Blood glucose and brain function: interactions with CNS cholinergic systems

Cognitive dysfunctions in individuals with diabetes mellitus

Some patients with type 1 and type 2 diabetes mellitus (DM) present with cognitive dysfunctions. The pathophysiology underlying this complication is not well understood. Type 1 DM has been associated with a decrease in the speed of information processing, psychomotor efficiency, attention, mental flexibility, and visual perception. Longitudinal epidemiological studies of type 1 DM have indicated that chronic hyperglycemia and microvascular disease, rather than repeated severe hypoglycemia, are associated with the pathogenesis of DM-related cognitive dysfunction. However, severe hypoglycemic episodes may contribute to cognitive dysfunction in high-risk patients with DM. Type 2 DM has been associated with memory deficits, decreased psychomotor speed, and reduced frontal lobe/executive function. In type 2 DM, chronic hyperglycemia, long duration of DM, presence of vascular risk factors (e.g., hypertension and obesity), and microvascular and macrovascular complications are associated with the increased risk of developing cognitive dysfunction. The pathophysiology of cognitive dysfunction in individuals with DM include the following: (1) role of hyperglycemia, (2) role of vascular disease, (3) role of hypoglycemia, and (4) role of insulin resistance and amyloid. Recently, some investigators have proposed that type 3 DM is correlated to sporadic Alzheimer's disease. The molecular and biochemical consequences of insulin and insulin-like growth factor resistance in the brain compromise neuronal survival, energy production, gene expression, plasticity, and white matter integrity. If patients claim that their performance is worsening or if they ask about the effects of DM on functioning, screening and assessment are recommended.

Diet composition modifies the toxicity of repeated soman exposure in rats

It was previously demonstrated that diet potently modulates the toxic effects of an acute lethal dose of the nerve agent soman. The current investigation was undertaken to examine the influence of diet on the cumulative toxicity of repeated soman administration. Rats were fed one of four distinct diets (standard, choline-enriched, glucose-enriched, or ketogenic) for four weeks prior to and throughout a repeated soman dosing and recovery regimen. Each diet group included animals exposed to an equivalent volume of saline that served as negative controls. In exposure Week 1, animals received three consecutive daily doses of 0.4 LD(50) soman. In exposure Week 2, animals received four consecutive daily doses of 0.5 LD(50) soman. In exposure Week 3, animals received five consecutive daily doses of 0.5 LD(50) soman. Week 4 constituted a post-exposure recovery evaluation. Throughout the experiment, behavioral function was assessed by a discriminated avoidance test that required intact sensory and motor function. Survival and body weight changes were recorded daily. Differences in toxicity as a function of diet composition became apparent during the first week. Specifically, rats fed the glucose-enriched diet showed pronounced intoxication during Week 1, resulting in imperfect survival, weight loss, and deteriorated avoidance performance relative to all other groups. All rats fed the glucose-enriched diet died by the end of exposure Week 2. In contrast, only 10% of animals fed the standard diet died by the end of Week 2. Also in Week 2, weight loss and disrupted avoidance performance were apparent for all groups except for those fed the ketogenic diet. This differential effect of diet composition became even more striking in Week 3 when survival in the standard and choline diet groups approximated 50%, whereas survival equaled 90% in the ketogenic diet group. Avoidance performance and weight loss measures corroborated the differential toxicity observed across diet groups. Upon cessation of soman exposure during the final week, recovery of weight and avoidance performance in survivors was comparable across diet groups. These results systematically replicate previous findings demonstrating that diet composition exacerbates or attenuates toxicity in rodents exposed acutely to organophosphorus compounds.

Diet composition exacerbates or attenuates soman toxicity in rats: implied metabolic control of nerve agent toxicity

To evaluate the role of diet composition on nerve agent toxicity, rats were fed four distinct diets ad libitum for 28 d prior to challenge with 110 μg/kg (1.0 LD(50), sc) soman. The four diets used were a standard rodent diet, a choline-enriched diet, a glucose-enriched diet, and a ketogenic diet. Body weight was recorded throughout the study. Toxic signs and survival were evaluated at key times for up to 72 h following soman exposure. Additionally, acquisition of discriminated shuttlebox avoidance performance was characterized beginning 24h after soman challenge and across the next 8 d (six behavioral sessions). Prior to exposure, body weight was highest in the standard diet group and lowest in the ketogenic diet group. Upon exposure, differences in soman toxicity as a function of diet became apparent within the first hour, with mortality in the glucose-enriched diet group reaching 80% and exceeding all other groups (in which mortality ranged from 0 to 6%). At 72 h after exposure, mortality was 100% in the glucose-enriched diet group, and survival approximated 50% in the standard and choline-enriched diet groups, but equaled 87% in the ketogenic diet group. Body weight loss was significantly reduced in the ketogenic and choline-enriched diet groups, relative to the standard diet group. At 1 and 4h after exposure, rats in the ketogenic diet group had significantly lower toxic sign scores than all other groups. The ketogenic diet group performed significantly better than the standard diet group on two measures of active avoidance performance. The exacerbated soman toxicity observed in the glucose-enriched diet group coupled with the attenuated soman toxicity observed in the ketogenic diet group implicates glucose availability in the toxic effects of soman. This increased glucose availability may enhance acetylcholine synthesis and/or utilization, thereby exacerbating peripheral and central soman toxicity.

Cognitive dysfunction and diabetes mellitus

The deleterious effects of diabetes mellitus on the retinal, renal, cardiovascular, and peripheral nervous systems are widely acknowledged. Less attention has been given to the effect of diabetes on cognitive function. Both type 1 and type 2 diabetes mellitus have been associated with reduced performance on numerous domains of cognitive function. The exact pathophysiology of cognitive dysfunction in diabetes is not completely understood, but it is likely that hyperglycemia, vascular disease, hypoglycemia, and insulin resistance play significant roles. Modalities to study the effect of diabetes on the brain have evolved over the years, including neurocognitive testing, evoked response potentials, and magnetic resonance imaging. Although much insightful research has examined cognitive dysfunction in patients with diabetes, more needs to be understood about the mechanisms and natural history of this complication in order to develop strategies for prevention and treatment.

Cognitive dysfunction and diabetes mellitus

The deleterious effects of diabetes mellitus on the retinal, renal, cardiovascular, and peripheral nervous systems are widely acknowledged. Less attention has been given to the effect of diabetes on cognitive function. Both type 1 and type 2 diabetes mellitus have been associated with reduced performance on numerous domains of cognitive function. The exact pathophysiology of cognitive dysfunction in diabetes is not completely understood, but it is likely that hyperglycemia, vascular disease, hypoglycemia, and insulin resistance play significant roles. Modalities to study the effect of diabetes on the brain have evolved over the years, including neurocognitive testing, evoked response potentials, and magnetic resonance imaging. Although much insightful research has examined cognitive dysfunction in patients with diabetes, more needs to be understood about the mechanisms and natural history of this complication in order to develop strategies for prevention and treatment.

