Scopolamine selectively disrupts the acquisition of contextual fear conditioning in rats

Differential role of GABAergic and cholinergic ventral pallidal neurons in behavioral despair, conditioned fear memory and active coping

The ventral pallidum (VP), a major component of the reward circuit, is well-associated with appetitive behaviors. Recent evidence suggests that this basal forebrain nucleus may have an overarching role in affective processing, including behavioral responses to aversive stimuli. We investigated this by utilizing selective immunotoxin lesions and a series of behavioral tests in adult male Wistar rats. We made bilateral GAT1-Saporin, 192-IgG-Saporin or PBS (vehicle) injections into the VP to respectively eliminate GABAergic and cholinergic neurons, and tested the animals in the forced swim test (FST), open field test (OFT), elevated plus maze (EPM), Morris water maze (MWM) and cued fear conditioning. Both GAT1-Saporin and 192-IgG-Saporin injections reduced behavioral despair without altering general locomotor activity. During the acquisition phase of cued fear conditioning, this antidepressant effect was accompanied by reduced freezing and increased darting in the 192-IgG-Saporin group, and increased jumping in the GAT1-Saporin group. In the extinction phase, cholinergic lesions impaired fear memory irrespective of the context, while GABAergic lesions reduced memory durability only during the early phases of extinction in a novel context. In line with this, selective cholinergic, but not GABAergic, lesions impaired spatial memory in the MWM. We observed no consistent effect in anxiety-like behavior assessed in the OFT and EPM. These findings indicate that both the GABAergic and cholinergic neuronal groups of the VP may contribute to emotion regulation through modulation of behavioral despair and acquired fear by suppressing active coping and promoting species-specific passive behaviors.

Muscarinic antagonists impair multiple aspects of operant discrimination learning and performance

Acetylcholine signaling can strengthen associations between environmental cues and reward availability. Diverse subtypes (M1-M5) of the muscarinic acetylcholine receptor (mAChR) family may have distinct roles in different learning and memory processes, such as encoding cue-reward associations and consolidating these associations in long-term memory. Using an operant discrimination learning task in which mice are trained to nose poke during a tone to receive a food reward, we found that acquisition of the task requires mAChR signaling in the central nervous system. In addition, post-session injections of a broad mAChR antagonist, scopolamine impaired consolidation of the cue-reward memory. Further, after successful learning of a cue-reward contingency across multiple training sessions, mice that received a single pre-session injection of scopolamine were unable to use the learned cue association to receive rewards. Taken together, these data demonstrate distinct roles for muscarinic signaling in acquisition, consolidation and recall of the operant discrimination learning task. Understanding mechanisms underlying natural reward-related responding may provide insight into other maladaptive forms of reward learning such as addiction.

D-ribose-L-cysteine enhances memory task, attenuates oxidative stress and acetyl-cholinesterase activity in scopolamine amnesic mice

d-Ribose-l-cysteine (DRLC) is an analogue of cysteine that has been shown to boost cellular antioxidant capacity by enhancing intracellular biosynthesis of glutathione (GSH). Deficiency of GSH has been implicated in the pathogenesis of Alzheimer's disease (AD), a neurodegenerative disorder associated with loss of memory. Thus, the use of antioxidants to prevent or retard the progression of memory deteriorations in persons with AD has been the focus of intense investigations. This study was carried out to evaluate the effects of DRLC on memory and scopolamine-induced amnesia, acetyl-cholinesterase activity, and oxidative stress in mice. Male Swiss mice were given oral administration of saline (10 ml/kg), DRLC (25, 50, and 100 mg/kg) or donepezil (1 mg/kg) 30 min before testing for memory performance using Y-maze and object recognition models. Another set of mice were also pretreated orally with saline, DRLC (25, 50, and 100 mg/kg) or donepezil (1 mg/kg) but in combination with scopolamine (3 mg/kg, i.p.) daily for 7 days. Thirty minutes after treatment on Day 7, memory function was then evaluated. The brain levels of acetyl-cholinesterase and oxidative stress parameters were assayed. DRLC significantly (p < .05) enhanced memory performance and attenuated scopolamine-induced amnesia. Increased acetyl-cholinesterase activity and oxidative stress, as shown by decreased antioxidant substrates (glutathione and catalase) and elevated malondialdehyde contents in mice with scopolamine amnesia were also attenuated by DRLC. Our findings suggest that inhibition of oxidative stress and acetyl-cholinesterase activity might contribute to the potential benefit of DRLC in persons with amnesia.

M(4) muscarinic receptors and locomotor activity regulation

M(4) muscarinic receptors (M(4) MR) represent a subfamily of G-protein coupled receptors serving a substantial role in spontaneous locomotor activity regulation, cognition and modulation of cholinergic system. With increasing body of literature discussing the role of M(4) MR some controversies arose. Thus, we try here to summarize the current evidence regarding the M(4) MR, with the special focus on their role in Locomotor activity control. We review the molecular function of M(4) MR in specific brain areas implicated in locomotor regulation, and shortly in other CNS processes that could be connected to locomotor activity. We also focus on brain areas implicated in locomotor activity biorhythm changes like suprachiasmatic nucleus, subparaventricular zone posterior hypothalamic area, striatum and thalamus. Gender-related aspects and differences in locomotor activity in males and females are discussed further.

Gene expression analysis of Arc mRNA as a neuronal cell activity marker in the hippocampus and amygdala in two-way active avoidance test in rats

INTRODUCTION

Immediate early genes are widely used as neuronal cell activity markers in neuroscience. The present study investigated the relationship between their expression and abnormality in context fear conditioning.

METHODS

The learning test (two-way active avoidance test) was conducted in male rats administered with nonselective muscarinic antagonist scopolamine or selective dopamine D1-like receptor antagonist SCH 23390 at a dose level of 2.0 or 0.1mg/kg, respectively, for 4days. Expression levels of Arc and Fos mRNA in the hippocampus and amygdala were also evaluated on the second day of dosing by fluorescent in situ hybridization (FISH) and reverse transcriptase-quantitative polymerase chain reaction (RT-qPCR).

RESULTS

Scopolamine had no effect on avoidance rate, but decreased freezing in the two-way active avoidance test. SCH 23390 decreased avoidance rate and increased freezing. In FISH and RT-qPCR assays, scopolamine decreased Arc mRNA in the hippocampus and amygdala, whereas SCH 23390 increased Arc mRNA in the hippocampus. By contrast, scopolamine and SCH 23390 did not change Fos mRNA expression compared to Arc mRNA expression.

