Critical role of the cholinergic system for object-in-place associative recognition memory

Neuronal circuitry for recognition memory of object and place in rodent models

Rats and mice are used for studying neuronal circuits underlying recognition memory due to their ability to spontaneously remember the occurrence of an object, its place and an association of the object and place in a particular environment. A joint employment of lesions, pharmacological interventions, optogenetics and chemogenetics is constantly expanding our knowledge of the neural basis for recognition memory of object, place, and their association. In this review, we summarize current studies on recognition memory in rodents with a focus on the novel object preference, novel location preference and object-in-place paradigms. The evidence suggests that the medial prefrontal cortex- and hippocampus-connected circuits contribute to recognition memory for object and place. Under certain conditions, the striatum, medial septum, amygdala, locus coeruleus and cerebellum are also involved. We propose that the neuronal circuitry for recognition memory of object and place is hierarchically connected and constructed by different cortical (perirhinal, entorhinal and retrosplenial cortices), thalamic (nucleus reuniens, mediodorsal and anterior thalamic nuclei) and primeval (hypothalamus and interpeduncular nucleus) modules interacting with the medial prefrontal cortex and hippocampus.

Rosinidin Protects Streptozotocin-Induced Memory Impairment-Activated Neurotoxicity by Suppressing Oxidative Stress and Inflammatory Mediators in Rats

Background and Objectives: To assess the antioxidant and neuroprotective role of rosinidin on rat memory impairment that is induced by streptozotocin. Materials and Methods: Wistar rats were given an intraperitoneal (i.p) injection of streptozotocin (60 mg/kg) followed by treatment with rosinidin at selective doses (10 and 20 mg/kg) for 30 days. The behavioral parameters were estimated by Y-maze test and Morris water test. Biochemical parameters such as acetylcholinesterase (AChE), choline aacetyltransferase (ChAT), and nitric oxide, and antioxidants such as glutathione transferase (GSH), superoxide dismutase (SOD) IL-6, IL-10, Nrf2, and BDNF, were determined. Results: The study results revealed that rosinidin improved cognition by reverting the behavioral parameters. The treatment with rosinidin restored the antioxidant enzymes and inflammatory cytokines. Conclusions: From the results, it has been proven that rosinidin possesses antioxidant, anti-amnesic, and anti-inflammatory activity. Rosinidin improved the cognitive and behavioral deficits that were induced by streptozotocin. Furthermore, 20 mg/kg rosinidin was found to have strong protective action against streptozotocin-induced toxicity.

TrkA-cholinergic signaling modulates fear encoding and extinction learning in PTSD-like behavior

Recent studies have suggested that the use of cognitive enhancers as adjuncts to exposure-based therapy in individuals suffering from post-traumatic stress disorder (PTSD) may be beneficial. Brain cholinergic signaling through basal forebrain projections to the hippocampus is an established pathway mediating fear response and cognitive flexibility. Here we employed a genetic strategy to enhance cholinergic activity through increased signaling of the NGF receptor TrkA. This strategy leads to increased levels of the marker of cholinergic activation, acetylcholine synthesizing enzyme choline acetyltransferase, in forebrain cholinergic regions and their projection areas such as the hippocampus. Mice with increased cholinergic activity do not display any neurobehavioral abnormalities except a selective attenuation of fear response and lower fear expression in extinction trials. Reduction in fear response is rescued by the GABA antagonist picrotoxin in mutant mice, and, in wild-type mice, is mimicked by the GABA agonist midazolam suggesting that GABA can modulate cholinergic functions on fear circuitries. Importantly, mutant mice also show a reduction in fear processing under stress conditions in a single prolonged stress (SPS) model of PTSD-like behavior, and augmentation of cholinergic signaling by the drug donepezil in wild-type mice promotes extinction learning in a similar SPS model of PTSD-like behavior. Donepezil is already in clinical use for the treatment of dementia suggesting a new translational application of this drug for improving exposure-based psychotherapy in PTSD patients.

Beyond the hippocampus: The role of parahippocampal-prefrontal communication in context-modulated behavior

Multiple paradigms indicate that the physical environment can influence spontaneous and learned behavior. In rodents, context-dependent behavior is putatively supported by the prefrontal cortex and the medial temporal lobe. A preponderance of the literature has targeted the role of the hippocampus. In addition to the hippocampus proper, the medial temporal lobe also comprises parahippocampal areas, including the perirhinal and postrhinal cortices. These parahippocampal areas directly connect with multiple regions in the prefrontal cortex. The function of these connections, however, is not well understood. This article first reviews the involvement of the perirhinal, postrhinal, and prefrontal cortices in context-dependent behavior in rodents. Then, based on functional and anatomical evidence, we suggest that perirhinal and postrhinal contributions to context-dependent behavior go beyond supporting context representation in the hippocampus. Specifically, we propose that the perirhinal and postrhinal cortices act as a contextual-support network that directly provides contextual and spatial information to the prefrontal cortex. In turn, the perirhinal and postrhinal cortices modulate prefrontal input to the hippocampus in the service of context-guided behavior.

