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.     

Blunted hippocampal, but not striatal, acetylcholine efflux parallels learning impairment in diencephalic-lesioned rats

A rodent model of diencephalic amnesia, pyrithiamine-induced thiamine deficiency (PTD), was used to investigate the dynamic role of hippocampal and striatal acetylcholine (ACh) efflux across acquisition of a nonmatching-to-position (NMTP) T-maze task. Changes in ACh efflux were measured in rats at different time points in the acquisition curve of the task (early=day 1, middle=day 5, and late=day 10). Overall, the control group had higher accuracy scores than the PTD group in the latter sessions of NMTP training. During the three microdialysis sampling points, all animals displayed significant increases in ACh efflux in both hippocampus and striatum, while performing the task. However, on day 10, the PTD group showed a significant behavioral impairment that paralleled their blunted hippocampal--but not striatal--ACh efflux during maze training. The results support selective diencephalic-hippocampal dysfunction in the PTD model. This diencephalic-hippocampal interaction appears to be critical for successful episodic and spatial learning/memory.     

Anterior thalamic lesions alter both hippocampal-dependent behavior and hippocampal acetylcholine release in the rat

The anterior thalamic nuclei (ATN) are important for learning and memory as damage to this region produces a persistent amnestic syndrome. Dense connections between the ATN and the hippocampus exist, and importantly, damage to the ATN can impair hippocampal functioning. Acetylcholine (ACh) is a key neurotransmitter in the hippocampus, and in vivo measures of ACh are correlated to learning and memory performance. In the present study, complete lesions of the ATN impaired performance on two measures of hippocampal-dependent learning and memory (spontaneous alternation and delayed alternation) and severely disrupted behaviorally evoked ACh efflux within the hippocampus of adult male rats. In contrast, incomplete ATN lesions did not impair spontaneous alternation performance but did impair delayed alternation performance while blunting hippocampal ACh efflux. Interestingly, ATN lesions of any size did not affect basal concentrations of ACh in the hippocampus. These results demonstrate that the ATN have the capacity to modulate behaviorally relevant neuronal transmission within the hippocampus.     

Acetylcholine release in hippocampus and striatum during testing on a rewarded spontaneous alternation task

The present experiment tested male Sprague-Dawley rats for spontaneous alternation performance in a food-rewarded Y-shaped maze. Microdialysis samples, later assessed for acetylcholine concentration, were collected from the hippocampus and striatum of each rat prior to and during testing; testing sessions lasted 20 min. Early in testing, rats alternated at a rate of 72%. Alternation scores increased throughout the 20-min testing session and reached 93% during the last 5 min. The behavioral findings suggest that, during testing, rats changed the basis for their performance from a spatial working memory strategy to a persistent turning strategy. ACh release in both hippocampus and striatum increased at the onset of testing. Increases in ACh release in the striatum began at 18% above baseline during the first 5 min of testing and steadily increased reaching 58% above baseline during the final 5 min. The progressive rise of striatum ACh release during testing occurred at about the time rats adopted a persistent turning strategy. In contrast, ACh release in the hippocampus increased by 50% with the onset of testing and remained at this level until declining slightly during the last 5 min of testing. The relative changes in ACh release in the striatum and hippocampus resulted in a close negative relationship between the ratio of ACh release in the hippocampus/striatum and alternation scores.     

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.     

Age-related changes in memory and in acetylcholine functions in the hippocampus in the Ts65Dn mouse, a model of Down syndrome

