Behavioral screening for cognition enhancers: from indiscriminate to valid testing: Part II

Hippocampal ensemble dynamics and memory performance are modulated by respiration during encoding

During offline brain states, such as sleep and memory consolidation, respiration coordinates hippocampal activity. However, the role of breathing during online memory traces remains unclear. Here, we show that respiration can be recruited during online memory encoding. Optogenetic manipulation was used to control activation of the primary inspiratory rhythm generator PreBötzinger complex (PreBötC) in transgenic mice. When intermittent PreBötC-induced apnea covered the object exploration time during encoding, novel object detection was impaired. Moreover, the mice did not exhibit freezing behavior during presentation of fear-conditioned stimuli (CS⁺) when PreBötC-induced apnea occurred at the exact time of encoding. This apnea did not evoke changes in CA3 cell ensembles between presentations of CS⁺ and conditioned inhibition (CS^-), whereas in normal breathing, CS⁺ presentations produced dynamic changes. Our findings demonstrate that components of central respiratory activity (e.g., frequency) during online encoding strongly contribute to shaping hippocampal ensemble dynamics and memory performance.

Introduction of a pharmacological neurovascular uncoupling model in rats based on results of mice

Our aim was to establish a pharmacologically induced neurovascular uncoupling (NVU) method in rats as a model of human cognitive decline. Pharmacologically induced NVU with subsequent neurological and cognitive defects was described in mice, but not in rats so far. We used 32 male Hannover Wistar rats. NVU was induced by intraperitoneal administration of a pharmacological "cocktail" consisting of N-(methylsulfonyl)-2-(2-propynyloxy)-benzenehexanamide (MSPPOH, a specific inhibitor of epoxyeicosatrienoic acid-producing epoxidases, 5 mg kg-1), L-NG-nitroarginine methyl ester (L-NAME, a nitric oxide synthase inhibitor, 10 mg kg-1) and indomethacin (a nonselective inhibitor of cyclooxygenases, 1 mg kg-1) and injected twice daily for 8 consecutive days. Cognitive performance was tested in the Morris water-maze and fear-conditioning assays. We also monitored blood pressure. In a terminal operation a laser Doppler probe was used to detect changes in blood-flow (CBF) in the barrel cortex while the contralateral whisker pad was stimulated. Brain and small intestine tissue samples were collected post mortem and examined for prostaglandin E2 (PGE2) level. Animals treated with the "cocktail" showed no impairment in their performance in any of the cognitive tasks. They had higher blood pressure and showed cca. 50% decrease in CBF. Intestinal bleeding and ulcers were found in some animals with significantly decreased levels of PGE2 in the brain and small intestine. Although we could evoke NVU by the applied mixture of pharmacons, it also induced adverse side effects such as hypertension and intestinal malformations while the treatment did not cause cognitive impairment. Thus, further refinements are still required for the development of an applicable model.

MAM and C99, key players in the pathogenesis of Alzheimer's disease

Inter-organelle communication is a rapidly-expanding field that has transformed our understanding of cell biology and pathology. Organelle-organelle contact sites can generate transient functional domains that act as enzymatic hubs involved in the regulation of cellular metabolism and intracellular signaling. One of these hubs is located in areas of the endoplasmic reticulum (ER) connected to mitochondria, called mitochondria-associated ER membranes (MAM). These MAM are transient lipid rafts intimately involved in cholesterol and phospholipid metabolism, calcium homeostasis, and mitochondrial function and dynamics. In addition, γ-secretase-mediated proteolysis of the amyloid precursor protein 99-aa C-terminal fragment (C99) to form amyloid β also occurs at the MAM. Our most recent data indicates that in Alzheimer's disease, increases in uncleaved C99 levels at the MAM provoke the upregulation of MAM-resident functions, resulting in the loss of lipid homeostasis, and mitochondrial dysfunction. Here, we discuss the relevance of these findings in the field, and the contribution of C99 and MAM dysfunction to Alzheimer's disease neuropathology.

Rescuing the attentional performance of rats with cholinergic losses by the M1 positive allosteric modulator TAK-071

RATIONALE

Loss of basal forebrain cholinergic neurons contributes to the severity of the cognitive decline in age-related dementia and, in patients with Parkinson's disease (PD), to impairments in gait and balance and the resulting risks for falls. Contrasting with the extensive evidence indicating an essential role of cholinergic activity in mediating cognitive, specifically attentional abilities, treatment with conventional acetylcholinesterase inhibitors (AChEIs) has not fulfilled the promise of efficacy of pro-cholinergic treatments.

OBJECTIVES

Here, we investigated the potential usefulness of a muscarinic M1 positive allosteric modulator (PAM) in an animal model of cholinergic loss-induced impairments in attentional performance. Given evidence indicating that fast, transient cholinergic signaling mediates the detection of cues in attentional contexts, we hypothesized that a M1 PAM amplifies such transient signaling and thereby rescues attentional performance.

RESULTS

Rats performed an operant sustained attention task (SAT), including in the presence of a distractor (dSAT) and during a post-distractor (post-dSAT) period. The post-dSAT period served to assess the capacity for recovering performance following a disruptive event. Basal forebrain infusions of the cholino-specific immunotoxin 192 IgG-saporin impaired SAT performance, and greater cholinergic losses predicted lower post-dSAT performance. Administration of TAK-071 (0.1, 0.3 mg/kg, p.o., administered over 6-day blocks) improved the performance of all rats during the post-dSAT period (main effect of dose). Drug-induced improvement of post-dSAT performance was relatively greater in lesioned rats, irrespective of sex, but also manifested in female control rats. TAK-071 primarily improved perceptual sensitivity (d') in lesioned rats and facilitated the adoption of a more liberal response bias (B˝_D) in all female rats.

CONCLUSIONS

These findings suggest that TAK-071 may benefit the attentional performance of patients with partial cholinergic losses and specifically in situations that tax top-down, or goal-driven, attentional control.

