Differential effects of lidocaine infusions into the ventral CA1/subiculum or the nucleus accumbens on the acquisition and retention of spatial information

Nucleus Accumbens Shell Neurons Encode the Kinematics of Reward Approach Locomotion

The nucleus accumbens (NAc) is considered an interface between motivation and action, with NAc neurons playing an important role in promoting reward approach. However, the encoding by NAc neurons that contributes to this role remains unknown. We recorded 62 NAc neurons in male Wistar rats (n = 5) running towards rewarded locations in an 8-arm radial maze. Variables related to locomotor approach kinematics were the best predictors of the firing rate for most NAc neurons. Nearly 18% of the recorded neurons were inhibited during the entire approach run (locomotion-off cells), suggesting that reduction in firing of these neurons promotes initiation of locomotor approach. 27% of the neurons presented a peak of activity during acceleration followed by a valley during deceleration (acceleration-on cells). Together, these neurons accounted for most of the speed and acceleration encoding identified in our analysis. In contrast, a further 16% of neurons presented a valley during acceleration followed by a peak just prior to or after reaching reward (deceleration-on cells). These findings suggest that these three classes of NAc neurons influence the time course of speed changes during locomotor approach to reward.

Ventral hippocampus lesions and allocentric spatial memory in the radial maze: Anterograde and retrograde deficits

Although the dorsal hippocampus (DHip) has been clearly implicated in spatial learning and memory, there is currently debate as to whether the ventral hippocampus (VHip) is also necessary in allocentric-based navigation tasks. To differentiate between these two subregions of the hippocampal dorsoventral axis, we examined the effect of neurotoxic lesions to the DHip and VHip in different learning situations, using a four-arm plus-shaped maze. In experiment 1 a spatial reference memory task was used, with results showing an acquisition deficit in DHip-lesioned rats but perfect learning in VHip-lesioned rats. However, in experiment 2 an acquisition deficit was found in VHip-lesioned rats using a doubly marked training protocol. In this case the position of the goal arm during training was marked simultaneously by the extramaze constellation of stimuli around the maze and an intramaze cue. The main results indicated that DHip and VHip groups presented significantly more allocentric errors in the probe test than the control rats. In experiments 3 and 4, animals with their brains still intact learned, respectively, a spatial reference memory task or a purely cue-guided navigation task, and DHip and VHip lesions were made 2-3 days after reaching learning criterion. Results indicated a profound retrograde deficit in both lesioned groups but only with regard to allocentric information. So, depending on the training protocol used, our results point to increased integration and cooperation throughout the hippocampal dorsoventral axis when allocentric learning and memory is involved. These data support the existence of a functional continuum from the dorsal to the ventral hippocampus.

Interdependence between dorsal and ventral hippocampus during spatial navigation

INTRODUCTION

The hippocampus is linked to the formation and retrieval of episodic memories and spatial navigation. In rats, it is an elongated structure divided into dorsal (septal) and ventral (temporal) regions paralleling the respective division in the posterior and anterior hippocampus in humans. The dorsal hippocampus has been suggested to be more important for spatial processing and the ventral to processing anxiety-based behaviors. Far less is known regarding the degree to which these different regions interact during information processing. The anatomical connectivity suggests a flow of information between the dorsal and ventral regions; conversely, there are also commissural connections to the contralateral hippocampus. The current study examined the extent to which information from the dorsal hippocampus interacts with processing in the ipsilateral and contralateral ventral hippocampus following the acquisition of a spatial task.

METHODS

Rats were well-trained on a spatial reference version of the water maze, followed by muscimol inactivation of different hippocampal subregions in a within-animal repeated design. Various combinations of bilateral, ipsilateral, and contralateral infusions were used.

RESULTS

Combined dorsal and ventral inactivation produced a severe impairment in spatial performance. Inactivation of only the dorsal or ventral regions resulted in intermediate impairment with performance levels falling between controls and combined inactivation. Performance was impaired during contralateral inactivation and was almost equivalent to bilateral dorsal and ventral hippocampus inactivation, while ipsilateral inactivation resulted in little impairment.

CONCLUSIONS

Taken together, results indicate that for spatial processing, the hippocampus functions as a single integrated structure along the longitudinal axis.

Ultra-High-Field fMRI Reveals a Role for the Subiculum in Scene Perceptual Discrimination

Recent "representational" accounts suggest a key role for the hippocampus in complex scene perception. Due to limitations in scanner field strength, however, the functional neuroanatomy of hippocampal-dependent scene perception is unknown. Here, we applied 7 T high-resolution functional magnetic resonance imaging (fMRI) alongside a perceptual oddity task, modified from nonhuman primate studies. This task requires subjects to discriminate highly similar scenes, faces, or objects from multiple viewpoints, and has revealed selective impairments during scene discrimination following hippocampal lesions. Region-of-interest analyses identified a preferential response in the subiculum subfield of the hippocampus during scene, but not face or object, discriminations. Notably, this effect was in the anteromedial subiculum and was not modulated by whether scenes were subsequently remembered or forgotten. These results highlight the value of ultra-high-field fMRI in generating more refined, anatomically informed, functional accounts of hippocampal contributions to cognition, and a unique role for the human subiculum in discrimination of complex scenes from different viewpoints.SIGNIFICANCE STATEMENT There is increasing evidence that the human hippocampus supports functions beyond just episodic memory, with human lesion studies suggesting a contribution to the perceptual processing of navigationally relevant, complex scenes. While the hippocampus itself contains several small, functionally distinct subfields, examining the role of these in scene processing has been previously limited by scanner field strength. By applying ultra-high-resolution 7 T fMRI, we delineated the functional contribution of individual hippocampal subfields during a perceptual discrimination task for scenes, faces, and objects. This demonstrated that the discrimination of scenes, relative to faces and objects, recruits the anterior subicular region of the hippocampus, regardless of whether scenes were subsequently remembered or forgotten.

