Lesion of the ventral and intermediate hippocampus abolishes anticipatory activity in the medial prefrontal cortex of the rat

Selective reactivation of value- and place-dependent information during sharp-wave ripples in the intermediate and dorsal hippocampus

Reactivating place cells during sharp-wave ripples in the hippocampus is important for memory consolidation. However, whether hippocampal reactivation is affected by the values of events experienced by the animal is largely unknown. Here, we investigated whether place cells in the dorsal (dHP) and intermediate hippocampus (iHP) of rats are differentially reactivated depending on the value associated with a place during the learning of places associated with higher-value rewards in a T-maze. Place cells in the iHP representing the high-value location were reactivated significantly more frequently than those representing the low-value location, characteristics not observed in the dHP. In contrast, the activities of place cells in the dHP coding the routes leading to high-value locations were replayed more than those in the iHP. Our findings suggest that value-based differential reactivation patterns along the septotemporal axis of the hippocampus may play essential roles in optimizing goal-directed spatial learning for maximal reward.

Border cells without theta rhythmicity in the medial prefrontal cortex

The medial prefrontal cortex (mPFC) is a key brain structure for higher cognitive functions such as decision-making and goal-directed behavior, many of which require awareness of spatial variables including one's current position within the surrounding environment. Although previous studies have reported spatially tuned activities in mPFC during memory-related trajectory, the spatial tuning of mPFC network during freely foraging behavior remains elusive. Here, we reveal geometric border or border-proximal representations from the neural activity of mPFC ensembles during naturally exploring behavior, with both allocentric and egocentric boundary responses. Unlike most of classical border cells in the medial entorhinal cortex (MEC) discharging along a single wall, a large majority of border cells in mPFC fire particularly along four walls. mPFC border cells generate new firing fields to external insert, and remain stable under darkness, across distinct shapes, and in novel environments. In contrast to hippocampal theta entrainment during spatial working memory tasks, mPFC border cells rarely exhibited theta rhythmicity during spontaneous locomotion behavior. These findings reveal spatially modulated activity in mPFC, supporting local computation for cognitive functions involving spatial context and contributing to a broad spatial tuning property of cortical circuits.

High-resolution awake mouse fMRI at 14 Tesla

High-resolution awake mouse fMRI remains challenging despite extensive efforts to address motion-induced artifacts and stress. This study introduces an implantable radiofrequency (RF) surface coil design that minimizes image distortion caused by the air/tissue interface of mouse brains while simultaneously serving as a headpost for fixation during scanning. Using a 14T scanner, high-resolution fMRI enabled brain-wide functional mapping of visual and vibrissa stimulation at 100×100×200μm resolution with a 2s per frame sampling rate. Besides activated ascending visual and vibrissa pathways, robust BOLD responses were detected in the anterior cingulate cortex upon visual stimulation and spread through the ventral retrosplenial area (VRA) with vibrissa air-puff stimulation, demonstrating higher-order sensory processing in association cortices of awake mice. In particular, the rapid hemodynamic responses in VRA upon vibrissa stimulation showed a strong correlation with the hippocampus, thalamus, and prefrontal cortical areas. Cross-correlation analysis with designated VRA responses revealed early positive BOLD signals at the contralateral barrel cortex (BC) occurring 2 seconds prior to the air-puff in awake mice with repetitive stimulation, which was not detectable with the randomized stimulation paradigm. This early BC activation indicated learned anticipation through the vibrissa system and association cortices in awake mice under continuous training of repetitive air-puff stimulation. This work establishes a high-resolution awake mouse fMRI platform, enabling brain-wide functional mapping of sensory signal processing in higher association cortical areas.

Alcohol-induced damage to the fimbria/fornix reduces hippocampal-prefrontal cortex connection during early abstinence

INTRODUCTION

Alcohol dependence is characterized by a gradual reduction in cognitive control and inflexibility to contingency changes. The neuroadaptations underlying this aberrant behavior are poorly understood. Using an animal model of alcohol use disorders (AUD) and complementing diffusion-weighted (dw)-MRI with quantitative immunohistochemistry and electrophysiological recordings, we provide causal evidence that chronic intermittent alcohol exposure affects the microstructural integrity of the fimbria/fornix, decreasing myelin basic protein content, and reducing the effective communication from the hippocampus (HC) to the prefrontal cortex (PFC). Using a simple quantitative neural network model, we show how disturbed HC-PFC communication may impede the extinction of maladaptive memories, decreasing flexibility. Finally, combining dw-MRI and psychometric data in AUD patients, we discovered an association between the magnitude of microstructural alteration in the fimbria/fornix and the reduction in cognitive flexibility. Overall, these findings highlight the vulnerability of the fimbria/fornix microstructure in AUD and its potential contribution to alcohol pathophysiology. Fimbria vulnerability to alcohol underlies hippocampal-prefrontal cortex dysfunction and correlates with cognitive impairment.

Differential behavior-related activity of distinct hippocampal interneuron types during odor-associated spatial navigation

Hippocampal pyramidal cells represent an animal's position in space together with specific contexts and events. However, it is largely unknown how distinct types of GABAergic interneurons contribute to such computations. We recorded from the intermediate CA1 hippocampus of head-fixed mice exhibiting odor-to-place memory associations during navigation in a virtual reality (VR). The presence of an odor cue and its prediction of a different reward location induced a remapping of place cell activity in the virtual maze. Based on this, we performed extracellular recording and juxtacellular labeling of identified interneurons during task performance. The activity of parvalbumin (PV)-expressing basket, but not of PV-expressing bistratified cells, reflected the expected contextual change in the working-memory-related sections of the maze. Some interneurons, including identified cholecystokinin-expressing cells, decreased activity during visuospatial navigation and increased activity during reward. Our findings suggest that distinct types of GABAergic interneuron are differentially involved in cognitive processes of the hippocampus.

