Differential effects of scopolamine and mecamylamine on working and reference memory in the rat

Systemic injection of nicotinic acetylcholine receptor antagonist mecamylamine affects licking, eyelid size, and locomotor and autonomic activities but not temporal prediction in male mice

Nicotinic acetylcholine receptors are thought to be associated with a wide range of phenomena, such as movement, learning, memory, attention, and addiction. However, the causal relationship between nicotinic receptor activity and behavior remains unclear. Contrary to the studies that examined the functions of muscarinic acetylcholine receptors, the role of the nicotinic acetylcholine receptors on behavior has not been examined as extensively. Here, we examined the effects of intraperitoneal injection of mecamylamine, a nicotinic acetylcholine receptor antagonist, on the performance of male mice in a head-fixed temporal conditioning task and a free-moving open-field task. The head-fixed experimental setup allowed us to record and precisely quantify the licking response while the mice performed the behavioral task with no external cues. In addition, by combining the utility of the head-fixed experimental design with computer vision analysis based on deep learning algorithms, we succeeded in quantifying the eyelid size of awake mice. In the temporal conditioning task, we delivered a 10% sucrose solution every 10 s using a blunt-tipped needle placed within the licking distance of the mice. After the training, the mice showed increased anticipatory licking toward the timing of sucrose delivery, suggesting that the mice could predict the timing of the reward. Systemic injection of mecamylamine decreased licking behavior and caused eye closure but had no effect on learned conditioned predictive behavior in the head-fixed temporal conditioning task. In addition, the injection of mecamylamine decreased spontaneous locomotor activity in a dose-dependent manner in the free-moving open-field task. The results in the open-field experiments further revealed that the effect of mecamylamine on fecal output and urination, suggesting the effects on autonomic activities. Our achievement of successful eyelid size recording has potential as a useful approach in initial screening for drug discovery. Our study paves a way forward to understanding the role of nicotinic acetylcholine receptors on learning and behavior.

Spatial reference learning deficits in absence of dysfunctional working memory in the TgF344-AD rat model of Alzheimer's disease

Alzheimer's disease (AD) is characterized by cognitive disorders and alterations of behavioral traits such as anhedonia and anxiety. Contribution of nonphysiological forms of amyloid and tau peptides to the onset of neurological dysfunctions remains unclear because most preclinical models only present one of those pathological AD-related biomarkers. A more recently developed model, the TgF344-AD rat has the advantage of overexpressing amyloid and naturally developing tauopathy, thus making it close to human familial forms of AD. We showed the presence of a learning dysfunction in a reference memory test, without spatial working memory impairment but with an increase in anxiety levels and a decrease in motivation to participate in the test. In the sucrose preference test, TgF344-AD rats did not show signs of anhedonia but did not increase the volume of liquid consumed when the water was replaced by sucrose solution. These behavioral phenomena were observed at an age when tau accumulation are absent, and where amyloid deposits are predominant in the hippocampus and the entorhinal cortex. Within the hippocampus itself, amyloid accumulation is heterogenous between the subiculum, the dorsal hippocampus and the ventral hippocampus. Thus, our data demonstrated heterogeneity in the appearance of various behavioral and neurochemical markers in the TgF344-AD rat. This multivariate analysis will therefore make it possible to define the stage of the pathology, to measure its evolution and the effects of future therapeutic treatments.

The contribution of transgenic and nontransgenic animal models in Alzheimer's disease drug research and development

Over the last few years, several papers have become available in the literature on both the main hallmarks of Alzheimer's disease (AD) and the several intracellular pathways whose alteration is responsible for its onset and progression. The use of transgenic and nontransgenic animal models has played a key role in achieving such a remarkable amount of preclinical data, allowing researchers to dissect the cellular changes occurring in the AD brain. In addition, the huge amount of preclinical evidence arising from these animal models was necessary for the further clinical development of pharmacological agents capable of interfering with most of the impaired neural pathways in AD patients. In this respect, a significant role is played by the dysfunction of excitatory and inhibitory neurotransmission responsible for the cognitive and behavioral symptoms described in AD patients. The aim of this review is to summarize the main animal models that contributed toward unraveling the pathological changes in neurotransmitter synthesis, release, and receptor binding in AD preclinical studies. The review also provides an updated description of the current pharmacological agents - still under clinical development - acting on the neurotransmitter systems.

Drugs Interfering with Muscarinic Acetylcholine Receptors and Their Effects on Place Navigation

Muscarinic acetylcholine receptors (mAChRs) have been found to regulate many diverse functions, ranging from motivation and feeding to spatial navigation, an important and widely studied type of cognitive behavior. Systemic administration of non-selective antagonists of mAChRs, such as scopolamine or atropine, have been found to have adverse effects on a vast majority of place navigation tasks. However, many of these results may be potentially confounded by disruptions of functions other than spatial learning and memory. Although studies with selective antimuscarinics point to mutually opposite effects of M1 and M2 receptors, their particular contribution to spatial cognition is still poorly understood, partly due to a lack of truly selective agents. Furthermore, constitutive knock-outs do not always support results from selective antagonists. For modeling impaired spatial cognition, the scopolamine-induced amnesia model still maintains some limited validity, but there is an apparent need for more targeted approaches such as local intracerebral administration of antagonists, as well as novel techniques such as optogenetics focused on cholinergic neurons and chemogenetics aimed at cells expressing metabotropic mAChRs.

Inhibitory learning is modulated by nicotinic acetylcholine receptors

Prior research has established that stimulating nicotinic acetylcholine receptors can facilitate learning and memory. However, most studies have focused on learning to emit a particular behavior, while little is known about the effects of nicotine on learning to withhold a behavioral response. The present study consisted of a dose response analysis of the effects of nicotine on negative occasion setting, a form of learned inhibition. In this paradigm, rats received one type of training trial in which presentation of a tone by itself was followed immediately by food reward. During the other type of trials, the tone was preceded by presentation of a light and no food was delivered after the tone. Rats gradually learned to approach the cup in anticipation of receiving food reward during presentations of the tone alone, but withheld that behavior when the tone was preceded by the light. Nicotine (0.35 mg/kg) facilitated negative occasion setting by reducing the number of sessions needed to learn the discrimination between trial types and by reducing the rate of responding on non-reinforced trials. Nicotine also increased the orienting response to the light, suggesting that nicotine may have affected the ability to withhold food cup behavior on non-reinforced trials by increasing attention to the light. In contrast to the effects of nicotine, rats treated with mecamylamine (0.125, 0.5, or 2 mg/kg) needed more training sessions to discriminate between reinforced and non-reinforced trials compared to saline-treated rats. The findings indicate that nicotinic acetylcholine receptors may be active during negative occasion setting and that nicotine can potentiate learned inhibition.

