Muscimol, AP5, or scopolamine infused into perirhinal cortex impairs two-choice visual discrimination learning in rats

The Granular Retrosplenial Cortex Is Necessary in Male Rats for Object-Location Associative Learning and Memory, But Not Spatial Working Memory or Visual Discrimination and Reversal, in the Touchscreen Operant Chamber

The retrosplenial cortex (RSC) is a hub of diverse afferent and efferent projections thought to be involved in associative learning. RSC shows early pathology in mild cognitive impairment and Alzheimer's disease (AD), which impairs associative learning. To understand and develop therapies for diseases such as AD, animal models are essential. Given the importance of human RSC in object-location associative learning and the success of object-location associative paradigms in human studies and in the clinic, it would be of considerable value to establish a translational model of object-location learning for the rodent. For this reason, we sought to test the role of RSC in object-location learning in male rats using the object-location paired-associates learning (PAL) touchscreen task. First, increased cFos immunoreactivity was observed in granular RSC following PAL training when compared with extended pretraining controls. Following this, RSC lesions following PAL acquisition were used to explore the necessity of the RSC in object-location associative learning and memory and two tasks involving only one modality: trial-unique nonmatching-to-location for spatial working memory and pairwise visual discrimination/reversal. RSC lesions impaired both memory for learned paired-associates and learning of new object-location associations but did not affect performance in either the spatial or visual single-modality tasks. These findings provide evidence that RSC is necessary for object-location learning and less so for learning and memory involving the individual modalities therein.

Relationship between hippocampal gene expression and cognitive performance differences in visual discrimination learning task of male rats

Learning to discriminate between environmental visual stimuli is essential to make right decisions and guide appropriate behaviors. Moreover, impairments in visual discrimination learning are observed in several neuropsychiatric disorders. Visual discrimination learning requires perception and memory processing, in which the hippocampus critically involved. To understand the molecular mechanisms underpinning hippocampus function in visual discrimination learning, we examined the hippocampal gene expression profiles of Sprague-Dawley rats with different cognitive performance (high cognition group vs. low cognition group) in the modified visual discrimination learning task, using high-throughput RNA sequencing technology. Compared with the low cognition group, bioinformatics analysis indicated that 319 genes were differentially expressed in the high cognition group with statistical significance, of which 253 genes were down-regulated and 66 genes were up-regulated. The functional enrichment analysis showed that protein translation and energy metabolism were up-regulated pathways, while transforming growth factor beta receptor signaling pathway, bone morphogenetic protein signaling pathway, apoptosis, inflammation response, transport, and glycosaminoglycan metabolism were down-regulated pathways, which were related to good cognitive performance in the visual discrimination learning task. Taken together, our finding reveals the differential gene expression and enrichment biological pathways related to cognitive performance differences in visual discrimination learning of rats, which provides us direct insight into the molecular mechanisms of hippocampus function in visual discrimination learning and may contribute to developing potential treatment strategies for neuropsychiatric disorders accompanied with cognitive impairments.

Viral vector-mediated upregulation of serine racemase expression in medial prefrontal cortex improves learning and synaptic function in middle age rats

An age-associated decrease in N-methyl-D-aspartate receptor (NMDAR)-mediated synaptic function contributes to impaired synaptic plasticity and is associated with cognitive impairments. Levels of serine racemase (SR), an enzyme that synthesizes D-serine, an NMDAR co-agonist, decline with age. Thus, enhancing NMDAR function via increased SR expression in middle age, when subtle declines in cognition emerge, was predicted to enhance performance on a prefrontal cortex-mediated task sensitive to aging. Middle-aged (~12 mo) male Fischer-344 rats were injected bilaterally in the medial prefrontal cortex (mPFC) with viral vector (LV), SR (LV-SR) or control (LV-GFP). Rats were trained on the operant attentional set-shift task (AST) to examine cognitive flexibility and attentional function. LV-SR rats exhibited a faster rate of learning compared to controls during visual discrimination of the AST. Extradimensional set shifting and reversal were not impacted. Immunohistochemical analyses demonstrated that LV-SR significantly increased SR expression in the mPFC. Electrophysiological characterization of synaptic transmission in the mPFC slices obtained from LV-GFP and LV-SR animals indicated a significant increase in isolated NMDAR-mediated synaptic responses in LV-SR slices. Thus, results of the current study demonstrated that prefrontal SR upregulation in middle age rats can improve learning of task contingencies for visual discrimination and increase glutamatergic synaptic transmission, including NMDAR activity.

Rodent mnemonic similarity task performance requires the prefrontal cortex

Mnemonic similarity task performance, in which a known target stimulus must be distinguished from similar lures, is supported by the hippocampus and perirhinal cortex. Impairments on this task are known to manifest with advancing age. Interestingly, disrupting hippocampal activity leads to mnemonic discrimination impairments when lures are novel, but not when they are familiar. This observation suggests that other brain structures support discrimination abilities as stimuli are learned. The prefrontal cortex (PFC) is critical for retrieval of remote events and executive functions, such as working memory, and is also particularly vulnerable to dysfunction in aging. Importantly, the medial PFC is reciprocally connected to the perirhinal cortex and neuron firing in this region coordinates communication between lateral entorhinal and perirhinal cortices to presumably modulate hippocampal activity. This anatomical organization and function of the medial PFC suggests that it contributes to mnemonic discrimination; however, this notion has not been empirically tested. In the current study, rats were trained on a LEGO object-based mnemonic similarity task adapted for rodents, and surgically implanted with guide cannulae targeting prelimbic and infralimbic regions of the medial PFC. Prior to mnemonic discrimination tests, rats received PFC infusions of the GABAA agonist muscimol. Analyses of expression of the neuronal activity-dependent immediate-early gene Arc in medial PFC and adjacent cortical regions confirmed muscimol infusions led to neuronal inactivation in the infralimbic and prelimbic cortices. Moreover, muscimol infusions in PFC impaired mnemonic discrimination performance relative to the vehicle control across all testing blocks when lures shared 50-90% feature overlap with the target. Thus, in contrast hippocampal infusions, PFC inactivation impaired target-lure discrimination regardless of the novelty or familiarity of the lures. These findings indicate the PFC plays a critical role in mnemonic similarity task performance, but the time course of PFC involvement is dissociable from that of the hippocampus.

