Effects of insulin on local cerebral glucose utilization in the rat

New Insights into the Role of Insulin and Hypothalamic-Pituitary-Adrenal (HPA) Axis in the Metabolic Syndrome

Recent data suggests that (pre)diabetes onset is preceded by a period of hyperinsulinemia. Consumption of the "modern" Western diet, over-nutrition, genetic background, decreased hepatic insulin clearance, and fetal/metabolic programming may increase insulin secretion, thereby causing chronic hyperinsulinemia. Hyperinsulinemia is an important etiological factor in the development of metabolic syndrome, type 2 diabetes, cardiovascular disease, polycystic ovarian syndrome, and Alzheimer's disease. Recent data suggests that the onset of prediabetes and diabetes are preceded by a variable period of hyperinsulinemia. Emerging data suggest that chromic hyperinsulinemia is also a driving force for increased activation of the hypothalamic-adrenal-pituitary (HPA) axis in subjects with the metabolic syndrome, leading to a state of "functional hypercortisolism". This "functional hypercortisolism" by antagonizing insulin actions may prevent hypoglycemia. It also disturbs energy balance by shifting energy fluxes away from muscles toward abdominal fat stores. Synergistic effects of hyperinsulinemia and "functional hypercortisolism" promote abdominal visceral obesity and insulin resistance which are core pathophysiological components of the metabolic syndrome. It is hypothesized that hyperinsulinemia-induced increased activation of the HPA axis plays an important etiological role in the development of the metabolic syndrome and its consequences. Numerous studies have demonstrated reversibility of hyperinsulinemia with lifestyle, surgical, and pharmaceutical-based therapies. Longitudinal studies should be performed to investigate whether strategies that reduce hyperinsulinemia at an early stage are successfully in preventing increased activation of the HPA axis and the metabolic syndrome.

Role of Cardiovascular Risk Factors on the Association Between Physical Activity and Brain Integrity Markers in Older Adults

BACKGROUND AND OBJECTIVES

Physical activity has been associated with a decreased risk for dementia, but the mechanisms underlying this association remain to be determined. Our objective was to assess whether cardiovascular risk factors mediate the association between physical activity and brain integrity markers in older adults.

METHODS

At baseline, participants from the Age-Well study completed a physical activity questionnaire and underwent cardiovascular risk factors collection (systolic blood pressure, body mass index [BMI], current smoker status, and high-density lipoprotein cholesterol, total cholesterol, and insulin levels) and multimodal neuroimaging (structural MRI, diffusion MRI, FDG-PET, and florbetapir PET). Multiple regressions were conducted to assess the association among physical activity, cardiovascular risk factors, and neuroimaging. Mediation analyses were performed to test whether cardiovascular risk factors mediated the associations between physical activity and neuroimaging.

RESULTS

A total of 134 cognitively unimpaired older adults (≥65 years) were included. Higher physical activity was associated with higher gray matter (GM) volume (β = 0.174, p = 0.030) and cerebral glucose metabolism (β = 0.247, p = 0.019) but not with amyloid deposition or white matter integrity. Higher physical activity was associated with lower insulin level and BMI but not with the other cardiovascular risk factors. Lower insulin level and BMI were related to higher GM volume but not to cerebral glucose metabolism. When controlling for insulin level and BMI, the association between physical activity and cerebral glucose metabolism remained unchanged, while the association with GM volume was lost. When insulin level and BMI were entered in the same model, only BMI remained a significant predictor of GM volume. Mediation analyses confirmed that insulin level and BMI mediated the association between physical activity and GM volume. Analyses were replicated within Alzheimer disease-sensitive regions and results remained overall similar.

DISCUSSION

The association between physical activity and GM volume is mediated by changes in insulin level and BMI. In contrast, the association with cerebral glucose metabolism seems to be independent from cardiovascular risk factors. Older adults engaging in physical activity experience cardiovascular benefits through the maintenance of a lower BMI and insulin level, resulting in greater structural brain integrity. This study has implications for understanding how physical activity affects brain health and may help in developing strategies to prevent or delay age-related decline.

TRIAL REGISTRATION INFORMATION

EudraCT: 2016-002,441-36; IDRCB: 2016-A01767-44; ClinicalTrials.gov Identifier: NCT02977819.

Prediabetes Is Associated With Brain Hypometabolism and Cognitive Decline in a Sex-Dependent Manner: A Longitudinal Study of Nondemented Older Adults

Although type 2 diabetes is a well-known risk factor for Alzheimer's disease (AD), little is known about how its precursor-prediabetes-impacts neuropsychological function and brain health. Thus, we examined the relationship between prediabetes and AD-related biological and cognitive/clinical markers in a well-characterized sample drawn from the Alzheimer's Disease Neuroimaging Initiative. Additionally, because women show higher rates of AD and generally more atherogenic lipid profiles than men, particularly in the context of diabetes, we examined whether sex moderates any observed associations. The total sample of 911 nondemented and non-diabetic participants [normal control = 540; mild cognitive impairment (MCI) = 371] included 391 prediabetic (fasting blood glucose: 100-125 mg/dL) and 520 normoglycemic individuals (age range: 55-91). Linear mixed effects models, adjusted for demographics and vascular and AD risk factors, examined the independent and interactive effects of prediabetes and sex on 2-6 year trajectories of FDG-PET measured cerebral metabolic glucose rate (CMRglu), hippocampal/intracranial volume ratio (HV/IV), cerebrospinal fluid phosphorylated tau-₁₈₁/amyloid-β_1-42 ratio (p-tau₁₈₁/Aβ_1-42), cognitive function (executive function, language, and episodic memory) and the development of dementia. Analyses were repeated in the MCI subsample. In the total sample, prediabetic status had an adverse effect on CMRglu across time regardless of sex, whereas prediabetes had an adverse effect on executive function across time in women only. Within the MCI subsample, prediabetic status was associated with lower CMRglu and poorer executive function and language performance across time within women, whereas these associations were not seen within men. In the total sample and MCI subsample, prediabetes did not relate to HV/IV, p-tau₁₈₁/Aβ_1-42, memory function or dementia risk regardless of sex; however, among incident dementia cases, prediabetic status related to earlier age of dementia onset in women but not in men. Results suggest that prediabetes may affect cognition through altered brain metabolism, and that women may be more vulnerable to the negative effects of glucose intolerance.

Mitochondria, OxPhos, and neurodegeneration: cells are not just running out of gas

Mitochondrial respiratory deficiencies have been observed in numerous neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. For decades, these reductions in oxidative phosphorylation (OxPhos) have been presumed to trigger an overall bioenergetic crisis in the neuron, resulting in cell death. While the connection between respiratory defects and neuronal death has never been proven, this hypothesis has been supported by the detection of nonspecific mitochondrial DNA mutations in these disorders. These findings led to the notion that mitochondrial respiratory defects could be initiators of these common neurodegenerative disorders, instead of being consequences of a prior insult, a theory we believe to be misconstrued. Herein, we review the roots of this mitochondrial hypothesis and offer a new perspective wherein mitochondria are analyzed not only from the OxPhos point of view, but also as a complex organelle residing at the epicenter of many metabolic pathways.