Behavioral and neurochemical characterization of new mouse model of hyperphenylalaninemia

Hyperphenylalaninemia (HPA) refers to all clinical conditions characterized by increased amounts of phenylalanine (PHE) in blood and other tissues. According to their blood PHE concentrations under a free diet, hyperphenylalaninemic patients are commonly classified into phenotypic subtypes: classical phenylketonuria (PKU) (PHE > 1200 µM/L), mild PKU (PHE 600-1200 µM/L) and persistent HPA (PHE 120-600 µM/L) (normal blood PHE < 120 µM/L). The current treatment for hyperphenylalaninemic patients is aimed to keep blood PHE levels within the safe range of 120-360 µM/L through a PHE-restricted diet, difficult to achieve. If untreated, classical PKU presents variable neurological and mental impairment. However, even mildly elevated blood PHE levels, due to a bad compliance to dietary treatment, produce cognitive deficits involving the prefrontal cortical areas, extremely sensible to PHE-induced disturbances. The development of animal models of different degrees of HPA is a useful tool for identifying the metabolic mechanisms underlying cognitive deficits induced by PHE. In this paper we analyzed the behavioral and biochemical phenotypes of different forms of HPA (control, mild-HPA, mild-PKU and classic-PKU), developed on the base of plasma PHE concentrations. Our results demonstrated that mice with different forms of HPA present different phenotypes, characterized by increasing severity of behavioral symptoms and brain aminergic deficits moving from mild HPA to classical PKU forms. In addition, our data identify preFrontal cortex and amygdala as the most affected brain areas and confirm the highest susceptibility of brain serotonin metabolism to mildly elevated blood PHE.

Cellular, molecular, and genetic substrates underlying the impact of nicotine on learning

Addiction is a chronic disorder marked by long-lasting maladaptive changes in behavior and in reward system function. However, the factors that contribute to the behavioral and biological changes that occur with addiction are complex and go beyond reward. Addiction involves changes in cognitive control and the development of disruptive drug-stimuli associations that can drive behavior. A reason for the strong influence drugs of abuse can exert on cognition may be the striking overlap between the neurobiological substrates of addiction and of learning and memory, especially areas involved in declarative memory. Declarative memories are critically involved in the formation of autobiographical memories, and the ability of drugs of abuse to alter these memories could be particularly detrimental. A key structure in this memory system is the hippocampus, which is critically involved in binding multimodal stimuli together to form complex long-term memories. While all drugs of abuse can alter hippocampal function, this review focuses on nicotine. Addiction to tobacco products is insidious, with the majority of smokers wanting to quit; yet the majority of those that attempt to quit fail. Nicotine addiction is associated with the presence of drug-context and drug-cue associations that trigger drug seeking behavior and altered cognition during periods of abstinence, which contributes to relapse. This suggests that understanding the effects of nicotine on learning and memory will advance understanding and potentially facilitate treating nicotine addiction. The following sections examine: (1) how the effects of nicotine on hippocampus-dependent learning change as nicotine administration transitions from acute to chronic and then to withdrawal from chronic treatment and the potential impact of these changes on addiction, (2) how nicotine usurps the cellular mechanisms of synaptic plasticity, (3) the physiological changes in the hippocampus that may contribute to nicotine withdrawal deficits in learning, and (4) the role of genetics and developmental stage (i.e., adolescence) in these effects.

Interactions between mecamylamine and alcohol in Long-Evans rats: flash-evoked potentials, body temperature, behavior, and blood alcohol concentration

Mecamylamine, a noncompetitive antagonist of nicotinic acetylcholine receptors, has many potential clinical applications, including treating alcohol dependency. However, little is known about the combined effects of mecamylamine and alcohol on visual system electrophysiology. We examined the separate and combined effects of mecamylamine (4.0mg/kg, ip) and alcohol (2.0 g/kg, ip) on flash-evoked potentials (FEPs) recorded from the visual cortex (VC) and superior colliculus (SC) of chronically implanted adult male Long-Evans rats. On separate days, either saline or mecamylamine was given 10 min prior to either saline or ethanol. FEPs were recorded 15 and 30 min after the second injection. In the VC, alcohol significantly decreased the amplitudes of components P23, N29, N39, P89, N143, and P237, but increased P46. N63 amplitude was not significantly altered. In contrast, mecamylamine increased the amplitude of P23, P46, and N63, but reduced the amplitude of N29 and P237. The combination of mecamylamine and alcohol resulted in amplitudes very similar to alcohol alone for components P23, N29, N63, P89, N143, and P237. However, mecamylamine pretreatment reduced the effects of alcohol on components N39 and P46. In the SC, FEP component amplitudes were generally decreased by alcohol but not significantly altered by mecamylamine. Mecamylamine pretreatment did not significantly alter the effects of alcohol on SC amplitudes. Latencies of nearly all components in both structures were significantly increased by all drug treatments, with the greatest increase produced by the combination treatment. Hypothermia was also produced by all drug treatments, with the greatest hypothermia (2.25 °C) produced by the combination treatment, most likely accounting for much of the drug-induced increase in latencies. All drug treatments reduced movement during FEP testing, but later in an open field alcohol increased ambulation while mecamylamine reduced movement. Separate groups of experimentally naïve adult male Holtzman albino and Long-Evans hooded rats were given (ip) either alcohol or mecamylamine plus alcohol. Tail vein samples were taken 30 min later. For both rat strains, blood alcohol concentration in the mecamylamine pretreatment group was significantly less at this time interval by about 50-60 mg/dL, suggesting a mechanism whereby mecamylamine can mitigate some of the acute effects of alcohol (e.g., on VC components N39 and P46).

Animal models in the drug discovery pipeline for Alzheimer's disease

With increasing feasibility of predicting conversion of mild cognitive impairment to dementia based on biomarker profiling, the urgent need for efficacious disease-modifying compounds has become even more critical. Despite intensive research, underlying pathophysiological mechanisms remain insufficiently documented for purposeful target discovery. Translational research based on valid animal models may aid in alleviating some of the unmet needs in the current Alzheimer's disease pharmaceutical market, which includes disease-modification, increased efficacy and safety, reduction of the number of treatment unresponsive patients and patient compliance. The development and phenotyping of animal models is indeed essential in Alzheimer's disease-related research as valid models enable the appraisal of early pathological processes - which are often not accessible in patients, and subsequent target discovery and evaluation. This review paper summarizes and critically evaluates currently available animal models, and discusses their value to the Alzheimer drug discovery pipeline. Models dealt with include spontaneous models in various species, including senescence-accelerated mice, chemical and lesion-induced rodent models, and genetically modified models developed in Drosophila melanogaster, Caenorhabditis elegans, Danio rerio and rodents. Although highly valid animal models exist, none of the currently available models recapitulates all aspects of human Alzheimer's disease, and one should always be aware of the potential dangers of uncritical extrapolating from model organisms to a human condition that takes decades to develop and mainly involves higher cognitive functions.

Muscarinic and nicotinic acetylcholine receptor agonists and allosteric modulators for the treatment of schizophrenia

Muscarinic and nicotinic acetylcholine (ACh) receptors (mAChRs and nAChRs) are emerging as important targets for the development of novel treatments for the symptoms associated with schizophrenia. Preclinical and early proof-of-concept clinical studies have provided strong evidence that activators of specific mAChR (M(1) and M(4)) and nAChR (α(7) and α(2)β(4)) subtypes are effective in animal models of antipsychotic-like activity and/or cognitive enhancement, and in the treatment of positive and cognitive symptoms in patients with schizophrenia. While early attempts to develop selective mAChR and nAChR agonists provided important preliminary findings, these compounds have ultimately failed in clinical development due to a lack of true subtype selectivity and subsequent dose-limiting adverse effects. In recent years, there have been major advances in the discovery of highly selective activators for the different mAChR and nAChR subtypes with suitable properties for optimization as potential candidates for clinical trials. One novel strategy has been to identify ligands that activate a specific receptor subtype through actions at sites that are distinct from the highly conserved ACh-binding site, termed allosteric sites. These allosteric activators, both allosteric agonists and positive allosteric modulators, of mAChR and nAChR subtypes demonstrate unique mechanisms of action and high selectivity in vivo, and may provide innovative treatment strategies for schizophrenia.