Organization of the Anterior Limb of the Internal Capsule in the Rat

Dysfunction of the orbitofrontal (OFC) and anterior cingulate (ACC) cortices has been linked with several psychiatric disorders, including obsessive-compulsive disorder, major depressive disorder, posttraumatic stress disorder, and addiction. These conditions are also associated with abnormalities in the anterior limb of the internal capsule, the white matter (WM) bundle carrying ascending and descending fibers from the OFC and ACC. Furthermore, deep-brain stimulation (DBS) for psychiatric disorders targets these fibers. Experiments in rats provide essential information on the mechanisms of normal and abnormal brain anatomy, including WM composition and perturbations. However, whereas descending prefrontal cortex (PFC) fibers in primates form a well defined and topographic anterior limb of the internal capsule, the specific locations and organization of these fibers in rats is unknown. We address this gap by analyzing descending fibers from injections of an anterograde tracer in the rat ACC and OFC. Our results show that the descending PFC fibers in the rat form WM fascicles embedded within the striatum. These bundles are arranged topographically and contain projections, not only to the striatum, but also to the thalamus and brainstem. They can therefore be viewed as the rat homolog of the primate anterior limb of the internal capsule. Furthermore, mapping these projections allows us to identify the fibers likely to be affected by experimental manipulations of the striatum and the anterior limb of the internal capsule. These results are therefore essential for translating abnormalities of human WM and effects of DBS to rodent models.SIGNIFICANCE STATEMENT Psychiatric diseases are linked to abnormalities in specific white matter (WM) pathways, and the efficacy of deep-brain stimulation relies upon activation of WM. Experiments in rodents are necessary for studying the mechanisms of brain function. However, the translation of results between primates and rodents is hindered by the fact that the organization of descending WM in rodents is poorly understood. This is especially relevant for the prefrontal cortex, abnormal connectivity of which is central to psychiatric disorders. We address this gap by studying the organization of descending rodent prefrontal pathways. These fibers course through a subcortical structure, the striatum, and share important organization principles with primate WM. These results allow us to model primate WM effectively in the rodent.

Selective Pharmacological Augmentation of Hippocampal Activity Produces a Sustained Antidepressant-Like Response without Abuse-Related or Psychotomimetic Effects

BACKGROUND

Selective augmentation of hippocampal activity in ways similar to that caused by ketamine may have therapeutic advantages over ketamine, which has psychotomimetic and reinforcing effects likely due to effects outside the hippocampus (i.e., off-target effects).

METHODS

Here we evaluated the antidepressant-like response to a negative allosteric modulator of α5 subunit- containing gamma aminobutyric acid subtype A receptors, L-655,708, as these receptors are expressed to a much greater extent in the hippocampus than in other brain areas.

RESULTS

Systemic administration of L-655,708 produced a sustained antidepressant-like effect in the forced swim test that was comparable with that of ketamine and was blocked by hippocampal inactivation with lidocaine. However, in contrast to ketamine, L-655,708 did not affect prepulse inhibition of startle, nor did it maintain responding in rats trained to self-administer i.v. ketamine.

CONCLUSION

Taken together, these findings suggest that activation of the hippocampus by L-655,708 produces an antidepressant-like effect in the absence of any psychotomimetic or abuse-related effects.

Activation of a ventral hippocampus-medial prefrontal cortex pathway is both necessary and sufficient for an antidepressant response to ketamine

A single sub-anesthetic dose of ketamine exerts rapid and sustained antidepressant effects. Here, we examined the role of the ventral hippocampus (vHipp)-medial prefrontal cortex (mPFC) pathway in ketamine's antidepressant response. Inactivation of the vHipp with lidocaine prevented the sustained, but not acute, antidepressant-like effect of ketamine as measured by the forced swim test (FST). Moreover, optogenetic as well as pharmacogenetic specific activation of the vHipp-mPFC pathway using DREADDs (designer receptors exclusively activated by designer drugs) mimicked the antidepressant-like response to ketamine; importantly, this was pathway specific, in that activation of a vHipp to nucleus accumbens circuit did not do this. Furthermore, optogenetic inactivation of the vHipp/mPFC pathway at the time of FST completely reversed ketamine's antidepressant response. In addition, we found that a transient increase in TrkB receptor phosphorylation in the vHipp contributes to ketamine's sustained antidepressant response. These data demonstrate that activity in the vHipp-mPFC pathway is both necessary and sufficient for the antidepressant-like effect of ketamine.

Impairing effect of amphetamine and concomitant ionotropic glutamate receptors blockade in the ventral striatum on spatial learning in mice

RATIONALE

Accumulating evidence supports the involvement of the ventral striatum (VS) in spatial information processing. The multiple cortical glutamatergic and mesolimbic dopaminergic (DAergic) afferences on the same neurons in the ventral striatum provide the neuroanatomical substrate for glutamate and dopamine functional interaction. However, there is little evidence in the literature on how this interaction affects the ability to encode spatial information.

OBJECTIVE

First, we evaluated the effect of intra-VS bilateral infusion of different doses of amphetamine (0.3, 0.75, and 1.5 μg/side) on the ability to detect spatial novelty in mice. Next, we examined the impact produced on the same abilities by intra-VS infusion of ineffective doses of amphetamine (0.3 μg/side) in association with N-methyl-D-aspartate (NMDA) (3.125 ng/side) or α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) (0.25 ng/side) receptor antagonist.

RESULTS

The results show that infusion of amphetamine impairs detection of spatial novelty, affecting also exploratory activity and marginally the detection of nonspatial novelty. In contrast, an association of subthreshold doses of amphetamine with NMDA or AMPA receptor antagonists exerted a selective effect on reactivity to a spatial change.

CONCLUSIONS

These findings demonstrate that enhanced DAergic activity in the VS enhances glutamate receptor antagonist-induced impairment in learning and memory.

Characterizing cognitive aging of working memory and executive function in animal models

Executive functions supported by prefrontal cortical (PFC) systems provide essential control and planning mechanisms to guide goal-directed behavior. As such, age-related alterations in executive functions can mediate profound and widespread deficits on a diverse array of neurocognitive processes. Many of the critical neuroanatomical and functional characteristics of prefrontal cortex are preserved in rodents, allowing for meaningful cross species comparisons relevant to the study of cognitive aging. In particular, as rodents lend themselves to genetic, cellular and biochemical approaches, rodent models of executive function stand to significantly contribute to our understanding of the critical neurobiological mechanisms that mediate decline of executive processes across the lifespan. Moreover, rodent analogs of executive functions that decline in human aging represent an essential component of a targeted, rational approach for developing and testing effective treatment and prevention therapies for age-related cognitive decline. This paper reviews behavioral approaches used to study executive function in rodents, with a focus on those assays that share a foundation in the psychological and neuroanatomical constructs important for human aging. A particular emphasis is placed on behavioral approaches used to assess working memory and cognitive flexibility, which are sensitive to decline with age across species and for which strong rodent models currently exist. In addition, other approaches in rodent behavior that have potential for providing analogs to functions that reliably decline to human aging (e.g., information processing speed) are discussed.

Where is my reward and how do I get it? Interaction between the hippocampus and the basal ganglia during spatial learning

Spatial learning has been recognized over the years to be under the control of the hippocampus and related temporal lobe structures. Hippocampal damage often causes severe impairments in the ability to learn and remember a location in space defined by distal visual cues. Recent experimental evidence in rodents demonstrates, however, that other brain areas might also be involved in the acquisition of spatial information. Amongst these, the cortex--basal ganglia loop is known to be involved in reinforcement learning and has been identified as an important contributor to spatial learning. In particular, it has been shown that altered activity of the basal ganglia striatal complex can impair the ability to perform spatial learning tasks. Until recently, little was known about how the basal ganglia and the hippocampus interact and how their activities evolve during learning. The present review, focusing on rodent studies, provides a glimpse of the findings obtained over the past decade that support a dialog between these two structures during spatial learning. Based on these studies, we propose a new functional spatial decision network with three separate loops encompassing hippocampus and specific basal ganglia regions. Each of the three loops serves a different aspect of spatial decision making and all three are linked by their mutual connections and are under the control of the dopaminergic learning signal.

