Functional neuroanatomy of the parahippocampal region: The lateral and medial entorhinal areas

Complementary Functional Organization of Neuronal Activity Patterns in the Perirhinal, Lateral Entorhinal, and Medial Entorhinal Cortices

It is commonly conceived that the cortical areas of the hippocampal region are functionally divided into the perirhinal cortex (PRC) and the lateral entorhinal cortex (LEC), which selectively process object information; and the medial entorhinal cortex (MEC), which selectively processes spatial information. Contrary to this notion, in rats performing a task that demands both object and spatial information processing, single neurons in PRC, LEC, and MEC, including those in both superficial and deep cortical areas and in grid, border, and head direction cells of MEC, have a highly similar range of selectivity to object and spatial dimensions of the task. By contrast, representational similarity analysis of population activity reveals a key distinction in the organization of information in these areas, such that PRC and LEC populations prioritize object over location information, whereas MEC populations prioritize location over object information. These findings bring to the hippocampal system a growing emphasis on population analyses as a powerful tool for characterizing neural representations supporting cognition and memory.

SIGNIFICANCE STATEMENT

Contrary to the common view that brain regions in the "what" and "where" streams distinctly process object and spatial cues, respectively, we found that both streams encode both object and spatial information but distinctly organize memories for objects and space. Specifically, perirhinal cortex and lateral entorhinal cortex represent objects and, within the object-specific representations, the locations where they occur. Conversely, medial entorhinal cortex represents relevant locations and, within those spatial representations, the objects that occupy them. Furthermore, these findings reach beyond simple notions of perirhinal cortex and lateral entorhinal cortex neurons as object detectors and MEC neurons as position detectors, and point to a more complex organization of memory representations within the medial temporal lobe system.

Excitatory Postrhinal Projections to Principal Cells in the Medial Entorhinal Cortex

The postrhinal cortex (POR) provides substantial input to the entorhinal cortex, mainly targeting superficial layers of the medial entorhinal cortex (MEC). Major inputs to POR originate in the visual and parietal cortex, thus providing neurons in MEC with a subset of cortical information relevant to their spatial firing properties. The POR takes a position that is comparable with that of the perirhinal cortex (PER) with regard to the lateral entorhinal cortex (LEC). Neurons in LEC and MEC show different functional properties likely reflecting differences in their respective inputs. Projections from PER to LEC exert a main inhibitory influence, which may relate to the sparse object-selective firing in LEC. In view of the continuous, spatially modulated firing properties of principal neurons in MEC, we tested in rats the hypothesis that projections from POR to MEC are functionally different from the PER-to-LEC counterpart in providing an excitatory drive to MEC. Our combined confocal and quantitative electron-microscopic observations indicated that POR projections target mainly principal cells in MEC, including neurons that project to the hippocampus. The ultrastructure of the majority of the synapses indicated that they are excitatory. Voltage-sensitive dye imaging in sagittal slices confirmed this morphologically derived conclusion, showing that the MEC network always responded with an overall depolarization, indicative for net excitatory transmission. In vitro single-cell recordings from principal cells showed only excitatory responses upon POR stimulation. These results show that POR provides an excitatory projection to MEC, differing fundamentally from the inhibitory projection of PER to LEC.

Head-Directional Tuning and Theta Modulation of Anatomically Identified Neurons in the Presubiculum

The presubiculum provides a major input to the medial entorhinal cortex (MEC) and contains cells that encode for the animal's head direction (HD), as well as other cells likely to be important for navigation and memory, including grid cells. To understand the mechanisms underlying HD cell firing and its effects on other parts of the circuit, it is important to determine the anatomical identity of these functionally defined cells. Therefore, we juxtacellularly recorded single cells in the presubiculum in freely moving rats, finding two classes of cells based on firing patterns and juxtacellular labeling (of a subset). Regular-firing cells had the anatomical characteristics of pyramidal cells and included most recorded HD cells. Therefore, HD cells are likely to be excitatory pyramidal cells. For one HD cell, we could follow an axon projecting directly to the MEC. Fast-spiking (FS) cells had the anatomical characteristics of interneurons and displayed weak HD tuning. Furthermore, FS cells displayed a surprising lack of theta-rhythmic firing, in strong contrast to the FS cells that we recorded in the MEC. Overall, we show that HD cells in the presubiculum are pyramidal cells, with FS interneurons only showing weak HD tuning; therefore, MEC may receive an excitatory HD input, as previously assumed by many models. The lack of theta rhythmicity in FS interneurons suggests that different mechanisms may underlie theta in different parts of the hippocampal formation.

SIGNIFICANCE STATEMENT

In freely moving rats, we recorded and labeled single neurons in the presubiculum, an area providing one of the major inputs to the medial entorhinal cortex and part of a network involved in spatial navigation and memory. Post hoc identification of labeled cells showed that (fast-spiking, FS) interneurons and pyramidal cells in the presubiculum can be distinguished based on physiological criteria. We found that both moderately and strongly tuned head-direction (HD) cells are pyramidal cells and therefore likely to provide an excitatory HD input to the entorhinal cortex. FS interneurons were weakly head directional and, surprisingly, showed no theta-rhythmic firing. Therefore, the presubiculum appears to encode HD information via excitatory pyramidal cells, possibly also involving FS interneurons, without using a theta-rhythmic temporal code.

Unilateral Resection of the Anterior Medial Temporal Lobe Impairs Odor Identification and Valence Perception

The anterior medial temporal lobe (TL), including the amygdala, has been implicated in olfactory processing, e.g., coding for intensity and valence, and seems also involved in memory. With this background, the present study evaluated whether anterior medial TL-resections in TL epilepsy affected intensity and valence ratings, as well as free and cued identification of odors. These aspects of odor perception were assessed in 31 patients with unilateral anterior medial TL-resections (17 left, 14 right) and 16 healthy controls. Results suggest that the anterior medial TL is in particular necessary for free, but also cued, odor identification. TL resection was also found to impair odor valence, but not intensity ratings. Left resected patients rated nominally pleasant and unpleasant odors as more neutral suggesting a special role for the left anterior TL in coding for emotional saliency in response to odors.

Neural Activity Patterns Underlying Spatial Coding in the Hippocampus

The hippocampus is well known as a central site for memory processing-critical for storing and later retrieving the experiences events of daily life so they can be used to shape future behavior. Much of what we know about the physiology underlying hippocampal function comes from spatial navigation studies in rodents, which have allowed great strides in understanding how the hippocampus represents experience at the cellular level. However, it remains a challenge to reconcile our knowledge of spatial encoding in the hippocampus with its demonstrated role in memory-dependent tasks in both humans and other animals. Moreover, our understanding of how networks of neurons coordinate their activity within and across hippocampal subregions to enable the encoding, consolidation, and retrieval of memories is incomplete. In this chapter, we explore how information may be represented at the cellular level and processed via coordinated patterns of activity throughout the subregions of the hippocampal network.

