Scientific Understanding of Consciousness
Consciousness as an Emergent Property of Thalamocortical Activity

 

 

 

Science 21 February 2014:  Vol. 343  no. 6173  pp. 846-847

Neuroscience: Charting the Islands of Memory

H. T. Blair

Psychology Department and Brain Research Institute, University of California, Los Angeles, CA 90095, USA.

 

Exquisitely structured microcircuits in the hippocampus and entorhinal cortex (EC) were first sketched more than a century ago by the great Spanish neuroanatomist Ramon y Cajal. It has since become known that these circuits are components of a memory system that allows us to recall facts and past events from our lives. Two reports in this issue describe newly discovered circuits formed by a population of neurons in the EC, which congregate in distinctive clusters referred to as “patches” by Ray et al. and as “islands” by Kitamura et al. Both studies suggest that these island cells may play important roles in learning and memory.

Island cells belong to a common class of cortical neurons called pyramidal cells, and are distinguished by their expression of a protein called calbindin, which other EC neurons lack. The EC comprises six distinct layers, each with its own pattern of input and output connections. Patches of island cell bodies reside in the second layer (ECII), which sends a major excitatory projection—called the perforant path—to the dentate gyrus (DG) and CA3 subregions of the hippocampus. However, the perforant path arises from ECII stellate cells that are interleaved between island cell patches, so island cell axons do not project to the DG or CA3.

Kitamura et al. show that island cell axons follow the temporoammonic pathway from the EC to the CA1 subregion of the hippocampus. This pathway contains many axons from neurons in the third layer of the EC (ECIII), which excite the apical dendrites of CA1 pyramidal cells. By contrast, island cell axons preferentially excite CA1 interneurons, which inhibit the same apical dendrites of CA1 pyramidal cells that are excited by ECIII axons. Hence, island cells appear well positioned to exert inhibitory control over the ability of ECIII neurons to excite CA1.

To investigate how island cells influence memory, Kitamura et al. engineered island cells to express a light-sensitive ion channel. Island cells were then artificially stimulated by light while mice were trained to fear an auditory tone by pairing the tone with an electric shock that occurred 20 s after the tone had ended. Stimulation of the island cell pathway impaired memory for trace fear evoked by the tone (thus, mice could not remember that the tone predicted the shock) but spared context fear evoked by the experimental chamber (that is, mice could still remember that a shock had occurred in the chamber). Prior evidence indicates that trace fear requires activation of CA1 by inputs from ECIII, whereas context fear depends on other inputs to CA1. Thus, artificial stimulation of island cells may have selectively switched off temporoammonic inputs to CA1 from ECIII, thereby inhibiting specific memory processes (such as trace fear) while sparing others (such as context fear).

By artificially manipulating island cells, it may thus be possible to selectively impair or enhance specific kinds of memories. However, the natural activity of island cells may serve functions more complex than simply acting as an “off switch” for ECIII inputs to CA1. CA1 contains spatially tuned neurons called place cells, each of which fires selectively when the animal visits a preferred location. The EC contains spatially tuned neurons called grid cells, each of which fires at multiple locations that form a hexagonal lattice on the floor of the environment, as well as border cells that fire along environmental boundaries. In rodents, firing patterns of neurons in CA1 and the EC are comodulated by 4- to 12-Hz theta oscillations, a prominent brain rhythm that becomes especially strong when the animal is moving through its environment. Ray et al. recorded island cells in freely behaving mice, and found that they were more strongly modulated by movement-related theta rhythm than neighboring stellate cells. This was unexpected because island cells are ECII pyramidal cells, which have been shown to exhibit less theta resonance of their intrinsic membrane currents than stellate cells. Theta rhythmicity of island cells may thus be driven by an external source of oscillatory input.

Further research will be needed to clarify the functional contributions of island cells to memory and spatial coding. But now that they have taken the stage, it appears likely that EC island cells will become a new focus for research on memory microcircuits.

 

 

Science 21 February 2014:  Vol. 343  no. 6173  pp. 891-896  

Grid-Layout and Theta-Modulation of Layer 2 Pyramidal Neurons in Medial Entorhinal Cortex

Saikat Ray,  Robert Naumann,  Andrea Burgalossi,  Qiusong Tang,  Helene Schmidt,  Michael Brecht

Bernstein Center for Computational Neuroscience, Humboldt University of Berlin, Philippstrasse 13 Haus 6, 10115 Berlin, Germany.