Scopolamine disrupts hippocampal activity during allocentric spatial memory in humans: an fMRI study using a virtual reality analogue of the Morris Water Maze

The role of the septohippocampal cholinergic system in memory disorders is well established. The effects of cholinergic challenge in animals have been extensively studied using the Morris Water Maze (MWM) which engages allocentric spatial memory. The present study investigated the effect of the centrally active muscarinic antagonist scopolamine on allocentric spatial memory in humans using a virtual reality analogue of the MWM task, the Arena task. Twenty right-handed healthy male adults with a mean age of 28 years (range 23-35 years) were studied using functional MRI in a randomized double-blind cross-over design with scopolamine bromide (0.4 mg i.m.) or placebo (saline) administered 70-90 min before the beginning of the functional scan. Scopolamine induced a significant reduction in the activation of the hippocampus/parahippocampal gyrus compared with placebo. Furthermore, there was dissociation between hippocampus-based and striatal-based memory systems, which were significantly more activated in the placebo and scopolamine conditions, respectively. The activation of the striatal system under scopolamine challenge was accompanied by the activation of the amygdala. In conclusion, the study extends the well-documented finding in animals of the attenuating effect of scopolamine on hippocampal activity during allocentric spatial memory to humans. Furthermore, the results call for further investigation of the dissociation between the hippocampal and neostriatal memory systems during allocentric spatial processing under cholinergic blockade in humans.

Beneficial effects of resveratrol on scopolamine but not mecamylamine induced memory impairment in the passive avoidance and Morris water maze tests in rats

Resveratrol (3,5,4-trihydroxy-trans-stilbene), which is found in grapes and red wine has been shown to protect neuronal cells with its antioxidant activity, improve memory function in dementia and reverse acetylcholine esterase (AChE) activity. The aim of this study was to investigate the effect of resveratrol on emotional and spatial memory in naive rats, as well as on scopolamine- and mecamylamine-induced memory impairment in the passive avoidance and Morris water maze (MWM) tests. Resveratrol (12.5, 25 and 50 mg/kg), scopolamine (0.6 mg/kg) and mecamylamine (10mg/kg) were administered to male Wistar rats. In the passive avoidance test, there was no significant difference in the first day latency between all groups, whereas scopolamine and mecamylamine significantly shortened the second day latency compared to the control group. Resveratrol reversed the effect of scopolamine at all doses used, but it had no effect on mecamylamine-induced memory impairment in the passive avoidance test. Both scopolamine and mecamylamine significantly decreased the time spent in the escape platform quadrant during the probe trial of the MWM test compared to the control group. Resveratrol reversed the effect of scopolamine at all doses, but did not change the effect of mecamylamine in the MWM test. There were no significant differences in the locomotor activities of any of the groups. In conclusion, we suggested that resveratrol had improving effects on learning and memory by acting on muscarinic cholinergic receptors and at least in part, may reverse AChE activity.

Ameliorative effect of traditional Japanese medicine yokukansan on age-related impairments of working memory and reversal learning in rats

Aging is thought to impair prefrontal cortical (PFC) structure-sensitive cognitive functions and flexibility, such as working memory and reversal learning. A traditional Japanese medicine, yokukansan (YKS), is frequently used to treat age-related neurodegenerative disorders such as Alzheimer's disease in Japan, but its pharmacological properties have not been elucidated. The present study was designed to examine whether YKS improves age-related cognitive deficits using aged rats. YKS was administered to 21-month-old rats for 3 months. The ability to learn initially a reward rule for a T-maze discrimination task (initial learning) was examined in young control (4-month-old), aged control (24-month-old) and YKS-treated aged (24-month-old) rats. Subsequently, working memory and reversal learning were examined in delayed alternation and reversal discrimination T-maze tasks, respectively. Locomotor activity was also measured in new environments. Although performance accuracy in the initial learning procedure did not differ among any experimental groups, accuracy in the delayed alternation task was significantly decreased in aged rats compared to young rats. Aged rats also showed significant decreases in accuracy in the reversal discrimination task. YKS treatment significantly ameliorated the age-related decreases in accuracy in the delayed alternation and reversal discrimination tasks. The ameliorative effects of YKS on impaired delayed alternation performance were reduced by intracranial infusions of a dopamine D1 receptor antagonist, SCH 23390, into the prelimbic cortical region of the PFC, and the YKS effects on impaired reversal learning were done by the infusions into the orbitofrontal cortex (OFC). Locomotor activity did not change in any experimental group. Thus, YKS ameliorated age-related impairments of working memory and reversal learning, which might be mediated by a dopaminergic mechanism in the PFC structure. These investigations provide information important for the treatment of brain dysfunctions in the elderly people.

Effects of mecamylamine on flash-evoked potentials, body temperature, and behavior in Long-Evans rats

This experiment examined the effects of mecamylamine, a nicotinic acetylcholine receptor antagonist, on flash-evoked potentials (FEPs) recorded from the visual cortex (VC) and superior colliculus (SC) of chronically implanted male Long-Evans rats, and on body temperature and open field behavior. FEPs were recorded at 20 and 35 min following intraperitoneal injections of saline, and of 0.3, 3.0, and 10.0 mg/kg mecamylamine on separate days. The 0.3 mg/kg dose did not produce significant effects. The amplitude of VC components N₃₀, P₄₈, and P₈₇ increased, N₁₅₀ and P₂₃₁ decreased, and P₂₃, N₄₀, N₅₈, and N₆₈ were unchanged following administration of the 10.0 mg/kg dose. In the SC, component P₂₈ was unaffected, P₃₉ was reduced, and N₄₉ was augmented by the 10.0 mg/kg dose. All component peak latencies were increased by the 3.0 and 10.0 mg/kg doses. Significant hypothermia was also produced by the 3.0 and 10.0 mg/kg doses, suggesting that this was the basis for the increased latencies. The 10.0 mg/kg dose produced a significant decrease in movement during the recording sessions. In subsequent open field observations, both line crossings and rearings were reduced by the 3.0 and 10.0 mg/kg doses. The results suggest that endogenous acetylcholine acting on nicotinic acetylcholine receptors plays at most a modest role in producing FEPs recorded from the VC and SC.

Desensitization of nicotinic acetylcholine receptors as a strategy for drug development

The specific pharmacological response evoked by a nicotinic acetylcholine receptor (nAChR) agonist is governed by the anatomical distribution and expression of each receptor subtype and by the stoichiometry of subunits comprising each subtype. Contributing to this complexity is the ability of agonists that bind to the orthosteric site of the receptor to alter the affinity state of the receptor and induce desensitization and the observation that, at low doses, some nAChR antagonists evoke agonist-like nicotinic responses. Brain concentrations of nicotine rarely increase to the low-mid micromolar concentrations that have been reported to evoke direct agonist-like responses, such as calcium influx or neurotransmitter release. Low microgram per kilogram doses of nicotine administered to humans or to nonhuman primates to improve cognition and working memory probably result only in low nanomolar brain concentrations--more in line with the ability of nicotine to induce receptor desensitization. Here we review data illustrating that nicotine, its major metabolite cotinine, and two novel analogs of choline, JWB1-84-1 [2-(4-(pyridin-3-ylmethyl)piperazin-1-yl)ethanol] and JAY2-22-33, JWB1-84-1 [2-(methyl(pyridine-3-ylmethyl)amino)-ethanol], improve working memory in macaques. The effectiveness of these four compounds in the task was linearly related to their effectiveness in producing desensitization of the pressor response to ganglionic stimulation evoked by a nAChR agonist in rats. Only nicotine evoked an agonist-like action (increased resting blood pressure). Therefore, it is possible to develop new chemical entities that have the ability to desensitize nAChRs without an antecedent agonist action. Because these "silent desensitizers" are probably acting allosterically, an additional degree of subtype specificity could be attained.

