This document summarizes research on the effects of intranasal administration of insulin to the brain. It discusses how intranasal insulin impacts both cognitive function and peripheral metabolism in the following ways: 1) Intranasal insulin improves memory function in studies with healthy subjects and memory-impaired patients by enhancing synaptic plasticity in the hippocampus and possibly increasing cerebral glucose metabolism. 2) It enhances neurobehavioral measures of brain activity and reduces activity in brain regions involved in food reward processing. 3) It reduces food intake and body fat content while increasing thermogenesis and fat oxidation, indicating effects on energy homeostasis. 4) It may regulate hepatic glucose production and insulin secretion from the pancreas through brain

1. Intranasal administration of insulin to the brain impacts cognitive function
and peripheral metabolism
Volker Ott1*, Christian Benedict2, Bernd Schultes3,
Jan Born4, Manfred Hallschmid1
1
Department 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 Behavioral
Neurobiology, University of Tübingen, Tübingen, Germany
Key Terms: Insulin, intranasal, central nervous system, glucose homeostasis, energy homeostasis,
thermogenesis, cognitive function, insulin resistance, hepatic glucose production
Word Count: 3487
* To whom correspondence and reprint requests should be addressed:
Department of Neuroendocrinology, Hs. 50.1
University of Luebeck
Ratzeburger Allee 160
23538 Luebeck, Germany
Phone: ++49-451-500-5375
Fax: ++49-451-500-3640
E-mail: ott@kfg.uni-luebeck.de
This 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 this
article as an "Accepted Article"; doi: 10.1111/j.1463-1326.2011.01490.x
1

2. Abstract
In 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 and
by experiments in humans that make use of the intranasal pathway of insulin administration to the
brain. Employing neurobehavioral and metabolic measurements as well as functional imaging
techniques, these studies have provided insight into a broad range of central and peripheral effects of
brain insulin. The present review focuses on CNS effects of insulin administered via the intranasal
route on cognition, in particular memory function, and whole-body energy homeostasis including
glucose metabolism. Furthermore, evidence is reviewed that suggests a pathophysiological role of
impaired brain insulin signaling in obesity and type 2 diabetes, which are hallmarked by peripheral
and possibly central nervous insulin resistance, as well as in conditions such as Alzheimer´s disease
where CNS insulin resistance might contribute to cognitive dysfunction.
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3. Introduction
In the mid-eighteen-hundreds, Claude Bernard demonstrated that puncture of the fourth cerebral
ventricle 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 the
regulation 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 rat
CNS [2], followed closely by the demonstration that insulin receptors are also expressed in the human
brain [3,4]. The insulin receptor, a tyrosine kinase receptor, is found in particularly high densities in
brain regions like the olfactory bulb, the cerebellum, the dentate gyrus, the pyriform cortex, the
hippocampus, the choroid plexus and the arcuate nucleus of the hypothalamus [5]. Some animal
studies suggest that insulin gene expression takes place within the CNS [6,7]. However, although
indicators of insulin transcription in human brain tissue have been presented [8], solid evidence for
local insulin production in the human CNS is still lacking [9]. It is rather assumed that peripheral
insulin 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 glucose
homeostasis [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 blood
stream in proportion to body fat stores, acts as an adiposity signal within in the CNS. In conjunction
with the adipokine leptin it provides the brain with negative feedback on the amount of peripheral
energy (i.e., fat) depots [17; for review see reference 18]. In line with this notion, CNS administration
of insulin reduces body adiposity by down-regulating food intake [12,19,20]. This catabolic effect has
been observed mainly in males, indicating that the role of insulin in central nervous body weight
regulation 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 cognitive
functions (Figure 1). Brain insulin signaling might even constitute a neuroendocrine link between both
domains and is therefore emerging as a potential target in the treatment of metabolic and cognitive
disorders [24].
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4. Enhancing central nervous insulin signaling by intranasal insulin administration in humans
Whereas in animals effective modes of insulin administration to the CNS, e.g., direct
intracerebroventricular (ICV) [25] or hypothalamic infusion [26] are routinely employed, insulin
administration to the human brain is more complicated. The conventional way of increasing CNS
concentrations of insulin to investigate effects of brain insulin relies on the intravenous (IV) infusion
of the hormone which has been shown to result in an increase in cerebrospinal fluid (CSF) insulin
concentrations [27]. This parenteral route, however, faces several serious drawbacks. The fall in blood
glucose levels resulting from systemic insulin infusion triggers the graded activation of endocrine axes
that 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 by
simultaneous continuous glucose infusion that per se may exert a biasing impact on (cognitive) brain
functioning. Moreover, the euglycemic-hyperinsulinemic clamp procedure implies considerable time
and labor investments and, generally, systemic insulin administration does not permit the dissection of
insulin’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 has
been shown in humans to bypass the BBB and effectively deliver insulin as well as other peptide
hormones to the CNS within one hour after administration in the absence of relevant systemic
absorption [32]. Accordingly, findings in animals demonstrate that intranasally administered
neuropeptides 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 several
hours [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 preferential
path of peptide transport into the CNS compartment [32,37], with additional transport along trigeminal
nerve 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