 

Little is known about how microcircuits are organized in layer 2 of the medial entorhinal cortex. We visualized principal cell microcircuits and determined cellular theta-rhythmicity in freely moving rats. Non–dentate-projecting, calbindin-positive pyramidal cells bundled dendrites together and formed patches arranged in a hexagonal grid aligned to layer 1 axons, parasubiculum, and cholinergic inputs. Calbindin-negative, dentate-gyrus–projecting stellate cells were distributed across layer 2 but avoided centers of calbindin-positive patches. Cholinergic drive sustained theta-rhythmicity, which was twofold stronger in pyramidal than in stellate neurons. Theta-rhythmicity was cell-type–specific but not distributed as expected from cell-intrinsic properties. Layer 2 divides into a weakly theta-locked stellate cell lattice and spatiotemporally highly organized pyramidal grid. It needs to be assessed how these two distinct principal cell networks contribute to grid cell activity.

Temporal and spatial discharge patterns in layer 2 of the medial entorhinal cortex (MEC) are related through phase precession and the correlation of gridness (hexagonal regularity) and theta-rhythmicity. Layer 2 principal neurons divide into pyramidal and stellate cells, the latter of which have been suggested to shape entorhinal theta and grid activity by their intrinsic properties. Progress in understanding entorhinal microcircuits has been limited because most though not all data stem from extracellular recordings of unidentified cells. Such recordings have characterized diverse functional cell types in layer 2. Clustering of grid cells points to spatial organization. It is not clear, however, how functionally defined cell types correspond to stellate and pyramidal cells, which differ in conductances, immunoreactivity, projections, and inhibitory inputs. We combined juxtacellular labeling with principal cell identification to visualize microcircuits in the MEC.

Calbindin immunoreactivity identifies a relatively homogeneous pyramidal neuron population in MEC layer 2. Parasagittal sections stained for calbindin showed that calbindin-positive (calbindin+) pyramidal cells were arranged in patches. Apical dendrites of calbindin+ pyramidal cells bundled together in layer 1 to form tent-like structures over the patches. The patchy structure is well defined at the layer 1/2 border, whereas a “salt-and-pepper” appearance of calbindin+ and calbindin– cells is observed deeper in layer 2.

We noted a striking hexagonal organization of calbindin+ patches and characterized this organization by means of three techniques.

We retrogradely labeled neurons from ipsilateral dentate gyrus.

Although most retrogradely labeled neurons were stellate cells, a small fraction had pyramidal morphologies.

Last, we assessed in freely moving animals how activity of identified neurons related to the entorhinal theta-rhythm. We recorded 31 layer 2 neurons in rats trained to explore open fields and classified them by morphology and immunoreactivity. Calbindin+ neurons (n = 12) were pyramidal cells, whereas calbindin– neurons (n = 19) had stellate morphologies. Firing rates were not different (calbindin+ = 2.1 ± 1.1 Hz; calbindin– = 2.3 ± 1.5 Hz; P > 0.5, Mann-Whitney test). We found, however, that calbindin+ neurons showed stronger theta-rhythmicity of spiking than that of calbindin– cells (P < 0.01, unpaired t test). Theta-rhythmicity was associated with locomotion of the animal. A similar twofold difference in theta-rhythmicity between calbindin+ (n = 14) and calbindin– (n = 20) cells was observed under urethane-ketamine anesthesia (P = 0.0003, Mann-Whitney test), which preserves cortico-hippocampal theta-rhythmicity. Pharmacological blockade of cholinergic transmission suppressed theta-rhythmicity in both calbindin+ and calbindin– cells. Specifically, we observed that cholinergic blockade led to a loss of the distinct peak at theta-frequency in the power spectra of spike discharges. Cells also differed in their phase-locking to entorhinal field potential theta: Calbindin+ cells were more strongly phase-locked (average Rayleigh vector length = 0.54 versus 0.22 in calbindin– cells; P < 0.0012, Mann-Whitney test) and fired near the trough of the theta-oscillation, whereas locking was weaker and more variable in calbindin– cells.

