelial cells forming the blood-brain barrier (BBB) express transporters regulating the efflux of Aβ out of the cerebral tissue. Age-related BBB dysfunctions, that have been identified in AD patients, might impair Aβ clearance from the brain. Thus, targeting BBB outward transport systems has been suggested as a way to stimulate the clearance of Aβ from the brain. Recent data indicate that the increase in soluble brain Aβ and behavioral impairments in 3×Tg-AD mice generated by months of intake of a high-fat diet can be acutely reversed by the administration of a single dose of insulin. A concomitant increase in plasma Aβ suggests that clearance from the brain through the BBB is a likely mechanism for this rapid effect of insulin. Here, we review how BBB insulin response pathways could be stimulated to decrease brain Aβ concentrations and improve cognitive performance, at least on the short term.

Insulin in the brain: its pathophysiological implications for States related with central insulin resistance, type 2 diabetes and Alzheimer's disease

Although the brain has been considered an insulin-insensitive organ, recent reports on the location of insulin and its receptors in the brain have introduced new ways of considering this hormone responsible for several functions. The origin of insulin in the brain has been explained from peripheral or central sources, or both. Regardless of whether insulin is of peripheral origin or produced in the brain, this hormone may act through its own receptors present in the brain. The molecular events through which insulin functions in the brain are the same as those operating in the periphery. However, certain insulin actions are different in the central nervous system, such as hormone-induced glucose uptake due to a low insulin-sensitive GLUT-4 activity, and because of the predominant presence of GLUT-1 and GLUT-3. In addition, insulin in the brain contributes to the control of nutrient homeostasis, reproduction, cognition, and memory, as well as to neurotrophic, neuromodulatory, and neuroprotective effects. Alterations of these functional activities may contribute to the manifestation of several clinical entities, such as central insulin resistance, type 2 diabetes mellitus (T2DM), and Alzheimer's disease (AD). A close association between T2DM and AD has been reported, to the extent that AD is twice more frequent in diabetic patients, and some authors have proposed the name "type 3 diabetes" for this association. There are links between AD and T2DM through mitochondrial alterations and oxidative stress, altered energy and glucose metabolism, cholesterol modifications, dysfunctional protein O-GlcNAcylation, formation of amyloid plaques, altered Aβ metabolism, and tau hyperphosphorylation. Advances in the knowledge of preclinical AD and T2DM may be a major stimulus for the development of treatment for preventing the pathogenic events of these disorders, mainly those focused on reducing brain insulin resistance, which is seems to be a common ground for both pathological entities.

Dietary components in the development of leptin resistance

Classically, leptin resistance has been associated with increased body fat and circulating leptin levels, and the condition is believed to contribute to the onset and/or maintenance of obesity. Although a great deal is known about the central nervous system mechanisms mediating leptin resistance, considerably less is known about the role of diet in establishing and maintaining this altered hormonal state. An exciting new finding has recently been published demonstrating the existence of leptin resistance in normal-weight rats with lean leptin levels by feeding them a high-concentration-fructose diet. This finding has opened the possibility that specific macronutrients may be capable of inducing leptin resistance, independently of the amount of body fat or circulating leptin present in the treated animals. This review describes several lines of research that have recently emerged indicating that specific types of dietary sugars and fats are capable of inducing leptin resistance in experimental rodent models. The results further show that diet-induced leptin resistance is capable of increasing energy intake and elevating body weight gain under appropriate dietary challenges. It appears that biological mechanisms on multiple levels may underlie the dietary induction of leptin resistance, including alterations in the leptin blood-to-brain transport system, in peripheral glucose metabolism, and in central leptin receptor signaling pathways. What is clear from the findings reviewed here is that diet-induced leptin resistance can occur in the absence of elevated circulating leptin levels and body weight, rendering it a potential cause and/or predisposing factor to excess body weight gain and obesity.

Cytokine signaling modulates blood-brain barrier function

The blood-brain barrier (BBB) provides a vast interface for cytokines to affect CNS function. The BBB is a target for therapeutic intervention. It is essential, therefore, to understand how cytokines interact with each other at the level of the BBB and how secondary signals modulate CNS functions beyond the BBB. The interactions between cytokines and lipids, however, have not been fully addressed at the level of the BBB. Here, we summarize current understanding of the localization of cytokine receptors and transporters in specific membrane microdomains, particularly lipid rafts, on the luminal (apical) surface of the microvascular endothelial cells composing the BBB. We then illustrate the clinical context of cytokine effects on the BBB by neuroendocrine regulation and amplification of inflammatory signals. Two unusual aspects discussed are signaling crosstalk by different classes of cytokines and genetic regulation of drug efflux transporters. We also introduce a novel area of focus on how cytokines may act through nuclear hormone receptors to modulate efflux transporters and other targets. A specific example discussed is the ATP-binding cassette transporter-1 (ABCA-1) that regulates lipid metabolism. Overall, cytokine signaling at the level of the BBB is a crucial feature of the dynamic regulation that can rapidly change BBB function and affect brain health and disease.