Cerebral perfusion alterations in type 2 diabetes and its relation to insulin resistance and cognitive dysfunction

To explore the effect of T2DM on cerebral perfusion, and the relationship between cerebral perfusion changes and cognitive impairment as well as diabetic variables, by using a whole-brain arterial spin-labeling (ASL) MRI technique. This prospective study was approved by the local institutional review board and was performed between November 2012 and October 2013. All subjects provided informed consent. Forty T2DM patients and 41 age-, sex- and education-matched healthy controls were included. Cerebral blood flow (CBF) map was obtained by pulsed ASL perfusion imaging at 3 T MRI. Voxel-wise comparisons on CBF maps with and without partial volume effects (PVEs) correction were performed between groups. Associations between CBF and cognitive functioning, and between CBF and diabetic variables were investigated by using voxel-wise, whole-brain correlation analyses. In T2DM patients, PVEs uncorrected CBF was decreased in the posterior cingulate cortex (PCC), precuneus and bilateral occipital lobe, and increased in the anterior cingulate cortex (corrected P < .05). These changes were largely unchanged after PVEs correction. Correlation analyses revealed that in patients, hypoperfusion in PCC and precuneus regions were related to higher insulin resistance level and deficits in clock-drawing performance, while the occipital hypoperfusion was associated with worse visual-memory performance, regardless of PVEs correction. The cerebral hypoperfusion pattern in T2DM resembles the pattern observed in the early stage of dementia, and increased insulin resistance might be an important risk factor as well as treatment target for such CBF dysregulation.

The underexplored question of β-amyloid monomers

Conceived more than 25 years ago, the amyloid cascade hypothesis of Alzheimer's disease has evolved to accommodate new findings, namely different forms of β-amyloid aggregates and downstream dysfunctions. Yet, the cascade does not mention its very beginning, the β-amyloid monomer. Here, I will discuss the monomer from a functional evolutionary perspective, highlighting the potential advantages of a native unfolded state that, however, involves an amyloidogenic risk. Finally, I will make a summary of what is known about its functional role in the brain and discuss the implications of its conceivable shortage in the development of Alzheimer's disease.

Diabesity and Brain Energy Metabolism: The Case of Alzheimer's Disease

It is widely accepted that high calorie diets and a sedentary lifestyle sturdily influence the incidence and outcome of type 2 diabetes and obesity, which can occur simultaneously, a situation called diabesity. Tightly linked with metabolic and energy regulation, a close association between diabetes and Alzheimer's disease (AD) has been proposed. Among the common pathogenic mechanisms that underpin both conditions, insulin resistance, brain glucose hypometabolism, and metabolic dyshomeostasis appear to have a pivotal role. This century is an unprecedented diabetogenic period in human history, so therapeutic strategies and/or approaches to control and/or revert this evolving epidemic is of utmost importance. This chapter will make a brief contextualization about the impact that diabetes and obesity can exert in brain structure and function alongside with a brief survey about the role of insulin in normal brain function, exploring its roles in cognition and brain glucose metabolism. Later, attention will be given to the intricate relation of diabesity, insulin resistance, and AD. Finally, both pharmacological and lifestyle interventions will also be reviewed as strategies aimed at fighting diabesity and/or AD-related metabolic effects.

Monomeric ß-amyloid interacts with type-1 insulin-like growth factor receptors to provide energy supply to neurons

ß-amyloid (Aß₁₋₄₂) is produced by proteolytic cleavage of the transmembrane type-1 protein, amyloid precursor protein. Under pathological conditions, Aß₁₋₄₂self-aggregates into oligomers, which cause synaptic dysfunction and neuronal loss, and are considered the culprit of Alzheimer's disease (AD). However, Aß₁₋₄₂ is mainly monomeric at physiological concentrations, and the precise role of monomeric Aß₁₋₄₂ in neuronal function is largely unknown. We report that the monomer of Aß₁₋₄₂ activates type-1 insulin-like growth factor receptors and enhances glucose uptake in neurons and peripheral cells by promoting the translocation of the Glut3 glucose transporter from the cytosol to the plasma membrane. In neurons, activity-dependent glucose uptake was blunted after blocking endogenous Aß production, and re-established in the presence of cerebrospinal fluid Aß. APP-null neurons failed to enhance depolarization-stimulated glucose uptake unless exogenous monomeric Aß₁₋₄₂ was added. These data suggest that Aß₁₋₄₂ monomers were critical for maintaining neuronal glucose homeostasis. Accordingly, exogenous Aß₁₋₄₂ monomers were able to rescue the low levels of glucose consumption observed in brain slices from AD mutant mice.

Reduced uptake of 18F-FDG and 15O-H2O in Alzheimer's disease-related regions after glucose loading

Increased plasma glucose levels are known to reduce fluorine-18-labeled fluorodeoxyglucose ((18)F-FDG) uptake in Alzheimer's disease (AD)-related regions, resulting in the appearance of an AD-like pattern. However, the relationships of its appearance with cerebral blood flow and insulin levels are uncertain. We performed (18)F-FDG and oxygen-15-labeled water ((15)O-H2O) positron emission tomography in the fasting and glucose-loading conditions on nine young healthy volunteers with no cognitive impairments. Measurement of plasma glucose and insulin levels confirmed that all subjects were free of insulin resistance, and that glucose loading significantly increased plasma glucose and insulin levels. Fluorine-18-labeled fluorodeoxyglucose and (15)O-H2O images were compared between the two conditions, focusing on AD-related regions: precuneus/posterior cingulate (PP), lateral parietal cortex (LPC), and frontal cortex (FC). Volume-of-interest analyses showed significantly lower uptake of both (18)F-FDG and (15)O-H2O in PP, LPC, and FC after glucose loading (P<0.05). Whole-brain voxel-wise analyses also revealed the PP, LPC, and FC areas where uptake of both (18)F-FDG and (15)O-H2O decreased (P<0.05, familywise error rate-corrected). We concluded that increased plasma glucose and insulin levels can cause the appearance of the AD-like pattern in both (18)F-FDG and (15)O-H2O images, and this phenomenon can occur even in subjects without insulin resistance.

Relationship between Alzheimer disease-like pattern of 18F-FDG and fasting plasma glucose levels in cognitively normal volunteers

Increased plasma glucose (PG) levels can alter the cerebral distribution pattern of (18)F-FDG uptake and reduce (18)F-FDG uptake, especially in the precuneus. The (18)F-FDG distribution pattern in cognitively normal subjects is described as an Alzheimer disease (AD)-like pattern. The aim of this study was to determine the fasting PG levels that can reduce (18)F-FDG uptake in the precuneus.