DISCUSSION

The results of the learning test indicated that scopolamine or SCH 23390 respectively inhibited fear or context conditioning in rats. Furthermore, alteration of the expression of Arc mRNA but not of Fos mRNA in the hippocampus and amygdala of the brain was suggested to be a sensitive neuronal cell activity marker to detect behavioral abnormality in the two-way active avoidance test.

Low-dose systemic scopolamine disrupts context conditioning in rats

Cholinergic neurotransmission plays a key role in learning and memory. Prior research with rats indicated that a low dose of pre-training scopolamine (0.1 mg/kg), a cholinergic receptor antagonist, did not affect cued fear conditioning, but did block renewal when injected before extinguishing a conditioned tone, opening up opportunities to pharmacologically improve exposure therapy for anxiety patients. Before translating these findings to the clinic, it is important to carefully examine how scopolamine affects contextual fear memories. Here, we investigated the effects of scopolamine on encoding of contextual anxiety and its generalization in male Wistar rats. We found a profound disruption of context conditioning, suggesting that, even at a low dose, systemic scopolamine may influence contextual encoding in the hippocampus, particularly when the context is the best predictor for the presence of shocks.

Neurobiological mechanisms of state-dependent learning

State-dependent learning (SDL) is a phenomenon relating to information storage and retrieval restricted to discrete states. While extensively studied using psychopharmacological approaches, SDL has not been subjected to rigorous neuroscientific study. Here we present an overview of approaches historically used to induce SDL, and highlight some of the known neurobiological mechanisms, in particular those related to inhibitory neurotransmission and its regulation by microRNAs (miR). We also propose novel cellular and circuit mechanisms as contributing factors. Lastly, we discuss the implications of advancing our knowledge on SDL, both for most fundamental processes of learning and memory as well as for development and maintenance of psychopathology.

Neural circuits involved in the renewal of extinguished fear

The last 10 years have witnessed a substantial progress in understanding the neural mechanisms for the renewal of the extinguished fear memory. Based on the theory of fear extinction, exposure therapy has been developed as a typical cognitive behavioral therapy for posttraumatic stress disorder. Although the fear memory can be extinguished by repeated presentation of conditioned stimulus without unconditioned stimulus, the fear memory is not erased and tends to relapse outside of extinction context, which is referred to as renewal. Therefore, the renewal is regarded as a great obstruction interfering with the effect of exposure therapy. In recent years, there has been a great deal of studies in understanding the neurobiological underpinnings of fear renewal. These offer a foundation upon which novel therapeutic interventions for the renewal may be built. This review focuses on behavioral, anatomical and electrophysiological studies that interpret roles of the hippocampus, prelimbic cortex and amygdala as well as the connections between them for the renewal of the extinguished fear. Additionally, this review suggests the possible pathways for the renewal: (1) the prelimbic cortex may integrate contextual information from hippocampal inputs and project to the basolateral amygdala to mediate the renewal of extinguished fear memory; the ventral hippocampus may innervate the activities of the basolateral amygdala or the central amygdala directly for the renewal. © 2017 IUBMB Life, 69(7):470-478, 2017.

Antagonism of muscarinic acetylcholine receptors in medial prefrontal cortex disrupts the context preexposure facilitation effect

Cholinergic function plays a role in a variant of context fear conditioning known as the context preexposure facilitation effect (CPFE; Robinson-Drummer, Dokovna, Heroux, & Stanton, 2016). In the CPFE, acquisition of a context representation, the context-shock association, and expression of context fear occur across successive phases, usually 24h apart. Systemic administration of scopolamine, a muscarinic acetylcholine receptor antagonist, prior to each phase (context preexposure, immediate-shock training, and testing) disrupts the CPFE in juvenile rats (Robinson-Drummer et al., 2016). Dorsal hippocampal (dHPC) cholinergic function contributes significantly to this effect, as local infusion of scopolamine into the dHPC prior to any individual phase of the CPFE produces a disruption identical to systemic administration (Robinson-Drummer et al., 2016). The current experiment extended these findings to another forebrain region implicated in the CPFE, the medial prefrontal cortex (mPFC). Adolescent rats received bilateral infusions of scopolamine (35μg/side) or PBS 10min before all three phases of the CPFE or only prior to a single phase. Intra-mPFC administration of scopolamine prior to all three phases significantly impaired fear conditioning suggesting that mPFC cholinergic function is necessary for successful CPFE performance. Analyses of the individual infusion days revealed a significant impairment of the CPFE when infusions occurred prior to preexposure or training (i.e. immediate footshock) but not prior to testing. In total, these findings suggests a role of mPFC cholinergic function in the acquisition and/or consolidation of a contextual representation and the context-shock association but not in retrieval or expression of fear memory. Implications for mPFC involvement in contextual fear conditioning and neurological dysfunction following neonatal alcohol exposure are discussed.

M1 muscarinic receptors are necessary for retrieval of remote context fear memory

Several studies have investigated the transition of consolidation of recent memory to remote memory in aversively motivated tasks, such as contextual fear conditioning (CFC) and inhibitory avoidance (IA). However, the mechanisms that serve the retrieval of remote memories, has not yet been fully understood. Some evidences suggest that the central cholinergic system appears be involved in the modulation of these processes. Therefore, the present study aimed to investigate the effects of a pre-test administration of dicyclomine, a high-affinity M1 muscarinic receptor antagonist, on the retrieval of remote memories in fear conditioning and IA tasks. Male Wistar rats were trained, and after 1 or 28days, the rats received dicyclomine (16 or 32mg/kg, intraperitoneally, i.p.) and were tested in CFC, tone fear conditioning (TFC) and IA tasks. At both time intervals, 32mg/kg dicyclomine induced impairment of CFC. In TFC task only the performance of the rats 28days after training was impaired. The IA task was not affected in any of the studied intervals. These findings suggest a differential contribution of muscarinic receptors on recent and remote memories retrieval revealing a more generalized role in remote memory.

