Extracellular levels of glucose in the hippocampus and striatum during maze training for food or water reward in male rats.

Glucose potently enhances cognitive functions whether given systemically or directly to the brain. The present experiments examined changes in brain extracellular glucose levels while rats were trained to solve hippocampus-sensitive place or striatum-sensitive response learning tasks for food or water reward. Because there were no task-related differences in glucose responses, the glucose results were pooled across tasks to form combined trained groups. During the first 1-3 min of training for food reward, glucose levels in extracellular fluid (ECF) declined significantly in the hippocampus and striatum; the declines were not seen in untrained, rewarded rats. When trained for water reward, similar decreases were observed in both brain areas, but these findings were less consistent than those seen with food rewards. After the initial declines in ECF glucose levels, glucose increased in most groups, approaching asymptotic levels ∼15-30 min into training. Compared to untrained food controls, training with food reward resulted in significant glucose increases in the hippocampus but not striatum; striatal glucose levels exhibited large increases to food intake in both trained and untrained groups. In rats trained to find water, glucose levels increased significantly above the values seen in untrained rats in both hippocampus and striatum. The decreases in glucose early in training might reflect an increase in brain glucose consumption, perhaps triggering increased brain uptake of glucose from blood, as evident in the increases in glucose later in training. The increased brain uptake of glucose may provide additional neuronal metabolic substrate for metabolism or provide astrocytic substrate for production of glycogen and lactate.

Aging is not equal across memory systems.

The present experiments compared the effects of aging on learning several hippocampus- and striatum-sensitive tasks in young (3-4 month) and old (24-28 month) male Fischer-344 rats. Across three sets of tasks, aging was accompanied not only by deficits on hippocampal tasks but also by maintained or even enhanced abilities on striatal tasks. On two novel object recognition tasks, rats showed impaired performance on a hippocampal object location task but enhanced performance on a striatal object replacement task. On a dual solution task, young rats predominately used hippocampal solutions and old rats used striatal solutions. In addition, on two maze tasks optimally solved using either hippocampus-sensitive place or striatum-sensitive response strategies, relative to young rats, old rats had impaired learning on the place version but equivalent learning on the response version. Because glucose treatments can reverse deficits in learning and memory across many tasks and contexts, levels of available glucose in the brain may have particular importance in cognitive aging observed across tasks and memory systems. During place learning, training-related rises in extracellular glucose levels were attenuated in the hippocampus of old rats compared to young rats. In contrast, glucose levels in the striatum increased comparably in young and old rats trained on either the place or response task. These extracellular brain glucose responses to training paralleled the impairment in hippocampus-sensitive learning and the sparing of striatum-sensitive learning seen as rats age, suggesting a link between age-related changes in learning and metabolic substrate availability in these brain regions.

Attenuation in rats of impairments of memory by scopolamine, a muscarinic receptor antagonist, by mecamylamine, a nicotinic receptor antagonist.

RATIONALE: Scopolamine, a muscarinic antagonist, impairs learning and memory for many tasks, supporting an important role for the cholinergic system in these cognitive functions. The findings are most often interpreted to indicate that a decrease in postsynaptic muscarinic receptor activation mediates the memory impairments. However, scopolamine also results in increased release of acetylcholine in the brain as a result of blocking presynaptic muscarinic receptors. OBJECTIVES: The present experiments assess whether scopolamine-induced increases in acetylcholine release may impair memory by overstimulating postsynaptic cholinergic nicotinic receptors, i.e., by reaching the high end of a nicotinic receptor activation inverted-U dose-response function. RESULTS: Rats tested in a spontaneous alternation task showed dose-dependent working memory deficits with systemic injections of mecamylamine and scopolamine. When an amnestic dose of scopolamine (0.15 mg/kg) was co-administered with a subamnestic dose of mecamylamine (0.25 mg/kg), this dose of mecamylamine significantly attenuated the scopolamine-induced memory impairments. We next assessed the levels of acetylcholine release in the hippocampus in the presence of scopolamine and mecamylamine. Mecamylamine injections resulted in decreased release of acetylcholine, while scopolamine administration caused a large increase in acetylcholine release. CONCLUSIONS: These findings indicate that a nicotinic antagonist can attenuate impairments in memory produced by a muscarinic antagonist. The nicotinic antagonist may block excessive activation of nicotinic receptors postsynaptically or attenuate increases in acetylcholine release presynaptically. Either effect of a nicotinic antagonist-to decrease scopolamine-induced increases in acetylcholine output or to decrease postsynaptic acetylcholine receptor activation-may mediate the negative effects on memory of muscarinic antagonists.