The Role of the Human Entorhinal Cortex in a Representational Account of Memory

Connectivity studies in animals form the basis for a representational view of medial temporal lobe (MTL) subregions. In this view, distinct subfields of the entorhinal cortex (EC) relay object-related and spatial information from the perirhinal and parahippocampal cortices (PRC, PHC) to the hippocampus (HC). Relatively recent advances in functional magnetic resonance imaging (fMRI) methodology allow examining properties of human EC subregions directly. Antero-lateral and posterior-medial EC subfields show remarkable consistency to their putative rodent and nonhuman primate homologs with regard to intra- and extra-MTL functional connectivity. Accordingly, there is now evidence for a dissociation of object-related vs. spatial processing in human EC subfields. Here, variance in localization may be integrated in the antero-lateral vs. posterior-medial distinction, but may additionally reflect process differences. Functional results in rodents further suggest material-specific representations may be more integrated in EC compared to PRC/PHC. In humans, however, evidence for such a dissociation between EC and PRC/PHC is lacking. Future research may elucidate on the unique contributions of human EC to memory, especially in light of its high degree of intrinsic and extrinsic connectivity. A thorough characterization of EC subfield function may not only advance our understanding of human memory, but also have important clinical implications.

Differences in Visual-Spatial Input May Underlie Different Compression Properties of Firing Fields for Grid Cell Modules in Medial Entorhinal Cortex

Firing fields of grid cells in medial entorhinal cortex show compression or expansion after manipulations of the location of environmental barriers. This compression or expansion could be selective for individual grid cell modules with particular properties of spatial scaling. We present a model for differences in the response of modules to barrier location that arise from different mechanisms for the influence of visual features on the computation of location that drives grid cell firing patterns. These differences could arise from differences in the position of visual features within the visual field. When location was computed from the movement of visual features on the ground plane (optic flow) in the ventral visual field, this resulted in grid cell spatial firing that was not sensitive to barrier location in modules modeled with small spacing between grid cell firing fields. In contrast, when location was computed from static visual features on walls of barriers, i.e. in the more dorsal visual field, this resulted in grid cell spatial firing that compressed or expanded based on the barrier locations in modules modeled with large spacing between grid cell firing fields. This indicates that different grid cell modules might have differential properties for computing location based on visual cues, or the spatial radius of sensitivity to visual cues might differ between modules.

How do we navigate from one location to another and how do we represent space to accomplish this task? Researchers have collected data from the entorhinal cortex in rodents to answer these questions, finding grid cells that fire whenever a rodent traverses through an array of locations falling on the vertices of tightly packed equilateral triangles. Grid cells with large spacing (large side lengths of the triangles between firing fields) are distorted when the environment is manipulated, e.g. by pushing walls or inserting walls in a box. In contrast, grid cells of small spacing remain largely unaffected by such manipulations. We present a computational model to explain this behavior of grid cells. In our model information about the motion of features on the ground, which are unaffected by wall manipulations, drive grid cells with small spacing between firing fields, while static features like landmarks, which are affected by wall manipulations, drive grid cells with large spacing between firing fields. These differences could correspond to different positions within the visual field of the animal. This model puts forth a testable hypothesis about the type of features that drive grid cells of different spacing.

Hierarchical organization of long-range circuits in the olfactory cortices

How sensory information is processed within olfactory cortices is unclear. Here, we examined long-range circuit wiring between different olfactory cortical regions of acute mouse brain slices using a channelrhodopsin-2 (ChR2)-based neuronal targeting approach. Our results provide detailed information regarding the synaptic properties of the reciprocal long-range monosynaptic glutamatergic projections (LRMGP) between and within anterior piriform cortex (aPC), posterior piriform cortex (pPC), and lateral entorhinal cortex (LEC), thereby creating a long-range inter- and intracortical circuit diagrams at the level of synapses and single cortical neurons. Our results reveal the following information regarding hierarchical intra- and intercortical organizations: (i) there is massive bottom-up (i.e., rostral-caudal) excitation within the LRMGP accompanied with strong feedforward (FF) inhibition; (ii) there are convergent FF connections onto LEC from both aPC and pPC; (iii) feedback (FB) intercortical connections are weak with a significant fraction of presumptive silent synapses; and (iv) intra and intercortical long-range connections lack layer specificity and their innervation of interneurons are stronger than neighboring pyramidal neurons. The elucidation of the distinct hierarchical organization of long-range olfactory cortical circuits paves the way for further understanding of higher order cortical processing within the olfactory system.

Extinction of Learned Fear Induces Hippocampal Place Cell Remapping

The extinction of learned fear is a hippocampus-dependent process thought to embody new learning rather than erasure of the original fear memory, although it is unknown how these competing contextual memories are represented in the hippocampus. We previously demonstrated that contextual fear conditioning results in hippocampal place cell remapping and long-term stabilization of novel representations. Here we report that extinction learning also induces place cell remapping in C57BL/6 mice. Specifically, we observed cells that preferentially remapped during different stages of learning. While some cells remapped in both fear conditioning and extinction, others responded predominantly during extinction, which may serve to modify previous representations as well as encode new safe associations. Additionally, we found cells that remapped primarily during fear conditioning, which could facilitate reacquisition of the original fear association. Moreover, we also observed cells that were stable throughout learning, which may serve to encode the static aspects of the environment. The short-term remapping observed during extinction was not found in animals that did not undergo fear conditioning, or when extinction was conducted outside of the conditioning context. Finally, conditioning and extinction produced an increase in spike phase locking to the theta and gamma frequencies. However, the degree of remapping seen during conditioning and extinction only correlated with gamma synchronization. Our results suggest that the extinction learning is a complex process that involves both modification of pre-existing memories and formation of new ones, and these traces coexist within the same hippocampal representation.

Activation of Phosphatidylinositol-Linked Dopamine Receptors Induces a Facilitation of Glutamate-Mediated Synaptic Transmission in the Lateral Entorhinal Cortex

The lateral entorhinal cortex receives strong inputs from midbrain dopamine neurons that can modulate its sensory and mnemonic function. We have previously demonstrated that 1 µM dopamine facilitates synaptic transmission in layer II entorhinal cortex cells via activation of D1-like receptors, increased cAMP-PKA activity, and a resulting enhancement of AMPA-receptor mediated currents. The present study assessed the contribution of phosphatidylinositol (PI)-linked D1 receptors to the dopaminergic facilitation of transmission in layer II of the rat entorhinal cortex, and the involvement of phospholipase C activity and release of calcium from internal stores. Whole-cell patch-clamp recordings of glutamate-mediated evoked excitatory postsynaptic currents were obtained from pyramidal and fan cells. Activation of D1-like receptors using SKF38393, SKF83959, or 1 µM dopamine induced a reversible facilitation of EPSCs which was abolished by loading cells with either the phospholipase C inhibitor U-73122 or the Ca2+ chelator BAPTA. Neither the L-type voltage-gated Ca2+ channel blocker nifedipine, nor the L/N-type channel blocker cilnidipine, blocked the facilitation of synaptic currents. However, the facilitation was blocked by blocking Ca2+ release from internal stores via inositol 1,4,5-trisphosphate (InsP3) receptors or ryanodine receptors. Follow-up studies demonstrated that inhibiting CaMKII activity with KN-93 failed to block the facilitation, but that application of the protein kinase C inhibitor PKC(19-36) completely blocked the dopamine-induced facilitation. Overall, in addition to our previous report indicating a role for the cAMP-PKA pathway in dopamine-induced facilitation of synaptic transmission, we demonstrate here that the dopaminergic facilitation of synaptic responses in layer II entorhinal neurons also relies on a signaling cascade dependent on PI-linked D1 receptors, PLC, release of Ca2+ from internal stores, and PKC activation which is likely dependent upon both DAG and enhanced intracellular Ca2+. These signaling pathways may collaborate to enhance sensory and mnemonic function in the entorhinal cortex during tonic release of dopamine.

