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.

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.

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.

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.

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.

Norepinephrine release in the amygdala after systemic injection of epinephrine or escapable footshock: contribution of the nucleus of the solitary tract

Several findings based largely on lesions and drug manipulations within the amygdala suggest that norepinephrine (NE) systems in the amygdala contribute to enhancement of memory processes by epinephrine (EPI). However, no studies to date have directly measured changes in the release of NE in the amygdala after EPI injection. In Experiment 1, in vivo microdialysis was used to assess amygdala NE release after systemic injection of saline, EPI (0.1 or 0.3 mg/kg), and administration of an escapable footshock (0.8 mA, 1 s). Both doses of EPI produced a significant elevation in NE release that persisted for up to 60 min. In Experiment 2, the local anesthetic lidocaine (2%) was infused (0.5 microl) into the nucleus of the solitary tract (NTS) immediately before injection of 0.3 mg/kg EPI. The EPI-induced elevation in amygdala NE release observed in Experiment I was attenuated by inactivation of the NTS. These findings indicate that systemic injection of EPI increases release of NE in the amygdala and suggest that the effects are mediated in part by activation of brainstem neurons in the NTS that project to the amygdala.

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.

Norepinephrine release in the amygdala after systemic injection of epinephrine or escapable footshock: contribution of the nucleus of the solitary tract

Several findings based largely on lesions and drug manipulations within the amygdala suggest that norepinephrine (NE) systems in the amygdala contribute to enhancement of memory processes by epinephrine (EPI). However, no studies to date have directly measured changes in the release of NE in the amygdala after EPI injection. In Experiment 1, in vivo microdialysis was used to assess amygdala NE release after systemic injection of saline, EPI (0.1 or 0.3 mg/kg), and administration of an escapable footshock (0.8 mA, 1 s). Both doses of EPI produced a significant elevation in NE release that persisted for up to 60 min. In Experiment 2, the local anesthetic lidocaine (2%) was infused (0.5 microl) into the nucleus of the solitary tract (NTS) immediately before injection of 0.3 mg/kg EPI. The EPI-induced elevation in amygdala NE release observed in Experiment I was attenuated by inactivation of the NTS. These findings indicate that systemic injection of EPI increases release of NE in the amygdala and suggest that the effects are mediated in part by activation of brainstem neurons in the NTS that project to the amygdala.

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 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.

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.

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.

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, 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.

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.

Muscimol increases acetylcholine release by directly stimulating adult striatal cholinergic interneurons

Because GabaA ligands increase acetylcholine (ACh) release from adult striatal slices, we hypothesized that activation of GabaA receptors on striatal cholinergic interneurons directly stimulates ACh secretion. Fractional [3H]ACh release was recorded during perifusion of acutely dissociated, [3H]choline-labeled, adult male rat striata. The GabaA agonist, muscimol, immediately stimulated release maximally approximately 300% with EC50 = approximately 1 microM. This action was enhanced by the allosteric GabaA receptor modulators, diazepam and secobarbital, and inhibited by the GabaA antagonist, bicuculline, by ligands for D2 or muscarinic cholinergic receptors or by low calcium buffer, tetrodotoxin or vesamicol. Membrane depolarization inversely regulated muscimol-stimulated secretion. Release of endogenous and newly synthesized ACh was stimulated in parallel by muscimol without changing choline release. Muscimol pretreatment inhibited release evoked by K+ depolarization or by receptor-mediated stimulation with glutamate. Thus, GabaA receptors on adult striatal cholinergic interneurons directly stimulate voltage- and calcium-dependent exocytosis of ACh stored in vesamicol-sensitive synaptic vesicles. The action depends on the state of membrane polarization and apparently depolarizes the membrane in turn. This functional assay demonstrates that excitatory GabaA actions are not limited to neonatal tissues. GabaA-stimulated ACh release may be prevented in situ by normal tonic dopaminergic and muscarinic input to cholinergic neurons.

