Acetylcholine modulation of neural systems involved in learning and memory

Extensive evidence supports the view that cholinergic mechanisms modulate learning and memory formation. This paper reviews evidence for cholinergic regulation of multiple memory systems, noting that manipulations of cholinergic functions in many neural systems can enhance or impair memory for tasks generally associated with those neural systems. While parallel memory systems can be identified by combining lesions with carefully crafted tasks, most-if not all-tasks require the combinatorial participation of multiple neural systems. This paper offers the hypothesis that the magnitude of acetylcholine (ACh) release in different neural systems may regulate the relative contributions of these systems to learning. Recent studies of ACh release, obtained with in vivo microdialysis samples during training, together with direct injections of cholinergic drugs into different neural systems, provide evidence that release of ACh is important in engaging these systems during learning, and that the extent to which the systems are engaged is associated with individual differences in learning and memory.

Impaired and spared cholinergic functions in the hippocampus after lesions of the medial septum/vertical limb of the diagonal band with 192 IgG-saporin

To lesion the cholinergic input to the hippocampus, rats received injections of 192 IgG-saporin into the medial septum/vertical limb of the diagonal band (MS/VDB). The lesions produced near-total loss of choline acetyltransferase (ChAT)-positive neurons in the MS/VDB. The loss was accompanied, however, by only partial decreases (to 40% of control levels) in acetylcholine (ACh) release in the hippocampus. Moreover, ACh release in the hippocampus increased when lesioned and control rats were tested on a spontaneous alternation task, indicating that there was significant residual cholinergic function in the hippocampus. The lesions were sufficient to impair spontaneous alternation scores. However, this impairment could be reversed by either systemic or intra-hippocampal injections of the indirect cholinergic agonist, physostigmine, providing additional evidence of residual and effective cholinergic functions in the hippocampus of lesioned rats. Moreover, systemic injections of physostigmine at doses that produced mild tremors in control rats led to more severe tremors in the lesioned rats, suggesting upregulation of cholinergic mechanisms after saporin lesions, likely in brain areas other than the hippocampus. Thus, these findings provide evidence for decreases in cholinergic input to the hippocampus accompanied by deficits on a spontaneous alternation tasks. The findings also provide evidence for considerable residual cholinergic input to the hippocampus after saporin lesions of the MS/VDB. Together, the results suggest that 192 IgG-saporin lesions of the MS/VDB, using methods often employed, do not fully remove septohippocampal cholinergic input to the hippocampus but are nonetheless sufficient to produce impairments on a task impaired by hippocampal lesions.

Inactivation of Dorsolateral Striatum Impairs Acquisition of Response Learning in Cue-Deficient, but Not Cue-Available, Conditions

Rats received bilateral injections of lidocaine or artificial cerebrospinal fluid (aCSF) into the doisolateral striatum 6 min prior to training in either a plus- or T-shaped maze under cue-poor or cue-available conditions. Lidocaine injections significantly impaired acquisition in the cue-poor environments, but not in the cue-available environments. In addition, aCSF control rats trained in a plus-maze in a cue-poor environment reached criterion much more rapidly than did rats trained in a cue-available environment. These findings suggest that cue availability can permit acquisition of response learning in a manner that is not dependent on activity of the striatum. However, in a cue-poor environment, alternate strategies may be less readily available, revealing more efficient striatal involvement in response learning.

Intra-hippocampal lidocaine injections impair acquisition of a place task and facilitate acquisition of a response task in rats

While hippocampal lesions impair learning and memory in many tasks, such lesions also enhance learning and memory in other tasks. The present experiment examines the effects of inactivation of the hippocampus with lidocaine prior to learning, to find food in a place or response version of a four-arm plus-shaped maze. Rats received lidocaine injections 6 min prior to training. Rats were trained in a single session to a criterion of 9/10 correct responses. Compared to artificial cerebrospinal fluid (aCSF)-injected controls, rats with intra-hippocampal injections of lidocaine exhibited significantly retarded acquisition of place learning. In marked contrast, rats with intra-hippocampal injections of lidocaine exhibited significantly enhanced acquisition of response learning compared to their controls. In addition to showing that the hippocampus is important for learning the place task, these findings suggest that processing of information by the hippocampus interferes with learning a task dependent on a different neural system.

Diencephalic damage decreases hippocampal acetylcholine release during spontaneous alternation testing

A rodent model of diencephalic amnesia, pyrithiamine-induced thiamine deficiency (PTD), was used to investigate diencephalic-hippocampal interactions. Acetylcholine (ACh) release, a marker of memory-related activation, was measured in the hippocampus of PTD-treated and control rats prior to, during, and after spontaneous alternation test. During behavioral testing, all animals displayed increases in ACh release. However, both the percent increase of ACh release during spontaneous alternation testing and the alternation scores were higher in control rats relative to PTD-treated rats. Thus, when rats are tested on a task with demands dependent on the hippocampus, it appears that the hippocampus is not fully activated after diencephalic damage.

Cooperation between memory systems: acetylcholine release in the amygdala correlates positively with performance on a hippocampus-dependent task

The present experiment examined the relationship between release of acetylcholine (ACh) in the amygdala and performance on a hippocampus-dependent spatial working memory task. Using in vivo microdialysis, the authors measured ACh release in rats during testing on a spontaneous alternation task. Amygdala ACh release was positively correlated with performance on the hippocampus-dependent task. These findings suggest that activation of the amygdala promotes processing in other neural systems important for learning and memory.

