Sensitivity to cholinergic drug treatments of aged rats with variable degrees of spatial memory impairment

Regulation of striatal cells and goal-directed behavior by cerebellar outputs

The cerebellum integrates descending motor commands and sensory information to generate predictions and detect errors during ongoing behaviors. Cerebellar computation has been proposed to control motor but also non-motor behaviors, including reward expectation and cognitive flexibility. However, the organization and functional contribution of cerebellar output channels are incompletely understood. Here, we elaborate the cell-type specificity of a broad connectivity matrix from the deep cerebellar nuclei (DCN) to the dorsal striatum in mice. Cerebello-striatal connections arise from all deep cerebellar subnuclei and are relayed through intralaminar thalamic nuclei (ILN). In the dorsal striatum, these connections target medium spiny neurons, but also ChAT-positive interneurons, a class of tonically active interneurons implicated in shifting and updating behavioral strategies. Chemogenetic silencing of cerebello-striatal connectivity modifies function of striatal ChAT-positive interneurons. We propose that cerebello-striatal connections relay cerebellar computation to striatal circuits for goal-directed behaviors.

Cerebellar outputs contribute to motor as well as cognitive behaviors. Here, the authors elucidate the connectivity between deep cerebellar nuclei and specific cell types in the striatum via the intralaminar thalamic nucleus and the participation of this circuit in striatum-dependent behavior.

A second wind for the cholinergic system in Alzheimer's therapy

Notwithstanding tremendous research efforts, the cause of Alzheimer's disease (AD) remains elusive and there is no curative treatment. The cholinergic hypothesis presented 35 years ago was the first major evidence-based hypothesis on the etiology of AD. It proposed that the depletion of brain acetylcholine was a primary cause of cognitive decline in advanced age and AD. It relied on a series of observations obtained in aged animals, elderly, and AD patients that pointed to dysfunctions of cholinergic basal forebrain, similarities between cognitive impairments induced by anticholinergic drugs and those found in advanced age and AD, and beneficial effects of drugs stimulating cholinergic activity. This review revisits these major results to show how this hypothesis provided the drive for the development of anticholinesterase inhibitor-based therapies of AD, the almost exclusively approved treatment in use despite transient and modest efficacy. New ideas for improving cholinergic therapies are also compared and discussed in light of the current revival of the cholinergic hypothesis on the basis of two sets of evidence from new animal models and refined imagery techniques in humans. First, human and animal studies agree in detecting signs of cholinergic dysfunctions much earlier than initially believed. Second, alterations of the cholinergic system are deeply intertwined with its reactive responses, providing the brain with efficient compensatory mechanisms to delay the conversion into AD. Active research in this field should provide new insight into development of multitherapies incorporating cholinergic manipulation, as well as early biomarkers of AD enabling earlier diagnostics. This is of prime importance to counteract a disease that is now recognized to start early in adult life.

Development of an in-vivo active reversible butyrylcholinesterase inhibitor

Alzheimer’s disease (AD) is characterized by severe basal forebrain cholinergic deficit, which results in progressive and chronic deterioration of memory and cognitive functions. Similar to acetylcholinesterase, butyrylcholinesterase (BChE) contributes to the termination of cholinergic neurotransmission. Its enzymatic activity increases with the disease progression, thus classifying BChE as a viable therapeutic target in advanced AD. Potent, selective and reversible human BChE inhibitors were developed. The solved crystal structure of human BChE in complex with the most potent inhibitor reveals its binding mode and provides the molecular basis of its low nanomolar potency. Additionally, this compound is noncytotoxic and has neuroprotective properties. Furthermore, this inhibitor moderately crosses the blood-brain barrier and improves memory, cognitive functions and learning abilities of mice in a model of the cholinergic deficit that characterizes AD, without producing acute cholinergic adverse effects. Our study provides an advanced lead compound for developing drugs for alleviating symptoms caused by cholinergic hypofunction in advanced AD.

Hippocampal theta, gamma, and theta-gamma coupling: effects of aging, environmental change, and cholinergic activation

Hippocampal theta and gamma oscillations coordinate the timing of multiple inputs to hippocampal neurons and have been linked to information processing and the dynamics of encoding and retrieval. One major influence on hippocampal rhythmicity is from cholinergic afferents. In both humans and rodents, aging is linked to impairments in hippocampus-dependent function along with degradation of cholinergic function. Cholinomimetics can reverse some age-related memory impairments and modulate oscillations in the hippocampus. Therefore, one would expect corresponding changes in these oscillations and possible rescue with the cholinomimetic physostigmine. Hippocampal activity was recorded while animals explored a familiar or a novel maze configuration. Reexposure to a familiar situation resulted in minimal aging effects or changes in theta or gamma oscillations. In contrast, exploration of a novel maze configuration increased theta power; this was greater in adult than old animals, although the deficit was reversed with physostigmine. In contrast to the theta results, the effects of novelty, age, and/or physostigmine on gamma were relatively weak. Unrelated to the behavioral situation were an age-related decrease in the degree of theta-gamma coupling and the fact that physostigmine lowered the frequency of theta in both adult and old animals. The results indicate that age-related changes in gamma and theta modulation of gamma, while reflecting aging changes in hippocampal circuitry, seem less related to aging changes in information processing. In contrast, the data support a role for theta and the cholinergic system in encoding and that hippocampal aging is related to impaired encoding of new information.