What is the cellular basis of theta-rhythmicity in MEC layer 2? Stellate cells have been prime candidates for theta discharges in layer 2 because intrinsic conductances make them resonate at theta-frequency. We found, however, that calbindin⁺ pyramidal cells showed twofold stronger theta-rhythmicity and theta-phase-locking than calbindin^– stellate neurons. The stronger theta-rhythmicity of calbindin⁺ pyramidal neurons, which have weaker sag-currents, opposite from what had been predicted on the basis of intrinsic properties. Hence, layer 2 theta-modulation is cell-type–specific but not distributed as expected from cell-intrinsic resonance properties. This finding agrees with other evidence that questioned the causal relationship between intrinsic properties and theta-rhythmicity in vivo. The membrane properties of calbindin⁺ neurons are not tuned to the generation of theta-rhythmicity. Their strongly rhythmic discharges suggest that calbindin⁺ neurons might correspond to a subset of cells with strong membrane potential theta-oscillations, which—in the absence of cell-intrinsic mechanisms—probably arise from synaptic interactions. Cholinergic innervation and effects of cholinergic blockade suggest cholinergic drive sustains theta-rhythmicity of calbindin⁺ cells.

We were not yet able to assess spatial modulation in a sufficient number of identified neurons to directly relate our results to grid cell function. The limited available evidence suggests that grid cells are recruited from a heterogeneous neuronal population in layer 2, possibly indicating weak structure-function relationships. Yet, we observed similarities between calbindin⁺ neurons and grid cells: Calbindin⁺ patches receive cholinergic inputs, which are required for grid cell activity according to preliminary data; calbindin⁺ cells have strong theta-rhythmicity, a feature that correlates with grid cell discharge; and, like grid cells, calbindin⁺ cells are clustered.

We have hypothesized that calbindin⁺ neurons form a “grid-cell-grid” —that their hexagonal arrangement might be an isomorphism to hexagonal grid cell activity, much like isomorphic cortical representations of body parts in tactile specialists. However, hexagonality often results from spacing constraints and hence might be unrelated to grid cell activity. Determining the spatial modulation patterns of identified entorhinal neurons will help clarifying whether and how the calbindin⁺ grid is related to grid cell activity.

 

 

Science 21 February 2014:  Vol. 343  no. 6173  pp. 896-901

Island Cells Control Temporal Association Memory

Takashi Kitamura,  Michele Pignatelli,  Junghyup Suh,  Keigo Kohara,  Atsushi Yoshiki,  Kuniya Abe,  Susumu Tonegawa

¹RIKEN-MIT Center for Neural Circuit Genetics at the Picower Institute for Learning and Memory, Department of Biology and Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology (MIT), Cambridge, MA 02139, USA.

²RIKEN BioResource Center, 3-1-1 Koyadai, Ibaraki 305-0074, Japan.

³Howard Hughes Medical Institute at MIT, Cambridge, MA 02139, USA.

 

Episodic memory requires associations of temporally discontiguous events. In the entorhinal-hippocampal network, temporal associations are driven by a direct pathway from layer III of the medial entorhinal cortex (MECIII) to the hippocampal CA1 region. However, the identification of neural circuits that regulate this association has remained unknown. In layer II of entorhinal cortex (ECII), we report clusters of excitatory neurons called island cells, which appear in a curvilinear matrix of bulblike structures, directly project to CA1, and activate interneurons that target the distal dendrites of CA1 pyramidal neurons. Island cells suppress the excitatory MECIII input through the feed-forward inhibition to control the strength and duration of temporal association in trace fear memory. Together, the two EC inputs compose a control circuit for temporal association memory.

Episodic memory consists of associations of objects, space, and time. In humans and animals, the entorhinal cortex (EC)–hippocampal (HPC) network plays an essential role in episodic memory, with medial EC (MEC) and lateral EC (LEC) inputs into HPC providing spatial and object information, respectively. Neural circuits have been identified in the EC-HPC network that mediate space and object associations. In contrast, the neural circuits for time-related aspects of episodic memory are only beginning to be studied. Direct inputs from MEC layer III (MECIII) cells to CA1 pyramidal cells drive the temporal association of discontiguous events. Like most cognitive and motor phenomena, temporal association memory must be regulated for optimal adaptive benefit, yet nearly nothing is known about the underlying mechanisms of this regulation. We investigated this issue by mapping and characterizing an unsuspected neuronal circuit within the EC-HPC network and examining the effect of its optogenetic manipulations on a temporal association memory.