Acetylcholine release in the hippocampus and prelimbic cortex during acquisition of a socially transmitted food preference.

Interference with cholinergic functions in hippocampus and prefrontal cortex impairs learning and memory for social transmission of food preference, suggesting that acetylcholine (ACh) release in the two brain regions may be important for acquiring the food preference. This experiment examined release of ACh in the hippocampus and prefrontal cortex of rats during training for social transmission of food preference. After demonstrator rats ate a food with novel flavor and odor, a social transmission of food preference group of rats was allowed to interact with the demonstrators for 30 min, while in vivo microdialysis collected samples for later measurement of ACh release with HPLC methods. A social control group observed a demonstrator that had eaten food without novel flavor and odor. An odor control group was allowed to smell but not ingest food with novel odor. Rats in the social transmission but not control groups preferred the novel food on a trial 48 h later. ACh release in prefrontal cortex, with probes that primarily sampled prelimbic cortex, did not increase during acquisition of the social transmission of food preference, suggesting that training-initiated release of ACh in prelimbic cortex is not necessary for acquisition of the food preference. In contrast, ACh release in the hippocampus increased substantially (200%) upon exposure to a rat that had eaten the novel food. Release in the hippocampus increased significantly less (25%) upon exposure to a rat that had eaten normal food and did not increase significantly in the rats exposed to the novel odor; ACh release in the social transmission group was significantly greater than that of the either of the control groups. Thus, ACh release in the hippocampus but not prelimbic cortex distinguished well the social transmission vs. control conditions, suggesting that cholinergic mechanisms in the hippocampus but not prelimbic cortex are important for acquiring a socially transmitted food preference.

Epinephrine converts long-term potentiation from transient to durable form in awake rats.

Neuroendocrine responses to an emotional or arousing experience modulate memory for the event. Extensive evidence suggests that epinephrine plays an important role in the regulation of memory formation by emotions and arousal. Some forms of synaptic plasticity are similarly responsive to modulation by stress and arousal. The present experiment examined the effects of epinephrine on induction and maintenance of long-term potentiation (LTP) in awake rats. Rats were prepared with bilaterally implanted electrodes for recording evoked field potentials in dentate granule cells following perforant pathway stimulation. LTP was induced with high-frequency stimulation parameters that resulted in modest early potentiation of the EPSP that decayed within 20 min. Epinephrine enhanced the magnitude of early LTP induction and also extended the durability of LTP from minutes to at least several days. Epinephrine did not alter baseline responses or modulate pre-LTP input-output curves. The enhancement of LTP by epinephrine was dose-dependent, following an inverted-U dose-response curve similar to that seen in memory enhancement experiments, suggesting considerable convergence of epinephrine modulation of memory and LTP. In extending substantially the maintenance of LTP after induction, the present finding offer potential means to study the neurobiology of rapid forgetting seen in aged rodents and other animals and the neurobiology of the impaired forgetting seen in post-traumatic stress disorder.

Fluctuations in brain glucose concentration during behavioral testing: dissociations between brain areas and between brain and blood.

Traditional beliefs about two aspects of glucose regulation in the brain have been challenged by recent findings. First, the absolute level of glucose in the brain's extracellular fluid appears to be lower than previously thought. Second, the level of glucose in brain extracellular fluid is less stable than previously believed. In vivo brain microdialysis was used, according to the method of zero net flux, to determine the basal concentration of glucose in the extracellular fluid of the striatum in awake, freely moving rats for comparison with recent hippocampal measurements. In addition, extracellular glucose levels in both the hippocampus and the striatum were measured before, during, and after behavioral testing in a hippocampus-dependent spontaneous alternation task. In the striatum, the resting extracellular glucose level was 0.71 mM, approximately 70% of the concentration measured previously in the hippocampus. Consistent with past findings, the hippocampal extracellular glucose level decreased by up to 30 +/- 4% during testing; no decrease, and in fact a small increase (9 +/- 3%), was seen in the striatum. Blood glucose measurements obtained during the same testing procedure and following administration of systemic glucose at a dose known to enhance memory in this task revealed a dissociation in glucose level fluctuations between the blood and both striatal and hippocampal extracellular fluid. These findings suggest, first, that glucose is compartmentalized within the brain and, second, that one mechanism by which administration of glucose enhances memory performance is via provision of increased glucose supply from the blood specifically to those brain areas involved in mediating that performance.