Insulin in the brain: there and back again

Insulin performs unique functions within the CNS. Produced nearly exclusively by the pancreas, insulin crosses the blood-brain barrier (BBB) using a saturable transporter, affecting feeding and cognition through CNS mechanisms largely independent of glucose utilization. Whereas peripheral insulin acts primarily as a metabolic regulatory hormone, CNS insulin has an array of effects on brain that may more closely resemble the actions of the ancestral insulin molecule. Brain endothelial cells (BECs), the cells that form the vascular BBB and contain the transporter that translocates insulin from blood to brain, are themselves regulated by insulin. The insulin transporter is altered by physiological and pathological factors including hyperglycemia and the diabetic state. The latter can lead to BBB disruption. Pericytes, pluripotent cells in intimate contact with the BECs, protect the integrity of the BBB and its ability to transport insulin. Most of insulin's known actions within the CNS are mediated through two canonical pathways, the phosphoinositide-3 kinase (PI3)/Akt and Ras/mitogen activated kinase (MAPK) cascades. Resistance to insulin action within the CNS, sometimes referred to as diabetes mellitus type III, is associated with peripheral insulin resistance, but it is possible that variable hormonal resistance syndromes exist so that resistance at one tissue bed may be independent of that at others. CNS insulin resistance is associated with Alzheimer's disease, depression, and impaired baroreceptor gain in pregnancy. These aspects of CNS insulin action and the control of its entry by the BBB are likely only a small part of the story of insulin within the brain.

Brain meets body: the blood-brain barrier as an endocrine interface

The blood-brain barrier (BBB) separates the central nervous system (CNS) from the peripheral tissues. However, this does not prevent hormones from entering the brain, but shifts the main control of entry to the BBB. In general, steroid hormones cross the BBB by transmembrane diffusion, a nonsaturable process resulting in brain levels that reflect blood levels, whereas thyroid hormones and many peptides and regulatory proteins cross using transporters, a saturable process resulting in brain levels that reflect blood levels and transporter characteristics. Protein binding, brain-to-blood transport, and pharmacokinetics modulate BBB penetration. Some hormones have the opposite effect within the CNS than they do in the periphery, suggesting that these hormones cross the BBB to act as their own counterregulators. The cells making up the BBB are also endocrine like, both responding to circulating substances and secreting substances into the circulation and CNS. By dividing a hormone's receptors into central and peripheral pools, the former of which may not be part of the hormone's negative feed back loop, the BBB fosters the development of variable hormone resistance syndromes, as exemplified by evidence that altered insulin action in the CNS can contribute to Alzheimer's disease. In summary, the BBB acts as a regulatory interface in an endocrine-like, humoral-based communication between the CNS and peripheral tissues.

Role of the blood-brain barrier in the evolution of feeding and cognition

The blood-brain barrier (BBB) regulates the blood-to-brain passage of gastrointestinal hormones, thus informing the brain about feeding and nutritional status. Disruption of this communication results in dysregulation of feeding and body weight control. Leptin, which crosses the BBB to inform the CNS about adiposity, provides an example. Impaired leptin transport, especially coupled with central resistance, results in obesity. Various substances/conditions regulate leptin BBB transport. For example, triglycerides inhibit leptin transport. This may represent an evolutionary adaptation in that hypertriglyceridemia occurs during starvation. Inhibition of leptin, an anorectic, during starvation could have survival advantages. The large number of other substances that influence feeding is explained by the complexity of feeding. This complexity includes cognitive aspects; animals in the wild are faced with cost/benefit analyses to feed in the safest, most economical way. This cognitive aspect partially explains why so many feeding substances affect neurogenesis, neuroprotection, and cognition. The relation between triglycerides and cognition may be partially mediated through triglyceride's ability to regulate the BBB transport of cognitively active gastrointestinal hormones such as leptin, insulin, and ghrelin.

