Local cerebral glucose utilization in the brain of old, learning impaired rats

Effects of Glucose, Insulin, and Supernatant from Pancreatic β-cells on Brain–Pancreas Relative Protein in Rat Hippocampus

Brain-pancreas relative protein (BPRP) is a novel protein that mainly expresses in brain and pancreas. In our previous study, we found that various stressors significantly decreased the expression of BPRP in pancreas in vivo, accompanied by changes in insulin and glucose levels, and that expression of BPRP in pancreas also decreased significantly in diabetic rats induced by Streptozocin (STZ). All these findings suggest that BPRP may be a glucose or insulin-sensitive protein. However, how the changes in insulin or glucose levels influence the expression of BPRP in hippocampus requires further study. Here, we investigated the effects of insulin or glucose on the expression of BPRP in primary cultured hippocampal neurons. We supplied hippocampal neurons with glucose, insulin, or supernatant from pancreatic beta-cells, which secrete insulin into the supernatant. Our data showed that insulin had beneficial effect on the viability while no significant effect on the expression of BPRP in hippocampal neurons. On the contrary, 40 mM glucose or free glucose culture significantly decreased the expression of BPRP, while had no significant effect on the viability and apoptosis of hippocampal neurons. Further study showed that levels of insulin in the supernatant collected from pancreatic beta-cells medium changed over days, and that supernatant increased the viability of hippocampal neurons, while it had no obvious effect on the expression of BPRP in hippocampal neurons. These results suggest that BPRP may be a glucose-sensitive protein.

Regulation of cytochrome oxidase activity in the rat forebrain throughout adulthood

Measures of metabolic activity can provide useful indices of the effects of aging on neural function, since sustained changes in neural activity alter metabolic demand and the activity of metabolic enzymes. Previous reports of effects of aging on key enzymes for oxidative metabolism are mixed, however, with some reports that activity declines in the aging brain and others that activity remains stable or increases. We used high-resolution, quantitative histochemistry to test whether cytochrome oxidase (CO) activity changes in the forebrain during adulthood and senescence, measuring activity in each layer of the hippocampus and several cerebral cortical areas. In most forebrain regions, average cytochrome oxidase activity was slightly higher in middle-aged than in young adult rats but did not differ between middle-aged and old rats. Thus, there was no significant change in cytochrome oxidase activity with senescence. Additional analyses indicated that cytochrome oxidase activity is regulated regionally in the brain, as well as focally, and that differences in regional regulation may contribute to variation in CO activity among individuals, which was greater in young and old rats than in middle-aged animals.

Intracerebroventricular injection of streptozotocin causes neurotoxicity to myelin that contributes to spatial memory deficits in rats

It has been reported that intracerebroventricular (icv) injection of streptozotocin (STZ) impairs spatial memory by disrupting glucose utilization through an insulin-dependent mechanism in the cerebral cortex and hippocampus. However, evidence of septal damage and microglosis induced by icv STZ suggested that its neurotoxic effects could contribute to the memory impairment. The present study examined the histopathological changes in adult rats following three icv STZ injections (0.25 mg into each lateral ventricle) and their effects on spatial memory in a Morris water maze task. STZ retarded acquisition of reference learning (progressive reduction in escape latency) and disrupted working memory (difference in escape latency between the two swims within a daily session). STZ caused selective injury to myelin and axons in the fornix and hippocampus in association with activation of microglia. The 3rd ventricle was enlarged by 100-150% because of a loss of ependymal cells and damage to hypothalamic periventricular myelin but the process involved in these changes is unclear. Our findings provide an alternative explanation for the decrease in glucose utilization in the hippocampus and cortex and the impairment of spatial memory induced by STZ. These could result from a disruption of the communication through myelinated axons in the fornix connecting the septum and the hippocampus, and through other myelinated axons adjacent to the ventricles. The selective damage to myelin may well result from oxidative stress.

[1-13C]Glucose entry in neuronal and astrocytic intermediary metabolism of aged rats. A study of the effects of nicergoline treatment by 13C NMR spectroscopy

