Atrial natriuretic peptide in the central nervous system of the rat

1. Studies of the presence of atrial natriuretic peptide immunoreactivity and receptor binding sites in the central nervous system have revealed unusual sites of interest. 2. As a result, numerous studies have appeared that indicate that brain atrial natriuretic peptide is implicated in the regulation of blood pressure, fluid and sodium balance, cerebral blood flow, brain microcirculation, blood-brain barrier function, and cerebrospinal fluid production. 3. Alteration of the atrial natriuretic peptide system in the brain could have important implications in hypertensive disease and disorders of water balance in the central nervous system.     

Entry of EGF into brain is rapid and saturable.

Epidermal growth factor (EGF) is a neurotrophic peptide produced both in the central nervous system and the periphery. Peripheral administration of EGF causes central nervous system-mediated changes. The central nervous system effects could be explained by the permeation of EGF across the blood-brain barrier (BBB). In this report, we show that 125I-EGF crosses the BBB rapidly, with an influx rate of about 2 microl/g x min, much faster than that for neurotrophins, cytokines, and most other bioactive peptides tested. The 125I-EGF was recovered intact in the brain 10 min after i.v. injection, and the majority of the peptide reaching the brain was present in the parenchyma. The fast rate of influx was significantly decreased by co-administration of nonradiolabeled EGF and transforming growth factor alpha, peptides that share the EGF receptor. By contrast, a monoclonal antibody against the EGF receptor failed to inhibit the entry of EGF. Furthermore, mice with a mutation in the EGF receptor had no significant decrease in the rapid rate of entry of 125I-EGF. By contrast to the fast rate of entry, 125I-EGF injected intracerebroventricularly (i.c.v.) only exited the brain with the bulk flow of cerebrospinal fluid. Thus, EGF has a saturable transport system at the BBB for rapid, unidirectional influx. The transport system does not require the entire EGF receptor and is susceptible to possible therapeutic manipulation.     

Peptidergic control of food intake in food-producing animals

Roles of brain and intestinal peptides in the control of food intake may vary among species for specific peptides depending on the degree of complexity of the gastrointestinal tract. Cholecystokinin (CCK) in the brain and intestine is the most widely studied of the peptides involved in the control of feeding. Although CCK released from the intestine may act on peripheral receptors in producing satiety in the pig, a monogastric animal, it has little effect on feeding after peripheral administration in sheep. CCK injected peripherally in chickens decreases food intake, but because of the delay in gastric emptying related to the crop and gizzard, it may be of minor importance. Possible roles for brain CCK have been suggested because CCK injected into the cerebrospinal fluid (CSF) decreases feeding in all three species. In sheep, food intake was stimulated by sequestration of endogenous CCK in CSF with specific CCK antibodies, which suggests a physiological role for brain CCK controlling food intake in this species. Opioid peptides increased feeding in sheep after i.v. and CSF injections. Only peripheral, and not CSF, injections of naloxone, a specific opiate antagonist, decreased feeding and blocked both peripheral and central opioid peptide-stimulated feeding. The balance of CCK and the opioid peptide activity in either the central nervous system or the periphery appears important in the control of feeding, but specific peptide functions and sites of action probably vary among species.     

The Brain Response to Peripheral Insulin Declines with Age: A Contribution of the Blood-Brain Barrier?

ObjectivesIt is a matter of debate whether impaired insulin action originates from a defect at the neural level or impaired transport of the hormone into the brain. In this study, we aimed to investigate the effect of aging on insulin concentrations in the periphery and the central nervous system as well as its impact on insulin-dependent brain activity.MethodsInsulin, glucose and albumin concentrations were determined in 160 paired human serum and cerebrospinal fluid (CSF) samples. Additionally, insulin was applied in young and aged mice by subcutaneous injection or intracerebroventricularly to circumvent the blood-brain barrier. Insulin action and cortical activity were assessed by Western blotting and electrocorticography radiotelemetric measurements.ResultsIn humans, CSF glucose and insulin concentrations were tightly correlated with the respective serum/plasma concentrations. The CSF/serum ratio for insulin was reduced in older subjects while the CSF/serum ratio for albumin increased with age like for most other proteins. Western blot analysis in murine whole brain lysates revealed impaired phosphorylation of AKT (P-AKT) in aged mice following peripheral insulin stimulation whereas P-AKT was comparable to levels in young mice after intracerebroventricular insulin application. As readout for insulin action in the brain, insulin-mediated cortical brain activity instantly increased in young mice subcutaneously injected with insulin but was significantly reduced and delayed in aged mice during the treatment period. When insulin was applied intracerebroventricularly into aged animals, brain activity was readily improved.ConclusionsThis study discloses age-dependent changes in insulin CSF/serum ratios in humans. In the elderly, cerebral insulin resistance might be partially attributed to an impaired transport of insulin into the central nervous system.     

