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.

Intra-amygdala kinase inhibitors disrupt retention of a learned avoidance response in rats

To assess the involvement of intra-amygdala kinase activity in aversively motivated learning, rats received intracranial injections of polymixin B sulfate (PMXB)--a protein kinase C (PKC) and calcium/calmodulin-dependent kinase II inhibitor--immediately after inhibitory avoidance training. When tested 48 h later, retention was significantly impaired relative to vehicle-injected controls. Delayed injections (2 h posttraining) and injections made dorsal to the amygdala were ineffective. Immediate posttraining injections of the more selective PKC inhibitor, NPC 15437, also impaired retention. These results suggest that intra-amygdala protein phosphorylation must occur soon after training for learned avoidance responses to be successfully retained.

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

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

Glucose enhancement of memory in patients with probable senile dementia of the Alzheimer's type

Attempts to attenuate the severe memory deficits in patients with SDAT have been largely unsuccessful, particularly in patients at advanced stages of the disease. Recent evidence indicates that glucose administration enhances memory in generally healthy aged rodents and humans. The present experiment demonstrates that glucose administration improves memory in moderately to severely demented patients with probable SDAT. Glucose ingestion significantly enhanced performance on several tests including orientation, word recognition and recall, narrative prose, and face recognition after glucose ingestion. Thus, the results extend enhancement of memory with glucose from generally healthy rodent and human populations to patients with probable SDAT.

Spontaneous alternation and inhibitory avoidance impairments with morphine injections into the medial septum. Attenuation by glucose administration

Peripheral glucose administration attenuates impairments produced by peripheral injections of the opioid agonist, morphine, on spontaneous alternation. Injections of opioid agonists directly into the medial septum also impair memory. The present experiments examined whether systemic and intraseptal glucose injections could attenuate deficits on spontaneous alternation and inhibitory avoidance in rats treated with intraseptal morphine. Morphine (3.95 nmol) injected into the medial septum significantly impaired performance on spontaneous alternation and inhibitory avoidance tasks. Both systemic (100 mg/kg, i.p.) and intraseptal (18.33 nmol) injections of glucose, administered concomitantly with intraseptal morphine, attenuated the morphine-induced impairments on these tasks in rats. These findings suggest that one brain region where glucose may act is the medial septum, possibly by releasing opioid inhibition of cholinergic activity.

Plasma glucose levels predict the disrupting effects of adrenoceptor antagonists on enhancement of memory storage

Adrenoceptor antagonists block the enhancement of memory storage produced by epinephrine injection, but not that produced by glucose injection. The present experiment determined whether adrenoceptor antagonists modify resting blood glucose levels or the magnitude of epinephrine-, glucose-, and footshock-induced increases in circulating glucose levels in a manner related to these previously observed effects on memory. The alpha- and beta-adrenoceptor antagonists, phenoxybenzamine and propranolol, respectively, were injected in rats 30 min prior to administration of epinephrine, glucose, or footshock. Plasma glucose levels were sampled during the next 30 min. Epinephrine-induced increases in plasma glucose levels were potentiated by phenoxybenzamine and were attenuated and delayed by propranolol. The adrenoceptor antagonists did not alter resting plasma glucose levels, or the increases in plasma glucose levels resulting from glucose injection or footshock. These findings suggest that phenoxybenzamine and propranolol alter blood glucose responses to epinephrine injection in a manner which may contribute to attenuation of epinephrine-induced enhancement of memory storage with peripheral injections of adrenoceptor antagonists.

Enhancement of REM sleep with auditory stimulation in young and old rats

Auditory stimulation applied during rapid eye movement (REM) sleep enhances the duration of REM sleep in cats and humans. The present experiment investigated whether auditory stimulation would enhance REM sleep in young (3-6 months) rats, and also in old (22-24 months) rats which have impaired REM sleep. Baseline sleep records were obtained on two days. Sleep patterns were then assessed during auditory stimulation test sessions. In young rats, auditory stimulation was administered during each REM sleep bout. In old rats, auditory stimulation was administered on a fixed schedule (10 min of stimulation alternating with 15 min quiet). The day after the stimulation session, an additional sleep record (Day 2) was obtained for each rat. In young rats, auditory stimulation enhanced both REM sleep duration and total REM sleep time. In the old rats, which showed impaired sleep measures as compared to young animals, auditory stimulation enhanced both total REM sleep time and the number of REM sleep periods. Residual proactive effects of auditory stimulation (Day 2) were observed in both young and old rats. Thus, auditory stimulation is an effective manipulation with which to augment REM sleep in both young and old rats, and partially attenuates REM sleep impairments in old rats.

