Insulina intranasal e cognição
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    Insulina intranasal e cognição Insulina intranasal e cognição Document Transcript

    • Intranasal administration of insulin to the brain impacts cognitive function and peripheral metabolism Volker Ott1*, Christian Benedict2, Bernd Schultes3, Jan Born4, Manfred Hallschmid11Department of Neuroendocrinology, University of Luebeck, Germany; 2Department of Neuroscience,Functional Pharmacology, Uppsala University, Uppsala, Sweden; 3Interdisciplinary Obesity Center,Cantonal Hospital St. Gallen, Switzerland; 4Department of Medical Psychology and BehavioralNeurobiology, University of Tübingen, Tübingen, GermanyKey Terms: Insulin, intranasal, central nervous system, glucose homeostasis, energy homeostasis, thermogenesis, cognitive function, insulin resistance, hepatic glucose productionWord Count: 3487* To whom correspondence and reprint requests should be addressed:Department of Neuroendocrinology, Hs. 50.1University of LuebeckRatzeburger Allee 16023538 Luebeck, GermanyPhone: ++49-451-500-5375Fax: ++49-451-500-3640E-mail: ott@kfg.uni-luebeck.deThis is an Accepted Article that has been peer-reviewed and approved for publication in the Diabetes,Obesity and Metabolism, but has yet to undergo copy-editing and proof correction. Please cite thisarticle as an "Accepted Article"; doi: 10.1111/j.1463-1326.2011.01490.x 1
    • AbstractIn recent years, the central nervous system (CNS) has emerged as a principle site of insulin action.This notion is supported by studies in animals relying on intracerebroventricular insulin infusion andby experiments in humans that make use of the intranasal pathway of insulin administration to thebrain. Employing neurobehavioral and metabolic measurements as well as functional imagingtechniques, these studies have provided insight into a broad range of central and peripheral effects ofbrain insulin. The present review focuses on CNS effects of insulin administered via the intranasalroute on cognition, in particular memory function, and whole-body energy homeostasis includingglucose metabolism. Furthermore, evidence is reviewed that suggests a pathophysiological role ofimpaired brain insulin signaling in obesity and type 2 diabetes, which are hallmarked by peripheraland possibly central nervous insulin resistance, as well as in conditions such as Alzheimer´s diseasewhere CNS insulin resistance might contribute to cognitive dysfunction. 2
    • IntroductionIn the mid-eighteen-hundreds, Claude Bernard demonstrated that puncture of the fourth cerebralventricle induces glucosuria in mice [1], giving rise to the assumption that the central nervous system(CNS) is involved in glucose homeostasis. However, interest in the role the brain might play in theregulation of glucose metabolism abated and was only sparked again more than a century later.Havrankova and coworkers demonstrated in 1978 that insulin receptors are present throughout the ratCNS [2], followed closely by the demonstration that insulin receptors are also expressed in the humanbrain [3,4]. The insulin receptor, a tyrosine kinase receptor, is found in particularly high densities inbrain regions like the olfactory bulb, the cerebellum, the dentate gyrus, the pyriform cortex, thehippocampus, the choroid plexus and the arcuate nucleus of the hypothalamus [5]. Some animalstudies suggest that insulin gene expression takes place within the CNS [6,7]. However, althoughindicators of insulin transcription in human brain tissue have been presented [8], solid evidence forlocal insulin production in the human CNS is still lacking [9]. It is rather assumed that peripheralinsulin crosses the blood-brain barrier (BBB) by a saturable, receptor-mediated transport mechanism[9-11] and by binding to brain insulin receptors affects functions as diverse as energy and glucosehomeostasis [12-14], reproduction [13], growth [15] and neuronal plasticity [16]. Woods and co-workers were the first to perform seminal studies indicating that insulin, circulating within the bloodstream in proportion to body fat stores, acts as an adiposity signal within in the CNS. In conjunctionwith the adipokine leptin it provides the brain with negative feedback on the amount of peripheralenergy (i.e., fat) depots [17; for review see reference 18]. In line with this notion, CNS administrationof insulin reduces body adiposity by down-regulating food intake [12,19,20]. This catabolic effect hasbeen observed mainly in males, indicating that the role of insulin in central nervous body weightregulation may have sex-specific properties [21-23]. Moreover, as will be outlined in this review,insulin’s impact on the brain exceeds its involvement in energy homeostasis and pertains to cognitivefunctions (Figure 1). Brain insulin signaling might even constitute a neuroendocrine link between bothdomains and is therefore emerging as a potential target in the treatment of metabolic and cognitivedisorders [24]. 3
