Central Nervous Insulin Signaling in Sleep-Associated Memory Formation and Neuroendocrine Regulation

Effects of Intranasal Insulin Pretreatment on Preoperative Sleep Quality and Postoperative Delirium in Patients Undergoing Valve Replacement for Rheumatic Heart Disease

Background

Postoperative delirium (POD) is a common neurological complication associated with valve replacement. Preoperative sleep disturbance is a risk factor for POD development, and nasal insulin modulates the sleep-wake cycle. This study investigated the beneficial effects of intranasal insulin pretreatment on preoperative sleep quality and reducing POD in patients undergoing valve replacement for rheumatic heart disease.

Patients and Methods

This prospective, single-center, randomized controlled trial (RCT) included 76 adult patients aged 18-65 years undergoing valve surgery with cardiopulmonary bypass who were randomly allocated to receive intranasal insulin or normal saline interventions two days before surgery. POD incidence was on postoperative days 1 (T3), 2 (T4), and 3 (T5). Before the first intervention (T0), 1 d before surgery (T1), and before anesthesia on the day of surgery (T2), sleep quality was assessed and serum cortisol concentrations were measured. At T1 and T2, sleep quality related indicators monitored by sleep monitoring watches from the previous night were recorded.

Results

Compared with the normal saline group, 3 days after surgery, the insulin group showed a significantly reduced incidence of POD; significantly increased deep sleep, REM sleep, deep sleep continuity, and total sleep quality scores at T1 and T2; and significantly reduced serum cortisol concentration, PSQI scale, light sleep ratio, and wakefulness at T1 and T2.

Conclusion

The administration of 20 U of intranasal insulin twice daily, from 2 days preoperatively until 10 minutes preanesthesia on the day of surgery, can improved preoperative sleep quality significantly and reduced POD incidence in patients with rheumatic heart disease undergoing valve replacement.

Clinical Trial Registration

This study was registered with the Chinese Clinical Trial Registry (www.chictr.org.cn, with the unique identifier ChiCTR2100048515; July 9, 2021).

Protective effect of intranasal insulin on postoperative cognitive dysfunction in elderly patients with metabolic syndrome undergoing noncardiac surgery: a randomized clinical trial

BACKGROUND

Insulin plays a crucial and multifactorial role in cognitive activity, with insulin resistance appearing in neurodegenerative and metabolic diseases. Insulin resistance contributes to the pathobiology of postoperative cognitive dysfunction (POCD) in experimental models, which can be rescued by intranasal insulin administration.

AIMS

To test the effect of intranasal insulin on the incidence of POCD in elderly patients with metabolic syndrome.

METHODS

The study was designed as a randomized, double-blind, placebo-controlled clinical trial. 116 elderly participants were randomly assigned to receive either 40 IU insulin (n = 58) or placebo (n = 58) for 7 days. The primary outcome was the incidence of POCD at 7 days and 3 months after surgery. Secondary outcomes included the degree of peripheral insulin resistance postoperatively, changes in peripheral inflammation levels and the safety of interventions.

RESULTS

The results showed that POCD occurred in the insulin group on the 7th postoperative day in 11 (20.8%) patients, which was fewer than the 23 (45.1%) patients in the placebo group (P = 0.008). The insulin group indicated better cognitive functional performance on language and memory test than the placebo group (P < 0.05). Mean peripheral plasma concentration of TNF-α (P < 0.05) and CRP (P < 0.001) in the insulin group was significantly declined compared with the placebo group on D3 and D7.

CONCLUSIONS

Intranasal insulin administration reduced the incidence of POCD and alleviated peripheral inflammatory levels in elderly patients with metabolic syndrome.

TRIAL REGISTRY

Chinese Clinical Trial Registry (ChiCTR1800015502).