Different effects of scopolamine and inhibition of prolyl oligopeptidase on mnemonic and motility functions of young and 8- to 9-month-old rats in the radial-arm maze

Prolyl oligopeptidase (POP) has been connected to memory and mood through regulation of the brain levels of its biologically active peptide substrates and phosphatidylinositol system. This is the first study in a radial-arm maze of the effects of a single dose of a novel potent prolyl oligopeptidase inhibitor, KYP-2047 (5 mg/kg, dissolved in 5% Tween 80), on memory and learning of scopolamine-treated (0.4 mg/kg, dissolved in saline) rats. Habituated (days 1 and 2) and trained (days 3-11) young (3 months) and old (8-9 months) male Wistar rats were given (i) saline + Tween, (ii) saline + KYP-2047, (iii) scopolamine + Tween or (iv) scopolamine + KYP-2047 30 min. prior to testing their memory. Food rewards located in four randomly chosen arms of the maze. The rat had 10 min. to find and eat the rewards. Time spent in the maze, visits to each arm and number of eaten rewards were measured. Old rats made generally more errors, spent more time and visited fewer arms per minute in the maze than young rats. The memory- and function-impairing effects of scopolamine were also seen more clearly in old than young rats. KYP-2047 had no or only a marginal effect on memory of either age group, but when given without scopolamine, it slightly increased the maze motility of young rats and decreased the motility of old rats. In a separate locomotor activity test, KYP-2047 enhanced the motility of young rats supporting a suggested role of POP in motor functions.

Long-term treatment of l-3-n-butylphthalide attenuated neurodegenerative changes in aged rats

It is shown that l-3-n-butylphthalide (l-NBP), the isomer of dl-NBP (racemic 3-n-butylphthalide, a new anti-cerebral ischemic agent) significantly attenuated cerebral hypoperfusion-induced learning dysfunction and brain damage in rats. In the present study, l-NBP (10 and 30 mg/kg) long-term (3-month) treatment of aged rat (21-month-old) significantly improved the learning and memory capability measured by the Morris water maze test. Hematoxylin-eosin-stained slices showed that both l-NBP at 30 mg/kg, and memantine as control at 20 mg/kg, attenuated the neurodegenerative changes in aged rats. L: -NBP treatment significantly increased the choline acetyltransferase activity and dose-dependently decreased the acetylcholinesterases activity in the hippocampus of aged rats. The immunohistological study demonstrated that expressions of beta-secretase and hyperphosphorylated tau protein were significantly increased in the hippocampus CA1 subfield and parietal cortex in aged rats. However, they were decreased significantly by treatment of l-NBP and memantine for 3 months. Our results indicated that long-term treatment with l-NBP might prevent age-related neurodegenerative changes by modulation of cholinergic system, reduction of phosphorylated tau and maintain structure and morphology of neurons. Therefore, l-NBP might be a potential drug for treatment of senile dementia.

Hippocampal lesions in rats impair learning and memory for locations on a touch-sensitive computer screen: the "ASAT" task

It has been repeatedly demonstrated across species that the hippocampus is critical for spatial learning and memory. Consequently, numerous paradigms have been created to study spatial learning in the rodent. Most of these tasks, such as the Morris water maze, 8-arm radial maze, and T-maze, are non-automated procedures. It was our goal to create an automated task in the rodent that is quickly learned, hippocampal-dependent, and minimizes the confounding variables present in most tests measuring hippocampal-dependent learning and memory. To accomplish this, we created a novel search task using a standard operant box fitted with a touch-sensitive computer monitor. Subjects were required to locate an S+ "hidden" amongst other identical stimuli on the monitor. In two versions of the task the S+ stayed in the same location within a session but shifted location between sessions. In a third version of the task the S+ was moved to a new location after every 10 trials. It was found that the location of the S+ was quickly acquired each day (within 10 trials), and that the hippocampal-lesion group was impaired when compared to their control cohort. With the benefits inherent in automation, these tasks confer significant advantages over traditional tasks used to study spatial learning and memory in the rodent. When combined with previously developed non-spatial cognitive tests that can also be run in the touch-screen apparatus, the result is a powerful cognitive test battery for the rodent.