Cardiovascular risk factors for Alzheimer's disease

There is now sizable literature on the association between traditional cardiovascular risk factors and Alzheimer's disease (AD). Based on epidemiologic studies, both cross-sectional and longitudinal, there are statistically significant correlations between the prevalence of AD and diabetes, hypercholesterolemia, hypertension, hyperhomocysteinemia, dietary saturated fats, cholesterol, antioxidants, alcohol consumption, smoking, physical activity, the presence of atrial fibrillation, atherosclerotic disease, and the plasma concentration of some hemostatic factors. Most of the cardiovascular risk factors found to be associated with AD are age-dependent, and the prevalence of AD increases with age. Therefore, the association could simply be attributed to aging. On the other hand, the common pathogenetic mechanisms for the generation of both atherosclerotic disease and AD, such as inflammation and the generation of free radicals, suggest a causal link. If this is the case, the identification of modifiable risk factors for dementia becomes a research priority and early intervention aimed at reducing those cardiovascular risk factors a therapeutic imperative.

Dietary modulation of parathion-induced neurotoxicity in adult and juvenile rats

Previous studies indicated that dietary glucose (15% in drinking water) could markedly exacerbate the toxicity of parathion in adult rats. The present study evaluated the effect of consumption of the commonly used sweetener, high fructose corn syrup (HFCS), on parathion toxicity in adult and juvenile rats. Animals were given free access to either water or 15% HFCS in drinking water for a total of 10 days and challenged with parathion (6 or 18 mg/kg, s.c., for juveniles or adults, respectively) on the 4th day. Signs of cholinergic toxicity, body weight and chow/fluid intake were recorded daily. Acetylcholinesterase (AChE) activity and immunoreactivity (AChE-IR) in frontal cortex and diaphragm were measured at 2, 4, and 7 days after parathion. As HFCS was associated with significant reduction in chow intake, adult rats were also pair-fed to evaluate the effect of similar reduced chow intake alone on parathion toxicity. The results indicated that the cholinergic toxicity of parathion was significantly increased by HFCS feeding in both age groups. The excess sugar consumption, however, did not significantly affect parathion-induced AChE inhibition in either tissue or either age group. Enzyme immunoreactivity in frontal cortex was generally not affected in either age group while diaphragm AChE-IR was significantly reduced by parathion and HFCS alone in adult animals at 2 and 4 days timepoints, and more so by the combination of sugar feeding and parathion exposure in both age groups. Food restriction alone did not exacerbate parathion toxicity. While the mechanism(s) remains unclear, we conclude that voluntary consumption of the common sweetener HFCS can markedly amplify parathion acute toxicity in both juvenile and adult rats.

Glucose improvement of memory: a review

The memory-improving action of glucose has now been studied for almost 20 years and the study of this phenomenon has led to a number of important developments in the understanding of memory, brain physiology and pathological consequences of impaired glucose tolerance. Glucose improvement of memory appears to involve two optimal doses in animals (100 mg/kg and 2 g/kg) that may correspond to two physiological mechanisms underlying glucose effects on memory. In humans, there have been few dose-response studies so the existence of more than one effective dose in humans is uncertain. Many tasks are facilitated by glucose in humans but tasks that are difficult to master or involve divided attention are improved more readily that easier tasks. There are a number of hypotheses about the physiological bases of the memory-improving action of glucose. Peripheral glucose injections could alleviate localized deficits in extracellular glucose in the hippocampus. These localized deficits may be due to changes in glucose transporters in that structure. Because certain neurotransmitters such as acetylcholine are directly dependent on the glucose supply for their synthesis, glucose is thought to facilitate neurotransmitter synthesis under certain circumstances. However, these hypotheses cannot account for the specificity of the dose-response effect of glucose. A number of peripheral mechanisms have been proposed, including the possibility that glucose-sensitive neurons in the brain or in the periphery may serve as glucose sensors and eventually produce neural changes that would facilitate memory processing. These latter results could be of importance because the mechanisms they suggest appear to be dose-dependent, a crucial characteristic to explain the dose-dependent effects of glucose. There may be an advantage to develop hypotheses that include both peripheral and central actions of glucose. There is evidence that impaired glucose regulation is associated with impaired cognition, particularly episodic memory. This impairment is minimal in young people but increases in older people (65 years and over) where it may compound other aging processes leading to reduced brain function. A small number of studies showed that glucose improvement of memory is associated with poor glucose regulation although this may not be the case for diabetic patients. Results of a few studies also suggest that drug treatments that improve glucose regulation also produce cognitive improvement in diabetic patients.

Modulation of memory by insulin and glucose: neuropsychological observations in Alzheimer's disease

Converging evidence has identified a potential association among Alzheimer's disease, glucose metabolism, insulin activity, and memory. Notably, type 2 diabetes, which is characterized by insulin resistance, may modulate the risk of Alzheimer's disease, and patients with Alzheimer's disease may have a greater risk for glucoregulatory impairments than do healthy older adults. In animal studies, it has been shown that raising blood glucose levels acutely can facilitate memory, in part, by increasing cholinergic activity, which is greatly diminished in patients with Alzheimer's disease. Other studies have confirmed that glucose administration can facilitate memory in healthy humans and in patients with Alzheimer's disease. Interestingly, glucose effects on memory appear to be modulated by insulin sensitivity (efficiency of insulin-mediated glucose disposal). Of course, the acute effects of glucose administration should be distinguished from the effects of chronic hyperglycemia (diabetes), which has been associated with cognitive impairments, at least in older adults. The relationship of insulin and memory has been more difficult to characterize. In animals, systemic insulin administration has been associated with memory deficits, likely due, in part, to hypoglycemia that occurs when exogenous insulin is not supplemented with glucose to maintain euglycemia. In healthy adults and patients with Alzheimer's disease, raising plasma insulin levels while maintaining euglycemia can improve memory; however, raising plasma glucose while suppressing endogenous insulin secretion may not improve memory, suggesting that adequate levels of insulin and glucose are necessary for memory facilitation. Clinical studies have corroborated findings that patients with Alzheimer's disease are more likely than healthy older adults to have reduced insulin sensitivity, and further suggest that apolipoprotein E genotype may modulate the effects of insulin on glucose disposal, memory facilitation, and amyloid precursor protein processing. Collectively, these findings support an association among Alzheimer's disease, impaired glucose metabolism, and reduced insulin sensitivity.

