Immediate but not long-term intranasal administration of insulin raises blood pressure in human beings

Effect of acute intranasal insulin administration on muscle sympathetic nerve activity in healthy young adults

Systemic insulin increases muscle sympathetic nerve activity (MSNA) via both central actions within the brainstem and peripheral activation of the arterial baroreflex. Augmented MSNA during hyperinsulinemia likely restrains peripheral vasodilation and contributes to the maintenance of blood pressure (BP). However, in the absence of insulin action within the peripheral vasculature, whether central insulin stimulation increases MSNA and influences peripheral hemodynamics in humans remains unknown. Herein, we hypothesized intranasal insulin administration would increase MSNA and BP in healthy young adults. Participants were assigned to time control [TC, n = 13 (5 females/8 males), 28 ± 1 yr] or 160 IU of intranasal insulin administered over 5 min [n = 15 (5 females/10 males), 26 ± 2 yr]; five (1 female/4 males) participants completed both conditions. MSNA (fibular microneurography), BP (finger photoplethysmography), and leg blood flow (LBF, femoral Doppler ultrasound) were assessed at baseline, and 15 and 30 min following insulin administration. Leg vascular conductance [LVC = (LBF ÷ mean BP) × 100] was calculated. Venous insulin and glucose concentrations remained unchanged throughout (P > 0.05). Following intranasal insulin administration, MSNA (burst frequency; baseline = 100%; minute 15, 121 ± 8%; minute 30, 118 ± 6%; P = 0.009, n = 7) and mean BP (baseline = 100%; minute 15, 103 ± 1%; minute 30, 102 ± 1%; P = 0.003) increased, whereas LVC decreased (baseline = 100%; minute 15, 93 ± 3%; minute 30, 99 ± 3%; P = 0.03). In contrast, MSNA, mean BP, and LVC were unchanged in TC participants (P > 0.05). We provide the first evidence that intranasal insulin administration in healthy young adults acutely increases MSNA and BP and decreases LVC. These results enhance mechanistic understanding of the sympathetic and peripheral hemodynamic response to insulin.NEW & NOTEWORTHY Systemic insulin increases muscle sympathetic nerve activity (MSNA) via central actions within the brainstem and peripheral activation of the arterial baroreflex. In the absence of peripheral insulin action, whether central insulin stimulation increases MSNA and influences peripheral hemodynamics in humans was unknown. We provide the first evidence that intranasal insulin administration increases MSNA and blood pressure and reduces leg vascular conductance. These results enhance mechanistic understanding of the sympathetic and hemodynamic response to insulin.

Insulin Delivery to the Brain via the Nasal Route: Unraveling the Potential for Alzheimer's Disease Therapy

This comprehensive review delves into the potential of intranasal insulin delivery for managing Alzheimer's Disease (AD) while exploring the connection between AD and diabetes mellitus (DM). Both conditions share features of insulin signalling dysregulation and oxidative stress that accelerate inflammatory response. Given the physiological barriers to brain drug delivery, including the blood-brain barrier, intranasal administration emerges as a non-invasive alternative. Notably, intranasal insulin has shown neuroprotective effects, impacting Aβ clearance, tau phosphorylation, and synaptic plasticity. In preclinical studies and clinical trials, intranasally administered insulin achieved rapid and extensive distribution throughout the brain, with optimal formulations exhibiting minimal systemic circulation. The detailed mechanism of insulin transport through the nose-to-brain pathway is elucidated in the review, emphasizing the role of olfactory and trigeminal nerves. Despite promising prospects, challenges in delivering protein drugs from the nasal cavity to the brain remain, including enzymes, tight junctions, mucociliary clearance, and precise drug deposition, which hinder its translation to clinical settings. The review encompasses a discussion of the strategies to enhance the intranasal delivery of therapeutic proteins, such as tight junction modulators, cell-penetrating peptides, and nano-drug carrier systems. Moreover, successful translation of nose-to-brain drug delivery necessitates a holistic understanding of drug transport mechanisms, brain anatomy, and nasal formulation optimization. To date, no intranasal insulin formulation has received regulatory approval for AD treatment. Future research should address challenges related to drug absorption, nasal deposition, and the long-term effects of intranasal insulin. In this context, the evaluation of administration devices for nose-to-brain drug delivery becomes crucial in ensuring precise drug deposition patterns and enhancing bioavailability.

Acute inorganic nitrate intake increases regional insulin action in the brain: Results of a double-blind, randomized, controlled cross-over trial with abdominally obese men

AIMS

Improving brain insulin sensitivity may be a promising approach in the prevention and treatment of metabolic and cognitive diseases. Our aim was to investigate acute effects of inorganic nitrate on regional cerebral blood flow (CBF) responses to intranasal insulin in abdominally obese men.