Drug delivery to the brain in Alzheimer's disease: consideration of the blood-brain barrier

The successful treatment of Alzheimer's disease (AD) will require drugs that can negotiate the blood-brain barrier (BBB). However, the BBB is not simply a physical barrier, but a complex interface that is in intimate communication with the rest of the central nervous system (CNS) and influenced by peripheral tissues. This review examines three aspects of the BBB in AD. First, it considers how the BBB may be contributing to the onset and progression of AD. In this regard, the BBB itself is a therapeutic target in the treatment of AD. Second, it examines how the BBB restricts drugs that might otherwise be useful in the treatment of AD and examines strategies being developed to deliver drugs to the CNS for the treatment of AD. Third, it considers how drug penetration across the AD BBB may differ from the BBB of normal aging. In this case, those differences can complicate the treatment of CNS diseases such as depression, delirium, psychoses, and pain control in the AD population.

Leptin in anorexia and cachexia syndrome

Leptin is a product of the obese (OB) gene secreted by adipocytes in proportion to fat mass. It decreases food intake and increases energy expenditure by affecting the balance between orexigenic and anorexigenic hypothalamic pathways. Low leptin levels are responsible for the compensatory increase in appetite and body weight and decreased energy expenditure (EE) following caloric deprivation. The anorexia-cachexia syndrome is a complication of many chronic conditions including cancer, chronic obstructive pulmonary disease, congestive heart failure, chronic kidney disease, and aging, where the decrease in body weight and food intake is not followed by a compensatory increase in appetite or decreased EE. Crosstalk between leptin and inflammatory signaling known to be activated in these conditions may be responsible for this paradox. This manuscript will review the evidence and potential mechanisms mediating changes in the leptin pathway in the setting of anorexia and cachexia associated with chronic diseases.

Systemic administration of urocortin after intracerebral hemorrhage reduces neurological deficits and neuroinflammation in rats

Background

Intracerebral hemorrhage (ICH) remains a serious clinical problem lacking effective treatment. Urocortin (UCN), a novel anti-inflammatory neuropeptide, protects injured cardiomyocytes and dopaminergic neurons. Our preliminary studies indicate UCN alleviates ICH-induced brain injury when administered intracerebroventricularly (ICV). The present study examines the therapeutic effect of UCN on ICH-induced neurological deficits and neuroinflammation when administered by the more convenient intraperitoneal (i.p.) route.

Methods

ICH was induced in male Sprague-Dawley rats by intrastriatal infusion of bacterial collagenase VII-S or autologous blood. UCN (2.5 or 25 μg/kg) was administered i.p. at 60 minutes post-ICH. Penetration of i.p. administered fluorescently labeled UCN into the striatum was examined by fluorescence microscopy. Neurological deficits were evaluated by modified neurological severity score (mNSS). Brain edema was assessed using the dry/wet method. Blood-brain barrier (BBB) disruption was assessed using the Evans blue assay. Hemorrhagic volume and lesion volume were assessed by Drabkin's method and morphometric assay, respectively. Pro-inflammatory cytokine (TNF-α, IL-1β, and IL-6) expression was evaluated by enzyme-linked immunosorbent assay (ELISA). Microglial activation and neuronal loss were evaluated by immunohistochemistry.

Results

Administration of UCN reduced neurological deficits from 1 to 7 days post-ICH. Surprisingly, although a higher dose (25 μg/kg, i.p.) also reduced the functional deficits associated with ICH, it is significantly less effective than the lower dose (2.5 μg/kg, i.p.). Beneficial results with the low dose of UCN included a reduction in neurological deficits from 1 to 7 days post-ICH, as well as a reduction in brain edema, BBB disruption, lesion volume, microglial activation and neuronal loss 3 days post-ICH, and suppression of TNF-α, IL-1β, and IL-6 production 1, 3 and 7 days post-ICH.

Conclusion

Systemic post-ICH treatment with UCN reduces striatal injury and neurological deficits, likely via suppression of microglial activation and inflammatory cytokine production. The low dose of UCN necessary and the clinically amenable peripheral route make UCN a potential candidate for development into a clinical treatment regimen.

Principles of strategic drug delivery to the brain (SDDB): development of anorectic and orexigenic analogs of leptin

The blood-brain barrier (BBB) presents a tremendous challenge for the delivery of drugs to the central nervous system (CNS). This includes drugs that target brain receptors for the treatment of obesity and anorexia. Strategic drug delivery to brain (SDDB) is an approach that considers in depth the relations among the BBB, the candidate therapeutic, the CNS target, and the disease state to be treated. Here, we illustrate principles of SDDB with two different approaches to developing drugs based on leptin. In normal body weight humans and in non-obese rodents, leptin is readily transported across the BBB and into the CNS where it inhibits feeding and enhances thermogenesis. However, in obesity, the transport of leptin across the BBB is impaired, resulting in a resistance to leptin. As a result, it is difficult to treat obesity with leptin or its analogs that depend on the leptin transporter for access to the CNS. To treat obesity, we developed a leptin agonist modified by the addition of pluronic block copolymers (P85-leptin). P85-leptin retains biological activity and is capable of crossing the BBB by a mechanism that is not dependent on the leptin transporter. As such, P85-leptin is able to cross the BBB of obese mice at a rate similar to that of native leptin in lean mice. To treat anorexia, we developed a leptin antagonist modified by pegylation (PEG-MLA) that acts primarily by blocking the BBB transporter for endogenous, circulating leptin. This prevents blood-borne, endogenous leptin from entering the CNS, essentially mimicking the leptin resistance seen in obesity, and resulting in a significant increase in adiposity. These examples illustrate two strategies in which an understanding of the interactions among the BBB, CNS targets, and candidate therapeutics under physiologic and diseased conditions can be used to develop drugs effective for the treatment of brain disease.