Effect of dentate gyrus disruption on remembering what happened where

Our previous studies using Bax knockout (Bax-KO) mice, in which newly generated granule cells continue to accumulate, disrupting neural circuitry specifically in the dentate gyrus (DG), suggest the involvement of the DG in binding the internally-generated spatial map with sensory information on external landmarks (spatial map-object association) in forming a distinct spatial context for each environment. In order to test whether the DG is also involved in binding the internal spatial map with sensory information on external events (spatial map-event association), we tested the behavior of Bax-KO mice in a delayed-non-match-to-place task. Performance of Bax-KO mice was indistinguishable from that of wild-type mice as long as there was no interruption during the delay period (tested up to 5 min), suggesting that on-line maintenance of working memory is intact in Bax-KO mice. However, Bax-KO mice showed profound performance deficits when they were removed from the maze during the delay period (interruption condition) with a sufficiently long (65 s) delay, suggesting that episodic memory was impaired in Bax-KO mice. Together with previous findings, these results suggest the role of the DG in binding spatial information derived from dead reckoning and nonspatial information, such as external objects and events, in the process of encoding episodic memory.

Functional topography of the human entorhinal cortex

Despite extensive research on the role of the rodent medial and lateral entorhinal cortex (MEC/LEC) in spatial navigation, memory and related disease, their human homologues remain elusive. Here, we combine high-field functional magnetic resonance imaging at 7 T with novel data-driven and model-based analyses to identify corresponding subregions in humans based on the well-known global connectivity fingerprints in rodents and sensitivity to spatial and non-spatial information. We provide evidence for a functional division primarily along the anteroposterior axis. Localising the human homologue of the rodent MEC and LEC has important implications for translating studies on the hippocampo-entorhinal memory system from rodents to humans.

DOI:http://dx.doi.org/10.7554/eLife.06738.001

In the early 1950s, an American named Henry Molaison underwent an experimental type of brain surgery to treat his severe epilepsy. The surgeon removed a region of the brain known as the temporal lobe from both sides of his brain. After the surgery, Molaison's epilepsy was greatly improved, but he was also left with a profound amnesia, unable to form new memories of recent events.

Subsequent experiments, including many with Molaison himself as a subject, have attempted to identify the roles of the various structures within the temporal lobes. The hippocampus—which is involved in memory and spatial navigation—has received the most attention, but in recent years a region called the entorhinal cortex has also come to the fore. Known as the gateway to the hippocampus, the entorhinal cortex relays sensory information from the outer cortex of the brain to the hippocampus.

In rats and mice the entorhinal cortex can be divided into two subregions that have distinct connections to other parts of the temporal lobe and to the rest of the brain. These are the medial entorhinal cortex, which is the subregion nearest the centre of the brain, and the lateral entorhinal cortex, which is to the left or right of the centre.

For many years researchers had assumed that human entorhinal subregions were located simply to the center or to the sides of the brain. However, it was difficult to check this as the entorhinal cortex measures less than 1 cm across, which placed it beyond the reach of most brain-imaging techniques. Now, two independent groups of researchers have used a technique called functional magnetic resonance imaging to show a different picture. The fMRI data—which were collected in a magnetic field of 7 Tesla, rather than the 1.5 Tesla used in previous experiments—reveal that the entorhinal cortex is predominantly divided from front-to-back in humans.

One of the groups—Navarro Schröder, Haak et al.—used three different sets of functional MRI data to show that the human entorhinal cortex has anterior-lateral and posterior-medial subregions. In one of these experiments, functional MRI was used to measure activity across the whole brain as subjects performed a virtual reality task: this task included some components that involved spatial navigation and other components that did not. The other group—Maass, Berron et al.—used the imaging data to show that the pattern of connections between the anterior-lateral subregion and the hippocampus was different to that between the posterior-medial subregion and the hippocampus.

The discovery of these networks in the temporal lobe in humans will help to bridge the gap between studies of memory in rodents and in humans. Given that the lateral entorhinal cortex is one of the first regions to be affected in Alzheimer's disease, identifying the specific properties and roles of these networks could also provide insights into disease mechanisms.

DOI:http://dx.doi.org/10.7554/eLife.06738.002

Pronounced differences in signal processing and synaptic plasticity between piriform-hippocampal network stages: a prominent role for adenosine

KEY POINTS

Extended trains of theta rhythm afferent activity lead to a biphasic response facilitation in field CA1 but not in the lateral perforant path input to the dentate gyrus. Processes that reverse long-term potentiation in field CA1 are not operative in the lateral perforant path: multiple lines of evidence indicate that this reflects differences in adenosine signalling. Adenosine A1 receptors modulate baseline synaptic transmission in the lateral olfactory tract but not the associational afferents of the piriform cortex. Levels of ecto-5'-nucleotidase (CD73), an enzyme that converts extracellular ATP into adenosine, are markedly different between regions and correlate with adenosine signalling and the efficacy of theta pulse stimulation in reversing long-term potentiation. Variations in transmitter mobilization, CD73 levels, and afferent divergence result in multivariate differences in signal processing through nodes in the cortico-hippocampal network.

ABSTRACT

The present study evaluated learning-related synaptic operations across the serial stages of the olfactory cortex-hippocampus network. Theta frequency stimulation produced very different time-varying responses in the Schaffer-commissural projections than in the lateral perforant path (LPP), an effect associated with distinctions in transmitter mobilization. Long-term potentiation (LTP) had a higher threshold in LPP field potential studies but not in voltage clamped neurons; coupled with input/output relationships, these results suggest that LTP threshold differences reflect the degree of input divergence. Theta pulse stimulation erased LTP in CA1 but not in the dentate gyrus (DG), although adenosine eliminated potentiation in both areas, suggesting that theta increases extracellular adenosine to a greater degree in CA1. Moreover, adenosine A1 receptor antagonism had larger effects on theta responses in CA1 than in the DG, and concentrations of ecto-5'-nucleotidase (CD73) were much higher in CA1. Input/output curves for two connections in the piriform cortex were similar to those for the LPP, whereas adenosine modulation again correlated with levels of CD73. In sum, multiple relays in a network extending from the piriform cortex through the hippocampus can be differentiated along three dimensions (input divergence, transmitter mobilization, adenosine modulation) that potently influence throughput and plasticity. A model that incorporates the regional differences, supplemented with data for three additional links, suggests that network output goes through three transitions during the processing of theta input. It is proposed that individuated relays allow the circuit to deal with different types of behavioural problems.

