accumulating information suggests several significant roles for insulin action in the brain . here , we briefly review selected studies that provoke exploration of this emerging field . more pointedly , we highlight seemingly serious deficiencies in our understanding of how insulin from the systemic circulation might actually get to the brain parenchyma , and suggest that addressing these deficiencies is requisite to both a basic understanding of insulin physiology and a rational consideration of therapeutics involving the delivery of insulin to the brain . because bulk brain glucose uptake is not affected by insulin in either rats ( 1 ) or humans ( 2,3 ) , the brain had long been considered  insulin insensitive .  while there is evidence for the expression and activity of glucose transport with the insulin - sensitive glut4 in a few selected nuclei , glucose transport into most neurons is glut3 dependent , while the glia and brain endothelial cells depend on glut1 activity for glucose uptake from brain interstitial fluid ( isf ) and plasma , respectively ( 4 ) . as insulin is not required for glut1- or glut3-mediated glucose transport , insulin is not needed for glucose transport into most brain cells . insulin does , however , play a role as a neuroregulatory peptide , and this role is slowly being unraveled ( 5 ) . early , provocative studies showed that chronic intracerebroventricular ( icv ) insulin administration markedly decreased food intake and body weight in primates ( 6 ) . in contrast , intravenous insulin administration to humans during an euglycemic clamp did not acutely affect food intake ( 7 ) . however , chronic intranasal insulin administration , which allows more direct access to the cerebrospinal fluid ( csf ) than systemic insulin administration , decreased food intake in fasting men and acutely affected postprandial selection of palatable food by women without causing hypoglycemia ( 8,9 ) . in addition , the correlation between adiposity and basal plasma insulin level led to the hypothesis that insulin in the central nervous system ( cns ) could , like leptin , be a chronic signal regulating or reporting on energy reserves , as opposed to an acute satiety signal ( 10 ) . consistent with an important role for cns insulin action , neuron - specific knockout of the insulin receptor ( nirko mouse ) enhanced diet - induced obesity and provoked insulin resistance , hypertriglyceridemia , and reproductive dysfunction ( 11 ) . thus , insulin in the brain appears to be important for the regulation of feeding behavior and monitoring energy stores . during the past decade , acute insulin action in the cns has been reported to regulate whole - body metabolic function . ( 12 ) demonstrated that acute delivery of insulin via icv injection suppressed hepatic glucose production ( hgp ) in conscious rats . moreover , icv - delivered antisense oligonucleotide knockdown of the insulin receptor ( ir ) blocked the ability of insulin to regulate hgp in rats ( 13 ) . in addition , the ability of icv insulin to regulate hgp is lost in nirko mice ( 11 ) . the arcuate nucleus in the hypothalamus appears to be critical , as ir deletion in agouti - related protein  expressing neurons from this area blunted the ability of both systemic and icv insulin to inhibit hgp in mice ( 14 ) . in contrast to these findings in rodents , studies in a canine model have convincingly demonstrated that insulin acts directly at the liver to suppress hgp , and , while insulin acting via the cns can influence the expression of certain gluconeogenic enzymes , it adds little to the acute physiologic regulation of hepatic glucose metabolism ( 1618 ) . species and significant methodological differences cloud the resolution of these disparate findings , and definitive addressing of the question in humans is technically beyond reach . beyond the cns effects of insulin on nutrient intake and acute metabolic effects , there is tantalizing emerging data on the effects of insulin on memory and cognition ( 19,20 ) . most particularly , insulin affects areas in the hippocampus that are active in reward recognition , as well as areas involved in more global cognitive and memory functions ( 21 ) . these insights have arisen from studies using either icv or nasal insulin delivery , circumventing the metabolic effects of peripherally delivered insulin that would limit such treatment to improve cognitive function . indeed , clinical trials examining the effect of intranasal insulin as a potential therapy in early alzheimer disease are ongoing ( see www.clinicaltrials.gov/ct2/results?term=nasal+insulin ) . the effects of insulin