Cholinergic regulation of fear learning and extinction

Cholinergic activation regulates cognitive function, particularly long-term memory consolidation. This Review presents an overview of the anatomical, neurochemical, and pharmacological evidence supporting the cholinergic regulation of Pavlovian contextual and cue-conditioned fear learning and extinction. Basal forebrain cholinergic neurons provide inputs to neocortical regions and subcortical limbic structures such as the hippocampus and amygdala. Pharmacological manipulations of muscarinic and nicotinic receptors support the role of cholinergic processes in the amygdala, hippocampus, and prefrontal cortex in modulating the learning and extinction of contexts or cues associated with threat. Additional evidence from lesion studies and analysis of in vivo acetylcholine release with microdialysis similarly support a critical role of cholinergic neurotransmission in corticoamygdalar or corticohippocampal circuits during acquisition of fear extinction. Although a few studies have suggested a complex role of cholinergic neurotransmission in the cellular plasticity essential for extinction learning, more work is required to elucidate the exact cholinergic mechanisms and physiological role of muscarinic and nicotinic receptors in these fear circuits. Such studies are important for elucidating the role of cholinergic neurotransmission in disorders such as posttraumatic stress disorder that involve deficits in extinction learning as well as for developing novel therapeutic approaches for such disorders. © 2016 Wiley Periodicals, Inc.

Cholinergic mechanisms of the context preexposure facilitation effect in adolescent rats

The context preexposure facilitation effect (CPFE) is a variant of contextual fear conditioning in which context learning, context-shock association, and expression of context conditioning occur in 3 separate phases-preexposure, training, and testing. During the preexposure phase, the CPFE is disrupted by hippocampal NMDA receptor blockade in juvenile rats (Schiffino et al., 2011), and a similar deficit is seen with a subcutaneous injection of the muscarinic receptor antagonist, scopolamine, in adult mice (Brown, Kennard, Sherer, Comalli, & Woodruff-Pak, 2011). As a foundation for further developmental research, the present study examined the role of cholinergic function in the CPFE in adolescent rats during each phase of the CPFE protocol. In Experiment 1, an i.p injection of either 0.5 or 1.0 mg/kg dose of scopolamine administered prior to all 3 phases of the CPFE protocol impaired the CPFE. Experiment 2 further showed that a 0.5 mg/kg injection prior to just 1 of the 3 phases of the CPFE also disrupted contextual fear conditioning. We further showed that the CPFE is impaired by localized scopolamine infusions into dorsal hippocampus on the preexposure day (Experiment 3a), training day (Experiment 3b), and test day (Experiment 3c). These findings demonstrate a role of cholinergic signaling in hippocampus during each of the 3 phases of the CPFE in adolescent rats. Implications for the development and neural basis of the CPFE are discussed. (PsycINFO Database Record

Rethinking Extinction

Extinction serves as the leading theoretical framework and experimental model to describe how learned behaviors diminish through absence of anticipated reinforcement. In the past decade, extinction has moved beyond the realm of associative learning theory and behavioral experimentation in animals and has become a topic of considerable interest in the neuroscience of learning, memory, and emotion. Here, we review research and theories of extinction, both as a learning process and as a behavioral technique, and consider whether traditional understandings warrant a re-examination. We discuss the neurobiology, cognitive factors, and major computational theories, and revisit the predominant view that extinction results in new learning that interferes with expression of the original memory. Additionally, we reconsider the limitations of extinction as a technique to prevent the relapse of maladaptive behavior and discuss novel approaches, informed by contemporary theoretical advances, that augment traditional extinction methods to target and potentially alter maladaptive memories.

M3 muscarinic receptor in the ventral medial prefrontal cortex modulating the expression of contextual fear conditioning in rats

RATIONALE

Basal forebrain cholinergic neurons modulate the activation of cortical neurons by several stimuli such as fear and anxiety. However, the role of the muscarinic receptor in the medial prefrontal cortex (MPFC) in the modulation of the conditioned emotional response (CER) evoked in the model contextual conditioned fear remains unclear.

OBJECTIVES

The objective of this study is to test the hypothesis that inhibition of the muscarinic receptor in ventral MPFC modulates CER observed during animal's re-exposure to the aversive context.

METHODS

Rats implanted with cannulae aimed at the prelimbic (PL) or the infralimbic (IL) were submitted to a high-intensity contextual fear conditioning protocol. Before the test session, they received microinjections of the hemicholinium (choline reuptake blocker), atropine (muscarinic antagonist), J104129 fumarate (M1-M3 muscarinic antagonists), pirenzepine (M1 muscarinic antagonist), neostigmine (inhibitor acetylcholinesterase enzyme), or the systemic administration of the FG7142 (inverse benzodiazepine agonist). Additional independent groups received the neostigmine or FG7142 before the ineffective doses of J104129 fumarate in the low-intensity protocol of contextual fear conditioning.

RESULTS

In the high-intensity protocol, the administration of hemicholinium (1 nmol), atropine (0.06-6 nmol), J104129 fumarate (6 nmol), or pirenzepine (6 nmol) attenuated the expression of CER in rats. However, in the low-intensity protocol, only J10129 fumarate (0.06 nmol) reduced the expression of the CER. Finally, neostigmine (0.1-1 nmol) or FG7142 (8 mg/Kg) increased CER expression, an effect inhibited by the low dose of the J10129 fumarate.

CONCLUSIONS

These results indicated that the blockade of M3 muscarinic receptor in the vMPFC attenuates the CER expression.

Electric foot shock stress: a useful tool in neuropsychiatric studies

Electric foot shock is a complex stressor with both physical and emotional components. It has been employed as an important tool to develop diverse animal models in the field of psychopharmacology. The electric foot shock paradigm includes acute or chronic exposures of shocks of varying intensity and duration on an electrified grid floor in an electric foot shock apparatus. Research evidence reveals that foot shocks of varying intensity produce behavioral and neurochemical changes reflecting depression, anxiety, and post-traumatic stress disorder (PTSD) in humans. Animals generally do not habituate to foot shocks in comparison to other stressors, including loud noise, bright light, and hot and cold temperatures. Additionally, it offers an experimental advantage of control over intensity and duration; therefore, by varying its application parameters, different disorder models have been created. Electric foot shock fear conditioning-induced ultrasonic vocalization and fear-potentiated startle have been explored to develop models of anxiety and panic. Similarly, fear conditioning in the form of foot shock exposure followed by situational reminders has been used to develop a model of PTSD. Electric foot shock-induced conflict has been explored to develop operant conflict models (Geller-Seifter and Vogel tests), which in turn are pharmacologically validated to screen potential anti-anxiety agents. Inescapable electric shock-induced ‘learned helplessness’ mimics the symptomology of depression, and this phenomenon has been employed to develop the model of depression. The present review describes the pharmacologically validated models of anxiety, depression, and PTSD involving electric foot shock as an aversive stimulus.