Fluorescent-protein stabilization and high-resolution imaging of cleared, intact mouse brains

In order to observe and quantify long-range neuronal connections in intact mouse brain by light microscopy, it is first necessary to clear the brain, thus suppressing refractive-index variations. Here we describe a method that clears the brain and preserves the signal from proteinaceous fluorophores using a pH-adjusted non-aqueous index-matching medium. Successful clearing is enabled through the use of either 1-propanol or tert-butanol during dehydration whilst maintaining a basic pH. We show that high-resolution fluorescence imaging of entire, structurally intact juvenile and adult mouse brains is possible at subcellular resolution, even following many months in clearing solution. We also show that axonal long-range projections that are EGFP-labelled by modified Rabies virus can be imaged throughout the brain using a purpose-built light-sheet fluorescence microscope. To demonstrate the viability of the technique, we determined a detailed map of the monosynaptic projections onto a target cell population in the lateral entorhinal cortex. This example demonstrates that our method permits the quantification of whole-brain connectivity patterns at the subcellular level in the uncut brain.

Cholinergic, but not NMDA, receptors in the lateral entorhinal cortex mediate acquisition in trace eyeblink conditioning

Anatomical and electrophysiological studies collectively suggest that the entorhinal cortex consists of several subregions, each of which is involved in the processing of different types of information. Consistent with this idea, we previously reported that the dorsolateral portion of the entorhinal cortex (DLE), but not the caudomedial portion, is necessary for the expression of a memory association between temporally discontiguous stimuli in trace eyeblink conditioning (Morrissey et al. (2012) J Neurosci 32:5356-5361). The present study examined whether memory acquisition depends on the DLE and what types of local neurotransmitter mechanisms are involved in memory acquisition and expression. Male Long-Evans rats experienced trace eyeblink conditioning, in which an auditory conditioned stimulus (CS) was paired with a mildly aversive electric shock to the eyelid (US) with a stimulus-free interval of 500 ms. Immediately before the conditioning, the rats received a microinfusion of neuroreactive substances into the DLE. We found that reversible inactivation of the DLE with GABAA receptor agonist, muscimol impaired memory acquisition. Furthermore, blockade of local muscarinic acetylcholine receptors (mACh) with scopolamine retarded memory acquisition while blockade of local NMDA receptors with APV had no effect. Memory expression was not impaired by either type of receptor blocker. These results suggest that the DLE is necessary for memory acquisition, and that acquisition depends on the integrity of local mACh receptor-dependent firing modulation, but not NMDA receptor-dependent synaptic plasticity.

Theta variation and spatiotemporal scaling along the septotemporal axis of the hippocampus

Hippocampal theta has been related to locomotor speed, attention, anxiety, sensorimotor integration and memory among other emergent phenomena. One difficulty in understanding the function of theta is that the hippocampus (HPC) modulates voluntary behavior at the same time that it processes sensory input. Both functions are correlated with characteristic changes in theta indices. The current review highlights a series of studies examining theta local field potential (LFP) signals across the septotemporal or longitudinal axis of the HPC. While the theta signal is coherent throughout the entirety of the HPC, the amplitude, but not the frequency, of theta varies significantly across its three-dimensional expanse. We suggest that the theta signal offers a rich vein of information about how distributed neuronal ensembles support emergent function. Further, we speculate that emergent function across the long axis varies with respect to spatiotemporal scale. Thus, septal HPC processes details of the proximal spatiotemporal environment while more temporal aspects process larger spaces and wider time-scales. The degree to which emergent functions are supported by the synchronization of theta across the septotemporal axis is an open question. Our working model is that theta synchrony serves to bind ensembles representing varying resolutions of spatiotemporal information at interdependent septotemporal areas of the HPC. Such synchrony and cooperative interactions along the septotemporal axis likely support memory formation and subsequent consolidation and retrieval.

Insular projections to the parahippocampal region in the rat

The insular cortex is involved in the perception of interoceptive signals, coding of emotional and affective states, and processing information from gustatory, olfactory, auditory, somatosensory, and nociceptive modalities. This information represents an important component of episodic memory, mediated by the parahippocampal-hippocampal region. A comprehensive description of insular projections to the latter region is lacking. Previous studies reported that insular projections do not target any of the subdivisions in the hippocampal formation (the dentate gyus, the cornu ammonis [CA] fields 1, 2, and 3 and the subiculum), but, in contrast, target the parahippocampal region (perirhinal, postrhinal, lateral and medial entorhinal cortices, and pre- and parasubiculum). The present study examined the topographical and laminar organization of insular projections to the parahippocampal region in the rat with the use of anterograde tracing. Notably, our results corroborated the absence of hippocampal projections. We further showed that the perirhinal and the lateral entorhinal cortices received extensive projections from the insular cortex, primarily from its agranular areas. With the exception of a weak projection to the postrhinal cortex, projections to the remaining parahippocampal areas were either absent or very sparse. The projections to the lateral entorhinal cortex displayed a preference for the deep layers of its most lateral subdivisions, known also to receive hippocampal inputs. Projections to the perirhinal cortex primarily targeted the superficial layers with a preference for its ventral subdivision, referred to as area 35. Our findings indicate that only processed information, reflecting emotional and affective states, but not primary gustatory and viscerosensory information, has direct access to the parahippocampal-hippocampal system.

Superadditive opercular activation to food flavor is mediated by enhanced temporal and limbic coupling

Food perception is characterized by a transition from initially separate sensations of the olfactory and gustatory properties of the object toward their combined sensory experience during consumption. The holistic flavor experience, which occurs as the smell and taste merge, extends beyond the mere addition of the two chemosensory modalities, being usually perceived as more object-like, intense and rewarding. To explore the cortical mechanisms which give rise to olfactory-gustatory binding during natural food consumption, brain activation during consumption of a pleasant familiar beverage was contrasted with presentation of its taste and orthonasal smell alone. Convergent activation to all presentation modes was observed in executive and chemosensory association areas. Flavor, but not orthonasal smell or taste alone, stimulated the frontal operculum, supporting previous accounts of its central role in the formation of the flavor percept. A functional dissociation was observed in the insula: the anterior portion was characterized by sensory convergence, while mid-dorsal sections activated exclusively to the combined flavor stimulus. psycho-physiological interaction analyses demonstrated increased neural coupling between the frontal operculum and the anterior insula during flavor presentation. Connectivity was also increased with the lateral entorhinal cortex, a relay to memory networks and central node for contextual modulation of olfactory processing. These findings suggest a central role of the insular cortex in the transition from mere detection of chemosensory convergence to a superadditive flavor representation. The increased connections between the frontal operculum and medial temporal memory structures during combined olfactory-gustatory stimulation point to a potential mechanism underlying the acquisition and modification of flavor preferences.