METHODS

Fifty-one cognitively normal volunteers (mean age ± SD, 69.7 ± 5.9 y) underwent (18)F-FDG PET scanning and were divided into 2 groups according to the level of fasting PG at the time of PET scanning: control (n = 31, 80 mg/dL ≤ fasting PG < 100 mg/dL) and impaired fasting glucose (IFG) (n = 20, 100 mg/dL ≤ fasting PG < 110 mg/dL). (18)F-FDG uptake was compared between the 2 groups using voxelwise analyses with a global normalization method and volume-of-interest (VOI)-based analyses. VOIs were placed on the precuneus, posterior cingulate, and visual cortex, and the ratio of the uptake value on the precuneus VOI to that on the visual cortex VOI (PreCne/VC ratios) and to that on the posterior cingulate VOI (PreCne/PostCin ratios) was calculated.

RESULTS

Whole-brain voxelwise analyses showed that (18)F-FDG uptake in the precuneus was significantly lower in the IFG group (P < 0.05, familywise error rate-corrected) than in the control group. VOI analyses showed significantly lower PreCne/VC ratios (P = 0.002) and PreCne/PostCin ratios (P = 0.004) in the IFG group than in the control group.

CONCLUSION

The present study confirmed that increased fasting PG levels decrease (18)F-FDG uptake, especially in the precuneus, as in the AD-like pattern. Furthermore, the study provided initial evidence that the AD-like pattern can appear even in an individual with a mildly higher level of fasting PG (100-110 mg/dL).

Real-time effects of insulin-induced hypoglycaemia on hippocampal glucose and oxygen

The hippocampus plays a vital role in learning and memory and is susceptible to damage following hypoglycaemic shock. The effect of an acute administration of insulin on hippocampal function has been described in terms of behavioural deficits but its effect on hippocampal oxygen and glucose is unclear. Glucose oxidase biosensors (detecting glucose) and carbon paste electrodes (detecting oxygen) were implanted into the hippocampus of Sprague Dawley rats. Animals were allowed to recover and real-time recordings were made in order to determine the effects of fasting, insulin administration (15 U/kg; i.p.) and reintroduction of food on hippocampal oxygen and glucose. Fasting caused a significant decrease in hippocampal glucose over the course of 24h. Insulin administration produced a significant decrease in hippocampal glucose along with a significant increase in hippocampal oxygen. Finally, the reintroduction of food resulted in glucose levels significantly increasing along with a transient but significant increase in oxygen levels. The findings presented here suggest that even a single acute period of hypoglycaemia may substantially disrupt hippocampal oxygen and glucose and therefore affect hippocampal function.

Insulin and insulin-like growth factor prevent brain atrophy and cognitive impairment in diabetic rats

There are an estimated 36 million dementia patients worldwide. The anticipated tripling of this number by year 2050 will negatively impact the capacity to deliver quality health care. The epidemic in diabetes is particularly troubling, because diabetes is a substantial risk factor for dementia independently of cerebrovascular disease. There is an urgent need to elucidate the pathogenesis of progressive brain atrophy, the cause of dementia, to allow rational design of new therapeutic interventions. This review summarizes recent tests of the hypothesis that the concomitant loss of insulin and insulin-like growth factors (IGFs) is the dominant cause for age-dependent, progressive brain atrophy with degeneration and cognitive decline. These tests are the first to show that insulin and IGFs regulate adult brain mass by maintaining brain protein content. Insulin and IGF levels are reduced in diabetes, and replacement of both ligands can prevent loss of total brain protein, widespread cell degeneration, and demyelination. IGF alone prevents retinal degeneration in diabetic rats. It supports synapses and is required for learning and memory. Replacement doses in diabetic rats can cross the blood-brain barrier to prevent hippocampus-dependent memory impairment. Insulin and IGFs are protective despite unabated hyperglycemia in diabetic rats, severely restricting hyperglycemia and its consequences as dominant pathogenic causes of brain atrophy and impaired cognition. These findings have important implications for late-onset alzheimer's disease (LOAD) where diabetes is a major risk factor, and concomitant decline in insulin and IGF activity suggest a similar pathogenesis for brain atrophy and dementia.

Beta-amyloid monomer and insulin/IGF-1 signaling in Alzheimer's disease

Alzheimer's disease is the most common form of dementia among older people and is still untreatable. While β-amyloid protein is recognized as the disease determinant with a pivotal role in inducing neuronal loss and dementia, an impaired brain insulin signaling seems to account in part for the cognitive deficit associated with the disease. The origin of this defective signaling is uncertain. Accumulating toxic species of β-amyloid, the so-called oligomers, has been proposed to be responsible for downregulation of neuronal insulin receptors. We have found that the nontoxic form of β-amyloid, the monomer, is able to activate insulin/insulin-like growth factor-1 (IGF-1) receptor signaling and thus behaves as a neuroprotectant agent. Our suggestion is that depletion of β-amyloid monomers, occurring in the preclinical phase of Alzheimer's disease, might be the cause of early insulin/IGF-1 signaling disturbances that anticipate cognitive decline.

Metabolic dysfunction in Alzheimer's disease and related neurodegenerative disorders

Alzheimer's disease and other related neurodegenerative diseases are highly debilitating disorders that affect millions of people worldwide. Efforts towards developing effective treatments for these disorders have shown limited efficacy at best, with no true cure to this day being present. Recent work, both clinical and experimental, indicates that many neurodegenerative disorders often display a coexisting metabolic dysfunction which may exacerbate neurological symptoms. It stands to reason therefore that metabolic pathways may themselves contain promising therapeutic targets for major neurodegenerative diseases. In this review, we provide an overview of some of the most recent evidence for metabolic dysregulation in Alzheimer's disease, Huntington's disease, and Parkinson's disease, and discuss several potential mechanisms that may underlie the potential relationships between metabolic dysfunction and etiology of nervous system degeneration. We also highlight some prominent signaling pathways involved in the link between peripheral metabolism and the central nervous system that are potential targets for future therapies, and we will review some of the clinical progress in this field. It is likely that in the near future, therapeutics with combinatorial neuroprotective and 'eumetabolic' activities may possess superior efficacies compared to less pluripotent remedies.

Relation between brain interstitial and systemic glucose concentrations after subarachnoid hemorrhage

OBJECT

The aim in the present investigation was to study the relation between brain interstitial and systemic blood glucose concentrations during the acute phase after subarachnoid hemorrhage (SAH). The authors also evaluated the effects of insulin administration on local brain energy metabolism.

METHODS

Nineteen patients with spontaneous SAH were prospectively monitored with intracerebral microdialysis (MD). The relation between plasma glucose and MD-measured interstitial brain glucose concentrations as well as the temporal pattern of MD glucose, lactate, pyruvate, glutamate, and glycerol was studied for 7 days after SAH. Using a target plasma glucose concentration of 5-10 mmol/L, the effect of insulin injection was also evaluated.