    • Enhancing central nervous insulin signaling by intranasal insulin administration in humansWhereas in animals effective modes of insulin administration to the CNS, e.g., directintracerebroventricular (ICV) [25] or hypothalamic infusion [26] are routinely employed, insulinadministration to the human brain is more complicated. The conventional way of increasing CNSconcentrations of insulin to investigate effects of brain insulin relies on the intravenous (IV) infusionof the hormone which has been shown to result in an increase in cerebrospinal fluid (CSF) insulinconcentrations [27]. This parenteral route, however, faces several serious drawbacks. The fall in bloodglucose levels resulting from systemic insulin infusion triggers the graded activation of endocrine axesthat can affect brain function [28], and below certain threshold levels inevitably impairs cognition[29]. Insulin-induced hypoglycemia and its potentially harmful effects can be prevented bysimultaneous continuous glucose infusion that per se may exert a biasing impact on (cognitive) brainfunctioning. Moreover, the euglycemic-hyperinsulinemic clamp procedure implies considerable timeand labor investments and, generally, systemic insulin administration does not permit the dissection ofinsulin’s effects on the CNS from its direct peripheral actions e.g. in liver [30] and adipose tissue [31].These methodological limitations are avoided by the intranasal (IN) route of administration that hasbeen shown in humans to bypass the BBB and effectively deliver insulin as well as other peptidehormones to the CNS within one hour after administration in the absence of relevant systemicabsorption [32]. Accordingly, findings in animals demonstrate that intranasally administeredneuropeptides reach brain structures involved in the regulation of metabolism and cognition [33,34].Intra-neuronal transport of neuropeptides from the nasal cavity to the olfactory bulb takes severalhours [35]. Thus, extra-neuronal passage through intercellular clefts of the olfactory epithelium,situated on the superior turbinate and opposite the nasal septum [36], is assumed to be the preferentialpath of peptide transport into the CNS compartment [32,37], with additional transport along trigeminalnerve branches to brainstem regions [38]. IN administration of insulin preparations for the purpose of systemic insulin substitution, i.e.,as an alternative approach to subcutaneous insulin injection, is not within the scope of this review. 4
    • Information on this aspect of nasal insulin administration can be found elsewhere [e.g. references39,40].Insulin modulates neurobehavioral measures of brain activity and cognition in humansInsulin effects on human brain activity have been revealed in a number of studies relying on differentmethodological approaches. CNS responses to IN insulin were observed in the form of distinctalterations in auditory evoked electroencephalographic brain potential responses during an oddball-paradigm in healthy men while peripheral blood glucose levels remained unchanged [41] . In a relatedstudy, the IN administration of 60 international units of insulin induced a negative shift in directcurrent brain potentials that was also found after IV bolus injection of the hormone [42]. Both the INand the intravenous effects emerged within 20 min after insulin administration, indicating thatincreases in systemic insulin concentrations are rapidly reported to the brain and that IN delivery ofthe compound bypassing the body periphery can have a comparable impact on brain activity. Theimpact of systemic insulin on cerebrocortical activity was likewise measured in euglycemic-hyperinsulinemic clamp studies that utilized magnetoencephalographic (MEG) recordings anddemonstrated that obesity [43,44] and the fat-mass and obesity associated (FTO) allele variantrs8050136 [45] modify insulin’s effects on cerebrocortical beta- and theta-wave activity . Experiments employing functional magnetic resonance imaging (fMRI) have shown a positiverelationship between plasma insulin levels and activation of the right hippocampus in response toviewing photographs of high-caloric food items [46]. In some contrast to these results, in anotherfMRI study, food picture-related activity of this and other brain regions was found to be reduced afterIN insulin in comparison to placebo administration [47], raising the question if insulin acting on thehippocampus is relevant for the regulation of ingestive behavior. On the other hand, the hippocampusis highly relevant for the formation and maintenance of declarative memory, i.e. memory for facts andepisodes that is accessible to conscious recollection [for review see reference 48]. The ability toacquire and retain memories depends on synaptic plasticity. Thus, long-term potentiation (LTP) andlong-term depression (LTD) of synaptic transmission, i.e. the augmentation or reduction of synapticefficacy are assumed to be important modulators of the strength of a memory representation [49,50]. 5