5. Information on this aspect of nasal insulin administration can be found elsewhere [e.g. references
39,40].
Insulin modulates neurobehavioral measures of brain activity and cognition in humans
Insulin effects on human brain activity have been revealed in a number of studies relying on different
methodological approaches. CNS responses to IN insulin were observed in the form of distinct
alterations in auditory evoked electroencephalographic brain potential responses during an oddball-
paradigm in healthy men while peripheral blood glucose levels remained unchanged [41] . In a related
study, the IN administration of 60 international units of insulin induced a negative shift in direct
current brain potentials that was also found after IV bolus injection of the hormone [42]. Both the IN
and the intravenous effects emerged within 20 min after insulin administration, indicating that
increases in systemic insulin concentrations are rapidly reported to the brain and that IN delivery of
the compound bypassing the body periphery can have a comparable impact on brain activity. The
impact of systemic insulin on cerebrocortical activity was likewise measured in euglycemic-
hyperinsulinemic clamp studies that utilized magnetoencephalographic (MEG) recordings and
demonstrated that obesity [43,44] and the fat-mass and obesity associated (FTO) allele variant
rs8050136 [45] modify insulin’s effects on cerebrocortical beta- and theta-wave activity .
Experiments employing functional magnetic resonance imaging (fMRI) have shown a positive
relationship between plasma insulin levels and activation of the right hippocampus in response to
viewing photographs of high-caloric food items [46]. In some contrast to these results, in another
fMRI study, food picture-related activity of this and other brain regions was found to be reduced after
IN insulin in comparison to placebo administration [47], raising the question if insulin acting on the
hippocampus is relevant for the regulation of ingestive behavior. On the other hand, the hippocampus
is highly relevant for the formation and maintenance of declarative memory, i.e. memory for facts and
episodes that is accessible to conscious recollection [for review see reference 48]. The ability to
acquire and retain memories depends on synaptic plasticity. Thus, long-term potentiation (LTP) and
long-term depression (LTD) of synaptic transmission, i.e. the augmentation or reduction of synaptic
efficacy are assumed to be important modulators of the strength of a memory representation [49,50].
5