Sleep-associated insulin resistance promotes neurodegeneration

Lifestyle modification can lead to numerous health issues closely associated with sleep. Sleep deprivation and disturbances significantly affect inflammation, immunity, neurodegeneration, cognitive depletion, memory impairment, neuroplasticity, and insulin resistance. Sleep significantly impacts brain and memory formation, toxin excretion, hormonal function, metabolism, and motor and cognitive functions. Sleep restriction associated with insulin resistance affects these functions by interfering with the insulin signalling pathway, neurotransmission, inflammatory pathways, and plasticity of neurons. So, in this review, We discuss the evidence that suggests that neurodegeneration occurs via sleep and is associated with insulin resistance, along with the insulin signalling pathways involved in neurodegeneration and neuroplasticity, while exploring the role of hormones in these conditions.

Outcomes and clinical implications of intranasal insulin on cognition in humans: A systematic review and meta-analysis

BACKGROUND

Aberrant brain insulin signaling has been posited to lie at the crossroads of several metabolic and cognitive disorders. Intranasal insulin (INI) is a non-invasive approach that allows investigation and modulation of insulin signaling in the brain while limiting peripheral side effects.

OBJECTIVES

The objective of this systematic review and meta-analysis is to evaluate the effects of INI on cognition in diverse patient populations and healthy individuals.

METHODS

MEDLINE, EMBASE, PsycINFO, and Cochrane CENTRAL were systematically searched from 2000 to July 2021. Eligible studies were randomized controlled trials that studied the effects of INI on cognition. Two independent reviewers determined study eligibility and extracted relevant descriptive and outcome data.

RESULTS

Twenty-nine studies (pooled N = 1,726) in healthy individuals as well as those with Alzheimer's disease (AD)/mild cognitive impairment (MCI), mental health disorders, metabolic disorders, among others, were included in the quantitative meta-analysis. Patients with AD/MCI treated with INI were more likely to show an improvement in global cognition (SMD = 0.22, 95% CI: 0.05-0.38 p = <0.00001, N = 12 studies). Among studies with healthy individuals and other patient populations, no significant effects of INI were found for global cognition.

CONCLUSIONS

This review demonstrates that INI may be associated with pro-cognitive benefits for global cognition, specifically for individuals with AD/MCI. Further studies are required to better understand the neurobiological mechanisms and differences in etiology to dissect the intrinsic and extrinsic factors contributing to the treatment response of INI.

Biological and Clinical Implications of Sex-Specific Differences in Alzheimer's Disease

Mounting evidence indicates that the female sex is a risk factor for Alzheimer's disease (AD), the most common cause of dementia worldwide. Decades of research suggest that sex-specific differences in genetics, environmental factors, hormones, comorbidities, and brain structure and function may contribute to AD development. However, although significant progress has been made in uncovering specific genetic factors and biological pathways, the precise mechanisms underlying sex-biased differences are not fully characterized. Here, we review several lines of evidence, including epidemiological, clinical, and molecular studies addressing sex differences in AD. In addition, we discuss the challenges and future directions in advancing personalized treatments for AD.

Cell Culture Media, Unlike the Presence of Insulin, Affect α-Synuclein Aggregation in Dopaminergic Neurons

There are several links between insulin resistance and neurodegenerative disorders such as Parkinson’s disease. However, the direct influence of insulin signaling on abnormal α-synuclein accumulation—a hallmark of Parkinson’s disease—remains poorly explored. To our best knowledge, this work is the first attempt to investigate the direct effects of insulin signaling on pathological α-synuclein accumulation induced by the addition of α-synuclein preformed fibrils in primary dopaminergic neurons. We found that modifying insulin signaling through (1) insulin receptor inhibitor GSK1904529A, (2) SHIP2 inhibitor AS1949490 or (3) PTEN inhibitor VO-OHpic failed to significantly affect α-synuclein aggregation in dopaminergic neurons, in contrast to the aggregation-reducing effects observed after the addition of glial cell line-derived neurotrophic factor. Subsequently, we tested different media formulations, with and without insulin. Again, removal of insulin from cell culturing media showed no effect on α-synuclein accumulation. We observed, however, a reduced α-synuclein aggregation in neurons cultured in neurobasal medium with a B27 supplement, regardless of the presence of insulin, in contrast to DMEM/F12 medium with an N2 supplement. The effects of culture conditions were present only in dopaminergic but not in primary cortical or hippocampal cells, indicating the unique sensitivity of the former. Altogether, our data contravene the direct involvement of insulin signaling in the modulation of α-synuclein aggregation in dopamine neurons. Moreover, we show that the choice of culturing media can significantly affect preformed fibril-induced α-synuclein phosphorylation in a primary dopaminergic cell culture.