Spatial working memory and the ability of a cholinesterase inhibitor to enhance memory were assessed at 4, 10, and 16 months of ages in control and Ts65Dn mice, a partial trisomy model of Down syndrome, with possibly significant relationships to Alzheimer's disease as well. In addition, ACh release during memory testing was measured in samples collected from the hippocampus using in vivo microdialysis at 4, 10, and 22-25 months of age. When tested on a four-arm spontaneous alternation task, the Ts65Dn mice exhibited impaired memory scores at both 4 and 10 months. At 16 months, control performance had declined toward that of the Ts65Dn mice and the difference in scores across genotypes was not significant. Physostigmine (50 microg/kg) fully reversed memory deficits in the Ts65Dn mice in the 4-month-old group but not in older mice. Ts65Dn and control mice exhibited comparable baseline levels of ACh release at all ages tested; these levels did not decline significantly across age in either genotype. ACh release increased significantly during alternation testing only in the young Ts65Dn and control mice. However, the increase in ACh release during alternation testing was significantly greater in control than Ts65Dn mice at this age. The controls exhibited a significant age-related decline in the testing-related increase in ACh release. With only a small increase during testing in young Ts65Dn mice, the age-related decline in responsiveness of ACh release to testing was not significant in these mice. Overall, these results suggest that diminished responsiveness of ACh release in the hippocampus to behavioral testing may contribute memory impairments in Ts65Dn mice.     

Acetylcholine release in the hippocampus and striatum during place and response training

These experiments examined the release of acetylcholine in the hippocampus and striatum when rats were trained, within single sessions, on place or response versions of food-rewarded mazes. Microdialysis samples of extra-cellular fluid were collected from the hippocampus and striatum at 5-min increments before, during, and after training. These samples were later analyzed for ACh content using HPLC methods. In Experiment 1, ACh release in both the hippocampus and striatum increased during training on both the place and response tasks. The magnitude of increase of training-related ACh release in the striatum was greater in rats trained on the response task than in rats trained on the place task, while the magnitude of ACh release in the hippocampus was comparable in the two tasks. Experiment 2 tested the possibility that the hippocampus was engaged and participated in learning the response task, as well as the place task, because of the availability of extra-maze cues. Rats were trained on a response version of a maze under either cue-rich or cue-poor conditions. The findings indicate that ACh release in the hippocampus increased similarly under both cue conditions, but declined during training on the cue-poor condition, when spatial processing by the hippocampus would not be suitable for solving the maze. In addition, high baseline levels of ACh release in the hippocampus predicted rapid learning in the cue-rich condition and slow learning in the cue-poor condition. These findings suggest that ACh release in the hippocampus augments response learning when extra-maze cues can be used to solve the maze but impairs response learning when extra-maze cues are not available for use in solving the maze.     

Rats depend on habit memory for discrimination learning and retention

We explored the circumstances in which rats engage either declarative memory (and the hippocampus) or habit memory (and the dorsal striatum). Rats with damage to the hippocampus or dorsal striatum were given three different two-choice discrimination tasks (odor, object, and pattern). These tasks differed in the number of trials required for learning (~10, 60, and 220 trials). Dorsal striatum lesions impaired discrimination performance to a greater extent than hippocampal lesions. Strikingly, performance on the task learned most rapidly (the odor discrimination) was severely impaired by dorsal striatum lesions and entirely spared by hippocampal lesions. These findings suggest that discrimination learning in the rat is primarily supported by the dorsal striatum (and habit memory) and that rats engage a habit-based memory system even for a task that takes only a few trials to acquire. Considered together with related studies of humans and nonhuman primates, the findings suggest that different species will approach the same task in different ways.     

GABA, glutamate, dopamine and serotonin transporters expression on memory formation and amnesia

Notwithstanding several neurotransmission systems are frequently related to memory formation, amnesia and/or therapeutic targets for memory alterations, the role of transporters γ-aminobutyric acid (GABA, GAT1), glutamate (neuronal glutamate transporter excitatory amino acid carrier; EACC1), dopamine (DAT) and serotonin (SERT) is poorly understood. Hence, in this paper Western-blot analysis was used to evaluate expression changes on them during memory formation in trained and untrained rats treated with the selective serotonin transporter inhibitor fluoxetine, the amnesic drug d-methamphetamine (METH) and fluoxetine plus METH. Transporters expression was evaluated in the hippocampus, prefrontal cortex and striatum. Data indicated that in addition of memory performance other behavioral parameters (e.g., explorative behavior, food-intake, etc.) that memory formation was recorded. Thus, memory formation in a Pavlovian/instrumental autoshaping was associated to up-regulation of prefrontal cortex GAT1 and EAAC1, striatal SERT, DAT and EACC1; while, hippocampal EACC1, GAT1 and SERT were down-regulated. METH impaired short (STM) and long-term memory (LTM), at 24 or 48h. The METH-induced amnesia down-regulated SERT, DAT, EACC1 and GAT1 in hippocampus and the GAT1 in striatum; no-changes were observed in prefrontal cortex. Post-training administration of fluoxetine improved LTM (48h), which was associated to DAT, GAT1 (prefrontal cortex) up-regulation, but GAT1 (striatum) and SERT (hippocampus) down-regulation. Fluoxetine plus METH administration was able to prevent amnesia, which was associated to DAT, EACC1 and GAT1 (prefrontal cortex), SERT and DAT (hippocampus) and EACC1 or DAT (striatal) up-regulation. Together these data show that memory formation, amnesia and anti-amnesic effects are associated to specific patters of transporters expression.     