Plasticity in Prefrontal Cortex Induced by Coordinated Synaptic Transmission Arising from Reuniens/Rhomboid Nuclei and Hippocampus

The nucleus reuniens and rhomboid nuclei of the thalamus (ReRh) are reciprocally connected to a range of higher order cortices including hippocampus (HPC) and medial prefrontal cortex (mPFC). The physiological function of ReRh is well predicted by requirement for interactions between mPFC and HPC, including associative recognition memory, spatial navigation, and working memory. Although anatomical and electrophysiological evidence suggests ReRh makes excitatory synapses in mPFC there is little data on the physiological properties of these projections, or whether ReRh and HPC target overlapping cell populations and, if so, how they interact. We demonstrate in ex vivo mPFC slices that ReRh and HPC afferent inputs converge onto more than two-thirds of layer 5 pyramidal neurons, show that ReRh, but not HPC, undergoes marked short-term plasticity during theta frequency transmission, and that HPC, but not ReRh, afferents are subject to neuromodulation by acetylcholine acting via muscarinic receptor M2. Finally, we demonstrate that pairing HPC followed by ReRh (but not pairing ReRh followed by HPC) at theta frequency induces associative, NMDA receptor dependent synaptic plasticity in both inputs to mPFC. These data provide vital physiological phenotypes of the synapses of this circuit and provide a novel mechanism for HPC-ReRh-mPFC encoding.

Altered acoustic startle, prepulse facilitation, and object recognition memory produced by corticosterone withdrawal in male rats

The symptoms of human depression often include cognitive deficits. However, cognition is not frequently included in the behavioral assessments conducted in preclinical models of depression. For example, it is well known that repeated corticosterone (CORT) injections in rodents produce depression-like behavior as measured by the forced swim test, sucrose preference test, and tail suspension test, but the cognitive impairments produced by repeated CORT have not been thoroughly examined. The purpose of this experiment was to assess the effect of repeated CORT injections on several versions of object recognition memory and modulation of the acoustic startle response by relatively low intensity prepulses, along with the more traditional assessment of depression-like behavior using the forced swim test. Rats received 21 days of CORT (40 mg/kg) or vehicle injections followed by a battery of behavioral tests. Importantly, during behavioral testing CORT treatment did not occur (CORT withdrawal). Corticosterone decreased body weight, increased immobility in the forced swim test, lowered startle amplitudes, and facilitated responding to trials with a short interval (30 ms) between the prepulse and pulse. Corticosterone also impaired both object location and object-in-place recognition memory, while sparing performance on object recognition memory. Collectively, our data suggest that CORT produces selective disruptions in prepulse facilitation, object location, and object-in-place recognition memory, and that these impairments should be considered as part of the phenotype produced by repeated CORT, and perhaps chronic stress.

How do stupendous cannabinoids modulate memory processing via affecting neurotransmitter systems?

In the present study, we wanted to review the role of cannabinoids in learning and memory in animal models, with respect to their interaction effects with six principal neurotransmitters involved in learning and memory including dopamine, glutamate, GABA (γ-aminobutyric acid), serotonin, acetylcholine, and noradrenaline. Cannabinoids induce a wide-range of unpredictable effects on cognitive functions, while their mechanisms are not fully understood. Cannabinoids in different brain regions and in interaction with different neurotransmitters, show diverse responses. Previous findings have shown that cannabinoids agonists and antagonists induce various unpredictable effects such as similar effect, paradoxical effect, or dualistic effect. It should not be forgotten that brain neurotransmitter systems can also play unpredictable roles in mediating cognitive functions. Thus, we aimed to review and discuss the effect of cannabinoids in interaction with neurotransmitters on learning and memory. In addition, we mentioned to the type of interactions between cannabinoids and neurotransmitter systems. We suggested that investigating the type of interactions is a critical neuropharmacological issue that should be considered in future studies.

The medial prefrontal cortex - hippocampus circuit that integrates information of object, place and time to construct episodic memory in rodents: Behavioral, anatomical and neurochemical properties

Rats and mice have been demonstrated to show episodic-like memory, a prototype of episodic memory, as defined by an integrated memory of the experience of an object or event, in a particular place and time. Such memory can be assessed via the use of spontaneous object exploration paradigms, variably designed to measure memory for object, place, temporal order and object-location inter-relationships. We review the methodological properties of these tests, the neurobiology about time and discuss the evidence for the involvement of the medial prefrontal cortex (mPFC), entorhinal cortex (EC) and hippocampus, with respect to their anatomy, neurotransmitter systems and functional circuits. The systematic analysis suggests that a specific circuit between the mPFC, lateral EC and hippocampus encodes the information for event, place and time of occurrence into the complex episodic-like memory, as a top-down regulation from the mPFC onto the hippocampus. This circuit can be distinguished from the neuronal component memory systems for processing the individual information of object, time and place.

Nicotinic Acetylcholine Receptors Control Encoding and Retrieval of Associative Recognition Memory through Plasticity in the Medial Prefrontal Cortex

Nicotinic acetylcholine receptors (nAChRs) expressed in the medial prefrontal cortex have critical roles in cognitive function. However, whether nAChRs are required for associative recognition memory and the mechanisms by which nAChRs may contribute to mnemonic processing are not known. We demonstrate that nAChRs in the prefrontal cortex exhibit subtype-specific roles in associative memory encoding and retrieval. We present evidence that these separate roles of nAChRs may rely on bidirectional modulation of plasticity at synaptic inputs to the prefrontal cortex that are essential for associative recognition memory.