6. Several studies indicate that insulin contributes to changes in hippocampal synaptic plasticity by
potentiating 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 that
process visual input [52]. In addition to these mechanims, insulin may promote glucose utilization of
neuronal 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 glucose
metabolism 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 improve
memory functions in studies in healthy humans [23,58,59]. In experiments performed in our lab, a
declarative memory test was conducted at the beginning and end of eight weeks of IN insulin
treatment (160 international units/d). In brief, lists of 30 words were presented and in addition to an
immediate recall 3 min after presentation, in a delayed recall one week later subjects wrote down all
words they still remembered. The delayed recall of words was significantly improved after eight
weeks of IN insulin administration whereas immediate word recall and non-declarative memory
functions were not affected [58]. In line with the strong accumulation of insulin receptors in
hippocampal and cortical brain structures [60], this finding indicates that insulin signaling contributes
to the formation of declarative, hippocampus-dependent memory contents. Noteworthy, beneficial
effects of IN insulin on declarative memory are not restricted to healthy subjects but have also been
shown in memory-impaired subjects [e.g. references 61,62]. Suzanne Craft and co-workers performed
a study in adults with mild cognitive impairments including amnestic symptoms (e.g., due to
Alzheimer´s disease) who were treated with IN insulin over a period of three weeks (2x20
international units/d) [61]. The primary outcome measure was the recall of a story containing 44
informational bits to which subjects listened and that they were asked to recall immediately and after a
20-minute delay. Patients treated with insulin showed significantly increased memory savings over the
21-day period compared to placebo. Considering reports of impaired brain glucose metabolism in
Alzheimer’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

7. [66] at least in part derive from increases in cerebral glucose metabolism. In related animal studies, IN
administration of the peptide slowed the development of diabetes-induced brain changes in a murine
model of type 1 diabetes [67].
CNS insulin signaling has been linked not only to cognitive but also to emotional functions of
the brain. Most recently, lentivirus-mediated downregulation of hypothalamic insulin receptor
expression in rats has been shown to elicit depressive and anxiety-like behaviors [68]. Vice versa, the
8-week IN insulin treatment described above induced an improvement in rated mood in our human
subjects [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 same
dose of IN insulin both effects were abrogated [69]. These findings suggest that disturbed CNS insulin
signaling/CNS insulin resistance might link metabolic disorders like obesity with cognitive
impairments and also depressive symptoms. Further evidence for this assumption is discussed below.
CNS insulin signaling and peripheral metabolism
In animal experiments, brain insulin signaling has emerged as an important regulator of energy
balance [70-72]. Insulin’s net effect on energy homeostasis depends on several factors. Whereas
intravenously administered insulin exerts direct peripheral and, after BBB transport, central nervous
effects, intransally administered insulin selectively targets the CNS. In this context it is interesting to
note that the central nervous action of insulin on energy homeostasis partly opposes its peripheral
effects. Whereas after peripheral (IV or subcutaneous) administration insulin acts as an anabolic
hormone by promoting weight gain in form of muscle and fat mass [73,74], IN and ICV insulin
administration 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]. In
parallel, central insulin exerts an anabolic impact on adipose tissue: in addition to the inhibition of
lipolysis by peripheral insulin [75,76], CNS insulin has been found to likewise inhibit lipolysis and
also to enhance lipogenesis [75,77,78]. Recent murine data also suggest that hypothalamic insulin
signaling 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
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8. postprandial thermogenesis [80]. Thus, the catabolic effect of IN insulin appears to stem from reduced
energy intake [21,23] and increased energy expenditure [79,80] alike.
Within the last decade, evidence has amounted that the impact of brain insulin signaling on
energy balance extends to glucose homeostasis. Hepatic glucose metabolism is an important
determinant of euglycemia [81]. By glycogenesis on the one hand and glycogenolysis and
gluconeogenesis on the other hand the liver stabilizes plasma glucose concentrations during
(postprandial) glucose abundance and (fasting) glucose depletion, respectively [82]. These processes
have long been known to be mediated by direct insulin action on hepatic insulin receptors and indirect
insulin effects on liver functions, including the downregulation of glucagon secretion and circulating
plasma nonesterified fatty acid concentrations [for review see reference 83]. However, hepatic
glucose metabolism also seems to be under the control of a brain-liver axis. Obici and co-workers
have shown in rodents that genetic downregulation of hypothalamic insulin receptor expression
disinhibits hepatic glucose production [84]. This finding clearly hints at a reduction in hepatic insulin
sensitivity as a consequence of impaired hypothalamic insulin signaling. Fittingly, insulin has been
found to open ATP-sensitive potassium channels on glucose-responsive hypothalamic neurons and
the resulting neuronal hyperpolarization seems to be responsible for the vagal transmission of a signal
that downregulates hepatic glucose production [14,84]. However, in an experiment in dogs,
quadrupling the concentration of circulating insulin selectively in brain afferent arteries did not
enhance the inhibition of hepatic glucose production [85] which leaves open the question whether the
contribution of hypothalamic insulin signaling to insulin’s hepatic effects has a species-dependent
component.
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 positive
feedback on its own secretion [86]. Interestingly, a similar effect was found for brain insulin over 30
years 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 been
hypothesized to be another regulator of blood glucose. While effects of CNS insulin on peripheral
insulin sensitivity and pancreatic insulin secretion in humans are largely unexplored, two recent
8