    • Several studies indicate that insulin contributes to changes in hippocampal synaptic plasticity bypotentiating LTD and LTP, respectively, at different synapses [for review see reference 51]. Moreover,insulin receptors have been found to increase synapse density and dendritic plasticity in structures thatprocess visual input [52]. In addition to these mechanims, insulin may promote glucose utilization ofneuronal networks [53]. Although globally glucose transport to the CNS is assumed to be insulin-independent [54-56], hyperinsulinemia has been shown in rodents to exert effects on glucosemetabolism in regions like the anterior hypothalamus and the basolateral amygdale [57]. In accordance with insulin’s effects on synaptic plasticity [52] and regional glucose uptake[52], central nervous administration of the hormone via the IN route has been shown to improvememory functions in studies in healthy humans [23,58,59]. In experiments performed in our lab, adeclarative memory test was conducted at the beginning and end of eight weeks of IN insulintreatment (160 international units/d). In brief, lists of 30 words were presented and in addition to animmediate recall 3 min after presentation, in a delayed recall one week later subjects wrote down allwords they still remembered. The delayed recall of words was significantly improved after eightweeks of IN insulin administration whereas immediate word recall and non-declarative memoryfunctions were not affected [58]. In line with the strong accumulation of insulin receptors inhippocampal and cortical brain structures [60], this finding indicates that insulin signaling contributesto the formation of declarative, hippocampus-dependent memory contents. Noteworthy, beneficialeffects of IN insulin on declarative memory are not restricted to healthy subjects but have also beenshown in memory-impaired subjects [e.g. references 61,62]. Suzanne Craft and co-workers performeda study in adults with mild cognitive impairments including amnestic symptoms (e.g., due toAlzheimer´s disease) who were treated with IN insulin over a period of three weeks (2x20international units/d) [61]. The primary outcome measure was the recall of a story containing 44informational bits to which subjects listened and that they were asked to recall immediately and after a20-minute delay. Patients treated with insulin showed significantly increased memory savings over the21-day period compared to placebo. Considering reports of impaired brain glucose metabolism inAlzheimer’s disease [63-65], it might be speculated that these effects and, in particular, acute insulin-induced enhancements of cognitive function in memory-impaired patients occurring within minutes 6
    • [66] at least in part derive from increases in cerebral glucose metabolism. In related animal studies, INadministration of the peptide slowed the development of diabetes-induced brain changes in a murinemodel of type 1 diabetes [67]. CNS insulin signaling has been linked not only to cognitive but also to emotional functions ofthe brain. Most recently, lentivirus-mediated downregulation of hypothalamic insulin receptorexpression in rats has been shown to elicit depressive and anxiety-like behaviors [68]. Vice versa, the8-week IN insulin treatment described above induced an improvement in rated mood in our humansubjects [58]. In mice, IN insulin enhanced object-memory and induced anxiolytic behavioral effects[69]. However, in mice with impaired glucose tolerance due to diet-induced obesity receiving the samedose of IN insulin both effects were abrogated [69]. These findings suggest that disturbed CNS insulinsignaling/CNS insulin resistance might link metabolic disorders like obesity with cognitiveimpairments and also depressive symptoms. Further evidence for this assumption is discussed below.CNS insulin signaling and peripheral metabolismIn animal experiments, brain insulin signaling has emerged as an important regulator of energybalance [70-72]. Insulin’s net effect on energy homeostasis depends on several factors. Whereasintravenously administered insulin exerts direct peripheral and, after BBB transport, central nervouseffects, intransally administered