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Prefrontal α7 nAChRs are critical for encoding of associative recognition memory

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Prefrontal α4β2 nAChRs are required for retrieval of associative recognition memory

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α7 and α4β2 nAChRs gate bidirectional plasticity at hippocampal-prefrontal synapses

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Bidirectional plasticity underlies the role of nAChR in associative recognition

Pharmacokinetics and acetylcholine releasing effects of ginsenoside Rg1 in hippocampus of beta-amyloid model rats

This study aimed at investigating the pharmacokinetics (PK) and acetylcholine (Ach) releasing effects of ginsenoside Rg1 in hippocampus (HIP) of soluble oligomeric assemblies of amyloid beta (ADDLs) induced Alzheimer's disease (AD) model rats. Extracellular fluid was collected by microdialysis technique. The concentrations of Rg1 and Ach in dialysates were detected by liquid chromatography-tandem mass spectrometry (LC-MS/MS) method. The results showed, after administration, Rg1 was soon detected in the HIP of model rats and extracellular Ach concentrations in the HIP were significantly increased.

Chronic nicotine improves cognitive and social impairment in mice overexpressing wild type α-synuclein

In addition to dopaminergic and motor deficits, patients with Parkinson's disease (PD) suffer from non-motor symptoms, including early cognitive and social impairment, that do not respond well to dopaminergic therapy. Cholinergic deficits may contribute to these problems, but cholinesterase inhibitors have limited efficacy. Mice over-expressing α-synuclein, a protein critically associated with PD, show deficits in cognitive and social interaction tests, as well as a decrease in cortical acetylcholine. We have evaluated the effects of chronic administration of nicotine in mice over-expressing wild type human α-synuclein under the Thy1-promoter (Thy1-aSyn mice). Nicotine was administered subcutaneously by osmotic minipump for 6 months from 2 to 8 months of age at 0.4 mg/kg/h and 2.0 mg/kg/h. The higher dose was toxic in the Thy1-aSyn mice, but the low dose was well tolerated and both doses ameliorated cognitive impairment in Y-maze performance after 5 months of treatment. In a separate cohort of Thy1-aSyn mice, nicotine was administered at the lower dose for one month beginning at 5 months of age. This treatment partially eliminated the cognitive deficit in novel object recognition and social impairment. In contrast, chronic nicotine did not improve motor deficits after 2, 4 or 6 months of treatment, nor modified α-synuclein aggregation, tyrosine hydroxylase immunostaining, synaptic and dendritic markers, or microglial activation in Thy1-aSyn mice. These results suggest that cognitive and social impairment in synucleinopathies like PD may result from deficits in cholinergic neurotransmission and may benefit from chronic administration of nicotinic agonists.

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.

Impaired object-location learning and recognition memory but enhanced sustained attention in M2 muscarinic receptor-deficient mice

RATIONALE

Muscarinic acetylcholine receptors are known to play key roles in mediating cognitive processes, and impaired muscarinic cholinergic neurotransmission is associated with normal ageing processes and Alzheimer's disease. However, the specific contributions of the individual muscarinic receptor subtypes (M1-M5) to cognition are presently not well understood.

OBJECTIVES

The aim of this study was to investigate the contribution of M2-type muscarinic receptor signalling to sustained attention, executive control and learning and memory.

METHODS

M2 receptor-deficient (M2^-/-) mice were tested on a touchscreen-operated task battery testing visual discrimination, behavioural flexibility, object-location associative learning, attention and response control. Spontaneous recognition memory for real-world objects was also assessed.

RESULTS

We found that M2^-/- mice showed an enhancement of attentional performance, but significant deficits on some tests of learning and memory. Executive control and visual discrimination were unaffected by M2-depletion.

CONCLUSIONS

These findings suggest that M2 activation has heterogeneous effects across cognitive domains, and provide insights into how acetylcholine may support multiple specific cognitive processes through functionally distinct cholinergic receptor subtypes. They also suggest that therapeutics involving M2 receptor-active compounds should be assessed across a broad range of cognitive domains, as they may enhance some cognitive functions, but impair others.

Lingering Cognitive States Shape Fundamental Mnemonic Abilities

Why are people sometimes able to recall associations in exquisite detail while at other times left frustrated by the deficiencies of memory? Although this apparent fickleness of memory has been extensively studied by investigating factors that build strong memory traces, researchers know less about whether memory success also depends on cognitive states that are in place when a cue is encountered. Motivating this possibility, neurocomputational models propose that the hippocampus's capacity to support associative recollection (pattern completion) is biased by persistent neurochemical states, which can be elicited by exposure to familiarity and novelty. We investigated these models' behavioral implications by assessing how recent familiarity influences different memory-retrieval processes. We found that recent familiarity selectively benefitted associative memory (Experiment 1) and that this effect decayed over seconds (Experiment 2), consistent with the timescale of hippocampal neuromodulation. Thus, we show that basic memory computations can be shaped by a subtle, biologically motivated manipulation.