RESULTS

The mean (± SD) correlation coefficient between plasma glucose and MD glucose was 0.27 ± 0.27 (p = 0.0005), with a high degree of individual variation. Microdialysis glucose, the MD/plasma glucose ratio, and MD glutamate concentrations decreased in parallel with a gradual increase in MD pyruvate and MD lactate concentrations. There were no significant changes in the MD L/P ratio or MD glycerol levels. Insulin administration induced a decrease in MD glucose and MD pyruvate.

CONCLUSIONS

After SAH, there was a positive correlation between plasma and MD glucose concentrations with a high degree of individual variation. A gradual decline in MD glucose and the MD/plasma glucose ratio and an increase in MD pyruvate and MD lactate concentrations during the 1st week after SAH suggest a transition to a hyperglycolytic state with increased cerebral glucose consumption. The administration of insulin was related to a lowering of MD glucose and MD pyruvate, often to low levels even though plasma glucose values remained above 6 mmol/L. After SAH, the administration of insulin could impede the glucose supply of the brain.

Insulin, synaptic function, and opportunities for neuroprotection

A steadily growing number of studies have begun to establish that the brain and insulin, while traditionally viewed as separate, do indeed have a relationship. The uptake of pancreatic insulin, along with neuronal biosynthesis, provides neural tissue with the hormone. As well, insulin acts upon a neuronal receptor that, although a close reflection of its peripheral counterpart, is characterized by unique structural and functional properties. One distinction is that the neural variant plays only a limited part in neuronal glucose transport. However, a number of other roles for neural insulin are gradually emerging; most significant among these is the modulation of ligand-gated ion channel (LGIC) trafficking. Notably, insulin has been shown to affect the tone of synaptic transmission by regulating cell-surface expression of inhibitory and excitatory receptors. The manner in which insulin regulates receptor movement may provide a cellular mechanism for insulin-mediated neuroprotection in the absence of hypoglycemia and stimulate the exploration of new therapeutic opportunities.

Insulin resistance and Alzheimer-like reductions in regional cerebral glucose metabolism for cognitively normal adults with prediabetes or early type 2 diabetes

BACKGROUND

Insulin resistance is a causal factor in prediabetes (PD) and type 2 diabetes (T2D) and increases the risk of developing Alzheimer disease (AD). Reductions in cerebral glucose metabolic rate (CMRglu) as measured by fludeoxyglucose F 18-positron emission tomography (FDG-PET) in parietotemporal, frontal, and cingulate cortices are associated with increased AD risk and can be observed years before dementia onset.

OBJECTIVES

To examine whether greater homeostasis model assessment insulin resistance (HOMA-IR) is associated with reduced resting CMRglu in areas vulnerable in AD in cognitively normal adults with newly diagnosed PD or T2D (PD/T2D), and to determine whether adults with PD/T2D have abnormal patterns of CMRglu during a memory encoding task.

DESIGN

Randomized crossover design of resting and activation FDG-PET.

SETTING

University imaging center and Veterans Affairs clinical research unit.

PARTICIPANTS

Twenty-three older adults (mean [SEM] age, 74.4 [1.4] years) with no prior diagnosis of diabetes but who met American Diabetes Association glycemic criteria for PD (n = 11) or diabetes (n = 12) based on fasting or 2-hour oral glucose tolerance test (OGTT) glucose values and 6 adults (mean [SEM] age, 74.3 [2.8] years) with normal fasting glucose values and glucose tolerance. No participant met Petersen criteria for mild cognitive impairment.

INTERVENTIONS

Fasting participants underwent resting and cognitive activation FDG-PET imaging on separate days. Following a 30-minute transmission scan, subjects received an intravenous injection of 5 mCi of FDG, and the emission scan commenced 40 minutes after injection. In the activation condition, a 35-minute memory encoding task was initiated at the time of tracer injection. Subjects were instructed to remember a repeating list of 20 words randomly presented in series through earphones. Delayed free recall was assessed once the emission scan was complete.

MAIN OUTCOME MEASURES

The HOMA-IR value was calculated using fasting glucose and insulin values obtained during OGTT screening and then correlated with CMRglu values obtained during the resting scan. Resting CMRglu values were also subtracted from CMRglu values obtained during the memory encoding activation scan to examine task-related patterns of CMRglu.

RESULTS

Greater insulin resistance was associated with an AD-like pattern of reduced CMRglu in frontal, parietotemporal, and cingulate regions in adults with PD/T2D. The relationship between CMRglu and HOMA-IR was independent of age, 2-hour OGTT glucose concentration, or apolipoprotein E ε4 allele carriage. During the memory encoding task, healthy adults showed activation in right anterior and inferior prefrontal cortices, right inferior temporal cortex, and medial and posterior cingulate regions. Adults with PD/T2D showed a qualitatively different pattern during the memory encoding task, characterized by more diffuse and extensive activation, and recalled fewer items on the delayed memory test.

CONCLUSIONS

Insulin resistance may be a marker of AD risk that is associated with reduced CMRglu and subtle cognitive impairments at the earliest stage of disease, even before the onset of mild cognitive impairment.

Effect of chronic hypoglycaemia on glucose concentration and glycogen content in rat brain: A localized 13C NMR study

While chronic hypoglycaemia has been reported to increase unidirectional glucose transport across the blood-brain barrier (BBB) and to increase GLUT1 expression at the endothelium, the effect on steady-state brain d-glucose and brain glycogen content is currently unknown. Brain glucose and glycogen concentrations were directly measured in vivo using localized 13C magnetic resonance spectroscopy (MRS) following 12-14 days of hypoglycaemia. Brain glucose content was significantly increased by 48%, which is consistent with an increase in the maximal glucose transport rate, Tmax, by 58% compared with the sham-treated animals. The localized 13C NMR measurements of brain glucose were directly validated by comparison with biochemically determined brain glucose content after rapid focused microwave fixation (1.4 s at 4 kW). Both in vivo MRS and biochemical measurements implied that brain glycogen content was not affected by chronic hypoglycaemia, consistent with brain glucose being a major factor controlling brain glycogen content. We conclude that the increased glucose transporter expression in chronic hypoglycaemia leads to increased brain glucose content at a given level of glycaemia. Such increased brain glucose concentrations can result in a lowered glycaemic threshold of counter-regulation observed in chronic hypoglycaemia.