in the cns raise questions regarding how much circulating insulin reaches brain tissue and the route by which this may occur . before addressing that , however , there is the recurring question of whether insulin is produced locally within the brain . in drosophila , three of the seven circulating insulin - like peptides are secreted from the brain ( 22 ) to act locally on feeding behavior and systemically to regulate metabolism . while early work ( 23 ) suggested that immunoreactive insulin was present in rat brain at concentrations up to 100 times higher than that in plasma , subsequent studies did not confirm this ( 24 ) , and definitive evidence for the brain synthesizing significant amounts of insulin is lacking . furthermore , as insulin can clearly cross the blood - brain barrier ( bbb ) and may be concentrated in cns tissues , simple immune detection will not provide definitive evidence for a brain insulin source . an early rt - pcr study ( 25 ) found very small amounts of rat insulin 2 ( ins2 ) mrna , but no rat insulin 1 ( ins1 ) gene expression throughout the rat brain at all stages of development . this was confirmed in rabbit brain where rt - pcr revealed that the insulin gene was selectively expressed at low levels in olfactory bulbs and hippocampal neurons ( 26 ) . transcription of the ins2 gene , but not the ins1 gene , in the cns was shown in mice ( 27,28 ) . the ins2 gene is the rodent equivalent of the human ins gene , which is indeed expressed in several regions of the human brain ( 27 ) . these studies did not provide any evidence for a specific function for insulin produced within the cns . ( 29 ) reported detecting insulin mrna in neurogliaform cells in the rat cortex using single - cell quantitative rt - pcr . importantly , they also observed that the quantity of ins2 mrna per cell was greater when the concentration of extracellular glucose was increased . the number of spontaneous excitatory postsynaptic potentials measured in cells neighboring neurogliaform cells was decreased when neurogliaform cells were subjected to increasing extracellular glucose concentrations . interestingly , this effect was blocked when cells were treated with the ir blocker s961 . a similar effect of insulin has been observed in mouse proopiomelanocortin neurons ( 30 ) . these data obtained from cerebral cortical neuroglial cells provide the strongest evidence to date for locally produced insulin - regulating brain function as a neurotransmitter in a glucose - regulated manner ( 29 ) . in multiple studies and several species ( including humans ) , csf has been used as a surrogate for brain isf . recent findings , however ( vide infra ) , underscore that the composition of csf measured in the cerebral ventricles or the subarachnoid space of the cisterna magna or lumbar spine is quite different from brain isf . csf contributes one component to brain isf , and solute transport across the bbb provides another ( fig . 1 ) . the relative contribution of these two potential routes for allowing peripherally produced insulin access to brain isf is unknown . studies in both experimental animals and humans uniformly indicate that a large gradient exists between plasma and csf insulin concentrations in healthy individuals , with plasma concentrations being 10- to 20-fold higher ( 3133 ) and this gradient being even greater in obese humans ( 33 ) . analogous findings are reported ( 34 ) for the leptin plasma / csf gradient , suggesting that the transport of both proteins into csf is quite restricted under healthy conditions and is further impaired by insulin resistance . such findings raise the question of whether impaired systemic insulin delivery to csf could contribute to impaired feeding behavior , dysregulation of hepatic and adipose tissue metabolism , and increase the risk for cognitive decline seen in type 2 diabetic and insulin - resistant patients ( 20 ) . clearly , this would only be the case if insulin delivery via csf is a major contributor to the overall movement of insulin into brain isf . pmol / l ) are not sufficient to initiate significant ir activation , assuming that the ir in the cns has kinetic properties similar to those found for ir in peripheral tissues . equally striking in this regard are the results of kinetic studies in dogs ( 35 ) and humans ( 32 ) showing that the transit of insulin from plasma to csf under hyperinsulinemic euglycemic clamp conditions is very slow . even after 4 h of pharmacologic elevations in plasma insulin levels , such slow transport of insulin to the brain via the csf circulation would render insulin an unlikely satiety signal . csf insulin