A retrograde tracer, cholera toxin subunit B (CTB), was injected into the dentate gyrus (DG) of C57BL6 mice. Although a majority of cells in EC layer II (ECII) were CTB-positive, a large proportion was CTB-negative and clustered in a series of about 130-μm-diameter bulblike structures. Hereafter, we refer to these CTB-negative cells as ECII island (ECIIi) cells. ECIIi cells are mostly pyramidal.

We created a Cre transgenic mouse line.

ECIIi cells appeared in a curvilinear matrix of bulblike structures in tangential MEC sections.

ECIIi cells projected primarily to the CA1 region via the temporoammonic pathway. Additional weaker projections were detected in the subiculum, parasubiculum, and contralateral CA1 and EC.

Although our study has demonstrated that the feed-forward inhibition of MECIII input to CA1 pyramidal cells by the ECIIi–SL-INs pathway serves as an important mechanism for the control of temporal association memory, other circuits and/or mechanisms may also contribute to this process.

CA1 pyramidal cells receive a multitude of other excitatory and inhibitory inputs, including the Schaffer collateral (SC) input from CA3 that originates from ECIIo cells. The in vitro interaction of ECIII and CA3 inputs on the activity and synaptic plasticity of CA1 pyramidal cells have been reported, but the inhibition of SC input does not seem to have a substantial effect on the TFC performance. Although the role of the direct pathway, ECIIi-CA1 pyramidal cells, has not been yet elucidated, we hypothesize that the indirect pathway from ECIIo to CA1 via the trisynaptic circuit primarily processes context and space, whereas the direct pathways from MECIII and ECIIi-SL-INs are responsible for temporal properties of episodic memory.

 

Episodic Memory consists of Associations of Objects, Space and Time

 

Science 21 February 2014:  Vol. 343  no. 6173  pp. 846-847

Neuroscience: Charting the Islands of Memory

H. T. Blair

Psychology Department and Brain Research Institute, University of California, Los Angeles, CA 90095, USA.

[paraphrase]

Exquisitely structured microcircuits in the hippocampus and entorhinal cortex (EC) were first sketched more than a century ago by the great Spanish neuroanatomist Ramon y Cajal. It has since become known that these circuits are components of a memory system that allows us to recall facts and past events from our lives. Two reports in this issue describe newly discovered circuits formed by a population of neurons in the EC, which congregate in distinctive clusters referred to as “patches” by Ray et al. and as “islands” by Kitamura et al. Both studies suggest that these island cells may play important roles in learning and memory.

Island cells belong to a common class of cortical neurons called pyramidal cells, and are distinguished by their expression of a protein called calbindin, which other EC neurons lack. The EC comprises six distinct layers, each with its own pattern of input and output connections. Patches of island cell bodies reside in the second layer (ECII), which sends a major excitatory projection—called the perforant path—to the dentate gyrus (DG) and CA3 subregions of the hippocampus. However, the perforant path arises from ECII stellate cells that are interleaved between island cell patches, so island cell axons do not project to the DG or CA3.

Kitamura et al. show that island cell axons follow the temporoammonic pathway from the EC to the CA1 subregion of the hippocampus. This pathway contains many axons from neurons in the third layer of the EC (ECIII), which excite the apical dendrites of CA1 pyramidal cells. By contrast, island cell axons preferentially excite CA1 interneurons, which inhibit the same apical dendrites of CA1 pyramidal cells that are excited by ECIII axons. Hence, island cells appear well positioned to exert inhibitory control over the ability of ECIII neurons to excite CA1.