METHODS

Eighteen apparently healthy men, aged 18-60 years and with a waist circumference ≥ 102 cm, participated in a randomized, double-blind, placebo-controlled cross-over trial. The study consisted of two test days separated by at least one week. Men received in random order a drink providing 10 mmol (i.e., 625 mg nitrate) potassium nitrate or an isomolar placebo drink with potassium chloride. Brain insulin action was assessed 120-150 min after the drinks by quantifying acute effects of nasal insulin on regional CBF using arterial spin labeling Magnetic Resonance Imaging. Glucose and insulin concentrations were measured at regular intervals, while blood pressure was determined fasted and at 240 min.

RESULTS

Inorganic nitrate intake increased regional insulin action in five brain clusters. The two largest clusters were located in the right temporal lobe (ΔCBF: 7.0 ± 3.8 mL/100 g/min, volume: 5296 mm³, P < 0.001; and ΔCBF: 6.5 ± 4.3 mL/100 g/min, volume: 3592 mm³, P < 0.001), while two other cortical clusters were part of the right frontal (ΔCBF: 9.0 ± 6.0 mL/100 g/min, volume: 1096 mm³, P = 0.007) and the left parietal lobe (ΔCBF: 6.1 ± 4.3 mL/100 g/min, volume: 1024 mm³, P = 0.012). One subcortical cluster was located in the striatum (ΔCBF: 5.9 ± 3.2 mL/100 g/min, volume: 1792 mm³, P < 0.001). No effects of nitrate were observed on CBF before administration. Following nitrate intake, circulating nitrate plus nitrite concentrations increased over time (P = 0.003), but insulin and glucose concentrations and blood pressure did not change.

CONCLUSION

Acute inorganic nitrate intake may improve regional brain insulin action in abdominally obese men. These regions are involved in the regulation of different metabolic and cognitive processes. The trial was registered on January 6th, 2021 at ClinicalTrials.gov as NCT04700241.

Double-Blind Placebo-Controlled Pilot Investigation of the Safety of a Single Dose of Rapid-Acting Intranasal Insulin in Down Syndrome

Background

Individuals with Down syndrome are likely to develop clinical and neuropathological brain changes resembling Alzheimer’s disease dementia by the ages of 35–40 years. Intranasal insulin is a potential treatment for neurodegenerative disease that has been shown to reduce amyloid plaque burden and improve verbal memory performance in normal as well as memory-impaired adults. Investigations have shown that rapid-acting insulins may result in superior cognitive benefits compared with regular insulin.

Objectives

The primary objective of this study was to measure the safety and feasibility of intranasal rapid-acting glulisine in subjects with Down syndrome. Secondarily, we estimated the effects of intranasal glulisine on cognition and memory in Down syndrome.

Methods

A single-center, single-dose, randomized, double-blind, placebo-controlled, cross-over pilot study was performed to test the safety of intranasal glulisine vs placebo in 12 subjects with Down syndrome aged ≥ 35 years. Intranasal administration utilized the Impel NeuroPharma I109 Precision Olfactory Delivery (POD^®) device. The primary outcomes were the occurrence of any or related adverse and serious adverse events. Secondary post-treatment cognitive outcome measures included performance on the Fuld Object-Memory Evaluation and Rivermead Behavioral Memory Test.

Results

Intranasal glulisine was safe and well tolerated in the Down syndrome population. No adverse or serious adverse events were observed.

Conclusions

Further investigations are necessary to better evaluate the potential cognitive-enhancing role of intranasal insulin in the Down syndrome population.

ClinicalTrials.gov ID

NCT02432716.

Electronic supplementary material

The online version of this article (10.1007/s40268-020-00296-2) contains supplementary material, which is available to authorized users.

Memory advancement by intranasal insulin in type 2 diabetes (MemAID) randomized controlled clinical trial: Design, methods and rationale

BACKGROUND

Type 2 diabetes mellitus (T2DM) accelerates brain aging and increases the risk for dementia. Insulin is a key neurotrophic factor in the brain, where it modulates energy metabolism, neurovascular coupling, and regeneration. Impaired insulin-mediated brain signaling and central insulin resistance may contribute to cognitive and functional decline in T2DM. Intranasal insulin (INI) has emerged as a potential therapy for treating T2DM-related cognitive impairment.

METHODS/DESIGN

Ongoing from 2015, a prospective, two-center, randomized, double-blind, placebo-controlled trial of 210 subjects (120 T2DM and 90 non-diabetic older adults) randomized into four treatment arms (60 T2DM-INI, 60 T2DM-Placebo, 45 Control-INI, and 45 Control-Placebo) evaluating the long-term effects of daily intranasal administration of 40 International Units (IU) of human insulin, as compared to placebo (sterile saline) over 24 weeks and 24 weeks of post-treatment follow-up. Study outcomes are: 1) long-term INI effects on cognition, daily functionality, and gait speed; 2) identifying a clinically relevant phenotype that predicts response to INI therapy; 3) long-term safety.

CONCLUSION

This study addresses an important knowledge gap about the long-term effects of intranasal insulin on memory and cognition in older people with T2DM and non-diabetic controls, and may provide a novel therapeutic target for prevention and treatment of cognitive and functional decline and dementia. Trial Registration NCT02415556.