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.

Delayed evacuatory function due to specific smooth muscle reactivity in the gastrointestinal tracts of tacrine-treated rats

Most of the side effects induced by tacrine are associated with the gastrointestinal (GI) tract. The aim of the study was to analyze the nature of radiographically registered, tacrine-induced changes in evacuatory function, as well as to find a possible correlation with the immediate in vitro action of the drug on smooth muscles from the GI tracts of rats. The tacrine dose we used reliably delayed GI passage: contrast matter was not fully evacuated, predominantly from the stomach and cecum. The delay resulted from changes in tone and peristaltic activity, specific for the various regions of the tract. These changes were associated with a superposing of the responses due to the anticholinesterase and noncholinergic action of tacrine.

Effects of pilocarpine on the cortical and hippocampal theta rhythm in different vigilance states in rats

It has been suggested that theta rhythm gates the flow of information between the hippocampus and cortex during memory processes. The cholinergic system plays an important role in regulating vigilance states and in generating theta rhythm. This study aims to analyse the effects of the muscarinic agonist pilocarpine (120 and 360 microg, i.c.v.) on hippocampal and frontal cortical theta rhythm during several vigilance states in rats. Pilocarpine injection increased the duration and number of episodes with theta activity, particularly when theta rhythm appeared during waking states in the cortex and hippocampus simultaneously. It seems that the effects of pilocarpine are related to the appearance of cortical theta activity in waking states, and suggest that pilocarpine could modify the transference rate of information from the hippocampus to cortex in rats during wakefulness states, in relation to the postulated effect of cholinergic system modulating memory consolidation.

Hippocampal gene expression profiling reveals the possible involvement ofHomer1andGABABreceptors in scopolamine-induced amnesia

Scopolamine-treated rats are commonly used as a psychopharmacological model of memory dysfunction and have been extensively studied to establish the effectiveness of acetylcholinesterase inhibitors in the treatment of Alzheimer's disease. Scopolamine is a muscarinic acetylcholine receptor antagonist that induces memory deficits in young subjects similar to those occurring during aging. The amnesic effect of scopolamine is well established but the molecular and cellular mechanisms that sustain its neuropharmacological action are still unclear. The present genome wide study investigates hippocampal gene expression profiling in scopolamine-treated adult rats following stimulation in a spatial memory task. Using microarray and quantitative real-time RT-PCR approaches, we identified several genes previously known to be associated with memory processes (Homer1, GABA(B) receptor, early growth response 1, prodynorphin, VGF nerve growth factor inducible) and multiple novel candidate genes possibly involved in cognition (including calcium/calmodulin-dependent protein kinase kinase 2, dual specificity phosphatase 5 and 6, glycophorin C) that were altered following scopolamine treatment. Moreover, we found that stable over-expression of glutamatergic components Homer1a and 1c in the hippocampus of adult rats induced by recombinant adeno-associated virus vector abolished memory improvement produced by the GABA(B) receptor antagonist SGS742 in scopolamine-treated rats. Taken together, these results reveal novel genes and mechanisms involved in scopolamine-induced amnesia, and demonstrate the involvement of both GABA and glutamate neurotransmission in this animal model of cognitive dysfunctions.

Animal models of behavioral dysfunctions: Basic concepts and classifications, and an evaluation strategy

In behavioral neurosciences, such as neurobiology and biopsychology, animal models make it possible to investigate brain-behavior relations, with the aim of gaining insight into normal and abnormal human behavior and its underlying neuronal and neuroendocrinological processes. Different types of animal models of behavioral dysfunctions are reviewed in this article. In order to determine the precise criteria that an animal model should fulfill, experts from different fields must define the desired characteristics of that model at the neuropathologic and behavioral level. The list of characteristics depends on the purpose of the model. The phenotype-abnormal behavior or behavioral dysfunctions-has to be translated into testable measures in animal experiments. It is essential to standardize rearing, housing, and testing conditions, and to evaluate the reliability, validity (primarily predictive and construct validity), and biological or clinical relevance of putative animal models of human behavioral dysfunctions. This evaluation, guided by a systematic strategy, is central to the development of a model. The necessity of animal models and the responsible use of animals in research are discussed briefly.