Age-related changes in glucose utilization through the TCA cycle were studied using [1-13C]glucose and 13C, 1H NMR spectroscopy on rat brain extracts. Significant increases in lactate levels, as well as in creatine/phosphocreatine ratios (Cr/PCr), and a decrease in N-acetyl-aspartate (NAA) and aspartate levels were observed in aged rat brains as compared to adult animals following glucose administration. The total amount of 13C from [1-13C]glucose incorporated in glutamate, glutamine, aspartate and GABA was significantly decreased in control aged rat brains as compared to adult brains. The results showed a decrease in oxidative glucose utilization of control aged rat brains. The long-term nicergoline treatment increased NAA and glutamate levels, and decreased the lactate levels as well as the Cr/PCr ratios in aged rat brains as compared to adult rats. The total amount of 13C incorporated in glutamate, glutamine, aspartate, NAA and GABA was increased by nicergoline treatment, showing an improvement in oxidative glucose metabolism in aged brains. A significant increase in pyruvate carboxylase/pyruvate dehydrogenase activity (PC/PDH) in the synthesis of glutamate in nicergoline-treated aged rats is consistent with an increase in the transport of glutamine from glia to neurons for conversion into glutamate. In adult rat brains, no effect of nicergoline on glutamate PC/PDH activity was observed, although an increase in PC/PDH activity in glutamine was, suggesting that nicergoline affects the glutamate/glutamine cycle between neurons and glia in different ways depending on the age of animals. These results provide new insights into the effects of nicergoline on the CNS.

Brain insulin: regulation, mechanisms of action and functions

1. While many questions remained unanswered, it is now well documented that, contrary to earlier views, insulin is an important neuromodulator, contributing to neurobiological processes, in particular energy homeostasis and cognition. A specific role on cognitive functions related to feeding is proposed, and it is suggested that brain insulin from different sources might be involved in the above vital functions in health and disease. 2. A molecule identical to pancreatic insulin, and specific insulin receptors, are found widely distributed in the central nervous system networks related to feeding, reproduction, or cognition. 3. The actions of insulin in the central nervous system may be under both multilevel and multifactorial controls. The amount of blood insulin reaching the brain, brain insulin stores and secretion, potential local biosynthesis and degradation of the peptide, and insulin receptors and signal transduction can be affected by metabolic factors induced by nutrients, hormones, neurotransmitters, and regulatory peptides, peripherally or in the central nervous system. 4. Glucose and serotonin regulate insulin directly in the hypothalamus and may be of importance for its biological effects. Central mechanisms regulating glucose-induced insulin secretion show some analogy with the mechanisms operating in the pancreas. 5. A cross-talk between insulin and leptin receptors has been observed in the brain, and a regulation of central insulin actions, potentially via serotonin modulation, by leptin, galanin, melancortins, and neuropeptide Y (NPY) is suggested. 6. A more complete knowledge of the biological role of insulin in brain function and dysfunction, and of the regulatory mechanisms involved in these processes, constitutes a real advancement in the understanding of the pathophysiology of metabolic and mental diseases and could lead to important medical benefits.

A potential role of central insulin in learning and memory related to feeding

1. Hypothalamic insulin (HI) is well known for its role in feeding regulation. In addition, its concentration is modified in response to meals. Recent studies suggest that brain insulin participates in memory processes, possibly through stimulation by glucose. 2. The present microdialysis study focused on local in vivo regulation of HI by glucose and on the effects of aging on HI, since aging is characterized by deterioration of memory, body weight regulation, and central glucose utilization. Glucose (8 mM) infused for 5 min increased extracellular HI levels rapidly, by 4.6-fold, and cerebellar insulin levels by 0.4-fold only, suggesting a specific area-dependent regulation of HI by glucose. Neither insulinemia nor glycemia were affected, suggesting a central mechanism. The same dose of glucose induced a modest (0.4-fold), delayed (45 min) increase in hypothalamic serotonin, suggesting that the effect of glucose on HI is independent of a previously defined local serotonin-induced insulin release. HI levels in old normal weight rats were half the levels of young rats. In genetically old obese (fa/fa) Zucker rats, HI concentration was 30% of that in young normal rats, suggesting a deterioration of HI availability when aging and obesity are combined. 3. The above results, in line with recent considerations on a potential role of central insulin in learning and memory, suggest particular effects of HI on feeding and memory and probably on a specific "memory for food."

