Effects of glucose on scopolamine-induced learning deficits in rats performing the Morris water maze task

Neurochemical changes observed by in vivo proton magnetic resonance spectroscopy in the mouse brain postadministration of scopolamine

RATIONALE AND OBJECTIVES

This study is aimed at investigating neurochemical changes in scopolamine (SCP)-induced memory impairment using spatially localized in vivo magnetic resonance spectroscopy (MRS) of the hippocampus.

MATERIALS AND METHODS

Four groups of mice (eight mice per group) were scanned after the injection of different SCP doses: 0, 1, 3, and 5 mg/kg (intraperitoneally). All the animals received (1)H MRS of their hippocampus at two time intervals: 30 minutes and 72 hours after SCP injection.

RESULTS

This work demonstrated that the doses of 3 mg/kg SCP or higher reduce the concentration of total choline-containing compounds, and these levels returned to baseline after 72 hours. These results are consistent with observations made by others using more invasive brain dialysis approaches. The levels of glutamate and glutamic compounds (glutamate + glutamine) were slightly changed at 3 and 5 mg/kg SCP dose, but the differences were not statistically significant (P > .05). These findings suggest that SCP produces transient, in vivo measurable alterations in the cholinergic system in the hippocampus.

CONCLUSIONS

On this basis, we conclude that in vivo MRS is a feasible noninvasive method to probe aspects of the alterations induced by SCP in the cholinergic neurotransmission pathways in both animal models and human studies of memory impairment.

Memory, learning, and participation of the cholinergic system in young rats exposed to environmental enrichment

The present study demonstrates the consequences of animal exposure to an enriched environment compared to animals living in a standard environment regarding learning and space memory. Male albino Wistar rats were exposed to an enriched environment for 4 weeks after the lactation period and tested in the Morris water maze in the distal and proximal clue version and in the arena. In the former test, the animals were tested at 50 days of age with 12 daily trials on two consecutive days. At the end of each session, scopolamine at the dose of 0.6 mg/kg/ml or saline solution was injected intraperitoneally. Twenty-eight days after the first phase, a new test consisting of a single trial was held (retest). An independent group of animals receiving no drug was subjected to the arena test and to the proximal clue version of the Morris maze. In the distal clue version the results did not show differences between groups in the first phase of the experiment. After 28 days (retest), the animals reared in a standard environment and treated with scopolamine exhibited a significant increase in latency compared to the group receiving the same drug and stimulated and to the group receiving saline. The arena data demonstrated a significant increase in exploratory activity in the group of animals reared in an enriched environment. The proximal clue version of the Morris maze did not show differences between groups. The results of the present study indicate that animals exposed to environmental enrichment react less to the amnesic effects of scopolamine and show an increase in exploratory activity.

Effects of early protein malnutrition and scopolamine on learning and memory in the Morris water maze

The present study investigated the effects of early protein malnutrition on the spatial learning and memory processes. The consequences of malnutrition for the cholinergic system were evaluated by comparing the performance of malnourished and control animals in the Morris water maze after treatment with scopolamine. The learning test consisted of placing the animal in the maze to escape to a submerged platform with 12 trials per day for two consecutive days. After 24 trials, the platform was removed, the rats were placed in the maze and the time spent by them in each quadrant was recorded. After 28 days the animals were tested in a single trial to verify the retention of the spatial information. In the first Experiment, scopolamine (0.0, 0.2, 0.4 and 0.6 mg/kg per ml. i.p.) was administered 20 min before the experimental sessions. In the second experiment, a dose of 0.6 mg/kg was administered after the sessions, during the period in which learning consolidation occurs. In the first experiment, there was a significant effect of the drug, with scopolamine impairing, learning in both nutritional conditions. In the saline condition, control animals presented a better performance when compared with malnourished animals. However, 28 days later, both groups increased their latencies. With 0.2 and 0.4 mg/kg of scopolamine, the performance of both nutritional groups was similar and with 0.6 mg/kg malnourished animals performed better than controls. In the second experiment, malnourished animals were also less reactive to the effects of scopolamine, resulting in lower impairments as compared to control animals. These data suggest long-term changes in learning and memory as the result of changes produced by protein malnutrition in the cholinergic neurotransmitter system.