Enhancing cognitive function across the life span

Glucose administration regulates many neural and behavioral processes in rodents, including learning and memory. Given the important role of glucose in brain function and the safety of glucose as a treatment, we have investigated the effects of glucose administration in humans of different ages. In previous work, we examined the effects of early-morning glucose consumption on cognitive functions in elderly individuals. In this population, glucose enhanced performance on specific measures, particularly on those tasks where mild age-related deficits appear (e.g., verbal declarative memory). Interestingly, glucose failed to enhance cognitive functions in young adults. Our recent work has examined three issues related to glucose enhancement of cognition: First, is glucose effective only in reversing impairments or can it also facilitate performance in highly functioning individuals? Second, are glucose effects dependent either on time of day or on interactions with other meals? Third, are typical breakfast foods as effective as glucose in enhancing cognitive performance? Our findings suggest that glucose can improve memory in highly functioning populations as it does in populations with deficits. However, enhancement by glucose may require sufficient levels of task difficulty and of blood glucose. In addition, like glucose, early morning consumption of cereal can improve performance on some cognitive tests. These results have important implications for the nature of glucose facilitation of memory and for the role of dietary factors in performance of many daily activities.

Increasing acetylcholine levels in the hippocampus or entorhinal cortex reverses the impairing effects of septal GABA receptor activation on spontaneous alternation

Intra-septal infusions of the gamma-aminobutyric acid (GABA) agonist muscimol impair learning and memory in a variety of tasks. This experiment determined whether hippocampal or entorhinal infusions of the acetylcholinesterase inhibitor physostigmine would reverse such impairing effects on spontaneous alternation performance, a measure of spatial working memory. Male Sprague-Dawley rats were given intra-septal infusions of vehicle or muscimol (1 nmole/0.5 microL) combined with unilateral intra-hippocampal or intra-entorhinal infusions of vehicle or physostigmine (10 microg/microL for the hippocampus; 7.5 microg/microL or 1.875 microg/0.25 microL for the entorhinal cortex). Fifteen minutes later, spontaneous alternation performance was assessed. The results indicated that intra-septal infusions of muscimol significantly decreased percentage-of-alternation scores, whereas intra-hippocampal or intra-entorhinal infusions of physostigmine had no effect. More importantly, intra-hippocampal or intra-entorhinal infusions of physostigmine, at doses that did not influence performance when administered alone, completely reversed the impairing effects of the muscimol infusions. These findings indicate that increasing cholinergic levels in the hippocampus or entorhinal cortex is sufficient to reverse the impairing effects of septal GABA receptor activation and support the hypothesis that the impairing effects of septal GABAergic activity involve cholinergic processes in the hippocampus and the entorhinal cortex.

Shuttle-box avoidance learning in mice: improvement by glucose combined with stimulant drugs

Glucose was tested alone or in combination with two stimulant drugs, amphetamine and nicotine, in mice of the CD-1 strain subjected to five daily shuttle-box training sessions. Pretraining intraperitoneal administration of glucose (50 or 100 mg/kg) had no effect, while amphetamine and nicotine, given alone, significantly improved avoidance acquisition at a dose of 0.5 mg/kg, but not 0.025 mg/kg. Significant improvement of avoidance learning was also produced by a combination of glucose with the lower dose of amphetamine or nicotine. This enhancing action, produced by a combination of glucose and stimulant drugs, at doses ineffective by themselves, might be due to a concomitant cholinergic and dopaminergic activation, induced by glucose and stimulant drugs, respectively.

Effects of glucose and fructose on recently reactivated and recently acquired memories

1. The effects of glucose and fructose on memory reactivation were investigated. 2. Rats were trained originally on a brightness discrimination passive avoidance task. 3. Memory reactivation treatment consisted of re-exposing the rats 24 hr later to the footshock unconditioned stimulus in the experimental room. Glucose or fructose (32, 100, 320, 1000, or 2000 mg/kg) was administered immediately after reactivation. 4. Twenty-four hr after reactivation (48 hr after training) the rats were tested for their ability to acquire an active avoidance (reversal) task. 5. The dose-response functions for the effects of both glucose and fructose on the reactivated memory followed identical cubic trends. However, a combined dose of glucose and fructose was significantly less effective at modulating memory than was an equimolar dose of either sugar alone. 6. We compared analytically the effects of combined glucose and fructose treatment on new versus old memories. The dose-response functions for both types of memories follow cubic trends, suggesting that similar multiple interacting mechanisms operate when memories are originally stored and when they are later re-encoded.

Glucose metabolism in cholinoceptive cortical rat brain regions after basal forebrain cholinergic lesion

To address the question whether the changes in cortical glucose metabolism observed in patients with Alzheimer's disease are interrelated with, or consequences of, basal forebrain cholinergic cell loss, an experimental approach was employed to produce cortical cholinergic dysfunction in rat brain by administration of the cholinergic immunotoxin 192IgG-saporin. [14C]D-glucose utilization in brain homogenates, D-glucose-displaceable [3H]cytochalasin B binding to glucose transporters (GLUT). Northern and Western analyses, as well as in vivo [14C]2-deoxyglucose autoradiography were used to quantify the regional glucose metabolism. Basal forebrain cholinergic lesion resulted in transient increases in glucose transporter binding in cortical regions displaying reduced acetylcholinesterase activity, already detectable seven days after lesion with peak values around 30 days post lesion. Western analysis revealed that the changes in total glucose transporter binding are mainly due to changes in the GLUT3 subtype only, while the levels of GLUT1 and GLUT3 mRNA (Northern analysis) were not affected by cholinergic lesion. Both immunocytochemistry and in situ hybridization demonstrated preferential localizations of GLUT1 on brain capillaries and GLUT3 on neurons, respectively. A lesion-induced transient decrease in [14C]D-glucose utilization seven days post lesion was detected in the lesion site, whereas cholinoceptive cortical regions were not affected. In vivo [14C]deoxyglucose uptake was transiently increased in cholinoceptive cortical regions and in the lesion site being highest between three to seven days after lesion. The cholinergic lesion-induced transient up-regulation of cortical glucose transporters and deoxyglucose uptake reflects an increased glucose demand in regions depleted by acetylcholine suggesting functional links between cortical cholinergic activity and glucose metabolism in cholinoceptive target regions.