Goal-directed interaction of stimulus and task demand in the parahippocampal region

The hippocampus and parahippocampal region are essential for representing episodic memories involving various spatial locations and objects, and for using those memories for future adaptive behavior. The “dual‐stream model” was initially formulated based on anatomical characteristics of the medial temporal lobe, dividing the parahippocampal region into two streams that separately process and relay spatial and nonspatial information to the hippocampus. Despite its significance, the dual‐stream model in its original form cannot explain recent experimental results, and many researchers have recognized the need for a modification of the model. Here, we argue that dividing the parahippocampal region into spatial and nonspatial streams a priori may be too simplistic, particularly in light of ambiguous situations in which a sensory cue alone (e.g., visual scene) may not allow such a definitive categorization. Upon reviewing evidence, including our own, that reveals the importance of goal‐directed behavioral responses in determining the relative involvement of the parahippocampal processing streams, we propose the Goal‐directed Interaction of Stimulus and Task‐demand (GIST) model. In the GIST model, input stimuli such as visual scenes and objects are first processed by both the postrhinal and perirhinal cortices—the postrhinal cortex more heavily involved with visual scenes and perirhinal cortex with objects—with relatively little dependence on behavioral task demand. However, once perceptual ambiguities are resolved and the scenes and objects are identified and recognized, the information is then processed through the medial or lateral entorhinal cortex, depending on whether it is used to fulfill navigational or non‐navigational goals, respectively. As complex sensory stimuli are utilized for both navigational and non‐navigational purposes in an intermixed fashion in naturalistic settings, the hippocampus may be required to then put together these experiences into a coherent map to allow flexible cognitive operations for adaptive behavior to occur.

Neurobiological substrates of animal personality and cognition in a nonhuman primate (Microcebus murinus)

INTRODUCTION

The gray mouse lemur (Microcebus murinus) is an important nonhuman primate model in biomedical research. Numerous studies investigated mouse lemur behavior and possible factors underlying interindividual variation in both, animal personality and cognitive performance. Some effects, such as an age-related decline in executive functioning, have robustly been found across laboratory colonies; however, little is known about the brain structural substrates in mouse lemurs.

METHODS

Here, we provide first exploratory data linking in vivo magnetic resonance imaging of 34 mouse lemurs to performance in a standardized, touchscreen-based task on object discrimination and reversal learning as well as to animal personality under different scenarios in an open field.

RESULTS

High interindividual variability in both brain morphometric and behavioral measurements was found, but only few significant correlations between brain structure and behavior were revealed: Object discrimination learning was linked to the volume of the hippocampus and to temporal lobe thickness, while reversal learning was linked to thalamic volume and the thickness of the anterior cingulate lobe. Emergence latency into the open field correlated with volume of the amygdala. General exploration-avoidance in the empty open-field arena correlated with thicknesses of the anterior cingulate lobe and fronto-parietal substructures. Neophilia, assessed as exploration of a novel object placed in the arena, among others, related to the volume of the caudate nucleus.

CONCLUSION

In summary, our data suggest a prominent role of temporal structures (including the hippocampus) for learning capability, as well as thalamic and anterior cingulate structures for cognitive flexibility and response inhibition. The amygdala, the anterior cingulate lobe, and the caudate nucleus are particularly linked to animal personality in the open-field setting. These findings are congruent with the comparative psychological literature and provide a valuable basis for future studies elucidating aspects of behavioral variation in this nonhuman primate model.

NMDA Receptors in Visual and Olfactory Sensory Integration in Male Long Evans Rats: A Role for the Orbitofrontal Cortex

Sensory integration (SI) is a cognitive process whereby the brain uses unimodal or multimodal sensory features to create a comprehensive representation of the environment. Integration of sensory input is necessary to achieve a coherent perception of the environment, and to subsequently plan and coordinate action. The neural mechanisms mediating SI are poorly understood; however, recent studies suggest that the regulation of SI involves N-methyl-d-aspartate receptors (NMDARs) in orbitofrontal cortex (OFC). Thus, we tested this hypothesis directly in two experiments using object oddity tests that require SI for visual and olfactory stimuli. First, we blocked NMDARs with acute CPP treatment (i.p., 10 mg/kg) and tested rats in unimodal visual and olfactory SI tests, and respective control unimodal oddity tests that do not require SI. Second, we used intra-OFC infusions of AP5 (30 mM) to examine the role of NMDARs in the OFC in the oddity tests requiring SI. Systemic blockade of NMDARs impaired performance on the visual tests regardless of whether SI was required for determining oddity. In the olfactory tests, systemic treatment with CPP impaired the test requiring SI while sparing olfactory oddity, demonstrating a selective impairment in the olfactory SI. Intra-OFC blockade of NMDARs impaired olfactory SI, without effect on visual SI, demonstrating that intra-OFC NMDARs are essential for olfactory, but not visual SI. The present results are discussed in the context of the function of the OFC and its associated circuitry.

Linking cognition to age and amyloid-β burden in the brain of a nonhuman primate (Microcebus murinus)

The gray mouse lemur (Microcebus murinus) is a valuable model in research on age-related proteopathies. This nonhuman primate, comparable to humans, naturally develops tau and amyloid-β proteopathies during aging. Whether these are linked to cognitive alterations is unknown. Here, standardized cognitive testing in pairwise discrimination and reversal learning in a sample of 37 aged (>5 years) subjects was combined with tau and amyloid-β histochemistry in individuals that died naturally. Correlation analyses in successfully tested subjects (n = 22) revealed a significant relation between object discrimination learning and age, strongly influenced by outliers, suggesting pathological cases. Where neuroimmunohistochemistry was possible, as subjects deceased, the naturally developed cortical amyloid-β burden was significantly linked to pretraining success (intraneuronal accumulations) and discrimination learning (extracellular deposits), showing that cognitive (pairwise discrimination) performance in old age predicts the natural accumulation of amyloid-β at death. This is the first description of a direct relation between the cortical amyloid-β burden and cognition in a nonhuman primate.

Orexin-1 receptor blockade differentially affects spatial and visual discrimination memory facilitation by intracranial self-stimulation

Intracranial self-stimulation (ICSS) of the medial forebrain bundle is an effective treatment to facilitate memory. Performance in both explicit and implicit memory tasks has been improved by ICSS, and this treatment has even been capable of recovering loss of memory function due to lesions or old age. Several neurochemical systems have been studied in regard to their role in ICSS effects on memory, however the possible involvement of the orexinergic system in this facilitation has yet to be explored. The present study aims to examine the relationship between the OX1R and the facilitative effects of ICSS on two different types of memory tasks, both carried out in the Morris Water Maze: spatial and visual discrimination. Results show that the OX1R blockade, by intraventricular administration of SB-334867, partially negates the facilitating effect of ICSS on spatial memory, whereas it hinders ICSS facilitation of the discrimination task. However, ICSS treatment was capable of compensating for the severe detrimental effects of OX1R blockade on both memory paradigms. These results suggest different levels of involvement of the orexinergic system in the facilitation of memory by ICSS, depending on the memory task.