Spatial and memory circuits in the medial entorhinal cortex

The large capacity of episodic memory is thought to be supported by the emergence of distinct hippocampal cell assemblies for unrelated memories, such that interference is minimized. In large-scale population recordings, the orthogonal nature of hippocampal representations across environments is evident in the complete reorganization of the firing locations of hippocampal place cells. Entorhinal grid cells provide inputs to the hippocampus, and their firing patterns shift relative to each other across different environments. Although this suggests that altered grid cell firing could generate distinct hippocampal population codes, it has recently been shown that new and distinct hippocampal place fields emerge while grid cell firing is compromised. We therefore propose that separate circuits within the medial entorhinal cortex are specialized for performing either spatial or memory-related computations.

Looking for the roots of cortical sensory computation in three-layered cortices

Despite considerable effort over a century and the benefit of remarkable technical advances in the past few decades, we are still far from understanding mammalian cerebral neocortex. With its six layers, modular architecture, canonical circuits, innumerable cell types, and computational complexity, isocortex remains a challenging mystery. In this review, we argue that identifying the structural and functional similarities between mammalian piriform cortex and reptilian dorsal cortex could help reveal common organizational and computational principles and by extension, some of the most primordial computations carried out in cortical networks.

The Potentiation of Associative Memory by Emotions: An Event-Related FMRI Study

Establishing associations between pieces of information is related to the medial temporal lobe (MTL). However, it remains unclear how emotions affect memory for associations and, consequently, MTL activity. Thus, this event-related fMRI study attempted to identify neural correlates of the influence of positive and negative emotions on associative memory. Twenty-five participants were instructed to memorize 90 pairs of standardized pictures during a scanned encoding phase. Each pair was composed of a scene and an unrelated object. Trials were neutral, positive, or negative as a function of the emotional valence of the scene. At the behavioral level, participants exhibited better memory retrieval for both emotional conditions relative to neutral trials. Within the right MTL, a functional dissociation was observed, with entorhinal activation elicited by emotional associations, posterior parahippocampal activation elicited by neutral associations, and hippocampal activation elicited by both emotional and neutral associations. In addition, emotional associations induced greater activation than neutral trials in the right amygdala. This fMRI study shows that emotions are associated with the performance improvement of associative memory, by enhancing activity in the right amygdala and the right entorhinal cortex. It also provides evidence for a rostrocaudal specialization within the MTL regarding the emotional valence of associations.

A model of grid cell development through spatial exploration and spike time-dependent plasticity

Grid cell responses develop gradually after eye opening, but little is known about the rules that govern this process. We present a biologically plausible model for the formation of a grid cell network. An asymmetric spike time-dependent plasticity rule acts upon an initially unstructured network of spiking neurons that receive inputs encoding animal velocity and location. Neurons develop an organized recurrent architecture based on the similarity of their inputs, interacting through inhibitory interneurons. The mature network can convert velocity inputs into estimates of animal location, showing that spatially periodic responses and the capacity of path integration can arise through synaptic plasticity, acting on inputs that display neither. The model provides numerous predictions about the necessity of spatial exploration for grid cell development, network topography, the maturation of velocity tuning and neural correlations, the abrupt transition to stable patterned responses, and possible mechanisms to set grid period across grid modules.

Topographic organization of orbitofrontal projections to the parahippocampal region in rats

The parahippocampal region, which comprises the perirhinal, postrhinal, and entorhinal cortices, as well as the pre- and parasubiculum, receives inputs from several association cortices and provides the major cortical input to the hippocampus. This study examined the topographic organization of projections from the orbitofrontal cortex (OFC) to the parahippocampal region in rats by injecting anterograde tracers, biotinylated dextran amine (BDA) and Phaseolus vulgaris-leucoagglutinin (PHA-L), into four subdivisions of OFC. The rostral portion of the perirhinal cortex receives strong projections from the medial (MO), ventral (VO), and ventrolateral (VLO) orbitofrontal areas and the caudal portion of lateral orbitofrontal area (LO). These projections terminate in the dorsal bank and fundus of the rhinal sulcus. In contrast, the postrhinal cortex receives a strong projection specifically from VO. All four subdivisions of OFC give rise to projections to the dorsolateral parts of the lateral entorhinal cortex (LEC), preferentially distributing to more caudal levels of LEC. The medial entorhinal cortex (MEC) receives moderate input from VO and weak projections from MO, VLO, and LO. The presubiculum receives strong projections from caudal VO but only weak projections from other OFC regions. As for the laminar distribution of projections, axons originating from OFC terminate more densely in upper layers (layers I-III) than in deep layers in the parahippocampal region. These results thus show a striking topographic organization of OFC-to-parahippocampal connectivity. Whereas LO, VLO, VO, and MO interact with perirhinal-LEC circuits, the interactions with postrhinal cortex, presubiculum, and MEC are mediated predominantly through the projections of VO.

Combined compared to dissociated oral and intestinal sucrose stimuli induce different brain hedonic processes

The characterization of brain networks contributing to the processing of oral and/or intestinal sugar signals in a relevant animal model might help to understand the neural mechanisms related to the control of food intake in humans and suggest potential causes for impaired eating behaviors. This study aimed at comparing the brain responses triggered by oral and/or intestinal sucrose sensing in pigs. Seven animals underwent brain single photon emission computed tomography (^99mTc-HMPAO) further to oral stimulation with neutral or sucrose artificial saliva paired with saline or sucrose infusion in the duodenum, the proximal part of the intestine. Oral and/or duodenal sucrose sensing induced differential cerebral blood flow changes in brain regions known to be involved in memory, reward processes and hedonic (i.e., pleasure) evaluation of sensory stimuli, including the dorsal striatum, prefrontal cortex, cingulate cortex, insular cortex, hippocampus, and parahippocampal cortex. Sucrose duodenal infusion only and combined sucrose stimulation induced similar activity patterns in the putamen, ventral anterior cingulate cortex and hippocampus. Some brain deactivations in the prefrontal and insular cortices were only detected in the presence of oral sucrose stimulation. Finally, activation of the right insular cortex was only induced by combined oral and duodenal sucrose stimulation, while specific activity patterns were detected in the hippocampus and parahippocampal cortex with oral sucrose dissociated from caloric load. This study sheds new light on the brain hedonic responses to sugar and has potential implications to unravel the neuropsychological mechanisms underlying food pleasure and motivation.