Combined neonatal stress and young-adult glucocorticoid stimulation in rats reduce BDNF expression in hippocampus: effects on learning and memory

Epidemiological studies suggest that multiple developmental disruptions are involved in the etiology of psychiatric illnesses including schizophrenia. In addition, altered expression of brain-derived neurotrophic factor (BDNF) has been implicated in these illnesses. In the present study, we examined the combined long-term effect of an early stress, in the form of maternal deprivation, and a later stress, simulated by chronic young-adult treatment with the stress hormone, corticosterone, on BDNF expression in the hippocampus of rats. To assess whether there were behavioral effects, which may correlate with the BDNF changes, learning and memory was tested in the Y-maze test for short term spatial memory, the Morris water maze for long-term spatial memory, and the T-maze test for working memory. Four groups of rats received either no stress, maternal deprivation, corticosterone treatment, or both. Dorsal hippocampus sections obtained from parallel groups were used for BDNF mRNA in situ hybridization. Rats which had undergone both maternal deprivation and corticosterone treatment displayed a unique and significant 25-35% reduction of BDNF expression in the dentate gyrus (DG), and similar trends in the CA1 and CA3 regions of the hippocampus. These "two-hit" animals exhibited a learning delay in the Morris water maze test, a marked deficit in the Y-maze, but little change in the T-maze test. However, some aspects of cognition were also altered in rats with either maternal deprivation or corticosterone treatment. This study demonstrates a persistent effect of two developmental disruptions on BDNF expression in the hippocampus, with parallel, but not completely correlative changes in learning and memory.

Serotonin depletion in the dorsal and ventral hippocampus: effects on locomotor hyperactivity, prepulse inhibition and learning and memory

We present an overview of our studies on the differential role of serotonergic projections from the median raphe nucleus (MRN) and dorsal raphe nucleus (DRN) in behavioural animal models with relevance to schizophrenia. Stereotaxic microinjection of the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) into the MRN or one of its main projections regions, the dorsal hippocampus, induced a marked enhancement of phencyclidine-induced locomotor hyperactivity and a disruption of prepulse inhibition (PPI) in rats. There was no enhancement of locomotor hyperactivity induced by amphetamine or MK-801 or after 5,7-DHT lesions of the DRN or ventral hippocampus. Rats with dorsal hippocampus lesions did not show significant changes in the Y-maze test for short-term spatial memory, the Morris water maze for long-term spatial memory, or in the T-maze delayed alternation test for working memory. These chronic lesion studies suggest a modulatory influence of serotonergic projections from the MRN to the dorsal hippocampus on phencyclidine effects and prepulse inhibition, but not on different forms of learning and memory. The results provide new insight into the role of serotonin in the dorsal hippocampus in aspects of schizophrenia.

Involvement of the sigma1 (sigma1) receptor in the anti-amnesic, but not antidepressant-like, effects of the aminotetrahydrofuran derivative ANAVEX1-41

BACKGROUND AND PURPOSE

Tetrahydro-N, N-dimethyl-5, 5-diphenyl-3-furanmethanamine hydrochloride (ANAVEX1-41) is a potent muscarinic and sigma(1) (sigma (1)) receptor ligand. The sigma (1) receptor modulates glutamatergic and cholinergic responses in the forebrain and selective agonists are potent anti-amnesic and antidepressant DRUGS. WE HAVE HERE ANALYSED THE SIGMA (1) COMPONENT IN THE BEHAVIOURAL EFFECTS OF ANAVEX1-41.

EXPERIMENTAL APPROACH

Binding of ANAVEX1-41 to muscarinic and sigma (1) receptors were measured using cell membranes. Behavioural effects of ANAVEX1-41 were tested in mice using memory (spontaneous alternation, passive avoidance, water-maze) and antidepressant-like activity (forced swimming) procedures.

KEY RESULTS

In vitro, ANAVEX1-41 was a potent muscarinic (M(1)>M(3), M(4)>M(2) with K(i) ranging from 18 to 114 nM) and selective sigma (1) ligand (sigma (1), K(i)=44 nM; sigma (2), K(i)=4 microM). In mice, ANAVEX1-41 failed to affect learning when injected alone (0.03-1 mg kg(-1)), but attenuated scopolamine-induced amnesia with a bell-shaped dose response (maximum at 0.1 mg kg(-1)). The sigma (1) antagonist BD1047 blocked the anti-amnesic effect of ANAVEX1-41 on both short- and long-term memories. Pretreatment with a sigma (1) receptor-directed antisense oligodeoxynucleotide prevented effects of ANAVEX1-41 only in the passive avoidance procedure, measuring long-term memory. ANAVEX1-41 reduced behavioural despair at 30 and 60 mg kg(-1), without involving the sigma (1) receptor, as it was not blocked by BD1047 or the antisense oligodeoxynucleotide.

CONCLUSIONS AND IMPLICATIONS

ANAVEX1-41 is a potent anti-amnesic drug, acting through muscarinic and sigma (1) receptors. The latter component may be involved in the enhancing effects of the drug on long-term memory processes.

Dissociation of cholinergic function in spatial and procedural learning in rats

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

Endogenous glucocorticoids are essential for maintaining prefrontal cortical cognitive function

Glucocorticoid hormones are important in the maintenance of many brain functions. Although their receptors are distributed abundantly throughout the brain, including the prefrontal cortex (PFC), it is not clear how glucocorticoid functions, particularly with regard to cognitive processing in the PFC. There is evidence of PFC cognitive deficits such as working memory impairment in several stress-related neuropsychiatric disorders, including depression, schizophrenia, and Parkinson's disease. Disruption of the hypothalamo-pituitary-adrenal (HPA) system, which is characterized by attenuated glucocorticoid negative feedback, is also observed. In rats, chronic stress induces working memory impairment as a result of decreased dopaminergic transmission in the PFC. These chronically stressed rats also show HPA disruption; this is caused in part by a reduced glucocorticoid response in the PFC. These findings implicate reduced glucocorticoid actions in working memory impairment. In the present study, we examined the effects of the suppression of endogenous glucocorticoids by adrenalectomy (ADX) on working memory in rats and explored the involvement of PFC dopaminergic activities in memory. The ADX impaired working memory, decreased dopamine release, and upregulated D1 receptors in the PFC. These dysfunctions were prevented by corticosterone replacement that reproduced normal physiological plasma levels, indicating that suppression of glucocorticoids causes these dysfunctions. Moreover, the ADX-induced working memory impairment was ameliorated by intra-PFC infusions of a D1 receptor agonist, SKF 81297. Thus, suppression of glucocorticoids impaired working memory through a D1 receptor-mediated hypodopaminergic mechanism in the PFC. This finding indicates that endogenous glucocorticoids are essential for maintaining PFC cognitive function and suggests that HPA disruption contributes to PFC cognitive deficits.