Parallel associative processing in the dorsal striatum: segregation of stimulus-response and cognitive control subregions

Although evidence suggests that the dorsal striatum contributes to multiple learning and memory functions, there nevertheless remains considerable disagreement on the specific associative roles of different neuroanatomical subregions. We review evidence indicating that the dorsolateral striatum (DLS) is a substrate for stimulus-response habit formation - incremental strengthening of simple S-R bonds - via input from sensorimotor neocortex while the dorsomedial striatum (DMS) contributes to behavioral flexibility - the cognitive control of behavior - via prefrontal and limbic circuits engaged in relational and spatial information processing. The parallel circuits through dorsal striatum interact with incentive/affective motivational processing in the ventral striatum and portions of the prefrontal cortex leading to overt responding under specific testing conditions. Converging evidence obtained through a detailed task analysis and neurobehavioral assessment is beginning to illuminate striatal subregional interactions and relations to the rest of the mammalian brain.

The ventral hippocampus is necessary for expressing a spatial memory

Current views posit the dorsal hippocampus (DHipp) as contributing to spatial memory processes. Conversely, the ventral hippocampus (VHipp) modulates stress, emotions and affects. Arguments supporting this segregation include differences in (i) connectivity: the DHipp is connected with the entorhinal cortex which receives visuospatial neocortical inputs; the VHipp is connected with both the amygdala and hypothalamus, (ii) electrophysiological characteristics: there is a larger proportion of place cells in the DHipp than in the VHipp, and an increasing dorsoventral gradient in the size of place fields, suggesting less refined spatial coding in the VHipp, and (iii) consequences of lesions: spatial memory is altered after DHipp lesions, less dramatically, sometimes not, after VHipp lesions. Using reversible inactivation, we report in rats, that lidocaine infusions into the DHipp or VHipp right before a probe trial impair retrieval performance in a water-maze task. This impairment was found at two post-acquisition delays compatible with recent memory (1 and 5 days). Pre-training blockade of the VHipp did not prevent task acquisition and drug-free retrieval, on the contrary to pre-training blockade of DHipp, which altered performance in a subsequent drug-free probe trial. Complementary experiments excluded possible locomotor, sensorimotor, motivational or anxiety-related biases from data interpretation. Our conclusion is that a spatial memory can be acquired with the DHipp, less efficiently with the VHipp, and that the retrieval of such a memory and/or the expression of its representation engages the dorsoventral axis of the hippocampus when the task has been learnt with an entirely functional hippocampus.

NIH-3T3 fibroblast transplants enhance host regeneration and improve spatial learning in ventral subicular lesioned rats

Transplants, besides providing neural replacement, also stimulate host regeneration, which could serve as a powerful means to establish functional recovery in CNS insults. Earlier, we have reported the H3-GFP transplant mediated recovery of cognitive functions in the ventral subicular lesioned rats. In the present study, we demonstrate the efficacy of a non-neural fibroblast transplants in mediating host regeneration and functional recovery in ventral subicular lesioned rats. Adult male Wistar rats were lesioned with ibotenic acid in the ventral subiculum (VSL) and were transplanted with NIH-3T3 fibroblast cells into CA1 region of the hippocampus. Ventral subicular lesioning impaired the spatial task performances in rats and produced considerable degree of dendritic atrophy of the hippocampal pyramidal neurons. Two months following transplantation, the transplants were seen in the dentate gyrus and expressed BDNF and bFGF. Further, the VSL rats with fibroblast transplants showed enhanced expression of BDNF in the hippocampus and enhanced dendritic branching and increased spine density in the CA1 hippocampal pyramidal neurons. Transplantation of fibroblast cells also helped to establish functional recovery and the rats with transplants showed enhanced spatial learning performances. We attribute the recovery of cognitive functions to the graft mediated host regeneration, although the mechanisms of functional recovery remain to be elucidated.

A novel touchscreen-automated paired-associate learning (PAL) task sensitive to pharmacological manipulation of the hippocampus: a translational rodent model of cognitive impairments in neurodegenerative disease

RATIONALE

Paired-associate learning (PAL), as part of the Cambridge Neuropsychological Test Automated Battery, is able to predict who from an at-risk population will develop Alzheimer's disease. Schizophrenic patients are also impaired on this same task. An automated rodent model of PAL would be extremely beneficial in further research into Alzheimer's disease and schizophrenia.

OBJECTIVE

The objective of this study was to develop a PAL task using touchscreen-equipped operant boxes and test its sensitivity to manipulations of the hippocampus, a brain region of interest in both Alzheimer's disease and schizophrenia.

MATERIALS AND METHODS

Previous work has shown that spatial and non-spatial memory can be tested in touchscreen-equipped operant boxes. Using this same apparatus, rats were trained on two variants of a PAL task differing only in the nature of the S- (the unrewarded stimuli, a combination of image and location upon the screen). Rats underwent cannulation of the dorsal hippocampus, and after recovery were tested under the influence of intra-hippocampally administered glutamatergic and cholinergic antagonists while performing the PAL task.

RESULTS

Impairments were seen after the administration of glutamatergic antagonists, but not cholinergic antagonists, in one of the two versions of PAL.

CONCLUSIONS

De-activation of the hippocampus caused impairments in a PAL task. The selective nature of this effect (only one of the two tasks was impaired), suggests the effect is specific to cognition and cannot be attributed to gross impairments (changes in visual learning). The pattern of results suggests that rodent PAL may be suitable as a translational model of PAL in humans.

Basal ganglia contributions to adaptive navigation

The striatum has long been considered to be selectively important for nondeclarative, procedural types of memory. This stands in contrast with spatial context processing that is typically attributed to hippocampus. Neurophysiological evidence from studies of the neural mechanisms of adaptive navigation reveals that distinct neural systems such as the striatum and hippocampus continuously process task relevant information regardless of the current cognitive strategy. For example, both striatal and hippocampal neural representations reflect spatial location, directional heading, reward, and egocentric movement features of a test situation in an experience-dependent way, and independent of task demands. Thus, continual parallel processing across memory systems may be the norm rather than the exception. It is suggested that neuromodulators, such as dopamine, may serve to differentially regulate learning-induced neural plasticity mechanisms within these memory systems such that the most successful form of neural processing exerts the strongest control over response selection functions. In this way, dopamine may serve to optimize behavioral choices in the face of changing environmental demands during navigation.