Scopolamine Impairs Appetitive But Not Aversive Trace Conditioning: Role of the Medial Prefrontal Cortex

The muscarinic acetylcholine receptor is an important modulator of medial prefrontal cortex (mPFC) functions, such as the working memory required to bridge a trace interval in associative leaning. Aversive and appetitive trace conditioning procedures were used to examine the effects of scopolamine (0.1 and 0.5 mg/kg, i.p.) in male rats. Follow-up experiments tested the effects of microinfusion of 0.15 μg of scopolamine (0.075 μg of in 0.5 μl/side) in infralimbic (IL) versus prelimbic regions of rat mPFC, in appetitive trace and locomotor activity (LMA) procedures. Systemic scopolamine was without effect in an aversive trace conditioning procedure, but impaired appetitive conditioning at a 2 s trace interval. This effect was demonstrated as reduced responding during presentations of the conditioned stimulus (CS) and during the interstimulus interval (ISI). There was no such effect on responding during food (unconditioned stimulus, US) responding or in the intertrial interval (ITI). In contrast, systemic scopolamine dose-relatedly increased LMA. Trace conditioning was similarly impaired at the 2 s trace (shown as reduced responding to the CS and during the ISI, but not during US presentations or in the ITI) after infusion in mPFC, whereas LMA was increased (after infusion in IL only). Therefore, our results point to the importance of cholinergic modulation in mPFC for trace conditioning and show that the observed effects cannot be attributed to reduced activity.SIGNIFICANCE STATEMENT Events are very often separated in time, in which case working memory is necessary to condition their association in "trace conditioning." The present study used conditioning variants motivated aversively with foot shock and appetitively with food. The drug scopolamine was used to block muscarinic acetylcholine receptors involved in working memory. The results show that reduced cholinergic transmission in medial prefrontal cortex (mPFC) impaired appetitive trace conditioning at a 2 s trace interval. However, scopolamine was without effect in the aversive procedure, revealing the importance of procedural differences to the demonstration of the drug effect. The finding that blockade of muscarinic receptors in mPFC impaired trace conditioning shows that these receptors are critical modulators of short-term working memory.

Scopolamine Induces Deficits in Spontaneous Object-Location Recognition and Fear-Learning in Marmoset Monkeys

The non-selective muscarinic receptor antagonist scopolamine (SCP) induces memory deficits in both animals and humans. However, few studies have assessed the effects of amnesic agents on memory functions of marmosets – a small-bodied neotropical primate that is becoming increasingly used as a translational model for several neuropathologies. Here we assessed the effects of an acute SCP administration (0.03 mg/kg, sc) on the behavior of adult marmoset monkeys in two tasks. In the spontaneous object-location (SOL) recognition task, two identical neutral stimuli were explored on the sample trial, after which preferential exploration of the displaced versus the stationary object was analyzed on the test trial. In the fear-motivated behavior (FMB) procedure, the same subjects were submitted to an initial baseline trial, followed by an exposure period to a snake model and lastly a post-exposure trial. All trials and inter-trial intervals lasted 10 min for both tests. Results showed that on the SOL test trial, the saline group explored the displaced object significantly longer than its identical stationary counterpart, whereas SCP-treated marmosets explored both objects equivalently. In the FMB test, the saline group – but not the SCP-treated animals – spent significantly less time where the stimulus had been specifically encountered and more time being vigilant of their surroundings, compared to pre-exposure levels. Drug-related effects on general activity, overall exploration (SOL task) and behavioral response to the aversive stimulus (FMB task) were not observed. SCP thus impaired the marmosets’ short-term ability to detect changes associated with the spatial location of ethologically irrelevant (SOL task) and relevant stimuli (FMB task). Similar results have been reported in other animal species. Marmosets may thus help reduce the translational gap between pre-clinical studies and memory-associated human pathologies.

Caffeic acid attenuates oxidative stress, learning and memory deficit in intra-cerebroventricular streptozotocin induced experimental dementia in rats

Oxidative stress has been implicated in cognitive decline as seen during normal aging and in sporadic Alzheimer's disease (AD). Caffeic acid, a polyphenolic compound, has been reported to possess potent antioxidant and neuroprotective properties. The role of caffeic acid in experimental dementia is not fully understood. Thus the present study was designed to investigate the therapeutic potential of caffeic acid in streptozotocin (STZ)-induced experimental dementia of Alzheimer's type in rats. Streptozotocin (STZ) was administered intracerebroventrically (ICV) on day 1 and 3 (3mg/kg, ICV bilaterally) in Wistar rats. Caffeic acid was administered (10, 20 and 40mg/kg/day p.o.) 1h following STZ infusion upto 21st day. Morris water maze and object recognition task were used to assess learning and memory in rats. Terminally, acetylcholinesterase (AChE) activity and the levels of oxido-nitrosative stress markers were determined in cortical and hippocampal brain regions of rats. STZ produced significant (p<0.001) learning and memory impairment, oxido-nitrosative stress and cholinergic deficit in rats. Whereas, caffeic acid treatment significantly (p<0.001) and dose dependently attenuated STZ induced behavioral and biochemical abnormalities in rats. The observed cognitive improvement following caffeic acid in STZ treated rats may be due to its antioxidant activity and restoration of cholinergic functions. Our results suggest the therapeutic potential of caffeic acid in cognitive disorders such as AD.

Glucose improves object-location binding in visual-spatial working memory

RATIONALE

There is evidence that glucose temporarily enhances cognition and that processes dependent on the hippocampus may be particularly sensitive. As the hippocampus plays a key role in binding processes, we examined the influence of glucose on memory for object-location bindings.

OBJECTIVE

This study aims to study how glucose modifies performance on an object-location memory task, a task that draws heavily on hippocampal function.

METHODS

Thirty-one participants received 30 g glucose or placebo in a single 1-h session. After seeing between 3 and 10 objects (words or shapes) at different locations in a 9 × 9 matrix, participants attempted to immediately reproduce the display on a blank 9 × 9 matrix. Blood glucose was measured before drink ingestion, mid-way through the session, and at the end of the session.