Specific changes in sleep oscillations after blocking human metabotropic glutamate receptor 5 in the absence of altered memory function

BACKGROUND

Sleep consolidates declarative memory by repeated replay linked to the cardinal oscillations of non-rapid eye movement (NonREM) sleep. However, there is so far little evidence of classical glutamatergic plasticity induced by this replay. Rather, we have previously reported that blocking N-methyl-D-aspartate (NMDA) or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors does not affect sleep-dependent consolidation of declarative memory.

AIMS

The aim of this study was to investigate the role of metabotropic glutamate receptor 5 (mGluR5) in memory processing during sleep.

METHODS

In two placebo-controlled within-subject crossover experiments with 20 healthy humans each, we used fenobam to block mGluR5 during sleep. In Experiment I, participants learned word-pairs (declarative task) and a finger sequence (procedural task) in the evening, then received the drug and recall was tested the next morning. To cover possible effects on synaptic renormalization processes during sleep, in Experiment II participants learned new word-pairs in the morning after sleep.

RESULTS/OUTCOMES

Surprisingly, fenobam neither reduced retention of memory across sleep nor new learning after sleep, although it severely altered sleep architecture and memory-relevant EEG oscillations. In NonREM sleep, fenobam suppressed 12-15 Hz spindles but augmented 2-4 Hz delta waves, whereas in rapid eye movement (REM) sleep it suppressed 4-8 Hz theta and 16-22 Hz beta waves. Notably, under fenobam NonREM spindles became more consistently phase-coupled to the slow oscillation.

CONCLUSIONS/INTERPRETATIONS

Our findings indicate that mGluR5-related plasticity is not essential for memory processing during sleep, even though mGlurR5 are strongly implicated in the regulation of the cardinal sleep oscillations.

Intranasal insulin and orexins to treat age-related cognitive decline

The intranasal (IN) administration of neuropeptides, such as insulin and orexins, has been suggested as a treatment strategy for age-related cognitive decline (ARCD). Because dysfunctional neuropeptide signaling is an observed characteristic of ARCD, it has been suggested that IN delivery of insulin and/or orexins may restore endogenous peptide signaling and thereby preserve cognition. IN administration is particularly alluring as it is a relatively non-invasive method that directly targets peptides to the brain. Several laboratories have examined the behavioral effects of IN insulin in young, aged, and cognitively impaired rodents and humans. These studies demonstrated improved performance on various cognitive tasks following IN insulin administration. Fewer laboratories have assessed the effects of IN orexins; however, this peptide also holds promise as an effective treatment for ARCD through the activation of the cholinergic system and/or the reduction of neuroinflammation. Here, we provide a brief overview of the advantages of IN administration and the delivery pathway, then summarize the current literature on IN insulin and orexins. Additional preclinical studies will be useful to ultimately uncover the mechanisms underlying the pro-cognitive effects of IN insulin and orexins, whereas future clinical studies will aid in the determination of the most efficacious dose and dosing paradigm. Eventually, IN insulin and/or orexin administration may be a widely used treatment strategy in the clinic for ARCD.