concentration is typical of that found in peripheral blood and in human lumbar csf in postprandial healthy humans . the brain microvascular endothelial cell ( bmec ) insulin concentration is estimated based on the observed ability of aortic endothelial cells to concentrate insulin ( 55 ) . kinetic studies also demonstrate that insulin movement from plasma to csf involves a saturable transport system ( 35 ) that is decreased in several insulin - resistant states ( 3638 ) . it is not known whether the ir is involved in moving insulin from blood to csf during csf formation by the choroid plexus , or whether the observed saturation results from the limited capacity of the more promiscuous transporter , megalin . this protein is known to mediate leptin transport across the choroid plexus ( 39 ) and can mediate insulin transport across the renal tubular epithelial cells ( 40 ) . mathematical modeling based on the transit time of radioactive insulin from plasma to the cisternal csf in dogs suggests an intermediate compartment between plasma and cisternal csf . whether that compartment is between the blood - csf barrier or simply reflects the mixing and flow through the ventricular system is unknown ( 41 ) . given the above issues , it is important to consider recent data that clarifies the relationship between the csf , as found in the cerebral ventricles or the subarachnoid space of the spinal cord , and the brain isf . in an elegant study , iliff et al . ( 42 ) injected several fluorescent markers of varying weight into the lateral ventricles of mice and traced their movement over time to examine their penetration into brain tissue . they identified a paravascular pathway involving the virchow - robin space , whereby csf produced by the choroid plexus in the ventricles first passes through the third and fourth ventricles and eventually enters the cisterna magna ( see fig . 1 in iliff et al . ) . from there , it enters the subarachnoid space and subsequently accesses the virchow - robin space , and this para - arteriolar pathway brings csf into contact with the blood vessel wall down to the level of the microvasculature . at this interface , water , nutrients ( e.g. , glucose , amino acids ) , electrolytes , and other solutes ( e.g. , possibly insulin ) that cross the bbb now mix with csf . the solutes from this admixture enter the isf of the brain by passing through glial foot processes from astrocytes that line the virchow - robin space ( fig . these foot processes act as filters that can restrict the entry of very large molecular weight solutes from brain isf . ( 43 ) suggest that the paravenous space also functions to clear waste from the brain interstitium , which is analogous to the lymphatic system in peripheral tissues . insulin added to the csf at the blood - csf barrier in the choroid plexus will traverse the csf circulation slowly as part of the bulk flow of csf . in humans , csf is made at a rate of 700 ml/24 h , with a total of 130 ml csf being present in the brain at any time . considering the relatively low insulin concentrations present in either the ventricular or lumbar subarachnoid csf ( see above ) , it can be estimated that the csf circulation could only deliver insulin to the brain parenchyma ( on a per gram of tissue basis ) at a rate 1/600th the rate of delivery to skeletal muscle or adipose tissues and < 1/30,000th the rate of delivery to the liver . this , of course , has implications for the body of work described above , in which insulin is infused into the csf . certainly , adding high doses of insulin to the csf will greatly raise csf and eventually brain isf insulin levels , which may account for the reported effects of icv - infused insulin on peripheral glucose and fat metabolism . this figure depicts a longitudinal section of a capillary , with blood and csf flowing from left to right . the tight junctions ( blocks ) and adherens junctions ( ovals ) between endothelial cells in the brain vasculature prevent paracellular transport through the endothelial layer . insulin transported across the endothelium would mix with the small amounts entering via csf in the virchow - robin space , and the mixture could then enter brain isf . the astrocytes act as the final sieve for insulin before entry into the brain isf . a few studies in rats have attempted to measure brain isf insulin concentration using microdialysis . these studies detected very low fasting insulin concentrations in the hypothalamus , which increased significantly 3060 min after a meal or peripheral insulin infusion , a rise more rapid than that seen in csf ( 44,45 ) . whether these measurements