Excitotoxic lesions restricted to the dorsal CA1 field of the hippocampus impair spatial memory and extinction learning in C57BL/6 mice

Recent studies in patients with hippocampal lesions have indicated that the degree of memory impairment is proportional to the extent of damage within the hippocampus. Particularly, patients with damage restricted to the CA1 field demonstrate moderate to severe anterograde amnesia with only slight retrograde amnesia. Comparable results are also seen in other species such as non-human primates and rats; however, the effect of selective damage to CA1 has not yet been characterized in mice. In the present study, we investigated the effects of excitotoxic (NMDA) lesions of dorsal CA1 on several aspects of learning and memory performance in mice. Our data indicate that dorsal CA1 lesioned mice are hyperactive upon exposure to a novel environment, have spatial working memory impairments in the Y-maze spontaneous alternation task, and display deficits in an 8-arm spatial discrimination learning task. Lesioned mice are able to acquire an operant lever-press task but demonstrate extinction learning deficits in this appetitive operant paradigm. Taken together, our results indicate that lesions to dorsal CA1 in mice induce selective learning and memory performance deficits similar to those observed in other species, and extend previous findings indicating that this region of the hippocampus is critically involved in the processing of spatial information and/or the processing of inhibitory responses.     

Locomotor, avoidance,and maze behavior in rats with selective disruption of hippocampal output.

Behavioral correlates of selective disruption of hippocampal output were investigated in a series of five experiments. In two experiments an attempt was made through behavioral investigation to determine whether the CA1 neurons project to the fimbria or to the subiculum. The results supported recent views that the subiculum is the recipient of CA1 axons. Disruption of the CA1 output in the dorsal hippocampus of rats produced increased open-field activity, whereas passive avoidance and spontaneous alternation behaviors remained unchanged. No differentiation was obtained between CA1 damage and neocortical lesions in maze learning. Blocking of the fimbrial CA3 output from the dorsal hippocampus improved passive avoidance performance and impaired active avoidance performance, whereas open-field and spontaneous alternation behaviors were unaffected. Interruption of the CA3 output from the ventral hippocampus improved active avoidance performance and reduced spontaneous alternation behavior. Open-field behavior and passive avoidance performance remained unchanged. Total fimbrial sections increased open-field activity, improved passive and active avoidance, and reduced spontaneous alternation. The results are discussed in terms of functional differentiation between the CA1 and CA3 of the dorsal hippocampus and in terms of functional differences in the fimbrial CA3 output from the dorsal and ventral hippocampus.     

Dynamic changes in acetylcholine output in the medial striatum during place reversal learning

The present studies explored the role of the medial striatum in learning when taskcontingencies change. Experiment 1 examined whether the medial striatum is involved in place reversal learning. Testing occurred in a modified cross-maze across two consecutive sessions. Injections of the local anesthetic, bupivacaine, into the medial striatum, did not impair place acquisition, but impaired place reversal learning. The reversal-learning deficit was due to an inability to maintain the new choice pattern following the initial shift. Experiment 2 determined whether changes in acetylcholine (ACh) output occur during the acquisition or reversal learning of a place discrimination. Extracellular ACh output from the medial striatum was assessed in samples collected at 6-min intervals using in vivo microdialysis during behavioral testing. ACh output did not change from basal levels during place acquisition. During reversal learning, ACh output significantly increased as rats began to learn the new choice pattern, and returned to near basal levels as a rat reliably executed the new place strategy. The present results suggest that the medial striatum may be critical for flexible adaptations involving spatial information, and that ACh actions in this area enable the shifting of choice patterns when environmental conditions change.     