Stress, sex, and neural adaptation to a changing environment: mechanisms of neuronal remodeling

The adult brain is much more resilient and adaptable than previously believed, and adaptive structural plasticity involves growth and shrinkage of dendritic trees, turnover of synapses, and limited amounts of neurogenesis in the forebrain, especially the dentate gyrus of the hippocampal formation. Stress and sex hormones help to mediate adaptive structural plasticity, which has been extensively investigated in the hippocampus and to a lesser extent in the prefrontal cortex and amygdala, all brain regions that are involved in cognitive and emotional functions. Stress and sex hormones exert their effects on brain structural remodeling through both classical genomic as well as non-genomic mechanisms, and they do so in collaboration with neurotransmitters and other intra- and extracellular mediators. This review will illustrate the actions of estrogen on synapse formation in the hippocampus and the process of stress-induced remodeling of dendrites and synapses in the hippocampus, amygdala, and prefrontal cortex. The influence of early developmental epigenetic events, such as early life stress and brain sexual differentiation, is noted along with the interactions between sex hormones and the effects of stress on the brain. Because hormones influence brain structure and function and because hormone secretion is governed by the brain, applied molecular neuroscience techniques can begin to reveal the role of hormones in brain-related disorders and the treatment of these diseases. A better understanding of hormone-brain interactions should promote more flexible approaches to the treatment of psychiatric disorders, as well as their prevention through both behavioral and pharmaceutical interventions.

Mouse models of neurological disorders: a view from the blood-brain barrier

The number of disease models that involve an aspect of blood-brain barrier (BBB) dysregulation have increased tremendously. The main factors contributing to this expansion have been an increased number of diseases in which the BBB is known to be involved, an increase in the known functions of the BBB, and an increase in the number of models and tools with which those diverse functions can be studied. In many cases, the BBB may be a target of disease; current thinking would include hypertensive encephalopathy and perhaps stroke in this category. Another category are those diseases in which special attributes of the BBB may predispose to disease; for example, the ability of a pathogen to cross the BBB often depends on the pathogen's ability to invoke transcytotic pathways in the brain endothelial or choroid plexus cell. Of special interest are those diseases in which the BBB may be the primary seat of disease or play a major role in the onset or progression of the disease. An increasing number of diseases are so categorized in which BBB dysfunction or dysregulation plays a major role; this review highlights such roles for the BBB including those proposed for Alzheimer's disease and obesity.

Pegylated leptin antagonist is a potent orexigenic agent: preparation and mechanism of activity

Leptin, a pleiotropic adipokine, is a central regulator of appetite and weight and a key immunomodulatory protein. Although inborn leptin deficiency causes weight gain, it is unclear whether induced leptin deficiency in adult wild-type animals would be orexigenic. Previous work with a potent competitive leptin antagonist did not induce a true metabolic state of leptin deficiency in mice because of a short circulating half-life. In this study, we increased the half-life of the leptin antagonist by pegylation, which resulted in significantly increased bioavailability and retaining of antagonistic activity. Mice administered the pegylated antagonist showed a rapid and dramatic increase in food intake with weight gain. Resulting fat was confined to the mesenteric region with no accumulation in the liver. Serum cholesterol, triglyceride, and hepatic aminotransferases remained unaffected. Weight changes were reversible on cessation of leptin antagonist treatment. The mechanism of severe central leptin deficiency was found to be primarily caused by blockade of transport of circulating leptin across the blood-brain barrier with antagonisms at the arcuate nucleus playing a more minor role. Altogether we introduce a novel compound that induces central and peripheral leptin deficiency. This compound should be useful in exploring the involvement of leptin in metabolic and immune processes and could serve as a therapeutic for the treatment of cachexia.