Choline pivaloyl ester strengthened the benefit effects of Tacrine and Galantamine on electroencephalographic and cognitive performances in nucleus basalis magnocellularis-lesioned and aged rats

The aim of the present work was the assessment of the effects produced on the electroencephalographic (EEG) activity and the cognitive and memory performances of nucleus basalis magnocellularis (NBM)-lesioned or aged rats by the combined treatment with [2-(2,2-dimethylpropionyloxy)ethyl]trimethylammonium 2,2-dimethylpropionate (choline pivaloyl ester) (CPE) and the Cholinesterase inhibitors (ChEIs) Tacrine (THA) and Galantamine (GAL). Intraperitoneal administration of CPE combined with THA or GAL to both NBM-lesioned or aged rats, produced EEG desynchronisation, and a significant decrease in the energy of the total EEG spectrum and the lower frequency bands (delta 0.25-3 and theta 4-7 Hz) lasting many minutes. Furthermore, drug associations reversed in aged rats the scopolamine (0.2 mg/kg, i.p.)-induced increase in EEG power, slow waves and high-voltage spindle (HVS). Furthermore, the combined administration of CPE and Cholinesterase inhibitors in both NBM-lesioned or aged animals, improved performances in all behavioural tasks, enhancing object discrimination, increasing locomotory activity and alternation choice in T-maze, ameliorating retention in passive avoidance and decreasing escape latency in Morris water maze. In all test, AChEIs and CPE combinations proved to be more effective than CPE, THA or GAL given alone. In conclusion, the present work shows the ability of choline pivaloyl ester in strengthening the positive cerebral activity of THA and GAL.

Two months makes a difference in spatial orientation learning in very old FBNF1 rats

Age-related changes in cognitive performance may be more pronounced in the period near or exceeding the median life span. Therefore, we compared the acquisition of a Morris water escape task by two groups of very old Fischer344 x Brown Norway hybrid rats. The mean age difference between the two groups of rats (a 33- to 34-month-old group versus a 35- to 36-month-old group) was about 2 months. Both groups of rats initially had the same level of performance, but then the younger group learned to escape onto the submerged platform faster, swimming a shorter distance, than the older group. By the fifth acquisition session, the younger rats needed only half the time and swam a shorter distance before they reached the platform than the older rats. These differences in learning were not due to different locomotor abilities as both groups had a similar swimming speed. These results suggest that age-related changes in cognitive performance are indeed more pronounced in the period around the median life span. We also discussed different set-ups to perform cross-sectional age-comparison studies. If there are not sufficient animals from one batch, it may be worthwhile to combine animals from different batches per age group, provided that breeding, rearing, housing, and testing conditions are highly standardized.

Pilocarpine improves olfactory discrimination and social recognition memory deficits in 24 month-old rats

Muscarinic receptor agonists have been suggested as potential drugs to counteract age-related cognitive decline since critical changes in cholinergic system occur with aging. Recently, we demonstrated that single administration of the non-selective muscarinic receptor agonist pilocarpine prevents age-related spatial learning impairments in rats. In addition, increasing evidence suggests that areas in the central nervous system processing olfactory information are affected at the early stages of age-related diseases, such as Alzheimer's disease, and that specific olfactory testing may represent an important tool in the diagnosis of these diseases. In the present study, olfactory discrimination and short-term social memory of 3 and 24 month-old rats were assessed with the olfactory discrimination and social recognition memory tasks, respectively. The actions of the repeated treatment with pilocarpine (30 mg/kg, i.p.; once per day for 21 days) in relation to age-related effects on olfactory and cognitive functions were also studied. The 24 month-old rats exhibited significantly impaired performance in both models, demonstrating deficits in their odour discrimination and in their ability to recognize a juvenile rat after a short period of time. The treatment with pilocarpine improved in a specific manner these age-related deficits in 24 month-old rats without altering their motor performance. The present results extend the notion of the participation of muscarinic receptors in control of olfactory functions and reinforce the potential of muscarinic receptor agonists for the treatment of age-related cognitive decline.

Protective effect of the inhibition of the renin-angiotensin system on aging

Experimental studies indicate that chronic long-term inhibition of the renin-angiotensin system (RAS) can prevent most of the deleterious effects due to aging in the cardiovascular system and in the kidney of the normal mouse and rat. In this review, all the information available on this subject provided by several studies performed by our research group during the last years is been described. Treatment was initiated either after weaning or at 12 months of age that is about half the normal life span of the rat. A converting enzyme inhibitor: enalapril or an angiotensin II type 1 (AT1) receptor blocker: losartan were used to inhibit the RAS. Cognitive behaviour, emotionality, and locomotor activity were also determined at 10 and 18 months of age in treated since weaning and untreated control rats to elucidate the participation of angiotensin II in memory disfunction. A similar observation was obtained in animals treated from 12 to 18 months of age. Results have demonstrated a significant protective effect on the function and the structure of the cardiovascular system, the kidney and the brain in all the treated animals. Damage observed at 12 months of age was not very significant, but treatment stop further deterioration that was evident in untreated animals. The similarity of the results detected with either enalapril or losartan treatment, clearly indicates that most of the effects are exerted through AT1 receptors. Analysis of the nitric oxide and antioxidant enzymes systems suggest that the protective effect is related to an antioxidant action of the RAS inhibitors and a reduced formation of reactive oxygen species. AngII inhibition might produce changes in the mechanisms of oxidative stress specially at the mitochondrial level. Prevention of mitochondrial decrease and/or damage would be related with the delay of the normal aging process.