The absence of effect of glucose on memory is associated with low susceptibility to the amnestic effects of scopolamine in a strain of mice

In this series of experiments, we examined the ability of post-training glucose injections to improve memory of the Balb/cAnNCrlBR strain of mice for a bar-pressing task. We could not replicate this effect which has been demonstrated in many other strains of mice including Balb/cbyJ, a related strain. We found that the Balb/cAnNCrlBR strain of mice is also much less sensitive to the disrupting effects produced by pre- or post-training injections of the competitive cholinergic antagonist scopolamine. This strain also shows altered glucoregulation compared to the Balb/cbyJ strain. The absence of glucose effects on memory in Balb/cAnNCrlBR mice appears to be associated with decreased sensitivity to cholinergic antagonists. These results can be contrasted with previous ones obtained in a related strain, the Balb/cbyJ, in which glucose was shown to improve memory while scopolamine could easily disrupt memory processes. Taken together, these data provide additional indirect support for the hypothesis that glucose improvement of memory is closely linked to a functional interaction with central cholinergic systems. The comparison of these two strains could be the basis for a useful animal model to investigate the relationship between age-related changes in memory and central cholinergic function.

Modulation of hippocampal acetylcholine release and spontaneous alternation scores by intrahippocampal glucose injections

Recent evidence indicates that systemic glucose treatment enhances memory while producing a corresponding increase in hippocampal acetylcholine (ACh) output. The present experiments examined whether unilateral intrahippocampal infusions of glucose would enhance spontaneous alternation performance and whether there would be a corresponding increase in ACh output in the ipsilateral and contralateral hippocampus. Extracellular ACh was assessed in samples collected at 12 min intervals using in vivo microdialysis with HPLC with electrochemical detection. Twelve minutes after a unilateral infusion of artificial cerebrospinal fluid (CSF) or glucose (6.6 mM), rats were tested in a cross maze for spontaneous alternation behavior with concurrent microdialysis collection. In two experiments, glucose infusions significantly increased alternation scores (67.5 and 59%) compared with CSF controls (42.4 and 39.5%, respectively). In both experiments, behavioral testing resulted in increased ACh output in the hippocampus. Glucose administration at the time of alternation tests enhanced ACh output beyond that of behavioral testing alone both ipsilateral (+93.8%) and contralateral (+85%) to the infusion site, as compared with ACh output (+36.1% and +55.5%) of CSF controls. Glucose infusions did not affect hippocampal ACh output in rats kept in a holding chamber. These results suggest that glucose may enhance alternation scores by modulating ACh release. The findings also indicate that unilateral glucose infusions increase hippocampal ACh output both ipsilateral and contralateral to the site of injection. Furthermore, glucose increased ACh output only during maze testing, suggesting that specific behavioral demands, perhaps requiring activation of cholinergic neurons, determine the efficacy of glucose in modulating ACh release.

Shuttle-box avoidance learning in mice: improvement by combined glucose and tacrine

Glucose and the acetylcholinesterase inhibitor tacrine were tested, alone and in combination, in mice of the CD-1 strain subjected to five daily shuttle-box training sessions. Pretraining intraperitoneal administration of glucose alone (50-400 mg/kg) had no significant effect, while tacrine alone (0.5-3 mg/kg) improved avoidance acquisition at the dose of 2 mg/kg only. Significant avoidance learning improvements were instead produced by 50 or 100 mg/kg glucose combined with 0.5 or 1 mg/kg tacrine. The effects on shuttle-box avoidance acquisition produced by glucose combined with a cholinomimetic agent support the hypothesis that cholinergic mechanisms may be involved in the action of glucose on learning and memory. However, the main finding of the present study is related to the enhancement by glucose of the learning improving action of a drug clinically used as cognitive enhancer.

Epinephrine effects on memory are not dependent on hepatic glucose release

Epinephrine released or administered soon after a given training task modulates memory processes. Since epinephrine does not readily cross the blood-brain barrier, studies have suggested that some of the central effects of epinephrine might be mediated by peripheral release of glucose. These experiments examined the involvement of blood glucose levels in the posttraining effects of peripherally administered epinephrine. The effects of the administration of epinephrine (25 and 625 microg/kg) [corrected] on memory of an inhibitory avoidance task were evaluated in fed and fasted rats (depleted glycogen stores in liver). Blood glucose levels after the task in each group were also measured. Female Wistar rats were divided in two groups. Fed and 48-h-fasted animals were submitted to the inhibitory avoidance task and received i.p. epinephrine or saline immediately after training. The test session was carried out 48 h after training. Epinephrine (25 or 625 microg/kg) [corrected] caused an increased glycemia in fed rats, but no effect was observed in fasted animals. Administration of epinephrine 25 microg/kg [corrected] induced a facilitation of memory, while epinephrine 625 microg/kg [corrected] impaired retention (either in fasted or in fed animals). There was no relation between increased glycemia induced by epinephrine and its effects on memory, since this drug presented its classical effects independently of the previous state of the animal (fed or fasted). The results of the present study suggest that the effects of systemic released or administered epinephrine on memory processes are not dependent on hepatic glucose release.

Hippocampal acetylcholine release during memory testing in rats: augmentation by glucose

Several lines of evidence indicate that a modest increase in circulating glucose levels enhances memory. One mechanism underlying glucose effects on memory may be an increase in acetylcholine (ACh) release. The present experiment determined whether enhancement of spontaneous alternation performance by systemic glucose treatment is related to an increase in hippocampal ACh output. Samples of extracellular ACh were assessed at 12-min intervals using in vivo microdialysis with HPLC-EC. Twenty-four minutes after an intraperitoneal injection of saline or glucose (100, 250, or 1000 mg/kg), rats were tested in a four-arm cross maze for spontaneous alternation behavior combined with microdialysis collection. Glucose at 250 mg/kg, but not 100 or 1000 mg/kg, produced an increase in spontaneous alternation scores (69.5%) and ACh output (121.5% versus baseline) compared to alternation scores (44.7%) and ACh output (58.9% versus baseline) of saline controls. The glucose-induced increase in alternation scores and ACh output was not secondary to changes in locomotor activity. Saline and glucose (100-1000 mg/kg) treatment had no effect on hippocampal ACh output when rats remained in the holding chamber. These findings suggest that glucose may enhance memory by directly or indirectly increasing the release of ACh. The results also indicate that hippocampal ACh release is increased in rats performing a spatial task. Moreover, because glucose enhanced ACh output only during behavioral testing, circulating glucose may modulate ACh release only under conditions in which cholinergic cells are activated.