RESULTS

Glucose significantly improves object-location binding (d = 1.08) and location memory (d = 0.83), but not object memory (d = 0.51). Increasing working memory load impairs object memory and object-location binding, and word-location binding is more successful than shape-location binding, but the glucose improvement is robust across all difficulty manipulations. Within the glucose group, higher levels of circulating glucose are correlated with better binding memory and remembering the locations of successfully recalled objects.

CONCLUSIONS

The glucose improvements identified are consistent with a facilitative impact on hippocampal function. The findings are discussed in the context of the relationship between cognitive processes, hippocampal function, and the implications for glucose's mode of action.

Endocannabinoids Mediate Muscarinic Acetylcholine Receptor-Dependent Long-Term Depression in the Adult Medial Prefrontal Cortex

Cholinergic inputs into the prefrontal cortex (PFC) are associated with attention and cognition; however there is evidence that acetylcholine also has a role in PFC dependent learning and memory. Muscarinic acetylcholine receptors (mAChR) in the PFC can induce synaptic plasticity, but the underlying mechanisms remain either opaque or unresolved. We have characterized a form of mAChR mediated long-term depression (LTD) at glutamatergic synapses of layer 5 principal neurons in the adult medial PFC. This mAChR LTD is induced with the mAChR agonist carbachol and inhibited by selective M₁ mAChR antagonists. In contrast to other cortical regions, we find that this M₁ mAChR mediated LTD is coupled to endogenous cannabinoid (eCB) signaling. Inhibition of the principal eCB CB₁ receptor blocked carbachol induced LTD in both rats and mice. Furthermore, when challenged with a sub-threshold carbachol application, LTD was induced in slices pretreated with the monoacylglycerol lipase (MAGL) inhibitor JZL184, suggesting that the eCB 2-arachidonylglyerol (2-AG) mediates M₁ mAChR LTD. Yet, when endogenous acetylcholine was released from local cholinergic afferents in the PFC using optogenetics, it failed to trigger eCB-LTD. However coupling patterned optical and electrical stimulation to generate local synaptic signaling allowed the reliable induction of LTD. The light—electrical pairing induced LTD was M₁ mAChR and CB₁ receptor mediated. This shows for the first time that connecting excitatory synaptic activity with coincident endogenously released acetylcholine controls synaptic gain via eCB signaling. Together these results shed new light on the mechanisms of synaptic plasticity in the adult PFC and expand on the actions of endogenous cholinergic signaling.

Imaging of Functional and Dysfunctional Episodic Memory

A foundational framework for understanding functional and dysfunctional imaging of episodic memory emerges from the last 3 decades of human and animal research. This comprehensive review is presented from the vantage point of the fornix, a white matter bridge that occupies a central position in this functional network. Salient insights are identified, spanning topics such as hippocampal efferent and afferent networks, input and processing streams, hemispheric specialization, dysfunctional effects of pathologic and surgical injury, optimization of functional magnetic resonance imaging design and neuropsychological tests, and rehabilitation strategies. Far-reaching implications are considered for radiologists, whose clinical effect stretches beyond imaging and interfaces with neurosurgeons, neuropsychologists, and other neurospecialists.

Muscarinic receptor control of pyramidal neuron membrane potential in the medial prefrontal cortex (mPFC) in rats

Damage to the cholinergic input to the prefrontal cortex has been implicated in neuropsychiatric disorders. Cholinergic endings release acetylcholine, which activates nicotinic and/or G-protein-coupled muscarinic receptors. Muscarinic receptors activate transduction systems, which control cellular effectors that regulate the membrane potential in medial prefrontal cortex (mPFC) neurons. The mechanisms responsible for the cholinergic-dependent depolarization of mPFC layer V pyramidal neurons in slices obtained from young rats were elucidated in this study. Glutamatergic and GABAergic transmission as well as tetrodotoxin (TTX)-sensitive Na(+) and voltage-dependent Ca(++) currents were eliminated. Cholinergic receptor stimulation by carbamoylcholine chloride (CCh; 100 μM) evoked depolarization (10.0 ± 1.3 mV), which was blocked by M1/M4 (pirenzepine dihydrochloride, 2 μM) and M1 (VU 0255035, 5 μM) muscarinic receptor antagonists and was not affected by a nicotinic receptor antagonist (mecamylamine hydrochloride, 10 μM). CCh-dependent depolarization was attenuated by extra- (20 μM) or intracellular (50 μM) application of an inhibitor of the βγ-subunit-dependent transduction system (gallein). It was also inhibited by intracellular application of a βγ-subunit-binding peptide (GRK2i, 10μM). mPFC pyramidal neurons express Nav1.9 channels. CCh-dependent depolarization was abolished in the presence of antibodies against Nav1.9 channels in the intracellular solution and augmented by the presence of ProTx-I toxin (100 nM) in the extracellular solution. CCh-induced depolarization was not affected by the following reagents: intracellular transduction system blockers, including U-73122 (10 μM), chelerythrine chloride (5 μM), SQ 22536 (100 μM) and H-89 (2 μM); channel blockers, including Ba(++) ions (200 μM), apamin (100 nM), flufenamic acid (200 μM), 2-APB (200 μM), SKF 96365 (50 μM), and ZD 7288 (50 μM); and a Na(+)/Ca(++) exchanger blocker, benzamil (20 μM). We conclude that muscarinic M1 receptor-dependent depolarization in mPFC pyramidal neurons is evoked by the activation of Nav1.9 channels and that the signal transduction pathway involves G-protein βγ subunits.