Safety of intranasal human insulin: A review

AIMS

To conduct a review in order to assess the safety of intranasal human insulin in clinical studies as well as the temporal stability of nasal insulin sprays.

MATERIAL AND METHODS

An electronic search was performed using MEDLINE. We selected original research on intranasal human insulin without further additives in humans. The studies included could be of any design as long as they used human intranasal insulin as their study product. All outcomes and adverse side effects were extracted.

RESULTS

A total of 38 studies in 1092 individuals receiving acute human intranasal insulin treatment and 18 studies in 832 individuals receiving human intranasal insulin treatment lasting between 21 days and 9.7 years were identified. No cases of symptomatic hypoglycaemia or severe adverse events (AEs) were reported. Transient local side effects in the nasal area were frequently experienced after intranasal insulin and placebo spray, while other AEs were less commonly reported. There were no reports of participants being excluded as a result of AEs. No instances of temporal stability of nasal insulin were reported in the literature. Tests on insulin that had been repacked into spray flasks showed that it had a chemical stability of up to 57 days.

CONCLUSIONS

Our retrospective review of published studies on intranasal insulin did not reveal any safety concerns; however, there were insufficient data to ensure the long-term safety of this method of chronic insulin administration. Improved insulin preparations that cause less nasal irritation would be desirable for future treatment.

Intranasal insulin in Alzheimer's disease: Food for thought

Accumulating evidence suggests that disrupted brain insulin signaling promotes the development and progression of Alzheimer's disease (AD), driving clinicians to target this circuitry. While both traditional and more modern antidiabetics show promise in combating insulin resistance, intranasal insulin appears to be the most efficient method of boosting brain insulin. Furthermore, intranasal delivery elegantly avoids adverse effects from peripheral insulin administration. However, there remain significant open questions regarding intranasal insulin's efficacy, safety, and potential as an adjunct or mono-therapy. Thus, this review aims to critically evaluate the present evidence and future potential of intranasal insulin as a meaningful treatment for AD. This article is part of the Special Issue entitled 'Metabolic Impairment as Risk Factors for Neurodegenerative Disorders.'

Metabolic and Cognitive Outcomes of Subchronic Once-Daily Intranasal Insulin Administration in Healthy Men

Insulin acts in the brain to limit food intake and improve memory function. We have previously shown that 8 weeks of intranasal insulin delivered in four daily doses of 40 IU decrease body weight and enhance word list recall. In the present study, we investigated the effect on body composition, endocrine parameters, and memory performance of 8 weeks of once-daily administration of 160 IU in healthy men. We assumed that intranasal insulin administered before nocturnal sleep, a period of relative metabolic inactivity that moreover benefits memory formation, would be superior to insulin delivery in the morning and placebo administration. After a 2-week baseline period, healthy male normal-weight subjects (mean age, 27.1 ± 0.9 years) received either placebo, 160 IU intranasal insulin in the morning, or 160 IU in the evening (n = 12 per group) for 8 consecutive weeks. Throughout the experiment, we measured body weight and body composition as well as circulating concentrations of glucose, insulin, adrenocorticotropin, cortisol, growth hormone, insulin-like growth-factor 1, adiponectin, and leptin. Declarative and procedural memory function was repeatedly assessed by means of, respectively, word list recall and word-stem priming. We found that neither morning nor evening insulin compared to placebo administration induced discernible changes in body weight and body composition. Delayed recall of words showed slight improvements by insulin administration in the evening, and serum cortisol concentrations were reduced after 2 weeks of insulin administration in the morning compared to the other groups. Results indicate that catabolic long-term effects of central nervous insulin delivery necessitate repetitive, presumably pre-meal delivery schedules. The observed memory improvements, although generally weaker than previously found effects, suggest that sleep after intranasal insulin administration may support its beneficial cognitive impact.

Nose-to-brain delivery of insulin enhanced by a nanogel carrier

Recent evidences suggest that insulin delivery to the brain can be an important pharmacological therapy for some neurodegenerative pathologies, including Alzheimer disease (AD). Due to the presence of the Blood Brain Barrier, a suitable carrier and an appropriate route of administration are required to increase the efficacy and safety of the treatment. Here, poly(N-vinyl pyrrolidone)-based nanogels (NG), synthetized by e-beam irradiation, alone and with covalently attached insulin (NG-In) were characterized for biocompatibility and brain delivery features in a mouse model. Preliminarily, the biodistribution of the "empty" nanocarrier after intraperitoneal (i.p.) injection was investigated by using a fluorescent-labeled NG. By fluorescence spectroscopy, SEM and dynamic light scattering analyses we established that urine clearance occurs in 24h. Histological liver and kidneys inspections indicated that no morphological alterations of tissues occurred and no immunological response was activated after NG injection. Furthermore, after administration of the insulin-conjugated nanogels (NG-In) through the intranasal route (i.n.) no alteration or immunogenic response of the nasal mucosa was observed, suggesting that the formulation is well tolerated in mouse. Moreover, an enhancement of NG-In delivery to the different brain areas and of its biological activity, measured as Akt activation levels, with reference to free insulin administration was demonstrated. Taken together, these results indicate that the synthesized NG-In enhances brain insulin delivery upon i.n. administration and strongly encourage its further evaluation as therapeutic agent against some neurodegenerative diseases.