Glucose regulation and cognitive functions: relation to Alzheimer's disease and diabetes

Glucose has been found to improve memory in animals and humans. Animal research has revealed that glucose may improve memory through a facilitation of acetylcholine (ACh) synthesis and release in the brain. This glucose-related memory improvement has prompted research in elderly humans. These studies have shown that the memory-improving action of glucose depends on each individuals' blood glucose regulation. Based on these data, researchers have evaluated the effect of glucose on memory in patients with Alzheimer's disease (AD). Results demonstrated that glucose could improve memory in a subset of patients that had abnormalities in their blood glucose regulation. Interestingly, these alterations in blood glucose regulation were believed to depend on the severity of the disease process. Another line of investigation has focused on alterations in brain glucose metabolism. Both animal models and studies with Type II diabetic elderly patients have shown that altered glucose regulation impairs learning and memory processes. It is possible that in AD patients, hyperglycemia exerts a deleterious effect by potentiating the neuronal death produced by other pathological processes taking place such as amyloid deposition. Based on these data, it appears important to find the prevalence of altered glucoregulation at various stages of AD. Secondly, it may be of interest to determine prospectively whether altered glucoregulation is linked to a faster progression of the disease. Finally, if such a relationship is observed, the next logical step would be to determine whether AD patients could benefit from treatments aimed at normalizing blood glucose regulation and improving insulin sensitivity.

Memory improvement following induced hyperinsulinemia in Alzheimer's disease

Dementia of the Alzheimer type (DAT) is accompanied by disruption in glucose regulation and utilization that may contribute to its characteristic memory impairment. Increasing glucose availability by raising plasma glucose improves memory in patients with DAT. Such memory improvement is associated with a secondary elevation in plasma insulin levels, raising the question of whether improvement is due to changes in insulin levels, independent of hyperglycemia. Distributions of insulin receptors in the hippocampus and insulin-mediated increases in glucose utilization in entorhinal cortex provide potential mechanisms for such improvement. We show that raising plasma insulin through intravenous infusion while keeping plasma glucose at a fasting baseline level produces striking memory enhancement for patients with DAT. Previous findings of hyperglycemic memory enhancement were also replicated. Patients with DAT also showed abnormal plasma levels of glucoregulatory hormones and metabolites at baseline and during metabolic manipulations. Our findings suggest that neuroendocrine factors play an important role in the pathophysiology of DAT.

Glucose injections into the medial septum reverse the effects of intraseptal morphine infusions on hippocampal acetylcholine output and memory

Morphine infusions into the medial septum produce memory deficits which can be attenuated by concurrent intraseptal injections of glucose. The mnemonic deficits following intraseptal morphine injections may be due, in part, to opioid inhibition of cholinergic neurons projecting to the hippocampus, with glucose reducing the effect. The present experiment determined whether glucose injections into the medial septum attenuate the effects of intraseptal morphine injections on hippocampal acetylcholine release and on memory. Samples of extracellular acetylcholine levels were assessed at 12 min intervals using in vitro microdialysis with high-performance liquid chromatography with electrochemical detection. Intraseptal morphine injections (4.0 nmol) reduced acetylcholine output starting at 12 min and lasting up to 72 min post-injection. Glucose (18.3 nmol) injected concomitantly with morphine reversed the drug infusions in the septum 20 min prior to spontaneous alternation testing. Intraseptal morphine infusions reduced alternation scores; this behavioral effect was reversed by concurrent glucose infusions. The effect of drugs infused into the septal area on spontaneous alternation performance and acetylcholine output were positively correlated. These findings suggest that memory deficits induced by intraseptal morphine injections may result, at least partially, from a decrease in the activity of cholinergic neurons and that this effect is reversed by glucose.

Glucose enhancement of scopolamine-induced increase of hippocampal high-affinity choline uptake in mice: relation to plasma glucose levels

The administration of glucose has been shown to improve memory for various learning tasks in rodents. In humans, glucose also increases declarative memory performance in elderly people and in some patients with mild Alzheimer's disease. One of the possible physiological bases for the effect of glucose on memory processes is a facilitation of cholinergic function through increased synthesis. In support of this hypothesis, glucose was shown to attenuate the amnesia induced by scopolamine and, in similar conditions, glucose increased extracellular levels of acetylcholine following a scopolamine injection. To further examine the interaction between glucose and cholinergic function, the present experiment measured the effects of combined injections of glucose and scopolamine on hippocampal sodium-dependent high-affinity choline uptake, an indirect index of cholinergic activity. Results showed that the injection of 3 g/kg glucose enhanced the increase in high affinity choline uptake in hippocampal synaptosomes produced by scopolamine. A regression analysis revealed the existence of a positive correlation between plasma blood glucose level and hippocampal choline uptake particularly in the animals receiving a combined injection of scopolamine and glucose. These data further support the hypothesis that glucose administration can facilitate acetylcholine synthesis under certain conditions and that this action could explain how glucose attenuates scopolamine-induced amnesia.

Memory-improving actions of glucose: involvement of a central cholinergic muscarinic mechanism

Post-training intraperitoneal administration of alpha-D[+]-glucose (10-300 mg/kg) facilitated 24-h retention, in male Swiss mice, of a one-trial step-through inhibitory avoidance task. The dose-response curve was an inverted U. Glucose did not increase the retention latencies of mice that had not received a footshock during training. The effect of glucose (30 mg/kg, ip) on retention was time-dependent, which suggests that the drug facilitated memory storage. The memory facilitation induced by glucose (30 mg/kg, ip) was prevented by atropine (0.5 mg/kg, ip) administered after training, but 10 min prior to glucose treatment. In contrast, neither methylatropine (0.5 mg/kg, ip), a peripherally acting muscarinic receptor blocker, nor mecamylamine (5 mg/kg, ip) or hexamethonium (5 mg/kg, ip), two cholinergic nicotinic receptor antagonists, prevented the effects of post-training glucose on retention. Low subeffective doses of the central acting anticholinesterase physostigmine (35 micrograms/kg, ip), administered immediately after training, and glucose (10 mg/kg, ip), given 10 min after training, acted synergistically to improve retention. The effects of glucose (10 mg/kg, ip) were not influenced by the peripherally acting anticholinesterase neostigmine (35 micrograms/kg, ip). Considered together, these findings suggest that the memory facilitation induced by post-training administration of glucose could result from an enhancement of brain acetylcholine synthesis and/or its release that, in turn, might modulate the activity of muscarinic cholinergic mechanisms that are critically involved in memory storage.

Glucose attenuates a morphine-induced decrease in hippocampal acetylcholine output: an in vivo microdialysis study in rats

Systemic injections of morphine impair performance in memory tests. Glucose administration ameliorates memory deficits produced by morphine treatment. The memory impairments induced by morphine may be related to opioid inhibition of acetylcholine release with reversal of this effect by glucose. The present experiment determined whether: (1) systemic morphine treatment decreases acetylcholine output in the hippocampal formation; and (2) systemic glucose administration attenuates the effect of morphine treatment. Employing microdialysis, samples were collected at 12-min intervals and assayed for acetylcholine using HPLC with electrochemical detection. Morphine (10 mg/kg)/saline injections resulted in an immediate decrease in acetylcholine output (20-35%) that was observed up to the third postinjection sample (36 min). Glucose (100 mg/kg) administered concurrently with morphine attenuated the reduction in acetylcholine output in the second and third samples. These findings suggest that glucose may attenuate morphine-induced memory impairments by reversing a decrease in acetylcholine output produced by morphine.