Endogenous insulin signaling protects cultured neurons from oxygen-glucose deprivation-induced cell death

Curiosity surrounding the physiological relevance of neural insulin signaling has gradually developed since the discovery that nervous tissue contains both the hormone and its receptor. Similar to other receptor tyrosine kinases, ligand interaction with the insulin receptor (IR) activates a variety of intracellular signaling pathways, particularly those relevant to cellular survival. Consequently, one explanation for the presence of the insulin pathway in the brain may involve participation in the response to neuronal injury. To investigate this possibility, the present study began by examining the effect of oxygen-glucose deprivation (OGD), a well-characterized in vitro model of ischemia, on ligand-binding, surface expression, and function of the IR in cultured rat neurons that were prepared under serum-free conditions. Reduced insulin-binding was observed following OGD, although surface expression of the receptor was not altered. However, OGD did significantly decrease the ability of insulin to stimulate phosphorylation of the transmembrane IR beta-subunit, without affecting protein expression of this subunit. Subsequent experiments focused on the manner in which pharmacologically manipulating IR function affected neuronal viability after OGD. Application of the IR sensitizer metformin moderately improved neuronal viability, while the specific IR tyrosine kinase inhibitor tyrphostin A47 was able to dramatically decrease viability; both compounds acted without affecting IR surface expression. Our study suggests that not only does the IR appear to play an important role in neuronal survival, but also that neurons may actively maintain IRs on the cell surface to compensate for the OGD-induced decrease in the ability of insulin to phosphorylate its receptor.

Insulin and neurodegenerative disease: shared and specific mechanisms

Insulin has functions in the brain and dysregulation of these functions may contribute to the expression of late-life neurodegenerative disease. We provide a brief summary of research on the influence of insulin on normal brain function. We then review evidence that perturbation of this role may contribute to the symptoms and pathogenesis of various neurodegenerative disorders, such as Alzheimer's disease, vascular dementia, Parkinson's disease, and Huntington's disease. We conclude by considering whether insulin dysregulation contributes to neurodegenerative disorders through disease-specific or general mechanisms.

The role of insulin in human brain glucose metabolism: an 18fluoro-deoxyglucose positron emission tomography study

The effect of basal insulin on global and regional brain glucose uptake and metabolism in humans was studied using 18-fluorodeoxyglucose and positron emission tomography (FDG-PET). Eight healthy male volunteers aged 49.3 +/- 5.1 years were studied twice in random order. On each occasion, they received an infusion of 0.1 mg. kg(-1). min(-1) somatostatin to suppress endogenous insulin production. In one study 0.3 mU. kg(-1). min(-1) insulin was infused to replace basal circulating insulin levels, and in the other study a saline infusion was used as control. We sought stimulatory effects of basal insulin on brain glucose metabolism particularly in regions with deficiencies in the blood-brain barrier and high density of insulin receptors. Insulin levels were 27.07 +/- 1.3 mU/l with insulin replacement and 3.51 +/- 0.4 mU/l without (P = 0.001). Mean global rate of brain glucose utilization was 0.215 +/- 0.030 mmol. kg(-1). min(-1) without insulin and 0.245 +/- 0.021 mmol. kg(-1). min(-1) with insulin (P = 0.008, an average difference of 15.3 +/- 12.5%). Regional analysis using statistical parametric mapping showed that the effect of basal insulin was significantly less in the cerebellum (Z = 5.53, corrected P = 0.031). We conclude that basal insulin has a role in regulating global brain glucose uptake in humans, mostly marked in cortical areas.

Co-localization of GLUT1 and GLUT4 in the blood-brain barrier of the rat ventromedial hypothalamus

The ventromedial hypothalamus (VMH) has been proposed to be a glucose sensor within the brain and appears to play a critical role in initiating the counterregulatory response to hypoglycemia. Transport of glucose across the brain capillaries and into neurons in this region is mediated by different isoforms of the sodium-independent glucose transporter gene family. The objective of the present study was to identify the specific glucose transporter isoforms present, as well as their cellular localization, within the VMH. Immunohistochemistry was performed for GLUT1, GLUT2 and GLUT4 in frozen sections of hypothalami from normal rats. GLUT1 was present on the endothelial cells of the blood-brain barrier (BBB) of the VMH. GLUT2 immunoreactivity was seen in the ependymal cells of the third ventricle and in scattered cells in the arcuate and periventricular nuclei. There was no GLUT2 expression in the VMH. The insulin-sensitive GLUT4 isoform was localized to vascular structures within the VMH. Double-labeled immunohistochemistry demonstrated co-localization of GLUT4 with GLUT1 and with the tight junction protein ZO-1 in the VMH and suggested that VMH GLUT4 expression was restricted to the BBB. The role of GLUT4 in the brain and within the VMH is unknown, but given its location on the BBB, it may participate in brain sensing of blood glucose concentrations.

Lack of effect of insulin on glucose utilization of the hypothalamus in normotensive and hypertensive rats

Hypertension is frequently associated with insulin resistance and enhanced sympathetic activity supposedly mediated by an effect of the hormone on the hypothalamus. In this study we sought to determine whether insulin modifies the functional activity of the hypothalamus and other brain areas of spontaneously hypertensive (SHR) and normotensive WKY rats. The study was carried out in control and hyperinsulinemic, normoglycemic rats. Insulin plasma levels were increased to 198 +/- 10 (WKY) or 220 +/- 10 microunits/ml (SHR). Brain functional activity was evaluated by the 2-[14C]deoxyglucose method for measuring local rates of glucose utilization. The results show that insulin has no effect on any of the brain areas examined including the hypothalamus, of both WKY and SHR rats. The two strains of rats have comparable cerebral metabolic rates also under basal conditions.

Glucose transport in brain and retina: implications in the management and complications of diabetes

Neural tissue is entirely dependent on glucose for normal metabolic activity. Since glucose stores in the brain and retina are negligible compared to glucose demand, metabolism in these tissues is dependent upon adequate glucose delivery from the systemic circulation. In the brain, the critical interface for glucose transport is at the brain capillary endothelial cells which comprise the blood-brain barrier (BBB). In the retina, transport occurs across the retinal capillary endothelial cells of the inner blood-retinal barrier (BRB) and the retinal pigment epithelium of the outer BRB. Because glucose transport across these barriers is mediated exclusively by the sodium-independent glucose transporter GLUT1, changes in endothelial glucose transport and GLUT1 abundance in the barriers of the brain and retina may have profound consequences on glucose delivery to these tissues and major implications in the development of two major diabetic complications, namely insulin-induced hypoglycemia and diabetic retinopathy. This review discusses the regulation of brain and retinal glucose transport and glucose transporter expression and considers the role of changes in glucose transporter expression in the development of two of the most devastating complications of long-standing diabetes mellitus and its management.