Effects of triglycerides, obesity, and starvation on ghrelin transport across the blood-brain barrier

Human ghrelin is transported across the blood-brain barrier (BBB) of normal mice. Here, we studied the effects of triglycerides, obesity, and starvation in retired breeder mice maintained on a high fat diet, mice age-matched to the retired breeders but maintained on normal chow, and 8-week-old mice maintained on breeder chow. The rate of ghrelin transport across the BBB was studied by both the intravenous administration method of multiple-time regression analysis and by the brain perfusion method. We found that (1) obese, aged mice lost the ability to transport intravenously administered ghrelin across the BBB, resulting in an inverse relation between body weight and ghrelin BBB permeability; (2) serum triglycerides promoted transport of intravenously administered ghrelin across the BBB, whereas epinephrine had no effect; (3) fasting tended to promote ghrelin transport across the BBB as most readily shown in brain perfusion studies; (4) evidence suggested that a serum factor promoted ghrelin transport in 8-week-old mice. Overall, these results show that serum factors and physiological states influence the rate at which ghrelin is transported across the blood-brain barrier.

Blood-brain barrier and feeding: regulatory roles of saturable transport systems for ingestive peptides

The two main ways for peptides in the peripheral body to enter the brain are by either saturable transport or passive diffusion across the blood-brain barrier (BBB). Saturable transport systems have the advantage of being responsive to physiological and pathological stimuli. Since saturable systems can regulate peptide entry into the brain, they have the potential to play controlling roles in feeding behavior. For therapeutic applications, however, saturable systems have the disadvantage of functioning as a threshold to limit access of large amounts of peptides into the brain. This pharmacological problem presumably would not be encountered for peptides crossing the BBB by passive diffusion, a process dependent on physicochemical properties. Thus, the gatekeeper function of the BBB can be expanded to a primary governing role, especially for entry of ingestive peptides subject to their respective saturable transport systems.

Starvation and triglycerides reverse the obesity-induced impairment of insulin transport at the blood-brain barrier

Insulin in the brain acts as a satiety factor, reduces appetite, and decreases body mass. Altered sensing by brain of insulin may be a leading cause of weight gain and insulin resistance. A decrease in the transport across the blood-brain barrier (BBB) of insulin may induce brain insulin resistance by inducing obesity. We here report that transport of iv administrated insulin across the BBB of obese mice, as measured by multiple-time regression analysis, was significantly lower than that in thin adult mice. The reduction in obese mice was reversed by starvation for 48 h. There were no differences in insulin transport rates across the BBB of obese, thin, or starved obese mice when studied by the brain perfusion model, demonstrating that BBB transport of insulin is modulated by circulating factors. In the brain perfusion study, the triglyceride triolein significantly increased the brain uptake of insulin, an effect opposite to that on leptin transport, in starved obese mice. Thus, circulating triglycerides are one of the systemic modulators for the transport of insulin across the BBB.

Astrocyte leptin receptor (ObR) and leptin transport in adult-onset obese mice

The agouti viable yellow (A vy) spontaneous mutation generates an unusual mouse phenotype of agouti-colored coat and adult-onset obesity with metabolic syndrome. Persistent production of agouti signaling protein in A vy mice antagonizes melanocortin receptors in the hypothalamus. To determine how this disruption of neuroendocrine circuits affects leptin transport across the blood-brain barrier (BBB), we measured leptin influx in A vy and B6 control mice after the development of obesity, hyperleptinemia, and increased adiposity. After iv bolus injection, (125)I-leptin crossed the BBB significantly faster in young (2 month old) B6 mice than in young A vy mice or in older (8 month old) mice of either strain. This difference was not observed by in situ brain perfusion studies, indicating the cause being circulating factors, such as elevated leptin levels or soluble receptors. Thus, A vy mice showed peripheral leptin resistance. ObRa, the main transporting receptor for leptin at the BBB, showed no change in mRNA expression in the cerebral microvessels between the age-matched (2 month old) A vy and B6 mice. Higher ObRb mRNA was seen in the A vy microvasculature with unknown significance. Immunofluorescent staining unexpectedly revealed that many of the ObR(+) cells were astrocytes and that the A vy mice showed significantly more ObR(+) astrocytes in the hypothalamus than the B6 mice. Although leptin permeation from the circulation was slower in the A vy mice, the increased ObR expression in astrocytes and increased ObRb mRNA in microvessels suggest the possibility of heightened central nervous system sensitivity to circulating leptin.