GABAergic mechanisms regulated by miR-33 encode state-dependent fear

Fear-inducing memories can be state dependent, meaning that they can best be retrieved if the brain states at encoding and retrieval are similar. Restricted access to such memories can present a risk for psychiatric disorders and hamper their treatment. To better understand the mechanisms underlying state-dependent fear, we used a mouse model of contextual fear conditioning. We found that heightened activity of hippocampal extrasynaptic GABAA receptors, believed to impair fear and memory, actually enabled their state-dependent encoding and retrieval. This effect required protein kinase C-βII and was influenced by miR-33, a microRNA that regulates several GABA-related proteins. In the extended hippocampal circuit, extrasynaptic GABAA receptors promoted subcortical, but impaired cortical, activation during memory encoding of context fear. Moreover, suppression of retrosplenial cortical activity, which normally impairs retrieval, had an enhancing effect on the retrieval of state-dependent fear. These mechanisms can serve as treatment targets for managing access to state-dependent memories of stressful experiences.

Adolescent and adult rats differ in the amnesic effects of acute ethanol in two hippocampus-dependent tasks: Trace and contextual fear conditioning

Experience-produced deficits in trace conditioning and context conditioning have been useful tools for examining the role of the hippocampus in learning. It has also been suggested that learning in these tasks is especially vulnerable to neurotoxic effects of alcohol during key developmental periods such as adolescence. In five experiments we systematically examined the presence and source of age-dependent vulnerability to the memory-disrupting effects of acute ethanol in trace conditioning and contextual fear conditioning. In Experiment 1a pre-training ethanol disrupted trace conditioning more strongly in adolescent (postnatal day, PD30-35) than adult rats (PD65-75). In Experiment 1b when pre-training ethanol was accompanied by pre-test ethanol no deficit in trace conditioning was observed in adolescents, suggesting that state-dependent retrieval failure mediated ethanol's disruption of trace conditioning at this age. Experiment 2a and b examined the effect of ethanol pretreatment on context conditioning. Here, adult but not adolescent rats were impaired in conditioned freezing to context cues. Experiment 2c explored state-dependency of this effect. Pre-training ethanol continued to disrupt context conditioning in adults even when ethanol was also administered prior to test. Collectively these findings reveal clear age-dependent and task-dependent vulnerabilities in ethanol's disruptive effects on hippocampus-dependent memory. Adolescents were more disrupted by ethanol in trace conditioning than adults, and adults were more disrupted by ethanol in context conditioning than adolescents. We suggest that adolescents may be more susceptible to changes in internal state (state-dependent retrieval failure) than adults and that ethanol disrupted performance in trace and context conditioning through different mechanisms. Relevance of these findings to theories of hippocampus function is discussed.

Japanese Huperzia serrata extract and the constituent, huperzine A, ameliorate the scopolamine-induced cognitive impairment in mice

Huperzia serrata has been used as a Chinese folk medicine for many years. It contains huperzine A, which has a protective effect against memory deficits in animal models; however, it is unclear if H. serrata extract exerts any effects in Alzheimer's disease (AD) models. We used H. serrata collected in Japan and determined its huperzine A content using HPLC. We determined its inhibitory effects on acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) activity. H. serrata extract (30 mg/kg/day) and donepezil (10 mg/kg/day) were orally administrated for 7 days. After repeated administration, we performed the Y-maze and passive avoidance tests. H. serrata extract contained 0.5% huperzine A; H. serrata extract inhibited AChE, but not BuChE. H. serrata extract ameliorated cognitive function in mice. These results indicate that Japanese H. serrata extract ameliorates cognitive function deficits by inhibiting AChE. Therefore, H. serrata extract may be valuable for the prevention or treatment of dementia in AD.

Neonatal alcohol exposure impairs contextual fear conditioning in juvenile rats by disrupting cholinergic function

The context preexposure facilitation effect (CPFE) is a variant of context fear conditioning in which context preexposure facilitates conditioning to immediate foot shock. Learning about context (preexposure), associating the context with shock (training), and expression of context fear (testing) occur in successive phases of the protocol. The CPFE develops postnatally, depends on hippocampal NMDA receptor function, and is highly sensitive to neonatal alcohol exposure during the weanling/juvenile period of development [15,16]. The present study examined some behavioral and pharmacological mechanisms through which neonatal alcohol impairs the CPFE in juvenile rats. We found that a 5-min context preexposure plus five 1-min preexposures greatly increases the levels of conditioned freezing compared to a single 5-min exposure or to five 1-min preexposures (Experiment 1). Increasing conditioned freezing with the multiple- exposure CPFE protocol does not alter the neonatal alcohol-induced deficit in the CPFE (Experiment 2). Finally, systemic administration of 0.01 mg/kg physostigmine prior to all three phases of the CPFE reverses this ethanol-induced deficit. These findings show that impairment of the CPFE by neonatal alcohol is not confined to behavioral protocols that produce low levels of conditioned freezing. They also support recent evidence that this impairment reflects a disruption of cholinergic function [18].

Cholinergic blockade frees fear extinction from its contextual dependency

BACKGROUND

Fears that are maladaptive or inappropriate can be reduced through extinction training. However, extinction is highly context-sensitive, resulting in the renewal of fear after shifts in context and limiting the clinical efficacy of extinction training. Lesion and inactivation studies have shown that the contextualization of extinction depends on the hippocampus. Parallel studies have found that intrahippocampal scopolamine (Scop) blocks contextual fear conditioning. Importantly, this effect was replicated with a noninvasive technique in which a low dose of Scop was administered systemically. We aimed to transfer the effects of this noninvasive approach to block the contextualization of fear extinction.

METHODS

Rats were tone fear conditioned and extinguished under various systemic doses of Scop or the saline vehicle. They were subsequently tested (off drug) for tone fear in a context that was the same (control subjects) or shifted (renewal group) with respect to the extinction context.

RESULTS

The lowest dose of Scop produced a significant attenuation of fear renewal when renewal was tested either in the original training context or a novel context. The drug also slowed the rate of long-term extinction memory formation, which was readily overcome by extending extinction training. Scopolamine only gave this effect when it was administered during but not after extinction training. Higher doses of Scop severely disrupted extinction learning.

CONCLUSIONS

We discovered that disrupting contextual processing during extinction with the cholinergic antagonist Scop blocked subsequent fear renewal. Low doses of Scop might be a clinically promising adjunct to exposure therapy by making extinction more relapse-resistant.