Functional interaction between the hippocampus and nucleus accumbens shell is necessary for the acquisition of appetitive spatial context conditioning

The nucleus accumbens (NAc) has been implicated in a variety of associative processes that are dependent on the integrity of the amygdala and hippocampus (HPC). However, the extent to which the two subregions of the NAc, the core and shell, form differentiated circuits within the amygdala- and hippocampal-ventral striatal circuitry remains unclear. The present study investigated the effects of selective excitotoxic lesions of the nucleus accumbens shell or core subregion on appetitive elemental cue and context conditioning, shown previously to be dependent on the basolateral amygdala and hippocampus, respectively. Rats were trained sequentially to acquire discrete conditioned stimulus-sucrose conditioning, followed by spatial context-sucrose conditioning in a place preference apparatus characterized by three topographically identical chambers, the chambers being discriminable only on the basis of path integration. NAc shell lesions selectively impaired the acquisition of conditioned place preference and the use of spatial information to retrieve information about a discrete cue, whereas, as expected, NAc core lesions attenuated the acquisition of cue conditioning compared with sham rats. In a subsequent experiment, disconnection of the HPC from the NAc shell using unilateral asymmetric lesions of each structure resulted in a pattern of impairment in place conditioning and context-dependent cue retrieval similar to that produced by NAc shell lesions. These data not only suggest that the NAc core and shell subregions subserve distinct associative processes but also that the NAc shell and HPC are important functional components of a limbic corticostriatal network involved in spatial context conditioning.

The dorsal subiculum mediates the acquisition of conditioned reinstatement of cocaine-seeking

Contextual stimuli present during a single lifetime cocaine self-administration experience acquire occasion-setting actions sufficient to persistently elicit cocaine-seeking behavior in rats, with effects lasting nearly 1 year. The goal of this study was to identify neural substrates mediating the acquisition of drug-related conditioning taking place during a single cocaine self-administration experience with focus on the subicular formation, a brain site that has been implicated in associative learning relevant for conditioned reward-seeking including conditioned reinstatement. Male Wistar rats were given 2 h of response-contingent access to intravenous cocaine or saline in the presence of distinct stimuli that served as contextual stimuli associated with the availability and subjective effects of cocaine (S(+)) vs saline (S(-)). Before onset of the sessions, rats received bilateral microinjections of tetrodotoxin (TTX) into the ventral subiculum (VSUB) or dorsal subiculum (DSUB). Following extinction of responding by withholding cocaine, rats were subjected to reinstatement tests in which exposure to the cocaine- (but not saline) associated stimulus produced strong recovery of responding. This effect was completely abolished in rats with transient TTX inactivation of the DSUB during the conditioning session. TTX inactivation of the VSUB during conditioning did not alter the response-reinstating effects of the cocaine cue. The results suggest that functional integrity of the DSUB, but not VSUB, is critical for the acquisition of conditioned cocaine-seeking controlled by contextual stimuli under conditions where such learning occurs during a single conditioning trial.

Ventral hippocampal involvement in temporal order, but not recognition, memory for spatial information

The hippocampus is critical for spatial memory. Recently, subregional differences in the function of hippocampus have been described in a number of behavioral tasks. The present experiments assessed the effects of reversibly lesioning either the dorsal (dHip) or ventral hippocampus (vHip) on spontaneous tests of spatial recognition and temporal order memory. We report that although the dHip is necessary for spatial recognition memory (RM) (distinguishing a novel from a familiar spatial location), the vHip is involved in temporal order memory (the capacity to distinguish between two spatial locations visited at different points in time), but not RM. These findings and others are consistent with the hypothesis that temporal order memory is supported by an integrated circuit of limbic areas including the vHip and the medial prefrontal cortex.

Effects of intra-accumbens NMDA and AMPA receptor antagonists on short-term spatial learning in the Morris water maze task

Glutamatergic transmission within the nucleus accumbens (Nac) is considered to subserve the transfer of different types of information from the cortical and limbic regions. In particular, it has been suggested that glutamatergic afferences from the hippocampus and the prefrontal cortex provide the main source of contextual information to the Nac. Accordingly, several authors have demonstrated that the blockade of glutamate receptors within the Nac impairs various spatial tasks. However, the exact role of the different classes of glutamate receptors within the Nac in short-term spatial memory is still not clear. In this study we investigated the involvement of two major classes of glutamate receptors, NMDA and AMPA receptors, within the Nac in the acquisition of spatial information, using the Morris water maze task. Focal injections of the NMDA antagonist, AP-5 (0.1 and 0.15 microg/side), and the AMPA antagonist, DNQX (0.005, 0.01 microg/side), were performed before a massed training phase, and mice were tested for retention immediately after. NMDA and AMPA receptor blockade induced no effect during training. On the contrary, injection of the two glutamatergic antagonists impaired spatial localization during the probe test. These data demonstrate an involvement of the Nac in short-term spatial learning. Moreover, they prove that within this structure the short-term processing of spatial information needs the activation of both NMDA and AMPA receptors.

The hippocampus plays a critical role at encoding discontiguous events for subsequent declarative memory expression in mice

The hypothesis that hippocampal activity at encoding is causally related to subsequent declarative memory expression is tested in the mouse, by using lidocaine inactivation of the hippocampus in combination with c-fos neuroimaging analysis. We employed a two-stage radial maze paradigm of spatial discrimination, which was previously shown to dissociate between declarative and nondeclarative expression of memory related to the same acquired material. In Stage 1 (encoding), mice learnt the constant location of food among a set of six arms (three baited, three unbaited) by being submitted repeatedly to discontiguous experiences with each arm separately ("go/no-go" discrimination). In Stage 2 (test-session), they are challenged with novel presentations of the arms, which are either combined into pairs of opposite valence ("two-choice" discrimination), or opened all six together ("six-choice" discrimination). Previous experiments have demonstrated that the "two-choice" situation is a critical test for declarative memory while "six-choice" discrimination may rely on procedural memory. We observed that (i) hippocampal activity measured by c-fos mRNA expression was increased by "go/no-go" learning, and this activation was blocked by pre-training local infusions of lidocaine; (ii) when performed just before each session of Stage 1, such inactivation spared the acquisition of "go/no-go" discrimination but produced, subsequently, a selective deficit in the "two-choice" test (not in the "six-choice" test). This study indicates that the hippocampus is "spontaneously" engaged in encoding processes necessary for long-term storage of discontiguous experiences under a form enabling flexible declarative memory expression.

Dissociating ventral and dorsal subicular dopamine D1 receptor involvement in instrumental learning, spontaneous motor behavior, and motivation

A series of experiments investigating the role of dopamine D1 receptors in the ventral subiculum (vSUB) and dorsal subiculum (dSUB), 2 subregions of the hippocampal formation, found that D1 receptor antagonism (3.0 nmol/0.5 microl SCH-23390 bilaterally) in the vSUB impaired instrumental learning and performance, reduced break point in progressive ratio (PR) tests, and produced an intrasession decline in responding during test sessions, but had no effect on spontaneous motor or food-directed behavior. In contrast, D1 receptor blockade in the dSUB had no effect on instrumental learning, performance, PR break point, or food-directed behavior, but reduced spontaneous motor behavior. These results suggest a dissociation between the vSUB and dSUB with respect to the role of dopamine in various aspects of motivated and motor behavior. Further, D1 activation in the vSUB may be a critical component of motivational arousal associated with learned contextual cues.