Peptide drug modifications to enhance bioavailability and blood-brain barrier permeability

Peptides have the potential to be potent pharmaceutical agents for the treatment of many central nervous system derived maladies. Unfortunately peptides are generally water-soluble compounds that will not enter the central nervous system, via passive diffusion, due to the existence of the blood-brain barrier. Peptides can also undergo metabolic deactivation by peptidases, thus further reducing their therapeutic benefits. In targeting peptides to the central nervous system consideration must be focused both on increasing bioavailability and enhancing brain uptake. To date multiple strategies have been examined with this focus. However, each strategy comes with its own complications and considerations. In this review we assess the strengths and weaknesses of many of the methods currently being examined to enhance peptide entry into the central nervous system.     

Making sense of it: roles of the sensory circumventricular organs in feeding and regulation of energy homeostasis

Obesity is associated with significant health risks including stroke and heart disease. The prevalence of obesity has dramatically increased over the past 20 years. Although the development of obesity is clearly related to changing lifestyles, the central nervous system plays a key role in regulation of energy balance. To develop effective strategies for treating obesity, we must gain a clearer understanding of the neuro-circuitry and signaling mechanisms involved. Toward this end, recent progress has been made in the understanding of the roles played by the sensory circumventricular organs (CVOs) of the brain. These areas lack the normal blood-brain barrier and thus act as transducers of signals between the blood, other centers in the brain, and the cerebrospinal fluid. This review focuses on the roles played by the sensory CVOs in detecting and responding to a number of signals that carry information regarding nutritional status, including cholecystokinin, amylin, ghrelin, peptide YY, pancreatic polypeptide, leptin, adiponectin, and glucose.     

Leukocyte-facilitated entry of intracellular pathogens into the central nervous system

Microbes use numerous strategies to invade the central nervous system. Leukocyte-facilitated entry is one such mechanism whereby intracellular pathogens establish infection by taking advantage of leukocyte trafficking to the central nervous system. Key components of this process include peripheral infection and activation of leukocytes, activation of cerebral endothelial cells with or without concomitant infection, and trafficking of infected leukocytes to and through the blood-brain or blood-cerebrospinal fluid barrier.     

Mast cells in neuroimmune function: Neurotoxicological and neuropharmacological perspectives

Mast cells are located in close proximity to neurons in the peripheral and central nervous systems, suggesting a functional role in normal and aberrant neurodegenerative states. They also possess many of the features of neurons, in terms of monoaminergic systems, responsiveness to neurotrophins and neuropeptides and the ability to synthesise and release bioactive neurotrophic factors. Mast cells are able to secrete an array of potent mediators which may orchestrate neuroinflammation and affect the integrity of the blood-brain barrier. The cross-talk between mast cells, lymphocytes, neurons and glia constitutes a neuroimmune axis which is implicated in a range of neurodegenerative diseases with an inflammatory and/or autoimmune component, such as multiple sclerosis and Alzheimer's disease. Mast cells appear to make an important contribution to developing, mature and degenerating nervous systems and this should now be recognised when assessing the neurotoxic potential of xenobiotics.Abbreviations AChE acetylcholinesterase - ALS amyotrophic lateral sclerosis - APP amyloid precursor protein - BBB blood-brain barrier - BDNF brain-derived neurotrophic factor - CGRF calcitonin gene-related peptide - CNS central nervous system - CNTF ciliary neurotrophic factor - CSF cerebrospinal fluid - C48/80 compound 48/80 - CTMC connective tissue mast cells - EAA excitatory amino acids - EAE experimental allergic encephalomyelitis - ECMA ethylcholine mustard aziridinium ion - FACS fluorescent activated cell sorter - 5HT 5-hydroxytryptamine (serotonin) - HMT histamine-N-methyltransferase - HPMC human placental mast cells - HRNGF human recombinant nerve growth factor - IgE immunoglobulin E - MMC methyl mercuric chloride - MAOI monoamine oxidase inhibitors - MDMA methylenedioxymetamphetamine - MS multiple sclerosis - NGF nerve growth factor - NT3 neurotrophin 3 - PNS peripheral nervous system - RBMC rat brain mast cells - ROS reactive oxygen species - RPMC rat peritoneal mast cells - SLE systemic lupus erythematosus - SP substance P - TCA trichloroacetic acid - THA tetrahydroacridine - TCA tricyclic antidepressants Special issue dedicated to Dr. Robert Balázs.     