Cerebral microvascular pathology in aging and Alzheimer's disease

The aging of the central nervous system and the development of incapacitating neurological diseases like Alzheimer's disease (AD) are generally associated with a wide range of histological and pathophysiological changes eventually leading to a compromised cognitive status. Although the diverse triggers of the neurodegenerative processes and their interactions are still the topic of extensive debate, the possible contribution of cerebrovascular deficiencies has been vigorously promoted in recent years. Various forms of cerebrovascular insufficiency such as reduced blood supply to the brain or disrupted microvascular integrity in cortical regions may occupy an initiating or intermediate position in the chain of events ending with cognitive failure. When, for example, vasoconstriction takes over a dominating role in the cerebral vessels, the perfusion rate of the brain can considerably decrease causing directly or through structural vascular damage a drop in cerebral glucose utilization. Consequently, cerebral metabolism can suffer a setback leading to neuronal damage and a concomitant suboptimal cognitive capacity. The present review focuses on the microvascular aspects of neurodegenerative processes in aging and AD with special attention to cerebral blood flow, neural metabolic changes and the abnormalities in microvascular ultrastructure. In this context, a few of the specific triggers leading to the prominent cerebrovascular pathology, as well as the potential neurological outcome of the compromised cerebral microvascular system are also going to be touched upon to a certain extent, without aiming at total comprehensiveness. Finally, a set of animal models are going to be presented that are frequently used to uncover the functional relationship between cerebrovascular factors and the damage to neural networks.

Effect of subchronic metrifonate treatment on cerebral glucose metabolism in young and aged rats

The effects of subchronic administration of metrifonate, a long-lasting cholinesterase inhibitor, on local cerebral glucose utilization were assessed in 3- and 27-month old Sprague-Dawley rats, using the autoradiographic [14C]2-deoxyglucose technique. Rats were treated twice daily with metrifonate (80 or 120 mg/kg) for 3 weeks. The [14C]2-deoxyglucose experiment was performed 18 h after the last metrifonate administration. In 3-month old rats, metrifonate 80 mg/kg increased the average hemispheric cerebral glucose utilization by 12% (P > 0.001). Significant effects were observed in 19 of the 54 regions studied, including cortical and limbic regions. The higher dose induced a larger effect (average increase 17%, 24 of the 54 regions affected). In 27-month old rats, very similar effects were obtained. These results show that repeated administration of metrifonate leads to a sustained metabolic activation in rat brain, at a level comparable to the activation observed previously after a single administration of the drug.

Distribution and levels of [125I]IGF-I, [125I]IGF-II and [125I]insulin receptor binding sites in the hippocampus of aged memory-unimpaired and -impaired rats

The insulin-like growth factors (IGF-I and IGF-II) and insulin are localized within distinct brain regions and their respective functions are mediated by specific membrane receptors. High densities of binding sites for these growth factors are discretely and differentially distributed throughout the brain, with prominent levels localized to the hippocampal formation. IGFs and insulin, in addition to their growth promoting actions, are considered to play important roles in the development and maintenance of normal cell functions throughout life. We compared the anatomical distribution and levels of IGF and insulin receptors in young (five month) and aged (25 month) memory-impaired and memory-unimpaired male Long Evans rats as determined in the Morris water maze task in order to determine if alterations in IGF and insulin activity may be related to the emergence of cognitive deficits in the aged memory-impaired rat. In the hippocampus, [125I]IGF-I receptors are concentrated primarily in the dentate gyrus (DG) and the CA3 sub-field while high amounts of [125I]IGF-II binding sites are localized to the pyramidal cell layer, and the granular cell layer of the DG. [125I]insulin binding sites are mostly found in the molecular layer of the DG and the CA1 sub-field. No significant differences were found in [125I]IGF-I. [125I]IGF-II or [125I]insulin binding levels in any regions or laminae of the hippocampus of young vs aged rats. and deficits in cognitive performance did not relate to altered levels of these receptors in aged memory-impaired vs aged memory-unimpaired rats. Other regions. including various cortical areas, were also examined and failed to reveal any significant differences between the three groups studied. It thus appears that IGF-I, IGF-II and insulin receptor sites are not markedly altered during the normal ageing process in the Long Evans rat, in spite of significant learning deficits in a sub-group (memory-impaired) of aged animals. Hence. recently reported changes in IGF-I receptor messenger RNA levels in aged memory-impaired rats are apparently not reflected at the level of the translated protein.

Effects of metrifonate, a cholinesterase inhibitor, on local cerebral glucose utilization in young and aged rats

The effects of the centrally acting anti-cholinesterase metrifonate (MFT) and its metabolite dichlorvos (2,2-dichlorovinyl dimethyl phosphate; DDVP) on local cerebral glucose utilization (LCGU) have been studied in 3- and 27-month-old rats, using the autoradiographic [14C]deoxyglucose technique. In 3-month-old rats, MFT (80 mg/kg i.p.) increased LCGU significantly in 17 of the 54 regions studied, including insular, cingulate, and temporal cortices, ventral hippocampus, thalamus, lateral habenula, substantia nigra, and superior colliculus. In these regions, the average MFT-induced increase in LCGU was 23% above control. The average hemispheric LCGU increased by 10% (p < 0.01). DDVP (5 mg/kg) increased LCGU in 19 regions (average increase 26%). The average hemispheric LCGU increased by 9% (p < 0.01). Regional distributions of MFT- and DDVP-induced increases in LCGU were similar and overlapped the distribution of the acetylcholinesterase activity. In 27-month-old rats, MFT was active in 18 regions (average increase 25%). The whole-brain mean LCGU increased by 10% (p < 0.01). MFT compensated for the age-related hypometabolism in some brain areas including insular, temporal, and retrosplenial cortices, substantia nigra, and superior colliculus. The effects of MFT on LCGU were preserved in old rats, at variance with other anticholinesterases (tacrine, physostigmine). Which are less active in the aged rat brain.