The blood-brain barrier: connecting the gut and the brain

The BBB prevents the unrestricted exchange of substances between the central nervous system (CNS) and the blood. The blood-brain barrier (BBB) also conveys information between the CNS and the gastrointestinal (GI) tract through several mechanisms. Here, we review three of those mechanisms. First, the BBB selectively transports some peptides and regulatory proteins in the blood-to-brain or the brain-to-blood direction. The ability of GI hormones to affect functions of the BBB, as illustrated by the ability of insulin to alter the BBB transport of amino acids and drugs, represents a second mechanism. A third mechanism is the ability of GI hormones to affect the secretion by the BBB of substances that themselves affect feeding and appetite, such as nitric oxide and cytokines. By these and other mechanisms, the BBB regulates communications between the CNS and GI tract.

Nitric oxide isoenzymes regulate lipopolysaccharide-enhanced insulin transport across the blood-brain barrier

Insulin transported across the blood-brain barrier (BBB) has many effects within the central nervous system. Insulin transport is not static but altered by obesity and inflammation. Lipopolysaccharide (LPS), derived from the cell walls of Gram-negative bacteria, enhances insulin transport across the BBB but also releases nitric oxide (NO), which opposes LPS-enhanced insulin transport. Here we determined the role of NO synthase (NOS) in mediating the effects of LPS on insulin BBB transport. The activity of all three NOS isoenzymes was stimulated in vivo by LPS. Endothelial NOS and inducible NOS together mediated the LPS-enhanced transport of insulin, whereas neuronal NOS (nNOS) opposed LPS-enhanced insulin transport. This dual pattern of NOS action was found in most brain regions with the exception of the striatum, which did not respond to LPS, and the parietal cortex, hippocampus, and pons medulla, which did not respond to nNOS inhibition. In vitro studies of a brain endothelial cell (BEC) monolayer BBB model showed that LPS did not directly affect insulin transport, whereas NO inhibited insulin transport. This suggests that the stimulatory effect of LPS and NOS on insulin transport is mediated through cells of the neurovascular unit other than BECs. Protein and mRNA levels of the isoenzymes indicated that the effects of LPS are mainly posttranslational. In conclusion, LPS affects insulin transport across the BBB by modulating NOS isoenzyme activity. NO released by endothelial NOS and inducible NOS acts indirectly to stimulate insulin transport, whereas NO released by nNOS acts directly on BECs to inhibit insulin transport.

Central leptin insufficiency syndrome: an interactive etiology for obesity, metabolic and neural diseases and for designing new therapeutic interventions

This review critically reappraises recent scientific evidence concerning central leptin insufficiency versus leptin resistance formulations to explain metabolic and neural disorders resulting from subnormal or defective leptin signaling in various sites in the brain. Research at various fronts to unravel the complexities of the neurobiology of leptin is surveyed to provide a comprehensive account of the neural and metabolic effects of environmentally imposed fluctuations in leptin availability at brain sites and the outcome of newer technology to restore leptin signaling in a site-specific manner. The cumulative new knowledge favors a unified central leptin insufficiency syndrome over the, in vogue, central resistance hypothesis to explain the global adverse impact of deficient leptin signaling in the brain. Furthermore, the leptin insufficiency syndrome delineates a novel role of leptin in the hypothalamus in restraining rhythmic pancreatic insulin secretion while concomitantly enhancing glucose metabolism and non-shivering thermogenic energy expenditure, sequelae that would otherwise promote fat accrual to store excess energy resulting from consumption of energy-enriched diets. A concerted effort should now focus on development of newer technologies for delivery of leptin or leptin mimetics to specifically target neural pathways for remediation of diverse ailments encompassing the central leptin insufficiency syndrome.

Physiology and neurobiology of stress and adaptation: central role of the brain

The brain is the key organ of the response to stress because it determines what is threatening and, therefore, potentially stressful, as well as the physiological and behavioral responses which can be either adaptive or damaging. Stress involves two-way communication between the brain and the cardiovascular, immune, and other systems via neural and endocrine mechanisms. Beyond the "flight-or-fight" response to acute stress, there are events in daily life that produce a type of chronic stress and lead over time to wear and tear on the body ("allostatic load"). Yet, hormones associated with stress protect the body in the short-run and promote adaptation ("allostasis"). The brain is a target of stress, and the hippocampus was the first brain region, besides the hypothalamus, to be recognized as a target of glucocorticoids. Stress and stress hormones produce both adaptive and maladaptive effects on this brain region throughout the life course. Early life events influence life-long patterns of emotionality and stress responsiveness and alter the rate of brain and body aging. The hippocampus, amygdala, and prefrontal cortex undergo stress-induced structural remodeling, which alters behavioral and physiological responses. As an adjunct to pharmaceutical therapy, social and behavioral interventions such as regular physical activity and social support reduce the chronic stress burden and benefit brain and body health and resilience.