Precise spatial coding is preserved along the longitudinal hippocampal axis

Compared with the dorsal hippocampus, relatively few studies have characterized neuronal responses in the ventral hippocampus. In particular, it is unclear whether and how cells in the ventral region represent space and/or respond to contextual changes. We recorded from dorsal and ventral CA1 neurons in freely moving mice exposed to manipulations of visuospatial and olfactory contexts. We found that ventral cells respond to alterations of the visuospatial environment such as exposure to novel local cues, cue rotations, and contextual expansion in similar ways to dorsal cells, with the exception of cue rotations. Furthermore, we found that ventral cells responded to odors much more strongly than dorsal cells, particularly to odors of high valence. Similar to earlier studies recording from the ventral hippocampus in CA3, we also found increased scaling of place cell field size along the longitudinal hippocampal axis. Although the increase in place field size observed toward the ventral pole has previously been taken to suggest a decrease in spatial information coded by ventral place cells, we hypothesized that a change in spatial scaling could instead signal a shift in representational coding that preserves the resolution of spatial information. To explore this possibility, we examined population activity using principal component analysis (PCA) and neural location reconstruction techniques. Our results suggest that ventral populations encode a distributed representation of space, and that the resolution of spatial information at the population level is comparable to that of dorsal populations of similar size. Finally, through the use of neural network modeling, we suggest that the redundancy in spatial representation along the longitudinal hippocampal axis may allow the hippocampus to overcome the conflict between memory interference and generalization inherent in neural network memory. Our results indicate that ventral population activity is well suited for generalization across locations and contexts.

Involvement of the lateral entorhinal cortex for the formation of cross-modal olfactory-tactile associations in the rat

While the olfactory and tactile vibrissal systems have been extensively studied in the rat, the neural basis of these cross-modal associations is still elusive. Here we tested the hypothesis that the lateral entorhinal cortex (LEC) could be particularly involved. In order to tackle this question, we have developed a new behavioral paradigm which consists in finding one baited cup (+) among three, each of the cups presenting a different and specific odor/texture (OT) combination. During the acquisition of a first task (Task OT1), the three cups were associated with the following OT combination: O1T1 for the baited cup; O2T1 and O1T2 for non-baited ones. Most rats learn this task within three training sessions (20 trials/session). In a second task (Task OT2) animals had to pair another OT combination with the reward using a new set of stimuli (O3T3+, O4T3, and O3T4). Results showed that rats manage to learn Task OT2 within one session only. In a third task (Task OT3) animals had to learn another OT combination based on previously learned items (e.g. O4T4+, O1T4 and O4T1). This task is called the "recombination task." Results showed that control rats solve the recombination task within one session. Animals bilaterally implanted with cannulae in the LEC were microinfused with d-APV (3 µg/0.6 µL) just before the acquisition or the test session of each task. The results showed that NMDA receptor blockade in LEC did not affect recall of Task OT1 but strongly impaired acquisition of both Task OT2 and OT3. Moreover, two control groups of animals infused with d-APV showed no deficit in the acquisition of unimodal olfactory and tactile tasks. Taken together, these data show that the NMDA system in the LEC is involved in the acquisition of association between an olfactory and a tactile stimulus during cross-modal learning task.

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.

Neuronal classes and their specialization in the corticoid complex of a food-storing bird, the Indian House Crow (Corvus splendens)

Neuronal classes and their specialization in the corticoid complex of a food-storing bird (the Indian House Crow, Corvus splendens Vieillot, 1817) have been investigated using Golgi and Cresyl-violet methods. The aim of present study is to observe the neuronal characteristics of corticoid complex of the House Crow (food-storing bird) and to compare them with that of a nonfood-storing bird (the Strawberry Finch, Estrilda amandava = Amandava amandava (L., 1758)). Three main neuronal classes, viz. projection neurons, local circuit neurons, and stellate neurons, have been identified in both intermediate corticoid area (CI) and dorsolateral corticoid area (CDL) based on soma shape, arrangement of dendrites around the soma, and axonal projection. Projection neurons have four neuronal subtypes: multipolar, pyramidal, pyramidal-like, and horizontal cells. It seems that the specialization in pyramidal, local circuit, and pyramidal-like neurons show advantages in the House Crow as a food-storing bird for better memory, cognition, and connectivity in corticoid complex. This is the first study of its kind that provides information regarding neuronal classes within the corticoid complex of a food-storing bird and a comparison between a food-storing bird (House Crow) and the only available study on a nonfood-storing bird (Strawberry Finch).

Slow oscillations in the mouse hippocampus entrained by nasal respiration

Different types of network oscillations occur in different behavioral, cognitive, or vigilance states. The rodent hippocampus expresses prominent θ oscillations at frequencies between 4 and 12 Hz, which are superimposed by phase-coupled γ oscillations (30-100 Hz). These patterns entrain multineuronal activity over large distances and have been implicated in sensory information processing and memory formation. Here we report a new type of oscillation at near-θ frequencies (2-4 Hz) in the hippocampus of urethane-anesthetized mice. The rhythm is highly coherent with nasal respiration and with rhythmic field potentials in the olfactory bulb: hence, we called it hippocampal respiration-induced oscillations. Despite the similarity in frequency range, several features distinguish this pattern from locally generated θ oscillations: hippocampal respiration-induced oscillations have a unique laminar amplitude profile, are resistant to atropine, couple differently to γ oscillations, and are abolished when nasal airflow is bypassed by tracheotomy. Hippocampal neurons are entrained by both the respiration-induced rhythm and concurrent θ oscillations, suggesting a direct interaction between endogenous activity in the hippocampus and nasal respiratory inputs. Our results demonstrate that nasal respiration strongly modulates hippocampal network activity in mice, providing a long-range synchronizing signal between olfactory and hippocampal networks.

Advancing the discovery of medications for autism spectrum disorder using new technologies to reveal social brain circuitry in rodents

INTRODUCTION

Autism spectrum disorder (ASD) is a heterogeneous neurodevelopmental condition characterized by core differences and impairments in social behavioral functioning. There are no approved medications for improving social cognition and behavior in ASD, and the underlying mechanisms needed to discover safer, more effective medications are unclear.

DISCUSSION

In this review, we diagram the basic neurocircuitry governing social behaviors in order to provide a neurobiological framework for the origins of the core social behavioral symptoms of ASD. In addition, we discuss recent technological innovations in research tools that provide unprecedented observation of cellular morphology and activity deep within the intact brain and permit the precise control of discrete brain regions and specific cell types at distinct developmental stages.

CONCLUSIONS

The use of new technologies to reveal the neural circuits underlying social behavioral impairments associated with ASD is advancing our understanding of the brain changes underlying ASD and enabling the discovery of novel and effective therapeutic interventions.