Age-related differences in an ecologically based study of route learning

Spatial learning abilities in younger adults and in healthy elderly adults were examined in 2 tasks. In the first task, participants were tested for their ability to recall relevant route information as well as to recognize and to order temporally landmark information observed along the route. Older participants had relatively greater difficulty retracing the route and temporospatially ordering landmarks but were equally good at recognition of landmarks occurring on the route. In the second task, participants memorized a 2-dimensional representation of a route and subsequently navigated the route from memory. Older participants had greater difficulty memorizing the route and navigating it. Errors of omission, commission, wrong, and forced choice were analyzed. Group differences in the pattern of errors differed by task.

Intraseptal infusions of muscimol impair spontaneous alternation performance: infusions of glucose into the hippocampus, but not the medial septum, reverse the deficit

As observed with intraseptal injections of opioid receptor agonists, direct infusions of GABAergic receptor agonists into the medial septum impair performance on several tasks that involve spatial or working memory processes in rats. Because the effects of opioid-induced impairments can be reliably reversed by concomitant intraseptal infusions of glucose, the experiments reported here determined whether impairments produced by GABAergic agonists would similarly be reversed by glucose. The findings of Experiment 1 showed, in male Sprague-Dawley rats, that intraseptal infusions of the GABA agonist muscimol (1 or 3 nmol/0.5 microliter) impaired spontaneous alternation performance. The results of Experiment 2 indicated that intraseptal infusions of glucose (8, 17, or 33 nmol) or glutamate (15 or 30 nmol) did not attenuate the muscimol-induced deficit on spontaneous alternation performance, whereas infusions of the GABAergic antagonist bicuculline methiodide (0.1 nmol) did. However, the findings of Experiment 3 indicated that glucose injections (50 nmol/0.5 microliter) into the hippocampus did reverse the impairing effect of the intraseptal muscimol infusions. Combined, these findings suggest that the neurochemical regulation of learning and memory may involve hierarchical interactions between particular neurotransmitter and neuroanatomical systems. Specifically, medial septal GABAergic effects on spontaneous alternation prevail over those of glucose or glutamate in the medial septum, but are overridden by the effects of glucose in the hippocampus.

Memory scores in middle-aged rats predict later deficits in memory, paradoxical sleep, and blood glucose regulation in old age

Age-related deficits in memory are correlated with deficits in paradoxical sleep and poor glucose tolerance in rats. The present experiment used a longitudinal design to determine whether memory or glucose tolerance in middle-aged rats could predict deficits in memory, sleep, and glucose tolerance in old age. Correlations were obtained between spontaneous alternation scores and glucose tolerance levels in middle age (14 months) and inhibitory avoidance, daytime sleep, and glucose tolerance levels in old age (24 months). Spontaneous alternation scores, but not glucose tolerance levels, predicted performance on all 3 behavioral and biological measures in old age. Measures of functional integrity, such as memory, may be sensitive predictors of subsequent age-related change in specific cognitive and neurobiological systems.

Age-related differences in an ecologically based study of route learning

Spatial learning abilities in younger adults and in healthy elderly adults were examined in 2 tasks. In the first task, participants were tested for their ability to recall relevant route information as well as to recognize and to order temporally landmark information observed along the route. Older participants had relatively greater difficulty retracing the route and temporospatially ordering landmarks but were equally good at recognition of landmarks occurring on the route. In the second task, participants memorized a 2-dimensional representation of a route and subsequently navigated the route from memory. Older participants had greater difficulty memorizing the route and navigating it. Errors of omission, commission, wrong, and forced choice were analyzed. Group differences in the pattern of errors differed by task.