Intrahippocampal infusions of k-atp channel modulators influence spontaneous alternation performance: relationships to acetylcholine release in the hippocampus.

One mechanism by which administration of glucose enhances cognitive functions may be by modulating central ATP-sensitive potassium (K-ATP) channels. K-ATP channels appear to couple glucose metabolism and neuronal excitability, with channel blockade increasing the likelihood of neurosecretion. The present experiment examined the effects of glucose and the direct K-ATP channel modulators glibenclamide and lemakalim on spontaneous alternation performance and hippocampal ACh release. Rats received either artificial CSF vehicle or vehicle plus drug for two consecutive 12 min periods via microdialysis probes (3 mm; flow rate of 2.1 microliter/min) implanted in the left hippocampus. During the second 12 min period, rats were tested for spontaneous alternation performance. Dialysate was simultaneously collected for later analysis of ACh content. Both glucose (6.6 mm) and glibenclamide (100 micrometer) significantly increased alternation scores compared with those of controls. Conversely, lemakalim (200 micrometer) significantly reduced alternation scores relative to those of controls. Simultaneous administration of lemakalim with either glucose or glibenclamide resulted in alternation scores not significantly different from control values. All drug treatments enhanced hippocampal ACh output relative to control values. The results demonstrate that K-ATP channel modulators influence behavior when administered directly into the hippocampus, with channel blockers enhancing and openers impairing spontaneous alternation performance, thus supporting the hypothesis that glucose enhances memory via action at central K-ATP channels. That lemakalim, as well as glibenclamide and glucose, increased hippocampal ACh output suggests a dissociation between the effects of K-ATP channel modulators on behavior and hippocampal ACh release.

Age-related differences in hippocampal extracellular fluid glucose concentration during behavioral testing and following systemic glucose administration.

Recent evidence indicates that the level of glucose in the brain's extracellular fluid (ECF) is not constant, as traditionally thought, but fluctuates. We determined the effect of aging on hippocampal ECF glucose before, during, and after spatial memory testing. Fischer-344 rats (24 months old) showed a greater decrease in ECF glucose than 3-month-old rats (48% vs 12%); the decrease seen in 24-month-old rats persisted for much longer following testing. These changes were associated with an age-related deficit in spontaneous alternation performance. Following systemic glucose administration, the decrease in ECF glucose was reversed in both aged and young rats, and performance in aged versus young rats following glucose administration did not differ. These findings suggest that increased susceptibility to depletion of ECF glucose in aged rats may contribute to age-related deficits in learning and memory and that administration of glucose may enhance memory by providing additional glucose to the brain at times of increased cognitive demand.

Decreases in rat extracellular hippocampal glucose concentration associated with cognitive demand during a spatial task.

Using in vivo microdialysis, we measured hippocampal extracellular glucose concentrations in rats while they performed spontaneous alternation tests of spatial working memory in one of two mazes. Extracellular glucose levels in the hippocampus decreased by 32% below baseline during the test period on the more complex maze, but by a maximum of 11% on the less complex maze. Comparable decreases were not observed in samples taken from rats tested on the more complex maze but with probes located near but outside of the hippocampus. Systemic glucose fully blocked any decrease in extracellular glucose and enhanced alternation on the more complex maze. These findings suggest that cognitive activity can deplete extracellular glucose in the hippocampus and that exogenous glucose administration reverses the depletion while enhancing task performance.

Epinephrine fails to enhance performance of food-deprived rats on a delayed spontaneous alternation task.