Hop-Derived Iso-α-Acids in Beer Improve Visual Discrimination and Reversal Learning in Mice as Assessed by a Touch Panel Operant System

Dementia and cognitive decline have become worldwide health problems due to rapid growth of the aged population in many countries. We previously demonstrated that single or short-term administration of iso-α-acids, hop-derived bitter acids in beer, improves the spatial memory of scopolamine-induced amnesia model mice in the Y-maze and enhances novel object recognition in normal mice via activation of the vagus nerve and hippocampal dopaminergic system. However, these behavioral tests do not replicate the stimulus conditions or response requirements of human memory tests, and so may have poor translational validity. In this report, we investigated the effects of iso-α-acids on visual discrimination (VD) and reversal discrimination (RD) using a touch panel-based operant system similar to that used for human working memory tests. In the VD task, scopolamine treatment reduced correct response rate and prolonged response latency in mice, deficits reversed by prior oral administration of iso-α-acids. In the RD task, administration of iso-α-acids significantly increased correct response rate compared to vehicle administration. Previous studies have reported that dopamine signaling is involved in both VD and RD learning, suggesting that enhancement of dopamine release contributes to improved memory performance in mice treated with iso-α-acids. Taken together, iso-α-acids improve VD and RD learning, which are considered high-order cognitive functions. Given the translational advantages of the touch panel-based operant system, the present study suggests that iso-α-acids could be effective for improvement of working memory in human dementia patients.

Prolonged exposure of rats to varenicline increases anxiety and alters serotonergic system, but has no effect on memory

Varenicline is a drug used for smoking addiction cessation treatment and acts as a partial agonist of nicotinic cholinergic receptors. Recent clinical trial data support use of varenicline for treatment of conditions/addictions that are not related to smoking cessation. Considering the importance of this issue and the need for new studies on its effects, especially on behavior, more studies using animal models are necessary. Thus, the aim of this study was to evaluate the effects of prolonged exposure to varenicline in anxiety-like behavior and memory, as well as in cerebral neurochemistry of rats. Male rats received three different doses of varenicline: 0.03 (therapeutic dose for humans), 0.1 and 0.3 mg/kg orally (gavage) for 30 days. Animal behavior was analyzed through open field, elevated plus maze, light/dark box, social interaction, Barnes maze and novel object recognition tests. Neurotransmitter levels and their metabolites in different brain structures (hippocampus, striatum and frontal cortex) were measured. Results showed that prolonged exposure of rats to varenicline: 1) did not interfere in motor activity, but caused an anxiogenic effect on elevated plus maze, light/dark box and social interaction testes; 2) did not alter memory; and 3) promoted alterations on serotoninergic system in the striatum and frontal cortex. In conclusion, compilation of the data indicates that prolonged exposure of rats to varenicline promoted anxiogenic effects and alteration in serotonergic system, which corroborated behavioral findings.

Chemogenetic activation of the perirhinal cortex reverses methamphetamine-induced memory deficits and reduces relapse

Prolonged use of methamphetamine (meth) has been associated with episodic memory deficits in humans, and preclinical rat models of meth self-administration indicate the memory deficits are a consequence of meth use. Others have suggested that the meth-induced memory deficits may promote a cyclical pattern of drug use, abstinence, and relapse, although preclinical evidence for this relationship is somewhat lacking. The memory deficits in preclinical models manifest as a loss of novel object recognition (NOR) memory. These deficits occur one to two weeks after cessation of meth use and involve the perirhinal cortex, a parahippocampal region essential to NOR memory. We hypothesized that a loss of perirhinal cortex function contributes to both the NOR memory deficits and increased vulnerability to relapse in a novel-cue reinstatement model. To test this, we attempted to restore NOR memory in meth rats using an excitatory Gq-DREADD in perirhinal neurons. Activation of these neurons not only reversed the meth-induced deficit in NOR memory, but also restored novelty salience in a novel-cue reinstatement model. Thus, perirhinal cortex functionality contributes to both memory deficits in relapse in a long-access model of meth self-administration in rats, and chemogenetic restoration of perirhinal function restores memory and reduces relapse.

Development of novel tasks for studying view-invariant object recognition in rodents: Sensitivity to scopolamine

The capacity to recognize objects from different view-points or angles, referred to as view-invariance, is an essential process that humans engage in daily. Currently, the ability to investigate the neurobiological underpinnings of this phenomenon is limited, as few ethologically valid view-invariant object recognition tasks exist for rodents. Here, we report two complementary, novel view-invariant object recognition tasks in which rodents physically interact with three-dimensional objects. Prior to experimentation, rats and mice were given extensive experience with a set of 'pre-exposure' objects. In a variant of the spontaneous object recognition task, novelty preference for pre-exposed or new objects was assessed at various angles of rotation (45°, 90° or 180°); unlike control rodents, for whom the objects were novel, rats and mice tested with pre-exposed objects did not discriminate between rotated and un-rotated objects in the choice phase, indicating substantial view-invariant object recognition. Secondly, using automated operant touchscreen chambers, rats were tested on pre-exposed or novel objects in a pairwise discrimination task, where the rewarded stimulus (S+) was rotated (180°) once rats had reached acquisition criterion; rats tested with pre-exposed objects re-acquired the pairwise discrimination following S+ rotation more effectively than those tested with new objects. Systemic scopolamine impaired performance on both tasks, suggesting involvement of acetylcholine at muscarinic receptors in view-invariant object processing. These tasks present novel means of studying the behavioral and neural bases of view-invariant object recognition in rodents.

Selective effects of 5-HT2C receptor modulation on performance of a novel valence-probe visual discrimination task and probabilistic reversal learning in mice

RATIONALE

Dysregulation of the serotonin (5-HT) system is a pathophysiological component in major depressive disorder (MDD), a condition closely associated with abnormal emotional responsivity to positive and negative feedback. However, the precise mechanism through which 5-HT tone biases feedback responsivity remains unclear. 5-HT2C receptors (5-HT2CRs) are closely linked with aspects of depressive symptomatology, including abnormalities in reinforcement processes and response to stress. Thus, we aimed to determine the impact of 5-HT2CR function on response to feedback in biased reinforcement learning.

METHODS

We used two touchscreen assays designed to assess the impact of positive and negative feedback on probabilistic reinforcement in mice, including a novel valence-probe visual discrimination (VPVD) and a probabilistic reversal learning procedure (PRL). Systemic administration of a 5-HT2CR agonist and antagonist resulted in selective changes in the balance of feedback sensitivity bias on these tasks.

RESULTS

Specifically, on VPVD, SB 242084, the 5-HT2CR antagonist, impaired acquisition of a discrimination dependent on appropriate integration of positive and negative feedback. On PRL, SB 242084 at 1 mg/kg resulted in changes in behaviour consistent with reduced sensitivity to positive feedback. In contrast, WAY 163909, the 5-HT2CR agonist, resulted in changes associated with increased sensitivity to positive feedback and decreased sensitivity to negative feedback.

CONCLUSIONS

These results suggest that 5-HT2CRs tightly regulate feedback sensitivity bias in mice with consequent effects on learning and cognitive flexibility and specify a framework for the influence of 5-HT2CRs on sensitivity to reinforcement.