Role of central cholinergic receptor sub-types in spatial working memory: a five-arm maze task in mice provides evidence for a functional role of nicotinic receptors in mediating trace access processes

A delayed-matching spatial working memory protocol in a 5-arm maze was used to test the hypothesis of differential roles for central nicotinic and muscarinic cholinergic receptors in mediating task performance. In experiment 1, using a within subjects-repeated design, groups of C57Bl/6 mice, previously trained to criterion with a 4 h retention interval separating presentation and test phases, received i.p. injections of either saline, scopolamine (0.8 mg/kg), mecamylamine (8.0 mg/kg), or the combination of scopolamine and mecamylamine before re-testing. Injections were given either, a) 15 min pre-presentation or, b) 30 s, c) 15 min, d) 3 h 45 min post-presentation in order to differentially affect the acquisition, trace maintenance and recall phases. Significant decreases in correct responses were observed for each drug treatment but the effects were a function of the time of treatment. Results of condition d), (i.e.15 min before retention test) confirm previous reports of severe disruption by each antagonist and their combination on retention. However, conditions a-c) show a constant disruption by scopolamine, increasing disruption by mecamylamine, whereas the combined treatment was without effect. Although the data show that central nicotinic and muscarinic antagonists both modulate working memory performance, they indicate first, that scopolamine-induced "amnesia" results, not from selective post-synaptic M1 muscarinic blockade but from indirect over-activation of nicotinic receptors. Second, the observation of high levels of retention although nicotinic and muscarinic receptors had undergone combined blockade during a large part of the retention interval is incompatible with the concept that test-induced activation of central cholinergic neurones mediates memory trace maintenance. Finally, taken with data from experiment 2, using a short (20 min) treatment-to-test interval, we conclude that central nicotinic receptors play a key role in attentional processes enabling working memory trace access during retrieval.

Associative and nonassociative learning after chronic imipramine in rats

We investigated effects of 15 daily injections of imipramine (20 mg/kg; in one experiment also 10 and 30 mg/kg). The associative learning types (place learning and object recognition) as well as nonassociative learning (habituation of exploration in an open field and within the object recognition test) were studied. Tests were performed immediately after the final injection (early test) and 24 h after the final injection (late test). The 5-HT(1A), 5-HT(1B/D), 5-HT(2A), beta-adrenergic, D(2) receptors were assayed 24 h after the final injection and the 5-HT(2A) and beta-adrenergic receptors were also measured 60 and 96 h after the final injection. While associative types of learning were impaired in early tests, they remained unaffected in late tests and, while the nonassociative learning (habituation of exploration) remained unaffected in early tests, it was changed in late tests. Measured 24 h after the final injection, imipramine (20 and 30 mg/kg per day) down-regulated the concentration of beta-adrenergic and 5-HT(2A) receptors, while leaving all other measured receptors unaffected. However, only the down-regulation of the 5-HT(2A) receptor outlasted the initial 24-h period after the final injection. On the basis of present and previous results, we interpret the impairment of associative types of learning in early tests as a reflection of anticholinergic effects of imipramine, while the modifications of habituation of exploration in late tests are likely primarily to be mediated by imipramine-provoked regulations of serotonergic receptors.

Mice lacking dopamine D2 and D3 receptors have spatial working memory deficits

Mice deficient for dopamine D(2) and D(3) receptors exhibit blunted c-fos responses to D(1) agonist stimulation. Stereologic cell counting revealed decreased numbers of medial prefrontal cortex neurons that express Fos immunoreactivity in all layers, particularly in the prelimbic and anterior cingulate subregions. Pretreatment of these mutants with a single, low dose of methamphetamine (METH) led to a sustained increase in the number of neurons that express Fos immunoreactivity in response to a D(1) agonist challenge, which was most significant in prelimbic and anterior cingulate subregions. The increased c-fos responses reached wild-type-like levels in METH-pretreated D(2) mutants but remained submaximal in METH-pretreated D(3) mutants. Additional studies tested the performance of wild type and mutants in a delayed alternation test, a cognitive task critically dependent on optimal activation of prefrontal cortical D(1) receptors by synaptically released dopamine. Both D(2) and D(3) mutants exhibited deficits in their spatial working memory, with increasing impairments at increasing delays. Whereas METH pretreatment rescued the spatial working memory of D(2) mutants, it had no effect on D(3) mutants. These data suggest that the sustained improvement of spatial working memory in METH-pretreated D(2) mutants is attributable to D(1) receptor-mediated mechanisms.

Place learning in scopolamine-treated rats: the roles of distal cues and catecholaminergic mediation

Experiments 1 and 2 tested the hypothesis that cholinergic receptor antagonists impair place learning in a water maze by interfering with the processing of distal, visual cues. Extramaze cues were offered to rats in the form of geometrical patterns arranged on the inner circumference of a curtain surrounding the water maze. In Experiment 1 the animals were offered both the distal cues and proximal cues in the form of pingpong balls in fixed positions on the surface of the water while only distal cues were present in Experiment 2. Animals were injected with either scopolamine (0.5 mg/kg body wt) or saline 20 min prior to the daily place learning sessions. Upon reaching criterion level performance the animals were tested on "rotation" sessions on which the distal cues were displaced. The outcome of such "rotations" demonstrated that-regardless of the presence or absence of proximal cues-scopolamine-treated rats relied at least as much as normal animals on the distal cues. The acquisition phase of both Experiments 1 and 2 demonstrated an almost complete lack of scopolamine-associated impairment in acquisition and performance of the place learning task. In Experiment 3 (when scopolamine was no longer administered) the subjects of Experiment 2 were exposed to a series of pharmacological "challenges" of their place learning performance and eventually to surgical ablation of the anteromedial prefrontal cortex. The outcome of the pharmacological challenges and the postoperative test of task performance demonstrated that the place learning performance of animals which had acquired the task under scopolamine was mediated by a neural substrate dissimilar to the substrate of task performance in normal animals. Rats acquiring the task while deprived of the cholinergic system demonstrated above-normal contributions to task mediation from catecholaminergic-probably dopaminergic-mechanisms and tentative results pointed to a "shift" toward prefrontal task mediation.

Cholinergic blockade impairs performance in operant DNMTP in two inbred strains of mice

Cholinergic blockade has been shown to impair performance in delayed nonmatching to position (DNMTP) paradigms in rats. In this study, a murine operant DNMTP task was used to assess the effects of cholinergic antagonism in two strains of mice (DBA/2 and C57BL/6) differing in spatial learning abilities. DNMTP was scheduled in operant chambers with retractable levers, where mice were trained until high levels of accuracy. Subsequently, proactive interference effects were assessed by manipulation of the intertrial interval (ITI), and animals were tested in this task under scopolamine (0.1-1.0 mg/kg) and mecamylamine (0.5-2.0 mg/kg) treatment. Data were analyzed according to the methods of signal detection theory. ITI manipulation decreased accuracy when the time between trials was reduced to 5 s. Cholinergic blockade failed to induce a pure mnemonic impairment but distinguishable effects of both receptor antagonists could be detected: scopolamine disrupted accuracy in a dose-dependent but delay-independent manner, whereas mecamylamine failed to impair accuracy, but decreased responsivity delay- and dose-dependently. Strains mainly differed in responsivity, with DBA/2 showing higher latencies to respond to the levers. These results are comparable to those obtained in rats. Thus, operant DNMTP can be applied to assess working memory in mice.