Switching memory systems during learning: changes in patterns of brain acetylcholine release in the hippocampus and striatum in rats

This experiment measured acetylcholine (ACh) release simultaneously in the hippocampus and striatum while rats were trained in a cross maze. Consistent with past findings, rats initially showed learning on the basis of place (i.e., turning to the correct position relative to the room), but after extensive training, rats shifted to learning on the basis of response (i.e., turning to the right/left to find the food). Profiles of ACh release in the hippocampus and striatum were markedly different during training. In the hippocampus, ACh release increased by approximately 60% at the onset of training and remained at that level of release throughout training, even after the rats began to show learning on the basis of turning rather than place. In the striatum, increases in ACh release occurred later, reaching asymptotic increases of 30-40%, coincident with a transition from expressing place learning to expressing response learning. These findings suggest that the hippocampal and striatal systems both participate in learning in this task, but in a manner characterized by differential activation of the neural systems. The hippocampal system is apparently engaged first before the striatum is activated and, to the extent the hippocampus is important for place learning, promotes the use of a place solution to the maze. Later in training, although the hippocampus remains activated, the striatum is also activated in a manner that may enable the use of a response strategy to solve the maze. These findings may offer a neurobiological marker of a transition during skill learning from declarative to procedural learning.

Patterns of brain acetylcholine release predict individual differences in preferred learning strategies in rats

Acetylcholine release was measured simultaneously in the hippocampus and dorsal striatum of rats before and during training on a maze that could be learned using either a hippocampus-dependent spatial strategy or a dorsal striatum-dependent turning strategy. A probe trial administered after rats reached a criterion of 9/10 correct responses revealed that about half of the rats used a spatial strategy and half a turning strategy to solve the task. Acetylcholine release in the hippocampus, as well as the ratio of acetylcholine release in the hippocampus vs. the dorsal striatum, measured either before or during training, predicted these individual differences in strategy selection during learning. These findings suggest that differences in release of acetylcholine across brain areas may provide a neurobiological marker of individual differences in selection of the strategies rats use to solve a learning task.

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.

Relationship of enhanced norepinephrine activity during memory consolidation to enhanced long-term memory in humans

OBJECTIVE

The purpose of this study was to investigate the effect of enhanced noradrenergic activity on memory consolidation in humans.

METHOD

Thirty healthy subjects (21 men and nine women) viewed a series of 12 slides that depicted an emotionally arousing story. Five minutes after viewing the slides, subjects received either intravenous yohimbine or intravenous placebo in a double-blind randomized fashion. Multiple blood samples were drawn for determining plasma free 3-methoxy-4-hydroxyphenylglycol (MHPG). One week later subjects took a surprise memory test for the slides.

RESULTS

There was no significant difference in memory score between yohimbine and placebo groups. Linear regression revealed a significant effect of MHPG on memory score for the group as a whole (subjects who had received yohimbine and those who had received placebo) and for the placebo group alone.

CONCLUSIONS

These findings strengthen support for the hypothesis that enhanced memory for emotionally arousing events in humans depends critically on postlearning adrenergic modulation.

Achieving and maintaining cognitive vitality with aging

Cognitive vitality is essential to quality of life and survival in old age. With normal aging, cognitive changes such as slowed speed of processing are common, but there is substantial interindividual variability, and cognitive decline is clearly not inevitable. In this review, we focus on recent research investigating the association of various lifestyle factors and medical comorbidities with cognitive aging. Most of these factors are potentially modifiable or manageable, and some are protective. For example, animal and human studies suggest that lifelong learning, mental and physical exercise, continuing social engagement, stress reduction, and proper nutrition may be important factors in promoting cognitive vitality in aging. Manageable medical comorbidities, such as diabetes, hypertension, and hyperlipidemia, also contribute to cognitive decline in older persons. Other comorbidities such as smoking and excess alcohol intake may contribute to cognitive decline, and avoiding these activities may promote cognitive vitality in aging. Various therapeutics, including cognitive enhancers and protective agents such as antioxidants and anti-inflammatories, may eventually prove useful as adjuncts for the prevention and treatment of cognitive decline with aging. The data presented in this review should interest physicians who provide preventive care management to middle-aged and older individuals who seek to maintain cognitive vitality with aging.

Competition between memory systems: acetylcholine release in the hippocampus correlates negatively with good performance on an amygdala-dependent task

Lesions of the amygdala impair acquisition of a food conditioned place preference (CPP) task. In contrast, lesions of the fornix facilitate acquisition on this task, showing that an intact hippocampal system can interfere with learning an amygdala-dependent task. Our recent findings indicate that acetylcholine (ACh) release in the hippocampus increases while rats perform a hippocampus-dependent spontaneous alternation task. To the extent that ACh output in the hippocampus reflects activation of that brain area in learning and memory, the results obtained with fornix lesions suggest that ACh release in the hippocampus might be negatively correlated with learning on a CPP task. Using in vivo microdialysis, release of ACh was measured in the hippocampus while rats learned and were tested on an amygdala-dependent CPP task and a hippocampus-dependent spontaneous alternation task. Release of ACh in the hippocampus increased when rats were tested on either task. The magnitude of the increase in release of hippocampal ACh was negatively correlated with good performance on the amygdala-dependent CPP task. These findings suggest that ACh release may reflect activation and participation of the hippocampus in learning and memory, but in a manner that can be detrimental to performance on a task dependent on another brain area.

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.