Spontaneous object recognition: a promising approach to the comparative study of memory

Spontaneous recognition of a novel object is a popular measure of exploratory behavior, perception and recognition memory in rodent models. Because of its relative simplicity and speed of testing, the variety of stimuli that can be used, and its ecological validity across species, it is also an attractive task for comparative research. To date, variants of this test have been used with vertebrate and invertebrate species, but the methods have seldom been sufficiently standardized to allow cross-species comparison. Here, we review the methods necessary for the study of novel object recognition in mammalian and non-mammalian models, as well as the results of these experiments. Critical to the use of this test is an understanding of the organism's initial response to a novel object, the modulation of exploration by context, and species differences in object perception and exploratory behaviors. We argue that with appropriate consideration of species differences in perception, object affordances, and natural exploratory behaviors, the spontaneous object recognition test can be a valid and versatile tool for translational research with non-mammalian models.

Tree shrews (Tupaia belangeri) exhibit novelty preference in the novel location memory task with 24-h retention periods

Novelty preference is pervasive in mammalian species, and describes an inherent tendency to preferentially explore novelty. The novel location memory task studied here assesses the ability of animals to form accurate memories of a spatial configuration, consisting of several identical objects placed within an arena. Tree shrews were first familiarized with a particular object configuration during several sessions, and then an object was displaced during a test session. Tree shrews exhibited enhanced exploration when confronted with this novel configuration. The most reliable indicator associated with novelty preference was an enhancement in directed exploration towards the novel object, although we also observed a non-specific overall increase in exploration in one experiment. During the test session, we also observed an exploration of the location, which had previously been occupied by the displaced object, an effect termed empty quadrant. Our behavioral findings suggest multiple stages of spatial memory formation in tree shrews that are associated with various forms of behavioral responses to novelty. Reduced novelty preference has been linked to major depressive disorder in human patients. Given the established social conflict depression model in tree shrews, we anticipate that the study of the neural circuits of novelty preference and their malfunction during depression may have implications for understanding or treating depression in humans.

The effects of pregnancy, lactation, and primiparity on object-in-place memory of female rats

Maternal physiology and behavior change dramatically over the course of pregnancy to nurture the fetus and prepare for motherhood. Further, the experience of motherhood itself continues to influence brain functioning well after birth, shaping behavior to promote the survival of offspring. To meet these goals, cognitive abilities, such as spatial memory and navigation, may be enhanced to facilitate foraging behavior. Existing studies on pregnant and maternal rats demonstrate enhanced cognitive function in specific spatial domains. We adopted a novel object-in-place task to assess the ability of female rats to integrate information about specific objects in specific locations, a critical element of foraging behavior. Using a longitudinal design to study changes in spatial memory across pregnancy and motherhood, an advantage in the object-in-place memory of primiparous female rats compared to nulliparous females emerged during lactation not during pregnancy, and was maintained after weaning at 42 days postpartum. This enhancement was not dependent on the non-mnemonic variables of anxiety or neophobia. Parity did not affect the type of learning strategy used by females to locate a cued escape platform on a dual-solution water maze task. Results indicate that the enhancement of object-in-place memory, a cognitive function that facilitates foraging, emerged after pregnancy during the postpartum period of lactation and persisted for several weeks after weaning of offspring.

Consolidation of object recognition memory requires simultaneous activation of dopamine D1/D5 receptors in the amygdala and medial prefrontal cortex but not in the hippocampus

The mesocorticolimbic dopaminergic system includes the ventral tegmental area (VTA) and its projections to the amygdala (AMY), the hippocampus (HIP) and the medial prefrontal cortex (mPFC), among others. Object recognition (OR) long-term memory (LTM) processing requires dopaminergic activity but, although some of the brain regions mentioned above are necessary for OR LTM consolidation, their possible dopamine-mediated interplay remains to be analyzed. Using adult male Wistar rats, we found that posttraining microinjection of the dopamine D1/D5 receptor antagonist SCH23390 in mPFC or AMY, but not in HIP, impaired OR LTM. The dopamine D2 receptor agonist quinpirole had no effect on retention. VTA inactivation also hindered OR LTM, and even though this effect was unaffected by co-infusion of the dopamine D1/D5 receptor agonist SKF38393 in HIP, mPFC or AMY alone, it was reversed by simultaneous activation of D1/D5 receptors in the last two regions. Our results demonstrate that the mesocorticolimbic dopaminergic system is indeed essential for OR LTM consolidation and suggest that the role played by some of its components during this process is much more complex than previously thought.

Recognition memory and synaptic plasticity in the perirhinal and prefrontal cortices

Work is reviewed that relates recognition memory to studies of synaptic plasticity mechanisms in perirhinal and prefrontal cortices. The aim is to consider evidence that perirhinal cortex and medial prefrontal cortex store rather than merely transmit information necessary for recognition memory and, if so, to consider what mechanisms are potentially available within these cortices for producing such storage through synaptic change. Interventions with known actions on plasticity mechanisms are reviewed in relation to their effects on recognition memory processes. These interventions importantly include those involving antagonism of glutamatergic and cholinergic receptors but also inhibition of plasticity consolidation and expression mechanisms. It is concluded that there is strong evidence that perirhinal cortex is involved in information storage necessary for object recognition memory and, moreover, that such storage involves synaptic weakening mechanisms including the removal of AMPA glutamate receptors from synapses. There is good evidence that medial prefrontal cortex is necessary for associative and temporal order recognition memory and that this cortex expresses plasticity mechanisms that potentially allow the storage of information. However, the case for medial prefrontal cortex acting as a store requires further support.