Preclinical Models of Alzheimer's Disease: Relevance and Translational Validity

The only drugs currently approved for the treatment of Alzheimer's Disease (AD) are four acetylcholinesterase inhibitors and the NMDA antagonist memantine. Apart from these drugs, which have minimal to no clinical benefit, the 40-year search for effective therapeutics to treat AD has resulted in a clinical failure rate of 100% not only for compounds that prevent brain amyloid deposition or remove existing amyloid plaques but also those acting by a variety of other putative disease-associated mechanisms. This indicates that the preclinical data generated from current AD targets to support the selection, optimization, and translation of new chemical entities (NCEs) and biologics to clinical trials is seriously compromised. While many of these failures reflect flawed hypotheses or a lack of adequate characterization of the preclinical pharmacodynamic and pharmacokinetic (PD/PK) properties of lead NCEs-including their bioavailability and toxicity-the conceptualization, validation, and interrogation of the current animal models of AD represent key limitations. The overwhelming majority of these AD models are transgenic, based on aspects of the amyloid hypothesis and the genetics of the familial form of the disease. As a result, these generally lack construct and predictive validity for the sporadic form of the human disease. The 170 or so transgenic models, perhaps the largest number ever focused on a single disease, use rodents, mainly mice, and in addition to amyloid also address aspects of tau causality with more complex multigene models including other presumed causative factors together with amyloid. This overview discusses the current animal models of AD in the context of both the controversies surrounding the causative role of amyloid in the disease and the need to develop validated models of cognitive function/dysfunction that more appropriately reflect the phenotype(s) of human aged-related dementias. © 2019 by John Wiley & Sons, Inc.

Contrasting effects of pre-training on acquisition of operant and radial arm maze tasks in rats

Performing multiple tasks either simultaneously, in rapid alternation or in succession, is routine in daily life. Further, testing rodents in a battery of tests is common both in drug discovery and behavioral phenotyping research. However, learning of new tasks can be influenced by prior experience(s). There has been some research on 'switching cost' involved in the transition from one behavior to another. However, there has been no specific assessment of the effect of learning an operant paradigm on performance in a spatial memory task and vice versa. Accordingly, we evaluated task switching between two forms of learning paradigms, operant conditioning and radial arm maze (RAM) tasks. In experiment 1, rats were trained for operant conditioning with food reward followed by a partially baited RAM task. In experiment 2, rats were trained first on a RAM task followed by operant learning. Pre-training on the operant task, impaired the acquisition of the RAM. On the contrary, pre-training on the RAM enhanced operant performance. Our study reveals significant effects of the test order on task-switching in rats. This knowledge can be useful when framing test sequences in test batteries for drug discovery research and screening genetically modified mice.

Predictive validity of a MK-801-induced cognitive impairment model in mice: implications on the potential limitations and challenges of modeling cognitive impairment associated with schizophrenia preclinically

Cognitive impairment associated with schizophrenia (CIAS) is a major and disabling symptom domain of the disease that is generally unresponsive to current pharmacotherapies. Critically important to the discovery of novel therapeutics for CIAS is the utilization of preclinical models with robust predictive validity. We investigated the predictive validity of MK-801-induced memory impairments in mouse inhibitory avoidance (MK-IA) as a preclinical model for CIAS by investigating compounds that have been tested in humans, including antipsychotics, sodium channel blocker mood stabilizers, and putative cognitive enhancers. The atypical antipsychotic clozapine, as well as risperidone and olanzapine (see Brown et al., 2013), had no effect on MK-801-induced memory impairments. For sodium channel blockers, carbamazepine significantly attenuated memory impairments induced by MK-801, whereas lamotrigine had no effect. Nicotine, donepezil, modafinil, and xanomeline all significantly attenuated MK-801-induced memory impairments, but the magnitude of effects and the dose-responses observed varied across compounds. Clinically, only acute administration of nicotine has demonstrated consistent positive effects on CIAS, while inconsistent results have been reported for lamotrigine, donepezil, and modafinil; atypical antipsychotics produce only moderate improvements at best. A positive clinical signal has been observed with xanomeline, but only in a small pilot trial. The results presented here suggest that the MK-IA model lacks robust predictive validity for CIAS as the model is likely permissive and may indicate false positive signals for compounds and mechanisms that lack clear clinical efficacy for CIAS. Our findings also highlight the potential limitations and challenges of using NMDA receptor antagonists in rodents to model CIAS.

A novel automated test battery reveals enduring behavioural alterations and cognitive impairments in survivors of murine pneumococcal meningitis

Pneumococcal meningitis, caused by Streptococcus pneumoniae infection, is a major form of lethal bacterial meningitis. Survivors are predisposed to developing lifelong disabling sequelae, including cognitive impairment, psychological problems and motor deficits. In our experimental model, ventricular inoculation of 10(5) colony-forming units of S. pneumoniae type 3 caused 90% of mice to develop life-threatening meningitis within 48 h. Antibiotic treatment with ceftriaxone 20 h post infection reduced the incidence of severe meningitis to <10%. At the time of treatment, upregulation of pro-inflammatory cytokines was detected, including interleukin-1β, interleukin-6 and tumour necrosis factor. We evaluated the long-term behavioural and cognitive sequelae in control mice and those surviving meningitis using an automated system (the IntelliCage) in which mice perform a range of behavioural and spatial tasks to obtain water rewards from conditioning units in their home cage. Surviving mice showed a number of altered behaviours relative to controls, including (i) hypoexploration when first exposed to the IntelliCage, (ii) altered activity patterns (fewer visits to conditioning stations during the light phase and more in the dark phase), (iii) avoidance of light (a constant or flashing LED stimulus), (iv) impaired spatial learning (a complex patrolling task), and (v) impaired discrimination reversal learning. Overall these results suggest photophobia and weakened learning ability in post-meningitic mice, particularly on tasks engaging hippocampal and prefrontal neural substrates. This study also demonstrates a standardised and comprehensive battery of tests that can be readily used to investigate neurological sequelae in undisturbed mice residing in a complex home cage environment.