Brain Insulin Resistance at the Crossroads of Metabolic and Cognitive Disorders in Humans

Ever since the brain was identified as an insulin-sensitive organ, evidence has rapidly accumulated that insulin action in the brain produces multiple behavioral and metabolic effects, influencing eating behavior, peripheral metabolism, and cognition. Disturbances in brain insulin action can be observed in obesity and type 2 diabetes (T2D), as well as in aging and dementia. Decreases in insulin sensitivity of central nervous pathways, i.e., brain insulin resistance, may therefore constitute a joint pathological feature of metabolic and cognitive dysfunctions. Modern neuroimaging methods have provided new means of probing brain insulin action, revealing the influence of insulin on both global and regional brain function. In this review, we highlight recent findings on brain insulin action in humans and its impact on metabolism and cognition. Furthermore, we elaborate on the most prominent factors associated with brain insulin resistance, i.e., obesity, T2D, genes, maternal metabolism, normal aging, inflammation, and dementia, and on their roles regarding causes and consequences of brain insulin resistance. We also describe the beneficial effects of enhanced brain insulin signaling on human eating behavior and cognition and discuss potential applications in the treatment of metabolic and cognitive disorders.

The Rationale for Insulin Therapy in Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia, with a prevalence that increases with age. By 2050, the worldwide number of patients with AD is projected to reach more than 140 million. The prominent signs of AD are progressive memory loss, accompanied by a gradual decline in cognitive function and premature death. AD is the clinical manifestation of altered proteostasis. The initiating step of altered proteostasis in most AD patients is not known. The progression of AD is accelerated by several chronic disorders, among which the contribution of diabetes to AD is well understood at the cell biology level. The pathological mechanisms of AD and diabetes interact and tend to reinforce each other, thus accelerating cognitive impairment. At present, only symptomatic interventions are available for treating AD. To optimise symptomatic treatment, a personalised therapy approach has been suggested. Intranasal insulin administration seems to open the possibility for a safe, and at least in the short term, effective symptomatic intervention that delays loss of cognition in AD patients. This review summarizes the interactions of AD and diabetes from the cell biology to the patient level and the clinical results of intranasal insulin treatment of cognitive decline in AD.

[Diabetes mellitus and the central nervous system]

The review considers the current views on the central nervous system (CNS) in diabetes mellitus (DM). It discusses an attitude towards the term 'diabetic encephalopathy'. The data of investigations of cognitive functions in types 1 and 2 DM and the brain structural imaging results obtained using up-to-date technologies are considered. The results of studies of the factors that induce cerebral changes in DM and their associated cognitive impairments are given. There is evidence that hyperglycemia has a more considerable impact on the above processes than hypoglycemia; other possible factors, apart from blood glucose control, are set out. The current views on the function of insulin in the CNS and the relationship of central insulin resistance to Alzheimer's disease are outlined. There are current data on intranasal insulin application that is still exploratory, but, as might be judged from the findings, may by a promising method for the treatment and prevention of cognitive decline in both patients with DM and those without this condition.

Impaired insulin action in the human brain: causes and metabolic consequences

Over the past few years, evidence has accumulated that the human brain is an insulin-sensitive organ. Insulin regulates activity in a limited number of specific brain areas that are important for memory, reward, eating behaviour and the regulation of whole-body metabolism. Accordingly, insulin in the brain modulates cognition, food intake and body weight as well as whole-body glucose, energy and lipid metabolism. However, brain imaging studies have revealed that not everybody responds equally to insulin and that a substantial number of people are brain insulin resistant. In this Review, we provide an overview of the effects of insulin in the brain in humans and the relevance of the effects for physiology. We present emerging evidence for insulin resistance of the human brain. Factors associated with brain insulin resistance such as obesity and increasing age, as well as possible pathogenic factors such as visceral fat, saturated fatty acids, alterations at the blood-brain barrier and certain genetic polymorphisms, are reviewed. In particular, the metabolic consequences of brain insulin resistance are discussed and possible future approaches to overcome brain insulin resistance and thereby prevent or treat obesity and type 2 diabetes mellitus are outlined.