SSR181507, a dopamine D(2) receptor antagonist and 5-HT(1A) receptor agonist, alleviates disturbances of novelty discrimination in a social context in rats, a putative model of selective attention deficit

RATIONALE

Selective attention deficit, characterised by the inability to differentiate relevant from irrelevant information, is considered to underlie many cognitive deficits of schizophrenia, and appears to be only marginally responsive to treatment with current antipsychotics.

OBJECTIVES

We compared the activity of the putative atypical antipsychotic SSR181507 (a dopamine D(2) receptor antagonist and 5HT(1A) receptor agonist) with reference compounds, on disturbances of novelty discrimination in a social context in rats, a behavioural paradigm that putatively models selective attention deficit.

METHODS

A first (familiar) juvenile rat was presented to an adult rat for a period (P1) of 30 min. A second (novel) juvenile was then introduced at the end of P1 for a period (P2) of 5 min. The ability of the adult rat to discriminate between the two juveniles, presented at the same time, was evaluated by measuring the ratio of the time spent in interaction with the novel vs the familiar juvenile during P2.

RESULTS

Adult rats spent more time exploring the novel than the familiar juvenile. This novelty discrimination capacity was disrupted by: (1) parametric modification of the procedure (reduction of time spent in contact with the familiar juvenile during P1); (2) acute injection of psychotomimetics that are known to induce schizophrenia-like symptoms in humans, such as phencyclidine (PCP; 3 mg/kg, i.p.) and d-amphetamine (1 mg/kg, i.p.) and (3) neonatal treatment with PCP (three injections of 10 mg/kg, s.c.), a model based on the neurodevelopmental hypothesis of schizophrenia. The potential atypical antipsychotic SSR181507 (0.03-3 mg/kg, i.p.) and the atypical antipsychotics clozapine (0.1-1 mg/kg, i.p.) and amisulpride (1-3 mg/kg, i.p.) attenuated deficits in novelty discrimination produced by parametric manipulation and by acute or neonatal treatment with PCP. The typical antipsychotic haloperidol (up to 0.3 mg/kg, i.p.) attenuated only deficits in novelty discrimination produced by parametric modification.

CONCLUSION

Collectively, these results suggest that SSR181507 can alleviate disturbances of novelty discrimination in a social context in rats, and that this paradigm may represent a suitable animal model of selective attention deficits observed in schizophrenia.

Cognitive deficits in aged rats correlate with levels of L-arginine, not with nNOS expression or 3,4-DAP-evoked transmitter release in the frontoparietal cortex

Aging is associated with altered neurotransmitter function in the brain. In this study, we measured release parameters for acetylcholine (ACh), norepinephrine and serotonin in the frontoparietal cortex of young and aged rats. We also determined cortical amino acid concentrations and nitric oxide (NO) synthase function. Prior to sacrifice, the rats had been tested for Morris water-maze performance. In aged, compared with young rats, we observed a reduction in both uptake of choline and acetylcholine release. Serotonin release and L-arginine concentrations (a precursor of NO) showed an aging-related increase; however, L-citrulline/L-arginine ratios were decreased in aged rats. Moreover, while most age-related changes in transmitter release or neurochemical markers were not related to the learning performance, L-arginine concentrations were positively correlated to cognitive deficits. NO synthase concentrations were not affected by aging. It is suggested that events related to L-arginine-to-L-citrulline/NO metabolism in the frontoparietal cortex may take part in age-related cognitive deficits.

Behavioral Effects of Basal Forebrain Cholinergic Lesions in Young Adult and Aging Rats

The interactive effects of age and cholinergic damage were assessed behaviorally in young and middle-aged rats. Rats were lesioned at either 3 or 17 months of age by injection of 192 IgG-saporin immunotoxin into the medial septum and the nucleus basalis magnocellularis, and they were then tested on a range of behavioral tasks: a nonmatching-to-position task in a T-maze, an object-recognition task, an object-location task, and an open-field activity test. Depending on the task used, only an age or a lesion effect was observed, but there was no Age X Lesion interaction. Middle-aged and young rats responded to the cholinergic lesions in the same manner. These results show that in the middle-aged rats in which cholinergic transmission was affected, additional injury to the system was not always accompanied by major cognitive dysfunctions.