Neural dynamics underlying associative learning in the dorsal and ventral hippocampus

Animals associate cues with outcomes and update these associations as new information is presented. This requires the hippocampus, yet how hippocampal neurons track changes in cue-outcome associations remains unclear. Using two-photon calcium imaging, we tracked the same dCA1 and vCA1 neurons across days to determine how responses evolve across phases of odor-outcome learning. Initially, odors elicited robust responses in dCA1, whereas, in vCA1, odor responses primarily emerged after learning and embedded information about the paired outcome. Population activity in both regions rapidly reorganized with learning and then stabilized, storing learned odor representations for days, even after extinction or pairing with a different outcome. Additionally, we found stable, robust signals across CA1 when mice anticipated outcomes under behavioral control but not when mice anticipated an inescapable aversive outcome. These results show how the hippocampus encodes, stores and updates learned associations and illuminates the unique contributions of dorsal and ventral hippocampus.

Projections of the insular cortex to orbitofrontal and medial prefrontal cortex: A tracing study in the rat

The dense fiber pathways that connect the insular cortex with frontal cortices are thought to provide these frontal areas with interoceptive information, crucial for their involvement in executive functions. Using anterograde neuroanatomical tracing, we mapped the detailed organization of the projections from the rat insular cortex to its targets in orbitofrontal (OFC) and medial prefrontal (mPFC) cortex. In OFC, main insular projections distribute to lateral and medial parts, avoiding ventral parts. Whereas projections from the primary gustatory cortex densely innervate dorsolateral OFC, likely corresponding to what in primates is known as the secondary gustatory cortex, these projections avoid mPFC. Instead, mPFC is targeted almost exclusively by projections from agranular fields of the insular cortex. Finally, "parietal" domains of the insular cortex project specifically to the dorsolateral OFC, and strongly innervate ventral portions of mPFC, i.e., the dorsal peduncular cortex.

Transcerebral information coordination in directional hippocampus-prefrontal cortex network during working memory based on bimodal neural electrical signals

Working memory (WM) is a kind of advanced cognitive function, which requires the participation and cooperation of multiple brain regions. Hippocampus and prefrontal cortex are the main responsible brain regions for WM. Exploring information coordination between hippocampus and prefrontal cortex during WM is a frontier problem in cognitive neuroscience. In this paper, an advanced information theory analysis based on bimodal neural electrical signals (local field potentials, LFPs and spikes) was employed to characterize the transcerebral information coordination across the two brain regions. Firstly, LFPs and spikes were recorded simultaneously from rat hippocampus and prefrontal cortex during the WM task by using multi-channel in vivo recording technique. Then, from the perspective of information theory, directional hippocampus-prefrontal cortex networks were constructed by using transfer entropy algorithm based on spectral coherence between LFPs and spikes. Finally, transcerebral coordination of bimodal information at the brain-network level was investigated during acquisition and performance of the WM task. The results show that the transfer entropy in directional hippocampus-prefrontal cortex networks is related to the acquisition and performance of WM. During the acquisition of WM, the information flow, local information transmission ability and information transmission efficiency of the directional hippocampus-prefrontal networks increase over learning days. During the performance of WM, the transfer entropy from the hippocampus to prefrontal cortex plays a leading role for bimodal information coordination across brain regions and hippocampus has a driving effect on prefrontal cortex. Furthermore, bimodal information coordination in the hippocampus → prefrontal cortex network could predict WM during the successful performance of WM.

Multicentric tracking of multiple agents by anterior cingulate cortex during pursuit and evasion

Successful pursuit and evasion require rapid and precise coordination of navigation with adaptive motor control. We hypothesize that the dorsal anterior cingulate cortex (dACC), which communicates bidirectionally with both the hippocampal complex and premotor/motor areas, would serve a mapping role in this process. We recorded responses of dACC ensembles in two macaques performing a joystick-controlled continuous pursuit/evasion task. We find that dACC carries two sets of signals, (1) world-centric variables that together form a representation of the position and velocity of all relevant agents (self, prey, and predator) in the virtual world, and (2) avatar-centric variables, i.e. self-prey distance and angle. Both sets of variables are multiplexed within an overlapping set of neurons. Our results suggest that dACC may contribute to pursuit and evasion by computing and continuously updating a multicentric representation of the unfolding task state, and support the hypothesis that it plays a high-level abstract role in the control of behavior.

Time as the fourth dimension in the hippocampus

Experiences of animal and human beings are structured by the continuity of space and time coupled with the uni-directionality of time. In addition to its pivotal position in spatial processing and navigation, the hippocampal system also plays a central, multiform role in several types of temporal processing. These include timing and sequence learning, at scales ranging from meso-scales of seconds to macro-scales of minutes, hours, days and beyond, encompassing the classical functions of short term memory, working memory, long term memory, and episodic memories (comprised of information about when, what, and where). This review article highlights the principal findings and behavioral contexts of experiments in rats showing: 1) timing: tracking time during delays by hippocampal 'time cells' and during free behavior by hippocampal-afferent lateral entorhinal cortex ramping cells; 2) 'online' sequence processing: activity coding sequences of events during active behavior; 3) 'off-line' sequence replay: during quiescence or sleep, orderly reactivation of neuronal assemblies coding awake sequences. Studies in humans show neurophysiological correlates of episodic memory comparable to awake replay. Neural mechanisms are discussed, including ion channel properties, plateau and ramping potentials, oscillations of excitation and inhibition of population activity, bursts of high amplitude discharges (sharp wave ripples), as well as short and long term synaptic modifications among and within cell assemblies. Specifically conceived neural network models will suggest processes supporting the emergence of scalar properties (Weber's law), and include different classes of feedforward and recurrent network models, with intrinsic hippocampal coding for 'transitions' (sequencing of events or places).

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.