Brain Insulin Signaling and Alzheimer's Disease: Current Evidence and Future Directions

Insulin receptors in the brain are found in high densities in the hippocampus, a region that is fundamentally involved in the acquisition, consolidation, and recollection of new information. Using the intranasal method, which effectively bypasses the blood-brain barrier to deliver and target insulin directly from the nose to the brain, a series of experiments involving healthy humans has shown that increased central nervous system (CNS) insulin action enhances learning and memory processes associated with the hippocampus. Since Alzheimer's disease (AD) is linked to CNS insulin resistance, decreased expression of insulin and insulin receptor genes and attenuated permeation of blood-borne insulin across the blood-brain barrier, impaired brain insulin signaling could partially account for the cognitive deficits associated with this disease. Considering that insulin mitigates hippocampal synapse vulnerability to amyloid beta and inhibits the phosphorylation of tau, pharmacological strategies bolstering brain insulin signaling, such as intranasal insulin, could have significant therapeutic potential to deter AD pathogenesis.     

Receptor sites for atrial natriuretic factors in brain and associated structures: an overview

1. Recent data have clearly shown the existence of specific receptor binding sites for atrial natriuretic factors (ANF) or polypeptides in mammalian brain tissues. 2. Ligand selectivity pattern and coupling to cGMP production suggest that brain ANF sites are similar to high-affinity/low-capacity sites found in various peripheral tissues (kidney, adrenal gland, blood vessels). These brain ANF sites possibly are of the B-ANP subtype. 3. High densities of ANF binding sites are found especially in areas of the central nervous system associated with the control of various cardiovascular parameters (such as the subfornical organ and area postrema). However, high densities of sites are also present in other regions such as the hippocampus, cerebellum, and thalamus in the brain of certain mammalian species, suggesting that brain ANF could act as a neuromodulator of noncardiovascular functions. 4. The density of brain ANF binding sites is modified in certain animal models of cardiovascular disorders and during postnatal ontogeny, demonstrating the plasticity of these sites in the central nervous system (CNS). 5. Specific ANF binding sites are also found in various other CNS-associated tissues such as the eye, pituitary gland, and adrenal medulla. In these tissues ANF appears to act as a modulator of fluid production and hormone release. 6. Thus, ANF-like peptides and ANF receptor sites are present in brain and various peripheral tissues, demonstrating the existence of a family of brain/heart peptides.     

Angiotensin-converting enzyme activity in isolated brain microvessels.