Retrograde amnesia for visual memories after hippocampal damage in rats

It is generally believed that the hippocampus is not required for simple discrimination learning. However, a small number of studies have shown that hippocampus damage impairs retention of a previously learned visual discrimination task. We propose that, although simple discrimination learning may proceed in the absence of the hippocampus, it plays an important role in this type of learning when it is intact. In order to test the role of the hippocampus in simple discrimination learning, we performed a series of experiments utilizing a two-choice picture discrimination task. Our experiments confirm that rats readily learn simple two-choice picture discriminations after hippocampus damage. However, if such discriminations are first learned while the hippocampus is intact, subsequent hippocampus damage causes severe retrograde amnesia for the discriminations. Furthermore, retrograde amnesia for simple picture discriminations was equally severe when the interval between training and damage was 1 d or 60 d; remote picture memories are not spared. Similarly, the rule or schema underlying a recently or remotely acquired picture discrimination learning set was lost after hippocampus damage. The severity of retrograde amnesia for simple picture discriminations is negatively correlated with the volume of spared hippocampus tissue. Thus, the hippocampus plays an essential role in long-term memories supporting simple picture discriminations.     

Attenuation of Morphine-Induced Behavioral Changes in Rodents byd-andl-Glucose

Administration ofd-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 administeredd-glucose with those ofl-glucose, a stereoisomer ofd-glucose that is not metabolized and does not readily cross the blood–brain barrier. Liked-glucose, though at somewhat different doses, peripherally administeredl-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 plasmad-glucose, suggesting that the effects ofl-glucose are not secondary to increased availability ofd-glucose. Using direct injections ofd- andl-glucose and morphine into the medial septum of rats, the findings indicate thatd-glucose but notl-glucose attenuated morphine-induced deficits in spontaneous alternation performance; indeed, intraseptal injections ofl-glucose alone impaired spontaneous alternation performance. These findings suggest that peripherall-glucose antagonizes morphine-induced behavioral effects by a peripheral signaling mechanism, one distinct from the mechanisms that mediate at least some of the effects ofd-glucose on brain function.     

Effects of parafascicular excitotoxic lesions on two-way active avoidance and odor-discrimination

To investigate whether the parafascicular (PF) nucleus of the thalamus is involved in different learning and memory tasks, two experiments were carried out in adult male Wistar rats that were submitted to pre-training bilateral N-methyl-d-aspartate PF infusions (0.15M, pH 7.4; 1.2 microl/side, 0.2 microl/min). In Experiment 1, we evaluated the effects of PF lesions in two identical 30-trial training sessions, separated by a 24-h interval, of a two-way active avoidance conditioning. PF-lesioned rats exhibited impaired performance in both sessions, measured by number of avoidance responses. In Experiment 2, the effects of PF lesions were assessed in a training session (5 trials) and a 24-h retention test (2 retention trials and 2 relearning trials) of an odor-discrimination task. PF lesions did not significantly disrupt the acquisition or the first retention trial, which was not rewarded. However, lesioned animals' performance was clearly affected in subsequent trials, following the introduction of the single non-rewarded trial. Current data are discussed considering evidence that lesions of the PF nucleus affect learning and memory functions mediated by anatomically related areas of the frontal cortex and striatum.     

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.     