Glucose attenuates the effect of combined muscarinic-nicotinic receptor blockade on spontaneous alternation

Glucose administration reverses the effects of both muscarinic and nicotinic cholinergic receptor antagonists on memory and other measures. In experiment 1, we found that glucose attenuated impairments on spontaneous alternation after muscarinic (scopolamine, 0.5 mg/kg) or nicotinic (mecamylamine, 5.0 mg/kg) receptor blockade. In experiment 2, we examined whether glucose could reverse the spontaneous alternation impairments produced by combined muscarinic-nicotinic receptor blockade. Scopolamine (0.1 mg/kg) and mecamylamine (2.5 mg/kg) when administered separately did not modify alternation performance, but when coadministered they decreased spontaneous alternation scores. This decrease was attenuated by glucose at 100, 300, 500 and 3000 mg/kg. These findings suggest that glucose may attenuate the behavioral impairment by enhancing cholinergic activity and/or other neurotransmitter systems.

Comparable dose-response functions for the effects of glucose and fructose on memory

A passive avoidance-to-active avoidance negative transfer paradigm was used to investigate in rats the effects of glucose and fructose on recently acquired memories. Immediate post-passive avoidance conditioning injections of glucose, fructose, or saline were followed 48 h later by active avoidance conditioning. Equimolar 10, 32, 100, and 2000 mg/kg sc doses of the two sugars significantly impaired acquisition of the reversal task, whereas 3.2 mg/kg doses of both sugars were without significant effect on subsequent performance and 320 mg/kg doses of both sugars significantly enhanced subsequent performance. The cubic trends for both dose-response functions were statistically significant and did not differ from each other. This is the first demonstration that glucose and fructose affect recently acquired memories in accord with comparable cubic dose-response functions, and that there are doses of both sugars that can enhance memory (as indicated by an increase in the number of trials required to reach criterion on the reversal task) and doses of both sugars that can impair memory (as indicated by a decrease in the number of trials required to reach criterion on the reversal task), compared to saline treatment. The similar cubic dose-response functions for glucose and fructose suggest that their mechanisms of action when they are injected peripherally are similar. In addition, because fructose does not readily pass the blood-brain barrier, the results suggest that these two monosaccharides may act through a common peripheral pathway.

Vasopressin deficiency and blood glucose interactions on passive avoidance behavior

The performance of a passive avoidance task (measured for two trials based upon number of complete step-downs and latency to respond) and blood glucose levels were examined in five groups of animals. The groups included vasopressin-deficient (DI) and vasopressin-containing (LE) rats under ad lib (AL) and food-restricted (FR) conditions, as well as DI-FR animals provided with access to an 8% sucrose solution (SUC). In the AL condition, no significant differences were found between DI and LE animals in either step-down occurrences or blood glucose levels. However, the DI animals were significantly slower in latency to respond in trial 1. With FR, the LE animals resembled the LE-AL animals in both passive avoidance behavior and blood glucose levels. The DI-FR animals that were not provided with SUC showed an impairment in passive avoidance behavior and low blood glucose levels, whereas DI-FR animals provided with SUC showed an amelioration of passive avoidance deficiencies and had blood glucose levels comparable to AL animals and LE-FR animals. On trial 2, a significant negative correlation was found between number of step-down occurrences and blood glucose levels, and a significant positive correlation was found between latency to respond and blood glucose levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholinergic neurons are distributed preferentially in areas rich in substance P-like immunoreactivity in the caudate nucleus of the adult cat

The distribution of cells stained immunocytochemically for the cholinergic marker choline acetyltransferase was compared to the pattern of substance P immunoreactivity in the caudate nucleus of adult cats using a double-label immunocytochemical protocol and three-dimensional reconstructions of adjacent sections single-labeled for either substance P or choline acetyltransferase. Substance P immunoreactivity was distributed in a highly complex mosaic within the caudate nucleus of the cat. In the dorsal caudate nucleus, substance P-rich zones consisting of either clusters of substance P-positive cell bodies or fibers were seen against a lighter staining background. The density of cholinergic neurons was found to be significantly greater within these substance P-rich patches in comparison to surrounding regions. The pattern of substance P immunoreactivity within the ventral caudate nucleus differed from that in more dorsal regions. Clear substance P-rich patches were not seen in this region, but a large substance P-rich area consisting of a dense plexus of substance P-containing fibers was visible. Embedded within this substance P-rich area were fairly discrete patches of light substance P staining. As in the dorsal caudate nucleus, increased numbers of cholinergic neurons and processes were associated with substance P-rich regions in the ventral caudate nucleus. Choline acetyltransferase-positive perikarya also appeared to be concentrated in substance P-rich areas in the nucleus accumbens and olfactory tubercle. The results of this study suggest that a close relationship exists between the distribution of substance P fibers and cholinergic perikarya in the striatum of the cat.

Age-related changes in correlation between behavioral and biochemical parameters in Lewis rats

This study evaluated aging by using a correlational analysis of behavior and biochemistry in young (4 months old) and old (24 months old) Lewis rats. The rats were subjected to different learning tasks (spatial discrimination learning in the Morris task and cone-field task, temporal discrimination learning, one-trial inhibitory avoidance task) and noncognitive tests (emotional reactivity, motor coordination, and food motivation) and the relation between the various parameters was assessed. In the learning tasks, except for the inhibitory avoidance task, the first part of the learning curve was taken as an index of learning. Blood glucose (baseline and blood glucose regulation) and hippocampal choline acetyltransferase (ChAT) activity were also measured. There was no correlation between the different parameters of learning in young and old rats. This indicates that there are individual differences in performance in different learning and memory tasks. Measures of noncognitive behavior (food motivation, emotional reactivity, and motor performance) did not predict performance in the learning tasks. Hippocampal ChAT activity did not correlate with learning performance in old rats, whereas blood glucose level was found to correlate with spatial learning in old rats. These results suggest that an impaired regulation of blood glucose may be related to cognitive performance in aging.