Intranasal insulin

The intranasal (IN) route enables the delivery of insulin to the central nervous system in the relative absence of systemic uptake and related peripheral side effects. Intranasally administered insulin is assumed to travel along olfactory and adjacent pathways and has been shown to rapidly accumulate in cerebrospinal fluid, indicating efficient transport to the brain. Two decades of studies in healthy humans and patients have demonstrated that IN insulin exerts functional effects on metabolism, such as reductions in food intake and body weight and improvements of glucose homeostasis, as well as cognition, ie, enhancements of memory performance both in healthy individuals and patients with mild cognitive impairment or Alzheimer's disease; these studies moreover indicate a favourable safety profile of the acute and repeated use of IN insulin. Emerging findings suggest that IN insulin also modulates neuroendocrine activity, sleep-related mechanisms, sensory perception and mood. Some, but not all studies point to sex differences in the response to IN insulin that need to be further investigated along with the impact of age. "Brain insulin resistance" is an evolving concept that posits impairments in central nervous insulin signalling as a pathophysiological factor in metabolic and cognitive disorders such as obesity, type 2 diabetes and Alzheimer's disease, and, notably, a target of interventions that rely on IN insulin. Still, the negative outcomes of longer-term IN insulin trials in individuals with obesity or Alzheimer's disease highlight the need for conceptual as well as methodological advances to translate the promising results of proof-of-concept experiments and pilot clinical trials into the successful clinical application of IN insulin.

Intranasal administration of 40 and 80 units of insulin does not cause hypoglycemia during cardiac surgery: a randomized controlled trial

PURPOSE

Intranasal insulin administration may improve cognitive function in patients with dementia and may prevent cognitive problems after surgery. Although the metabolic effects of intranasal insulin in non-surgical patients have been studied, its influence on glucose concentration during surgery is unknown.

METHODS

We conducted a randomized, double-blind, placebo-contolled trial in patients scheduled for elective cardiac surgery. Patients with type 2 diabetes mellitus (T2DM) and non-T2DM patients were randomly allocated to one of three groups (normal saline, 40 international units [IU] of intranasal insulin, and 80 IU intranasal insulin). Insulin was given after the induction of general anesthesia. Glucose and plasma insulin concentrations were measured in ten-minute intervals during the first hour and every 30 min thereafter. The primary outcome was the change in glucose concentration 30 min after intranasal insulin administration.

RESULTS

A total of 115 patients were studied, 43 of whom had T2DM. In non-T2DM patients, 40 IU intranasal insulin did not affect glucose concentration, while 80 IU intranasal insulin led to a statistically significant but not clinically important decrease in blood glucose levels (mean difference, 0.4 mMol·L^-1; 95% confidence interval, 0.1 to 0.7). In T2DM patients, neither 40 IU nor 80 IU of insulin affected glucose concentration. No hypoglycemia (< 4.0 mMol·L^-1) was observed after intranasal insulin administration in any patients. In non-T2DM patients, changes in plasma insulin were similar in the three groups. In T2DM patients, there was an increase in plasma insulin concentrations ten minutes after administration of 80 IU of intranasal insulin compared with saline.

CONCLUSIONS

In patients with and without T2DM undergoing elective cardiac surgery, intranasal insulin administration at doses as high as 80 IU did not cause clinically important hypoglycemia.

TRIAL REGISTRATION

www.ClinicalTrials.gov (NCT02729064); registered 5 April 2016.

Intranasal Insulin for Alzheimer's Disease

Brain insulin signaling contributes to memory function and might be a viable target in the prevention and treatment of memory impairments including Alzheimer's disease. This short narrative review explores the potential of central nervous system (CNS) insulin administration via the intranasal pathway to improve memory performance in health and disease, with a focus on the most recent results. Proof-of-concept studies and (pilot) clinical trials in individuals with mild cognitive impairment or Alzheimer's disease indicate that acute and prolonged intranasal insulin administration enhances memory performance, and suggest that brain insulin resistance is a pathophysiological factor in Alzheimer's disease with or without concomitant metabolic dysfunction. Intranasally administered insulin is assumed to trigger improvements in synaptic plasticity and regional glucose uptake as well as alleviations of Alzheimer's disease neuropathology; additional contributions of changes in hypothalamus-pituitary-adrenocortical axis activity and sleep-related mechanisms are discussed. While intranasal insulin delivery has been conclusively demonstrated to be effective and safe, the recent outcomes of large-scale clinical studies underline the need for further investigations, which might also yield new insights into sex differences in the response to intranasal insulin and contribute to the optimization of delivery devices to grasp the full potential of intranasal insulin for Alzheimer's disease.