Hippocampal GABAA antagonism reverses the novel object recognition deficit in sub-chronic phencyclidine-treated rats

BACKGROUND

Abnormalities in prefrontal cortical and hippocampal GABAergic function are postulated to be major causes of the cognitive impairment associated with schizophrenia (CIAS). There are conflicting views on whether diminished or enhanced GABAergic activity contributes to the deficit in short-term novel object recognition (NOR) in the sub-chronic phencyclidine (scPCP) rodent model of CIAS. This study assessed the role of GABA_A signaling in the medial prefrontal cortex (mPFC) and ventral hippocampus (vHPC) in NOR in saline (scSAL)- and scPCP-treated rats.

METHODS

The effects of local administration of a GABA_A agonist (muscimol) into the vHPC or mPFC and an antagonist (bicuculline) or a GABA_A/benzodiazepine partial agonist (bretazenil) into the vHPC on NOR in scSAL and scPCP-treated rats were determined.

RESULTS

In scSAL-treated rats, injection of muscimol into the vHPC, but not mPFC, induced a deficit in NOR. The scPCP-induced NOR deficit was significantly reversed by intra-vHPC bicuculline, while intra-vHPC bretazenil produced a non-significant trend for reversal (p = .06). scPCP treatment increased mRNA expression of GABA_A γ₂ in PFC and GABA_A α₅ and GABA_A β₁ in the HPC. However, GABA concentration in the PFC or HPC was not altered.

CONCLUSIONS

These findings indicate that the scPCP-induced NOR deficit can be rescued by reducing GABA_A receptor stimulation in vHPC, indicating that increased vHPC GABA_A inhibition may contribute to the scPCP-induced NOR deficit in rats. These results also indicate that excessive GABA_A receptor signalling in the vHPC has a deleterious effect on NOR in normal rats.

Medial prefrontal cortex and dorsomedial striatum are necessary for the trial-unique, delayed nonmatching-to-location (TUNL) task in rats: role of NMDA receptors

The trial-unique, delayed nonmatching-to-location (TUNL) task is a recently developed behavioral task that measures spatial working memory and a form of pattern separation in touchscreen-equipped operant conditioning chambers. Limited information exists regarding the neurotransmitters and neural substrates involved in the task. The present experiments tested the effects of systemic and intracranial injections of NMDA receptor antagonists on the TUNL task. After training, male Long Evans rats systemically injected with the competitive NMDA receptor antagonist CPP (10 mg/kg) had impaired accuracy regardless of the degree of stimuli separation or length of delay between the sample and test phases. Injections of Ro 25-6981 (6 or 10 mg/kg), an antagonist selective for GluN2B subunit-containing NMDA receptors, did not affect accuracy on the task. Direct infusion of the competitive NMDA receptor antagonist AP5 into mPFC or dmSTR reduced overall accuracy on the TUNL task. These results demonstrate that TUNL task performance depends on NMDA receptors within the mPFC and dmSTR.

Rodent age-related impairments in discriminating perceptually similar objects parallel those observed in humans

The ability to accurately remember distinct episodes is supported by high-level sensory discrimination. Performance on mnemonic similarity tasks, which test high-level discrimination, declines with advancing age in humans and these deficits have been linked to altered activity in hippocampal CA3 and dentate gyrus. Lesion studies in animal models, however, point to the perirhinal cortex as a brain region critical for sensory discriminations that serve memory. Reconciliation of the contributions of different regions within the cortical-hippocampal circuit requires the development of a discrimination paradigm comparable to the human mnemonic similarity task that can be used in rodents. In the present experiments, young and aged rats were cross-characterized on a spatial water maze task and two variants of an object discrimination task: one in which rats incrementally learned which object of a pair was rewarded and different pairs varied in their similarity (Experiment 1), and a second in which rats were tested on their ability to discriminate a learned target object from multiple lure objects with an increasing degree of feature overlap (Experiment 2). In Experiment 1, aged rats required more training than young to correctly discriminate between similar objects. Comparably, in Experiment 2, aged rats were impaired in discriminating a target object from lures when the pair shared more features. Discrimination deficits across experiments were correlated within individual aged rats, though, for the cohort tested, aged rats were not impaired overall in spatial learning and memory. This could suggest discrimination deficits emerging with age precede declines in spatial or episodic memory, an observation that has been made in humans. Findings of robust impairments in object discrimination abilities in the aged rats parallel results from human studies, supporting use of the developed tasks for mechanistic investigation of cortical-hippocampal circuit dysfunction in aging and disease.

Temporary inactivation reveals that the CA1 region of the mouse dorsal hippocampus plays an equivalent role in the retrieval of long-term object memory and spatial memory

Recognition of a previously experienced item or object depends upon the successful retrieval of memory for the object. The neural mechanisms that support object recognition memory in the mammalian brain are not well understood. The rodent hippocampus plays a well-established role in spatial memory, and we previously demonstrated that temporary inactivation of the mouse hippocampus impairs object memory, as assessed with a novel object preference (NOP) test. The present studies were designed to test some remaining issues regarding the contribution of the CA1 sub-region of the mouse dorsal hippocampus to long-term object memory. Specifically, we examined whether the retrieval of spatial memory (as assessed by the Morris water maze; MWM) and object recognition memory are differentially sensitive to inactivation of the CA1 region. The current study used pre-test local microinfusion of muscimol directly into the CA1 region of dorsal hippocampus to temporarily interrupt its function during the respective retrieval phases of both behavioral tasks, in order to compare the contribution of the CA1 to object memory and spatial memory. Histological analyses revealed that local intra-CA1 injection of muscimol diffused within, and not beyond, the CA1 region of dorsal hippocampus. The degree of memory retrieval impairment induced by muscimol was comparable in the two tasks, supporting the view that object memory and spatial memory depend similarly on the CA1 region of rodent hippocampus. Further, we confirmed that the muscimol-induced impairment of CA1 function is temporary. First, mice that exhibited impaired object memory retrieval immediately after intra-CA1 muscimol, subsequently exhibited unimpaired retrieval of object memory when tested 24h later. Secondly, a cohort of mice that exhibited impaired object memory retrieval after intra-CA1 muscimol later acquired spatial memory in the MWM comparable to that of control mice. Together, these results offer further support for the involvement of the CA1 region of mouse hippocampus in object recognition memory, and provide evidence to suggest that the NOP task is as much a test of hippocampal function as the classic MWM test.

Comparing the effects of subchronic phencyclidine and medial prefrontal cortex dysfunction on cognitive tests relevant to schizophrenia

RATIONALE

It is becoming increasingly clear that the development of treatments for cognitive symptoms of schizophrenia requires urgent attention, and that valid animal models of relevant impairments are required. With subchronic psychotomimetic agent phencyclidine (scPCP), a putative model of such impairment, the extent to which changes following scPCP do or do not resemble those following dysfunction of the prefrontal cortex is of importance.