Sex and age differentially affect GABAergic neurons in the mouse prefrontal cortex and hippocampus following chronic intermittent hypoxia

Obstructive sleep apnea (OSA), a chronic sleep disorder characterized by repetitive reduction or cessation of airflow during sleep, is widely prevalent and is associated with adverse neurocognitive sequelae including increased risk of Alzheimer's disease (AD). In humans, OSA is more common in elderly males. OSA is characterized by sleep fragmentation and chronic intermittent hypoxia (CIH), and recent epidemiological studies point to CIH as the best predictor of neurocognitive sequelae associated with OSA. The sex- and age- specific effects of OSA-associated CIH on specific cell populations such as γ-aminobutyric acid (GABA)-ergic neurons in the hippocampus and the medial prefrontal cortex (mPFC), regions important for cognitive function, remain largely unknown. The present study examined the effect of 35 days of either moderate (10% oxygen) or severe (5% oxygen) CIH on GABAergic neurons in the mPFC and hippocampus of young and aged male and female mice as well as post-accelerated ovarian failure (AOF) female mice. In the mPFC and hippocampus, the number of GABA-labeled neurons increased in aged and young severe CIH males compared to controls but not in young moderate CIH males. This change was not representative of the individual GABAergic cell subpopulations, as the number of parvalbumin-labeled neurons decreased while the number of somatostatin-labeled neurons increased in the hippocampus of severe CIH young males only. In all female groups, the number of GABA-labeled cells was not different between CIH and controls. However, in the mPFC, CIH increased the number of parvalbumin-labeled neurons in young females and the number of somatostatin-labeled cells in AOF females but decreased the number of somatostatin-labeled cells in aged females. In the hippocampus, CIH decreased the number of somatostatin-labeled neurons in young females. CIH decreased the density of vesicular GABA transporter in the mPFC of AOF females only. These findings suggest sex-specific changes in GABAergic neurons in the hippocampus and mPFC with males showing an increase of this cell population as compared to their female counterparts following CIH. Age at exposure and severity of CIH also differentially affect the GABAergic cell population in mice.

Hippocampus-nidopallium caudolaterale interactions exist in the goal-directed behavior of pigeon

Avian hippocampus (Hp) and nidopallium caudolaterale (NCL) are believed to play key roles in goal-directed behavior. However, it is still unclear whether there are interactions between the two brain regions in the goal-directed behavior of pigeons. To investigate the interactions between the Hp and the NCL in the goal-directed behavior, we recorded local field potential (LFP) signals from the two regions simultaneously when the pigeons performed a goal-directed decision-making task. Amplitude-amplitude coupling analysis revealed that the coupling value between the LFP recorded from the Hp and that from the NCL increased significantly (P < 0.05) in slow gamma-band (40-60 Hz) during the turning area. In addition, the LFP functional network analysis demonstrated the LFP functional connections between the Hp and the NCL increased significantly (P < 0.05) in the turning area. The result of partial directed coherence (PDC) analysis showed that the predominant direction of information flow is thought to be from the Hp to the NCL. These findings suggest that there are causal functional interactions between the Hp and the NCL by which information is transmitted between the two regions relevant to goal-directed behavior.

Structural connectivity and risk for anhedonia after trauma: A prospective study and replication

Anhedonia emerges in some people after psychological trauma, reflected by a loss of interest, diminished affect, and detachment. Structural abnormalities in specific neural pathways at the time of trauma may influence the development of these posttraumatic anhedonia (PTA) symptoms. In this prospective study, we determined whether white matter connectivity at around one month post-trauma predicts PTA and other PTSD symptoms at six months post-trauma. Thirty men and women aged 19-62 were recruited from the emergency department of a Level I trauma center. Participants received diffusion tensor imaging at approximately one month post-trauma and clinical assessments at one and six months post-trauma. Probabilistic tractography was used to examine connectivity of select pathways. A replication sample (N = 57) in an independent, cross-sectional dataset of traumatized women was similarly analyzed. Logistic regression results indicated that, after accounting for early PTSD symptoms (at one month) and other clinical risk factors, the integrity of the uncinate fasciculus (UF) uniquely predicted the presence of PTA at six months post-trauma (Beta = -225.6, p < .05). Together, these factors contributed to 76% of the variance in PTA. Integrity of the UF also predicted re-experiencing PTSD symptoms at six months post-trauma. These results were supported in our replication analyses. Our findings indicate that the integrity of the UF around 1 month post-trauma affects vulnerability for the development of anhedonic PTSD symptoms as well as re-experiencing symptoms. Connectivity of this amygdala-ventromedial prefrontal pathway appears to be a salient predictor of anhedonia, above and beyond clinical risk factors.

Role of corticosterone in anxiety- and depressive-like behavior and HPA regulation following prenatal alcohol exposure

Prenatal alcohol exposure (PAE) is known to cause dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis, including hyperresponsivity to stressors. Dysregulation of the HPA axis plays a role in vulnerability to stress-related disorders, such as anxiety and depression. Thus, the effects of PAE on HPA function may result in increased vulnerability to the effects of stress and, in turn, lead to the development of stress-related disorders. Indeed, individuals prenatally exposed to alcohol have an increased risk of developing anxiety and depression. However, it is unclear whether hypersecretion of corticosterone (CORT) in response to stress per se is involved with mediating differential effects of stress in PAE and control animals. To investigate the role of CORT in mediating effects of stress in both adult females and males following PAE, adrenalectomy with CORT replacement (ADXR) was utilized to produce similar CORT levels among prenatal treatment groups before exposure to chronic unpredictable stress (CUS). Anxiety-like behavior was evaluated using the open field and elevated plus maze, and depressive-like behavior was examined in the forced swim test. Mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) mRNA expression was assessed in the medial prefrontal cortex (mPFC), amygdala, and hippocampal formation. Under the non-CUS condition, PAE alone differentially altered anxiety-like behavior in sham but not ADXR females and males, with females showing decreased anxiety-like behavior but males exhibiting increased anxiety-like behavior compared to their control counterparts. There were no effects of PAE alone on depressive-like in females or males. PAE also decreased GR mRNA expression in the hippocampal formation in females but had no effects on MR or GR mRNA expression in any brain region in males. CUS had differential effects on anxiety- and depressive-like behavior in PAE and control animals, and these effects were sex dependent. Importantly, ADXR unmasked differences between PAE and control animals, demonstrating that CORT may play a differential role in modulating behavior and HPA activity/regulation in PAE and control animals, and may do so in a sex-dependent manner.