truly reflect isf or a combination of isf , csf , and vascular leakage in the region of the acutely placed microdialysis catheter is uncertain . these concerns , as well as the technical limitations that affect microdialysis measurements of large molecules ( 46 ) , have limited the use of this method . while the majority of work addressing insulin access to brain tissue has used csf as a surrogate for brain isf , some studies ( 47 ) have directly examined the  tissue content  of radiolabeled insulin in either brain sections or tissue samples as a function of time after intravenous injection in mice . these latter studies do not discriminate between insulin delivered via the choroid plexus ( csf circulation ) and insulin delivered directly across the bbb to specific nuclei . in addition , these studies have not been conducted in a fashion that allows estimation of the brain isf insulin concentration ; rather , they report on radioactivity in specific regions . such studies have indicated that intravenously administered insulin reaches many areas of the brain seemingly quickly , and appears to most readily penetrate the hypothalamus , the pons / medulla , and the hippocampus ( 48 ) . considering the possible direct movement of insulin across the bbb , investigators have also demonstrated that insulin binds to isolated brain microvessels with high affinity , which is consistent with participation by the ir ( 49 ) and that there is little , if any , degradation of insulin incubated with brain microvessels . consequently , whether the rapid access of radiolabeled ( i ) insulin to the brain parenchyma simply reflects insulin binding to endothelial cells in the brain microvasculature , its uptake by the endothelial cell , or its actual delivery to the brain isf is not resolved by these studies . this notwithstanding , these findings with i - insulin suggest that transit across the bbb may be the preferred route of insulin entry into the brain . however , if this is the case , it appears fair to say that we know virtually nothing about the cell biology of this process and how it might  or might not  be regulated . this would appear to be an important area for future investigation . whether the primary route for insulin transfer from the plasma to the brain isf happens at the bbb throughout the brain , or is restricted to the choroid plexus and csf , it is likely that the endothelium is involved . we have been working to unravel aspects of the kinetics , cell biology , and regulation of insulin transport by endothelial cells from peripheral vessels ( 50 ) . it begins with insulin binding to its receptor ( or at high insulin concentrations , the igf-1 receptor ) ( 51 ) . in aortic endothelial cells , the ir is associated with caveolae , and the knockdown of caveolin-1 ( the principal structural protein of caveolae ) or chemical disruption of caveolae interferes with endothelial cell insulin transport ( 52 ) . likewise , interfering with insulin action in the endothelial cell by inhibiting signaling through the pi3 kinase , src kinase , or map kinase pathway impedes endothelial cell insulin uptake ( 53 ) . it will be of great interest to see whether the brain endothelium behaves in a similar fashion . in closing , it is interesting to consider that this question of how insulin reaches targets within the cns may be part of a larger question of how a host of peptides synthesized in the periphery navigate this voyage . peptides such as leptin , glp-1 , ghrelin , cholecystokinin , and others each act centrally , and must reach brain isf to exert their specific effects . it will be fascinating to unravel just how this occurs for insulin and these other peptides .
we have learned over the last several decades that the brain is an important target for insulin action . 
 insulin in the central nervous system ( cns ) affects feeding behavior and body energy stores , the metabolism of glucose and fats in the liver and adipose , and various aspects of memory and cognition . 
 insulin may even influence the development or progression of alzheimer disease . yet , a number of seemingly simple questions ( e.g. , what is the pathway for delivery of insulin to the brain ? 
 is insulin s delivery to the brain mediated by the insulin receptor and is it a regulated process ? 
 is brain insulin delivery affected by insulin resistance ? ) are unanswered . 
 here we briefly review accumulated findings affirming the importance of insulin as a cns regulatory peptide , examine the current understanding of how peripheral insulin is delivered to the brain , and identify key gaps in the current understanding of this process .