OBJECTIVES

The present study carried out a comparison of the most common scPCP dosing regimen with excitotoxin-induced medial prefrontal cortex (mPFC) dysfunction in rats, across several cognitive tests relevant to schizophrenia.

METHODS

ScPCP subjects were dosed intraperitoneal with 5 mg/kg PCP or vehicle twice daily for 1 week followed by 1 week washout prior to behavioural testing. mPFC dysfunction was induced via fibre-sparing excitotoxin infused into the pre-limbic and infralimbic cortex. Subjects were tested on spontaneous novel object recognition, touchscreen object-location paired-associates learning and touchscreen reversal learning.

RESULTS

A double-dissociation was observed between object-location paired-associates learning and object recognition: mPFC dysfunction impaired acquisition of the object-location task but not spontaneous novel object recognition, while scPCP impaired spontaneous novel object recognition but not object-location associative learning. Both scPCP and mPFC dysfunction resulted in a similar facilitation of reversal learning.

CONCLUSIONS

The pattern of impairment following scPCP raises questions around its efficacy as a model of cognitive impairment in schizophrenia, particularly if importance is placed on faithfully replicating the effects of mPFC dysfunction.

Animal paradigms to assess cognition with translation to humans

Cognition is a complex brain function that represents processes such as learning and memory, attention, working memory, and executive functions amongst others. Impairments in cognition are prevalent in many neuropsychiatric and neurological disorders with few viable treatment options. The development of new therapies is challenging, and poor efficacy in clinical development continues to be one of the most consistent reasons compounds fail to advance, suggesting that traditional animal models are not predictive of human conditions and behavior. An effort to improve the construct validity of neuropsychological testing across species with the intent of facilitating therapeutic development has been strengthening over recent years. With an emphasis on understanding the underlying biology, optimizing the use of appropriate systems (e.g., transgenic animals) to model targeted disease states, and incorporating non-rodent species (e.g., non-human primates) that may enable a closer comparison to humans, an improvement in the translatability of the results will be possible. This chapter focuses on some promising translational cognitive paradigms for use in rodents, non-human primates, and humans.

Re-evaluating the PCP challenge as a pre-clinical model of impaired cognitive flexibility in schizophrenia

NMDA-R antagonists are a popular translational pharmacological challenge to induce cognitive deficits associated with schizophrenia. Amongst their many cognitive and non-cognitive effects is an ability to impair cognitive flexibility in general, and reversal learning in particular. Here, we test the hypothesis that the NMDA-R antagonist phencyclidine when given acutely selectively effects reversal learning by simultaneously measuring reversal learning and baseline responding, or acquisition and baseline responding, under identical conditions. Animals were trained to simultaneously perform two different visual discriminations in a touch-screen equipped operant box. Accordingly the reward contingencies associated with one pair could be altered, while the second pair acted as an experimental control. As such, the effect of a manipulation on reversal learning, stimuli acquisition, or baseline responding can be more accurately evaluated through the use of a double visual discrimination. A similar approach was also used to investigate the influence of sub-chronic phencyclidine administration on cognitive flexibility. Phencyclidine (1mg/kg) given before testing caused a slowing in acquisition and reversal learning, while having a minimal effect on secondary measures. Sub-chronic phencyclidine administration had no significant effect on any of the measures used within this study. While acute phencyclidine impairs reversal learning, it is clear from these results that other aspects of cognition (learning/relearning) are also impaired, potentially questioning the specificity of acute phencyclidine in conjunction with reversal learning paradigms as a model of impaired cognitive flexibility.

Touchscreen-based cognitive tasks reveal age-related impairment in a primate aging model, the grey mouse lemur (Microcebus murinus)

Mouse lemurs are suggested to represent promising novel non-human primate models for aging research. However, standardized and cross-taxa cognitive testing methods are still lacking. Touchscreen-based testing procedures have proven high stimulus control and reliability in humans and rodents. The aim of this study was to adapt these procedures to mouse lemurs, thereby exploring the effect of age. We measured appetitive learning and cognitive flexibility of two age groups by applying pairwise visual discrimination (PD) and reversal learning (PDR) tasks. On average, mouse lemurs needed 24 days of training before starting with the PD task. Individual performances in PD and PDR tasks correlate significantly, suggesting that individual learning performance is unrelated to the respective task. Compared to the young, aged mouse lemurs showed impairments in both PD and PDR tasks. They needed significantly more trials to reach the task criteria. A much higher inter-individual variation in old than in young adults was revealed. Furthermore, in the PDR task, we found a significantly higher perseverance in aged compared to young adults, indicating an age-related deficit in cognitive flexibility. This study presents the first touchscreen-based data on the cognitive skills and age-related dysfunction in mouse lemurs and provides a unique basis to study mechanisms of inter-individual variation. It furthermore opens exciting perspectives for comparative approaches in aging, personality, and evolutionary research.

Stress facilitates late reversal learning using a touchscreen-based visual discrimination procedure in male Long Evans rats

The stress response is essential to the survival of all species as it maintains internal equilibrium and allows organisms to respond to threats in the environment. Most stress research has focused on the detrimental impacts of stress on cognition and behavior. Reversal learning, which requires a change in response strategy based on one dimension of the stimuli, is one type of behavioral flexibility that is facilitated following some brief stress procedures. The current study investigated a potential mechanism underlying this facilitation by blocking glucocorticoid receptors (GRs) during stress. Thirty-seven male Long Evans rats learned to discriminate between two images on a touchscreen, one of which was rewarded. Once a criterion was reached, rats received stress (30 min of restraint stress or no stress) and drug (GR antagonist RU38486 or vehicle) administration prior to each of the first 3 days of reversal learning. We expected that stress would facilitate reversal learning and RU38486 (10 mg/kg) would prevent this facilitation in both early (<50% correct in one session) and late (>50% correct in one session) stages of reversal learning. Results showed that stressed rats performed better than unstressed rats (fewer days for late reversal, fewer correction trials, and fewer errors) in the late but not early stage of reversal learning. RU38486 did not block the facilitation of RL by stress, although it dramatically increased response, but not reward, latencies. These results confirm the facilitation of late reversal by stress in a touchscreen-based operant task in rats and further our understanding of how stress affects higher level cognitive functioning and behavior.