An ensemble code in medial prefrontal cortex links prior events to outcomes during learning

The prefrontal cortex is implicated in learning the rules of an environment through trial and error. But it is unclear how such learning is related to the prefrontal cortex’s role in short-term memory. Here we ask if the encoding of short-term memory in prefrontal cortex is used by rats learning decision rules in a Y-maze task. We find that a similar pattern of neural ensemble activity is selectively recalled after reinforcement for a correct decision. This reinforcement-selective recall only reliably occurs immediately before the abrupt behavioural transitions indicating successful learning of the current rule, and fades quickly thereafter. We could simultaneously decode multiple, retrospective task events from the ensemble activity, suggesting the recalled ensemble activity has multiplexed encoding of prior events. Our results suggest that successful trial-and-error learning is dependent on reinforcement tagging the relevant features of the environment to maintain in prefrontal cortex short-term memory.

Prefrontal cortex is involved in flexibly learning the correct behavioural strategies but the neural correlates of this process are not well understood. Here the authors show that reinforcement for a correct decision at behavioural transitions evokes ensemble firing patterns related to prior events.

Rat anterior cingulate cortex recalls features of remote reward locations after disfavoured reinforcements

The anterior cingulate cortex (ACC) encodes information supporting mnemonic and cognitive processes. We show here that a rat’s position can be decoded with high spatiotemporal resolution from ACC activity. ACC neurons encoded the current state of the animal and task, except for brief excursions that sometimes occurred at target feeders. During excursions, the decoded position became more similar to a remote target feeder than the rat’s physical position. Excursions recruited activation of neurons encoding choice and reward, and the likelihood of excursions at a feeder was inversely correlated with feeder preference. These data suggest that the excursion phenomenon was related to evaluating real or fictive choice outcomes, particularly after disfavoured reinforcements. We propose that the multiplexing of position with choice-related information forms a mental model isomorphic with the task space, which can be mentally navigated via excursions to recall multimodal information about the utility of remote locations.

Regulation of actions and habits by ventral hippocampal trkB and adolescent corticosteroid exposure

In humans and rodents, stress promotes habit-based behaviors that can interfere with action-outcome decision-making. Further, developmental stressor exposure confers long-term habit biases across rodent-primate species. Despite these homologies, mechanisms remain unclear. We first report that exposure to the primary glucocorticoid corticosterone (CORT) in adolescent mice recapitulates multiple neurobehavioral consequences of stressor exposure, including long-lasting biases towards habit-based responding in a food-reinforced operant conditioning task. In both adolescents and adults, CORT also caused a shift in the balance between full-length tyrosine kinase receptor B (trkB) and a truncated form of this neurotrophin receptor, favoring the inactive form throughout multiple corticolimbic brain regions. In adolescents, phosphorylation of the trkB substrate extracellular signal-regulated kinase 42/44 (ERK42/44) in the ventral hippocampus was also diminished, a long-term effect that persisted for at least 12 wk. Administration of the trkB agonist 7,8-dihydroxyflavone (7,8-DHF) during adolescence at doses that stimulated ERK42/44 corrected long-lasting corticosterone-induced behavioral abnormalities. Meanwhile, viral-mediated overexpression of truncated trkB in the ventral hippocampus reduced local ERK42/44 phosphorylation and was sufficient to induce habit-based and depression-like behaviors. Together, our findings indicate that ventral hippocampal trkB is essential to goal-directed action selection, countering habit-based behavior otherwise facilitated by developmental stress hormone exposure. They also reveal an early-life sensitive period during which trkB-ERK42/44 tone determines long-term behavioral outcomes.

Differential involvement of the medial prefrontal cortex across variants of contextual fear conditioning

The context preexposure facilitation effect (CPFE) is a contextual fear conditioning paradigm in which learning about the context, acquiring the context-shock association, and retrieving/expressing contextual fear are temporally dissociated into three distinct phases. In contrast, learning about the context and the context-shock association happens concurrently in standard contextual fear conditioning (sCFC). By infusing the GABA_A receptor agonist muscimol into medial prefrontal cortex (mPFC) in adolescent Long-Evans rats, the current set of experiments examined the functional role of the mPFC in each phase of the CPFE and sCFC. In the CPFE, the mPFC is necessary for the following: acquisition and/or consolidation of context memory (Experiment 1), reconsolidation of a context memory to include shock (Experiment 2), and expression of contextual fear memory during a retention test (Experiment 3). In contrast to the CPFE, inactivation of the mPFC prior to conditioning in sCFC has no effect on acquisition, consolidation, or retention of a contextual fear memory (Experiment 4). Interestingly, the mPFC is not required for acquiring a context-shock association (measured by post-shock freezing) in the CPFE or sCFC (Experiment 2b and 4). Taken together, these results indicate that the mPFC is differentially recruited across stages of learning and variants of contextual fear conditioning (CPFE versus sCFC). More specifically, separating out learning about the context and the context-shock association necessitates activation of the medial prefrontal cortex during early learning and/or consolidation.

The ventral hippocampal muscarinic cholinergic system plays a key role in sexual dimorphisms of spatial working memory in rats

Sex differences in cognitive processing and function have been documented in human and animal studies. Females have been found to perform better than males on non-spatial memory tasks, while males tend to outperform females on spatial memory tasks. The neural mechanisms underlying these sexual dimorphisms are unclear. However, it is known that the cholinergic system is critically involved in memory processes, and there are notable differences between males and females in cholinergic system function and receptor expression. In particular, there are sex differences in the processing of information in the frontal cortex and hippocampus. In this study, we examined the roles of muscarinic and nicotinic acetylcholine receptors in the medial frontal cortex (MfC) and ventral hippocampus (VH) on spatial working memory in male and female rats. Local infusions of scopolamine (SCOP) and mecamylamine (MEC) (10, 20, 50 μg/side) were used to antagonize these receptors in each respective brain region during performance in the 16-arm radial arm maze. Infusions of SCOP into the VH caused a significant increase in memory errors in female rats, but had no significant effect on males, while infusions of MEC into the VH had no effect on either sex. Infusions of both SCOP (50 μg/side) and MEC (20 μg/side) into the MfC caused working memory impairments in both sexes. These results show that muscarinic acetylcholine receptors in the VH are differentially vulnerable to spatial memory impairments in females. Ventral hippocampal muscarinic acetylcholine receptors may play a key role in male-female differences in spatial memory.