Non-Invasive Strategies for Nose-to-Brain Drug Delivery

Intranasal drug administration is a promising method for delivering drugs directly to the brain. Animal studies have described pathways and potential brain targets, but nose-to-brain delivery and treatment efficacy in humans remains debated. We describe the proposed pathways and barriers for nose-to-brain drug delivery in humans, drug properties that influence central nervous system delivery, clinically tested methods to enhance absorption, and the devices used in clinical trials. This review compiles the available evidence for nose-to-brain drug delivery in humans and summarizes the factors involved in nose-to-brain drug delivery.

Molecular Mechanisms of Intranasal Insulin in SAMP8 Mice

Research on intranasal delivery of drugs, peptides, and proteins has grown over the past decade as an alternate way to deliver substrates to the brain. Recent work has shown intranasal (INL) delivery of insulin improves memory and cognition in healthy subjects as well as patients with Alzheimer's disease (AD) and in AD mouse models. However, the molecular mechanism(s) for the beneficial effect of insulin on memory are still unclear. Using the SAMP8 mouse model of AD, we investigated the impact of INL insulin on protein and gene expression in brain regions including the olfactory bulb, hypothalamus, and hippocampus. We found genes and proteins in the insulin receptor signaling pathway were not activated by the doses tested. However, we did find the expression of genes present in the hippocampus involved in other pathways, especially those related to inflammation, were altered due to age and with a dose of INL insulin previously shown to improve cognition. These alternate pathways could be targets of insulin when delivered via the INL route to aid in memory improvement.

Origins of Brain Insulin and Its Function

The brain or central nervous system (CNS) utilizes a vast amount of energy to sustain its basic functions, and most of the energy in the brain is derived from glucose. Whole-body energy and glucose homeostasis in the periphery of the human body are regulated by insulin, while the brain had been considered as an "insulin-insensitive" organ, because bulk brain glucose uptake is not affected by insulin in either rodents and humans. However, recently it has become clear that the actions of insulin are more widespread in the CNS and are a critical part of normal development, food intake, and energy balance, as well as plasticity throughout adulthood. Moreover, there are substantial evidence demonstrating that brain insulin is derived from pancreas, neurons, and astrocytes. In this chapter, I reviewed recent progress in roles of insulin in the brain, expression of insulin genes, and multiple origins of the brain insulin.

Insulin Modulates Excitatory Synaptic Transmission and Synaptic Plasticity in the Mouse Hippocampus

The administration of exogenous insulin into the hippocampus has the potential to enhance cognitive function and exert other beneficial effects. Elucidating the neurobiological substrates of insulin action and its underlying physiological mechanisms may further improve treatment efficacy. Previous work has shown that insulin affects synaptic plasticity, however there are discrepancies and contradictory conclusions between studies. Here, we used extracellular field recordings in mouse hippocampal slices to investigate how insulin acutely modulates synaptic transmission and synaptic plasticity, both of which are correlated with learning and memory processes. Our data demonstrate that insulin application inhibited basal excitatory synaptic transmission and promoted long-term potentiation (LTP) induction at hippocampal Schaffer collateral-CA1 synapses. Under similar conditions, insulin strongly activated the PI3K/AKT pathway, but had only a weak effect on the MAPK/ERK pathway. Although insulin-induced inhibition of field excitatory post-synaptic potentials (fEPSPs) was previously termed insulin-long-term depression (insulin-LTD), insulin application potentiated recovery from classically induced LTD. Further analysis suggests suppression of presynaptic neurotransmitter release contributed to the insulin-LTD. At low concentrations, insulin primarily inhibited fEPSPs; however, at high concentration, its effects were of mixed inhibition and enhancement in different recordings. Moreover, a broad spectrum protein kinase C blocker, cannabinoid receptor type 1 activator, or a high glucose concentration inhibited fEPSPs per se, and disturbed insulin's effect on fEPSP. Insulin also caused depotentiation during LTP expression and triggered depression during LTD recovery. Given the essential roles of dynamic synaptic transmission and plasticity in learning and memory, our data provide more evidence that insulin application may actively modulate hippocampal-dependent cognitive events.