Optimization of touchscreen-based behavioral paradigms in mice: implications for building a battery of tasks taxing learning and memory functions

Although many clinical pathological states are now detectable using imaging and biochemical analyses, neuropsychological tests are often considered as valuable complementary approaches to confirm diagnosis, especially for disorders like Alzheimer’s or Parkinson’s disease, and schizophrenia. The touchscreen-based automated test battery, which was introduced two decades ago in humans to assess cognitive functions, has recently been successfully back-translated in monkeys and rodents. We focused on optimizing the protocol of three distinct behavioral paradigms in mice: two variants of the Paired Associates Learning (PAL) and the Visuo-Motor Conditional Learning (VMCL) tasks. Acquisition of these tasks was assessed in naive versus pre-trained mice. In naive mice, we managed to define testing conditions allowing significant improvements of learning performances over time in the three aforementioned tasks. In pre-trained mice, we observed differential acquisition rates after specific task combinations. Particularly, we identified that animals previously trained in the VMCL paradigm subsequently poorly learned the sPAL rule. Together with previous findings, these data confirm the feasibility of using such behavioral assays to evaluate the power of different models of cognitive dysfunction in mice. They also highlight the risk of interactions between tasks when rodents are run through a battery of different cognitive touchscreen paradigms.

Dynamic functional brain networks involved in simple visual discrimination learning

Visual discrimination tasks have been widely used to evaluate many types of learning and memory processes. However, little is known about the brain regions involved at different stages of visual discrimination learning. We used cytochrome c oxidase histochemistry to evaluate changes in regional brain oxidative metabolism during visual discrimination learning in a water-T maze at different time points during training. As compared with control groups, the results of the present study reveal the gradual activation of cortical (prefrontal and temporal cortices) and subcortical brain regions (including the striatum and the hippocampus) associated to the mastery of a simple visual discrimination task. On the other hand, the brain regions involved and their functional interactions changed progressively over days of training. Regions associated with novelty, emotion, visuo-spatial orientation and motor aspects of the behavioral task seem to be relevant during the earlier phase of training, whereas a brain network comprising the prefrontal cortex was found along the whole learning process. This study highlights the relevance of functional interactions among brain regions to investigate learning and memory processes.

Different roles for M1 and M2 receptors within perirhinal cortex in object recognition and discrimination

Recognition and discrimination of objects and individuals are critical cognitive faculties in both humans and non-human animals, and cholinergic transmission has been shown to be essential for both of these functions. In the present study we focused on the role of M1 and M2 muscarinic receptors in perirhinal cortex (PRh)-dependent object recognition and discrimination. The selective M1 antagonists pirenzepine and the snake toxin MT-7, and a selective M2 antagonist, AF-DX 116, were infused directly into PRh. Pre-sample infusions of both pirenzepine and AF-DX 116 significantly impaired object recognition memory in a delay-dependent manner. However, pirenzepine and MT-7, but not AF-DX 116, impaired oddity discrimination performance in a perceptual difficulty-dependent manner. The findings indicate distinct functions for M1 and M2 receptors in object recognition and discrimination.

The prelimbic cortex and subthalamic nucleus contribute to cue-guided behavioral switching

Frontal cortex-basal ganglia circuitry supports behavioral switching when a change in outcome information is used to adapt response patterns. Less is known about whether specific frontal cortex-basal ganglia circuitry supports behavioral switching when cues signal that a change in response patterns should occur. The present experiments investigated whether the prelimbic cortex and subthalamic nucleus in male Long-Evans rats supports cue-guided switching in a conditional discrimination test. Rats learned in a cross-maze that a start arm cue (black or white) signaled which of two maze arms to enter for a food reward. The cue was switched every 3-6 trials. Baclofen and muscimol infused into the prelimbic cortex significantly impaired performance by increasing switch trial errors, as well as trials immediately following a switch trial (perseveration) and after initially making a correct switch (maintenance error). NMDA receptor blockade in the subthalamic nucleus significantly impaired performance by increasing switch errors and perseveration. Contralateral disconnection of these areas significantly reduced conditional discrimination performance by increasing switch and perseverative errors. These findings suggest that the prelimbic area and subthalamic nucleus support the use of cue information to facilitate an initial switch away from a previously relevant response pattern.

The touchscreen operant platform for testing learning and memory in rats and mice

An increasingly popular method of assessing cognitive functions in rodents is the automated touchscreen platform, on which a number of different cognitive tests can be run in a manner very similar to touchscreen methods currently used to test human subjects. This methodology is low stress (using appetitive rather than aversive reinforcement), has high translational potential and lends itself to a high degree of standardization and throughput. Applications include the study of cognition in rodent models of psychiatric and neurodegenerative diseases (e.g., Alzheimer's disease, schizophrenia, Huntington's disease, frontotemporal dementia), as well as the characterization of the role of select brain regions, neurotransmitter systems and genes in rodents. This protocol describes how to perform four touchscreen assays of learning and memory: visual discrimination, object-location paired-associates learning, visuomotor conditional learning and autoshaping. It is accompanied by two further protocols (also published in this issue) that use the touchscreen platform to assess executive function, working memory and pattern separation.

Object-in-place associative recognition memory depends on glutamate receptor neurotransmission within two defined hippocampal-cortical circuits: a critical role for AMPA and NMDA receptors in the hippocampus, perirhinal, and prefrontal cortices

Object-in-place associative recognition memory depends on an interaction between the hippocampus (HPC), perirhinal (PRH), and medial prefrontal (mPFC) cortices, yet the contribution of glutamate receptor neurotransmission to these interactions is unknown. NMDA receptors (NMDAR) in the HPC were critical for encoding of object-in-place memory but not for single-item object recognition. Next, a disconnection procedure was used to examine the importance of “concurrent” glutamate neurotransmission in the HPC-mPFC and HPC-PRH. Contralateral unilateral infusions of NBQX (AMPAR antagonist), into the HPC-mPFC, or HPC-PRH, either before acquisition or test, impaired object-in-place performance. Thus, both circuits are necessary for encoding and retrieval. Crossed unilateral AP5 (NMDAR antagonist) infusions into the HPC-mPFC or HPC-PRH impaired encoding, but not retrieval. Specifically crossed HPC-mPFC infusions impaired both short-term (5 min) and longer term (1 h) memory while HPC-PRH infusions impaired longer term memory only. This delay-dependent effect of AP5 in the HPC-PRH on object-in-place memory, accords with its effects in the PRH, on single item object recognition memory, thereby suggesting that a single PRH synaptic plasticity mechanism underpins different recognition memory processes. Further, blocking excitatory neurotransmission in any pair of structures within the networks impaired “both” encoding and retrieval, thus object-in-place memory clearly requires network interdependency across multiple structures.