Inhibitory avoidance impairments induced by intra-amygdala propranolol are reversed by glutamate but not glucose

Both systemic and central injections of glucose can enhance memory. For example, glucose reverses impairments on inhibitory avoidance resulting from intra-amygdala injections of morphine. The present experiment investigated the ability of glucose to reverse memory impairments resulting from intra-amygdala injections of propranolol, a beta-noradrenergic antagonist. Pretraining administration of 10 microg propranolol significantly reduced inhibitory avoidance retention latencies but had no effect on performance in a spontaneous alternation task. Coadministration of glucose into the amygdala at 3 doses (1.5, 3.0, and 6.0 microg) did not reverse the propranolol-induced inhibitory avoidance deficits. However, coadministration of 2.5 microg of glutamate with the propranolol did reverse these deficits. The ability of glucose to reverse impairments following intra-amygdala injections of morphine but not propranolol may reflect the neurotransmitter system or systems through which glucose exerts its effects.

Intra-septal infusions of glucose potentiate inhibitory avoidance deficits when co-infused with the GABA agonist muscimol

This experiment examined the effects of co-infusions of glucose with the gamma-aminobutyric acid (GABA) agonist muscimol into the medial septum on memory for inhibitory avoidance learning. Co-infusions of muscimol (3 nmol) and glucose (33 nmol) impaired memory, but neither drug did so when administered alone. Thus, although glucose typically reverses memory deficits, these results indicate that glucose potentiates the memory-impairing effects of a GABA agonist.

Prenatal exposure to alcohol in adult rats: relationships between sleep and memory deficits, and effects of glucose administration on memory

Previous studies show that prenatal exposure to alcohol results in sleep deficits in rats, including reductions in paradoxical sleep. Little is known, however, about the extent or duration of sleep impairments beyond the neonatal period. The present experiment examined effects of prenatal exposure on sleep in young adulthood. Three-hour, daytime sleep EEGs were obtained in 6-month-old female rats prenatally exposed to alcohol. Compared to isocaloric pair-fed and ad libitum control groups, the alcohol-exposed group showed reduced paradoxical sleep. Non-paradoxical sleep did not differ between groups. Concurrent deficits were obtained in radial arm maze, but not inhibitory (passive) avoidance, performance. One year later, at the age of 18 months, alcohol-exposed rats showed deficits in spontaneous alternation behavior which were reversed by administration of glucose (100 mg/kg). Deficits in paradoxical sleep at 6 months of age were highly correlated with deficits in spontaneous alternation behavior at 18 months of age, in individual, alcohol-exposed animals. These results provide the first evidence that prenatal exposure to alcohol results in selective and persistent deficits in sleep. They also show that measures of paradoxical sleep can predict impaired memory over a large portion of the life span, and suggest that glucose can attenuate memory deficits in this population.

Catecholamines, stress, and disease: a psychobiological perspective

OBJECTIVE

Research on the relationship between physiological responses to stressful stimulation and the onset of psychosomatic illnesses has been an area of intense interest for many years. Studies using animal models have contributed significantly to this field of inquiry by taking several complementary approaches.

METHOD

Three specific research strategies taken in our laboratory will be highlighted here. Each involves studies in conscious, freely behaving animals.

RESULTS

Genetically selected animals have been exposed to acute stressors to unmask neuroendocrine and autonomic abnormalities related to disease susceptibility. In addition, studies of aged animals suggest that exaggerated physiological responses to acute stress may underlie some age-related pathologies. Finally, a series of studies has revealed that exposure of laboratory animals to stressful stimulation may exert long-lasting influences on the ways in which these subjects respond in the future to the same or novel stressors.

CONCLUSIONS

These findings illustrate how studies with laboratory animals have the potential for refining the questions that are posed in research with clinical populations and for providing insight into the underlying physiological mechanisms of individual variability in disease susceptibility and the development of appropriate therapeutic interventions.