Movement Enhances the Nonlinearity of Hippocampal Theta

The nonlinear, metastable dynamics of the brain are essential for large-scale integration of smaller components and for the rapid organization of neurons in support of behavior. Therefore, understanding the nonlinearity of the brain is paramount for understanding the relationship between brain dynamics and behavior. Explicit quantitative descriptions of the properties and consequences of nonlinear neural networks, however, are rare. Because the local field potential (LFP) reflects the total activity across a population of neurons, nonlinearites of the nervous system should be quantifiable by examining oscillatory structure. We used high-order spectral analysis of LFP recorded from the dorsal and intermediate regions of the rat hippocampus to show that the nonlinear character of the hippocampal theta rhythm is directly related to movement speed of the animal. In the time domain, nonlinearity is expressed as the development of skewness and asymmetry in the theta shape. In the spectral domain, nonlinear dynamics manifest as the development of a chain of harmonics statistically phase coupled to the theta oscillation. This evolution was modulated across hippocampal regions, being stronger in the dorsal CA1 relative to more intermediate areas. The intensity and timing of the spiking activity of pyramidal cells and interneurons was strongly correlated to theta nonlinearity. Because theta is known to propagate from dorsal to ventral regions of the hippocampus, these data suggest that the nonlinear character of theta decreases as it travels and supports a hypothesis that activity dissipates along the longitudinal axis of the hippocampus.

SIGNIFICANCE STATEMENT

We describe the first explicit quantification regarding how behavior enhances the nonlinearity of the nervous system. Our findings demonstrate uniquely how theta changes with increasing speed due to the altered underlying neuronal dynamics and open new directions of research on the relationship between single-neuron activity and propagation of theta through the hippocampus. This work is significant because it will encourage others to consider the nonlinear nature of the nervous system and higher-order spectral analyses when examining oscillatory interactions.

The Cognitive Architecture of Spatial Navigation: Hippocampal and Striatal Contributions

Spatial navigation can serve as a model system in cognitive neuroscience, in which specific neural representations, learning rules, and control strategies can be inferred from the vast experimental literature that exists across many species, including humans. Here, we review this literature, focusing on the contributions of hippocampal and striatal systems, and attempt to outline a minimal cognitive architecture that is consistent with the experimental literature and that synthesizes previous related computational modeling. The resulting architecture includes striatal reinforcement learning based on egocentric representations of sensory states and actions, incidental Hebbian association of sensory information with allocentric state representations in the hippocampus, and arbitration of the outputs of both systems based on confidence/uncertainty in medial prefrontal cortex. We discuss the relationship between this architecture and learning in model-free and model-based systems, episodic memory, imagery, and planning, including some open questions and directions for further experiments.

The Anatomy and Physiology of Eyeblink Classical Conditioning

This chapter reviews the past research toward identifying the brain circuit and its computation underlying the associative memory in eyeblink classical conditioning. In the standard delay eyeblink conditioning paradigm, the conditioned stimulus (CS) and eyeblink-eliciting unconditioned stimulus (US) converge in the cerebellar cortex and interpositus nucleus (IPN) through the pontine nuclei and inferior olivary nucleus. Repeated pairings of CS and US modify synaptic weights in the cerebellar cortex and IPN, enabling IPN neurons to activate the red nucleus and generate the conditioned response (CR). In a variant of the standard paradigm, trace eyeblink conditioning, the CS and US are separated by a brief stimulus-free trace interval. Acquisition in trace eyeblink conditioning depends on several forebrain regions, including the hippocampus and medial prefrontal cortex as well as the cerebellar-brainstem circuit. Details of computations taking place in these regions remain unclear; however, recent evidence supports a view that the forebrain encodes a temporal sequence of the CS, trace interval, and US in a specific environmental context and signals the cerebellar-brainstem circuit to execute the CR when the US is likely to occur. Together, delay eyeblink conditioning represents one of the most successful cases of understanding the neural substrates of long-term memory in mammals, while trace eyeblink conditioning demonstrates its utility for uncovering detailed computations in the whole brain network underlying long-term memory.

Prefrontal-Hippocampal Interactions in Memory and Emotion

The hippocampal formation (HPC) and medial prefrontal cortex (mPFC) have well-established roles in memory encoding and retrieval. However, the mechanisms underlying interactions between the HPC and mPFC in achieving these functions is not fully understood. Considerable research supports the idea that a direct pathway from the HPC and subiculum to the mPFC is critically involved in cognitive and emotional regulation of mnemonic processes. More recently, evidence has emerged that an indirect pathway from the HPC to the mPFC via midline thalamic nucleus reuniens (RE) may plays a role in spatial and emotional memory processing. Here we will consider how bidirectional interactions between the HPC and mPFC are involved in working memory, episodic memory and emotional memory in animals and humans. We will also consider how dysfunction in bidirectional HPC-mPFC pathways contributes to psychiatric disorders.

Hippocampal-prefrontal input supports spatial encoding in working memory

Spatial working memory, the caching of behaviourally relevant spatial cues on a timescale of seconds, is a fundamental constituent of cognition. Although the prefrontal cortex and hippocampus are known to contribute jointly to successful spatial working memory, the anatomical pathway and temporal window for the interaction of these structures critical to spatial working memory has not yet been established. Here we find that direct hippocampal-prefrontal afferents are critical for encoding, but not for maintenance or retrieval, of spatial cues in mice. These cues are represented by the activity of individual prefrontal units in a manner that is dependent on hippocampal input only during the cue-encoding phase of a spatial working memory task. Successful encoding of these cues appears to be mediated by gamma-frequency synchrony between the two structures. These findings indicate a critical role for the direct hippocampal-prefrontal afferent pathway in the continuous updating of task-related spatial information during spatial working memory.

Long-range neural synchrony in behavior

Long-range synchrony between distant brain regions accompanies multiple forms of behavior. This review compares and contrasts the methods by which long-range synchrony is evaluated in both humans and model animals. Three examples of behaviorally relevant long-range synchrony are discussed in detail: gamma-frequency synchrony during visual perception, hippocampal-prefrontal synchrony during working memory, and prefrontal-amygdala synchrony during anxiety. Implications for circuit mechanism, translation, and clinical relevance are discussed.