Effects of scopolamine and D-cycloserine on non-spatial reference memory in rats

The aim of the present work was to use a three-choice simultaneous brightness discrimination test to examine the retention of non-spatial reference memory in rats treated with scopolamine (0.5 mg/kg) alone and in combination with various concentrations of D-cycloserine (DCS) (5, 15 and 50 mg/kg). Scopolamine given 1 h before testing for retention was found to increase both the number of errors and the number of trials necessary to reach criterion. The three doses of DCS reduced this increase significantly and resulted in a U-shaped dose-response curve, where 15 mg/kg had a stronger compensatory effect than both 5 and 50 mg/kg. Control experiments were performed to confirm that the observed response was not due to non-mnemonic factors. Our results indicate that scopolamine and DCS affect non-spatial reference memory in a similar manner as shown for spatial reference memory.

Interaction between the cholinergic system and CRH in the modulation of spatial discrimination learning in mice

Both cholinergic and CRH systems have been linked to cognitive processes such as learning and memory, and neuroanatomical as well as neurochemical evidence suggests important interactions between these two systems. Moreover, recent reports of pro-mnestic effects of CRH open the possibility that CRH could have beneficial effects in animals with cholinergic dysfunction. In a first experiment, spatial discrimination of C57BL/6 mice treated with various doses of scopolamine (0.5--2.0 mg/kg IP) was tested in a two-choice water maze task. Scopolamine, but not methylscopolamine, impaired accuracy and decreased responsivity. In contrast, similar doses of the nicotinic antagonist mecamylamine had no effect on choice accuracy but altered responsivity, as indicated by increased errors of omission and a reduction in swim speed during early experimental stages. ICV CRH (0.5--1.0 microg) also failed to significantly affect accuracy, but a strong tendency was observed to impair percentage correct responses. Measures of responsivity, such as errors of omission, choice latency and distance traveled, and of thigmotaxis were not significantly affected by CRH. However, initial swim speed was reduced by the peptide. Combined treatment with scopolamine (0.5 mg/kg IP) and CRH (0.5 microg ICV) had only mild, and primarily independent, effects, but overall suggested that concomitant blockade of muscarinic receptors and activation of the CRH system would rather act synergistically to disrupt spatial discrimination learning. Synergistic effects were also observed when animals receiving a combination of mecamylamine (2.0 mg/kg IP) and CRH (0.5 microg ICV) were tested, both in terms of responsivity and thigmotaxis, and there was limited evidence that part of these effects were potentiating. Thus, the cholinergic and CRH systems interact in the modulation of learning, but CRH, contrary to prediction, worsens the impairment caused by cholinergic blockade.

Chronic stress induces impairment of spatial working memory because of prefrontal dopaminergic dysfunction

Although the mechanism responsible for cognitive deficits in stress-related neuropsychiatric disorders has been obscure, prefrontal cortical (PFC) dopaminergic dysfunction is thought to be involved. In animals, the mesoprefrontal dopaminergic system is particularly vulnerable to stress, and chronic stress induces working memory impairment. However, the relation between the working memory impairment and altered dopaminergic activity in chronically stressed rats is unclear. Furthermore, the change of dopaminergic activity in the PFC induced by stress is thought to express as a stress response, not as a disorder of organic function. We have previously reported that chronic stress administered by water immersion and restraint for 4 weeks induces a organic disorder such as hippocampal neuronal degeneration. We therefore examined whether chronically stressed (4 weeks) and recovered (10 d) rats show a working memory impairment caused by reduced dopamine (DA) transmission in the PFC, as suspected in the neuropsychiatric disorders. The stress impaired the spatial working memory evaluated by T-maze task and induced a marked reduction of DA transmission concomitant with an increase in DA D1 receptor density in the PFC. This memory impairment was sufficiently ameliorated by intra-PFC infusion of 10 ng SKF 81297, a D1 receptor-specific agonist. Pretreatment with intraperitoneal injection of 20 microgram/kg SCH 23390, a D1 receptor antagonist, reversed the SKF 81297 response. These results indicate that chronic stress induces working memory impairment through a D1 receptor-mediated hypodopaminergic mechanism in the PFC. These findings provide important information for understanding of the mechanisms underlying PFC dysfunction in stress-related neuropsychiatric disorders.

Fetal alcohol-exposed rats exhibit differential response to cholinergic drugs on a delay-dependent memory task

Fetal alcohol exposure in human and rodents produces a number of cognitive deficits including impairments in learning and memory. Recent evidence in our laboratory has shown that fetal alcohol-exposed (FAE) rats respond differently to systemic administration of cholinergic drugs when tested for vigilance and locomotor activity. The present study examined the effects of muscarinic and nicotinic agonists and antagonists on memory performance in a delayed alternation task. Subjects were male offspring of Sprague-Dawley rats fed a 35% ethanol-derived caloric diet, pair-fed with sucrose, or chow-fed with lab chow during the last 2 weeks of gestation. Rats (3 months old) were food-deprived prior to training in the T-maze. Rats were first trained in the alternation task at no delay for five sessions. Rats were then trained at longer delays (20, 60, 180 s) until all groups showed similar performance for two consecutive sessions. Each animal was then tested following systemic injections of the cholinergic antagonists scopolamine and mecamylamine (60-s delay) and the cholinergic agonists pilocarpine and nicotine (180-s delay). Rats received saline injections on alternate days of testing. The results revealed that FAE rats exhibited no impairments in alternation performance at the no delay and 20-s delay, but showed impairments on both the 60- and 180-s delays during the initial sessions. However, with additional training, FAE rats showed performance similar to that of control groups at these delays. Following both pilocarpine and nicotine injections, control groups, but not the FAE group, showed significant memory enhancement in the alternation task. Following scopolamine injections, the FAE rats showed a significant impairment, while control groups showed a nonsignificant decrease in performance. All three groups showed impairments in the alternation task following administration of mecamylamine compared to saline treatment. These findings suggest that alterations in the cholinergic system in FAE rats may underlie some of the cognitive deficits observed with prenatal alcohol exposure.

A structural basis for memory storage in mammals

It is proposed that altered dendrite length and de novo formation of new dendrite branches in cholinoceptive cells are responsible for long-term memory storage, a process enabled by the degradation of microtubule-associated protein-2. These memories are encoded as modality-specific associable representations. Accordingly, associable representations are confined to cytoarchitectonic modules of the cerebral cortex, hippocampus, and amygdala. The proposed sequence of events leading to long-term storage in cholinoceptive dendrites begins with changes in neuronal activity, then in neurotrophin release, followed by enhanced acetylcholine release, muscarinic response, calcium influx, degradation of microtubule-associated protein-2, and finally new dendrite structure. Hypothetically, each associable representation consists of altered dendrite segments from approximately 5000-15,000 cholinoceptive cells contained within one or a few module(s). Simultaneous restructuring during consolidation of long-term memory is hypothesized to result in a similar infrastructure among dendrite sets, facilitating co-activation of those dendrite sets by neurotransmitters such as acetylcholine, and conceivably enabling high energy interactions between those dendrites by phenomena such as quantum optical coherence. Based on the specific architecture proposed, it is estimated that the human telecephalon contains enough dendrites to encode 50 million associable representations in a lifetime, or put another way, to encode one new associable representation each minute. The implications that this proposal has regarding treatments for Alzheimer's disease are also discussed.