Leveraging the cortical cholinergic system to enhance attention

Attentional impairments are found in a range of neurodegenerative and neuropsychiatric disorders. However, the development of procognitive enhancers to alleviate these impairments has been hindered by a lack of comprehensive hypotheses regarding the circuitry mediating the targeted attentional functions. Here we discuss the role of the cortical cholinergic system in mediating cue detection and attentional control and propose two target mechanisms for cognition enhancers: stimulation of prefrontal α4β2* nicotinic acetylcholine receptors (nAChR) for the enhancement of cue detection and augmentation of tonic acetylcholine levels for the enhancement of attentional control. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Effects of pregnanolone and flunitrazepam on the retention of response sequences in rats

Neuroactive steroids produce effects similar to other GABA(A) modulators (e.g., benzodiazepines and barbiturates) and have a large therapeutic potential; however, a greater understanding of the effects of these substances on learning and memory is needed. To specifically assess the effects of a neurosteroid on memory, pregnanolone (1-18 mg/kg) was administered to male Long-Evans rats responding under a repeated acquisition and delayed-performance procedure in which different 4-response sequences were acquired and then retested after varying delays. Responding was maintained under a second-order fixed-ratio (FR) 2 schedule of food reinforcement, and incorrect responses (errors) produced a 5-sec timeout. For comparison purposes, both a high (flunitrazepam) and low efficacy agonist/antagonist (flumazenil) of the GABA(A) receptor complex were also administered both alone and in combination. Retention of each sequence was quantified as percent savings in errors-to-criterion and this dependent measure was shown to be sensitive to increases in delay. When administered 15 min prior to the end of either a 30- or 180-minute delay, pregnanolone produced both dose- and delay-dependent decreases in percent savings, response rate and accuracy; this effect was selective in that decreases in retention occurred at doses lower than those that disrupted response rate or accuracy. Flunitrazepam (0.056-1mg/kg) produced similar disruptions in retention and these disruptions were antagonized by 5.6 mg/kg of flumazenil. Both an ineffective (0.056 mg/kg) and an effective (0.18 mg/kg) dose of flunitrazepam also potentiated the dose- and delay-dependent disruptions in retention produced by pregnanolone. These data indicate that the neurosteroid pregnanolone disrupts retention in a manner similar to the benzodiazepine flunitrazepam, and suggests that the interaction of flunitrazepam and pregnanolone on retention may be mediated by the GABA(A) receptor complex.

The inhibitory avoidance test optimized for discovery of cognitive enhancers

In the present article, we describe a new protocol for the inhibitory avoidance test, with a dual purpose: (1) to provide a less variable and more reliable assessment of the efficacy of potential cognitive enhancers in antagonizing scopolamine-induced long-term-memory deficits, and (2) to secure a high throughput for pharmacological screening of cognitive enhancers. The new protocol consists of two acquisition trials that are followed 24 h later by a single retention trial. In the present study, this protocol clearly dissociated the frequency distributions of retention latencies between scopolamine- and vehicle-treated groups and allowed validation by means of two acetylcholinesterase inhibitors-tacrine and donepezil-that proved to be active in counteracting the scopolamine-induced memory deficit. This protocol also produced stability of the behavioral response to pharmacological agents over a 3-year period. A statistical power analysis indicated that, depending on the efficacy of the drug/dose, a sample size of 5-12 mice was required in order to show a reversal of the scopolamine-induced memory deficit. The double-trial acquisition protocol is suitable for testing cognitive enhancers, while also providing a clearly enhanced throughput.

nAChR agonist-induced cognition enhancement: integration of cognitive and neuronal mechanisms

The identification and characterization of drugs for the treatment of cognitive disorders has been hampered by the absence of comprehensive hypotheses. Such hypotheses consist of (a) a precisely defined cognitive operation that fundamentally underlies a range of cognitive abilities and capacities and, if impaired, contributes to the manifestation of diverse cognitive symptoms; (b) defined neuronal mechanisms proposed to mediate the cognitive operation of interest; (c) evidence indicating that the putative cognition enhancer facilitates these neuronal mechanisms; (d) and evidence indicating that the cognition enhancer facilitates cognitive performance by modulating these underlying neuronal mechanisms. The evidence on the neuronal and attentional effects of nAChR agonists, specifically agonists selective for alpha4beta2* nAChRs, has begun to support such a hypothesis. nAChR agonists facilitate the detection of signals by augmenting the transient increases in prefrontal cholinergic activity that are necessary for a signal to gain control over behavior in attentional contexts. The prefrontal microcircuitry mediating these effects include alpha4beta2* nAChRs situated on the terminals of thalamic inputs and the glutamatergic stimulation of cholinergic terminals via ionotropic glutamate receptors. Collectively, this evidence forms the basis for hypothesis-guided development and characterization of cognition enhancers.

Detection of the moderately beneficial cognitive effects of low-dose treatment with haloperidol or clozapine in an animal model of the attentional impairments of schizophrenia

The absence of effective cognition enhancers for the treatment of patients with schizophrenia limits the validation of animal models and behavioral tests used for drug finding and characterization. However, low doses of haloperidol and clozapine were documented to produce moderately beneficial effects in patients. Therefore, this experiment was designed to determine the attentional effects of such treatments in a repeated-amphetamine (AMPH) animal model. Animals were trained in an operant-sustained attention task and underwent a 40-day pretreatment period with saline or increasing doses (1-10 mg per kg) of AMPH. After regaining baseline performance following 10 days of saline treatment, animals were treated with haloperidol (0.025 mg per kg), clozapine (2.5 mg per kg), or vehicle for 10 days. Furthermore, the effects of AMPH challenges (1.0 mg per kg) were assessed. In AMPH-pretreated animals, the administration of AMPH challenges resulted in the disruption of attentional performance. Treatment with haloperidol and clozapine attenuated the detrimental performance effects of these challenges, with clozapine exhibiting more robust attenuation. Furthermore, clozapine, but not haloperidol, impaired the performance of control animals. In contrast, the performance of AMPH-pretreated animals remained unaffected by clozapine. As this animal model detects the moderately beneficial cognitive effects of haloperidol and clozapine, it may be useful for preclinical research designed to detect and characterize treatments for the cognitive symptoms of schizophrenia.

Behavioral phenotyping strategies for mutant mice

Comprehensive behavioral analyses of transgenic and knockout mice have successfully identified the functional roles of many genes in the brain. Over the past 10 years, strategies for mouse behavioral phenotyping have evolved to maximize the scope and replicability of findings from a cohort of mutant mice, minimize the interpretation of procedural artifacts, and provide robust translational tools to test hypotheses and develop treatments. This Primer addresses experimental design issues and offers examples of high-throughput batteries, learning and memory tasks, and anxiety-related tests.