Role of the thalamic nucleus reuniens in mediating interactions between the hippocampus and medial prefrontal cortex during spatial working memory

Despite decades of research, the neural mechanisms of spatial working memory remain poorly understood. Although the dorsal hippocampus is known to be critical for memory-guided behavior, experimental evidence suggests that spatial working memory depends not only on the hippocampus itself, but also on the circuit comprised of the hippocampus and the medial prefrontal cortex (mPFC). Disruption of hippocampal-mPFC interactions may result in failed transfer of spatial and contextual information processed by the hippocampus to the circuitry in mPFC responsible for decision making and goal-directed behavior. Oscillatory synchrony between the hippocampus and mPFC has been shown to increase in tasks with high spatial working memory demand. However, the mechanisms and circuitry supporting hippocampal-mPFC interactions during these tasks is unknown. The midline thalamic nucleus reuniens (RE) is reciprocally connected to both the hippocampus and the mPFC and has been shown to be critical for a variety of working memory tasks. Therefore, it is likely that hippocampal-mPFC oscillatory synchrony is modulated by RE activity. This article will review the anatomical connections between the hippocampus, mPFC and RE along with the behavioral studies that have investigated the effects of RE disruption on working memory task performance. The article will conclude with suggestions for future directions aimed at identifying the specific role of the RE in regulating functional interactions between the hippocampus and the PFC and investigating the degree to which these interactions contribute to spatial working memory.

Retroactive interference of object-in-context long-term memory: role of dorsal hippocampus and medial prefrontal cortex

Retroactive interference (RI) is a type of amnesia in which a new learning experience can impair the expression of a previous one. It has been studied in several types of memories for over a century. Here, we aimed to study in the long-term memory (LTM) formation of an object-in-context task, defined as the recognition of a familiar object in a context different to that in which it was previously encountered. We trained rats with two sample trials, each taking place in a different context in association with different objects. Test sessions were performed 24 h later, to evaluate LTM for both object-context pairs using separate groups of trained rats. Furthermore, given the involvement of hippocampus (Hp) and medial prefrontal cortex (mPFC) in several recognition memories, we also analyzed the participation of these structures in the LTM formation of this task by the local infusion of muscimol. Our results show that object-in-context LTM formation is sensitive to RI by a different either familiar or novel object-context pair trial, experienced 1 h later. This interference occurs in a restricted temporal window and works on the LTM consolidation phase, leaving intact short-term memory expression. The second sample trial did not affect the object recognition part of the memory. Besides, muscimol treatment before the second sample trial blocks its object-in-context LTM and restores the first sample trial memory. We hypothesized that LTM-RI amnesia is probably caused by resources or cellular machinery competition in these brain regions when they are engaged in memory formation of the traces. In sum, when two different object-in-context memory traces are being processed, the second trace interferes with the consolidation of the first one requiring mPFC and CA1 dorsal Hp activation.

Double dissociation between the contributions of the septal and temporal hippocampus to spatial learning: the role of prior experience

The mammalian hippocampus is anatomically heterogeneous along its longitudinal axis, and there is evidence that distinct functions are executed by different septotemporal subregions. The best documented example is the dependency of spatial learning on the septal, but not the temporal, hippocampus. Here, we carried out a watermaze memory task in rats with partial lesions of the septal or temporal hippocampus made either before or after training. We then studied memory retention, reversal, and new spatial learning in a novel environment. This resulted in the surprising finding that spatial learning in a new environment is dependent on the temporal hippocampus in rats with preoperative experience of a different pool. Rats with septal hippocampal lesions made after learning not only retained the focused search strategy that was acquired during preoperative training, but were also capable of rapid spatial learning in a second pool. This demonstrates that once spatial information has been acquired in one context, related new learning in a different context can be mediated by the temporal hippocampus, a result that challenges the widely held view that spatial memory is an exclusive function of the septal hippocampus.

Local and regional heterogeneity underlying hippocampal modulation of cognition and mood

While the hippocampus has been classically studied for its role in learning and memory, there is significant support for a role of the HPC in regulating emotional behavior. Emerging research suggests these functions may be segregated along the dorsoventral axis of the HPC. In addition to this regional heterogeneity, within the HPC, the dentate gyrus is one of two areas in the adult brain where stem cells continuously give rise to new neurons. This process can influence and be modulated by the emotional state of the animal, suggesting that adult neurogenesis within the DG may contribute to psychiatric disorders and cognitive abilities. Yet, the exact mechanism by which these newborn neurons influence behavior remains unknown. Here, we will examine the contribution of hippocampal neurogenesis to the output of the HPC, and suggest that the role of neurogenesis may vary along the DV axis. Next, we will review literature indicating that anatomical connectivity varies along the DV axis of the HPC, and that this underlies the functional segregation along this axis. This analysis will allow us to synthesize novel hypotheses for the differential contribution of the HPC to cognition and mood.

Learning-facilitated synaptic plasticity occurs in the intermediate hippocampus in association with spatial learning

The dorsoventral axis of the hippocampus is differentiated into dorsal, intermediate, and ventral parts. Whereas the dorsal part is believed to specialize in processing spatial information, the ventral may be equipped to process non-spatial information. The precise role of the intermediate hippocampus is unclear, although recent data suggests it is functionally distinct, at least from the dorsal hippocampus. Learning-facilitated synaptic plasticity describes the ability of hippocampal synapses to respond with robust synaptic plasticity (>24 h) when a spatial learning event is coupled with afferent stimulation that would normally not lead to a lasting plasticity response: in the dorsal hippocampus novel space facilitates robust expression of long-term potentiation (LTP), whereas novel spatial content facilitates long-term depression (LTD). We explored whether the intermediate hippocampus engages in this kind of synaptic plasticity in response to novel spatial experience. In freely moving rats, high-frequency stimulation at 200 Hz (3 bursts of 15 stimuli) elicited synaptic potentiation that lasted for at least 4 h. Coupling of this stimulation with the exploration of a novel holeboard resulted in LTP that lasted for over 24 h. Low frequency afferent stimulation (1 Hz, 900 pulses) resulted in short-term depression (STD) that was significantly enhanced and prolonged by exposure to a novel large orientational (landmark) cues, however LTD was not enabled. Exposure to a holeboard that included novel objects in the holeboard holes elicited a transient enhancement of STD of the population spike (PS) but not field EPSP, and also failed to facilitate the expression of LTD. Our data suggest that the intermediate dentate gyrus engages in processing of spatial information, but is functionally distinct to the dorsal dentate gyrus. This may in turn reflect their assumed different roles in synaptic information processing and memory formation.