The role of the dorsal raphe-serotonergic system and cholinergic receptors in the modulation of working memory

The present study investigated the role of the dorsal raphe-serotonergic system and its interaction with muscarinic or nicotinic receptors in the modulation of working memory and motor activity by assessing the effects of serotonin lesion with pCA and cholinergic receptor blockade on the performance of rats in a working memory (delayed non-matching to position, DNMTP) task. The pCA lesion did not impair the choice accuracy or motor activity of rats in the DNMTP-task. The lower dose of scopolamine (0.075 mg/kg) impaired percent correct responses already at the shortest delay which is not indicative of a working memory impairment per se. Scopolamine also disrupted motor activity markedly. The effects of scopolamine 0.075 mg/kg on the choice accuracy were aggravated by pCA treatment. Furthermore, the effects of N-methylscopolamine (0.150 mg/kg) were comparable with scopolamine. The higher dose of mecamylamine (3.0 mg/kg) also interfered with motor activity and it decreased the choice accuracy. The performance disruption induced by mecamylamine was not as severe as that seen with scopolamine. Mecamylamine did not reveal any interaction with the serotonergic lesion. Hexamethonium slightly decreased the percent correct responses, while not interfering with motor activity of rats. The present results suggest that: (i) lesion of serotonergic fibers with pCA does not significantly impair the choice accuracy or interfere with motor activity of rats; (ii) the blockade of cholinergic receptors does not impair working memory per se, but disrupts motor activity, and (iii) pCA lesion of serotonergic fibers aggravates the non-mnemonic choice accuracy impairment induced by central muscarinic blockade, while not interacting with the cholinolytics in modulation of motor activity.

Recognition memory in rats--III. Neurochemical substrates

In the first part of three overviews on recognition memory in the rat, we discussed the tasks employed to study recognition memory. In the second part, we discussed the neuroanatomical systems thought to be of importance for the mediation of recognition memory in the rat. In particular, we delineated two parallel-distributed neuronal networks, one that is essential for the processing of non-spatial/item recognition memory processes and incorporates the cortical association areas such as TE1, TE2 and TE3, the rhinal cortices, the mediodorsal thalamic nucleus and prefrontal cortical areas (Network 1), the other comprising of the hippocampus, mamillary bodies, anterior thalamic nuclei and medial prefrontal areas (Network 2), suggested to be pivotal for the processing of spatial recognition memory. The next step will progress to the level of the neurotransmitters thought to be involved. Current data suggest that the majority of drugs have non-specific, i.e. delay-independent effects in tasks measuring recognition memory. This may be due to attentional, motivational or motoric changes. Alternatively, delay-independent effects may result from altered acquisition/encoding rather than from altered retention. Furthermore, the neurotransmitter systems affected by these drugs could be important as modulators rather than as mediators of recognition memory per se. It could, of course, also be the case that systemic treatment induces non-specific effects which overshadow any specific, delay-dependent, effect. This possibility receives support from lesion experiments (for example, of the septohippocampal cholinergic system) or studies employing local intracerebral infusion techniques. However, it is evident that those delay-dependent effects are relatively subtle and more readily seen in delayed response paradigms, which tax spatial recognition memory. One interpretation of these results could be that some neurotransmitter systems are more involved in spatial than in item recognition memory processes. However, performance in delayed response tasks can be aided by mediating strategies. Drugs or lesions can alter those strategies, which could equally explain some of the (delay-dependent) drug effects on delayed responding. Thus, it is evident that neither of the neurotransmitter systems reviewed (glutamate, GABA, acetylcholine, serotonin, dopamine and noradrenaline) can be viewed as being directly and exclusively concerned with storage/retention. Rather, our model of recognition memory suggests that information about previously encountered items is differentially processed by distinct neural networks and is not mediated by a single neurotransmitter type.

Chronic nicotinic agonist and antagonist effects on T-maze alternation

A variety of studies have found that nicotine improves working memory function. However, other studies have either not found improvements or have found nicotine-induced deficits. The demands of the particular memory test may be critical for the expression of the nicotine effects. In several studies, we have found that chronic nicotine administration improves working memory performance in the radial arm maze. Chronic mecamylamine coadministration reversed this effect. The current study was conducted to determine the effects of chronic nicotine and mecamylamine on choice accuracy in a T-maze spatial alternation task. The same dose and duration of nicotine administration that we have previously found to significantly improve choice accuracy in the radial-arm maze was not effective in altering T-maze spatial alternation. The critical difference in task demands may be the presence with T-maze alternation of proactive interference. During a session, a choice alternative repeatedly changes valence from correct to incorrect and back again. In contrast, with the radial-arm maze as run in our studies, in a session the valence of an arm only changes once from correct to incorrect. Previous work with nicotine effects on spatial alternation in an operant task found evidence that nicotine increased the negative effect of proactive interference on performance. In the current study, chronic mecamylamine caused a significant deficit in T-maze spatial alternation. This same dose did not produce a deficit in the radial-arm maze and, in fact, caused an improvement during the first week of administration.

Nicotinic, muscarinic and dopaminergic actions in the ventral hippocampus and the nucleus accumbens: effects on spatial working memory in rats

Acetylcholine (ACh) systems have been widely shown to be important for memory. In particular, ACh hippocampal neurons are critical for memory formation, though ACh innervation of other areas such as the nucleus accumbens may also be important. There has also been increasing interest in ACh and dopaminergic (DA) interactions with regard to short-term spatial memory. In a series of studies, we have found that ACh and DA agonists and antagonists given systemically interact to influence memory. The critical neural loci of these interactions are not currently known. In the present study, we used local infusion techniques to examine the role of ACh and DA transmitter systems in the nucleus accumbens and the ventral hippocampus on radial-arm maze (RAM) working memory performance. Into the nucleus accumbens of rats, we infused the nicotinic ACh agonist nicotine, the nicotinic ACh antagonist mecamylamine, the DA agonist apomorphine, or the DA antagonist haloperidol. Into the ventral hippocampus, we infused nicotine, mecamylamine, the muscarinic ACh agonist pilocarpine, or the muscarinic ACh antagonist, scopolamine. The nicotinic ACh and DA interaction was tested by a hippocampal infusion of mecamylamine alone or together with the DA D2 agonist quinpirole given via subcutaneous injection. The results confirmed that both nicotinic and muscarinic ACh receptors in the ventral hippocampus play a significant role in spatial working memory. Blockade of either nicotinic or muscarinic ACh receptors caused significant impairments in RAM choice accuracy. However, infusion of either nicotinic or muscarinic agonists failed to improve choice accuracy. The interaction of DA D2 systems in different with hippocampal nicotinic blockade than with general nicotinic blockade. Systemic administration of quinpirole potentiated the amnestic effect of mecamylamine infused into the ventral hippocampus, whereas it was previously found to reverse the amnestic effect of systemically administered mecamylamine. In contrast to the significant effects of mecamylamine in the hippocampus, no effects were found after infusion into the nucleus accumbens. Nicotine also was not found to have a significant effect on memory after intra-accumbens infusion. Neither the DA agonist apomorphine nor the DA antagonist haloperidol had a significant effect on memory after infusion into the nucleus accumbens. This study provides support for the involvement of nicotinic and muscarinic receptors in the ventral hippocampus in memory function. Ventral hippocampal nicotinic systems have significant interactions with D2 systems, but these differ from their systemic interactions. In contrast, nicotinic ACh and DA systems in the nucleus accumbens were not found in the current study to be important for working memory performance in the RAM.

Pharmacology of nicotine and its therapeutic use in smoking cessation and neurodegenerative disorders

During the last decade, nicotine has been used increasingly as an aid to smoking cessation and has been found to be a safe and efficacious treatment for the symptoms of nicotine withdrawal. This period has also seen significant advances in our understanding of the mechanisms underlying the psychopharmacological responses to nicotine, including, particularly, those that have been implicated in nicotine addiction. This paper reviews this decade of progress in the specific context of the therapeutic application of nicotine to the treatment of smoking cessation. Other putative future applications, particularly in the treatment of neurodegenerative disorders, are also reviewed.