Adipokines and the blood-brain barrier

Just as the blood-brain barrier (BBB) is not a static barrier, the adipocytes are not inert storage depots. Adipokines are peptides or polypeptides produced by white adipose tissue; they play important roles in normal physiology as well as in the metabolic syndrome. Adipokines secreted into the circulation can interact with the BBB and exert potent CNS effects. The specific transport systems for two important adipokines, leptin and tumor necrosis factor alpha, have been characterized during the past decade. By contrast, transforming growth factor beta-1 and adiponectin do not show specific permeation across the BBB, but modulate endothelial functions. Still others, like interleukin-6, may reach the brain but are rapidly degraded. This review summarizes current knowledge and recent findings of the rapidly growing family of adipokines and their interactions with the BBB.

Regulation of KATP channel subunit gene expression by hyperglycemia in the mediobasal hypothalamus of female rats

The ATP-sensitive potassium (K(ATP)) channels are gated by intracellular adenine nucleotides coupling cell metabolism to membrane potential. Channels comprised of Kir6.2 and SUR1 subunits function in subpopulations of mediobasal hypothalamic (MBH) neurons as an essential component of a glucose-sensing mechanism in these cells, wherein uptake and metabolism of glucose leads to increase in intracellular ATP/ADP, closure of the channels, and increase in neuronal excitability. However, it is unknown whether glucose and/or insulin may also regulate the gene expression of the channel subunits in the brain. The present study investigated whether regulation of K(ATP) channel subunit gene expression might be a mechanism by which neuronal populations adapt to prolonged changes in glucose and/or insulin levels in the periphery. Ovariectomized, steroid-replaced rats were fitted with indwelling jugular catheters and infused for 48 h with saline, glucose (hyperglycemia-hyperinsulinemia), insulin and glucose (hyperinsulinemia), diazoxide (control), or glucose and diazoxide (hyperglycemia). At the end of infusions, the MBH, preoptic area, and pituitary were dissected for RNA isolation and RT-PCR. Hyperglycemia decreased Kir6.2 mRNA levels in the MBH in both the presence and absence of hyperinsulinemia. These same conditions also produced a trend toward decreased SUR1 mRNA levels in the MBH; however, it did not exceed statistical significance. Hyperglycemia increased whereas hyperinsulinemia reduced neuropeptide Y mRNA levels when these groups were compared with each other. However, neither was significantly different from values observed in saline-infused controls. In conclusion, hyperglycemia per se may alter expression of K(ATP) channels and thereby induce changes in the excitability of some MBH neurons.

The blood-brain barrier as a regulatory interface in the gut-brain axes

The blood-brain barrier (BBB) prevents the unrestricted movement of peptides and proteins between the brain and blood. However, some peptides and regulatory proteins can cross the BBB by saturable and non-saturable mechanisms. Leptin and insulin each cross the BBB by their own transporters. Impaired transport of leptin occurs in obesity and accounts for peripheral resistance; that is, the condition wherein an obese animal loses weight when given leptin directly into the brain but not when given leptin peripherally. Leptin transport is also inhibited in starvation and by hypertriglyceridemia. Since hypertriglyceridemia occurs in both starvation and obesity, we have postulated that the peripheral resistance induced by hypertriglyceridemia may have evolved as an adaptive mechanism in response to starvation. Insulin transport is also regulated. For example, treatment of mice with lipopolysaccharide (LPS) increases insulin transport across the BBB by about threefold. Since many of the actions of CNS insulin oppose those of peripheral insulin and since LPS releases proinflammatory cytokines, enhanced transport of insulin across the BBB could be a mechanism which promotes insulin resistance in sepsis. The brain endothelial cells which comprise the BBB secrete many substances including cytokines. Such secretion can be stimulated from one side of the BBB with release into the other side. For example, it appears that adiponectin can inhibit release of interleukin-6 from brain endothelial cells. Overall, the BBB represents an important interface in mediating gut-brain axes.

CNS drug delivery: opioid peptides and the blood-brain barrier

Peptides are key regulators in cellular and intercellular physiological responses and possess enormous promise for the treatment of pathological conditions. Opioid peptide activity within the central nervous system (CNS) is of particular interest for the treatment of pain owing to the elevated potency of peptides and the centrally mediated actions of pain processes. Despite this potential, peptides have seen limited use as clinically viable drugs for the treatment of pain. Reasons for the limited use are primarily based in the physiochemical and biochemical nature of peptides. Numerous approaches have been devised in an attempt to improve peptide drug delivery to the brain, with variable results. This review describes different approaches to peptide design/modification and provides examples of the value of these strategies to CNS delivery of peptide drugs. The various modes of modification of therapeutic peptides may be amalgamated, creating more efficacious "hybrid" peptides, with synergistic delivery to the CNS. The ongoing development of these strategies provides promise that peptide drugs may be useful for the treatment of pain and other neurologically-based disease states in the future.