Intranasal Insulin and Insulin-Like Growth Factor 1 as Neuroprotectants in Acute Ischemic Stroke

Treatment options for stroke remain limited. Neuroprotective therapies, in particular, have invariably failed to yield the expected benefit in stroke patients, despite robust theoretical and mechanistic background and promising animal data. Insulin and insulin-like growth factor 1 (IGF-1) play a pivotal role in critical brain functions, such as energy homeostasis, neuronal growth, and differentiation. They may exhibit neuroprotective properties in acute ischemic stroke based upon their vasodilatory, anti-inflammatory and antithrombotic effects, as well as improvements of functional connectivity, neuronal metabolism, neurotransmitter regulation, and remyelination. Intranasally administered insulin has demonstrated a benefit for prevention of cognitive decline in older people, and IGF-1 has shown potential benefit to improve functional outcomes in animal models of acute ischemic stroke. The intranasal route presents a feasible, tolerable, safe, and particularly effective administration route, bypassing the blood-brain barrier and maximizing distribution to the central nervous system (CNS), without the disadvantages of systemic side effects and first-pass metabolism. This review summarizes the neuroprotective potential of intranasally administered insulin and IGF-1 in stroke patients. We present the theoretical background and pathophysiologic mechanisms, animal and human studies of intranasal insulin and IGF-1, and the safety and feasibility of intranasal route for medication administration to the CNS.

Intranasal insulin enhanced resting-state functional connectivity of hippocampal regions in type 2 diabetes

Type 2 diabetes mellitus (T2DM) alters brain function and manifests as brain atrophy. Intranasal insulin has emerged as a promising intervention for treatment of cognitive impairment. We evaluated the acute effects of intranasal insulin on resting-state brain functional connectivity in older adults with T2DM. This proof-of-concept, randomized, double-blind, placebo-controlled study evaluated the effects of a single 40 IU dose of insulin or saline in 14 diabetic and 14 control subjects. Resting-state functional connectivity between the hippocampal region and default mode network (DMN) was quantified using functional MRI (fMRI) at 3Tesla. Following insulin administration, diabetic patients demonstrated increased resting-state connectivity between the hippocampal regions and the medial frontal cortex (MFC) as compared with placebo (cluster size: right, P = 0.03) and other DMN regions. On placebo, the diabetes group had lower connectivity between the hippocampal region and the MFC as compared with control subjects (cluster size: right, P = 0.02), but on insulin, MFC connectivity was similar to control subjects. Resting-state connectivity correlated with cognitive performance. A single dose of intranasal insulin increases resting-state functional connectivity between the hippocampal regions and multiple DMN regions in older adults with T2DM. Intranasal insulin administration may modify functional connectivity among brain regions regulating memory and complex cognitive behaviors.

Insulin, aging, and the brain: mechanisms and implications

There is now an impressive body of literature implicating insulin and insulin signaling in successful aging and longevity. New information from in vivo and in vitro studies concerning insulin and insulin receptors has extended our understanding of the physiological role of insulin in the brain. However, the relevance of these to aging and longevity remains to be elucidated. Here, we review advances in our understanding of the physiological role of insulin in the brain, how insulin gets into the brain, and its relevance to aging and longevity. Furthermore, we examine possible future therapeutic applications and implications of insulin in the context of available models of delayed and accelerated aging.

Intranasal insulin therapy for cognitive impairment and neurodegeneration: current state of the art

INTRODUCTION

Growing evidence supports the concept that insulin resistance plays an important role in the pathogenesis of cognitive impairment and neurodegeneration, including in Alzheimer's disease (AD). The metabolic hypothesis has led to the development and utilization of insulin- and insulin agonist-based treatments. Therapeutic challenges faced include the ability to provide effective treatments that do not require repeated injections and also the ability to minimize the potentially hazardous off-target effects.

AREAS COVERED

This review covers the role of intranasal insulin therapy for cognitive impairment and neurodegeneration, particularly AD. The literature reviewed focuses on data published within the past 5 years as this field is evolving rapidly. The review provides evidence that brain insulin resistance is an important and early abnormality in AD, and that increasing brain supply and utilization of insulin improves cognition and memory. Emphasis was placed on discussing outcomes of clinical trials and interpreting discordant results to clarify the benefits and limitations of intranasal insulin therapy.

EXPERT OPINION

Intranasal insulin therapy can efficiently and directly target the brain to support energy metabolism, myelin maintenance, cell survival and neuronal plasticity, which begin to fail in the early stages of neurodegeneration. Efforts must continue toward increasing the safety, efficacy and specificity of intranasal insulin therapy.

Enhancement of vasoreactivity and cognition by intranasal insulin in type 2 diabetes

OBJECTIVE

To determine acute effects of intranasal insulin on regional cerebral perfusion and cognition in older adults with type 2 diabetes mellitus (DM).

RESEARCH DESIGN AND METHODS

This was a proof-of-concept, randomized, double-blind, placebo-controlled intervention evaluating the effects of a single 40-IU dose of insulin or saline on vasoreactivity and cognition in 15 DM and 14 control subjects. Measurements included regional perfusion, vasodilatation to hypercapnia with 3-Tesla MRI, and neuropsychological evaluation.