The localization of angiotensin-converting enzyme (kininase II; ACE) in bovine cerebral cortex was studied by mechanically isolating microvessels from surrounding brain parenchyma. ACE specific activity, as assayed by generation of L-histidyl-L-leucine from the synthetic substrate hippuryl-L-histidyl-L-leucine, was enriched approximately 30 times in microvessels compared to homogenates of intact cerebral cortical gray matter. The nonapeptide 9a, SQ20,881), the orally active anti-hypertensive drug, 2-D-methyl-3-mercaptopropanoyl-L-proline (SQ14,225), and the vasoactive peptides bradykinin and angiotensin II inhibited this activity in a dose-dependent fashion. Brain microvessel ACE required chloride for optimal activity, was potentiated by cobalt nitrate, and was inhibited by the chelating agents EDTA and o-phenanthroline. Enzymatic generation of histidyl-leucine also was observed with the naturally occurring decapeptide substrate angiotensin I. In addition, microvessels obtained from bovine cerebellar cortex, hippocampus and corpus striatum, as well as from the cerebral cortex of Sprague-Dawley rats, were enriched in ACE activity. The presence of angiotensin-converting enzyme in brain microvessels suggests that cellular components of the blood-brain barrier may participate in the metabolism of peptide hormones such as angiotensin I and bradykinin within the central nervous system.     

A cell culture model of the blood-brain barrier

Endothelial cells that make up brain capillaries and constitute the blood-brain barrier become different from peripheral endothelial cells in response to inductive factors found in the nervous system. We have established a cell culture model of the blood-brain barrier by treating brain endothelial cells with a combination of astrocyte-conditioned medium and agents that elevate intracellular cAMP. These cells form high resistance tight junctions and exhibit low rates of paracellular leakage and fluid-phase endocytosis. They also undergo a dramatic structural reorganization as they form tight junctions. Results from these studies suggest modes of manipulating the permeability of the blood-brain barrier, potentially providing the basis for increasing the penetration of drugs into the central nervous system.     

中枢神经系统疾病治疗性抗体药物应用进展

单克隆抗体药物是一种新兴的治疗药物,具有高选择性,被用于多种疾病的治疗,如肿瘤、免疫疾病等,也可以用于中枢神经系统疾病,如阿尔茨海默病、帕金森病、中风和脑肿瘤等。然而,因为血脑屏障低通透性,限制了抗体药物在中枢神经系统疾病治疗中的应用,在很多神经系统疾病临床试验中,抗体药物并没有取得预期效果。如今,人们利用血脑屏障上内源性转运蛋白介导,设计了可以通过血脑屏障的抗体药物。对通过血脑屏障治疗性抗体药物研发进展及其应用前景进行了综述。     

Evolutionary combinatorial chemistry, a novel tool for SAR studies on peptide transport across the blood-brain barrier. Part 2. Design, synthesis and evaluation of a first generation of peptides.

The use of high-throughput methods in drug discovery allows the generation and testing of a large number of compounds, but at the price of providing redundant information. Evolutionary combinatorial chemistry combines the selection and synthesis of biologically active compounds with artificial intelligence optimization methods, such as genetic algorithms (GA). Drug candidates for the treatment of central nervous system (CNS) disorders must overcome the blood-brain barrier (BBB). This paper reports a new genetic algorithm that searches for the optimal physicochemical properties for peptide transport across the blood-brain barrier. A first generation of peptides has been generated and synthesized. Due to the high content of N-methyl amino acids present in most of these peptides, their syntheses were especially challenging due to over-incorporations, deletions and DKP formations. Distinct fragmentation patterns during peptide cleavage have been identified. The first generation of peptides has been studied by evaluation techniques such as immobilized artificial membrane chromatography (IAMC), a cell-based assay, log Poctanol/water calculations, etc. Finally, a second generation has been proposed.     

Towards the therapeutic use of intranasal neuropeptide administration in metabolic and cognitive disorders

The nose provides an effective way for delivering neuropeptides to the central nervous system, bypassing the blood-brain barrier and avoiding systemic side effects. Thereby intranasal neuropeptide administration enables the modulation of central nervous signaling pathways of body weight regulation and cognitive functions. Central nervous control of energy homeostasis is assumed to rely on hypothalamic neuropeptidergic pathways that are triggered by the peripheral adiposity signals insulin and leptin conveying the amount of body fat to the brain. Melanocortins, including alpha-melanocyte stimulating hormone (alpha-MSH), are essential for inducing anorexigenic/catabolic effects, i.e. for inhibiting caloric intake and increasing energy expenditure. Insulin, in addition to its function as an adiposity signal, also influences memory formation. Here we present a series of studies on the intranasal administration of MSH/ACTH(4-10), a melanocortin receptor agonist, and of insulin. Prolonged administration of MSH/ACTH(4-10) induced weight loss in normal-weight, but not in overweight humans. Intranasal insulin reduced body fat and improved memory functions in the absence of adverse peripheral side effects. Our results may contribute to the future development of therapeutic strategies in disorders like obesity and cognitive impairments that derive from dysfunctions of central nervous neuropeptidergic pathways.     