Cortical odor processing in health and disease

The olfactory system has a rich cortical representation, including a large archicortical component present in most vertebrates, and in mammals neocortical components including the entorhinal and orbitofrontal cortices. Together, these cortical components contribute to normal odor perception and memory. They help transform the physicochemical features of volatile molecules inhaled or exhaled through the nose into the perception of odor objects with rich associative and hedonic aspects. This chapter focuses on how olfactory cortical areas contribute to odor perception and begins to explore why odor perception is so sensitive to disease and pathology. Odor perception is disrupted by a wide range of disorders including Alzheimer's disease, Parkinson's disease, schizophrenia, depression, autism, and early life exposure to toxins. This olfactory deficit often occurs despite maintained functioning in other sensory systems. Does the unusual network of olfactory cortical structures contribute to this sensitivity?

The cognitive and neuroanatomical underpinnings of destination memory

The ability to remember the destination to whom a piece of information has been addressed (e.g., did I tell you about the weekend?) has been labelled destination memory. Although this topic has been relatively scarcely studied, recent studies support the notion that destination recall can be the subject of important distortions in healthy younger and older adults and in individuals with Alzheimer’s disease. This research also links destination recall to several cognitive domains such as episodic memory, executive function, and self-referential processes (e.g., did I tell you about the weekend?). The present review aims to assemble these findings into a comprehensive framework and shed light onto potential neuroanatomical underpinnings of destination memory, thus providing a promising venue for future exploration and research.

Parallel and convergent processing in grid cell, head-direction cell, boundary cell, and place cell networks

The brain is able to construct internal representations that correspond to external spatial coordinates. Such brain maps of the external spatial topography may support a number of cognitive functions, including navigation and memory. The neuronal building block of brain maps are place cells, which are found throughout the hippocampus of rodents and, in a lower proportion, primates. Place cells typically fire in one or few restricted areas of space, and each area where a cell fires can range, along the dorsoventral axis of the hippocampus, from 30 cm to at least several meters. The sensory processing streams that give rise to hippocampal place cells are not fully understood, but substantial progress has been made in characterizing the entorhinal cortex, which is the gateway between neocortical areas and the hippocampus. Entorhinal neurons have diverse spatial firing characteristics, and the different entorhinal cell types converge in the hippocampus to give rise to a single, spatially modulated cell type—the place cell. We therefore suggest that parallel information processing in different classes of cells—as is typically observed at lower levels of sensory processing—continues up into higher level association cortices, including those that provide the inputs to hippocampus. WIREs Cogn Sci 2014, 5:207–219. doi: 10.1002/wcs.1272

Coordinated learning of grid cell and place cell spatial and temporal properties: multiple scales, attention and oscillations

A neural model proposes how entorhinal grid cells and hippocampal place cells may develop as spatial categories in a hierarchy of self-organizing maps (SOMs). The model responds to realistic rat navigational trajectories by learning both grid cells with hexagonal grid firing fields of multiple spatial scales, and place cells with one or more firing fields, that match neurophysiological data about their development in juvenile rats. Both grid and place cells can develop by detecting, learning and remembering the most frequent and energetic co-occurrences of their inputs. The model's parsimonious properties include: similar ring attractor mechanisms process linear and angular path integration inputs that drive map learning; the same SOM mechanisms can learn grid cell and place cell receptive fields; and the learning of the dorsoventral organization of multiple spatial scale modules through medial entorhinal cortex to hippocampus (HC) may use mechanisms homologous to those for temporal learning through lateral entorhinal cortex to HC ('neural relativity'). The model clarifies how top-down HC-to-entorhinal attentional mechanisms may stabilize map learning, simulates how hippocampal inactivation may disrupt grid cells, and explains data about theta, beta and gamma oscillations. The article also compares the three main types of grid cell models in the light of recent data.

Architecture of spatial circuits in the hippocampal region

The hippocampal region contains several principal neuron types, some of which show distinct spatial firing patterns. The region is also known for its diversity in neural circuits and many have attempted to causally relate network architecture within and between these unique circuits to functional outcome. Still, much is unknown about the mechanisms or network properties by which the functionally specific spatial firing profiles of neurons are generated, let alone how they are integrated into a coherently functioning meta-network. In this review, we explore the architecture of local networks and address how they may interact within the context of an overarching space circuit, aiming to provide directions for future successful explorations.

Transgenically targeted rabies virus demonstrates a major monosynaptic projection from hippocampal area CA2 to medial entorhinal layer II neurons

The enormous potential of modern molecular neuroanatomical tools lies in their ability to determine the precise connectivity of the neuronal cell types comprising the innate circuitry of the brain. We used transgenically targeted viral tracing to identify the monosynaptic inputs to the projection neurons of layer II of medial entorhinal cortex (MEC-LII) in mice. These neurons are not only major inputs to the hippocampus, the structure most clearly implicated in learning and memory, they also are "grid cells." Here we address the question of what kinds of inputs are specifically targeting these MEC-LII cells. Cell-specific infection of MEC-LII with recombinant rabies virus results in unambiguous labeling of monosynaptic inputs. Furthermore, ratios of labeled neurons in different regions are largely consistent between animals, suggesting that label reflects density of innervation. While the results mostly confirm prior anatomical work, they also reveal a novel major direct input to MEC-LII from hippocampal pyramidal neurons. Interestingly, the vast majority of these direct hippocampal inputs arise not from the major hippocampal subfields of CA1 and CA3, but from area CA2, a region that has historically been thought to merely be a transitional zone between CA3 and CA1. We confirmed this unexpected result using conventional tracing techniques in both rats and mice.

Dopaminergic enhancement of excitatory synaptic transmission in layer II entorhinal neurons is dependent on D₁-like receptor-mediated signaling

The modulatory neurotransmitter dopamine induces concentration-dependent changes in synaptic transmission in the entorhinal cortex, in which high concentrations of dopamine suppress evoked excitatory postsynaptic potentials (EPSPs) and lower concentrations induce an acute synaptic facilitation. Whole-cell current-clamp recordings were used to investigate the dopaminergic facilitation of synaptic responses in layer II neurons of the rat lateral entorhinal cortex. A constant bath application of 1 μM dopamine resulted in a consistent facilitation of EPSPs evoked in layer II fan cells by layer I stimulation; the size of the facilitation was more variable in pyramidal neurons, and synaptic responses in a small group of multiform neurons were not modulated by dopamine. Isolated inhibitory synaptic responses were not affected by dopamine, and the facilitation of EPSPs was not associated with a change in paired-pulse facilitation ratio. Voltage-clamp recordings of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) glutamate receptor-mediated excitatory postsynaptic currents (EPSCs) were facilitated by dopamine, but N-methyl-D-aspartate receptor-mediated currents were not. Bath application of the dopamine D₁-like receptor blocker SCH23390 (50 μM), but not the D₂-like receptor blocker sulpiride (50 μM), prevented the facilitation, indicating that it is dependent upon D₁-like receptor activation. Dopamine D₁ receptors lead to activation of protein kinase A (PKA), and including the PKA inhibitor H-89 or KT 5720 in the recording pipette solution prevented the facilitation of EPSCs. PKA-dependent phosphorylation of inhibitor 1 or the dopamine- and cAMP-regulated protein phosphatase (DARPP-32) can lead to a facilitation of AMPA receptor responses by inhibiting the activity of protein phosphatase 1 (PP1) that reduces dephosphorylation of AMPA receptors, and we found here that inhibition of PP1 occluded the facilitatory effect of dopamine. The dopamine-induced facilitation of AMPA receptor-mediated synaptic responses in layer II neurons of the lateral entorhinal cortex is therefore likely mediated via a D₁ receptor-dependent increase in PKA activity and a resulting inhibition in PP1-dependent dephosphorylation of AMPA receptors.