Regional differences in cerebral blood flow and glucose utilization in diabetic man: the effect of insulin

To determine the effect of insulin on regional cerebral blood flow (rCBF) and glucose metabolism (CMRglu), we performed quantitative dynamic PET scanning of labeled water (H215O) and deoxyglucose (18FDG) using two protocols in 10 diabetic men. In protocol A, to test reproducibility of the technique, insulin was infused at 1.5 mU.kg-1.min-1 twice (n = 5). In protocol B, low (0.3 mU.kg-1.min-1) and high (3 mU.kg-1.min-1) dose insulin was given on separate occasions (n = 5). Euglycemia (5 mmol/L) was maintained by glucose infusion. In protocol A, CMRglu was 6% higher during the first infusion, and catecholamines were also increased, indicating stress. Blood flow was not different. Changing free insulin levels from 20.5 +/- 4.8 to 191 +/- 44.5 mU/L (P < 0.001, low versus high dose, protocol B) did not alter total or regional CMRglu (whole brain 36.6 +/- 4.0 versus 32.8 +/- 6.2 mumol.100 g-1.min-1, P = 0.32) or CBF (41.7 +/- 5.1 and 45.6 +/- 9.7 mL.100 g-1.min-1, P = 0.4) or rCBF. In cerebellum, CMRglu was lower than in cortex and the ratio between rate constants for glucose uptake and phosphorylation (K1 and k3) was reversed. There are regional differences in cerebral metabolic capacity that may explain why cerebral cortex is more sensitive to hypoglycemia than cerebellum. Brain glucose metabolism is not sensitive to insulin concentration within the physiologic range. This suggests that intracerebral insulin receptors have a different role from those in the periphery.

Posttraumatic hyperemia in immature, mature, and aged rats: autoradiographic determination of cerebral blood flow

Clinical studies suggest that increased cerebral blood flow (CBF), or hyperemia, after traumatic brain injury (TBI) is commonly found in children and young adults, but is less often found in adults older than 40 years. However, whether posttraumatic cerebral hyperemia is truly an age-related phenomenon has not been proven. Using a model of focal percussive TBI, we hypothesized that (1) local CBF (ICBF) is increased by 24 after injury, and (2) the magnitude of the ICBF increase is age-related and is greatest in immature rats. Wistar rats that were immature (3.5-4.5 weeks), mature (2-3 months), and aged (14.5-15.5 months) were anesthetized and ventilated. TBI was produced by dropping a weight on the exposed right parietal cortex. LCBF was determined by [(14)C]iodoan-tipyrine autoradiography at 24 h posttrauma in all three age groups, at 48 h posttrauma in immature and mature rats, and at 7 days posttrauma in mature rats. In all age groups, low ICBF (<50 mL 100 g(-1) min(-1)) was present in the area of impact at all times studied. At 24 h, hyperemia was observed (vs. corresponding regions of age-matched control rats) in immature and mature rats (7/17 and 5/17 regions, respectively, both p < 0.05), but not in aged rats. Comparisons of ICBF between the three age groups revealed a hyperemic response in the peritrauma region in immature rats. Hyperemia persisted to 48 h in both immature and mature rats (2 and 7 of 17 structures with increased ICBF in immature and mature rats, respectively, both p < .05). By 7 days posttrauma no regions of increased ICBF were found. Posttraumatic hyperemia appears to be an age-dependent phenomenon. These results suggest possible age-related differences in vasoreactivity or regional metabolism after TBI.

Four-day hyperinsulinemia in euglycemic conditions alters local cerebral glucose utilization in specific brain nuclei of freely moving rats

Although insulin is a well known regulator of peripheral tissue glucose metabolism, there is little agreement over its effects on brain glucose metabolism. Several investigators report that peripheral insulin may enter the brain via several routes. The presence of insulin receptors specific to brain, coupled to diverse reports of the effect of acute insulin administration on brain glucose use, led us to carry out a 4-day hyperinsulinemic euglycemic clamp in freely moving rats with subsequent labelled 2-deoxyglucose metabolic mapping studies. It was found that after 4 days of peripheral insulin infusion, several brain regions (Anterior Hypothalamic area, Suprachiasmatic nucleus, Basolateral Amygdaloid nucleus, Supramammillary bodies, Medial Geniculate nucleus and Locus Coeruleus) had an altered local cerebral glucose utilization. Upon subsequent analysis of their anatomical and functional connections it is proposed that insulin may regulate an integrated circuit of pathways within the central nervous system.

Neuropeptide Y, the hypothalamus, and diabetes: insights into the central control of metabolism

Neuropeptide Y (NPY), a major brain neurotransmitter, is expressed in neurons of the hypothalamic arcuate nucleus (ARC) that project mainly to the paraventricular nucleus (PVN), an important site of NPY release. NPY synthesis in the ARC is thought to be regulated by several factors, notably insulin, which may exert an inhibitory action. The effects of NPY injected into the PVN and other sites include hyperphagia, reduced energy expenditure and enhanced weight gain, insulin secretion, and stimulation of corticotropin and corticosterone release. The ARC-PVN projection appears to be overactive in insulin-deficient diabetic rats, and could contribute to the compensatory hyperphagia and reduced energy expenditure, and pituitary dysfunction found in these animals; overactivity of these NPY neurons may be due to reduction of insulin's normal inhibitory effect. The ARC-PVN projection is also stimulated in rat models of obesity +/- non-insulin diabetes, possibly because the hypothalamus is resistant to inhibition by insulin; in these animals, enhanced activity of ARC NPY neurons could cause hyperphagia, reduced energy expenditure, and obesity, and perhaps contribute to hyperinsulinemia and altered pituitary secretion. Overall, these findings suggest that NPY released in the hypothalamuss, especially from the ARC-PVN projection, plays a key role in the hypothalamic regulation of energy balance and metabolism. NPY is also found in the human hypothalamus. Its roles (if any) in human homeostasis and glucoregulation remain enigmatic, but the animal studies have identified it as a potential target for new drugs to treat obesity and perhaps NIDDM.

Effects of neutropenia on edema, histology, and cerebral blood flow after traumatic brain injury in rats

Neutrophils accumulate during the acute inflammatory response to brain injury, but their role in the injury process remains controversial. We tested the hypothesis that neutrophils contribute to cerebral edema, tissue injury, and disturbed cerebral blood flow (CBF) (hyperemia or ischemia) during the first 24 h after traumatic brain injury. Wistar rats (n = 51) were injected with either vinblastine sulfate to induce neutropenia or the saline vehicle. Five days later, under halothane anesthesia, right hemispheric trauma was produced by weight drop (10 g x 5 cm) onto exposed dura. At 24 h after trauma, brain water (wet-dry weight), traumatic infarct size (percent of hemispheric section infarcted), or local CBF (lCBF, 14C-iodoantipyrine autoradiography) was assessed. Vinblastine treatment produced profound neutropenia on the day of trauma (absolute neutrophil count 0.024 +/- 0.008 x 10(9)/L vs 1.471 +/- 0.322 x 10(9)/L, p < 0.05 in neutropenic vs saline, respectively, mean +/- SEM). Neutropenia did not reduce the development of brain edema in the injured hemisphere (brain water 82.38 +/- 0.29% vs 82.73 +/- 0.37% in neutropenic and saline, respectively, mean +/- SEM) or traumatic infarct size (34.5 +/- 3.3% vs 33.2 +/- 2.1% in neutropenic vs saline respectively). In contrast, neutropenic rats exhibited 52%, 41%, and 57% reductions in lCBF in the frontal cortex, parietal cortex, and amygdala, respectively, of the injured hemisphere 24 h after trauma (all p < 0.05 vs nonneutropenic controls). These data suggest that neutrophils and the acute inflammatory process contribute to the level of CBF observed 24 h after trauma, but effects on edema or early posttraumatic infarct size could not be demonstrated.