Inhibitory avoidance impairments induced by intra-amygdala propranolol are reversed by glutamate but not glucose

Both systemic and central injections of glucose can enhance memory. For example, glucose reverses impairments on inhibitory avoidance resulting from intra-amygdala injections of morphine. The present experiment investigated the ability of glucose to reverse memory impairments resulting from intra-amygdala injections of propranolol, a beta-noradrenergic antagonist. Pretraining administration of 10 microg propranolol significantly reduced inhibitory avoidance retention latencies but had no effect on performance in a spontaneous alternation task. Coadministration of glucose into the amygdala at 3 doses (1.5, 3.0, and 6.0 microg) did not reverse the propranolol-induced inhibitory avoidance deficits. However, coadministration of 2.5 microg of glutamate with the propranolol did reverse these deficits. The ability of glucose to reverse impairments following intra-amygdala injections of morphine but not propranolol may reflect the neurotransmitter system or systems through which glucose exerts its effects.

Pyruvate infusions into the septal area attenuate spontaneous alternation impairments induced by intraseptal morphine injections

Glucose infusions into the medial septal area attenuate memory impairments produced by concurrent intraseptal morphine injections. One possible explanation for these effects of glucose on memory is that the treatment modulates regional energy metabolism. As a test of this hypothesis, the present experiment determined whether intraseptal pyruvate injections could attenuate a spontaneous alternation impairment seen after intraseptal morphine injections. Intraseptal injections of morphine (4.0 nmol) 30 min prior to testing produced spontaneous alternation scores significantly lower than those in control groups. Morphine injections near, but outside, the septal region did not impair spontaneous alternation performance. The morphine-induced impairment was similarly reversed by coadministration of either glucose (18 nmol) or pyruvate (18 nmol) into the septum. These findings suggest that glucose may act through the tricarboxylic acid cycle by increasing the availability of ATP, augmenting the synthesis of certain neurotransmitters, or both.

Age and stress history effects on spatial performance in a swim task in Fischer-344 rats

This study determined whether prior habituation to water immersion would ameliorate age-related deficits in learning and memory in a swim task. Aged (22 months) and young adult (3 months) rats were immersed in water (30 degrees C) for 15 min on each of 28 consecutive days before training in the swim task. Additional groups of age-matched animals served as handled controls. Training on a spatial discrimination version of the water task was conducted over 5 days with two trials per day (1-h intertrial interval). A probe trial was substituted for the last trial on the fifth day to assess the rats' use of spatial information. Three days later, rats received cue discrimination training to find a visible platform. In the spatial task, prior habituation to water immersion ameliorated deficits in acquisition within each day (i.e., at a 1-h intertrial interval) but not across days (at 24 h). The results obtained with the 24-h interval confirm the rapid forgetting characteristic of aged rats in many tasks. The stress-habituation procedures reduced age-related deficits seen on the probe trial and on cue discrimination training. These findings indicate that several aspects of age-related impairments in the swim task, often attributed to primary age-related deficits in learning and memory processes per se, may instead be secondary to age-related differences in stress responses to water immersion.

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.

Pyruvate infusions into the septal area attenuate spontaneous alternation impairments induced by intraseptal morphine injections

Glucose infusions into the medial septal area attenuate memory impairments produced by concurrent intraseptal morphine injections. One possible explanation for these effects of glucose on memory is that the treatment modulates regional energy metabolism. As a test of this hypothesis, the present experiment determined whether intraseptal pyruvate injections could attenuate a spontaneous alternation impairment seen after intraseptal morphine injections. Intraseptal injections of morphine (4.0 nmol) 30 min prior to testing produced spontaneous alternation scores significantly lower than those in control groups. Morphine injections near, but outside, the septal region did not impair spontaneous alternation performance. The morphine-induced impairment was similarly reversed by coadministration of either glucose (18 nmol) or pyruvate (18 nmol) into the septum. These findings suggest that glucose may act through the tricarboxylic acid cycle by increasing the availability of ATP, augmenting the synthesis of certain neurotransmitters, or both.