Metabolic response to tacrine (THA) and physostigmine in the aged rat brain

The effects of the centrally acting anti-cholinesterases tacrine (tetrahydroaminoacridine, THA) and physostigmine (PHY), on local cerebral glucose utilization (LCGU) have been studied in 27-month-old rats, using the autoradiographic [14C]deoxyglucose technique. THA (10 mg/kg i.p.) increased LCGU significantly in 13 of the 54 regions studied (24%) including insular, parietal, temporal, and retrosplenial cortices, septohippocampal system, thalamus, lateral habenula, and superior colliculus. In these regions, the average THA-induced increase in LCGU was 24% above control. The whole brain mean LCGU was not significantly increased. PHY (0.5 mg/kg) increased LCGU in 18% of the regions (average elevation, 23%). The whole brain mean LCGU increased by 7% (p < 0.05). The regional distributions of THA- and PHY-induced increases in LCGU were extremely similar and overlapped the distribution of the M2 muscarinic receptors and that of acetylcholinesterase activity, suggesting that the major effects of THA and PHY on LCGU result from their anticholinesterase action. As compared to those of 3-month-old rats, both the number of regions affected and the amplitude of the metabolic activation were significantly less in aged rats. However, the drugs were still active in old rats and compensated for the age-related hypometabolism in some brain areas.

Chronic depression of glucose metabolism in postischemic rat brains

BACKGROUND AND PURPOSE

The present investigation aimed to quantify functional activity in rat brains after long-term recovery from transient forebrain ischemia.

METHODS

With the use of the [14C]2-deoxyglucose method, local cerebral glucose utilization was measured in 62 cortical and subcortical brain regions in postischemic rat brains. Transient forebrain ischemia of 10 minutes' duration was induced by clamping the common carotid arteries and simultaneously lowering blood pressure to 40 mm Hg. Rats survived the insults for 1 week, 2 weeks, 3 weeks, or 3 months.

RESULTS

Reductions predominated in the majority of gray matter structures at all time points investigated (P < .05). Except for a few areas, recoveries of local cerebral glucose utilization to preischemic levels did not occur.

CONCLUSIONS

The data illustrate that widespread alterations of functional activity prevail in postischemic brains beyond the selectively vulnerable regions. The present functional data are in line with previous stereological results of reduced fresh volumes in the majority of postischemic brain structures. The data suggest that chronic alterations of ischemic brains are not confined to the selectively vulnerable regions.

Spatial discrimination learning and choline acetyltransferase activity in streptozotocin-treated rats: effects of chronic treatment with acetyl-L-carnitine

Treatment of rats with i.c.v. injected streptozotocin (STREP) may provide a relevant model of neurodegeneration that is induced by a decrease in the central metabolism of glucose. Acetyl-L-carnitine (ALCAR) enhances the utilization of alternative energy sources and by such a mechanism of action ALCAR could antagonize the effects of STREP treatment. In this study the effects of chronic treatment with ALCAR were evaluated on spatial discrimination learning in the Morris task and choline acetyltransferase (ChAT) activity of middle-aged STREP-treated rats. Chronic treatment with ALCAR attenuated both the STREP-induced impairment in spatial bias and the decrease in hippocampal ChAT activity. These findings indicate that ALCAR treatment has a neuroprotective effect, although further studies are needed to characterize the mechanism of action of ALCAR in this model.

Behavioral and biochemical effects of an ICV injection of streptozotocin in old Lewis rats

Intracerebroventricularly (ICV) injected streptozotocin (STREP) decreases central glucose metabolism and energy metabolism, which has also been observed in patients with dementia. In the present study we examined the behavioral (open-field behavior and two-way active avoidance learning) and biochemical (hippocampal ChAT activity) effects of STREP treatment in old Lewis rats. The results suggest that hippocampal function was affected by STREP. STREP-treated rats acquired the two-way active avoidance task faster than the control rats, which indicates that STREP treatment does not lead to a general learning deficit. Hippocampal ChAT activity was decreased in STREP-treated rats. The present results suggest also that susceptibility to STREP may not be related to age in Lewis rats.