Triglycerides induce leptin resistance at the blood-brain barrier

Obesity is associated with leptin resistance as evidenced by hyperleptinemia. Resistance arises from impaired leptin transport across the blood-brain barrier (BBB), defects in leptin receptor signaling, and blockades in downstream neuronal circuitries. The mediator of this resistance is unknown. Here, we show that milk, for which fats are 98% triglycerides, immediately inhibited leptin transport as assessed with in vivo, in vitro, and in situ models of the BBB. Fat-free milk and intralipid, a source of vegetable triglycerides, were without effect. Both starvation and diet-induced obesity elevated triglycerides and decreased the transport of leptin across the BBB, whereas short-term fasting decreased triglycerides and increased transport. Three of four triglycerides tested intravenously inhibited transport of leptin across the BBB, but their free fatty acid constituents were without effect. Treatment with gemfibrozil, a drug that specifically reduces triglyceride levels, reversed both hypertriglyceridemia and impaired leptin transport. We conclude that triglycerides are an important cause of leptin resistance as mediated by impaired transport across the BBB and suggest that triglyceride-mediated leptin resistance may have evolved as an anti-anorectic mechanism during starvation. Decreasing triglycerides may potentiate the anorectic effect of leptin by enhancing leptin transport across the BBB.

The source of cerebral insulin

Insulin and its receptor are found throughout the central nervous system (CNS). Insulin administered into the CNS can exert powerful effects, yet the consensus is that little or no insulin is produced in the CNS. Therefore, CNS insulin is essentially dependent on the ability of peripheral insulin to cross the blood-brain barrier (BBB). Insulin is known to cross the BBB by a saturable transport mechanism. This transporter shows some thematic similarities to other transporters for peptides or regulatory proteins. It is unevenly distributed throughout the CNS with the olfactory bulbs having the fastest transport rate of any brain region. It is partially saturated at euglycemic levels, suggesting that its main signaling function occurs at physiological blood levels, rather than as a brake to hypoglycemic events. One probable function of the BBB transporter is to allow CNS insulin to act as a counter-regulatory hormone to peripheral insulin. The transporter is regulated, with the transport rate of insulin being altered during development and by fasting, obesity, hibernation, diabetes mellitus and Alzheimer's disease. Enhancement of insulin transport by lipopolysaccharide could be the basis for the insulin resistance seen with bacterial infections. Inhibition of insulin transport across the BBB by dexamethasone could be the basis for the enhanced appetite seen with glucocorticoid treatments. Insulin itself also has effects on the BBB, altering enzymatic and transporter functions. Overall, BBB transport of insulin provides a mechanism for peripheral insulin to act within the CNS as a regulatory peptide.

The many lives of leptin

Leptin is a 16,000-Da protein which is secreted by fat but acts within the brain to regulate adiposity. Our Peptides Classic addressed the mystery of how such a large molecule could negotiate the blood-brain barrier (BBB), a structure which normally excludes proteins from the brain. We found that leptin was transported across the BBB by a saturable transport system. This finding was important to understanding how satiety-related peptides and proteins worked, but it was also important to the concept that the BBB is a regulatory interface important in brain-body communication. Obesity in humans and many animals is associated with a leptin resistant state rather than a leptin deficiency. Subsequent work has shown that a defect in the BBB transport of leptin is key in producing and reinforcing this state of resistance. Leptin is pluripotent and the concept of it being primarily an adipostat is being discarded for more encompassing views. Consideration of the BBB data would favor the view that ancestral levels of leptin were much lower than those currently considered normal and are consistent with leptin acting as a metabolic switch, informing the brain when fat reserves are adequate to direct energy expenditures towards activities other than seeking calories.

Seasonal Regulation of Reproductive Activity in Sheep

Sheep in temperate latitudes are seasonal breeders. In female sheep, ovarian activity decreases during the anestrous period due to modification of secretion of luteinizing hormone (LH). The seasonal changes in the hormonal LH pattern mainly reflect an increase in the brain responsiveness to the negative feedback exerted by estradiol during long days (LD) on the frequency of pulsatile LH secretion, under neurohormonal GnRH control. The resulting seasonal inhibition of LH secretion mainly involves the activation of dopaminergic systems by E2, which in turn inhibits the GnRH cells from the preoptico-hypothalamic structures. The increased responsiveness of the brain during LD could lead to increased expression of central E2 receptors. In addition, our study shows that steroid access to the brain could be modulated by photoperiodism, thus increasing the availability of steroids to the nervous structures during LD.