Novel allosteric agonists of M1 muscarinic acetylcholine receptors induce brain region-specific responses that correspond with behavioral effects in animal models

M(1) muscarinic acetylcholine receptors (mAChRs) represent a viable target for treatment of multiple disorders of the central nervous system (CNS) including Alzheimer's disease and schizophrenia. The recent discovery of highly selective allosteric agonists of M(1) receptors has provided a major breakthrough in developing a viable approach for the discovery of novel therapeutic agents that target these receptors. Here we describe the characterization of two novel M(1) allosteric agonists, VU0357017 and VU0364572, that display profound differences in their efficacy in activating M(1) coupling to different signaling pathways including Ca(2+) and β-arrestin responses. Interestingly, the ability of these agents to differentially activate coupling of M(1) to specific signaling pathways leads to selective actions on some but not all M(1)-mediated responses in brain circuits. These novel M(1) allosteric agonists induced robust electrophysiological effects in rat hippocampal slices, but showed lower efficacy in striatum and no measureable effects on M(1)-mediated responses in medial prefrontal cortical pyramidal cells in mice. Consistent with these actions, both M(1) agonists enhanced acquisition of hippocampal-dependent cognitive function but did not reverse amphetamine-induced hyperlocomotion in rats. Together, these data reveal that M(1) allosteric agonists can differentially regulate coupling of M(1) to different signaling pathways, and this can dramatically alter the actions of these compounds on specific brain circuits important for learning and memory and psychosis.

Muscarinic receptor pharmacology and circuitry for the modulation of cognition

The muscarinic cholinergic system constitutes an important part of the neuronal circuitry that modulates normal cognition. Muscarinic receptor antagonists are well known to produce or exacerbate impairments in attention, learning, and memory. Conversely, both direct-acting muscarinic receptor agonists and indirect-acting muscarinic cholinergic agonists, such as acetylcholinesterase inhibitors, have shown cognition-enhancing properties, including improvements in normal cognitive function, reversal of cognitive deficits induced by muscarinic receptor antagonists, and attenuation of cognitive deficits in psychiatric and neurological disorders, such as Alzheimer's disease and schizophrenia. However, until recently, the lack of small molecule ligands that antagonize or activate specific muscarinic acetylcholine receptor (mAChR) subtypes with high selectivity has been a major obstacle in defining the relative contributions of individual mAChRs to different aspects of cognitive function and for the development of novel therapeutic agents. These limitations may be potentially overcome by the recent discovery of novel mAChR subtype-selective compounds, notably allosteric agonists and positive allosteric modulators, which exhibit greater selectivity for individual mAChR subtypes than previous mAChR orthosteric agonists. In preclinical studies, these novel ligands have shown promising efficacy in several models for the enhancement of cognition. In this chapter, we will review the muscarinic cholinergic circuitry and pharmacology of mAChR agonists and antagonists relevant to the modulation of different aspects of cognition in animals and clinical populations.

The context preexposure facilitation effect in mice: a dose-response analysis of pretraining scopolamine administration

The context preexposure facilitation effect (CPFE) is an elaboration of contextual fear conditioning and refers to enhanced contextual conditioning resulting from preexposure to the context prior to a separate, brief context-shock episode. A version of the CPFE developed by Rudy and colleagues in rats has demonstrated greater sensitivity to pre-training hippocampal insult relative to standard contextual fear conditioning preparations. Our aim was to adapt the Rudy CPFE procedures to mice. In Experiment 1 we compared performance of young adult male C57BL6/J mice on two versions of the CPFE. One version - not previously used in mice - adapted methods established by Rudy and colleagues, and the other CPFE task replicated procedures previously established in this mouse strain by Gould and colleagues. In Experiment 2 we compared the effects of pre-training intraperitoneal administration of moderate levels of scopolamine or methylscopolamine on contextual conditioning between mice trained using the Rudy CPFE method and a separate group trained using standard contextual fear procedures. Scopolamine is a muscarinic cholinergic receptor antagonist that impairs hippocampal function. Robust freezing to the conditioning context was observed in mice trained using the Rudy CPFE method (Experiment 1), and greater scopolamine-induced impairments in contextual freezing were observed using this CPFE method relative to mice trained using standard contextual fear procedures (Experiment 2). These findings support use of the Rudy CPFE task as a behavioral assay for hippocampal function in mice.

Involvement of the cholinergic system in conditioning and perceptual memory

The cholinergic systems play a pivotal role in learning and memory, and have been the centre of attention when it comes to diseases containing cognitive deficits. It is therefore not surprising, that the cholinergic transmitter system has experienced detailed examination of its role in numerous behavioural situations not least with the perspective that cognition may be rescued with appropriate cholinergic 'boosters'. Here we reviewed the literature on (i) cholinergic lesions, (ii) pharmacological intervention of muscarinic or nicotinic system, or (iii) genetic deletion of selective receptor subtypes with respect to sensory discrimination and conditioning procedures. We consider visual, auditory, olfactory and somatosensory processing first before discussing more complex tasks such as startle responses, latent inhibition, negative patterning, eye blink and fear conditioning, and passive avoidance paradigms. An overarching reoccurring theme is that lesions of the cholinergic projection neurones of the basal forebrain impact negatively on acquisition learning in these paradigms and blockade of muscarinic (and to a lesser extent nicotinic) receptors in the target structures produce similar behavioural deficits. While these pertain mainly to impairments in acquisition learning, some rare cases extend to memory consolidation. Such single case observations warranted replication and more in-depth studies. Intriguingly, receptor blockade or receptor gene knockout repeatedly produced contradictory results (for example in fear conditioning) and combined studies, in which genetically altered mice are pharmacological manipulated, are so far missing. However, they are desperately needed to clarify underlying reasons for these contradictions. Consistently, stimulation of either muscarinic (mainly M(1)) or nicotinic (predominantly α7) receptors was beneficial for learning and memory formation across all paradigms supporting the notion that research into the development and mechanisms of novel and better cholinomimetics may prove useful in the treatment of neurodegenerative or psychiatric disorders with cognitive endophenotypes.