Sleep deficits in rats after NMDA receptor blockade

N-Methyl-D-aspartate (NMDA) receptor blockade disrupts a variety of functions associated with neural plasticity, including acquisition of learned responses and long-term potentiation. Deficits in memory are significantly correlated with deficits in measures of paradoxical sleep in several amnesic populations. The present experiment therefore assessed whether NPC 12626, a competitive NMDA receptor antagonist, also disrupts sleep. NPC 12626 (1, 10, 50, and 100 mg/kg) or saline was administered to Sprague-Dawley rats 30 min prior to 3-h daytime recording periods. Paradoxical sleep was selectively impaired at all but the highest dose, which prevented all sleep during the recording period. Some deficits in nonparadoxical sleep first appeared at the 10 mg/kg dose but did not became prominent until the 50 mg/kg dose. The results thus show that NPC 12626 impairs sleep states in rats and demonstrate that paradoxical sleep is particularly susceptible to the effects of NMDA receptor blockade. These findings, along with previous evidence that NMDA antagonists impair waking measures of arousal, provide evidence that all sleep-wake states are impaired by NMDA receptor blockade. More generally, the results suggest that some brain mechanisms underlying sleep and memory may share common elements.

Anterograde and retrograde enhancement of 24-h memory by glucose in elderly humans

The present experiment examined anterograde and retrograde enhancement of memory storage by glucose in elderly humans. Glucose (50 g) or saccharin was administered shortly before or immediately after acquisition of a narrative prose passage. Recall was tested 24 h later. Glucose administration before or after presentation of the material to be learned significantly improved recall 24 h later compared to performance in the saccharin condition. These findings suggest that glucose retroactively enhances memory storage processing in elderly humans and that the enhancement of memory outlasts the transient elevations in blood glucose levels after glucose ingestion.

Impairment of spontaneous alternation performance by an NMDA antagonist: attenuation with non-NMDA treatments

N-Methyl-D-aspartate (NMDA) receptor antagonists disrupt learning on a variety of tasks. Previous findings indicate that glucose, naloxone, and physostigmine ameliorate learning deficits produced by several treatments. The present experiment examines whether these agents also reverse the amnestic effects of NMDA receptor blockade. Mice were tested for spontaneous alternation performance in a Y-maze. The animals received either saline or the NMDA antagonist, NPC 12626 (35 mg/kg, IP), 50 min prior to testing and received an additional injection of saline, glucose, naloxone, or physostigmine 30 min prior to testing. NPC 12626 significantly decreased alternation scores. Glucose (250 mg/kg), physostigmine (0.01 mg/kg), and naloxone (1 mg/kg) reversed the effects of NPC 12626. Thus, impairments of learning after NMDA receptor blockade share with other amnestic conditions the susceptibility to attenuation by glucose, naloxone, and physostigmine.

Glucose enhancement of memory in elderly humans: an inverted-U dose-response curve

In animals, enhancement of memory with glucose and many other treatments is characterized by an inverted-U dose-response curve. The present experiment examined the dose-response curve for glucose enhancement of memory in elderly humans. Using a repeated measures, counterbalanced, crossover design, the subjects (60-82 year olds) were tested on four sessions, separated by 1 week or more, for performance on the Wechsler Logical Memory Test after ingestion of a fruit drink sweetened with glucose (0, 10, 25, and 50 g) and saccharin matched to comparable taste. The findings indicate that glucose enhanced performance on this test in an inverted-U dose-response manner, with optimal enhancement obtained at the 25 g glucose dose. These findings provide further demonstration of glucose enhancement of memory in elderly humans and also describe an additional analogy between the characteristics of glucose enhancement of memory in animals and humans.

Phlorizin enhancement of memory in rats and mice

Glucose administration near the time of training or testing elevates blood glucose levels and enhances memory in rodents and humans. The magnitude of increases in circulating glucose levels predicts later retention performance in these and several other situations. Thus, circulating glucose levels appear to contribute to the regulation of memory storage processes. Phlorizin is an inhibitor of glucose transport, which, in view of the effects of glucose on memory, should impair memory. However, rats and mice injected with phlorizin before training in an inhibitory (passive) avoidance task demonstrated significantly enhanced memory performance compared to that of control animals. The effective dose of phlorizin did not significantly change regional brain-relative 3H-2-deoxyglucose uptake or plasma glucose levels. To summarize, phlorizin is a potent memory-enhancing drug. While the mechanism of this enhancement is unknown, it does not appear to include changes in blood glucose levels or brain glucose uptake.

Glucose attenuation of paradoxical sleep deficits in old rats

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

Scopolamine-induced deficits in spontaneous alternation performance: attenuation with lateral ventricle injections of glucose

This experiment determined whether centrally administered glucose can attenuate scopolamine-induced deficits in spontaneous alternation performance. All rats were surgically prepared with indwelling cannulae directed at the lateral ventricle. Thirty min prior to alternation tests, rats received systemic (ip) injections of saline or scopolamine (3 mg/kg). Ten or thirty min prior to training, the rats also received a direct injection into the lateral ventricle of either artificial cerebrospinal fluid (CSF) or glucose (3 micrograms in 1 microliter). Scopolamine significantly impaired spontaneous alternation performance relative to controls. Additional treatment with ICV glucose 30 min, but not 10 min prior to testing, significantly attenuated the scopolamine-induced deficit. These results add support to the view that glucose acts directly on brain systems to attenuate behavioral effects of cholinergic antagonists.

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

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