Clinical attrition due to biased preclinical assessments of potential efficacy

Unless it is carefully controlled, bias often distorts the results of clinical trials, usually exaggerating the magnitude of true efficacy. For that reason, procedures to limit bias have been mandated by the FDA when assessing efficacy in clinical trials. The present review shows that the effects of bias in preclinical studies are at least as large as in clinical trials, and since bias is not usually controlled in preclinical proof of concept studies, compounds that actually have little or no therapeutic potential may often be advanced into clinical trials. This possibility is supported by the fact that lack of efficacy is the single biggest reason why compounds fail in the clinic. The shift to target-based discovery during the last 10-15 years may have further increased the effects of bias on preclinical assessments of potential efficacy, and contributed to the continuing decline in clinical success rates. Procedures are available to control for bias during preclinical assessments of potential efficacy, and their use could dramatically increase clinical success rates and substantially reduce the costs of drug discovery and development.

Preclinical research into cognition enhancers

The preclinical development of drugs to treat the cognitive symptoms of neuropsychiatric and neurological disorders is a formidable challenge. Evidence from a wide range of preclinical behavioral and neuropharmacological tests has formed the basis for predicting drug-induced cognition enhancement in normal volunteers and in patients with cognitive impairments. However, the limited validity of preclinical predictions of this enhancement in humans indicates that conventional screening for "broadly active" compounds represents a below-optimal research strategy. This article conceptualizes the evidence needed to improve the predictive validity of preclinical research designed to discover and characterize cognition enhancers. We suggest that the investigation of reciprocal relationships among molecular, cellular, behavioral and cognitive processes modulated by candidate drugs represents the core of such research. By contrast, the usefulness of simple and high-throughput screening tests for the detection of cognition enhancers might be restricted to advanced drug-finding programs that are guided by evidence of the modulation of neurocognitive relationships by cognition enhancers and that are informed by iterative preclinical-clinical cross-validation of research approaches. We stress the need for basic biopsychological research approaches in preclinical programs to find and characterize drugs to treat cognitive disorders.

Neurotoxicity test validation, positive controls and proficiency: Are chemicals necessary?

The USEPA neurotoxicity guidelines require the use of positive control data in support of toxicology studies submitted to the Agency and emphasize the use of chemicals to accomplish this requirement. These guidelines, though, propose a number of different rationales for the use of chemicals as positive control agents. We re-evaluated the potential roles of positive control data in addressing three questions: 1) what does the test measure? 2) is the performing laboratory proficient in the use of the test? 3) do the complementary data submitted in support of neurotoxicity studies conducted with the test material provide enough context for the interpretation of the biological significance of an effect? While, for most types of guideline neurotoxicity tests, the use of test chemicals has been emphasized for positive control testing, the use of non-chemical procedures (i.e., systematic manipulation of the experimental parameters of a test, which poses less risk of adverse effects to the test animals) should be strongly considered as a potential alternative.

Animal cognition: defining the issues

The assessment of cognitive functions in rodents represents a critical experimental variable in many research fields, ranging from the basic cognitive neurosciences to psychopharmacology and neurotoxicology. The increasing use of animal behavioral tests as 'assays' for the assessment of effects on learning and memory has resulted in a considerable heterogeneity of data, particularly in the field of behavioral and psycho pharmacology. The limited predictive validity of changes in behavioral performance observed in standard animal tests of learning and memory indicates that a renewed effort to scrutinize the validity of these tests is warranted. In humans, levels of processing (effortful vs. automatic) and categories of information (procedural vs. episodic/declarative) are important variables of cognitive operations. The design of tasks that assess the recall of 'episodic' or 'declarative' information appears to represent a particular challenge for research using laboratory rodents. For example, the hypothesis that changes in inspection time for a previously encountered place or object are based on the recall of declarative/episodic information requires substantiation. In order to generalize findings on the effects of neuronal or pharmacological manipulations on learning and memory, obtained from one species and one task, to other species and other tasks, the mediating role of important sets of variables which influence learning and memory (e.g. attentional, affective) needs to be determined. Similar to the view that a neuronal manipulation (e.g. a lesion) represents a theory of the condition modeled (e.g. a degenerative disorder), an animal behavioral task represents a theory of the behavioral/cognitive process of interest. Therefore, the test of hypotheses regarding the validity of procedures used to assess cognitive functions in animals is an inherent part of the research process.

Selective 5-HT1A antagonists WAY 100635 and NAD-299 attenuate the impairment of passive avoidance caused by scopolamine in the rat

Systemic administration of the muscarinic-receptor antagonists atropine and scopolamine produces cognitive deficits in humans, nonhuman primates and rodents. In humans, these deficits resemble symptoms of dementia seen in Alzheimer's disease. The passive avoidance (PA) task has been one of the most frequently used animal models for studying cholinergic mechanisms in learning and memory. The present study examined the ability of two selective 5-HT(1A) receptor antagonists WAY 100635 and NAD-299 (robalzotan) and two acetylcholinesterase (AChE) inhibitors tacrine and donepezil to attenuate the impairment of PA retention caused by the nonselective muscarinic receptor antagonist scopolamine in the rat. Although demonstrating differences in their temporal kinetics, both WAY 100635 and NAD-299 attenuated the impairment of PA caused by scopolamine (0.3 mg/kg s.c.). Donepezil did not block the PA deficit caused by the 0.3 mg/kg dose of scopolamine, but it prevented the inhibitory effects of the 0.2 mg/kg dose of scopolamine. In contrast, tacrine was effective vs both the 0.2 and 0.3 mg/kg doses of scopolamine. These results indicate that (1). a functional 5-HT(1A) receptor antagonism can attenuate the anterograde amnesia produced by muscarinic-receptor blockade, and (2). the AChE inhibitors tacrine and donepezil differ in their ability to modify muscarinic-receptor-mediated function in vivo. These results suggest that 5-HT(1A) receptor antagonists may have a potential in the treatment of cognitive symptoms in psychopathologies characterized by reduced ACh transmission such as Alzheimer's disease.

A common role for psychotropic medications: memory impairment

The psychopathologic profile of mental disorders is very diverse and psychotropic medications used to treat them differ in their chemical structure. Nevertheless, these drugs share these four characteristics: delayed onset of clinical response, not one of them can be said to cure, there is a high number of non-responders, and the mechanism responsible for their therapeutic action is not known. It is hypothesized that the action of psychotropic medications is memory impairment, understanding memory as the trace left in the nervous system not only by individual experiences but also by genetic and epigenetic phenomena. It is suggested that it would be beneficial to translate some research strategies from the neurobiology of learning and memory to the study of the effects of psychotropic medications. The hypothesis is briefly assessed according to the following three criteria: (a). the comparison between the molecular effects of psychotropic medications and the so-called molecular biology of learning and memory, (b). the effects of these drugs, preferentially after chronic use, on memory tests, and (c). the effects of drugs that impair memory on tests used for screening psychotropic medications. Finally, some general suggestions for future research are pointed out.