Acetylcholine release from dissociated striatal cells

To study the regulation of striatal acetylcholine (ACH) release, adult male rat striata were dissociated and incubated with 3H-choline to synthesize 3H-ACH. Fractional 3H-ACH efflux per min during continuous perifusion was: (1) tightly regulated; (2) dependent on calcium influx; (3) stimulated by 10 mM K+ and 1 mM glutamate; and (4) comparable to ACH release detected by HPLC. Thus, acutely dissociated striata exhibit calcium-sensitive, voltage-dependent secretion of 3H-ACH and direct receptor-mediated stimulation of release through the glutamate receptor family. This new approach toward cholinergic secretory physiology will help clarify complex striatal circuitry.

Glucose attenuation of atropine-induced deficits in paradoxical sleep and memory

When administered systemically, glucose attenuates deficits in memory produced by several classes of drugs, including cholinergic antagonists and opiate agonists. Glucose also enhances memory in aged rats, mice, and humans. In addition, glucose ameliorates age-related reductions in paradoxical sleep. Because deficits in paradoxical sleep are most marked in those individual aged rats that also have deficits in memory, treatments which improve one of these functions may similarly improve the other. The present experiments show that glucose attenuates deficits in paradoxical sleep and memory after atropine administration, with similar dose-response curves for both actions. In the first experiment, rats received saline, atropine (1 mg/kg), glucose (100 mg/kg) or combinations of atropine + glucose (10, 100, 250, and 500 mg/kg) 30 min before assessment on a spontaneous alternation task. In the second experiment, 3-h EEGs were assessed for spontaneous daytime sleep in rats administered saline, atropine (1 mg/kg), glucose (100 mg/kg) or combinations of atropine + glucose (10, 100 and 250 mg/kg). In both experiments, glucose significantly attenuated deficits at an optimal dose of 100 mg/kg. A third experiment assessed blood glucose levels after injections of atropine + glucose (100 mg/kg) and determined that blood glucose levels were similar to those produced by other treatments which enhance memory. These results are consistent with the view that paradoxical sleep and at least one test of memory are similarly influenced by atropine and glucose.

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.

Morphine-induced deficits in sleep patterns: attenuation by glucose

Morphine effects on many neural and behavioral measures, including tests of learning and memory, are attenuated by increased circulating glucose levels. Using systemic injections, we investigated the ability of glucose to attenuate sleep deficits induced by morphine administration in the rat. Morphine at 1 mg/kg produced a moderate decrease in slow wave sleep which was prevented by concomitant administration of 100 mg/kg of glucose. A higher dose of morphine (10 mg/kg) severely delayed the onset of both slow wave sleep and REM sleep. These delays were attenuated by concurrent administration of 250 mg/kg of glucose. Thus, glucose reversals of morphine effects are also extended to measures of sleep.

Age-related changes in plasma catecholamine and glucose responses of F-344 rats to a single footshock as used in inhibitory avoidance training

Young adult (3 months) and aged (22 months) Fischer 344 male rats were prepared with chronic tail artery catheters. Two days after surgery, rats were transferred to a test chamber and exposed to a single footshock (0, 0.25, 0.50, or 1.0 mA for 1 s). Blood samples were obtained from each rat under basal conditions and at timed intervals after exposure to footshock. Basal plasma levels of norepinephrine (NE) and epinephrine (EPI) were similar for 3- and 22-month-old rats. In contrast, plasma glucose (GLU) levels were significantly lower in aged rats compared to young adults. Increments in plasma levels of EPI were greater in aged rats compared to young adult controls following transfer of rats to the test chamber. In addition, aged rats had potentiated plasma EPI responses to footshock. Finally, aged rats had greater plasma levels of both catecholamines up to 5 min after a single training footshock compared to young adult controls. However, the increased responsiveness of EPI in aged rats was not accompanied by proportionate increases in plasma GLU levels, i.e., the EPI-GLU relationship was uncoupled in aged rats. These findings point to dramatic differences between young adult and aged rats in their plasma EPI responses to inhibitory avoidance training. Age-related increases in EPI secreted from the adrenal medulla, together with decreased blood GLU responses, may contribute in part to age-related deficits in memory modulatory processes.