Dissociating Basal Forebrain and Medial Temporal Amnesic Syndromes: Insights from Classical Conditioning

In humans, anterograde amnesia can result from damage to the medial temporal (MT) lobes (including hippocampus), as well as to other brain areas such as basal forebrain. Results from animal classical conditioning studies suggest that there may be qualitative differences in the memory impairment following MT vs. basal forebrain damage. Specifically, delay eyeblink conditioning is spared after MT damage in animals and humans, but impaired in animals with basal forebrain damage. Recently, we have likewise shown delay eyeblink conditioning impairment in humans with amnesia following anterior communicating artery (ACoA) aneurysm rupture, which damages the basal forebrain. Another associative learning task, a computer-based concurrent visual discrimination, also appears to be spared in MT amnesia while ACoA amnesics are slower to learn the discriminations. Conversely, animal and computational models suggest that, even though MT amnesics may learn quickly, they may learn qualitatively differently from controls, and these differences may result in impaired transfer when familiar information is presented in novel combinations. Our initial data suggests such a two-phase learning and transfer task may provide a double dissociation between MT amnesics (spared initial learning but impaired transfer) and ACoA amnesics (slow initial learning but spared transfer). Together, these emerging data suggest that there are subtle but dissociable differences in the amnesic syndrome following damage to the MT lobes vs. basal forebrain, and that these differences may be most visible in non-declarative tasks such as eyeblink classical conditioning and simple associative learning.     

Dissociating basal forebrain and medial temporal amnesic syndromes: Insights from classical conditioning

In humans, anterograde amnesia can result from damage to the medical temporal (MT) lobes (including hippocampus), as well as to other brain areas such as basal forebrain. Results from animal classical conditioning studies suggest that there may be qualitative differences in the memory impairment following MT vs. basal forebrain damage. Specifically, delay eyeblink conditioning is spared after MT damage in animals and humans, but impaired in animals with basal forebrain damage. Recently, we have likewise shown delay eyeblink conditioning impairment in humans with amnesia following anterior communicating artery (ACoA) aneurysm rupture, which damages the basal forebrain. Another associative learning task, a computer-based concurrent visual discrimination, also appears to be spared in MT amnesia while ACoA amnesics are slower to learn the discriminations. Conversely, animal and computational models suggest that, even though MT amnesics may learn quickly, they may learn qualitatively differently from controls, and these differences may result in impaired transfer when familiar information is presented in novel combinations. Our initial data suggests such a two-phase learning and transfer task may provide a double dissociation between MT amnesics (spared initial learning but impaired transfer) and ACoA amnesics (slow initial learning but spared transfer). Together, these merging data suggest that there are subtle but dissociable differences in the amnesic syndrome following damage to the MT lobes vs. basal forebrain, and that these differences may be most visible in non-declarative tasks such as eyeblink classical conditioning and simple associative learning.     

Disproportionate deficit in associative recognition relative to item recognition in global amnesia

In two experiments, we tested the hypothesis that medial temporal lobe (MTL) amnesic patients and, likewise, diencephalic (DNC) amnesic patients evidence a disproportionate deficit in memory for associations in comparison with memory for single items. In Experiment 1, we equated item recognition in amnesic and control participants and found that, under these conditions, associative recognition remained impaired both for MTL patients and for DNC patients. To rule out an alternative interpretation of the results of Experiment 1, in Experiment 2 we compared the performance of amnesic and control participants on a one-item recognition task and a two-item recognition task that required no memory for the association between members of word pairs. In the MTL group, when single-item recognition was equated to that of the controls, two-item nonassociative pair memory was equivalent as well. In the DNC group, nonassociative pair memory was impaired, but this impairment did not fully account for the impairment in associative memory. These findings indicate that memory for novel associations between items is disproportionately impaired in comparison with memory for single items in amnesia.     

Rat hippocampus and memory for places of changing significance

Rats were tested once daily on a four-choice delayed match from sample task with a water reward. Each day the correct place changed, and a single exposure to it was provided on information trials. Lesions of the hippocampal formation that involved the fornix, or dorsal hippocampus bilaterally, produced a severe impairment in the performance of previously trained rats. By contrast, lesions of the ventral hippocampus did not preclude reacquisition of the place-memory task. Some otherwise impaired rats with fornical lesions were able to find the water when aided by nonplace cues that consistently signaled reward. Reducing the number of choices from four to two did not aid the impaired rats. Certain lesions of the hippocampal formation in the rat produce a deficit appropriately described as amnesia. The memory deficit is consistent with a role for the hippocampus in processing of place information and shows some parallels to the amnesia seen in persons with temporal lobe lesions.