Effects of chronic administration of huperzine A on memory in guinea pigs

Effects of subchronic administration of huperzine A, a cholinesterase inhibitor, on spatial memory were studied in guinea pig. Spatial memory was appreciated by the Morris water maze test. At a dose of 0.25 microgram/h, inhibiting 36% of blood AChE and 14-20% of central AChE, no effect on spatial learning was found. At a dose of 1 microgram/h, inhibiting 20% of blood AChE and 14-20% of central AChE, no memory impairment was found, on the other hand, a memory enhancing effect, limited to the first day was shown. It thus appears that subchronic administration of huperzine A did not induce deleterious effects on spatial memory.

The fallacy of behavioral phenotyping without standardisation

Behavioral phenotyping of mutant mice is a new and challenging task for the behavioral neuroscientist. Therefore, standardisation of the experimental conditions is required to permit comparisons between the results of experiments within and between laboratories. Once mutation-induced behavioral changes have been identified, phenotyping of mouse mutants should be performed along a systematic trajectory, which allows for an in-depth characterisation of the mutant under investigation.

Behavioural phenotyping of mouse mutants

Behavioural phenotyping of mouse mutants is not a goal in itself but serves to characterise the behavioural effects of naturally occurring or experimentally induced mutations. Genetically engineered mouse mutants are valuable tools to elucidate the genetic control of behaviour and the interaction between genetic and environmental factors. However, a prerequisite for their use is the ability to assess different elements of behaviour. To this end, a battery of tests, which should be flexible enough to meet the needs of a particular study, should be used to characterise the behavioural phenotype. Detailed and extensive information about the effects of gene mutations is crucial for model building and model evaluation. Model building is an iterative process, switching between experimental data and theory formation. In order to facilitate this process and to allow comparison of results within and between laboratories, the standardisation of breeding, housing, and testing conditions is essential. The development and standardisation of sensitive, valid behavioural tests which are suited to phenotype mouse mutants is both a responsibility and a challenge to investigators of mouse behaviour.

Effects of molsidomine on scopolamine-induced amnesia and hypermotility in the rat

Nitric oxide (NO) is hypothesized to be a novel intracellular messenger in the central nervous system. Recently, NO involvement in learning and memory processes has been proposed. Compounds that inhibit nitric oxide synthase, the key synthesizing enzyme, may block cognition, while NO donors may facilitate it. The aim of this study was to assess in the rat the effects of the NO donor molsidomine (2 and 4 mg/kg, i.p.) on memory deficits caused by scopolamine. For this purpose, the object recognition task and the step-through passive avoidance procedure were chosen. In addition, the effects of molsidomine in antagonizing the scopolamine-induced hypermotility were also examined. Scopolamine at 0.2 mg/kg (object recognition) and 0.75 mg/kg (passive avoidance) disrupted acquisition in both the tasks and induced locomotor hyperactivity at the dose of 0.2 mg/kg. Molsidomine at either dose reversed the scopolamine-induced deficits in the object recognition paradigm but did not counteract the hypermotility and the deficits occurred in the passive avoidance test. These results suggest that to some extent, the NO donor molsidomine is involved in memory processing.

Nootropic and anxiolytic activity of saponins of Albizzia lebbeck leaves

The effect of saponin containing, n-butanolic fraction (BF), extracted from dried leaves of Albizzia lebbeck, was studied on cognitive behavior and anxiety in albino mice. The elevated plus maze was used for assessment of both nootropic and anxiolytic activity. The nootropic activity was evaluated by recording the effect of BF (0, 10, 25, and 50 mg/kg) on the transfer latency, whereas anxiolytic activity was assessed by studying its effect on the duration of occupancy in the closed arm. Results showed significant improvement in the retention ability of the normal and amnesic mice as compared to their respective controls. Animals treated with BF (25 mg/kg) spent more time in the open arm in a dose-dependent manner. The BF was without any significant effect on motor coordination. However, it significantly inhibited passivity and hypothermia induced by baclofen (10 mg/kg), a GABA(B) agonist. The data emanated in the present study suggests involvement of gamma-aminobutyric acid (GABA) in the nootropic and anxiolytic activity of saponins obtained from A. lebbeck.

Preclinical psychopharmacology of AIDS-associated dementia: lessons to be learned from the cognitive psychopharmacology of other dementias

Following a brief discussion of the epidemiology, underlying neuropathological mechanisms, neuropsychological symptoms and present treatment strategies of AIDS-associated dementia (AAD), parallels are drawn between the longer standing research on drugs for the treatment of other cognitive disorders, particularly senile dementia, and ongoing efforts to develop psychopharmacological approaches for the treatment of the cognitive impairments in AAD. Important aspects of hypotheses designed to guide such a research are indicated with the help of a speculative, paradigmatic hypothesis concerning the role of cortical cholinergic inputs in AAD. Furthermore, aspects of validity of animal models, and cognition as a crucial intervening variable in the effects of potential treatments, are evaluated.

Pharmacological evidence supporting a role for galanin in cognition and affect

1. Galanin is localized in brain pathways involved in both cognition and affect. 2. Galanin has inhibitory actions on a variety of memory tasks including the Morris water maze, delayed nonmatching to position, T-maze delayed alternation, starburst maze, passive avoidance, active avoidance, and spontaneous alternation. 3. Galanin may inhibit learning and memory by inhibiting neurotransmitter release and neuronal firing rate. 4. Two signal transduction mechanisms through which galanin exerts its inhibitory actions are the inhibition of phosphatidyl inositol hydrolysis and the inhibition of adenylate cyclase. 5. Galanin released during periods of burst firing from noradrenergic locus coeruleus terminals in the ventral tegmental area (VTA) may lead to symptoms of depression through inhibition of dopaminergic VTA neurons. 6. Intraventricular galanin has anxiolytic effects in a punished drinking test. Intra-amygdala galanin has anxiogenic effects in a punished drinking test.