The hippocampal-prefrontal pathway: the weak link in psychiatric disorders?

While the hippocampal formation and the prefrontal cortex each have a well-established role in cognitive and mnemonic processes, the extent and manner in which these structures interact to achieve these functions has not been fully delineated. Recent research in rodents compellingly supports the idea that the projection of neurons extending from the CA1 region of the hippocampus and from the subiculum to the prefrontal cortex, referred to here as the H-PFC pathway, is critically involved in aspects of cognition related to executive function and to emotional regulation. Concurrently, it is becoming evident that persons suffering from schizophrenia, depression, and post-traumatic stress disorder display structural anomalies and aberrant functional coupling within the hippocampal-prefrontal circuit. Considering that these disorders involve varying degrees of cognitive impairment and emotional dysregulation, dysfunction in the H-PFC pathway might therefore be the common element of their pathophysiology. This overlap might also be intertwined with the pathway's evident susceptibility to stress and with its relationship to the amygdala. In consequence, the H-PFC pathway is a potentially crucial element of the pathophysiology of several psychiatric diseases, and it offers a specific target for therapeutic intervention, which is consistent with the recent emphasis on reframing psychiatric diseases in terms of brain circuits.

Hippocampal signatures of episodic memory: evidence from single-unit recording studies

What hippocampal neural firing patterns signal memory and, more importantly, how is this memory code used by associated structures to translate a memory into a decision or action? Candidate hippocampal activity patterns will be discussed including (1) trajectory-specific firing of place cells with place fields on an overlapping segment of two (or more) distinct trajectories (2) prospective firing of hippocampal neurons that signal an upcoming event or action, and (3) place cell remapping to changes in environment and task. To date, there has not been compelling evidence for any of these activity patterns being the neural substrate of episodic memory. New findings suggest that learning and memory processes are emergent properties of interregional interactions and not localized within any one discrete brain region. Therefore, the next step in understanding how remapping and trajectory coding participate in memory coding may be to investigate how these activity patterns relate to activity in anatomically connected structures such as the prefrontal cortex.

Modeling of grid cell activity demonstrates in vivo entorhinal 'look-ahead' properties

Recent in vivo data show ensemble activity in medial entorhinal neurons that demonstrates 'look-ahead' activity, decoding spatially to reward locations ahead of a rat deliberating at a choice point while performing a cued, appetitive T-Maze task. To model this experiment's look-ahead results, we adapted previous work that produced a model where scans along equally probable directions activated place cells, associated reward cells, grid cells, and persistent spiking cells along those trajectories. Such look-ahead activity may be a function of animals performing scans to reduce ambiguity while making decisions. In our updated model, look-ahead scans at the choice point can activate goal-associated reward and place cells, which indicate the direction the virtual rat should turn at the choice point. Hebbian associations between stimulus and reward cell layers are learned during training trials, and the reward and place layers are then used during testing to retrieve goal-associated cells based on cue presentation. This system creates representations of location and associated reward information based on only two inputs of heading and speed information which activate grid cell and place cell layers. We present spatial and temporal decoding of grid cell ensembles as rats are tested with perfect and imperfect stimuli. Here, the virtual rat reliably learns goal locations through training sessions and performs both biased and unbiased look-ahead scans at the choice point. Spatial and temporal decoding of simulated medial entorhinal activity indicates that ensembles are representing forward reward locations when the animal deliberates at the choice point, emulating in vivo results.

Prefrontal cortex focally modulates hippocampal place cell firing patterns

Previous work shows that medial prefrontal cortex (mPFC) cells exhibit spatio-selective activity at a goal location when rats are trained in a goal-oriented navigation task. Damaging the ventral and intermediate hippocampal regions severely disrupts both mPFC goal firing and behavioral performance in the same task. Additionally, hippocampal place cells tend to develop a secondary place field at the goal location, suggesting that goal locations can be encoded by local changes in firing rate, within an otherwise stable spatial representation. Therefore, it has been suggested that the coordinated activity of a large fraction of hippocampal cells at the goal location may interact with the mPFC to compute accurate planning trajectories, relying on both precise location-specific firing of place cells and the coarse-coded, goal-trajectory planning function of the prefrontal cortex. To test this hypothesis, we inactivated the mPFC and recorded hippocampal place cell activity while animals were performing the navigation task. The results show that post-training inactivation of the prefrontal cortex does not affect behavioral performance, suggesting that this structure is no longer required when animals are overtrained. The goal-related activity of place cells was not affected at either single unit or local field potential level. Conversely, profound modifications of place cell firing variability (overdispersion) were observed after suppression of prefrontal input, suggesting a possible mechanism underlying behavioral flexibility.

The role of medial prefrontal cortex in memory and decision making

Some have claimed that the medial prefrontal cortex (mPFC) mediates decision making. Others suggest mPFC is selectively involved in the retrieval of remote long-term memory. Yet others suggests mPFC supports memory and consolidation on time scales ranging from seconds to days. How can all these roles be reconciled? We propose that the function of the mPFC is to learn associations between context, locations, events, and corresponding adaptive responses, particularly emotional responses. Thus, the ubiquitous involvement of mPFC in both memory and decision making may be due to the fact that almost all such tasks entail the ability to recall the best action or emotional response to specific events in a particular place and time. An interaction between multiple memory systems may explain the changing importance of mPFC to different types of memories over time. In particular, mPFC likely relies on the hippocampus to support rapid learning and memory consolidation.