insulin selectively targets the CNS. In this context it is interesting tonote that the central nervous action of insulin on energy homeostasis partly opposes its peripheraleffects. Whereas after peripheral (IV or subcutaneous) administration insulin acts as an anabolichormone by promoting weight gain in form of muscle and fat mass [73,74], IN and ICV insulinadministration in humans and animals, respectively, induces catabolic effects by reducing food intake[19,23] and as a consequence body fat content [20,21] particularly in the male organism [21-23]. Inparallel, central insulin exerts an anabolic impact on adipose tissue: in addition to the inhibition oflipolysis by peripheral insulin [75,76], CNS insulin has been found to likewise inhibit lipolysis andalso to enhance lipogenesis [75,77,78]. Recent murine data also suggest that hypothalamic insulinsignaling potentiates brown adipose tissue thermogenesis through inhibition of warm sensitive neurons[79]. Our group corroborated these findings in humans by demonstrating that IN insulin enhances 7
    • postprandial thermogenesis [80]. Thus, the catabolic effect of IN insulin appears to stem from reducedenergy intake [21,23] and increased energy expenditure [79,80] alike. Within the last decade, evidence has amounted that the impact of brain insulin signaling onenergy balance extends to glucose homeostasis. Hepatic glucose metabolism is an importantdeterminant of euglycemia [81]. By glycogenesis on the one hand and glycogenolysis andgluconeogenesis on the other hand the liver stabilizes plasma glucose concentrations during(postprandial) glucose abundance and (fasting) glucose depletion, respectively [82]. These processeshave long been known to be mediated by direct insulin action on hepatic insulin receptors and indirectinsulin effects on liver functions, including the downregulation of glucagon secretion and circulatingplasma nonesterified fatty acid concentrations [for review see reference 83]. However, hepaticglucose metabolism also seems to be under the control of a brain-liver axis. Obici and co-workershave shown in rodents that genetic downregulation of hypothalamic insulin receptor expressiondisinhibits hepatic glucose production [84]. This finding clearly hints at a reduction in hepatic insulinsensitivity as a consequence of impaired hypothalamic insulin signaling. Fittingly, insulin has beenfound to open ATP-sensitive potassium channels on glucose-responsive hypothalamic neurons andthe resulting neuronal hyperpolarization seems to be responsible for the vagal transmission of a signalthat downregulates hepatic glucose production [14,84]. However, in an experiment in dogs,quadrupling the concentration of circulating insulin selectively in brain afferent arteries did notenhance the inhibition of hepatic glucose production [85] which leaves open the question whether thecontribution of hypothalamic insulin signaling to insulin’s hepatic effects has a species-dependentcomponent. In humans it has recently been shown that IV pretreatment with insulin potentiates glucose-induced pancreatic insulin secretion by 40%, suggesting that circulating insulin exerts a direct positivefeedback on its own secretion [86]. Interestingly, a similar effect was found for brain insulin over 30years ago in dogs, where ICV insulin administration increased pancreatic insulin secretion via a feed-forward mechanism [87,88]. This brain-pancreatic crosstalk involving the vagal nerve has beenhypothesized to be another regulator of blood glucose. While effects of CNS insulin on peripheralinsulin sensitivity and pancreatic insulin secretion in humans are largely unexplored, two recent 8
    • studies have gathered evidence that insulin delivery to the brain does affect peripheral glucosemetabolism. IN insulin administration before intake of a liquid meal reduced postprandial circulatinginsulin levels in healthy subjects while plasma glucose levels were unchanged in comparison toplacebo [80]. This finding suggests that brain insulin administration can enhance postprandialperipheral insulin sensitivity, adding to the feed-forward effect of brain insulin on pancreatic insulinsecretion observed in animals. Another recent set of experiments performed by Stockhorst andcolleagues indicates that such brain-pancreatic cross-talk is accessible to classical conditioning [89].On day 1, the investigators administered IN insulin vs. placebo that both have the same specific odordue to the formulation with meta-cresol (a stabilizing agent in insulin solutions). They found anincrease in serum insulin concentrations and a reduction in blood glucose levels (within theeuglycemic range) after insulin compared with placebo, suggesting that activation of