Perirhinal cortical inactivation impairs object-in-place memory and disrupts task-dependent firing in hippocampal CA1, but not in CA3

Objects and their locations can associatively define an event and a conjoint representation of object-place can form an event memory. Remembering how to respond to a certain object in a spatial context is dependent on both hippocampus and perirhinal cortex (PER). However, the relative functional contributions of the two regions are largely unknown in object-place associative memory. We investigated the PER influence on hippocampal firing in a goal-directed object-place memory task by comparing the firing patterns of CA1 and CA3 of the dorsal hippocampus between conditions of PER muscimol inactivation and vehicle control infusions. Rats were required to choose one of the two objects in a specific spatial context (regardless of the object positions in the context), which was shown to be dependent on both hippocampus and PER. Inactivation of PER with muscimol (MUS) severely disrupted performance of well-trained rats, resulting in response bias (i.e., choosing any object on a particular side). MUS did not significantly alter the baseline firing rates of hippocampal neurons. We measured the similarity in firing patterns between two trial conditions in which the same target objects were chosen on opposite sides within the same arm [object-in-place (O-P) strategy] and compared the results with the similarity in firing between two trial conditions in which the rat chose any object encountered on a particular side [response-in-place (R-P) strategy]. We found that the similarity in firing patterns for O-P trials was significantly reduced with MUS compared to control conditions (CTs). Importantly, this was largely because MUS injections affected the O-P firing patterns in CA1 neurons, but not in CA3. The results suggest that PER is critical for goal-directed organization of object-place associative memory in the hippocampus presumably by influencing how object information is associated with spatial information in CA1 according to task demand.

Role of cholinergic-muscarinic receptors in visual discrimination performance of rats: importance of stimulus load

Central cholinergic transmission has long been implicated in various cognitive processes, including memory acquisition, consolidation, and attentional processes. Here, we examined the role of muscarinic receptors in visual discrimination performance under conditions of altered visual information availability. Adult rats were trained to discriminate two visual cues (indicating the presence and absence of a hidden escape platform, respectively) in a water maze-based, trapezoidal-shaped apparatus. Following task acquisition, testing continued with two types of trials: regular trials (RTs; both visual cues present, identical to training conditions) and probe trials (PTs; only one of the two cues present). In Experiment 1, removal of one visual cue on PTs impaired discrimination performance. Moreover, scopolamine administration (0.125-1.0 mg/kg, i.p.) tended to further suppress performance in a dose-dependent manner on PTs, while discriminations on RTs were left intact. In Experiment 2, these results were confirmed and extended by showing that PT (one visual cue) performance could improve with training in undrugged, but not in scopolamine-treated rats. Together, these experiments reveal that visual discrimination performance of rats benefits from the concurrent availability of two visual cues that provide complimentary and consistent information. Furthermore, muscarinic receptors are particularly important under conditions of reduced visual information availability, as well as in the adoption of new behavioral strategies, such as switching from two-cue to single-cue guided navigation.

Intersection of reward and memory in monkey rhinal cortex

In humans and other animals, the vigor with which a reward is pursued depends on its desirability, that is, on the reward's predicted value. Predicted value is generally context-dependent, varying according to the value of rewards obtained in the recent and distant past. Signals related to reward prediction and valuation are believed to be encoded in a circuit centered around midbrain dopamine neurons and their targets in the prefrontal cortex and basal ganglia. Notably absent from this hypothesized reward pathway are dopaminergic targets in the medial temporal lobe. Here we show that a key part of the medial temporal lobe memory system previously reported to be important for sensory mnemonic and perceptual processing, the rhinal cortex (Rh), is required for using memories of previous reward values to predict the value of forthcoming rewards. We tested monkeys with bilateral Rh lesions on a task in which reward size varied across blocks of uncued trials. In this experiment, the only cues for predicting current reward value are the sizes of rewards delivered in previous blocks. Unexpectedly, monkeys with Rh ablations, but not intact controls, were insensitive to differences in predicted reward, responding as if they expected all rewards to be of equal magnitude. Thus, it appears that Rh is critical for using memory of previous rewards to predict the value of forthcoming rewards. These results are in agreement with accumulating evidence that Rh is critical for establishing the relationships between temporally interleaved events, which is a key element of episodic memory.

The pharmacological sensitivity of a touchscreen-based visual discrimination task in the rat using simple and perceptually challenging stimuli

RATIONALE

Cognitive testing with touchscreen-equipped operant boxes ('touchscreens') is becoming increasingly popular. Tasks, such as paired associate learning or reversal learning of visual stimuli, have the discrimination of visual stimuli as a fundamental component. However, the effect of drugs commonly used in the study of cognitive mechanisms has yet to be described in a visual discrimination.

OBJECTIVE

The objective of the study was to profile a range of psychoactive agents (glutamatergic, dopaminergic, and cholinergic agonists and antagonists) known to be important in cognitive processing on visual discrimination performance using a touch sensitive computer monitor.

METHODS

Male Lister Hooded rats were trained to a stable level of performance in a simple visual discrimination. In Experiment 1, the effect of MK-801, phencyclidine, memantine, dextroamphetamine sulphate (D-amphetamine) and scopolamine was assessed. In Experiment 2, the stimuli were blended together resulting in a perceptually more demanding discrimination and a reduction in accuracy. The rats used in Experiment 1 were then retested with these 'morphed' stimuli under the influence of the above compounds.

RESULTS

MK-801, PCP, and D-amphetamine induced selective deficits in accuracy in both versions of the task. In contrast, scopolamine and memantine produced non-selective deficits in accuracy. Morphing the stimuli reduced accuracy, but did not alter the observed behavioural profile after compound administration.

CONCLUSION

These data improve our understanding of the basic neuropharmacology of a visual discrimination in cognitive tests employing touchscreens and will aid in the interpretation of pharmacological studies with more cognitively demanding methodologies.

Spontaneous object recognition and its relevance to schizophrenia: a review of findings from pharmacological, genetic, lesion and developmental rodent models

RATIONALE

Spontaneous (novel) object recognition (SOR) is one of the most widely used rodent behavioural tests. The opportunity for rapid data collection has made SOR a popular choice in studies that explore cognitive impairment in rodent models of schizophrenia, and that test the efficacy of drugs intended to reverse these deficits.

OBJECTIVES

We provide an overview of the many recent studies that have used SOR to explore the mnemonic effects of manipulation of the key transmitter systems relevant to schizophrenia-the dopamine, glutamate, GABA, acetylcholine, serotonin and cannabinoid systems-alone or in combination. We also review the use of SOR in studying memory in genetically modified mouse models of schizophrenia, as well as in neurodevelopmental and lesion models. We end by discussing the construct and predictive validity, and translational relevance, of SOR with respect to cognitive impairment in schizophrenia.

RESULTS

Perturbation of the dopamine or glutamate systems can generate robust and reliable impairment in SOR. Impaired performance is also seen following antagonism of the muscarinic acetylcholine system, or exposure to cannabinoid agonists. Cognitive enhancement has been reported using alpha7-nicotinic acetylcholine receptor agonists and 5-HT(6) antagonists. Among non-pharmacological models, neonatal ventral hippocampal lesions and maternal immune activation can impair SOR, while mixed results have been obtained with mice carrying mutations in schizophrenia risk-associated genes, including neuregulin and COMT.

CONCLUSIONS

While SOR is not without its limitations, the task represents a useful method for studying manipulations with relevance to cognitive impairment in schizophrenia, as well as the interactions between them.