Age-related changes in plasma catecholamine responses to chronic intermittent stress

We examined habituation and sensitization of plasma catecholamine responses to stressful stimulation in young adult (3 months) and aged (22 months) Fischer 344 (F-344) male rats. Aged rats had greater elevations in plasma levels of norepinephrine (NE) and epinephrine (EPI) following exposure to restraint stress compared to young adult controls. Within ages, plasma catecholamine responses were similar in rats stressed for the first time compared to those stressed for the 27th time. When chronically stressed young adult and aged F-344 rats were exposed to a novel stressor, swim stress at 25 degrees C, plasma catecholamine responses were significantly greater than for age-matched handled controls. The magnitude of sensitization of plasma catecholamine responses to the novel stressor was similar for young adult and aged F-344 rats. These results indicate that aged rats have enhanced plasma catecholamine responses to acute restraint stress compared to young adults. In addition, rats of both ages displayed comparable levels of sensitization of plasma catecholamine responses to a novel stressor. These findings emphasize that aged rats differ from young adult rats in some but not all aspects of sympathetic-adrenal medullary regulation. Further, these age-related differences in sympathetic-adrenal medullary responses are unmasked when animals are exposed to stressful stimulation.

Age-related changes in plasma catecholamine responses to acute swim stress

Young adult (3 months) and aged (22 months) Fischer 344 male rats were prepared with chronic tail artery catheters. Three days after surgery, rats were exposed acutely to swim stress at 20, 25, 30, or 35 degrees C for 15 min. Blood samples were obtained from each rat under basal conditions, at the end of the swim stress episode, and 15, 30, and 45 min after swim stress. Basal plasma levels of norepinephrine and epinephrine (EPI) were similar for 3- and 22-month-old rats. In contrast, plasma catecholamine responses of aged rats were significantly greater than those of young adult rats following swim stress at 20 and 25 degrees C. Plasma catecholamine responses were similar for rats of the two ages following swim stress at 30 or 35 degrees C. These findings indicate that aged rats have exaggerated sympathetic-adrenal medullary responses to acute swim stress at the lower water temperatures. Given the modulatory effects of plasma EPI on memory, these age-related alterations in plasma catecholamine responses to acute swim stress may influence spatial memory performance of rats in the Morris water maze.

Role of glucose in regulating the brain and cognition

Extensive evidence indicates that relatively modest increases in circulating glucose concentrations enhance learning and memory processes in rodents and humans. In rats, systemic injections of glucose enhance learning and memory under many conditions. When microinjected into specific brain sites, glucose has selective behavioral and pharmacological effects, with behavioral effects that are specific to the brain site injected and pharmacological effects that are largely specific to interactions with opiate agonists. Recent evidence suggests that glucose may attenuate opiate inhibition of acetylcholine release in the hippocampus. The relative safety of glucose has permitted tests of glucose effects on cognitive functions in humans. Glucose also enhances learning and memory in healthy aged humans and enhances several other cognitive functions in subjects with severe cognitive pathologies, including individuals with Alzheimer's disease and Down syndrome. Thus, increases in circulating glucose concentrations have robust and broad influences on brain functions that span many neural and behavioral measures and cross readily from rodents to humans.