RESULTS

Intranasal insulin administration was well tolerated and did not affect systemic glucose levels. No serious adverse events were reported. Across all subjects, intranasal insulin improved visuospatial memory (P ≤ 0.05). In the DM group, an increase of perfusion after insulin administration was greater in the insular cortex compared with the control group (P = 0.0003). Cognitive performance after insulin administration was related to regional vasoreactivity. Improvements of visuospatial memory after insulin administration in the DM group (R(2)adjusted = 0.44, P = 0.0098) and in the verbal fluency test in the control group (R(2)adjusted = 0.64, P = 0.0087) were correlated with vasodilatation in the middle cerebral artery territory.

CONCLUSIONS

Intranasal insulin administration appears safe, does not affect systemic glucose control, and may provide acute improvements of cognitive function in patients with type 2 DM, potentially through vasoreactivity mechanisms. Intranasal insulin-induced changes in cognitive function may be related to vasodilatation in the anterior brain regions, such as insular cortex that regulates attention-related task performance. Larger studies are warranted to identify long-term effects and predictors of positive cognitive response to intranasal insulin therapy.

Brain insulin and leptin signaling in metabolic control: from animal research to clinical application

Besides the well-characterized effects of brain insulin and leptin in regulating food intake, insulin and leptin signaling to the central nervous system modulates a variety of metabolic processes, such as glucose and lipid homeostasis, as well as energy expenditure. This review summarizes the current literature on the contribution of central nervous insulin and leptin action to metabolic control in animals and humans. Potential therapeutic options based on the direct delivery of these peptides to the brain by, for example, intranasal administration, are discussed.

Intranasal treatment of central nervous system dysfunction in humans

One of the most challenging problems facing modern medicine is how to deliver a given drug to a specific target at the exclusion of other regions. For example, a variety of compounds have beneficial effects within the central nervous system (CNS), but unwanted side effects in the periphery. For such compounds, traditional oral or intravenous drug delivery fails to provide benefit without cost. However, intranasal delivery is emerging as a noninvasive option for delivering drugs to the CNS with minimal peripheral exposure. Additionally, this method facilitates the delivery of large and/or charged therapeutics, which fail to effectively cross the blood-brain barrier (BBB). Thus, for a variety of growth factors, hormones, neuropeptides and therapeutics including insulin, oxytocin, orexin, and even stem cells, intranasal delivery is emerging as an efficient method of administration, and represents a promising therapeutic strategy for the treatment of diseases with CNS involvement, such as obesity, Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, depression, anxiety, autism spectrum disorders, seizures, drug addiction, eating disorders, and stroke.

Nasal insulin changes peripheral insulin sensitivity simultaneously with altered activity in homeostatic and reward-related human brain regions

AIMS/HYPOTHESIS

Impaired insulin sensitivity is a major factor leading to type 2 diabetes. Animal studies suggest that the brain is involved in the regulation of insulin sensitivity. We investigated whether insulin action in the human brain regulates peripheral insulin sensitivity and examined which brain areas are involved.

METHODS

Insulin and placebo were given intranasally. Plasma glucose, insulin and C-peptide were measured in 103 participants at 0, 30 and 60 min. A subgroup (n = 12) was also studied with functional MRI, and blood sampling at 0, 30 and 120 min. For each time-point, the HOMA of insulin resistance (HOMA-IR) was calculated as an inverse estimate of peripheral insulin sensitivity.

RESULTS

Plasma insulin increased and subsequently decreased. This excursion was accompanied by slightly decreased plasma glucose, resulting in an initially increased HOMA-IR. At 1 h after insulin spray, the HOMA-IR subsequently decreased and remained lower up to 120 min. An increase in hypothalamic activity was observed, which correlated with the increased HOMA-IR at 30 min post-spray. Activity in the putamen, right insula and orbitofrontal cortex correlated with the decreased HOMA-IR at 120 min post-spray.

CONCLUSIONS/INTERPRETATION

Central insulin action in specific brain areas, including the hypothalamus, may time-dependently regulate peripheral insulin sensitivity. This introduces a potential novel mechanism for the regulation of peripheral insulin sensitivity and underlines the importance of cerebral insulin action for the whole organism.

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.

Inhaled insulin-intrapulmonary, intranasal, and other routes of administration: mechanisms of action

BACKGROUND

After discovery of insulin as a hypoglycemic agent in 1921 various routes of administration to control blood glucose were attempted. These included subcutaneous, oral, rectal, sublingual, buccal, transdermal, vaginal, intramuscular, intrapulmonary and intranasal delivery systems. While each delivery system controlled hyperglycemia the subcutaneous route was given priority until 2006 when the Federal Drug Administration (FDA) approved the first commercially available pulmonary inhaled insulin.

METHODS

A review of major publications dealing with intrapulmonary administration of insulin was made to understand the physiological basis for its use, its efficacy in controlling hyperglycemia, its side effects and a comparison of its efficacy with other delivery methods.