Pentyl-4-yn-valproic acid reverses age-associated memory impairment in the Wistar rat

Pentyl-4-yn-valproic acid (VPA), a cognition-enhancing agent whose mode of action has been attributed to cell adhesion molecule-mediated neuritogenesis, has been shown to enhance hippocampus-dependent spatial learning. Here, we investigated its potential to reverse age-related memory impairment that relates mainly to declarative memory. Aged spatial learning deficits in the water maze paradigm were demonstrated by swim angle analysis, the angle between axes of start-to-platform and start-to-animal position, and latency to reach a submerged platform. Chronic pentyl-4-yn-VPA administration mediated a significant improvement in both search strategy and latency to find the submerged platform in aged animals. Pentyl-4-yn-VPA also facilitated task recall in aged animals as evidenced by increased time in the target quadrant during a probe trial 3 days following the final training session. The action of pentyl-4-yn-VPA on platform latency, search strategy and task recall suggests that this agent may have great benefit in the treatment of age-dependent cognitive decline.

3,4-DAP-evoked transmitter release in hippocampal slices of aged rats with impaired memory

Based on a slice superfusion technique, this study investigated the release of acetylcholine, noradrenaline and serotonin in the hippocampus of aged rats (25-27 months) showing no or severe deficits in a spatial reference-memory task (water maze). Young adults (3-5 months) were used as controls. 3,4-diaminopyridine (3,4-DAP), a potassium channel antagonist which increases neuronal excitability, was used to evoke the overflow of the three neurotransmitters. The release of [3H]noradrenaline induced by stimulation of presynaptic nicotinic receptors was also assessed. The experiment compared the accumulation and 3,4-DAP-evoked (or nicotine-evoked) overflow of [3H] in hippocampal slices preincubated with [3H]choline, [3H]noradrenaline, or [3H]serotonin. In aged rats, only the accumulation of [3H]serotonin was reduced significantly (-17%). In percent of tissue-[3H], the 3,4-DAP-evoked overflow of [3H]serotonin was increased (+28%), and that of [3H]acetylcholine was reduced (-23%) in the aged rats. The nicotine-evoked overflow of [3H]noradrenaline was not altered in aged rats. There was a significant correlation of water-maze performance (distance to platform) and evoked overflow of [3H]serotonin. It is concluded that hippocampal cholinergic functions are more altered by aging than noradrenergic or serotonergic ones. Excessive excitability of serotonergic terminals, perhaps in addition to cholinergic dysfunction, might be a crucial factor accounting for age-related cognitive deficits in the present population of rats.

Protective effect of 5-HT1B receptor gene deletion on the age-related decline in spatial learning abilities in mice

We previously observed that 5 months old serotonin 1B receptor knockout (5-HT1BKO) mice exhibited a facilitation of learning in a long-term spatial memory task in a water maze. In this study, we attempted to assess whether this effect might persist during aging. We compared the performances of young-adult (3 months old) and aged (22 months old) 5-HT1BKO and wild type (WT) mice in the same task. Young-adult and aged KO mice exhibited facilitated acquisition of the reference memory task as compared to their respective WT controls. Generally, the performance of aged KO was similar to that of young-adult WT on the parameters defining performance and motor (swim speed) aspects of the task. During probe trials, all mice presented a spatial selectivity, which was, however, less pronounced in aged than in young-adult WT. No such age-related effect was observed in KO mice. In a massed spatial learning task, aged KO and WT mice globally exhibited the same level of performance. Nevertheless, young-adult and aged KO mice were superior to their WT controls as concerns the working memory component of the task. The data suggest that 5-HT1BKO mice are more resistant than WT to age-related memory decline as concerns both reference/long-term and working/short-term spatial memory.

Presynaptic modulation of acetylcholine, noradrenaline, and serotonin release in the hippocampus of aged rats with various levels of memory impairments

Aged (25-27 months) Long-Evans female rats were distinguished according to whether they showed no significant impairment (AU), moderate impairment (AMI), or severe impairment (ASI) in a spatial reference-memory task. Young (3-5 months) rats served as controls. Electrically evoked overflow of tritium was assessed in hippocampal slices preloaded with [3H]choline or [3H]serotonin (5-HT). Nicotine-evoked overflow of tritium was measured after preloading with [3H]noradrenaline (NA). Choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activity, and concentration of monoamines were assessed in homogenates. Aged rats exhibited reduced accumulation of [3H]choline and [3H]5-HT, increased accumulation of [3H]NA, and weaker electrically evoked overflow of [3H]acetylcholine ([3H]ACh) and [3H]5-HT. The overflow of [3H]NA was not altered consistently by aging. Roughly, drugs acting presynaptically had comparable effects in aged rats: oxotremorine and CP 93,129 inhibited the overflow of [3H]ACh, CP 93,129 and UK 14,304 reduced that of [3H]5-HT. ChAT or AChE activity, and 5-HT concentration were not changed by age; NA concentration was reduced. When significant, changes were comparable in AU, AMI, and ASI rats. Data show that aging alters cholinergic and serotonergic hippocampal innervations, release of ACh and 5-HT, but not presynaptic release-modulating mechanisms. These alterations do not account for variability in water-maze performance of aged rats.