To investigate how island cells influence memory, Kitamura et al. engineered island cells to express a light-sensitive ion channel. Island cells were then artificially stimulated by light while mice were trained to fear an auditory tone by pairing the tone with an electric shock that occurred 20 s after the tone had ended. Stimulation of the island cell pathway impaired memory for trace fear evoked by the tone (thus, mice could not remember that the tone predicted the shock) but spared context fear evoked by the experimental chamber (that is, mice could still remember that a shock had occurred in the chamber). Prior evidence indicates that trace fear requires activation of CA1 by inputs from ECIII, whereas context fear depends on other inputs to CA1. Thus, artificial stimulation of island cells may have selectively switched off temporoammonic inputs to CA1 from ECIII, thereby inhibiting specific memory processes (such as trace fear) while sparing others (such as context fear).

By artificially manipulating island cells, it may thus be possible to selectively impair or enhance specific kinds of memories. However, the natural activity of island cells may serve functions more complex than simply acting as an “off switch” for ECIII inputs to CA1. CA1 contains spatially tuned neurons called place cells, each of which fires selectively when the animal visits a preferred location. The EC contains spatially tuned neurons called grid cells, each of which fires at multiple locations that form a hexagonal lattice on the floor of the environment, as well as border cells that fire along environmental boundaries. In rodents, firing patterns of neurons in CA1 and the EC are comodulated by 4- to 12-Hz theta oscillations, a prominent brain rhythm that becomes especially strong when the animal is moving through its environment. Ray et al. recorded island cells in freely behaving mice, and found that they were more strongly modulated by movement-related theta rhythm than neighboring stellate cells. This was unexpected because island cells are ECII pyramidal cells, which have been shown to exhibit less theta resonance of their intrinsic membrane currents than stellate cells. Theta rhythmicity of island cells may thus be driven by an external source of oscillatory input.

Further research will be needed to clarify the functional contributions of island cells to memory and spatial coding. But now that they have taken the stage, it appears likely that EC island cells will become a new focus for research on memory microcircuits.

[end of paraphrase]

 

 

Science 21 February 2014:  Vol. 343  no. 6173  pp. 891-896  

Grid-Layout and Theta-Modulation of Layer 2 Pyramidal Neurons in Medial Entorhinal Cortex

Saikat Ray,  Robert Naumann,  Andrea Burgalossi,  Qiusong Tang,  Helene Schmidt,  Michael Brecht

Bernstein Center for Computational Neuroscience, Humboldt University of Berlin, Philippstrasse 13 Haus 6, 10115 Berlin, Germany.

[paraphrase]

Little is known about how microcircuits are organized in layer 2 of the medial entorhinal cortex. We visualized principal cell microcircuits and determined cellular theta-rhythmicity in freely moving rats. Non–dentate-projecting, calbindin-positive pyramidal cells bundled dendrites together and formed patches arranged in a hexagonal grid aligned to layer 1 axons, parasubiculum, and cholinergic inputs. Calbindin-negative, dentate-gyrus–projecting stellate cells were distributed across layer 2 but avoided centers of calbindin-positive patches. Cholinergic drive sustained theta-rhythmicity, which was twofold stronger in pyramidal than in stellate neurons. Theta-rhythmicity was cell-type–specific but not distributed as expected from cell-intrinsic properties. Layer 2 divides into a weakly theta-locked stellate cell lattice and spatiotemporally highly organized pyramidal grid. It needs to be assessed how these two distinct principal cell networks contribute to grid cell activity.

Temporal and spatial discharge patterns in layer 2 of the medial entorhinal cortex (MEC) are related through phase precession and the correlation of gridness (hexagonal regularity) and theta-rhythmicity. Layer 2 principal neurons divide into pyramidal and stellate cells, the latter of which have been suggested to shape entorhinal theta and grid activity by their intrinsic properties. Progress in understanding entorhinal microcircuits has been limited because most though not all data stem from extracellular recordings of unidentified cells. Such recordings have characterized diverse functional cell types in layer 2. Clustering of grid cells points to spatial organization. It is not clear, however, how functionally defined cell types correspond to stellate and pyramidal cells, which differ in conductances, immunoreactivity, projections, and inhibitory inputs. We combined juxtacellular labeling with principal cell identification to visualize microcircuits in the MEC.