Local glucose utilization and local blood flow in hearts of awake rats

Local cardiac glucose utilization and local cardiac blood flow in rat heart were measured in vivo by quantitative autoradiographic techniques with 2-[14C] deoxyglucose and [14C] iodoantipyrine, respectively. [14C]methylmethacrylate standards were calibrated for quantitative autoradiography of dried sections of heart tissue; the calibration values for heart tissue differed from those for brain by 8%, probably because of differences in self-absorption within the tissues. The lumped constant required by the deoxyglucose method was determined in isolated, perfused, working rat hearts and found to be 1.11 +/- 0.36 (mean +/- SD, n = 21). The heart: blood partition coefficient for iodoantipyrine required by the [14C]iodoantipyrine method was measured and found to be 1.25. The results obtained in awake rats showed: 1) overall cardiac glucose utilization varied considerably among animals with a mean of 53 (left ventricle) and 30 (right ventricle) mumol/100 g/min; 2) cardiac blood flow was less variable among animals with a mean of 592 (left ventricle) and 420 (right ventricle) ml/100 g/min; 3) glucose utilization was found to be particularly high in the papillary muscle; 4) systematic gradients of glucose utilization or blood flow in the ventricular wall were not observed; 5) glucose utilization and blood flow were not closely correlated on a local level. It is concluded that autoradiographic methods are suitable for the quantification of local glucose utilization and local blood flow in the rat heart in vivo. These methods could not demonstrate transmural gradients for glucose utilization and blood flow between epi- and endocardium in awake rats.

In vivo location of the rate-limiting step of hexose uptake in muscle and brain tissue of rats

The uptake of glucose proceeds via facilitated transport from the plasma followed by phosphorylation of intracellular glucose. We have quantified the relative contribution of transport and phosphorylation to the overall rate of hexose utilization into the quadriceps muscle (red and white) and cerebellum of rats anesthetized with pentobarbital sodium. The method employed simultaneous infusions of radiolabeled 3-O-methyl-D-glucose and 2-deoxy-D-glucose. Results were expressed in terms of a parameter ft*, which has theoretical limits of 0 and 1 corresponding to phosphorylation and transport limitation, respectively. In cerebellum, basal rates of transport and phosphorylation were comparable (ft* = 0.32 +/- 0.02). Under conditions of hyperglycemia plus maximum insulin stimulation, phosphorylation limited glucose utilization to a greater extent (ft* = 0.12 +/- 0.02). No effect of hyperinsulinemia alone was observed. In red muscle, transport determined overall glucose utilization in the basal (ft* = 0.96 +/- 0.05) and euglycemic insulin-stimulated states (ft* = 0.90 +/- 0.02). A shift of the rate-limiting step from transport toward phosphorylation was observed in insulin-stimulated red muscle when blood glucose (ft* = 0.64 +/- 0.05) or epinephrine levels (ft* = 0.66 +/- 0.07) were elevated. Neither effect was seen in white muscle. We conclude that the transport step dominates but is not the only determinant of muscle hexose utilization under all conditions.

Relation of hyperglycemia early in ischemic brain infarction to cerebral anatomy, metabolism, and clinical outcome

We studied the relation of serum glucose level measured in the first 12 hours of symptoms to the clinical findings, results of computed tomography (CT), and patterns of cerebral metabolism in 39 patients who had acute ischemic cerebral infarction. Structural damage was assessed by CT. Metabolic disruption was assessed using 18F-fluorodeoxyglucose and positron emission tomography (PET). Median initial serum glucose concentration was 155 mg/dl (6.7 mM). Clinical recovery was significantly poorer in patients with initial serum glucose levels higher than the median (p less than 0.05, chi square). PET tended to show normal results or minor abnormalities in patients with initial glucose levels less than the median, as opposed to lobar or multilobe abnormalities in patients with levels that were higher than the median (p less than 0.05, Kendall's Tau b). The severity of hypometabolism in the ischemic region, expressed as the percent asymmetry of local cerebral glucose metabolism between homologous brain regions, was greater in patients with initial glycemia concentrations higher than the median (p less than 0.001, t test). Relationships of serum glucose level with metabolic derangement and structural damage, but not outcome, held true in patients without a history of diabetes mellitus.

The lumped constant of the deoxyglucose method in hypoglycemia: effects of moderate hypoglycemia on local cerebral glucose utilization in the rat

The applicability of the [14C]deoxyglucose method for measuring local cerebral glucose utilization (lCMRglc) has been extended for use in hypoglycemia by determination of the values of the lumped constant to be used in rats with plasma glucose concentrations ranging from approximately 2 to 6 mM. Lumped constant values were higher in hypoglycemia and declined from a value of 1.2 at the lowest arterial plasma glucose level (1.9 mM) to about 0.48 in normoglycemia. The distribution of glucose, and therefore also of the lumped constant, was found to remain relatively uniform throughout the brain at the lowest plasma glucose levels studied. lCMRglc in moderate, insulin-induced hypoglycemia (mean arterial plasma glucose concentration +/- SD of 2.4 +/- 0.3 mM) was determined with the appropriate lumped constant corresponding to the animal's plasma glucose concentration and compared with the results obtained in six normoglycemic rats. The weighted average rate of glucose utilization for the brain as a whole was significantly depressed by 14% in the hypoglycemic animals, i.e., 61 mumols/100 g/min in hypoglycemia compared to 71 mumols/100 g/min in the normoglycemic controls (p less than 0.05). lCMRglc was lower in 47 of 49 structures examined but statistically significantly below the rate in normoglycemic rats in only six structures (p less than 0.05) by multiple comparison statistics. Regions within the brainstem were most prominently affected. The greatest reductions, statistically significant or not, occurred in structures in which glucose utilization is normally high, suggesting that glucose delivery and transport to the tissue became rate-limiting first in those structures with the greatest metabolic demands for glucose.