Presynaptic regulation of neurotransmitter release in the cortex of aged rats with differential memory impairments

Cluster analysis of water-maze reference-memory performances of 25-27-month-old (compared to 3-5-month-old) rats distinguished subpopulations of young adult rats (YOUNG), aged rats with no significant impairment (AU), aged rats with moderate impairment (AMI), and aged rats with severe impairment (ASI). In the frontoparietal cortex, we subsequently assessed the electrically evoked release of tritium in slices preloaded with [3H]choline, [3H]noradrenaline (NA), or [3H]serotonin (5-HT) and the effects of an agonist (oxotremorine, UK 14,304, and CP 93,129) of the respective autoreceptors. Cholinergic and monoaminergic markers were measured in homogenates. Overall, aged rats exhibited reduced accumulation of [3H]choline (-25%) and weaker evoked transmitter release (in % of accumulated tritium: -44%, -20%, and -34%, for [3H]acetylcholine, [3H]NA, and [3H]5-HT, respectively). In all rats, the inhibitory effects of the autoreceptor agonists on the evoked release of [3H] were comparable. Acetylcholinesterase (AChE), not choline acetyltransferase (ChAT), activity was reduced. The results suggest age-related modifications in the cholinergic, noradrenergic, and serotonergic innervation of the frontoparietal cortex, alterations of evoked transmitter release, but no interference with presynaptic autoinhibition of the release. Neither of these alterations seemed to account for the cognitive impairment assessed.

The memory effects of general anesthesia persist for weeks in young and aged rats

Studies demonstrate lasting cognitive impairment in elderly persons after anesthesia and surgery. We tested the hypothesis that general anesthesia contributes to this cognitive impairment. Six- and 18-mo-old Fischer 344 rats were trained in a 12-arm radial arm maze and were then randomized to anesthesia for 2 h with 1.2% isoflurane/70% nitrous oxide/30% oxygen or a control treatment consisting of 30% oxygen. Rats recovered for 24 h and then were tested daily on the radial arm maze for 8 wk. Performance of young control rats was stable throughout the experiment. In contrast, aged control rats improved their performance as measured by time to complete the maze but not by error rate. After anesthesia, time to complete the maze did not change in young rats, but error rate decreased (P < 0.05 at 1 and 3 wk), indicating improved performance. In contrast, previously anesthetized aged rats failed to improve with repeated testing and took longer to complete the maze than aged control rats (P < 0.05 at 1 and 3 wk). These data demonstrate that general anesthesia with isoflurane and nitrous oxide improves the memory performance on an established spatial memory task in young rats, but in aged rats it attenuates the improvement in performance that otherwise occurs with repeated testing. Therefore, isoflurane and nitrous oxide anesthesia produces a sustained learning impairment in aged rats.

IMPLICATIONS

This study demonstrates that general anesthesia with isoflurane and nitrous oxide improves spatial memory in young rats but impairs it in aged rats for at least 3 wk, indicating that it can influence memory for much longer than previously recognized and may adversely affect memory processes in the aged.

Baseline and 8-OH-DPAT-induced release of acetylcholine in the hippocampus of aged rats with different levels of cognitive dysfunction

During aging, neurotransmission systems such as the cholinergic and serotonergic ones are altered. Using rats aged 3 or 24-26 months, this study investigated whether the well-described 8-OH-DPAT-induced increase of hippocampal acetylcholine release was altered in aged rats and whether it may vary according to the magnitude of age-related cognitive deficits. Long-Evans female rats aged 24-26 months were classified as good or bad performers on the basis of their reference-memory performance in a Morris water-maze task. Subsequently, the efficiency of 5-HT(1A) receptor agonist 8-OH-DPAT (0.5 mg/kg, s.c.) in triggering hippocampal acetylcholine release was evaluated by in vivo microdialysis and high performance liquid chromatography analysis. Besides a reduced baseline release in aged rats and a correlation between the baseline release and probe-trial performance in all rats, the results demonstrated that 8-OH-DPAT produced a significant increase of hippocampal acetylcholine release (peak value) in all rats, whether aged or young. While significant in bad performers (+56%), this increase did not reach significance in good performers (+32%). The results suggest that (i) some aspects of cognitive alterations related to aging might be linked to the baseline release of acetylcholine in the hippocampus, and (ii) the cholinergic innervation of the hippocampus of aged rats responds almost normally to systemic activation of 5-HT(1A) receptors, and (iii) differential alterations of cholinergic/serotonergic interactions assessed by determination of the 8-OH-DPAT-induced release of acetylcholine in the hippocampus could not be linked with clarity to the cognitive status of aged rats.