Raised glucose levels enhance scopolamine-induced acetylcholine overflow from the hippocampus: an in vivo microdialysis study in the rat

Behavioural studies in both humans and animals have shown that an acute rise in circulating glucose levels at or around the time of training enhances subsequent retention performance and can also afford protection from the amnesia produced by posttraining injections of scopolamine. In an attempt to directly investigate the neurochemical basis for these effects of glucose we have tested the hypothesis that raised glucose levels may enhance acetylcholine (ACh) synthesis and release in the brain during conditions of increased neuronal activity, induced either by training or pharmacological challenge, via a microdialysis study using rats. Microdialysate concentrations of ACh overflow from the hippocampus of fasted rats induced by i.p. injections of scopolamine (1 mg/kg) combined with concurrent s.c. injections of either glucose (2 g/kg) or saline were compared in successive 15-min samples using an on-line HPLC system. Scopolamine injections resulted in an immediate 10-20-fold increase in hippocampal ACh overflow which subsequently progressively declined over a 4-h period to pretreatment baseline levels. The combined injection of glucose with scopolamine resulted in a highly significant enhancement (19.4%; P less than 0.01) in ACh content of the first two samples as compared to saline-injected controls. These results provide the first direct experimental evidence that raised glucose levels, via increased availability of acetyl-coenzyme A (acetyl-coA), transiently facilitates ACh synthesis and release during conditions of increased neuronal activity. This enhancement of ACh availability during states of cholinergic neuronal activation may underlie the previously observed facilitatory effects of glucose on memory performance and its protection from scopolamine-induced amnesia.

Memory modulation with peripherally acting cholinergic drugs

Physostigmine (PHYSO), in doses as low as 0.003 mg/kg IP, antagonized scopolamine (SCOP, 3 mg/kg) induced amnesia of step-through passive avoidance in mice. The peripherally acting acetylcholinesterase (AChE) inhibitor neostigmine (NEO) was also found to reliably, though less strongly, antagonize the SCOP induced amnesia at a dose of 0.03 mg/kg. The NEO antagonism of the SCOP amnesia could be reversed with SCOP (0.3, 1, and 3 mg/kg) and mecamylamine (MECA, 1, 3, and 10 mg/kg), muscarinic and nicotinic antagonists, respectively, which are active both peripherally and centrally, as well as with M-SCOP (0.3 and 1 mg/kg) and hexamethonium (HEX, 1 and 3 mg/kg), muscarinic and nicotinic antagonists, respectively, which are active only in the periphery. In contrast to the ability of these four compounds to attenuate the SCOP amnesia, only the centrally acting compounds SCOP (3 mg/kg) and MECA (10 mg/kg) induced an amnesia when administered alone. These findings suggest that the induction of amnesia of passive avoidance involves central cholinergic systems, whereas the NEO, and possibly PHYSO, reversal of the SCOP induced amnesia is mediated peripherally by both muscarinic and nicotinic receptors. It is hypothesized that the release of adrenal catecholamines, the influence of which on memory processes is well known, and secondarily glucose, may be responsible for the NEO antagonism of the SCOP amnesia.

Glucose attenuation of paradoxical sleep deficits in old rats

Glucose administration enhances memory in several amnestic populations, including old humans and rodents. The present experiment demonstrates that glucose also enhances measures of sleep in old rats. Three-hour day-time sleep EEGs were assessed in 3- and 24-month-old rats. The animals received injections of saline or glucose (100, 500, and 1000 mg/kg) on different days in a counter-balanced order. At doses of 100 and 500 mg/kg, glucose augmented the duration of paradoxical sleep bouts and total paradoxical sleep time in old, but not young, rats. Within 2 weeks after the sleep tests, measures of several brain neurotransmitter functions were obtained. Glucose was more effective in enhancing paradoxical sleep in those individual aged rats with high levels of hippocampal choline acetyltransferase and occipital cortex serotonin concentrations than in aged rats with lower levels on these neurochemical measures. The findings suggest that glucose attenuates selective age-related sleep deficits in old rats. More generally, these results add to a growing body of evidence indicating that moderate doses of peripheral glucose can influence a variety of CNS measures.

Effects of systemic glucose injection on the development of amygdala kindling in rats

The effects of systemic glucose administration on the development of electric amygdaloid kindling seizures were examined in Wistar rats. Daily intraperitoneal injections of glucose 100 mg/kg, 20 min prior to each amygdaloid stimulation did affect the rate of kindling development in comparison with the saline-injected group. The number of stimulations required to reach stage 5 was lower in animals treated with glucose. The facilitation of kindling development found in glucose-treated rats is related to a specific decrease in the number of stimulations needed to evolve from stage 2 to stage 3. These results indicate that systemic glucose administration facilitates amygdaloid kindling in rats.

Glucose effects on mecamylamine-induced memory deficits and decreases in locomotor activity in mice

Peripheral glucose administration attenuates the effects of muscarinic cholinergic antagonists on several measures, including spontaneous alternation, inhibitory avoidance, and locomotor activity. The present study examined glucose interactions with mecamylamine, a nicotinic cholinergic antagonist, on these measures. Mecamylamine (5 mg/kg, sc) significantly impaired spontaneous alternation performance. Glucose (100 mg/kg, ip) administered with mecamylamine attenuated the impairment. Treatment with hexamethonium (5 and 10 mg/kg, sc), a peripheral nicotinic blocker, did not impair performance. Pretraining treatment with mecamylamine, but not hexamethonium, significantly reduced later retention latencies on inhibitory avoidance tests. Glucose, administered with mecamylamine prior to training, significantly attenuated the impaired test performance. Mecamylamine, but not hexamethonium, significantly decreased locomotor activity. In contrast to the attenuating effects of glucose on the other measures above, glucose administered with mecamylamine potentiated the decreased locomotor activity. These findings demonstrate that glucose influences the behavioral effects of a nicotinic cholinergic antagonist in a manner generally similar to that of muscarinic cholinergic antagonists, and supports previous evidence that circulating glucose interacts with central cholinergic functions.

Muscarinic receptor subtypes and sexual behavior in female rats

Cholinergic muscarinic systems are involved in the regulation of female sexual behavior in rats and hamsters. This series of experiments was designed to determine whether sexual behavior in female rats is controlled preferentially by one of the traditional muscarinic receptor subtypes. Intraventricular infusion of the muscarinic antagonist scopolamine (10 micrograms bilaterally) which binds with high affinity to both M1 and M2 subtypes inhibited sexual behavior, as indicated by the incidence of lordosis, in ovariectomized rats treated with estrogen and progesterone. In contrast, the M1-selective antagonist pirenzepine failed to reduce the incidence of lordosis following intraventricular infusion (10 to 80 micrograms bilaterally). Biochemical analyses revealed that intraventricular infusion of scopolamine (10 micrograms bilaterally) inhibited both M1 and M2 binding in brain tissues while intraventricular infusion of pirenzepine (10 micrograms bilaterally) completely inhibited M1 binding without affecting M2 binding. Intraventricular infusions of the acetylcholinesterase inhibitor physostigmine (10 micrograms bilaterally), the cholinergic agonist carbachol (1 microgram bilaterally), and the muscarinic agonist oxotremorine-M (0.1 micrograms bilaterally) activated lordosis in ovariectomized females primed with low doses of estrogen. In contrast, the putative M1 agonist McN-A-343 failed to significantly increase lordosis following intraventricular infusions (1, 10, 20 micrograms bilaterally). According to biochemical results, the ability of these agents to activate lordosis in female rats was related to their affinities for M2 binding sites not M1 binding sites. In a final experiment, estrogen treatment of ovariectomized rats did not alter muscarinic subtype binding in several brain areas as measured by the M1-selective ligand [3H] pirenzepine and the M2-selective ligand [3H] oxotremorine-M. The results of these experiments confirm that muscarinic systems contribute to the regulation of lordosis in female rats and indicate that M2 binding sites rather than M1 binding sites may be a critical component of this regulation.