Cholinergic involvement and manipulation approaches in multiple system disorders

Within the autonomic system, acetylcholine signaling contributes simultaneously and interactively to cognitive, behavioral, muscle and immune functions. Therefore, manipulating cholinergic parameters such as the activities of the acetylcholine hydrolyzing enzymes in body fluids or the corresponding transcript levels in blood leukocytes can change the global status of the autonomic system in treated individuals. Specifically, cholinesterase activities are subject to rapid and effective changes. The enzyme activity baseline increases with age and body mass index and depends on gender and ethnic origin. Also, the corresponding DNA (for detecting mutations) and RNA (for measuring specific mRNA transcripts) of cholinergic genes present individual variability. In leukocytes, acetylcholine inhibits the production of pro-inflammatory cytokines, suggesting relevance of cholinergic parameters to both the basal levels and to disease-induced inflammation. Inversely, acetylcholine levels increase under various stress stimuli, inducing changes in autonomic system molecules (e.g., pro-inflammatory cytokines) which can penetrate the brain; therefore, manipulating these levels can also effect brain reactions, mainly of anxiety, depression and pain. Additionally, neurodegenerative diseases often involve exacerbated inflammation, depression and anxiety, providing a focus interest group for cholinergic manipulations. In Alzheimer's disease, the systemic cholinergic impairments reflect premature death of cholinergic neurons. The decline of cholinesterases in the serum of Parkinson's disease and post- stroke patients, discovery of the relevant microRNAs and the growing range of use of anticholinesterase medications all call for critical re-inspection of established and novel approaches for manipulating cholinergic parameters.

What pharmacological interventions indicate concerning the role of the perirhinal cortex in recognition memory

Findings of pharmacological studies that have investigated the involvement of specific regions of the brain in recognition memory are reviewed. The particular emphasis of the review concerns what such studies indicate concerning the role of the perirhinal cortex in recognition memory. Most of the studies involve rats and most have investigated recognition memory for objects. Pharmacological studies provide a large body of evidence supporting the essential role of the perirhinal cortex in the acquisition, consolidation and retrieval of object recognition memory. Such studies provide increasingly detailed evidence concerning both the neurotransmitter systems and the underlying intracellular mechanisms involved in recognition memory processes. They have provided evidence in support of synaptic weakening as a major synaptic plastic process within perirhinal cortex underlying object recognition memory. They have also supplied confirmatory evidence that that there is more than one synaptic plastic process involved. The demonstrated necessity to long-term recognition memory of intracellular signalling mechanisms related to synaptic modification within perirhinal cortex establishes a central role for the region in the information storage underlying such memory. Perirhinal cortex is thereby established as an information storage site rather than solely a processing station. Pharmacological studies have also supplied new evidence concerning the detailed roles of other regions, including the hippocampus and the medial prefrontal cortex in different types of recognition memory tasks that include a spatial or temporal component. In so doing, they have also further defined the contribution of perirhinal cortex to such tasks. To date it appears that the contribution of perirhinal cortex to associative and temporal order memory reflects that in simple object recognition memory, namely that perirhinal cortex provides information concerning objects and their prior occurrence (novelty/familiarity).

Cognitive deficits in a mouse model of pre-manifest Parkinson's disease

Early cognitive deficits are increasingly recognized in patients with Parkinson's disease (PD), and represent an unmet need for the treatment of PD. These early deficits have been difficult to model in mice, and their mechanisms are poorly understood. α-Synuclein is linked to both familial and sporadic forms of PD, and is believed to accumulate in brains of patients with PD before cell loss. Mice expressing human wild-type α-synuclein under the Thy1 promoter (Thy1-aSyn mice) exhibit broad overexpression of α-synuclein throughout the brain and dynamic alterations in dopamine release several months before striatal dopamine loss. We now show that these mice exhibit deficits in cholinergic systems involved in cognition, and cognitive deficits in domains affected in early PD. Together with an increase in extracellular dopamine and a decrease in cortical acetylcholine at 4-6 months of age, Thy1-aSyn mice made fewer spontaneous alternations in the Y-maze and showed deficits in tests of novel object recognition (NOR), object-place recognition, and operant reversal learning, as compared with age-matched wild-type littermates. These data indicate that cognitive impairments that resemble early PD manifestations are reproduced by α-synuclein overexpression in a murine genetic model of PD. With high power to detect drug effects, these anomalies provide a novel platform for testing improved treatments for these pervasive cognitive deficits.

Induction of activity-dependent LTD requires muscarinic receptor activation in medial prefrontal cortex

The medial prefrontal cortex (mPFC) forms part of a neural circuit involved in the formation of lasting associations between objects and places. Cholinergic inputs from the basal forebrain innervate the mPFC and may modulate synaptic processes required for the formation of object-in-place memories. To investigate whether acetylcholine regulates synaptic function in the rat mPFC, whole-cell voltage-clamp recordings were made from pyramidal neurons in layer V. Bath application of the cholinergic agonist carbachol caused a potent and long-term depression (LTD) of synaptic responses that was blocked by the muscarinic receptor antagonist scopolamine and was mimicked, in part, by the M(1) receptor agonists McN-A-343 or AF102B. Furthermore, inhibition of PKC blocked carbachol-mediated LTD. We next determined the requirements for activity-dependent LTD in the prefrontal cortex. Synaptic stimulation that was subthreshold for producing LTD did, however, result in LTD when acetylcholine levels were enhanced by inhibition of acetylcholinesterase or when delivered in the presence of the M(1)-selective positive allosteric modulator BQCA. Increasing the levels of synaptic stimulation resulted in M(1) receptor-dependent LTD without the need for pharmacological manipulation of acetylcholine levels. These results show that synaptic stimulation of muscarinic receptors alone can be critical for plastic changes in excitatory synaptic transmission in the mPFC. In turn, these muscarinic mediated events may be important in the formation of object-in-place memories. A loss of basal forebrain cholinergic neurons is a classic hallmark of Alzheimer's dementia and our results provide a potential explanation for the loss of memory associated with the disease.