SK-PC-B70M from Pulsatilla koreana improves scopolamine-induced impairments of memory consolidation and spatial working memory

Previous studies have shown that hederacolchiside-E from Pulsatilla koreana has neuroprotective effects and cognition-enhancing effects. Subsequently, in the current study, we demonstrate that oral administrations of oleanolic-glycoside saponins enriched fraction from P. koreana, designated as SK-PC-B70M, improve impairments in memory consolidation and spatial working memory by systemic injection of scopolamine, a muscarinic cholinergic receptor antagonist. In a step-through avoidance task, when the rats stepped through a dark chamber in a shuttle box, an electric shock was given and then SK-PC-B70M was administered 30 min later. Twenty-four hours later, the rats were placed in an illuminated chamber. The rats with SK-PC-B70M treatments showed longer response latencies than rats with only scopolamine. Spatial working memory was measured with a trial-unique matching-to-place task in a water maze which assessed memory for place information over varying lengths of delays. Three delay lengths were used: 1 min, 5 min, and 3 h. In comparison with the control rats, the rats with scopolamine treatments took significantly longer to find the platform in the second trial with 1- and 5-min delays. The rats with both scopolamine and SK-PC-B70M had significantly less search error compared with the rats with scopolamine only. These findings indicate that SK-PC-B70M has effects on reversing impairments of memory consolidation and working memory impairments induced by scopolamine.

Dissociation of cholinergic function in spatial and procedural learning in rats

The cholinergic system has long been known for its role in acquisition and retention of new information. Scopolamine, a muscarinic acetylcholine receptor antagonist impairs multiple memory systems, and this has promoted the notion that drug-induced side effects are responsible for diminished task execution rather than selective impairments on learning and memory per se. Here, we revisit this issue with the aim to dissociate the effects of scopolamine (0.2-1.0 mg/kg) on spatial learning in the water maze. Experiments 1 and 2 showed that acquisition of a reference memory paradigm with constant platform location is compromised by scopolamine independent of whether the animals are pre-trained or not. Deficits were paralleled by drug induced side-effects on sensorimotor parameters. Experiment 3 explored the role of muscarinic receptors in acquisition of an episodic-like spatial delayed matching to position (DMTP) protocol, and scopolamine still caused a learning deficit and side-effects on sensorimotor performance. Rats extensively pre-trained in the DMTP protocol with 30 s and 1 h delays over several months in experiment 4 and tested in a within-subject design under saline and scopolamine had no sensorimotor deficits, but spatial working memory remained compromised. Experiment 5 used the rising Atlantis platform in the DMTP paradigm. Intricate analysis of the amount of dwelling and its location revealed a clear deficit in spatial working memory induced by scopolamine, but there was no effect on sensorimotor or procedural task demands. Apart from the well-known contribution to sensorimotor and procedural learning, our findings provide compelling evidence for an important role of muscarinic acetylcholine receptor signaling in spatial episodic-like memory.

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

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

Long-term dietary restriction causes negative effects on cognitive functions in rats

Long-term dietary restriction is reported to increase life span and improve age-related cognitive deficits. The present study shows that the restriction increases the life span of rats but decreases their cognitive ability. Thirty-two rats were divided into restricted and ad lib feeding groups at 2.5 months of age. The restricted rats were kept at a weight of 280g. The restricted rats were poor in performing the Morris water maze task at 7-12 months. At 17-18 months, they were poor in performing the delayed matching-to-place task. At 24-27 months, the surviving 13 restricted and 5 ad lib rats performed the spatial discrimination task. The restricted rats were also poor in performing this task. Injection of glucose prior to the discrimination task improved their performance to the level of the ad lib rats. These results suggest that dietary restriction is beneficial for longevity but has negative effects on the performance of cognitive tasks, and that the cause of the negative effects may be a reduced availability of glucose in the food-restricted aged rats.

Vagotomy attenuates effects of L-glucose but not of D-glucose on spontaneous alternation performance

Two peripheral signaling routes have been proposed to account for the ability of peripheral substances such as glucose to modulate memory processing in the brain. One possible signaling route is by crossing the blood-brain barrier to act directly on brain. A second route involves activation of peripheral nerves with resulting changes in neural activity carried by peripheral nerves to the brain. Because the vagus nerve is a major neural pathway between the periphery and brain, peripherally acting modulators of memory modulators may act via vagal afferents to the brain to enhance memory processing. In the present experiments, systemic injections of either D-glucose or L-glucose, a metabolically inactive enantiomer, facilitated performance of rats on a four-arm alternation task, but at very different doses (D-glucose, 250 mg/kg; L-glucose, 3,000 mg/kg). The enhanced performance seen with L-glucose, but not that seen with D-glucose, was attenuated by vagotomy. These findings suggest that the mechanisms by which these enantiomers act to enhance memory are quite different, with L-glucose acting via vagal afferents but D-glucose acting by other means, including direct modulation of central nervous system (CNS) processes by D-glucose.