9. studies have gathered evidence that insulin delivery to the brain does affect peripheral glucose
metabolism. IN insulin administration before intake of a liquid meal reduced postprandial circulating
insulin levels in healthy subjects while plasma glucose levels were unchanged in comparison to
placebo [80]. This finding suggests that brain insulin administration can enhance postprandial
peripheral insulin sensitivity, adding to the feed-forward effect of brain insulin on pancreatic insulin
secretion observed in animals. Another recent set of experiments performed by Stockhorst and
colleagues 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 odor
due to the formulation with meta-cresol (a stabilizing agent in insulin solutions). They found an
increase in serum insulin concentrations and a reduction in blood glucose levels (within the
euglycemic range) after insulin compared with placebo, suggesting that activation of the brain-liver
axis enhanced pancreatic insulin secretion. On day 2, the procedure was repeated but placebo was
administered 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 was
sufficient to cause an even enhanced increasing effect on serum insulin concentrations, which points to
a significant contribution of neurocognitive learning mechanisms to the regulation of peripheral
glucose homeostasis by brain insulin.
Central nervous system insulin resistance
Peripheral insulin resistance is a well-known feature of type 2 diabetes and obesity. Insulin resistance
in central nervous structures might likewise contribute to the development not only of these metabolic
disorders but also of cognitive impairments. Raising systemic insulin levels by IV infusion results in
increased CSF insulin concentrations in healthy, normal-weight subjects [27]. Obese subjects and
Alzheimer patients seem to display relatively decreased CSF insulin concentrations suggesting
reduced 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 of
peripheral insulin resistance [43]. These findings support the notion that in obesity both BBB insulin
transport and the central nervous sensitivity to insulin are reduced. Related studies relying on acute
9

10. and prolonged IN administration of the hormone have refined this picture. In further MEG-based
experiments, IN insulin acutely increased cerebrocortical activity in response to food vs. non-food
pictures in lean but not in obese subjects [92]. Long-term (8 weeks) administration of IN insulin in
obese subjects failed to affect body weight and fat mass [93] but still enhanced declarative memory
and 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 glucose
homeostasis might be particularly prone to develop insulin resistance in obesity. In accordance with
these findings, in rats diet-induced obesity abolishes the catabolic actions of ICV insulin
administration, and a reduction in insulin receptor density in the hypothalamic arcuate nucleus causes
hyperphagia (and, as mentioned above, also disinhibits hepatic glucose production) [84]. On a
molecular level, activation of the PI-3 signaling cascade subsequent to the binding of insulin to its
receptor mediates the majority of central nervous insulin effects on energy homeostasis [for review see
reference 95], while disturbances of this pathway are regarded as a likely cause of neuronal insulin
resistance [for review see 96].
Reduced central nervous sensitivity might represent a pathophysiological link between
obesity, peripheral insulin resistance and cognitive disorders that have been found to be significantly
related in epidemiological studies [97,98]. Central insulin resistance seems to impair neuronal
plasticity via detrimental effects on glutamatergic and cholinergic pathways [99,100]. Such processes
are assumed to be influenced by genetic predisposition. One indicator for this assumption is that IN
insulin administration to patients with memory impairments improved memory functions
predominantly in non-carriers of the APOE*E4 allele, a risk factor for the development of Alzheimer’s
disease [62,66; for review see reference 101]. In this context, several further genetic polymorphisms
associated with reduced central nervous responsiveness to insulin have been characterized. Subjects
with the FTO gene polymorphism rs8050136 as well as carriers of the Gly972Arg polymorphism of
the 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 a
hyperphagic phenotype [102] characterised by a predilection for energy-dense foods [103], which
might involve decreased insulin sensitivity of food reward-related brain pathways [104].
10