Overview and perspective on the therapy of Alzheimer's disease from a preclinical viewpoint

1. Drugs effective in Alzheimer's disease (AD) should have several aims: to improve the cognitive impairment, control the behavioural and neurological symptoms, delay the progression of the disease, and prevent the onset. In order to attain these targets, cell and animal models are needed on which to test pathogenetic hypothesis and demonstrate the potential effectiveness of new drugs. This overview examines the results obtained in animal models. They are the link between the molecular and biochemical studies on the disease and the reality of human pathology. 2. The development of animal models reproducing the complexity of AD pathogenetic mechanisms and clinical symptoms still represents a challenge for the preclinical investigators. Moreover, the succession of different animal models well documents the progressive widening of our knowledge of the disease with the identification of new therapeutic targets. 3. The main animal models are listed, and their contribution to the understanding of the pathogenic mechanisms and development of the drugs presently used in AD therapy is described. Moreover, their role in the study of future drugs is analysed 4. Preclinical studies on cholinesterases and animal models mimicking the cholinergic hypofunction occurring in AD have been instrumental in developing cholinesterase inhibitors, which are the only recognised drugs for the symptomatic treatment of AD. 5. Artificially created beta-amyloid (A beta) deposits in normal rats, and transgenic mice overexpressing amyloid precursor protein (APP) are the models on which the future treatment are tested. They are aimed to prevent formation of A beta deposits or its transformation in neuritic plaques. 6. Models of brain inflammation, aging animals, and models of brain glucose and energy metabolism impairment make it possible to identify and assess the activity of anti-inflammatory agents, antioxidants, ampakines and other potentially active agents. 7. It is concluded that the present level of information on AD could never have been reached without preclinical studies, and the development of new drugs will always require extensive preclinical investigations.

Cognitive effects of neurotoxic lesions of the nucleus basalis magnocellularis in rats: differential roles for corticopetal versus amygdalopetal projections

The cholinergic hypothesis states that cholinergic neurons of the basal forebrain nucleus basalis magnocellularis (nbm) that project to cortical and amygdalar targets play an important role in memory. Biochemical studies have shown that these target areas are differentially sensitive to different excitotoxins (e.g., ibotenate vs. quisqualate). This observation might explain the finding from many behavioural studies of memory that different excitotoxins affect memory differentially even though they produce about the same level of depletion of cholinergic markers in the cortex and similar cortical electrophysiological effects. Thus, the magnitude of mnemonic impairment might be related to the extent of damage to cholinergic projections to the amygdala more than to the extent of damage to corticopetal cholinergic projections. This explanation might similarly apply to the observation that the immunotoxin 192 IgG-saporin produces mild effects on memory when injected into the nbm. This is because it damages cholinergic neurons projecting to the cortex but not those projecting to the amygdala. Studies comparing the effects on memory of ibotenic acid vs. quisqualic acid lesions of the nbm are reviewed as are studies of the mnemonic effects of 192 IgG-saporin. Results support the cholinergic hypothesis and suggest that amygdalopetal cholinergic neurons of the nbm play an important role in the control of memory.

A performance-dependent adjustment of the retention interval in a delayed non-matching-to-position paradigm differentiates effects of amnestic drugs in rats

Operant delayed non-matching-to-position (delayed non-matching-to-position) tasks have been widely used as tests of working memory in rats, but have suffered some loss in sensitivity to differentiating selective mnemonic from non-mnemonic deficits due to floor and ceiling effects. To circumvent this problem, a novel delayed non-matching-to-position was developed in which the retention interval was adjusted on a trial-by-trial basis to hold performance accuracy at an intermediate value. The present study assessed the effects of three amnestic drugs in this delayed non-matching-to-position. Rats were administered (i.p.) NMDA receptor antagonist ((5R,10S)-(+)-5-Methyl-10, 11-dihydro-5H-dibenzo[a,d,] cyclohepten-5,10-imine (Dizocilpine or MK-801), muscarinic receptor antagonist (-)-scopolamine hydrobromide (scopolamine), or cannabinoid receptor agonist ((R)-(+)-[2, 3-Dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1, 4-benzoxazin-6-yl]-1-naphthalenylmethanone) (WIN 55, 212-2). At high doses, both MK-801 (0.12-0.25 mg/kg) and scopolamine (0.25 mg/kg) produced deficits not selective to working memory. At low doses, scopolamine (0.06-0.12 mg/kg) and MK-801 (0.06 mg/kg) produced no deficits in any mnemonic or secondary measures. WIN 55, 212-2 produced deficits at 2.0 mg/kg that were consistent with a specific impairment of working memory. Using this particular delayed non-matching-to-position revealed that consistent changes in performance accuracy at the short retention interval were evident for scopolamine and MK-801, at times in the absence of changes in response tendency, which are consistent with an interpretation that these drugs produce general deficits in reference or procedural memory. In contrast, cannabinoid-induced deficits in choice accuracy support previous reports of delay-dependent deficits. Together, these data suggest that this delayed non-matching-to-position task is able to differentiate deficit patterns of amnestic drugs, and isolate the effects of motivational side effects of drugs from working memory measurement.

Effects of acute and chronic maprotiline administration on inhibitory avoidance in male mice

The effects of acute and chronic administration of maprotiline (5, 10 or 20 mg/kg, intraperitoneally) were assessed on inhibitory avoidance in male mice. Acute administration of maprotiline before training did not effect training phase latencies, but impaired performance (i.e. produced shorter latencies) in the test at doses of 5 and 20 mg/kg. When given after training, the drug did not modify test latencies at any of the doses used. Chronic administration for 21 days (interrupted 24 h before training) also shortened latencies in the test but not in training. An experiment on the acute effects of maprotiline on analgesia (determination of flinch and jump thresholds for increasing electric foot shock levels), at the doses stated, was carried out on naive animals. No analgesic effect of the drug was found. Taken together, the results indicate that acute maprotiline produces anterograde amnesia, and tolerance does not appear after 21 days of treatment.

In vivo neurochemical correlates of cognitive processes: methodological and conceptual challenges

The advent of the use of in vivo microdialysis and voltammetry techniques in behaving animals has ushered in a great deal of research on the neurochemistry of cognition. While studies exploring the relationship between neurotransmitter release and cognitive processing are quite feasible, a number of methodological and conceptual issues pose challenges to the interpretation of experimental results. These challenges include: 1) a demonstration that the behavioral task highlights the particular cognitive construct under study; 2) a determination of the role of non-cognitive variables (i.e. transfer effects, sensory stimulation, motivational variables, and motor activity) in affecting transmitter release, and 3) a recognition of the value of a distributed systems approach to studying the neurochemistry of cognition. This review summarizes the data on the validity of microdialysis and voltammetry as correlates of neurotransmitter release and then illustrates the impact that the above challenges can have on the conclusions drawn from various studies.