New translational assays for preclinical modelling of cognition in schizophrenia: the touchscreen testing method for mice and rats

We describe a touchscreen method that satisfies a proposed 'wish-list' of desirables for a cognitive testing method for assessing rodent models of schizophrenia. A number of tests relevant to schizophrenia research are described which are currently being developed and validated using this method. These tests can be used to study reward learning, memory, perceptual discrimination, object-place associative learning, attention, impulsivity, compulsivity, extinction, simple Pavlovian conditioning, and other constructs. The tests can be deployed using a 'flexible battery' approach to establish a cognitive profile for a particular mouse or rat model. We have found these tests to be capable of detecting not just impairments in function, but enhancements as well, which is essential for testing putative cognitive therapies. New tests are being continuously developed, many of which may prove particularly valuable for schizophrenia research.

On the dynamic nature of the engram: evidence for circuit-level reorganization of object memory traces following reactivation

Research has implicated the perirhinal cortex (PRh) in several aspects of object recognition memory. The specific role of the hippocampus (HPC) remains controversial, but its involvement in object recognition may pertain to processing contextual information in relation to objects rather than object representation per se. Here we investigated the roles of the PRh and HPC in object memory reconsolidation using the spontaneous object recognition task for rats. Intra-PRh infusions of the protein synthesis inhibitor anisomycin immediately following memory reactivation prevented object memory reconsolidation. Similar deficits were observed when a novel object or a salient contextual change was introduced during the reactivation phase. Intra-HPC infusions of anisomycin, however, blocked object memory reconsolidation only when a contextual change was introduced during reactivation. Moreover, disrupting functional interaction between the HPC and PRh by infusing anisomycin unilaterally into each structure in opposite hemispheres also impaired reconsolidation when reactivation was done in an altered context. These results show for the first time that the PRh is critical for reconsolidation of object memory traces and provide insight into the dynamic process of object memory storage; the selective requirement for hippocampal involvement following reactivation in an altered context suggests a substantial circuit level object trace reorganization whereby an initially PRh-dependent object memory becomes reliant on both the HPC and PRh and their interaction. Such trace reorganization may play a central role in reconsolidation-mediated memory updating and could represent an important aspect of lingering consolidation processes proposed to underlie long-term memory modulation and stabilization.

Intact attentional processing but abnormal responding in M1 muscarinic receptor-deficient mice using an automated touchscreen method

Cholinergic receptors have been implicated in schizophrenia, Alzheimer's disease, Parkinson's disease, and Huntington's disease. However, to better target therapeutically the appropriate receptor subsystems, we need to understand more about the functions of those subsystems. In the current series of experiments, we assessed the functional role of M(1) receptors in cognition by testing M(1) receptor-deficient mice (M1R(-/-)) on the five-choice serial reaction time test of attentional and response functions, carried out using a computer-automated touchscreen test system. In addition, we tested these mice on several tasks featuring learning, memory and perceptual challenges. An advantage of the touchscreen method is that each test in the battery is carried out in the same task setting, using the same types of stimuli, responses and feedback, thus providing a high level of control and task comparability. The surprising finding, given the predominance of the M(1) receptor in cortex, was the complete lack of effect of M(1) deletion on measures of attentional function per se. Moreover, M1R(-/-) mice performed relatively normally on tests of learning, memory and perception, although they were impaired in object recognition memory with, but not without an interposed delay interval. They did, however, show clear abnormalities on a variety of response measures: M1R(-/-) mice displayed fewer omissions, more premature responses, and increased perseverative responding compared to wild-types. These data suggest that M1R(-/-) mice display abnormal responding in the face of relatively preserved attention, learning and perception.

Visual perception and memory systems: from cortex to medial temporal lobe

Visual perception and memory are the most important components of vision processing in the brain. It was thought that the perceptual aspect of a visual stimulus occurs in visual cortical areas and that this serves as the substrate for the formation of visual memory in a distinct part of the brain called the medial temporal lobe. However, current evidence indicates that there is no functional separation of areas. Entire visual cortical pathways and connecting medial temporal lobe are important for both perception and visual memory. Though some aspects of this view are debated, evidence from both sides will be explored here. In this review, we will discuss the anatomical and functional architecture of the entire system and the implications of these structures in visual perception and memory.

Impaired conditioned emotional response and object recognition are concomitant to neuronal damage in the amygdala and perirhinal cortex in middle-aged ischemic rats

The current study characterizes fear conditioning responses following global ischemia and evaluates neuronal damage affecting discrete extra-hippocampal areas susceptible to contribute to post ischemic emotional and memory impairments. Conditioned emotional response, Barnes Maze and object recognition tests were used to assess emotional, spatial and recognition memory, respectively. Behavioural testing was initiated in middle-aged animals (10-12 month old) 1 week following sham (n=16) or 4VO occlusion (n=18). Post-mortem cellular assessment was performed in the hippocampal CA1 layer, the perirhinal cortex and basolateral amygdala. Middle-aged ischemic animals showed impaired spatial memory in the initial three testing days in the Barnes Maze and deficit in recognition memory. Of interest, ischemic rats demonstrated a significant reduction of freezing and increased locomotion during the contextual fear testing period, suggesting reduced fear in these animals. Assessment of neuronal density 40 days following global ischemia revealed that CA1 neuronal injury was accompanied by 20-25% neuronal loss in the basolateral nucleus of the amygdala and perirhinal cortex in middle-aged ischemic compared to sham-operated animals. This study represents the first demonstration of altered conditioned fear responses following ischemia. Our findings also indicate a vulnerability of extra-hippocampal neurons to ischemic injury, possibly contributing to discrete emotional and/or memory impairments post ischemia.

FK962 and donepezil act synergistically to improve cognition in rats: potential as an add-on therapy for Alzheimer's disease

FK962 is a member of a novel class of compounds that promote somatostatin production in the brain, and is being developed as a treatment for patients with Alzheimer's disease. As acetylcholinesterase inhibitors such as Aricept© (donepezil) are widely used to treat these patients, it is important to confirm that potential new medicines in this disease area can be co-administered with drugs such as Aricept. To study the effect of FK962 in combination with donepezil, touchscreen methodology was used to measure the effect on cognition in rats. Doses of FK962 and donepezil were identified that resulted in minimal cognition enhancement when given separately. There was strong evidence (p=0.002) of a treatment difference between the combination of FK962/donepezil and FK962 alone: the estimated treatment difference is 5.47 (95% CI: 2.19-8.75). There was also evidence (p=0.017) of a treatment difference between the combination of FK962/donepezil and donepezil alone: the estimated treatment difference is 4.01 (95% CI: 0.77-7.26). Therefore, a combination of low doses of FK962 and donepezil showed a significantly greater effect on cognition than low doses of either compound alone. This is the first time that FK962 has shown activity in a reward-based model of cognition. In addition, these data suggest that this compound could beneficially be given in addition to Aricept to treat Alzheimer's disease patients.