the brain-liveraxis enhanced pancreatic insulin secretion. On day 2, the procedure was repeated but placebo wasadministered in both groups with the smell of meta-cresol functioning as a conditioned stimulus. Here,the presentation of the conditioned stimulus alone after pretreatment with IN insulin on day 1 wassufficient to cause an even enhanced increasing effect on serum insulin concentrations, which points toa significant contribution of neurocognitive learning mechanisms to the regulation of peripheralglucose homeostasis by brain insulin.Central nervous system insulin resistancePeripheral insulin resistance is a well-known feature of type 2 diabetes and obesity. Insulin resistancein central nervous structures might likewise contribute to the development not only of these metabolicdisorders but also of cognitive impairments. Raising systemic insulin levels by IV infusion results inincreased CSF insulin concentrations in healthy, normal-weight subjects [27]. Obese subjects andAlzheimer patients seem to display relatively decreased CSF insulin concentrations suggestingreduced insulin transport across the BBB [90,91]. Likewise, in comparison to normal-weight subjects,overweight humans show a decrease in MEG-recorded cortical activity during hyperinsulinemic-euglycemic clamp experiments that is directly related to the amount of body fat and the degree ofperipheral insulin resistance [43]. These findings support the notion that in obesity both BBB insulintransport and the central nervous sensitivity to insulin are reduced. Related studies relying on acute 9
    • and prolonged IN administration of the hormone have refined this picture. In further MEG-basedexperiments, IN insulin acutely increased cerebrocortical activity in response to food vs. non-foodpictures in lean but not in obese subjects [92]. Long-term (8 weeks) administration of IN insulin inobese subjects failed to affect body weight and fat mass [93] but still enhanced declarative memoryand dampened HPA-axis activity to a degree comparable with that observed in normal-weight subjects[58,94]. This differential insulin response implies that brain regions involved in energy and glucosehomeostasis might be particularly prone to develop insulin resistance in obesity. In accordance withthese findings, in rats diet-induced obesity abolishes the catabolic actions of ICV insulinadministration, and a reduction in insulin receptor density in the hypothalamic arcuate nucleus causeshyperphagia (and, as mentioned above, also disinhibits hepatic glucose production) [84]. On amolecular level, activation of the PI-3 signaling cascade subsequent to the binding of insulin to itsreceptor mediates the majority of central nervous insulin effects on energy homeostasis [for review seereference 95], while disturbances of this pathway are regarded as a likely cause of neuronal insulinresistance [for review see 96]. Reduced central nervous sensitivity might represent a pathophysiological link betweenobesity, peripheral insulin resistance and cognitive disorders that have been found to be significantlyrelated in epidemiological studies [97,98]. Central insulin resistance seems to impair neuronalplasticity via detrimental effects on glutamatergic and cholinergic pathways [99,100]. Such processesare assumed to be influenced by genetic predisposition. One indicator for this assumption is that INinsulin administration to patients with memory impairments improved memory functionspredominantly in non-carriers of the APOE*E4 allele, a risk factor for the development of Alzheimer’sdisease [62,66; for review see reference 101]. In this context, several further genetic polymorphismsassociated with reduced central nervous responsiveness to insulin have been characterized. Subjectswith the FTO gene polymorphism rs8050136 as well as carriers of the Gly972Arg polymorphism ofthe Insulin Receptor Substrate 1 (IRS1) exhibit a decreased cerebrocortical response to IV insulin[43,45]. Interestingly, these data support the suggested connection between the FTO variant and ahyperphagic phenotype [102] characterised by a predilection for energy-dense foods [103], whichmight involve decreased insulin sensitivity of food reward-related brain pathways [104]. 10