Bacopa monniera Attenuates Scopolamine-Induced Impairment of Spatial Memory in Mice

Scopolamine, an anticholinergic, is an attractive amnesic agent for discerning the action of candidate antiamnesic drugs. Bacopa monniera Linn (Syn. Brahmi) is one such antiamnesic agent that is frequently used in the ancient Indian medical system. We have earlier reported the reversal of diazepam-induced amnesia with B. monniera. In this study we wanted to test if scopolamine-induced impairment of spatial memory can also be ameliorated by B. monniera using water maze mouse model. The objective of study was to study the effect of B. monniera on scopolamine-induced amnesia. We employed Morris water maze scale to test the amnesic effect of scopolamine and its reversal by B. monniera. Rotarod test was conducted to screen muscle coordination activity of mice. Scopolamine significantly impaired the acquisition and retrieval of memory producing both anterograde and retrograde amnesia. Bacopa monniera extract was able to reverse both anterograde and retrograde amnesia. We propose that B. monniera's effects on cholinergic system may be helpful for developing alternative therapeutic approaches for the treatment of Alzheimer's disease.

Nicotine ameliorates NMDA receptor antagonist-induced deficits in contextual fear conditioning through high-affinity nicotinic acetylcholine receptors in the hippocampus

NMDA glutamate receptors (NMDARs) and nicotinic acetylcholine receptors (nAChRs) are both involved in learning and synaptic plasticity. Increasing evidence suggests processes mediated by these receptors may interact to modulate learning; however, little is known about the neural substrates involved in these interactive processes. The present studies investigated the effects of nicotine on MK-801 hydrogen maleate (MK-801) and DL-2-Amino-5-phosphonovaleric acid (APV)-induced disruption of contextual fear conditioning in male C57BL/6J mice, using direct drug infusion and selective nAChR antagonists to define the brain regions and the nAChR subtypes involved. Mice treated with MK-801 showed a deficit in contextual fear conditioning that was ameliorated by nicotine. Direct drug infusion demonstrated that the NMDAR antagonists disrupted hippocampal function and that nicotine acted in the dorsal hippocampus to ameliorate the deficit in learning. The high-affinity nAChR antagonist Dihydro-β-erythroidine hydrobromide (DhβE) blocked the effects of nicotine on MK-801-induced deficits while the α7 nAChR antagonist methyllycaconitine citrate salt hydrate (MLA) did not. These results suggest that NMDARs and nAChRs may mediate similar hippocampal processes involved in contextual fear conditioning. Furthermore, these results may have implications for developing effective therapeutics for the cognitive deficits associated with schizophrenia because a large subset of patients with schizophrenia exhibit cognitive deficits that may be related to NMDAR dysfunction and smoke at much higher rates than the healthy population, which may be an attempt to ameliorate cognitive deficits.

Ameliorating effects of essential oil from Acori graminei rhizoma on learning and memory in aged rats and mice

Although there are normal cognitive changes that take place as a person becomes older, ageing in humans is generally associated with a deterioration of cognitive performance, in particular of learning and memory. There are a number of herbal medicines that are reported to improve brain function and intelligence. In the present study, the ameliorating effects of an essential oil extracted from Acori graminei rhizoma on learning and memory in aged, dysmnesia rats and mice were determined using the step-down passive avoidance test and Y maze. Oral administration of the essential oil (0.02, 0.04 and 0.08 g kg−1) to rats for 30 days and to mice for 15 days improved the latency and number of errors in aged, dysmnesia rats and mice. The cerebral neurotransmitters in aged rats given the essential oil (0.02, 0.04, 0.08 g kg−1) for 30 days were also investigated, and increased levels of norepinephrine, dopamine and serotonin, and decreased levels of acetylcholinesterase activity were found. The results suggest that the essential oil improves cognitive function in aged animals possibly by increasing norepinephrine, dopamine and serotonin relative levels, and by decreasing the activity of acetylcholinesterase in the cerebra.

Anti-amnesic effect of Ficus religiosa in scopolamine-induced anterograde and retrograde amnesia

CONTEXT

Ficus religiosa Linn (Moraceae) is a variety of fig tree. Its figs are known to contain a high serotonergic content, and modulation of serotonergic neurotransmission plays a crucial role in the pathogenesis of amnesia. Thus, the present study was envisaged.

OBJECTIVE

To investigate the effect of the methanol extract of figs of Ficus religiosa (FRFE) on scopolamine-induced anterograde and retrograde amnesia in mice.

MATERIALS AND METHODS

Transfer latency (TL) to the preferred niche in the elevated plus-maze (EPM) and learning avoidance of passive behavior to avoid punishment in the modified passive avoidance paradigm (MPA) served as behavioral models for the assessment of memory. Scopolamine (1 mg/kg, i.p.) was administered before training for induction of anterograde amnesia and before retrieval for induction of retrograde amnesia in both models. TL in the EPM, step down latency (SDL), number of trials, and number of mistakes in the MPA were determined in vehicle control, FRFE treated (10, 50, and 100 mg/kg, i.p.), and standard groups (piracetam 200 mg/kg, i.p.). Cyproheptadine, a non-selective 5-HT(1/2) blocker (4 mg/kg, i.p.), was administered along with the FRFE to investigate the involvement of serotonergic pathways in the anti-amnesic effect of FRFE.

RESULTS AND DISCUSSION

FRFE resulted in a significant improvement of memory, as its treatment attenuated the scopolamine-induced anterograde and retrograde amnesia dose-dependently. Further, cyproheptadine pretreatment significantly reversed the anti-amnesic effect of FRFE.

CONCLUSION

FRFE has anti-amnesic activity against scopolamine-induced amnesia, in a dose-dependent manner. Inhibition of the anti-amnesic effect of FRFE by cyproheptadine substantiates the involvement of serotonergic pathways for its activity.

A cross-species investigation of acetylcholine, attention, and feature binding

The binding problem is the brain's fundamental challenge to integrate sensory information to form a unified representation of a stimulus. A recent nonhuman animal model suggests that acetylcholine serves as the neuromodulatory substrate for feature binding. We hypothesized that this animal model of cholinergic contributions to feature binding may be an analogue of human attention. To test this hypothesis, we conducted a cross-species study in which rats and humans learned comparable intramodal feature-conjunction (FC) and feature-singleton (FS) tasks. We challenged the cholinergic system of rats using the muscarinic antagonist scopolamine (0.2 mg/kg) and challenged the attentional system of humans by dividing attention. The two manipulations yielded strikingly similar patterns of behavior, impairing FC acquisition, while sparing FS acquisition and FC retrieval. These cross-species findings support the hypothesis that cholinergically driven attentional processes are essential to feature binding at encoding, but are not required for retrieval of neural representations of bound stimuli.