Altered object-in-place recognition memory, prepulse inhibition, and locomotor activity in the offspring of rats exposed to a viral mimetic during pregnancy

Infection during pregnancy (i.e., prenatal infection) increases the risk of psychiatric illnesses such as schizophrenia and autism in the adult offspring. The present experiments examined the effects of prenatal immune challenge on behavior in three paradigms relevant to these disorders: prepulse inhibition (PPI) of the acoustic startle response, locomotor responses to an unfamiliar environment and the N-methyl-d-aspartate antagonist MK-801, and three forms of recognition memory. Pregnant Long-Evans rats were exposed to the viral mimetic polyinosinic-polycytidylic acid (PolyI:C; 4 mg/kg, i.v.) on gestational day 15. Offspring were tested for PPI and locomotor activity before puberty (postnatal days (PNDs)35 and 36) and during young adulthood (PNDs 56 and 57). Four prepulse-pulse intervals (30, 50, 80, and 140 ms) were employed in the PPI test. Recognition memory testing was performed using three different spontaneous novelty recognition tests (object, object location, and object-in-place recognition) after PND 60. Regardless of sex, offspring of PolyI:C-treated dams showed disrupted PPI at 50-, 80-, and 140-ms prepulse-pulse intervals. In the prepubescent rats, we observed prepulse facilitation for the 30-ms prepulse-pulse interval trials that was selectively retained in the adult PolyI:C-treated offspring. Locomotor responses to MK-801 were significantly reduced before puberty, whereas responses to an unfamiliar environment were increased in young adulthood. Both male and female PolyI:C-treated offspring showed intact object and object location recognition memory, whereas male PolyI:C-treated offspring displayed significantly impaired object-in-place recognition memory. Females were unable to perform the object-in-place test. The present results demonstrate that prenatal immune challenge during mid/late gestation disrupts PPI and locomotor behavior. In addition, the selective impairment of object-in-place recognition memory suggests tasks that depend on prefrontal cortex may be particularly vulnerable following prenatal immune challenge.

The Rho GTPase activating CNF1 improves associative working memory for object-in-place

Cerebral Rho GTPases are crucially involved in cognitive abilities. This activity is thought to be related to the regulation of actin polymerization and, thereby, of the shape of the dendritic tree. Here we report that Cytotoxic Necrotizing Factor 1 (CNF1, 1fmol/kgicv), a bacterial protein endowed with Rho GTPase activating properties, enhances working memory for object location/discrimination in C57BL/6 mice. CNF1 selectively increased the exploration of a specific familiar object moved to a position that had been previously occupied by another familiar object. Conversely, the treatment left unaffected (i) exploration of a familiar object moved to a location that was previously unoccupied and (ii) exploration of a novel object. The effects were associated with changes in Rho GTPase status, since CNF1 C866S, a recombinant CNF1 in which the enzymatic activity was abolished through substitution of serine to cysteine at position 866, was ineffective in all the experiments. The study suggests that working memory for specific object-location associations critically depends on neural connectivity. It also confirms the therapeutic potential of the manipulation of Rho GTPase signaling in the modulation of memory processes.

Functional interactions within the parahippocampal region revealed by voltage-sensitive dye imaging in the isolated guinea pig brain

The massive transfer of information from the neocortex to the entorhinal cortex (and vice versa) is hindered by a powerful inhibitory control generated in the perirhinal cortex. In vivo and in vitro experiments performed in rodents and cats support this conclusion, further extended in the present study to the analysis of the interaction between the entorhinal cortex and other parahippocampal areas, such as the postrhinal and the retrosplenial cortices. The experiments were performed in the in vitro isolated guinea pig brain by a combined approach based on electrophysiological recordings and fast imaging of optical signals generated by voltage-sensitive dyes applied to the entire brain by arterial perfusion. Local stimuli delivered in different portions of the perirhinal, postrhinal, and retrosplenial cortex evoked local responses that did not propagate to the entorhinal cortex. Neither high- and low-frequency-patterned stimulation nor paired associative stimuli facilitated the propagation of activity to the entorhinal region. Similar stimulations performed during cholinergic neuromodulation with carbachol were also ineffective in overcoming the inhibitory network that controls propagation to the entorhinal cortex. The pharmacological inactivation of GABAergic transmission by local application of bicuculline (1 mM) in area 36 of the perirhinal cortex facilitated the longitudinal (rostrocaudal) propagation of activity into the perirhinal/postrhinal cortices but did not cause propagation into the entorhinal cortex. Bicuculline injection in both area 35 and medial entorhinal cortex released the inhibitory control and allowed the propagation of the neural activity to the entorhinal cortex. These results demonstrate that, as for the perirhinal-entorhinal reciprocal interactions, also the connections between the postrhinal/retrosplenial cortices and the entorhinal region are subject to a powerful inhibitory control.