    • Potential therapies aimed at overcoming CNS insulin resistance might include the INadministration of insulin but could also rely on the insulin-sensitizing properties of the peroxisomeproliferator-activated receptor-ƴ (PPAR-ƴ) agonist rosiglitazone [105-108] and of metformin [109].Regarding glucose homeostasis, enhancing central nervous/hypothalamic insulin signaling by insulinadministration to the brain might reinforce a vagally transmitted inhibitory signal on hepaticgluconeogenesis [110], whose disinhibition represents a hallmark of type 2 diabetes and peripheralinsulin resistance [84]. The latter, moreover, appears to be highly associated with central nervousinsulin resistance [111]. IN insulin has been found to reduce HPA axis activity [58,93,94], thuspotentially opposing visceral adiposity and cognitive impairments due to stress-induced chronic HPA-axis overactivation [112-114]. Moreover, in light of IN insulin’s acute memory-improving effects inpatients with mild cognitive impairments [61,62], the compound might ameliorate the harmful effectson cognition that obesity and diabetes are suspected to engender [97]. Notwithstanding these encouraging results, some caveats need to be addressed. In light of thehyperinsulinemia that accompanies peripheral insulin resistance, it might be argued that the reductionof CSF insulin levels observed in obese subjects [91] could represent a protective mechanism thatlimits central nervous hyperinsulinemia and the potentially detrimental sequelae of cellular insulinresistance inside the brain. Although speculative, this assumption is in line with findings that hint at adose-dependent directionality of central insulin’s impact on memory function. For example, acute INinsulin administration to Alzheimer patients improved verbal memory recall only at lower doses (20international units), whereas higher doses (up to 60 international units) were not effective and, incarriers of the APOE*E4 allele, even induced a decline in memory performance [62]. In healthysubjects, the induction of acute moderate euglycemic hyperinsulinemia has been found to triggercentral nervous system inflammation and beta-amyloid formation [115], both of which are known riskfactors for the development of cognitive impairments. The notion that brain hyperinsulinemia mightpromote central nervous insulin resistance is supported by a recent in vitro-study showing thatprolonged (4-24 h) exposure of hypothalamic cells to high concentrations of insulin led to inactivationand degradation of the insulin receptor and IRS-1 [116]. Against this background and considering thatlong-term data on therapeutic and side effects of IN insulin in humans are so far lacking, obviously 11
    • much work is still necessary to sound the potential of brain insulin administration in the treatment ofcognitive and metabolic disorders.ConclusionInsulin binding to its receptors in the brain impacts a number of pivotal physiological functions,including energy uptake and expenditure, glucose metabolism, adipocyte function and cognition. Theexperimental data briefly summarized here clearly implicate CNS insulin resistance as a potentiallyimportant factor in the pathophysiology of obesity and systemic insulin resistance as well as ofcognitive impairments like Alzheimer’s disease. Moreover, the association between these disordersfound in epidemiological investigations may at least partly rely on dysregulated central nervousinsulin signaling. Although further studies are needed to substantiate the promising results of proof-of-concept experiments on central nervous insulin administration, overcoming insulin resistance in thebrain may prove a viable therapeutic option in the treatment of these increasingly prevalent afflictions.AcknowledgmentsThis work was supported by Deutsche Forschungsgemeinschaft (KFO126/B5), Germany. The fundingsource had no input in the preparation, review, or approval of the manuscript. 12
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    • Figure 1. Insulin-mediated crosstalk between brain and body peripheryIntranasal/CNS and systemic insulin affect hepatic glucose production, pancreatic insulin secretion,adipocyte function, energy homeostasis, and cognitive function. Respective citations in text boxesrefer to the reference list of the main text. 21
    • 2-4-1-9-5-8-3 Moon-Night Intranasal/CNS insulin enhances Intranasal insulin declarative (e.g. word pairs) and bypasses the BBB and working memory (e.g. number achieves maximal CSF sequences) learning [23;58;59].concentrations within 40 minutes [32]. Intranasal Insulin reaches the brain via receptor-mediated saturable insulin transport across the blood- Intranasal/CNS insulin decreases brain-barrier [9;10]. CNS insulin, like food intake and enhances CNS insulin inhibits systemic insulin, postprandial thermogenesis hepatic gluconeogenesis inhibits lipolysis and [21;23;80]. CNS insulin increases via vagal efferences pancreatic insulin secretion stimulates lipogenesis [14;110]. (feed-forward loop [87;88].) [77;78]. Liver Pancreas Systemic insulin Adipocytes inhibits gluco- Systemic insulin neogenesis via inhibits lipolysis and hepatic insulin stimulates receptors [82;83]. lipogenesis [75;76]. 22