Attenuation by nimodipine of amitriptyline-induced avoidance impairment in mice

The effects of the dihydropyridine calcium channel blocker nimodipine on avoidance impairment induced by the tricyclic antidepressant amitriptyline were assessed during shuttle-box training and in previously trained mice of the DBA/2 strain. Nimodipine (0, 0.5, 1, 2.5, or 5 mg/kg) had no effect alone, but attenuated the avoidance impairment induced by 5 mg/kg amitriptyline on avoidance acquisition, as well as on a previously learned avoidance response. The avoidance improving action of the calcium channel blocker was less evident in mice receiving a larger dose (7.5 mg/kg) of the antidepressant drug. The effect of nimodipine did not appear to be specifically related to the avoidance impairment induced by amitriptyline, because the calcium antagonist also attenuated the avoidance impairing action of the neuroleptic chlorpromazine. The avoidance impairment induced by amitriptyline and chlorpromazine, and the related ameliorating action of nimodipine, seem imputable to drug effects on the performance of the avoidance response, rather than to interferences with learning processes. The results suggest that, in the case of concomitant administration, nimodipine could alleviate adverse side effects of tricyclic antidepressant, i.e., psychomotor disturbances.

A comparative study in rats of the in vitro and in vivo pharmacology of the acetylcholinesterase inhibitors tacrine, donepezil and NXX-066

The in vitro and in vivo effects of the novel acetylcholinesterase inhibitors donepezil and NXX-066 have been compared to tacrine. Using purified acetylcholinesterase from electric eel both tacrine and donepezil were shown to be reversible mixed type inhibitors, binding to a similar site on the enzyme. In contrast, NXX-066 was an irreversible non-competitive inhibitor. All three compounds were potent inhibitors of rat brain acetylcholinesterase (IC50 [nM]; tacrine: 125 +/- 23; NXX-066: 148 +/- 15; donepezil: 33 +/- 12). Tacrine was also a potent butyrylcholinesterase inhibitor. Donepezil and tacrine displaced [3H]pirenzepine binding in rat brain homogenates (IC50 values [microM]; tacrine: 0.7; donepezil: 0.5) but NXX-066 was around 80 times less potent at this M1-muscarinic site. Studies of carbachol stimulated increases in [Ca2+]i in neuroblastoma cells demonstrated that both donepezil and tacrine were M1 antagonists. Ligand binding suggested little activity of likely pharmacological significance with any of the drugs at other neurotransmitter sites. Intraperitoneal administration of the compounds to rats produced dose dependent increases in salivation and tremor (ED50 [micromol/kg]; tacrine: 15, NXX-066: 35, donepezil: 6) with NXX-066 having the most sustained effect on tremor. Following oral administration, NXX-066 had the slowest onset but the greatest duration of action. The relative potency also changed, tacrine having low potency (ED50 [micromol/kg]; tacrine: 200, NXX-066: 30, donepezil: 50). Salivation was severe only in tacrine treated animals. Using in vivo microdialysis in cerebral cortex, both NXX-066 and tacrine were found to produce a marked (at least 30-fold) increase in extracellular acetylcholine which remained elevated for more than 2 h after tacrine and 4 h after NXX-066.

Animal models in the development of symptomatic and preventive drug therapies for Alzheimer's disease

Dementia of the Alzheimer type (AD) is clinically characterized by a progressive deterioration of intellect, memory, judgment, and abstract thinking. It is incurable, and causal therapy is not yet available. For the development of therapeutic drugs, valid animal models are needed that mimic the pathophysiological change in brain functions and the concomitant behavioural deterioration seen in AD patients. This article provides an overview of the animal models that are used most often to study the substrates and mechanisms of the pathological changes underlying AD and to identify, characterize and develop putative neuroprotective, antidegenerative, revalidation-supporting and/or cognition-enhancing compounds or treatments. The first generation of agents for the symptomatic treatment of the disease has been developed on the basis of results obtained with these models. These drugs are presently undergoing clinical testing or are already used therapeutically. There is, however, no single animal model that can mimic the full range of pathophysiological alterations and key symptoms of AD. New, genetically engineered mouse models that mimic at least some of the key pathological changes of AD are expected to provide tools that will facilitate the development of symptomatic and preventive drug therapies.

Is there a rationale for the use of acetylcholinesterase inhibitors in the therapy of Alzheimer's disease?

Since the 1980s, the cholinergic hypothesis of the pathogenesis of Alzheimer's disease has proven to be a strong stimulus to pharmacological strategies aimed at correcting the cognitive deficit by manipulating cholinergic neurotransmission. Among these strategies, the one based on acetylcholinesterase inhibition is currently the most extensively developed for the therapy of Alzheimer's disease. The inhibitors' mechanisms of action are complex, including changes in the release of acetylcholine, and modulation of acetylcholine receptors. Various clinical trials of various inhibitors have shown that, on the whole, their effects were modest and, in the case of some drugs, were associated with frequent adverse reactions. Among the conceivable reasons for the limited efficacy of these drugs, those related to the pharmacological target deserve particular attention. This review, therefore, focuses on the complex nature of the acetylcholine system, the alterations of acetylcholinesterase and muscarinic receptor signal transduction in Alzheimer's disease, and the involvement of other neurotransmitters.