Muscarinic receptors in perirhinal cortex control trace conditioning

Trace conditioning requires that a transient representation of the conditional stimulus (CS) persists during the time interval between the CS offset and the onset of the unconditional stimulus. According to one hypothesis, this transient CS representation is supported by endogenous activity in "persistent-firing" neurons of perirhinal cortex (PR). By definition, persistent-firing neurons discharge for tens of seconds or minutes after the termination of the original spike-initiating stimulus. This continued spiking does not depend on recurrent circuit activity and can be reliably and completely blocked by muscarinic receptor antagonists. The present study evaluated the role of PR muscarinic receptors in trace fear conditioning. Before conditioning, rats received bilateral intra-PR infusions with either saline or scopolamine, a nonselective muscarinic receptor antagonist. Scopolamine infusions profoundly impaired trace conditioning but had no effect on delay conditioning or context conditioning. The results encourage a more general understanding of muscarinic receptors in PR and they motivate additional tests of the emerging theory that persistent-firing neurons support aspects of transient memory.

Antiamnesic and neuroprotective effects of the aminotetrahydrofuran derivative ANAVEX1-41 against amyloid beta(25-35)-induced toxicity in mice

The antiamnesic and neuroprotective activities of the new aminotetrahydrofuran derivative tetrahydro-N,N-dimethyl-5,5-diphenyl-3-furanmethanamine hydrochloride (ANAVEX1-41), a nonselective muscarinic receptor ligand and sigma1 protein activator, were examined in mice injected intracerebroventricularly with amyloid beta(25-35) (Abeta(25-35)) peptide (9 nmol). Abeta(25-35) impaired significantly spontaneous alternation performance, a spatial working memory, and passive avoidance response. When ANAVEX1-41 (1-1000 microg/kg i.p.) was administered 7 days after Abeta(25-35), ie, 20 min before the behavioral tests, it significantly reversed the Abeta(25-35)-induced deficits, the most active doses being in the 3-100 microg/kg range. When the compound was preadministered 20 min before Abeta(25-35), ie, 7 days before the tests, it prevented the learning impairments at 30-100 microg/kg. Morphological analysis of corticolimbic structures showed that Abeta(25-35) induced a significant cell loss in the CA1 pyramidal cell layer of the hippocampus that was prevented by ANAVEX1-41 (100 microg/kg). Increased number of glial fibrillary acidic protein immunopositive cells in the retrosplenial cortex or throughout the hippocampus revealed an Abeta(25-35)-induced inflammation that was prevented by ANAVEX1-41. The drug also prevented the parameters of Abeta(25-35)-induced oxidative stress measured in hippocampus extracts, ie, the increases in lipid peroxidation and protein nitration. ANAVEX1-41, however, failed to prevent Abeta(25-35)-induced caspase-9 expression. The compound also blocked the Abeta(25-35)-induced caspase-3 expression, a marker of apoptosis. Both the muscarinic antagonist scopolamine and the sigma1 protein inactivator BD1047 prevented the beneficial effects of ANAVEX1-41 (30 or 100 microg/kg) against Abeta(25-35)-induced learning impairments, suggesting that muscarinic and sigma1 targets are involved in the drug effect. A synergic effect could indeed account for the very low active doses measured in vivo. These data outline the therapeutic potential of ANAVEX1-41 as a neuroprotective agent in Alzheimer's disease.

A novel selective muscarinic acetylcholine receptor subtype 1 antagonist reduces seizures without impairing hippocampus-dependent learning

Previous studies suggest that selective antagonists of specific subtypes of muscarinic acetylcholine receptors (mAChRs) may provide a novel approach for the treatment of certain central nervous system (CNS) disorders, including epileptic disorders, Parkinson's disease, and dystonia. Unfortunately, previously reported antagonists are not highly selective for specific mAChR subtypes, making it difficult to definitively establish the functional roles and therapeutic potential for individual subtypes of this receptor subfamily. The M(1) mAChR is of particular interest as a potential target for treatment of CNS disorders. We now report the discovery of a novel selective antagonist of M(1) mAChRs, termed VU0255035 [N-(3-oxo-3-(4-(pyridine-4-yl)piperazin-1-yl)propyl)-benzo[c][1,2,5]thiadiazole-4 sulfonamide]. Equilibrium radioligand binding and functional studies demonstrate a greater than 75-fold selectivity of VU0255035 for M(1) mAChRs relative to M(2)-M(5). Molecular pharmacology and mutagenesis studies indicate that VU0255035 is a competitive orthosteric antagonist of M(1) mAChRs, a surprising finding given the high level of M(1) mAChR selectivity relative to other orthosteric antagonists. Whole-cell patch-clamp recordings demonstrate that VU0255035 inhibits potentiation of N-methyl-D-aspartate receptor currents by the muscarinic agonist carbachol in hippocampal pyramidal cells. VU0255035 has excellent brain penetration in vivo and is efficacious in reducing pilocarpine-induced seizures in mice. We were surprised to find that doses of VU0255035 that reduce pilocarpine-induced seizures do not induce deficits in contextual freezing, a measure of hippocampus-dependent learning that is disrupted by nonselective mAChR antagonists. Taken together, these data suggest that selective antagonists of M(1) mAChRs do not induce the severe cognitive deficits seen with nonselective mAChR antagonists and could provide a novel approach for the treatment certain of CNS disorders.

Cholinergic deafferentation of the neocortex using 192 IgG-saporin impairs feature binding in rats

The binding problem refers to the fundamental challenge of the CNS to integrate sensory information registered by distinct brain regions to form a unified neural representation of a stimulus. Although the human cognitive literature has established that attentional processes in frontoparietal cortices support feature binding, the neurochemical and specific downstream neuroanatomical contributions to feature binding remain unknown. Using systemic pharmacology in rats, it has been shown that the neuromodulator acetylcholine is essential for feature binding at encoding, but the neural source of such critical cholinergic neurotransmission has yet to be identified. Cholinergic efferents from the nucleus basalis magnocellularis (NBM) of the basal forebrain provide the majority of the cholinergic input to the neocortex. Accordingly, it was hypothesized that the NBM is the neural source that provides the critical neuromodulatory support for feature binding. To test this hypothesis, rats received bilateral 192 IgG-saporin lesions of the NBM, and their feature binding performance was tested using a forced-choice digging paradigm. Relative to sham-lesioned rats, NBM-lesioned rats were significantly impaired at acquiring a crossmodal feature conjunction (FC) stimulus set that required feature binding, whereas their ability to retrieve an FC stimulus set and to acquire two crossmodal feature singleton stimulus sets, one of greater difficulty than the other but neither requiring feature binding, remained intact. These behavioral findings, along with histological analyses demonstrating positive relationships between feature-binding acquisition and markers of cholinergic activity in frontoparietal regions, reveal the importance of neocortical cholinergic input from the NBM to feature binding at encoding.