Cannabinoids and prefrontal cortical function: Insights from preclinical studies

Marijuana use has been associated with disordered cognition across several domains influenced by the prefrontal cortex (PFC). Here, we review the contribution of preclinical research to understanding the effects of cannabinoids on cognitive ability, and the mechanisms by which cannabinoids may affect the neurochemical processes in the PFC that are associated with these impairments. In rodents, acute administration of cannabinoid agonists produces deficits in working memory, attentional function and reversal learning. These effects appear to be largely dependent on CB1 cannabinoid receptor activation. Preclinical studies also indicate that the endogenous cannabinoid system may tonically regulate some mnemonic processes. Effects of cannabinoids on cognition may be mediated via interaction with neurochemical processes in the PFC and hippocampus. In the PFC, cannabinoids may alter dopaminergic, cholinergic and serotonergic transmission. These mechanisms may underlie cognitive impairments observed following marijuana intake in humans, and may also be relevant to other disorders of cognition. Preclinical research will further enhance our understanding of the interactions between the cannabinoid system and cognitive functioning.

Post insult enriched housing improves the 8-arm radial maze performance but not the Morris water maze task in ventral subicular lesioned rats

The present study attempted to evaluate the efficacy of enriched housing conditions in promoting behavioral recovery following subicular lesion. Rats with bilateral ibotenic acid lesions of ventral subiculum were exposed to either enriched housing conditions or standard housing for 6 h daily for 10 days. The performance of the lesioned rats reared in standard housing conditions was severely impaired in both 8-arm radial maze and water maze tasks. Lesioned rats exposed to enriched housing showed behavioral recovery in the 8-arm radial maze but not in the water maze. Our study suggests the possibility that recovery may be based on the functional demand of spatial tasks and may require more appropriate environmental stimulation.

Dissociation of the effect of spatial behaviors on the phosphorylation of cAMP-response element binding protein (CREB) within the nucleus accumbens

Several studies have reported a role for the nucleus accumbens (NAcc) in learning and synaptic plasticity. Many of them suggest that the NAcc is involved in translating cortico-limbic information to the motor system mediating spatial learning and memory processes. Previous studies from our laboratory have shown that protein kinase C is activated following training in a food search spatial learning task. The present study further characterizes the molecular substrates associated with NAcc-dependent spatial behavior. The cyclic AMP-response element binding protein (CREB), a transcription factor implicated in the formation of long-term memory, was studied in the NAcc following spatial training in a food search spatial learning task. Western blots were performed to detect phosphorylated (activated) and total CREB protein levels. Our results show that CREB is significantly phosphorylated in the NAcc 48 h after habituation and at 5 min and 1 h after the first spatial training session in comparison with the naive animals that remained in their home cages. Since published data show that NAcc plays a role in novelty detection and reactivity, we conducted further experiments in order to dissociate the effect on CREB phosphorylation and expression of spatial novelty (single exposure), exploration, and spatial learning in the food search apparatus. Results show that CREB phosphorylation is significantly increased 48 h after exposure to a novel environment. The present study suggests that CREB phosphorylation observed in the NAcc during habituation and spatial training may be mainly triggered by detection of spatial novelty.

Transient inactivation of perirhinal cortex disrupts encoding, retrieval, and consolidation of object recognition memory

Damage to perirhinal cortex (PRh) impairs object recognition memory in humans, monkeys, and rats when tested in tasks such as delayed nonmatching to sample, visual paired comparison, and its rodent analog, the spontaneous object recognition task. In the present study, we have capitalized on the discrete one-trial nature of the spontaneous object recognition task to investigate the role of PRh in several distinct stages of object recognition memory. In a series of experiments, transient inactivation of PRh was accomplished with bilateral infusions of lidocaine directly into PRh immediately before the sample phase (encoding), immediately before the choice phase (retrieval), or within the retention delay after the sample phase (storage-consolidation). Compared with performance on trials in which they received saline infusions, rats were significantly impaired when lidocaine was infused before the sample phase, regardless of the length of the retention delay. Similarly, delay-independent deficits were observed after immediate pre-choice infusions of lidocaine. Finally, PRh inactivation immediately and 20 min after the sample phase, but not 40, 60, or 80 min after, also disrupted subsequent object recognition when the retention delay was sufficiently long to ensure the dissipation of the actions of lidocaine during the choice phase. The effects of pre-sample and pre-choice inactivation indicate involvement of PRh in encoding and retrieval stages of object recognition, and the time course of post-sample inactivation effects suggests a role for PRh in the maintenance of the object trace during memory consolidation.

Nucleus accumbens dopamine receptors in the consolidation of spatial memory

Nucleus accumbens dopamine is known to play an important role in motor activity and in behaviours governed by drugs and natural reinforcers, as well as in non-associative forms of learning. At the same time, activation of D1 and D2 dopamine receptors has been suggested to promote intracellular events related to neural plasticity. Therefore, in this study we wished to investigate the role of the two classes of dopamine receptors within the nucleus accumbens on the consolidation of spatial information. On day 1, CD1 male mice were placed in an open field containing five different objects and, immediately after three sessions of habituation, the animals were focally injected within the nucleus accumbens with either the D1 antagonist SCH 23390 (12.5, 25 or 50 ng/side), or the D2 antagonist sulpiride (25, 50, 75 or 100 ng/side). Twenty-four hours later the ability of mice to discriminate an object displacement was assessed. Both the D1 and the D2 antagonists impaired the ability of mice to detect the spatial change. If the highest doses of the two antagonists were injected 2 h after the end of the last of the habituation sessions, no effect was observed in the reactivity to spatial change examined 24 h later. These data demonstrate that activation of both D1 and D2 receptors within the accumbens is necessary in the early stages of the consolidation of spatial information. The data are discussed in terms of involvement of nucleus accumbens dopamine in information processing in the absence of explicit reinforcers.