Increases in blood glucose levels after epinephrine injection appear to contribute to the hormone's effects on learning and memory. The present experiment evaluated whether epinephrine-induced enhancement of spontaneous alternation performance would be attenuated in fasted rats that had blunted increases in circulating glucose levels after injections of epinephrine. Rats deprived of food for 24 h prior to injection of epinephrine exhibited significant attenuation of the increase in blood glucose levels seen in fed rats. When the rats were tested on a delayed spontaneous alternation task, epinephrine enhanced performance in fed rats but not in rats deprived of food for 24 h. These findings are consistent with the view that hyperglycemia subsequent to epinephrine injections contributes to the memory-enhancing effects of epinephrine.

ATP-sensitive potassium channel blockade enhances spontaneous alternation performance in the rat: a potential mechanism for glucose-mediated memory enhancement.

Peripheral and central injections of D-glucose enhance learning and memory in rats, and block memory impairments produced by morphine. The mechanism(s) for these effects is (are) as yet unknown. One mechanism by which glucose might act on memory and other brain functions is by regulating the ATP-sensitive potassium channel. This channel may couple glucose metabolism and neuronal excitability, with channel blockade increasing the likelihood of stimulus-evoked neurotransmitter release. The present experiments explored the effects of intra-septal injections of glucose and the ATP-sensitive potassium channel blocker glibenclamide on spontaneous alternation behavior in the rat. Intra-septal injections of glucose (20 nmol) or glibenclamide (10 nmol), 30 min prior to plus-maze spontaneous alternation performance, significantly enhanced alternation scores compared to rats receiving vehicle injections. Glibenclamide enhanced spontaneous alternation performance in an inverted-U dose-response manner. Individually sub-effective doses of glucose (5 nmol) and glibenclamide (5 nmol) significantly enhanced plus-maze alternation scores when co-injected into the septal area. Glibenclamide (10 nmol), when co-administered with morphine (4 nmol) 30 min prior to Y-maze spontaneous alternation performance, attenuated the performance-impairing effects of morphine alone. The present findings show that intra-septal injections of the direct ATP-sensitive potassium channel blocker glibenclamide, both alone and in conjunction with a sub-effective dose of glucose, enhance spontaneous alternation performance and attenuate the performance-impairing effects of morphine. The similarity of the results obtained with glibenclamide and glucose, together with their similar actions on ATP-sensitive potassium channel function, suggests that glucose may modulate memory-dependent behavior in the rat by regulating the ATP-sensitive potassium channel.

Enhanced release of norepinephrine in rat hippocampus during spontaneous alternation tests.

Recent evidence suggests that release of acetylcholine (ACh) in the hippocampus is associated with performance on a spontaneous alternation task and with enhancement of that performance by systemic and central injections of glucose. The present study extended these findings by examining norepinephrine (NE) release in the hippocampus using in vivo microdialysis while rats were tested for spontaneous alternation performance with and without prior injections (ip) of glucose. Microdialysis samples were collected every 12 min and assayed for NE content by HPLC-ECD. Like ACh, NE release in hippocampus increased during spontaneous alternation testing. As in past experiments, administration of glucose (250 mg/kg) significantly enhanced alternation scores. However, glucose did not influence NE release either during behavioral testing or at rest. These findings contrast with prior evidence showing that glucose augments testing-related increases in ACh release. The findings suggest that norepinephrine is released within the hippocampus while rats are engaged in alternation performance. However, increased release of norepinephrine apparently does not contribute to the enhancement of alternation scores produced by glucose.

Extracellular glucose concentrations in the rat hippocampus measured by zero-net-flux: effects of microdialysis flow rate, strain, and age.

The concentration of glucose in the brain's extracellular fluid remains controversial, with recent estimates and measurements ranging from 0.35 to 3.3 mM. In the present experiments, we used the method of zeronet-flux microdialysis to determine glucose concentration in the hippocampal extracellular fluid of awake, freely moving rats. In addition, the point of zero-net-flux was measured across variations in flow rate to confirm that the results for glucose measurement were robust to such variations. In 3-month-old male Sprague-Dawley rats, the concentration of glucose in the hippocampal extracellular fluid was found to be 1.00 +/- 0.05 mM, which did not vary with changes in flow rate. Three-month-old and 24-month-old Fischer-344 rats both showed a significantly higher hippocampal extracellular fluid glucose concentration, at 1.24 +/- 0.07 and 1.21 +/- 0.04 mM, respectively; there was no significant difference between the two age groups. The present data demonstrate variation in extracellular brain glucose concentration between rat strains. When taken together with previous data showing a striatal extracellular glucose concentration on the order of 0.5 mM, the data also demonstrate variation in extracellular glucose between brain regions. Traditional models of brain glucose transport and distribution, in which extracellular concentration is assumed to be constant, may require revision.