Acetylcholine: a neurotransmitter for learning and memory?

The cholinergic hypothesis claims that the decline in cognitive functions in dementia is predominantly related to a decrease in cholinergic neurotransmission. This hypothesis has led to great interest in the putative involvement of the cholinergic neurotransmission in learning and memory processes. This review aims to assess the data of studies in which the role of acetylcholine (ACh) in cognitive functions was investigated. For this purpose, studies from three different fields of research, namely: (1) behavioral pharmacology (effects of drugs on behavior); (2) behavioral neuroscience (effects of brain lesions on behavior); and (3) dementia, are discussed separately. The experimental tools that have been used in pharmacological studies may appear to be inadequate to enable conclusions to be drawn about the involvement of ACh in learning and memory processes. Especially, the use of scopolamine as a pharmacological tool is criticized. In the field of behavioral neuroscience a highly specific cholinergic toxin has been developed. It appears that the greater and more specific the cholinergic damage, the fewer effects can be observed at the behavioral level. The correlation between the decrease in cholinergic markers and the cognitive decline in dementia may not be as clearcut as has been assumed. The involvement of other neurotransmitter systems in cognitive functions is briefly discussed. Taking into account the results of the different fields of research, the notion that ACh plays a pivotal role in learning and memory processes seems to be overstated. Even when the role of other neurotransmitter systems in learning and memory is taken into consideration, it is unlikely that ACh has a specific role in these processes. On basis of the available data, ACh seems to be more specifically involved in attentional processes than in learning and memory processes.

The ambivalent behaviour "stretched approach posture" in the rat as a paradigm to characterize anxiolytic drugs

The effect of various psychotropic drugs on the ambivalent behaviour "stretched approach posture" (SAP) in the rat was assessed. SAP was elicited after a mild startle reaction due to physical contact with an electrified prod at one end of a straight runway. Using ethological observation methods, SAP as well as intention movements, prod contact, crossings, rearing, exploration, grooming and immobility were recorded. The benzodiazepine receptor agonists chlordiazepoxide, diazepam and alprazolam, the 5-HT1A receptor agonists flesinoxan and ipsapirone and the 5-HT uptake inhibitor clomipramine selectively (no effect on crossings) reduced SAP. Except for alprazolam, these drugs also reduced intention movements. In addition, chlordiazepoxide and diazepam enhanced prod contact. Reductions of SAP and intentions with concomitant reductions of crossings (nonspecific antiambivalent effects) were established for the alpha 2-adrenoceptor agonist clonidine and the MAO inhibitor clorgyline. The 5-HT uptake inhibitor fluvoxamine suppressed intention movements, but not SAP. The mixed 5-HT/NA uptake inhibitor imipramine did not significantly affect SAP or intentions, but reduced crossings. The 5-HT2C/1B receptor agonist m-CPP, the inverse BZD receptor agonists FG 7142 and DMCM, and the alpha 2-adrenoceptor antagonist yohimbine, to all of which putative anxiogenic effects have been ascribed, had no effect on SAP directed towards the prod. m-CPP, however, produced an increase in the stretched posture directed away from the prod (SAwayP). FG 7142 reduced intentions while strongly enhancing immobility (freezing). SAwayP and/or freezing may possibly reflect anxiogenic properties of drugs. The putative anxiogenic drug pentylenetetrazol false positively reduced SAP while increasing exploration. The dopamine-D2 receptor antagonist haloperidol and the catecholamine releaser dl-amphetamine had no effect on ambivalent behaviour. The muscarine receptor antagonist scopolamine reduced SAP and intentions while stimulating crossings. Finally, the 5-HT2C receptor antagonist ritanserine, the CCKA receptor antagonist devazepide, the CCKB receptor antagonist L-365.260 and the strychnine-insensitive glycine site antagonist 7-Cl-kynurenic acid were without effect on the behaviours in this paradigm using single doses. In conclusion, SAP and intention movements were reduced selectively by anxiolytic agents from different classes, including benzodiazepine receptor agonists, 5-HT1A receptor agonists and a 5-HT uptake inhibitor, whereas an alpha 2-adrenoceptor agonist and a MAO inhibitor reduced SAP non-selectively. SAP in relation to other behaviours may therefore serve as a valuable paradigm to characterize anxiolytic drugs.

Cholinergic modulation of a decrement in social investigation following repeated contacts between mice

Social recognition has been inferred from a decline in olfactory investigation of conspecific intruders during repeated or protracted confrontation with a resident rat. A stimulus-response relationship defined by lack of response remains somewhat ambiguous. Since it is likely that behavior continues to be emitted by the resident animal, how behavior reorganizes as the resident becomes familiar with an intruder represents an important issue in the characterization of recognition. We examined the decline in olfactory investigation of ovariectomized females by adult male mice. The duration and frequency of olfactory investigation was measured during four 1 minute confrontations with 10-min intertrial intervals (Training trials). If the same female was presented in each trial, investigation declined to less than 50% of initial levels. Aggressive behavior gradually increased with repeated trials. No decline in investigation or increased aggression was measured when females were changed in each trial. Administration of doses of scopolamine (0.16-1.0 mg/kg, IP) blocked decrements in olfactory investigation in repeated confrontations and significantly reduced aggression. Co-administration of heptylphysostigmine (0.32-5.0 mg/kg, IP) reversed scopolamine's effects on olfactory investigation but not aggression. Acetylcholinesterase inhibitors heptylphysostigmine, galanthamine (0.63-2.5 mg/kg, IP) and tacrine (0.63-10.0 mg/kg, IP) all enhanced the rate of decrement of olfactory investigation when administered alone, but had differential effects on aggression. The decline in investigation corresponds to criteria for habituation. Increased responsivity expressed as aggression indicates recognition may also be characterized as a change in behavioral strategy dependent on the sexual and social status of the stimulus animal. Pharmacological data support a role for acetylcholine release in the development of social recognition as an olfactory memory, or through modulation of olfactory perception.

Nicotinic and muscarinic receptors in the rat prefrontal cortex: differential roles in working memory, response selection and effortful processing

The aim of the present study was to evaluate the effects of cholinergic receptor blockade in the rat prefrontal cortex on cognitive processes. The nicotinic antagonists neuronal bungarotoxin and dihydro-beta-erythroidine and the muscarinic antagonist scopolamine were injected into the prelimbic area of the prefrontal cortex. Their behavioural effects were assessed in a T-maze to test reference memory (visual discrimination task) and working memory in delayed matching (MTS) and non-matching to sample (NMTS) tasks. Neuronal bungarotoxin produced a significant decrease in working memory performance in the MTS task but not in the NMTS task. In contrast, scopolamine impaired working memory in both MTS and NMTS tasks. Reference memory was not altered by any of the cholinergic antagonists. These results demonstrate a differential role of nicotinic and muscarinic receptors in the rat prefrontal cortex. Nicotinic transmission appears to be important in delayed response tasks requiring effortful processing for response selection, while the muscarinic system is involved in general working memory processes.

Pharmacology of memory: cholinergic-glutamatergic interactions

Both acetylcholine and glutamate are now thought to play important roles in memory. Recent evidence suggests that the interaction of these two neurotransmitters may be important for some forms of memory, and that acetylcholine, in particular, may function to facilitate glutamate activity by coordinating states of acquisition and recall in the cortex and hippocampus.