Transient inactivation of the rat nucleus accumbens does not impair guidance of instrumental behaviour by stimuli predicting reward magnitude

The involvement of the nucleus accumbens (NAc) in the determination of reaction times (RTs) of instrumental responses by the expectancy of future reward was investigated. A simple RT task demanding conditioned lever release was used, in which the upcoming reward magnitude (5 versus 1 pellet) was signalled in advance by discriminative cues. In rats which acquired the task, RTs of instrumental responses were significantly shorter to the discriminative cue predictive of high reward magnitude. Inactivation of the NAc by lidocaine had no effect on RTs and their determination by cue-associated reward magnitudes, and did not affect the rate of correct responses. In keeping with an earlier study, intra-NAc infusion of amphetamine decreased RTs, impaired RT determination by cue-associated reward magnitudes and reduced the rate of correct responses. The unexpected finding that lidocaine inactivation of the NAc had no effect parallels previous data showing that lesions of NAc did not impair RT performance, while manipulation of intra-NAc glutamate or dopamine transmission impaired various aspects of RT performance in comparable tasks. It is suggested that experimental manipulations such as transient and permanent inactivation, which almost completely inhibit NAc neuronal output, allow alternative routes to be used to effectively control behaviour in the task employed here.

Spatial learning in rats is impaired by microinfusions of protein kinase C-gamma antisense oligodeoxynucleotide within the nucleus accumbens

The nucleus accumbens (NAcc) has been shown to play a role in motor and spatial learning. Protein kinase C (PKC) has been implicated in the mechanisms of initiation and maintenance of long-term potentiation that is thought to be involved in the storage of long-term memory. In the present study, the importance of de novo synthesis of PKC-gamma within the NAcc in the acquisition and retention of spatial discrimination learning was assessed using an antisense knockdown approach. Separate groups of Long-Evans rats were exposed to acute microinfusions (6microg/microl) of PKC-gamma antisense oligodeoxynucleotide (AS-ODN), control oligodeoxynucleotide (C-ODN) or vehicle into the NAcc at 24 and 3h before each training session. Behavioral findings showed that the blockade of NAcc-PKC-gamma translation caused impairments in the early phase of learning and retention of spatial information. Biochemical experiments showed that PKC-gamma expression was reduced and Ca(2+)/phospholipid-dependent protein kinase C (PKC) activity was blocked significantly in the AS-ODN-treated rats in comparison with control rats. The present findings suggest that NAcc-PKC-gamma plays a role during the early acquisition of spatial learning. Also, retention test results suggest that NAcc-PKC-gamma may be working as an intermediate factor involved in the onset of molecular mechanisms necessary for spatial memory consolidation within the NAcc.

Hippocampal modulation of sensorimotor processes

While the hippocampus makes unique contributions to memory, it has also long been associated with sensorimotor processes, i.e. innate processes involving control of motor responses to sensory stimuli. Moreover, hippocampal dysfunction has been implicated in neuropsychiatric diseases, such as schizophrenia and anxiety disorders, primarily characterized by non-mnemonic deficits in the processing of and responding to sensory information. This review is concerned with the hippocampal modulation of three sensorimotor processes in rats-locomotor activity, prepulse inhibition (PPI) of the startle reflex, and the startle reflex itself-whose alterations are related to human psychosis or anxiety disorders. Its main purpose is to present and discuss the picture emerging from studies examining the effects of pharmacological manipulations of the dorsal and ventral hippocampus by local drug microinfusions. While a role of the hippocampus in regulating locomotor activity, PPI, and startle reactivity has also been suggested based on the effects of hippocampal lesions, the microinfusion studies have revealed additional important details of this role and suggest modifications of notions based on lesion studies. In summary, the microinfusion studies corroborate that hippocampal mechanisms can directly influence locomotor activity, PPI, and startle reactivity, and that aberrant hippocampal function may contribute to neuropsychiatric diseases, in particular psychosis. The relation between different sensorimotor processes and hippocampal neurotransmission, the role of ventral and dorsal hippocampus, and the extrahippocampal mechanisms mediating the hippocampal modulation of different sensorimotor processes can partly be dissociated. Thus, the hippocampal modulation of these sensorimotor processes appears to reflect multiple operations, rather than one unitary operation.

A differential involvement of the shell and core subterritories of the nucleus accumbens of rats in memory processes

The role of the core and the shell subterritories of the nucleus accumbens in conditioned freezing and spatial learning was investigated by means of selective N-methyl-D-aspartate lesions. Shell-lesioned rats showed reduced conditioned freezing to context and a tendency toward reduced freezing to the discrete stimulus compared with controls. However, lesions of the core did not modify the freezing response either to the context or to the discrete stimuli. Although spatial memory, as assessed by a water-maze paradigm, was not disrupted by the lesions, in a 4-arm baited, 4-arm unbaited radial-arm maze paradigm, the shell-lesioned rats showed selective deficits in working memory, but not in reference memory. In contrast, core-lesioned rats showed no memory deficits.

Effects of intra-accumbens focal administrations of glutamate antagonists on object recognition memory in mice

Generally recognition memory is distinguished into spatial and object memories that have been suggested to relay at a cortical level on different neural substrates. Recent studies point to a possible involvement of the nucleus accumbens (Nac) in spatial memory, demonstrating that blockade of glutamate antagonists within this structure impairs acquisition and consolidation of spatial information, while not many data are available on the potential role of this structure in object recognition. Thus in this study we wanted to investigate the effects of intra-accumbens focal administrations of NMDA antagonist, AP-5 (0.05, 0.1, 0.15 or 0.2 microg per side), and AMPA antagonist, DNQX (0.0005 or 0.001 microg per side), in object recognition memory. The spontaneous preference displayed by mice for novel objects was taken as an index for measuring object recognition. Pre-training focal administrations of both antagonists impaired the ability of mice to selectively explore the novel object in test session. However, the AMPA antagonist induced also a decrease in exploration and locomotion. In order to assess whether glutamate receptors located within the Nac were also involved in subsequent steps of object information processing, we performed additional experiments injecting AP-5 and DNQX immediately after training and testing the animals 24-h later. In this case, AP-5 but not the AMPA antagonist impaired exploration of the novel object. These results demonstrate that the Nac is involved in object recognition, and confirm that the different glutamate receptors mediate different component of information processing within the accumbens.

Changes in synaptic plasticity in the rat hippocampo-medial prefrontal cortex pathway induced by repeated treatments with fluvoxamine

The present studies were conducted to examine the effects of single and repeated treatments with fluvoxamine, which is a selective serotonin reuptake inhibitor (SSRI), on the synaptic efficacy and synaptic plasticity in the rat hippocampo-medial prefrontal cortex (mPFC) pathway in vivo. It has been reported that the projections arising from the hippocampal structures to the mPFC are involved in the execution of higher cognitive functions in rats. The evoked potentials were recorded in the mPFC by stimulation of the CA1/subicular region of the ventral hippocampus in halothane-anesthetized rats. Single administration of fluvoxamine (10 and 30 mg/kg, i.p.) enhanced synaptic efficacy in the hippocampo-mPFC pathway in a dose-dependent manner. Although repeated treatments with fluvoxamine (30 mg/kg, i.p. after 30 mg/kg/dayx21 days, p.o.) caused an enhancement of synaptic efficacy, there was no significant difference between single and repeated treatments. The input/output characteristics showed hypersensitivity to stimulation intensity in the group with repeated fluvoxamine treatments. The establishment of long-term potentiation (LTP) in the hippocampo-mPFC pathway after a single administration of fluvoxamine was not different from that in the saline-injected group. On the other hand, the hippocampo-mPFC LTP was significantly augmented by repeated treatments with fluvoxamine when compared to a single treatment. These findings suggest that the serotonergic system could modulate the synaptic plasticity at hippocampal-mPFC synapses. The present study, furthermore, suggests that the enhancement of LTP in the hippocampo-mPFC pathway produced by repeated treatments with fluvoxamine may be implicated in the SSRI-induced therapeutic effect on psychiatric disorders.