Outcomes and clinical implications of intranasal insulin administration to the central nervous system

Insulin signaling in the brain plays a critical role in metabolic control and cognitive function. Targeting insulinergic pathways in the central nervous system via peripheral insulin administration is feasible, but associated with systemic effects that necessitate tight supervision or countermeasures. The intranasal route of insulin administration, which largely bypasses the circulation and thereby greatly reduces these obstacles, has now been repeatedly tested in proof-of-concept studies in humans as well as animals. It is routinely used in experimental settings to investigate the impact on eating behavior, peripheral metabolism, memory function and brain activation of acute or long-term enhancements in central nervous system insulin signaling. Epidemiological and experimental evidence linking deteriorations in metabolic control such as diabetes with neurodegenerative diseases imply pathophysiological relevance of dysfunctional brain insulin signaling or brain insulin resistance, and suggest that targeting insulin in the brain holds some promise as a therapy or adjunct therapy. This short narrative review gives an overview over recent findings on brain insulin signaling as derived from human studies deploying intranasal insulin, and evaluates the potential of therapeutic interventions that target brain insulin resistance.

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.'

Slow-Wave Activity Enhancement to Improve Cognition

Slow-wave activity (SWA), and its coupling with other sleep features, reorganizes cortical circuitry, supporting cognition. This raises the question: can cognition be improved through SWA enhancement? SWA enhancement techniques range from behavioral interventions (such as exercise), which have high feasibility but low specificity, to laboratory-based techniques (such as transcranial stimulation), which have high specificity but are less feasible for widespread use. In this review we describe the pathways through which SWA is enhanced. Pathways encompass enhanced neural activity, increased energy metabolism, and endocrine signaling during wakefulness; also direct enhancement during sleep. We evaluate the robustness and practicality of SWA-enhancement techniques, discuss approaches for determining a causal role of SWA on cognition, and present questions to clarify the mechanisms of SWA-dependent cognitive improvements.

Insulin Resistance as a Therapeutic Target in the Treatment of Alzheimer's Disease: A State-of-the-Art Review

Research in animals and humans has shown that type 2 diabetes and its prodromal state, insulin resistance, promote major pathological hallmarks of Alzheimer's disease (AD), such as the formation of amyloid plaques and neurofibrillary tangles (NFT). Worrisomely, dysregulated amyloid beta (Aβ) metabolism has also been shown to promote central nervous system insulin resistance; although the role of tau metabolism remains controversial. Collectively, as proposed in this review, these findings suggest the existence of a mechanistic interplay between AD pathogenesis and disrupted insulin signaling. They also provide strong support for the hypothesis that pharmacologically restoring brain insulin signaling could represent a promising strategy to curb the development and progression of AD. In this context, great hopes have been attached to the use of intranasal insulin. This drug delivery method increases cerebrospinal fluid concentrations of insulin in the absence of peripheral side effects, such as hypoglycemia. With this in mind, the present review will also summarize current knowledge on the efficacy of intranasal insulin to mitigate major pathological symptoms of AD, i.e., cognitive impairment and deregulation of Aβ and tau metabolism.