Intrastriatal infusions of methoctramine improve memory in cognitively impaired aged rats

Alterations of striatal cholinergic markers may correlate with cognitive impairments in aged rats. M2 muscarinic receptors were found to be presynaptic inhibitory autoreceptors on striatal cholinergic interneurons. The effect of bilateral intrastriatal infusions of the M2 muscarinic receptor antagonist methoctramine was assessed, in cognitively impaired aged (24-26 months) Long-Evans female rats, on memory performances in a water maze. Compared with vehicle infusions, methoctramine injected bilaterally (1 microg/side) in the dorsolateral striatum, significantly improved procedural memory performance while having no effect on spatial working memory. Our results suggest that, in cognitively impaired aged rats, the blockade of M2 muscarinic receptors in the dorsolateral striatum improves procedural memory probably by enhancing the release of acetylcholine.

Applications of the Morris water maze in the study of learning and memory

The Morris water maze (MWM) was described 20 years ago as a device to investigate spatial learning and memory in laboratory rats. In the meanwhile, it has become one of the most frequently used laboratory tools in behavioral neuroscience. Many methodological variations of the MWM task have been and are being used by research groups in many different applications. However, researchers have become increasingly aware that MWM performance is influenced by factors such as apparatus or training procedure as well as by the characteristics of the experimental animals (sex, species/strain, age, nutritional state, exposure to stress or infection). Lesions in distinct brain regions like hippocampus, striatum, basal forebrain, cerebellum and cerebral cortex were shown to impair MWM performance, but disconnecting rather than destroying brain regions relevant for spatial learning may impair MWM performance as well. Spatial learning in general and MWM performance in particular appear to depend upon the coordinated action of different brain regions and neurotransmitter systems constituting a functionally integrated neural network. Finally, the MWM task has often been used in the validation of rodent models for neurocognitive disorders and the evaluation of possible neurocognitive treatments. Through its many applications, MWM testing gained a position at the very core of contemporary neuroscience research.

Immunohistochemical and neurochemical correlates of learning deficits in aged rats

This study examined whether cholinergic and monoaminergic dysfunctions in the brain could be related to spatial learning capabilities in 26-month-old, as compared to three-month-old, Long-Evans female rats. Performances were evaluated in the water maze task and used to constitute subgroups with a cluster analysis statistical procedure. In the first experiment (histological approach), the first cluster contained young rats and aged unimpaired rats, the second one aged rats with moderate impairment and the third one aged rats with severe impairment. Aged rats showed a reduced number of choline acetyltransferase- and p75(NTR)-positive neurons in the nucleus basalis magnocellularis, and choline acetyltransferase-positive neurons in the striatum. In the second experiment (neurochemical approach), the three clusters comprised young rats, aged rats with moderate impairment and aged rats with severe impairment. Alterations related to aging consisted of reduced concentration of acetylcholine, norepinephrine and serotonin in the striatum, serotonin in the occipital cortex, dopamine and norepinephrine in the dorsal hippocampus, and norepinephrine in the ventral hippocampus. In the first experiment, there were significant correlations between water maze performance and the number of; (i) choline acetyltransferase- and p75(NTR)-positive neurons in the nucleus basalis magnocellularis; (ii) choline acetyltransferase-positive neurons in the striatum and; (iii) p75(NTR)-positive neurons in the medial septum. In the second experiment, water maze performance was correlated with the concentration of; (i) acetylcholine and serotonin in the striatum; (ii) serotonin and norepinephrine in the dorsal hippocampus; (iii) norepinephrine in the frontoparietal cortex and; (iv) with other functional markers such as the 5-hydroxyindoleacetic acid/serotonin ratio in the striatum, 3,4-dihydroxyphenylacetic acid/dopamine ratio in the dorsal hippocampus, 5-hydroxyindoleacetic acid/serotonin and homovanillic acid/dopamine ratios in the frontoparietal cortex, and 3,4-dihydroxyphenylacetic acid/dopamine ratio in the occipital cortex. The results indicate that cognitive deficits related to aging might involve concomitant alterations of various neurochemical systems in several brain regions such as the striatum, the hippocampus or the cortex. It also seems that these alterations occur in a complex way which, in addition to the loss of cholinergic neurons in the basal forebrain, affects dopaminergic, noradrenergic and serotonergic processes.