Rabies virus vector transgene expression level and cytotoxicity improvement induced by deletion of glycoprotein gene

The glycoprotein (G) of rabies virus (RV) is required for binding to neuronal receptors and for viral entry. G-deleted RV vector is a powerful tool for investigating the organization and function of the neural circuits. It gives the investigator the ability to genetically target initial infection to particular neurons and to control trans-synaptic propagation. In this study we have quantitatively evaluated the effect of G gene deletion on the cytotoxicity and transgene expression level of the RV vector. We compared the characteristics of the propagation-competent RV vector (rHEP5.0-CVSG-mRFP) and the G-deleted RV vector (rHEP5.0-ΔG-mRFP), both of which are based on the attenuated HEP-Flury strain and express monomeric red fluorescent protein (mRFP) as a transgene. rHEP5.0-ΔG-mRFP showed lower cytotoxicity than rHEP5.0-CVSG-mRFP, and within 16 days of infection we found no change in the basic electrophysiological properties of neurons infected with the rHEP5.0-ΔG-mRFP. The mRFP expression level of rHEP5.0-ΔG-mRFP was much higher than that of rHEP5.0-CVSG-mRFP, and 3 days after infection the retrogradely infected neurons were clearly visualized by the expressed fluorescent protein without any staining. This may be due to the low cytotoxicity and/or the presumed change in the polymerase gene (L) expression level of the G-deleted RV vector. Although the mechanisms remains to be clarified, the results of this study indicate that deletion of the G gene greatly improves the usability of the RV vector for studying the organization and function of the neural circuits by decreasing the cytotoxicity and increasing the transgene expression level.

Organization of multisynaptic inputs to the dorsal and ventral dentate gyrus: retrograde trans-synaptic tracing with rabies virus vector in the rat

Behavioral, anatomical, and gene expression studies have shown functional dissociations between the dorsal and ventral hippocampus with regard to their involvement in spatial cognition, emotion, and stress. In this study we examined the difference of the multisynaptic inputs to the dorsal and ventral dentate gyrus (DG) in the rat by using retrograde trans-synaptic tracing of recombinant rabies virus vectors. Three days after the vectors were injected into the dorsal or ventral DG, monosynaptic neuronal labeling was present in the entorhinal cortex, medial septum, diagonal band, and supramammillary nucleus, each of which is known to project to the DG directly. As in previous tracing studies, topographical patterns related to the dorsal and ventral DG were seen in these regions. Five days after infection, more of the neurons in these regions were labeled and labeled neurons were also seen in cortical and subcortical regions, including the piriform and medial prefrontal cortices, the endopiriform nucleus, the claustrum, the cortical amygdala, the medial raphe nucleus, the medial habenular nucleus, the interpeduncular nucleus, and the lateral septum. As in the monosynaptically labeled regions, a topographical distribution of labeled neurons was evident in most of these disynaptically labeled regions. These data indicate that the cortical and subcortical inputs to the dorsal and ventral DG are conveyed through parallel disynaptic pathways. This second-order input difference in the dorsal and ventral DG is likely to contribute to the functional differentiation of the hippocampus along the dorsoventral axis.

Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus

The CA2 area is an important, although relatively unexplored, component of the hippocampus. We used various tracers to provide a comprehensive analysis of CA2 connections in C57BL/6J mice. Using various adeno-associated viruses that express fluorescent proteins, we found a vasopressinergic projection from the paraventricular nuclei of the hypothalamus (PVN) to the CA2 as well as a projection from pyramidal neurons of the CA2 to the supramammillary nuclei. These projections were confirmed by retrograde tracing. As expected, we observed CA2 afferent projections from neurons in ipsilateral entorhinal cortical layer II as well as from bilateral dorsal CA2 and CA3 using retrograde tracers. Additionally, we saw CA2 neuronal input from bilateral medial septal nuclei, vertical and horizontal limbs of the nucleus of diagonal band of Broca, supramammillary nuclei (SUM), and median raphe nucleus. Dorsal CA2 injections of adeno-associated virus expressing green fluorescent protein revealed axonal projections primarily to dorsal CA1, CA2, and CA3 bilaterally. No projection was detected to the entorhinal cortex from the dorsal CA2. These results are consistent with recent observations that the dorsal CA2 forms disynaptic connections with the entorhinal cortex to influence dynamic memory processing. Mouse dorsal CA2 neurons send bilateral projections to the medial and lateral septal nuclei, vertical and horizontal limbs of the diagonal band of Broca, and SUM. Novel connections from the PVN and to the SUM suggest important regulatory roles for CA2 in mediating social and emotional input for memory processing.

Hemispheric lateralization interrupted: material-specific memory deficits in temporal lobe epilepsy

The hemispheric lateralization of memory has largely been informed through the study of patients with temporal lobe epilepsy originating from medial temporal sources (mTLE). The material-specific model of memory relies on the basic framework that the left temporal lobe mediates verbal memories, while the right temporal lobe mediates non-verbal memories. Over the years, this model has been refined, and even challenged, as our understanding of the material-specific memory deficits in mTLE has been further elaborated in the neuropsychological and neuroimaging literature. The first goal of this mini-review is to highlight the major findings in the mTLE literature that have advanced and expanded our understanding of material-specific memory deficits in mTLE. Second, we will review how functional neuroimaging patterns of material-specific hemispheric lateralization in mTLE are being translated into the innovative clinical application of preoperative fMRI memory mapping.

Effects of inactivating the agranular or granular insular cortex on the acquisition of the morphine-induced conditioned place preference and naloxone-precipitated conditioned place aversion in rats

Recent studies have indicated that the insula underlies affective learning. Although affective learning is well-established in the development of opiate addiction, the role of insula in this context remains unclear. To elucidate the organization of opiate-related affective learning within the insular cortex, we reversibly inactivated each of two major subdivisions of the insula in rats and tested the effects of this inactivation on the acquisition of morphine-induced conditioned place preference (CPP) and conditioned place aversion (CPA) induced by naloxone-precipitated acute morphine withdrawal. Results showed that inactivation of the primary interoceptive posterior granular insula (GI), but not that of the high-order anterior agranular insula (AI), disrupted the acquisition of CPP and that both GI and AI inactivation impaired the acquisition of CPA. These data suggest that the insular cortex is involved in positive and negative affective learning related to opiate addiction. In particular, the GI appears to be critical for both forms of affective learning, whereas the AI is crucial for learning associated with negative affects induced by opiate withdrawal.