Stress activation of glutamate neurotransmission in the prefrontal cortex: implications for dopamine-associated psychiatric disorders

In most psychiatric disorders, stress is the major nongenomic factor that contributes to the expression or exacerbation of acute symptoms, recurrence or relapse after a period of remission, and treatment outcome. Delineation of mechanisms by which stress contributes to these processes is fundamental to understanding the disease process and for improving outcome. In this article, evidence is reviewed to indicate that many central aspects of stress response, including activation of the hypothalmic-pituitary-adrenal (HPA) axis and dopamine neurotransmission, are modulated, and in some cases mediated, by glutamate neurotransmission in the prefrontal cortex (PFC). It is suggested that activation of glutamatergic neurotransmission in the PFC presents a common mechanism by which stress influences normal and abnormal processes that sustain affect and cognition. Although monoamines, in particular dopamine, have been considered the major culprits in the adverse effects of stress in disorders such as addiction and schizophrenia, it is likely that in a vulnerable brain with an underlying PFC pathophysiology, abnormal stress-activated monoaminergic neurotransmission is secondary to anomalies in cortical glutamate neurotransmission. Thus, understanding the contribution of glutamate-mediated processes to stress response through the use of experimental models that involve disrupted PFC function can provide insights to the fundamental pathophysiology of stress-sensitive psychiatric disorders and lead to novel strategies for treatment and prevention.

Position sensitivity in phasically discharging nucleus accumbens neurons of rats alternating between tasks requiring complementary types of spatial cues

To determine how hippocampal location-selective discharges might influence downstream structures for navigation, nucleus accumbens neurons were recorded in rats alternating between two tasks guided respectively by lit cues in the maze or by extramaze room cues. Of 144 phasically active neurons, 80 showed significant behavioral correlates including displacements, immobility prior to, or after reward delivery, as well as turning, similar to previous reports. Nine neurons were position-selective, 22 were sensitive to task and platform changes and 40 others were both. Although the accumbens neurons showed the same behavioral correlate in two or four functionally equivalent locations, these responses were stronger at some of these places, evidence for position sensitivity. To test whether position responses were selective for room versus platform cues, the experimental platform was rotated while the rat performed each of the two tasks. This revealed responses to changes in position relative to both platform and room cues, despite the fact that previous studies had shown that place responses of hippocampal neurons recorded in the same task are anchored to room cues only. After these manipulations and shifts between the two tasks, the responses varied among simultaneously recorded neurons, and even in single neurons in alternating visits to reward sites. Again this contrasts with the uniformity of place responses of hippocampal neurons recorded in this same task. Thus accumbens position responses may derive from hippocampal inputs, while responses to context changes are more likely to derive from other signals or intrinsic processing. Considering the accumbens as a limbic-motor interface, we conclude that position-modulated behavioral responses in the accumbens may be intermediate between the allocentric reference frame of position-selective discharges in the hippocampus and the egocentric coding required to organize movement control. The conflicting responses among simultaneously recorded neurons could reflect competition processes serving as substrates for action selection and learning.

Spatial learning deficits without hippocampal neuronal loss in a model of early-onset epilepsy

Studies were undertaken to examine the effects recurrent early-life seizures have on the ability of rats to acquire spatial memories in adulthood. A minute quantity of tetanus toxin was injected unilaterally into the hippocampus on postnatal day 10. Within 48 h, rats developed recurrent seizures that persisted for 1 week. Between postnatal days 57 and 61, rats were trained in a Morris water maze. Toxin-injected rats were markedly deficient in learning this task. While these rats showed gradual improvement in escape latencies over 20 trials, their performance always lagged behind that of controls. Poor performance could not be explained by motor impairments or motivational difficulties since swimming speed was similar for the groups. Only eight of 16 toxin-injected animals showed focal interictal spikes in the hippocampus during electroencephalographic recordings. This suggests that learning deficiencies and chronic epilepsy may be independent products of recurrent early-life seizures. A quantitative analysis of hippocampus revealed a significant decrease in neuronal density in stratum pyramidale of experimental rats. However, the differences were largely explained by a concomitant increase in the area of stratum pyramidale. Studies of glial fibrillary acidic protein expression and spread of horseradish peroxidase-conjugated tetanus toxin in the hippocampus suggest that the dispersion of cell bodies in stratum pyramidale can neither be explained by a reactive gliosis nor the direct action of the toxin itself. Taken together, we suggest that recurrent seizures beginning in early life can lead to a significant deficiency in spatial learning without ongoing hippocampal synchronized network discharging or a substantial loss of hippocampal pyramidal cells.

Re-evaluation of the spatial memory deficits induced by hippocampal short lasting inactivation reveals the need for cortical co-operation

Evidence has accumulated that the rat hippocampus plays a central role in spatial memory. In complement to lesion studies, reversible lidocaïne-induced inactivations have been used to investigate the time-course of the memory processes mediated by the hippocampus. A number of studies suggest that, in some conditions, the hippocampus is not necessary for online acquisition of spatial information. To test this hypothesis, we examined the effects of bilateral lidocaïne-induced inactivations of the dorsal hippocampus in the acquisition of new spatial information. After initial learning of a place navigation task in the water maze, rats were tested for acquisition of a new platform location and received injections of lidocaïne in the hippocampus prior to each daily four-trial block. The training blocks were separated by a 24-h period allowing the hippocampus to recover from inactivation. The results show that lidocaïne-injected rats were able to learn the new platform location like controls. Inactivations, however, was found to induce a within-block learning impairment. This suggests that the hippocampus can perform off-line processing and that another structure is able to handle spatial information during hippocampal inactivations. Parietal-lesioned rats that received an injection of lidocaïne were still able to learn the new platform location suggesting that the parietal cortex does not sustain this role. Overall, our results suggest that the hippocampus is not necessary for all stages of memory formation and co-operates with other brain, possibly cortical, structures which remain to be determined.