Attenuation of morphine-induced behavioral changes in rodents by D- and L-glucose.

Administration of d-glucose enhances learning and memory in several tasks and also attenuates memory impairments and other behavioral effects of several drugs, including morphine. The present experiment compared the effects of peripherally administered d-glucose with those of l-glucose, a stereoisomer of d-glucose that is not metabolized and does not readily cross the blood-brain barrier. Like d-glucose, though at somewhat different doses, peripherally administered l-glucose attenuated morphine-induced deficits in spontaneous alternation performance in rats and mice and attenuated morphine-induced hyperactivity in mice. l-Glucose did not raise circulating levels of plasma d-glucose, suggesting that the effects of l-glucose are not secondary to increased availability of d-glucose. Using direct injections of d- and l-glucose and morphine into the medial septum of rats, the findings indicate that d-glucose but not l-glucose attenuated morphine-induced deficits in spontaneous alternation performance; indeed, intraseptal injections of l-glucose alone impaired spontaneous alternation performance. These findings suggest that peripheral l-glucose antagonizes morphine-induced behavioral effects by a peripheral signaling mechanism, one distinct from the mechanisms that mediate at least some of the effects of d-glucose on brain function.

Intra-amygdala infusions of scopolamine impair performance on a conditioned place preference task but not a spatial radial maze task.

Lesions of the amygdala impair performance on a conditioned place preference (CPP) but not a spatial radial maze task. The role of cholinergic receptors within the amygdala in performance of these tasks was evaluated using intra-amygdala injections of the muscarinic receptor antagonist, scopolamine. Food deprived rats were trained on a CPP task, which consisted of four training trials on two arms of a radial eight-arm maze. One arm was consistently paired with a large amount of food (14 g) while the other arm was never baited. Prior to the fourth trial, rats received bilateral intra-amygdala infusions of the muscarinic receptor antagonist, scopolamine (SCOP; 5 microg/0.5 microl) or vehicle. On a retention test 24 h later, unoperated and vehicle-infused rats, but not SCOP-treated rats, spent significantly more time in the paired arm than chance (50%). Therefore, the scopolamine treatment appeared to block learning and/or memory on trial 4. The same rats were then trained on a radial maze task on the same apparatus, in which rats had access to all eight arms but only four were baited with food (1 pellet). Rats were trained until they reached criterion and then infusions were given prior to testing. SCOP treatment did not affect performance on the radial maze task. Thus, intact cholinergic mechanisms in the amygdala are necessary for learning or memory on a CPP task with a high reward component but not performance on a spatial radial maze task with a lower reward component.

Intra-septal injections of glucose and glibenclamide attenuate galanin-induced spontaneous alternation performance deficits in the rat.

Injection of the neuroactive peptide galanin into the rat hippocampus and medial septal area impairs spatial memory and cholinergic system activity. Conversely, injection of glucose into these same brain regions enhances spatial memory and cholinergic system activity. Glucose and galanin may both modulate neuronal activity via opposing actions at ATP-sensitive K+ (K-ATP) channels. The experiments described in this report tested the ability of glucose and the direct K-ATP channel blocker glibenclamide to attenuate galanin-induced impairments in spontaneous alternation performance in the rat. Intra-septal injection of galanin (2.5 microgram), 30 min prior to plus-maze spontaneous alternation performance, significantly decreased alternation scores compared to those of rats receiving injections of vehicle solution. Co-injection of glucose (20 nmol) or the K-ATP channel blocker glibenclamide (5 nmol) attenuated the galanin-induced performance deficits. Glibenclamide produced an inverted-U dose-response curve in its interaction with galanin, with doses of 0.5 and 10 nmol having no effect on galanin-induced spontaneous alternation deficits. Drug treatments did not alter motor activity, as measured by overall number of arm entries during spontaneous alternation testing, relative to vehicle injected controls. These findings support the hypothesis that, in the septal region, galanin and glucose act via K-ATP channels to modulate neural function and behavior.