The hippocampo-cortical loop: spatio-temporal learning and goal-oriented planning in navigation

We present a neural network model where the spatial and temporal components of a task are merged and learned in the hippocampus as chains of associations between sensory events. The prefrontal cortex integrates this information to build a cognitive map representing the environment. The cognitive map can be used after latent learning to select optimal actions to fulfill the goals of the animal. A simulation of the architecture is made and applied to learning and solving tasks that involve both spatial and temporal knowledge. We show how this model can be used to solve the continuous place navigation task, where a rat has to navigate to an unmarked goal and wait for 2 seconds without moving to receive a reward. The results emphasize the role of the hippocampus for both spatial and timing prediction, and the prefrontal cortex in the learning of goals related to the task.

The role of medial prefrontal cortex in memory and decision making

Some have claimed that the medial prefrontal cortex (mPFC) mediates decision making. Others suggest mPFC is selectively involved in the retrieval of remote long-term memory. Yet others suggests mPFC supports memory and consolidation on time scales ranging from seconds to days. How can all these roles be reconciled? We propose that the function of the mPFC is to learn associations between context, locations, events, and corresponding adaptive responses, particularly emotional responses. Thus, the ubiquitous involvement of mPFC in both memory and decision making may be due to the fact that almost all such tasks entail the ability to recall the best action or emotional response to specific events in a particular place and time. An interaction between multiple memory systems may explain the changing importance of mPFC to different types of memories over time. In particular, mPFC likely relies on the hippocampus to support rapid learning and memory consolidation.

A goal-directed spatial navigation model using forward trajectory planning based on grid cells

A goal-directed navigation model is proposed based on forward linear look-ahead probe of trajectories in a network of head direction cells, grid cells, place cells and prefrontal cortex (PFC) cells. The model allows selection of new goal-directed trajectories. In a novel environment, the virtual rat incrementally creates a map composed of place cells and PFC cells by random exploration. After exploration, the rat retrieves memory of the goal location, picks its next movement direction by forward linear look-ahead probe of trajectories in several candidate directions while stationary in one location, and finds the one activating PFC cells with the highest reward signal. Each probe direction involves activation of a static pattern of head direction cells to drive an interference model of grid cells to update their phases in a specific direction. The updating of grid cell spiking drives place cells along the probed look-ahead trajectory similar to the forward replay during waking seen in place cell recordings. Directions are probed until the look-ahead trajectory activates the reward signal and the corresponding direction is used to guide goal-finding behavior. We report simulation results in several mazes with and without barriers. Navigation with barriers requires a PFC map topology based on the temporal vicinity of visited place cells and a reward signal diffusion process. The interaction of the forward linear look-ahead trajectory probes with the reward diffusion allows discovery of never-before experienced shortcuts towards a goal location.

Excess folate during adolescence suppresses thyroid function with permanent deficits in motivation and spatial memory

Cognitive and memory deficits can be caused or exacerbated by dietary folate deficiency, which has been combatted by the addition of folate to grains and dietary supplements. The recommended dose of the B9 vitamin folate is 400 µg/day for adolescents and non-pregnant adults, and consumption above the recommended daily allowance is not considered to be detrimental. However, the effects of excess folate have not been tested in adolescence when neuro and endocrine development suggest possible vulnerability to long-term cognitive effects. We administered folate-supplemented (8.0 mg folic acid/kg diet) or control lab chow (2.7 mg folic acid/kg diet) to rats ad libitum from 30 to 60 days of age, and subsequently tested their motivation and learning and memory in the Morris water maze. We found that folate-supplemented animals had deficits in motivation and spatial memory, but they showed no changes of the learning- and memory-related molecules growth-associated protein-43 or Gs-α subunit protein in the hippocampus. They had decreased levels of thyroxine (T4) and triiodothyronine (T3) in the periphery and decreased protein levels of thyroid receptor-α1 and -α2 (TRα1 and TRα2) in the hippocampus. The latter may have been due to an observed increase of cytosine-phosphate-guanosine island methylation within the putative thyroid hormone receptor-α promoter, which we have mapped for the first time in the rat. Overall, folate supplementation in adolescence led to motivational and spatial memory deficits that may have been mediated by suppressed thyroid hormone function in the periphery and hippocampus.

Single units in the medial prefrontal cortex with anxiety-related firing patterns are preferentially influenced by ventral hippocampal activity

The medial prefrontal cortex (mPFC) and ventral hippocampus (vHPC) functionally interact during innate anxiety tasks. To explore the consequences of this interaction, we examined task-related firing of single units from the mPFC of mice exploring standard and modified versions of the elevated plus maze (EPM), an innate anxiety paradigm. Hippocampal local field potentials (LFPs) were simultaneously monitored. The population of mPFC units distinguished between safe and aversive locations within the maze, regardless of the nature of the anxiogenic stimulus. Strikingly, mPFC units with stronger task-related activity were more strongly coupled to theta-frequency activity in the vHPC LFP. Lastly, task-related activity was inversely correlated with behavioral measures of anxiety. These results clarify the role of the vHPC-mPFC circuit in innate anxiety and underscore how specific inputs may be involved in the generation of behaviorally relevant neural activity within the mPFC.

Gene regulation in the rat prefrontal cortex after learning with or without cholinergic insult

The prefrontal cortex is essential for a wide variety of higher functions, including attention and memory. Cholinergic neurons are thought to be of prime importance in the modulation of these processes. Degeneration of forebrain cholinergic neurons has been linked to several neurological disorders. The present study was designed to identify genes and networks in rat prefrontal cortex that are associated with learning and cholinergic-loss-memory deficit. Affymetrix microarray technology was used to screen gene expression changes in rats submitted or not to 192 IgG-saporin immunolesion of cholinergic basal forebrain and trained in spatial/object novelty tasks. Results showed learning processes were associated with significant expression of genes, which were organized in several clusters of highly correlated genes and would be involved in biological processes such as intracellular signaling process, transcription regulation, and filament organization and axon guidance. Memory loss following cortical cholinergic deafferentation was associated with significant expression of genes belonging to only one clearly delineated cluster and would be involved in biological processes related to cytoskeleton organization and proliferation, and glial and vascular remodeling, i.e., in processes associated with brain repair after injury.