Learning and memory performance following acute intranasal insulin administration in abstinent smokers

The highest incidence of relapse to smoking occurs within the first 2 weeks of a cessation attempt. In addition to enhanced nicotine craving, this phase of smoking cessation is also marked by learning and memory dysfunction. Many smokers are not able to overcome these symptoms, and they relapse to smoking shortly after trying to quit. In two clinical studies, we evaluated intranasal insulin for efficacy in improving learning and memory function during nicotine withdrawal. Our first study was a crossover evaluation (N = 19) following 20 hr of smoking abstinence. Study 2 was a parallel design study (N = 50) following 16 hr of abstinence. Intranasal insulin (60 IU) dose was administered in both studies and cognitive function was measured using California Verbal Learning Test-II. Intranasal insulin did not improve learning over the 5 verbal learning trials. In addition, intranasal insulin did not improve either short- or long-delay recall in either study. In summary, the one-time administration of intranasal insulin does not improve verbal learning and memory in smokers. Whether longer administration schedules may be of benefit should be evaluated in future studies.

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.

Intranasal insulin decreases circulating cortisol concentrations during early sleep in elderly humans

Aging is associated with increases in hypothalamic-pituitary-adrenal (HPA) axis activity that can predispose to metabolic and cognitive impairments. We investigated in elderly and young subjects whether intranasal insulin administration to the human brain reduces early-sleep nadir concentrations of adrenocorticotropin and cortisol, that is, indicators of baseline HPA axis activity. In within-subject comparisons, intranasal insulin (160 IU) or placebo was administered to 14 elderly (mean age 70.0 years) and 30 young (23.6 years) healthy subjects before bedtime. Sleep was polysomnographically assessed and blood samples were repeatedly collected. Elderly compared with young participants displayed increased early-sleep cortisol concentrations (p < 0.04) and reductions in slow wave and REM sleep (p < 0.001). Insulin administration reduced cortisol levels between 2300 hours and 0020 hours in the elderly (p = 0.03) but not young participants (p = 0.56; p = 0.003 for interaction). Findings indicate that central nervous insulin acts as an inhibitory signal in basal HPA axis activity regulation and suggest that intranasal insulin may normalize sleep-associated stress axis activity in older age.

Central Nervous Insulin Administration before Nocturnal Sleep Decreases Breakfast Intake in Healthy Young and Elderly Subjects

Peripheral insulin acts on the brain to regulate metabolic functions, in particular decreasing food intake and body weight. This concept has been supported by studies in humans relying on the intranasal route of administration, a method that permits the direct permeation of insulin into the CNS without substantial absorption into the blood stream. We investigated if intranasal insulin administration before nocturnal sleep, a period of reduced metabolic activity and largely absent external stimulation, affects food intake and energy turnover on the subsequent morning. Healthy participants who were either young (16 men and 16 women; mean age ± SEM, 23.68 ± 0.40 years, mean BMI ± SEM, 22.83 ± 0.33 kg/m2) or elderly (10 men, 9 women; 70.79 ± 0.81 years, 25.27 ± 0.60 kg/m2) were intranasally administered intranasal insulin (160 IU) or placebo before a night of regular sleep that was polysomnographically recorded. Blood was repeatedly sampled for the determination of circulating glucose, insulin, leptin and total ghrelin. In the morning, energy expenditure was assessed via indirect calorimetry and subjects were offered a large standardized breakfast buffet from which they could eat ad libitum. Insulin compared to placebo reduced breakfast size by around 110 kcal (1,054.43 ± 50.91 vs. 1,162.36 ± 64.69 kcal, p = 0.0095), in particular decreasing carbohydrate intake (502.70 ± 25.97 vs. 589.82 ± 35.03 kcal, p = 0.0080). This effect was not dependent on sex or age (all p > 0.11). Sleep architecture, blood glucose and hormonal parameters as well as energy expenditure were not or only marginally affected. Results show that intranasal insulin administered to healthy young and elderly humans before sleep exerts a delayed inhibitory effect on energy intake that is not compensated for by changes in energy expenditure. While the exact underlying mechanisms cannot be derived from our data, findings indicate a long-lasting catabolic effect of central nervous insulin delivery that extends across sleep and might be of particular relevance for potential therapeutic applications.

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.