Calbindin immunoreactivity identifies a relatively homogeneous pyramidal neuron population in MEC layer 2. Parasagittal sections stained for calbindin showed that calbindin-positive (calbindin+) pyramidal cells were arranged in patches. Apical dendrites of calbindin+ pyramidal cells bundled together in layer 1 to form tent-like structures over the patches. The patchy structure is well defined at the layer 1/2 border, whereas a “salt-and-pepper” appearance of calbindin+ and calbindin– cells is observed deeper in layer 2.

We noted a striking hexagonal organization of calbindin+ patches and characterized this organization by means of three techniques.

We retrogradely labeled neurons from ipsilateral dentate gyrus.

Although most retrogradely labeled neurons were stellate cells, a small fraction had pyramidal morphologies.

Last, we assessed in freely moving animals how activity of identified neurons related to the entorhinal theta-rhythm. We recorded 31 layer 2 neurons in rats trained to explore open fields and classified them by morphology and immunoreactivity. Calbindin+ neurons (n = 12) were pyramidal cells, whereas calbindin– neurons (n = 19) had stellate morphologies. Firing rates were not different (calbindin+ = 2.1 ± 1.1 Hz; calbindin– = 2.3 ± 1.5 Hz; P > 0.5, Mann-Whitney test). We found, however, that calbindin+ neurons showed stronger theta-rhythmicity of spiking than that of calbindin– cells (P < 0.01, unpaired t test). Theta-rhythmicity was associated with locomotion of the animal. A similar twofold difference in theta-rhythmicity between calbindin+ (n = 14) and calbindin– (n = 20) cells was observed under urethane-ketamine anesthesia (P = 0.0003, Mann-Whitney test), which preserves cortico-hippocampal theta-rhythmicity. Pharmacological blockade of cholinergic transmission suppressed theta-rhythmicity in both calbindin+ and calbindin– cells. Specifically, we observed that cholinergic blockade led to a loss of the distinct peak at theta-frequency in the power spectra of spike discharges. Cells also differed in their phase-locking to entorhinal field potential theta: Calbindin+ cells were more strongly phase-locked (average Rayleigh vector length = 0.54 versus 0.22 in calbindin– cells; P < 0.0012, Mann-Whitney test) and fired near the trough of the theta-oscillation, whereas locking was weaker and more variable in calbindin– cells.

What is the cellular basis of theta-rhythmicity in MEC layer 2? Stellate cells have been prime candidates for theta discharges in layer 2 because intrinsic conductances make them resonate at theta-frequency. We found, however, that calbindin⁺ pyramidal cells showed twofold stronger theta-rhythmicity and theta-phase-locking than calbindin^– stellate neurons. The stronger theta-rhythmicity of calbindin⁺ pyramidal neurons, which have weaker sag-currents, opposite from what had been predicted on the basis of intrinsic properties. Hence, layer 2 theta-modulation is cell-type–specific but not distributed as expected from cell-intrinsic resonance properties. This finding agrees with other evidence that questioned the causal relationship between intrinsic properties and theta-rhythmicity in vivo. The membrane properties of calbindin⁺ neurons are not tuned to the generation of theta-rhythmicity. Their strongly rhythmic discharges suggest that calbindin⁺ neurons might correspond to a subset of cells with strong membrane potential theta-oscillations, which—in the absence of cell-intrinsic mechanisms—probably arise from synaptic interactions. Cholinergic innervation and effects of cholinergic blockade suggest cholinergic drive sustains theta-rhythmicity of calbindin⁺ cells.

We were not yet able to assess spatial modulation in a sufficient number of identified neurons to directly relate our results to grid cell function. The limited available evidence suggests that grid cells are recruited from a heterogeneous neuronal population in layer 2, possibly indicating weak structure-function relationships. Yet, we observed similarities between calbindin⁺ neurons and grid cells: Calbindin⁺ patches receive cholinergic inputs, which are required for grid cell activity according to preliminary data; calbindin⁺ cells have strong theta-rhythmicity, a feature that correlates with grid cell discharge; and, like grid cells, calbindin⁺ cells are clustered.

We have hypothesized that calbindin⁺ neurons form a “grid-cell-grid” —that their hexagonal arrangement might be an isomorphism to hexagonal grid cell activity, much like isomorphic cortical representations of body parts in tactile specialists. However, hexagonality often results from spacing constraints and hence might be unrelated to grid cell activity. Determining the spatial modulation patterns of identified entorhinal neurons will help clarifying whether and how the calbindin⁺ grid is related to grid cell activity.