Morphological hippocampal changes during normal aging and their relation to cognitive deterioration

Cognitive and memory capacities were assessed in two strains of rats of various age groups prior to histological evaluation of their brains. Male Wistar rats, at the age of 3, 12, 17 and 24 months, were tested using the 8-arm radial maze and male Fischer 344 rats, aged 3, 12 and 16 months, were tested in the Morris water maze. Significant memory impairments were found in both strains already at the age of 12 months in about 50% of the population. Morphological analysis of the brains revealed age-related structural changes in the hippocampal formation starting with the middle-age group. Degenerative CA1 and CA3 pyramidal cells characterized the hippocampus of cognitive-impaired rats, while non-impaired animals exhibited intact hippocampus irrespective of age. This characteristic was supported by quantitative morpho-analysis. The best correlation between the decrease of area or number of cells and working memory impairment was found for CA3 region in both strains. Age-related decline in the density of muscarinic receptors in Wistar rats' brain corresponded with the pattern of cognitive deficit. The results of the present study support the hypothesis which associates hippocampal integrity with normal memory function. It is concluded that chronological age by itself is not an adequate indicator of age-related brain alterations and individual evaluation of performance, based on behavioral scores, is recommended.

Spatial learning deficit and reduced hippocampal ChAT activity in rats after an ICV injection of streptozotocin

ICV injections of streptozotocin (STREP) lower the glucose utilization of the brain and affect the cholinergic system. The present study was designed to evaluate whether STREP-treated rats have an impaired spatial discrimination performance in the Morris spatial navigation task. Performance in this task is sensitive to treatment with cholinergic antagonists. In contrast to young rats, middle-aged STREP-treated rats tended to have an impaired spatial discrimination performance in the Morris task at the end of training. In middle-aged STREP-treated rats, but not in control rats, spatial discrimination performance was associated with hippocampal choline acetyltransferase (ChAT) activity. The correlation between spatial discrimination performance in the Morris task and the decrease in hippocampal ChAT activity resembles the relation between cognitive and biochemical changes observed in Alzheimer's disease. Our findings suggest that STREP treatment of middle-aged rats may provide a relevant model for dementia.

Long-Evans and Sprague-Dawley rats differ in their spatial navigation performance during ontogeny and at maturity

The Morris maze has become a popular method for the assessment of spatial navigation. However, its use to study the development of spatial abilities has been limited to pigmented rats. Thus, the aim of the present study was to compare albino Sprague-Dawley and pigmented Long-Evans rats using this test during postnatal Days 20 through 27, and Day 90. It was found that Long-Evans rats showed significantly shorter escape latencies and swim distances than the Sprague-Dawley rats on Days 20-25 but not on Days 26-27. However, when tested at Day 90, the Long-Evans rats again showed more rapid location of the escape platform and shorter swim distances than the Sprague-Dawley rats. Probe trial analysis (platform removed) indicated that Long-Evans rats were generally more accurate in their localization of the former platform location than Sprague-Dawley rats. In a second experiment in which 21-day-old rats of both strains were tested in a proximal-cue version of the maze, the question of whether this performance difference might have related to visuo-perceptual differences was considered. Since no dissimilarity in performance was observed, a spatial-learning difference between the two strains would seem best able to explain the preceding data.

Individual differences in the cognitive and neurobiological consequences of normal aging

Defining the neural basis of age-related cognitive dysfunction is a major goal of current research on aging. Compelling evidence from laboratory animals and humans indicates that aging does not inevitably lead to cognitive decline. Conducting neurobiological investigations in subjects that have previously undergone behavioral characterization has therefore emerged as a promising strategy for identifying those alterations in brain structure and function that are specifically associated with age-related cognitive impairment.

Hippocampal nerve growth factor levels are related to spatial learning ability in aged rats

Brain nerve growth factor (NGF) was determined in two groups of aged rats: 'good' and 'poor' performers. The animals were selected out of a population of 40 aged rats (26-28 months old) trained in a spatial learning task. Animals performing well in the test had significantly higher NGF in the hippocampus when compared to 'poor' performers. No differences in the levels of NGF were found in the cortex, septum and cerebellum. The results implicate hippocampal NGF in cognitive functioning of aged rats, and suggests that the forebrain cholinergic neuronal atrophy which has been observed in cognitively impaired aged rats may be due to reduced availability of target-derived NGF.