RESULTS

The large surface area of the lung, its good vascularization, capacity for solute exchange and ultra thin membranes of alveolar epithelia are unique features that facilitate pulmonary insulin delivery. Large lung surface area ( approximately 75 m(2)) and thin alveolar epithelium ( approximately 0.1-0.5 microm) permit rapid drug absorption. First pass metabolism avoids gastrointestinal tract metabolism. Lung drug delivery depends upon a complex of factors including size, shape, density, charge and pH of delivery entity, velocity of entry, quality of aerosol deposition, character of alveoli, binding characteristics of aerosol on the alveolar surface, quality of alveolar capillary bed and its subsequent vascular tree. Many studies were performed to optimize each of these factors using several delivery systems to enhance pulmonary absorption. Availability was about 80% of subcutaneous administration with peak activity within 40-60 min of administration. Intranasal insulin delivery faces a smaller surface area ( approximately 180 cm(2)) with quite different absorption characteristics in nasal epithelium and its associated vasculature. Absorption depends upon many factors including composition and character of nasal mucus. Absorption of intranasal insulin resulted in a faster absorption time course than with subcutaneous insulin.

INTERPRETATION

After many studies the FDA approved Pfizer's product, Exubera, for intrapulmonary insulin delivery. While the system was effective its expense and putative side effects caused the drug company to withdraw the drug from the marketplace. Attempts by other pharmaceutical companies to use intrapulmonary insulin delivery are presently being made as well as some minor attempts to use intranasal delivery systems.

Neuroadrenergic dysfunction in obesity: an overview of the effects of weight loss

PURPOSE OF REVIEW

The prevalence of obesity is rising to epidemic proportions worldwide, and in tandem so is that of type 2 diabetes. Neuroadrenergic abnormalities, comprising increased resting sympathetic nervous system activity and blunted sympathetic neural responsiveness are recognized features of metabolic syndrome obesity, which contribute importantly to both the pathophysiology and adverse clinical prognosis of this high-risk population. Weight loss is recommended as first-line treatment for obesity. This review examines the effects of nonpharmacological weight loss on sympathetic nervous system function under basal and stimulated conditions.

RECENT FINDINGS

Human weight loss trials show that even moderate weight reduction is accompanied by significant attenuation in resting whole-body norepinephrine spillover rate and muscle sympathetic nerve activity, an improvement in cardiac autonomic modulation, and a reversal of blunted sympathetic responsiveness at both peripheral and central nervous system levels. Recent findings underscore the relevance of insulin resistance in mediating blunted sympathetic responsiveness to endogenous hyperinsulinemia induced by glucose ingestion. Impaired insulin transport across the blood-brain barrier may be one mechanism mediating these effects. Weight loss reverses blunted sympathetic responsiveness to glucose, which has implications for postprandial energy expenditure and body weight homeostasis.

SUMMARY

The autonomic dysfunction of obesity is reversible with weight loss, highlighting the importance of lifestyle intervention as a key therapeutic modality.

Weight loss may reverse blunted sympathetic neural responsiveness to glucose ingestion in obese subjects with metabolic syndrome

OBJECTIVE

The purpose of this study was to examine the effects of weight loss on sympathetic nervous system responsiveness to glucose ingestion in obese subjects with metabolic syndrome, in whom such responses are reportedly blunted.

RESEARCH DESIGN AND METHODS

Thirty four subjects, 19 insulin resistant and 15 insulin sensitive and aged 55 +/- 1 years (mean +/- SE) with BMI 31.6 +/- 0.6 kg/m2, who fulfilled the Adult Treatment Panel III criteria for metabolic syndrome participated. Simultaneous measurements of whole-body norepinephrine spillover rate, calf blood flow, and intra-arterial blood pressure were made at times 0, 30, 60, 90, and 120 min postglucose (75 g). The experiment was repeated after a 3-month hypocaloric diet with or without an exercise program.

RESULTS

Body weight decreased by 8.1 +/- 0.9 and 8.4 +/- 1.1 kg and resting norepinephrine spillover by 94 +/- 31 and 166 +/- 58 ng/min (all P < or = 0.01) in insulin-resistant and insulin-sensitive subjects, respectively. Weight loss was accompanied by a marked increase in sympathetic responsiveness after glucose but only in insulin-resistant subjects. In this subgroup, comparative increases in norepinephrine spillover rates at baseline and after weight loss averaged -3 +/- 25 versus 73 +/- 24 ng/min at 30 min (P = 0.039), 36 +/- 21 versus 115 +/- 28 ng/min at 60 min (P = 0.045), 9 +/- 21 versus 179 +/- 50 ng/min at 90 min (P < 0.001), and 40 +/- 48 versus 106 +/- 39 ng/min at 120 min (P = 0.24).

CONCLUSIONS

Weight loss reverses blunted sympathetic responsiveness to glucose ingestion in insulin-resistant subjects with metabolic syndrome, which is relevant to postprandial energy utilization and body weight homeostasis.