Towards an animal model of an antipsychotic drug-resistant cognitive impairment in schizophrenia: scopolamine induces abnormally persistent latent inhibition, which can be reversed by cognitive enhancers but not by antipsychotic drugs

Schizophrenia symptoms segregate into positive, negative and cognitive, which exhibit differential sensitivity to drugs. Recent efforts to identify treatments targeting cognitive impairments in schizophrenia have directed attention to the cholinergic system for its well documented role in cognition. Relatedly, muscarinic antagonists (e.g. scopolamine) produce an 'antimuscarinic syndrome', characterized by psychosis and cognitive impairments. Latent inhibition (LI) is the poorer conditioning to a stimulus resulting from its non-reinforced pre-exposure. LI indexes the ability to ignore irrelevant stimuli and aberrations of this capacity produced by pro-psychotic agents (e.g. amphetamine, MK-801) are used extensively to model attentional impairments in schizophrenia. We recently showed that LI was disrupted by scopolamine at low doses, and this was reversed by typical and atypical antipsychotic drugs (APDs) and the acetylcholinesterase inhibitor physostigmine. Here, at a higher dose (1.5 mg/kg), scopolamine produced an opposite pole of attentional impairment, namely, attentional perseveration, whereby scopolamine-treated rats persisted in expressing LI under strong conditioning that prevented LI expression in controls. Scopolamine-induced persistent LI was reversed by cholinergic and glycinergic cognitive enhancers (physostigmine and glycine) but was resistant to both typical and atypical APDs (haloperidol and clozapine). The latter sets scopolamine-induced persistent LI apart from scopolamine- and amphetamine-induced disrupted LI, which are reversed by both typical and atypical APDs, as well as from other cases of abnormally persistent LI including MK-801-induced persistent LI, which is reversed by atypical APDs. Thus, scopolamine-induced persistent LI may provide a pharmacological LI model for screening cognitive enhancers that are efficient for the treatment of APD-resistant cognitive impairments in schizophrenia.

Memory and psychostimulants: modulation of Pavlovian fear conditioning by amphetamine in C57BL/6 mice

RATIONALE AND OBJECTIVES

With the use of prescription stimulants on the rise, it is important to examine the cognitive effects of low and moderate doses of stimulants rather than only those typical of addicts.

MATERIALS AND METHODS

The present study examined the effects a range of doses (0.005-8 mg/kg) of D: -amphetamine sulfate on cued and contextual Pavlovian fear conditioning in mice.

RESULTS

In agreement with previous research, subjects administered with a moderately high dose of amphetamine (8 mg/kg) pre-training, typical of what addicts might take, displayed impaired conditioned freezing when tested off-drug. Alternately, subjects injected with a very low dose of amphetamine (0.005, 0.025, or 0.05 mg/kg) pre-training, similar to the therapeutic doses for attention deficit hyperactivity disorder, displayed enhanced memory when tested off-drug. A control study showed that these effects were not due to state-dependent learning.

CONCLUSIONS

Thus, dose is a critical determinant of the cognitive effects of psychostimulants.

Involvement of dopamine D2 receptors in the induction of long-term potentiation in the basolateral amygdala-dentate gyrus pathway of anesthetized rats

We have previously found that synaptic pathway from the basolateral amygdala (BLA) to the dentate gyrus (DG) displays N-methyl-D-aspartate (NMDA) receptor-independent form of long-term potentiation (LTP), which should be a valuable model for elucidating neural mechanisms linking emotion and memory. To explore its cellular mechanisms, we investigated possible involvement of the beta-adrenergic, muscarinic cholinergic and dopaminergic systems on LTP in this pathway of anesthetized rats. The induction of BLA-DG LTP was not affected by administration of the beta-adrenoceptor antagonist propranolol (50-150nmol, i.c.v.), the muscarinic receptor antagonist scopolamine (2-6mg/kg, i.p.), the cholinesterase inhibitor physostigmine (50 nmol, i.c.v.) or the dopamine D(1) receptor antagonist SCH23390 (100nmol, i.c.v.), but significantly inhibited by the dopamine D2 receptor antagonists, chlorpromazine (15nmol, i.c.v.) and haloperidol (0.15-0.5mg/kg, i.p.), and significantly promoted by the dopamine D2 receptor agonist quinpirole (78nmol, i.c.v.). Furthermore, lesioning with 6-hydroxydopamine of the ventral tegmental area (VTA), the origin of mesolimbic dopaminergic neurons, resulted in attenuated BLA-DG LTP. These results suggest that the D2-dopaminergic system, but not the beta-adrenergic, muscarinic or D1-dopaminergic system, is involved in the induction of BLA-DG LTP. In addition, inhibition of BLA-DG LTP by haloperidol or VTA lesion was abolished by blockade of GABAergic inhibition with picrotoxin. It is probable that the D2-dopaminergic system promotes the induction of BLA-DG LTP by suppressing GABAergic inhibition.

Interaction between glutamatergic-NMDA and cholinergic-muscarinic systems in classical fear conditioning

A number of studies have suggested that the glutamatergic and cholinergic systems are both involved in learning and memory processes and that they interact in order to facilitate these processes. However, the role of M1-muscarinic receptors in mediating this interaction has not been elucidated. The aim of this study was to determine whether the concomitant administration of MK-801 (non-competitive NMDA antagonist) and dicyclomine (M1-muscarinic antagonist--DIC) in sub-effective doses impairs contextual fear conditioning (hippocampal-dependent task) and tone fear conditioning tasks (hippocampal-independent task). The results showed that concomitant pre-training administration of DIC (8.0 mg/kg) and MK-801 (0.07 mg/kg)--two sub-effectives doses for the contextual fear conditioning task--does impair the performance of animals on this task (as measured by freezing behavior time). Tone fear conditioning tasks were not affected by the drugs either administered separately or concurrently. The pre-training administration of sub-effective doses of MK-801 and DIC in combination impairs performance on contextual fear conditioning task (hippocampal-dependent), but not on tone fear conditioning task (hippocampal-independent). These data support the hypothesis that the interaction between glutamatergic and cholinergic systems in hippocampus-dependent learning and memory processes probably occurs through M1 receptor.