Glucose enhancement of 24-h memory retrieval in healthy elderly humans.

When administered soon before or after training, glucose facilitates memory in rodents and in several populations of humans, including healthy elderly people. Thus, glucose appears to enhance memory formation in a time- and dose-dependent manner. By assessing the effects of glucose at the time of memory tests, the present experiment examined the role of glucose on memory retrieval in healthy elderly people. On four sessions separated by a week, glucose or saccharin were administered immediately before hearing a narrative prose passage, as in previous experiments, or immediately before being tested for recall of the passage (24 h after training). Subjects recalled significantly more information after glucose ingestion than after saccharin ingestion whether the glucose was given before acquisition or memory tests. In addition, recall was significantly better in the preacquisition glucose condition relative to recall in the retrieval glucose condition. These findings provide evidence that glucose enhances both memory storage and retrieval.

Glucose effects on cognition in adults with Down's syndrome.

Glucose enhances memory in a variety of individuals, including people with Alzheimer's disease. By 35 years of age, adults with Down's syndrome (DS) develop the characteristic plaques and tangles found in Alzheimer's disease, despite findings indicating that not all older DS individuals meet criteria for dementia. To examine the possibility that glucose enhances memory in adults with DS (mean age = 35 years, range = 19-55 years), adults with DS were given a battery of tests specifically designed for individuals with DS in glucose and control conditions. No participant met criteria for dementia, regardless of age. Glucose enhanced performance on tests requiring both long-term memory and auditory processing. In addition, increased age was associated with poorer performance on the majority of tests in the control condition, indicating that cognitive decline with aging may be more prevalent in DS than previously believed.

Memory modulation across neural systems: intra-amygdala glucose reverses deficits caused by intraseptal morphine on a spatial task but not on an aversive task.

Based largely on dissociations of the effects of different lesions on learning and memory, memories for different attributes appear to be organized in independent neural systems. Results obtained with direct injections of drugs into one brain region at a time support a similar conclusion. The present experiments investigated the effects of simultaneous pharmacological manipulation of two neural systems, the amygdala and the septohippocampal system, to examine possible interactions of memory modulation across systems. Morphine injected into the medial septum impaired memory both for avoidance training and during spontaneous alternation. When glucose was concomitantly administered to the amygdala, glucose reversed the morphine-induced deficits in memory during alternation but not for avoidance training. These results suggest that the amygdala is involved in modulation of spatial memory processes and that direct injections of memory-modulating drugs into the amygdala do not always modulate memory for aversive events. These findings are contrary to predictions from the findings of lesion studies and of studies using direct injections of drugs into single brain areas. Thus, the independence of neural systems responsible for processing different classes of memory is less clear than implied by studies using lesions or injections of drugs into single brain areas.

Glucose, memory, and aging.

Circulating glucose concentrations regulate many brain functions, including learning and memory. Much of the evidence for this view comes from experiments assessing stress-related release of epinephrine with subsequent increases in blood glucose concentrations. One application of this work has been to investigate whether age-related memory impairments result from dysfunctions in the neuroendocrine regulation of the brain processes responsible for memory. Like humans, aged rodents exhibit some memory impairments that can be reversed by administration of epinephrine or glucose. In elderly humans, ingestion of glucose enhances some cognitive functions, with effects best documented thus far on tests of verbal contextual and noncontextual information. Glucose also effectively enhances cognition in persons with Alzheimer disease or Down syndrome. Although earlier evidence suggested that glucose does not enhance cognitive function in healthy young adults, more recent findings suggest that glucose is effective in this population, provided the tests are sufficiently difficult. In college students, glucose consumption significantly enhanced memory of material in a paragraph. Glucose also appeared to enhance attentional processes in these students. Neither face and word recognition nor working memory was influenced by treatment with glucose. The neurobiological mechanisms by which glucose acts are under current investigation. Initial evidence suggests that glucose or a metabolite may activate release of the neurotransmitter acetylcholine in rats when they are engaged in learning. Consequently, the issue of nutrition and cognition becomes increasingly important in light of evidence that circulating glucose concentrations have substantial effects on brain and cognitive functions.