Sympathetic nervous system behavior in human obesity

The sympathetic nervous system (SNS) plays an essential role in the regulation of metabolic and cardiovascular homeostasis. Low SNS activity has been suggested to be a risk factor for weight gain and obesity development. In contrast, SNS activation is characteristic of a number of metabolic and cardiovascular diseases that occur more frequently in obese individuals. Until recently, the relation between obesity and SNS behavior has been controversial because previous approaches for assessing SNS activity in humans have produced inconsistent findings. Beginning in the early 1990s, many studies using state of the art neurochemical and neurophysiological techniques have provided important insight. The purpose of the present review is to provide an overview of our current understanding of the region specific alterations in SNS behavior in human obesity. We will discuss findings from our own laboratory which implicate visceral fat as an important depot linking obesity with skeletal muscle SNS activation. The influence of weight change on SNS behavior and the potential mechanisms and consequences of region specific SNS activation in obesity will also be considered.

Obese men respond to cognitive but not to catabolic brain insulin signaling

CONTEXT AND OBJECTIVE

Insulin acts in the brain to reduce food intake and body weight and is considered a major adiposity signal in energy homeostasis. In normal-weight men, intranasal insulin administration reduces body fat and improves declarative memory. The present experiments aimed to generalize these findings to obese patients, with a view to evaluate the therapeutic potential of the compound.

DESIGN, SUBJECTS AND MEASUREMENTS

Insulin and placebo, respectively, were intranasally administered four times a day (amounting to 160 IU day(-1)) over 8 weeks to two groups of 15 obese men each.

RESULTS

Contrasting with the catabolic effects in normal-weight men, insulin treatment did not induce any significant reduction of body weight (P>0.50) and body fat (P>0.44) in the obese subjects. However, in accordance with the effects in normal-weight men, declarative memory and mood were improved (P<0.05) and hypothalamic-pituitary-adrenal axis activity as assessed by circulating ACTH (P<0.01) and cortisol levels (P<0.04) was reduced.

CONCLUSIONS

Our results indicate that in obese men, intranasal insulin is functionally active in the central nervous system but fails to affect the neuronal networks critically involved in body weight regulation. We conclude that obesity in men is associated with central nervous resistance to the adiposity signal insulin. This defect likely contributes to the persistence of obesity in spite of elevated levels of circulating insulin in obese patients.

Targeting metabolic and cognitive pathways of the CNS by intranasal insulin administration

Intranasal administration effectively delivers neuropeptides to the CNS, bypassing the blood-brain barrier and avoiding systemic side effects. Using this route of administration, direct manipulations of central nervous signalling pathways involved in body weight regulation and cognition are possible. Specifically, the subchronic intranasal administration of insulin has been shown to reduce body fat and improve memory function in the absence of adverse peripheral side effects. These results may fuel the future development of therapeutic strategies in disorders such as obesity and Alzheimer's disease that are promoted by dysfunctions of central nervous neuropeptidergic pathways.

Intranasal insulin to improve memory function in humans

BACKGROUND

Compelling evidence indicates that central nervous insulin enhances learning and memory and in particular benefits hippocampus-dependent (i.e., declarative) memory. Intranasal administration of insulin provides an effective way of delivering the compound to the central nervous system, bypassing the blood-brain barrier and avoiding systemic side effects.

METHODS

Here we review a series of recent studies on the effects of intranasally administered insulin on memory functions in humans. In accordance with the beneficial effects of intravenously administered insulin on hippocampus-dependent declarative memory observed in hyperinsulinemic-euglycemic clamp studies, intranasal insulin administration similarly improves this type of memory, but in the absence of adverse peripheral side effects.

RESULT AND CONCLUSION

Considering that cerebrospinal fluid insulin levels are reduced in patients suffering from Alzheimer's disease, these results may be of considerable relevance for future clinical applications of insulin in the treatment of memory disorders.

Managing psychiatric disorders with antidiabetic agents: translational research and treatment opportunities

The objective of this paper is to synthesise extant studies describing the neurotherapeutic effects of antidiabetic agents in neuropsychiatric disorders. The authors conducted a MedLine search of all English-language articles published between 1966 and March 2006. The search terms were the nonproprietary names of established and putative antidiabetic agents (e.g., insulin, insulin secretagogues and sensitisers) cross-referenced with the individual names of Diagnostic and Statistical Manual of Mental Disorders (DSM)-III-R/IV/-TR-defined mood, psychotic, anxiety and dementing disorders. The search was augmented with a manual review of article reference lists. Contemporary models of disease pathophysiology in major depressive disorder, bipolar disorder and several dementing disorders (e.g., Alzheimer's disease) emphasise alterations in cellular plasticity and cytoarchitecture, with associated regional abnormalities in neuronal and glial density and morphology. Antidiabetic treatments (e.g., thiazolidinediones) may be capable of attenuating this pathological process via disparate mechanisms (e.g., neuroprotective, neurotrophic, anti-inflammatory). Enhanced insulin signalling with antidiabetic treatments may preserve and/or augment cognitive function in several neuropsychiatric disorders. Antidiabetic treatments, which maintain euglycaemia, hold promise as potent and clinically significant therapeutic interventions for several neuropsychiatric disorders.