[end of paraphrase]

 

 

Science 21 February 2014:  Vol. 343  no. 6173  pp. 896-901

Island Cells Control Temporal Association Memory

Takashi Kitamura,  Michele Pignatelli,  Junghyup Suh,  Keigo Kohara,  Atsushi Yoshiki,  Kuniya Abe,  Susumu Tonegawa

¹RIKEN-MIT Center for Neural Circuit Genetics at the Picower Institute for Learning and Memory, Department of Biology and Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology (MIT), Cambridge, MA 02139, USA.

²RIKEN BioResource Center, 3-1-1 Koyadai, Ibaraki 305-0074, Japan.

³Howard Hughes Medical Institute at MIT, Cambridge, MA 02139, USA.

[paraphrase]

Episodic memory requires associations of temporally discontiguous events. In the entorhinal-hippocampal network, temporal associations are driven by a direct pathway from layer III of the medial entorhinal cortex (MECIII) to the hippocampal CA1 region. However, the identification of neural circuits that regulate this association has remained unknown. In layer II of entorhinal cortex (ECII), we report clusters of excitatory neurons called island cells, which appear in a curvilinear matrix of bulblike structures, directly project to CA1, and activate interneurons that target the distal dendrites of CA1 pyramidal neurons. Island cells suppress the excitatory MECIII input through the feed-forward inhibition to control the strength and duration of temporal association in trace fear memory. Together, the two EC inputs compose a control circuit for temporal association memory.

Episodic memory consists of associations of objects, space, and time. In humans and animals, the entorhinal cortex (EC)–hippocampal (HPC) network plays an essential role in episodic memory, with medial EC (MEC) and lateral EC (LEC) inputs into HPC providing spatial and object information, respectively. Neural circuits have been identified in the EC-HPC network that mediate space and object associations. In contrast, the neural circuits for time-related aspects of episodic memory are only beginning to be studied. Direct inputs from MEC layer III (MECIII) cells to CA1 pyramidal cells drive the temporal association of discontiguous events. Like most cognitive and motor phenomena, temporal association memory must be regulated for optimal adaptive benefit, yet nearly nothing is known about the underlying mechanisms of this regulation. We investigated this issue by mapping and characterizing an unsuspected neuronal circuit within the EC-HPC network and examining the effect of its optogenetic manipulations on a temporal association memory.

A retrograde tracer, cholera toxin subunit B (CTB), was injected into the dentate gyrus (DG) of C57BL6 mice. Although a majority of cells in EC layer II (ECII) were CTB-positive, a large proportion was CTB-negative and clustered in a series of about 130-μm-diameter bulblike structures. Hereafter, we refer to these CTB-negative cells as ECII island (ECIIi) cells. ECIIi cells are mostly pyramidal.

We created a Cre transgenic mouse line.

ECIIi cells appeared in a curvilinear matrix of bulblike structures in tangential MEC sections.

ECIIi cells projected primarily to the CA1 region via the temporoammonic pathway. Additional weaker projections were detected in the subiculum, parasubiculum, and contralateral CA1 and EC.

Although our study has demonstrated that the feed-forward inhibition of MECIII input to CA1 pyramidal cells by the ECIIi–SL-INs pathway serves as an important mechanism for the control of temporal association memory, other circuits and/or mechanisms may also contribute to this process.

CA1 pyramidal cells receive a multitude of other excitatory and inhibitory inputs, including the Schaffer collateral (SC) input from CA3 that originates from ECIIo cells. The in vitro interaction of ECIII and CA3 inputs on the activity and synaptic plasticity of CA1 pyramidal cells have been reported, but the inhibition of SC input does not seem to have a substantial effect on the TFC performance. Although the role of the direct pathway, ECIIi-CA1 pyramidal cells, has not been yet elucidated, we hypothesize that the indirect pathway from ECIIo to CA1 via the trisynaptic circuit primarily processes context and space, whereas the direct pathways from MECIII and ECIIi-SL-INs are responsible for temporal properties of episodic memory.

[end of paraphrase]