Fasting prior to transient cerebral ischemia reduces delayed neuronal necrosis

A transient brain ischemia of 30-min duration was induced by the four-vessel occlusion technique in normally fed and in 48-hr-fasted rats. Evaluation of brain damage 72 hr after ischemia showed that fasting reduced neuronal necrosis in the striatum, the neocortex, and the lateral part of the CA1 sector of hippocampus. Signs of status spongiosis in the pars reticulata of the substantia nigra were seen in 75% of fed rats and in only 19% of fasted rats. The protective effect was associated with reduction in mortality and in postischemic seizure incidence. The metabolic changes induced by fasting were evaluated before and during ischemia. After 30 min of four-vessel occlusion, fasted rats showed a marked decrease in brain lactate level (14.7 vs 22.5 mumol/g in fed rats; P less than 0.001). The decrease in brain lactate concentration might explain the beneficial effect of fasting by minimizing the neuropathological consequences of lactic acidosis. Several factors may account for lower lactate production during ischemia in fasted rats: hypoglycemia, reduction in preischemic stores of glucose and glycogen, or increased utilization of ketone bodies aiming at reducing the glycolytic rate.

Refinement of the kinetic model of the 2-[14C]deoxyglucose method to incorporate effects of intracellular compartmentation in brain

A translocase to transport hexose phosphate formed in the cytosol into the cisterns of the endoplasmic reticulum, where the phosphatase resides, is absent in brain (Fishman and Karnovsky, 1986). 2-Deoxyglucose-6-phosphate (DG-6-P) may therefore have limited access to glucose-6-phosphatase (G-6-Pase), and transport of the DG-6-P across the endoplasmic reticular membrane may be rate limiting to its dephosphorylation. To take this compartmentation into account, a five-rate constant (5K) model was developed to describe the kinetic behavior of 2-deoxyglucose (DG) and its phosphorylated product in brain. Loss of DG-6-P was modeled as a two-step process: (a) transfer of DG-6-P from the cytosol into the cisterns of the endoplasmic reticulum; (b) hydrolysis of DG-6-P by G-6-Pase and subsequent return of the free DG to the precursor pool. Local CMRglc (LCMRglc) was calculated in the rat on the basis of this model and compared with values calculated on the basis of the three-rate constant (3K) and the four-rate constant (4K) models of the DG method. The results show that under normal physiological conditions all three models yield values of LCMRglc that are essentially equivalent for experimental periods between 25 and 45 min. Therefore, the simplest model, the 3K model, is sufficient. For experimental periods from 60 to 120 min, the 4K and 5K models do not correct completely for loss of product, but the 5K model does yield estimates of LCMRglc that are closer to the values at 45 min than those obtained with the 3K and 4K models.

Insulin administration protects neurologic function in cerebral ischemia in rats

Hyperglycemia exacerbates neurologic damage in clinical and experimental central nervous system ischemia. The purpose of our study was to determine if insulin administration before significantly alters neurologic deficit and survival after ischemia using a newly developed rat cerebral ischemia model. One hour before the onset of ischemia, 40 200-300-g Sprague-Dawley rats received intraperitoneal injections of either 1 ml normal saline or 0.4, 0.5, or 0.6 units regular insulin in 1 ml normal saline. Rats were then intubated and ventilated with 1-1.5% halothane. The aortic arch was exposed, and snares were placed on the innominate, left carotid, and left subclavian arteries. A 20-minute occlusion was begun, and anesthesia was discontinued. Baseline plasma glucose concentration was similar (p = 0.48, Student's t test) in both groups, but it subsequently was significantly lower in the 0.4 unit insulin-treated group up to 4 hours after occlusion (p less than or equal to 0.0035, Student's t test). Neurologic deficit was scored on a 50-point scale (0 = normal, 50 = severe deficit) 1, 4, 18, and 24 hours after occlusion. In the 0.4 unit insulin-treated group the neurologic deficit score was significantly lower than in the saline-treated group 1, 4, 18, and 24 hours after occlusion (p less than or equal to 0.005, Student's t test). Survival was significantly higher (p = 0.001) in the 0.4 unit insulin-treated (1.7 unit/kg dose) group than in the saline-treated group. No rats died when preocclusion plasma glucose concentration was between 65 and 175 mg/dl.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional cerebral blood flow and glucose utilization during hyperinsulinemia

Regional cerebral blood flow and glucose utilization were measured in awake-restrained rats during euglycemic hyperinsulinemia and were found to be normal. This indicates that elevated plasma insulin does not alter cerebral blood flow and cannot explain decreases in cerebral blood flow measured during acute hyperglycemia.

Local cerebral glucose utilization in controlled graded levels of hyperglycemia in the conscious rat

Local cerebral glucose utilization assayed by the [14C]deoxyglucose ([14C]DG) method and calculated by means of its operational equation with values for the rate constants and lumped constant determined in rats under physiological conditions remains relatively stable with variations in arterial plasma glucose concentration within the normoglycemic range. Large changes in arterial plasma glucose level may, however, significantly alter the values of these constants and lead to artifactual results. Values for the lumped constant have been measured and reported for a wide range of arterial plasma glucose concentrations ranging from hypoglycemia to hyperglycemia in the rat (Schuier et al., 1981; Suda et al., 1981; Pettigrew et al., 1983). In the present study we have redetermined the rate constants in rats with arterial plasma glucose levels clamped at approximately 350, 450, and 550 mg/dl (i.e., 19, 25, and 31 mM) by a glucose clamp technique. The rate constants for the transport of DG from plasma to brain, K1*, and its phosphorylation in tissue, k3*, were found to decline with increasing plasma glucose levels, while the rate constant for its transport back from brain to plasma, k*2, remained relatively unchanged from its value in normoglycemia. These rate constants were used together with the previously determined values for the lumped constants to calculate local rates of cerebral glucose utilization in three groups of rats in which arterial plasma glucose levels were clamped at approximately 350, 450, and 550 mg/dl (i.e., 19, 25, and 31 mM). Average glucose utilization in the brain as a whole was unchanged in hyperglycemia from the values calculated in normoglycemic rats with the standard normal set of constants. Changes in the rate of glucose utilization were found, however, in the hypothalamus, globus pallidus, and amygdala during hyperglycemia.

Insulin administration protects from paraplegia in the rat aortic occlusion model

The effect of insulin induced hypoglycemia was evaluated in a rat aortic occlusion model of ischemic paraplegia. One hour before aortic occlusion, 200-250 g Sprague-Dawley rats received either 1 cc of saline or 0.5 units regular insulin in 1 cc saline. Rats were then anesthetized, intubated, and ventilated with halothane (1-1.5%). The aortic arch was exposed and snares were placed on the right and left subclavian arteries and the aorta distal to the left subclavian. The three vessels were occluded for 10 min. Lower extremity neurologic deficit was evaluated at 1, 4, 18, and 24 hr postocclusion based on a 15-point scale (0 = normal, 15 = severe deficit). Lower extremity neurologic deficit was significantly decreased in the insulin-treated group at 18 and 24 hr postocclusion (P = 0.005 and 0.006, respectively, Student's test). Blood glucose concentration was significantly lower at the time of occlusion in the insulin-treated group when compared to the saline-treated group (P = 0.001). We conclude that in this rat model of paraplegia, insulin induced hypoglycemia is associated with a reduction in lower extremity neurologic deficit produced by temporary aortic occlusion.