11. Potential therapies aimed at overcoming CNS insulin resistance might include the IN
administration of insulin but could also rely on the insulin-sensitizing properties of the peroxisome
proliferator-activated receptor-ƴ (PPAR-ƴ) agonist rosiglitazone [105-108] and of metformin [109].
Regarding glucose homeostasis, enhancing central nervous/hypothalamic insulin signaling by insulin
administration to the brain might reinforce a vagally transmitted inhibitory signal on hepatic
gluconeogenesis [110], whose disinhibition represents a hallmark of type 2 diabetes and peripheral
insulin resistance [84]. The latter, moreover, appears to be highly associated with central nervous
insulin resistance [111]. IN insulin has been found to reduce HPA axis activity [58,93,94], thus
potentially 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 in
patients with mild cognitive impairments [61,62], the compound might ameliorate the harmful effects
on cognition that obesity and diabetes are suspected to engender [97].
Notwithstanding these encouraging results, some caveats need to be addressed. In light of the
hyperinsulinemia that accompanies peripheral insulin resistance, it might be argued that the reduction
of CSF insulin levels observed in obese subjects [91] could represent a protective mechanism that
limits central nervous hyperinsulinemia and the potentially detrimental sequelae of cellular insulin
resistance inside the brain. Although speculative, this assumption is in line with findings that hint at a
dose-dependent directionality of central insulin’s impact on memory function. For example, acute IN
insulin administration to Alzheimer patients improved verbal memory recall only at lower doses (20
international units), whereas higher doses (up to 60 international units) were not effective and, in
carriers of the APOE*E4 allele, even induced a decline in memory performance [62]. In healthy
subjects, the induction of acute moderate euglycemic hyperinsulinemia has been found to trigger
central nervous system inflammation and beta-amyloid formation [115], both of which are known risk
factors for the development of cognitive impairments. The notion that brain hyperinsulinemia might
promote central nervous insulin resistance is supported by a recent in vitro-study showing that
prolonged (4-24 h) exposure of hypothalamic cells to high concentrations of insulin led to inactivation
and degradation of the insulin receptor and IRS-1 [116]. Against this background and considering that
long-term data on therapeutic and side effects of IN insulin in humans are so far lacking, obviously
11

12. much work is still necessary to sound the potential of brain insulin administration in the treatment of
cognitive and metabolic disorders.
Conclusion
Insulin 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. The
experimental data briefly summarized here clearly implicate CNS insulin resistance as a potentially
important factor in the pathophysiology of obesity and systemic insulin resistance as well as of
cognitive impairments like Alzheimer’s disease. Moreover, the association between these disorders
found in epidemiological investigations may at least partly rely on dysregulated central nervous
insulin 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 the
brain may prove a viable therapeutic option in the treatment of these increasingly prevalent afflictions.
Acknowledgments
This work was supported by Deutsche Forschungsgemeinschaft (KFO126/B5), Germany. The funding
source had no input in the preparation, review, or approval of the manuscript.
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20

21. Figure 1. Insulin-mediated crosstalk between brain and body periphery
Intranasal/CNS and systemic insulin affect hepatic glucose production, pancreatic insulin secretion,
adipocyte function, energy homeostasis, and cognitive function. Respective citations in text boxes
refer to the reference list of the main text.
21

22. 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