Glucose does not reverse impairments on spontaneous alternation induced by the noncompetitive NMDA antagonist MK-801

N-Methyl-D-aspartate (NMDA) antagonists have been demonstrated to impair acquisition in a variety of tasks, including maze learning. It was previously reported from this laboratory that glucose can reverse the deficits on spontaneous alternation resulting from administration of the competitive NMDA antagonist NPC 12626 in mice. The present study tested the ability of glucose to reverse deficits induced by the noncompetitive NMDA antagonist MK-801. Although subcutaneous administration of 0.10 mg/kg of MK-801 resulted in a deficit on spontaneous alternation, glucose (100 and 250 mg/kg) did not reverse the impairment. This difference in the ability of glucose to reverse the impairment caused by the two NMDA antagonists may reflect their different modes of actions at the NMDA receptor complex.

Task-dependent effects of intra-amygdala morphine injections: attenuation by intra-amygdala glucose injections

Intraseptal injections of morphine impair learning and memory in rats, and these impairments are reversed by intraseptal injections of glucose. With evidence that injections of morphine into the amygdala also impair memory for some tasks, the present experiment determined whether (1) intra-amygdala morphine injections impair performance in inhibitory avoidance and spontaneous alternation tasks, and (2) intra-amygdala glucose injections attenuate the effects of intra-amygdala morphine injections. Rats receiving bilateral injections of morphine (4.0 nmol) into the amygdala, 30 min prior to training in inhibitory avoidance, had retention latencies significantly lower than those of unoperated and CSF controls when tested 24 hr later. Bilateral morphine injections (4.0 or 8.0 nmol) 30 min prior to testing in a spontaneous alternation task did not alter performance. The morphine-induced impairment observed in inhibitory avoidance was not due to diffusion up the cannulas, altered sensitivity to shock, or seizure activity. A glucose dose of 16.67 nmol, but not 8.33 nmol, injected into the amygdala attenuated the morphine-induced deficit in inhibitory avoidance. Rats receiving CSF into the amygdala exhibited decreased retention latencies in inhibitory avoidance compared to those of unoperated controls. This decrease was not attenuated by glucose at doses of 8.33 and 16.67 nmol. Therefore, these findings suggest that the amygdala is another brain region in which glucose affects brain functions, possibly by interacting with the opioid system and/or other neurotransmitter systems.

Intrahippocampal administration of both the D- and the L-isomers of AP5 disrupt spontaneous alternation behavior and evoked potentials

We previously reported that systemically administered N-methyl-D-aspartate (NMDA) antagonists significantly impair spontaneous alternation behavior. Others have reported that the restricted blockade of hippocampal NMDA receptors disrupts performance on different tests of spatial learning and have suggested that the resulting impairments are attributable to a disruption of endogenous NMDA-dependent long-term potentiation (LTP). In the present study, we determined whether spontaneous alternation performance was disrupted by circumscribed blockade of hippocampal NMDA receptors as well as by a second class of compounds which disrupt LTP, protein kinase inhibitors. The effect of hippocampal NMDA blockade on inhibitory avoidance was also examined insofar as this behavior too is disrupted by systemically administered NMDA antagonists. When injected into the hippocampus 15 min prior to spontaneous alternation testing, the NMDA antagonists CPP and D,L-AP5 each decreased alternation rates. The specific protein kinase C (PKC) inhibitor, NPC 15437, also disrupted spontaneous alternation, whereas the more general kinase inhibitor, PMXB, did not. When injected 15 min prior to inhibitory avoidance training, CPP also impaired inhibitory avoidance learning as assessed during a subsequent test session, 48 h later. Interpretation of these data was complicated by the additional findings that intrahippocampal infusion of L-AP5 (which is inactive with respect to NMDA receptors) also disrupted alternation performance, and that both the D- and the L-isomers of AP5 as well as each kinase inhibitor dramatically disrupted evoked responses (i.e., population spike amplitude, spike latency, and EPSP slope), as recorded in the dentate gyrus and evoked by perforant path stimulation. These data indicate that behaviorally effective doses of AP5 may have effects which extend beyond NMDA blockade. Moreover, the effects of these compounds on hippocampal transmission, in general, suggest that attribution of the amnestic consequences of their administration to impaired LTP may be unwarranted.

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.