NMDA and AMPA antagonist infusions into the ventral striatum impair different steps of spatial information processing in a nonassociative task in mice

Most of the research on ventral striatal functions has been focused on their role in modulating reward and motivation. More recently, a possible role of this structure in cognitive functions has been suggested. However, very little information is available on the involvement of the nucleus accumbens in the different stages of the consolidation process. In this study, the effect of focal injections of AP-5 and DNQX, competitive antagonists at the NMDA and AMPA receptors, respectively, was examined in a nonassociative task designed to estimate the ability of mice to react to spatial changes. The task consists of placing the animals in an open field containing five objects; after three sessions of habituation, their reactivity to object displacement was examined 24 hr later. AP-5 injections administered after training impaired the ability of mice to detect the spatial novelty but did not affect response when injected 120 min after training or before testing. On the contrary, DNQX did not affect response when administered immediately or 120 min after training but did impair spatial discrimination when administered before training or testing. These data demonstrate a double dissociation between glutamate receptor subtypes, such that accumbens NMDA receptors are important for consolidation and not ongoing discrimination of spatial information, whereas AMPA receptors have an opposite role in these processes.

Using fos imaging in the rat to reveal the anatomical extent of the disruptive effects of fornix lesions

Activity of the immediate early gene c-fos was compared across hemispheres in rats with unilateral fornix lesions. To engage Fos production, rats first performed a radial arm maze task that is severely disrupted by bilateral fornix lesions. Using immunohistochemical techniques, Fos-positive cells were visualized and counted in 39 sites in both hemispheres. Fornix lesions led to a significant reduction in Fos in all ipsilateral hippocampal subfields, as well as the entorhinal cortex and most of the subicular complex. Other sites that showed reduced activity included the ipsilateral retrosplenial, anterior cingulate, and postrhinal cortices. Subcortical regions showing significant Fos decreases included the anterior thalamic nuclei, supramammillary nucleus, diagonal band of Broca, and lateral septum. Thus, the effects of fornix lesions extended beyond the hippocampal formation and included sites not directly innervated by the tract. These changes were nevertheless selective, as shown by the lack of hemispheric difference in any of the preselected control sites, the perirhinal cortex, or nucleus accumbens. Furthermore, there were no hemispheric differences in an additional group of animals with unilateral fornix lesions that were killed directly from the home cage. The location of Fos changes closely corresponded to those brain regions that when lesioned disrupt spatial working memory. Moreover, there was a correspondence between those brain regions that show increased Fos production in normal animals performing the radial arm maze task and those affected by fornix lesions. These results show that fornix transection has widespread, but selective, effects on a network of structures normally activated by spatial memory processes, with these effects extending beyond the hippocampal formation.

Lesions of the medial shell of the nucleus accumbens impair rats in finding larger rewards, but spare reward-seeking behavior

The goal of this study was to help better understand the importance of the nucleus accumbens (Nacc) in the processing of position and reward value information for goal-directed orientation behaviors. Sixteen male Long-Evans rats, under partial water deprivation, were trained in a plus-maze to find water rewards in the respective arms which were lit in pseudo-random sequence (training trials). Each day one reward arm was selected to deliver six drops of water (at 1 s intervals) the others provided only one drop per visit. After 32 visits, probe trials were intermittently presented among training trials. Here, all four arms were lit and offered the previously assigned reward. The rats rapidly learned to go to the highly rewarded arm. Six trained rats were given bilateral electrolytic lesions in the Nacc shell, two others had unilateral lesions and eight had sham operations (with approved protocols). Field potentials evoked by fornix stimulation were recorded in lesion electrodes to guide placements. Only the lesioned rats showed significant impairments (P<0.05) in selecting the greater reward on probe trials. However on training trials, lesioned (and sham-operated) rats made only rare errors. While the motivation to drink and the capacity for cue-guided goal-directed orientation behavior was spared, lesioned rats were impaired in learning the location of the larger reward. The accumbens lesions apparently impaired integration of position and reward value information, consistent with anatomical and electrophysiological data showing the convergence of hippocampal, amygdalar, ventral tegmental area (VTA) and prefrontal cortical inputs there.

Contextual-dependent effects of nucleus accumbens lesions on spatial learning in mice

The effect of nucleus accumbens lesions on radial maze performance of C57BL/6 and DBA/2 mice was assessed under distinct extra-maze cuing conditions. Among sham-lesioned mice, C57BL/6 performed better under rich than poor cuing conditions whereas DBA performed in the same fashion under both conditions. In C57BL/6, a disruptive effect of lesions was found only in mice tested under rich cuing. Conversely, in DBA/2, the lesions improved performance under poor cuing and disrupted performance under rich cuing. In that strain, a possible lesion-induced enhancement of attention to background stimuli improving performance under poor cuing but producing interference under rich cuing is suggested. In general, the lesions effect seemed to depend on the strain predisposition to implement configural or cue-based responding.

Sensory and memory properties of hippocampal place cells

The rat hippocampus contains place cells whose firing is location-specific. These cells fire only when the rat enters a restricted region of the environment called the firing field. In this review, we examine the sensory information that is fundamental to the place cell system for producing spatial firing. While visual information takes precedence in the control of firing fields when it is available, local (olfactory and/or tactile) cues combined with motion-related cues can permit stable spatial firing. Motion-related cues are integrated by hippocampal place cells, but in the absence of external cues do not support stable firing over long periods. While firing fields are based on a variety of sensory cues, they do not strictly depend on such cues. Rather, sensory information is important for activating the representation appropriate to the current environment as reflected by the firing properties of place cell ensembles. Specific sensory channels as well as the memory properties of place cells can support ongoing firing under manipulations of the environment. These memory features raise the question of the role of the place cell system in the acquisition, storage and retrieval of spatial information. Based on the existing literature about the effects of hippocampal lesions and about the metabolic activations in spatial memory tasks, we suggest that a function of the place cell system is to automatically provide the organism with information about its current location so as to allow for the rapid acquisition of novel information.

A neural systems analysis of adaptive navigation

In the field of the neurobiology of learning, significant emphasis has been placed on understanding neural plasticity within a single structure (or synapse type) as it relates to a particular type of learning mediated by a particular brain area. To appreciate fully the breadth of the plasticity responsible for complex learning phenomena, it is imperative that we also examine the neural mechanisms of the behavioral instantiation of learned information, how motivational systems interact, and how past memories affect the learning process. To address this issue, we describe a model of complex learning (rodent adaptive navigation) that could be used to study dynamically interactive neural systems. Adaptive navigation depends on the efficient integration of external and internal sensory information with motivational systems to arrive at the most effective cognitive and/or behavioral strategies. We present evidence consistent with the view that during navigation: 1) the limbic thalamus and limbic cortex is primarily responsible for the integration of current and expected sensory information, 2) the hippocampal-septal-hypothalamic system provides a mechanism whereby motivational perspectives bias sensory processing, and 3) the amygdala-prefrontal-striatal circuit allows animals to evaluate the expected reinforcement consequences of context-dependent behavioral responses. Although much remains to be determined regarding the nature of the interactions among neural systems, new insights have emerged regarding the mechanisms that underlie flexible and adaptive behavioral responses.