Naloxone modulates the behavioral effects of cholinergic agonists and antagonists

Peripheral glucose administration enhances memory in rodents and humans. Recent findings suggest that glucose may affect behavior, in part, by augmenting central cholinergic functions and by attenuating central opiate functions. The present experiments examined interactions between an opiate antagonist, naloxone, and cholinergic agents to determine whether the effects would parallel those found with glucose. Three behavioral measures were assessed: tremors, hyperactivity, and spontaneous alternation. Naloxone (1 mg/kg) significantly augmented tremors elicited by physostigmine (0.3 mg/kg). Naloxone (1 mg/kg) also attenuated increases in locomotor activity and impairments in spontaneous alternation performance elicited by scopolamine (1 and 3 mg/kg for activity and alternation measures, respectively). Thus, across three diverse measures, naloxone produced effects similar to those previously reported for glucose. These findings are consistent with the hypothesis that release of cholinergic activity from opiate inhibition may contribute to glucose effects on behavior.

Glucose and physostigmine effects on morphine- and amphetamine-induced increases in locomotor activity in mice

Recent findings indicate that glucose antagonizes several behavioral effects of cholinergic antagonists and augments those of cholinergic agonists. For example, scopolamine elicits increased locomotor activity, an action which is attenuated by glucose and by combined treatment with glucose and physostigmine at doses which are individually without effect. Opiate and catecholamine agonists, such as morphine and amphetamine, also elicit hyperactivity. The present study examined interactions of glucose and physostigmine with morphine- and amphetamine-induced hyperactivity. Mice received saline, morphine (10 mg/kg), or amphetamine (1 mg/kg) 50 min prior to testing, followed by saline, physostigmine (0.01, 0.05, 0.1, or 0.2 mg/kg), or glucose (10, 50, 100, or 500 mg/kg) administered 20 min prior to activity testing in an open field. Physostigmine significantly attenuated both morphine- and amphetamine-induced increases in activity, but higher doses were required to attenuate the effects of amphetamine. Like physostigmine, glucose significantly attenuated morphine-induced activity levels, but unlike physostigmine, glucose did not attenuate amphetamine-induced activity. Thus, the behavioral effects of morphine were more susceptible to modification by physostigmine and glucose than were the effects of amphetamine. The attenuation of morphine-induced hyperactivity demonstrates a similarity between glucose and cholinergic agonists, and also indicates that glucose may inhibit, directly or indirectly, opiate functions. More generally, these findings add to the evidence that circulating glucose levels selectively influence a growing list of behavioral and neurobiological functions.

Memory-improving action of glucose: indirect evidence for a facilitation of hippocampal acetylcholine synthesis

The effect of a 3 g/kg glucose injection on the velocity of the sodium-dependent high-affinity choline uptake mechanism in the hippocampus was both measured in quiet control mice and in mice immediately after training in an operant bar pressing task. Glucose did not significantly change high-affinity choline uptake in resting animals. High-affinity choline uptake in the hippocampus was increased by training in the operant bar pressing task. Glucose significantly reduced the amplitude of the increase in high-affinity choline uptake observed in the trained animals. Similarly, a 3 g/kg glucose injection also attenuated the increase in high-affinity choline uptake observed in animals injected with 1 mg/kg scopolamine. Finally, a 3 g/kg glucose injection significantly attenuated the amnesia produced by a post-training 1 mg/kg scopolamine injection in mice trained for an operant bar pressing task. These results provide additional evidence for an action of glucose on hippocampal cholinergic activity under conditions of high acetylcholine demand. This action may be mediated via an increase in acetyl coenzyme A availability, one of the precursors of acetylcholine. This facilitative effect of glucose on hippocampal acetylcholine synthesis may constitute the physiological basis for its facilitative action on memory and its attenuation of scopolamine amnesia.

Adrenalectomy-induced memory deficits: role of plasma glucose levels

Circulating glucose levels regulate memory storage under several conditions. This study examined the contribution of blood glucose levels to the transient memory impairment seen in adrenalectomized rats. Inhibitory (passive) avoidance retention performance, blood glucose levels, and glycemic responses to footshock were tested 1, 2, and 8 days after adrenalectomy. Adrenalectomized animals demonstrated a transient inhibitory avoidance deficit 1 and 2 days after surgery which recovered by 8 days. The adrenalectomy-induced memory deficit was accompanied by decreased resting blood glucose levels. In animals tested 2 days after adrenalectomy, this decrease in baseline blood glucose levels was exacerbated by further reductions, rather than the normal increases, in circulating glucose levels after training. The magnitude of blood glucose increases after glucose injection was decreased in adrenalectomized animals tested 2 days after surgery. Posttraining glucose injections restored the retention performance of animals trained 2 days after adrenalectomy to that of sham-operated animals. These findings suggest that abnormalities in blood glucose regulation may contribute, in part, to the transient memory impairment seen after adrenalectomy. Additionally, the results further implicate blood glucose in the regulation of CNS information processing systems.

Glucose enhancement of performance on memory tests in young and aged humans

Recent findings indicate that glucose administration enhances memory processes in rodents. This study examined the effects of glucose on memory in humans. After drinking glucose- or saccharin-flavored beverages, college-aged and elderly humans were tested with modified versions of the Wechsler Memory Scale. Beverages and tests were administered in a counter-balanced, crossover design, enabling within subject comparisons. The major findings were: (1) glucose enhanced memory in elderly and, to a lesser extent, in young subjects; and (2) glucose tolerance in individual subjects predicted memory in elderly, but not in young subjects on both glucose and saccharin test days.

Neuroendocrine effects on memory in aged rodents and humans

Considerable evidence indicates that epinephrine regulates memory storage processing in young animals. Recent findings suggest that hyperglycemia subsequent to epinephrine release or injection may mediate the hormone's effects on memory. This paper reviews findings demonstrating that epinephrine and glucose treatments attenuate age-related memory impairments in rodents and humans. Additional results suggest that, in aged human and animal subjects, poor glucose regulation predicts memory performance of individual subjects.