Streptozotocin-induced diabetes progressively increases blood-brain barrier permeability in specific brain regions in rats

This study investigated the effects of streptozotocin-induced diabetes on the functional integrity of the blood-brain barrier in the rat at 7, 28, 56, and 90 days, using vascular space markers ranging in size from 342 to 65,000 Da. We also examined the effect of insulin treatment of diabetes on the formation and progression of cerebral microvascular damage and determined whether observed functional changes occurred globally throughout the brain or within specific brain regions. Results demonstrate that streptozotocin-induced diabetes produced a progressive increase in blood-brain barrier permeability to small molecules from 28 to 90 days and these changes in blood-brain barrier permeability were region specific, with the midbrain most susceptible to diabetes-induced microvascular damage. In addition, results showed that insulin treatment of diabetes attenuated blood-brain barrier disruption, especially during the first few weeks; however, as diabetes progressed, it was evident that microvascular damage occurred even when hyperglycemia was controlled. Overall, results of this study suggest that diabetes-induced perturbations to cerebral microvessels may disrupt homeostasis and contribute to long-term cognitive and functional deficits of the central nervous system.     

Brain targeting through the autonomous nervous system: lessons from prion diseases

The human central nervous system (CNS) is targeted by diverse pathogens that use distinct pathways to bypass the blood-brain barrier, such as trafficking into the brain via infected blood cells or using retrograde axonal transport through sensory or motor fibers. Prions are transmissible agents that induce a devastating subacute neurodegeneration when they successfully reach the CNS. Two recent studies focusing on pathways of prion neuroinvasion provide converging evidence that, in the case of peripheral transmission, such as human consumption of contaminated tissue, the infectious agent uses the sympathetic noradrenergic neurons to reach the CNS after early replication in lymphoid tissues.     

Tumour necrosis factor-alpha mediates blood-brain barrier damage in HIV-1 infection of the central nervous system

The pathogenesis of brain inflammation and damage by human immunodeficiency virus (HIV) infection is unclear. Because blood-brain barrier damage and impaired cerebral perfusion are common features of HIV-1 infection, we evaluated the role of tumour necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) in mediating disruption of the blood-brain barrier. Levels of TNF-alpha were more elevated in cerebrospinal fluid (CSF) than in serum of HIV-1 infected patients and were mainly detected in those patients who had neurologic involvement. Intrathecal TNF-alpha levels correlated with signs of blood-brain barrier damage, manifested by high CSF to serum albumin quotient, and with the degree of barrier impairment. In contrast, intrathecal IL-1beta levels did not correlate with blood-brain barrier damage in HIV-1 infected patients. TNF-alpha seems to be related to active neural inflammation and to blood-brain barrier damage. The proinflammatory effects of TNF-alpha in the nervous system are dissociated from those of IL-1beta.     

Blood-brain barrier and atrial natriuretic factor

In brain, binding sites for atrial natriuretic factor (ANF) have been characterized in areas such as circumventricular organs that lack the tight capillary endothelial junctions of the blood-brain barrier and therefore are exposed to circulating peptides. Since atrial natriuretic factor acts directly on vascular endothelium and has been proposed to be actively involved in blood pressure regulation and fluid homeostasis, it is interesting to know whether ANF receptors exist on brain capillaries that constitute the blood-brain barrier and participate in the constant fluid exchange between blood and brain. The present paper reports recent evidence of the presence of ANF receptors located on the structure. It assesses the specific binding of 125I-labelled ANF on bovine brain microvessel preparations and its coupling with a guanylate cyclase system. The potential physiological role of ANF on brain microcirculation and blood-brain barrier functions is discussed.