The effects of an extract of Ginkgo biloba, EGb 761, on cognitive behavior and longevity in the rat

Extracts of the leaves of the Ginkgo biloba tree are widely used throughout the world for their purportedly beneficial effects on brain function. In the present investigation, a standardized extract, EGb 761, was self-administered orally by male Fischer 344 rats that were then tested in an eight-arm radial maze. The tasks employed were a) continuous learning and b) delayed nonmatching to position. Chronic postsession administration of EGb 761 at a dose of 50 mg/kg had no effect on continuous learning but the same dose given presession resulted in a trend toward fewer sessions to reach criterion performance as well as fewer errors. In addition, it was observed that rats chronically treated with EGb 761 lived significantly longer than vehicle-treated subjects. In a delayed nonmatching to position task using a 30-min delay in 20-month-old rats. EGb 761 administered presession produced a dose-related decrease in total, retroactive, and proactive errors; a repeated-measures design was used, with subjects serving as their own controls. Following the dose-response determination, the group, now 26 months of age, was divided in two with half receiving EGb 761 at a dose of 200 mg/kg presession and the other half vehicle (sweetened condensed milk). A statistically significant positive effect of treatment with EGb-761 was observed. The present data are consistent with the beneficial effects on cognitive performance which have been widely reported in human subjects. In addition, the data suggest that the methods employed, i.e., continuous learning and delayed nonmatching to position tasks in aged rats, are capable of detecting drugs of possible value in the treatment of human cognitive impairment. Finally, the present results encourage a search for the pharmacologically active principles of EGb 761 and for their mechanisms of action.

Prevention of amitriptyline-induced avoidance impairment by tacrine in mice

The effects of two cognition enhancers on avoidance impairment induced by the tricyclic antidepressant amitriptyline were assessed during shuttle-box avoidance acquisition and in previously trained mice of the DBA/2 strain. The nootropic agent piracetam (50, 100 or 200 mg/kg, i.p.) had slight or no effect in mice receiving amitriptyline (5 or 10 mg/kg, i.p.). Conversely, the acetylcholinesterase inhibitor tacrine (0.5, 1, 2 or 3 mg/kg, i.p.) prevented the avoidance impairment induced by 5 mg/kg amitriptyline on shuttle-box avoidance acquisition as well as on a previously learned avoidance response. The avoidance disrupting action produced by 10 mg/kg of the antidepressant drug was not affected by the anticholinesterase drug. The preventing action of tacrine seems specifically related to the avoidance impairment induced by amitriptyline, since the acetylcholinesterase inhibitor did not reduce, but enhanced the avoidance impairing action of the neuroleptic chlorpromazine. Taken together, the results indicate that amitriptyline-induced avoidance impairment, and the related preventing action of tacrine, may be ascribed to drug effects on the performance of the avoidance response, rather than to interferences with learning processes.

A proposed test battery and constellations of specific behavioral paradigms to investigate the behavioral phenotypes of transgenic and knockout mice

Behavioral phenotyping of transgenic and knockout mice requires rigorous, formal analyses. Well-characterized paradigms can be chosen from the established behavioral neuroscience literature. This review describes (1) a series of neurological and neuropsychological tests which are effectively used as a first screen for behavioral abnormalities in mutant mice, and (2) a series of specific behavioral paradigms, clustered by category. Included are multiple paradigms for each category, including learning and memory, feeding, analgesia, aggression, anxiety, depression, schizophrenia, and drug abuse models. Examples are given from the experiences of the authors, in applying these experimental designs to transgenic and knockout mice. Extensive references for each behavioral paradigm are provided, to allow new investigators to access the relevant literature on behavioral methodology.

Nimodipine on shuttle-box avoidance learning in mice: no impairment but slight improvement

The dihydropyridine calcium channel antagonist nimodipine was tested in mice of CD-1, C57BL/6, and DBA/2 strains subjected to shuttle-box avoidance training. In contrast with some findings of other authors showing impairment of shuttle-box avoidance learning by low doses of the drug in rats, nimodipine given IP before each training session at doses of 0.25, 0.5, 1, 2.5, or 5 mg/kg never impaired avoidance acquisition in mice. On the contrary, one dose of nimodipine (1 mg/kg) significantly improved avoidance acquisition in mice of the DBA/2 strain. The drug failed to impair avoidance performance in DBA/2 mice even if given acutely in the middle (third session) or at the end (fifth session) of the training period. The results contradict studies showing cognitive impairment induced by calcium channel blockers, and may provide some limited evidence in support of improved cognitive function in normal animals, although this effect is much less evident than in aged or brain-damaged subjects.

Effects of cholinergic drug infusions into the dorsal part of the medial prefrontal cortex on delayed conditional discrimination performance in the rat

Effects of bilateral infusions of cholinergic drugs into the dorsal part of the medial prefrontal cortex (dmPFC) on performance in a delayed conditional discrimination (DCD) task were examined in rats. Scopolamine dose-dependently impaired performance. No delay-dependent effect was found indicating that scopolamine did not specifically affect working memory (WM). Physostigmine alone induced a slight improvement of DCD performance independent of delay and co-administration of physostigmine with scopolamine attenuated the scopolamine-induced impairment of DCD performance. Infusion of the muscarinic M2 antagonist AQRA-471, the M3 antagonist 4-DAMP and the mixed M1-M3 antagonist UH-AH 37 did not affect performance in the DCD task, suggesting that the effect of scopolamine is not mediated by a single muscarinic receptor subtype. The results furthermore indicate that the cholinergic system in the dmPFC does not play a specific role in WM processes in the DCD task. Furthermore, the results suggest that the dmPFC cholinergic system plays a role in the attentional processes involved in the DCD task.

Modulation of cognitive processes by transsynaptic activation of the basal forebrain

Each of the neurotransmitter-specific afferents to the basal forebrain (BF) carry different types of information which converge to regulate the activity of cholinergic projections to telencephalic areas. Brainstem monoaminergic and cholinergic inputs are critical for context-dependent arousal. GABAergic afferents are gated by a variety of ascending and descending systems, and in addition provide an intrinsic control of BF output excitability. Corticofugal glutamatergic inputs represent reciprocal connections from sites to which BF afferents project, and carry information about the current level of cortical processing intensity and capacity. Peptidergic inputs arise from hypothalamic sources and locally modulate BF output as a function of motivational and homeostatic processes. The significance of these afferent systems can be studied by examining the behavioral consequences of infusion into the BF of drugs that act on the specific receptor systems. Although traditional analyses suggest that the BF has many behavioral functions that can be subdivided regionally, an analysis of studies employing transsynaptic approaches lead to the conceptualization of the BF as having a uniform function, that of maximizing cortical processing efficiency. The BF is conditionally active during specific episodes of acquisition and processing of behaviorally significant, externally-derived information, and drives cortical targets into a state of readiness by reducing interference and amplifying the processing of relevant stimuli and associations, thus allowing for more efficient processing. This paper describes the transsynaptic approach to studying BF function, reviews the neurobiological and behavioral consequences of altering neurotransmitter-specific inputs to the BF, and explores the functional significance of the BF.