The orbitofrontal cortex maps future navigational goals.

Accurate navigation to a desired goal requires consecutive estimates of spatial relationships between the current position and future destination throughout the journey. Although neurons in the hippocampal formation can represent the position of an animal as well as its nearby trajectories1-7, their role in determining the destination of the animal has been questioned8,9. It is, thus, unclear whether the brain can possess a precise estimate of target location during active environmental exploration. Here we describe neurons in the rat orbitofrontal cortex (OFC) that form spatial representations persistently pointing to the subsequent goal destination of an animal throughout navigation. This destination coding emerges before the onset of navigation, without direct sensory access to a distal goal, and even predicts the incorrect destination of an animal at the beginning of an error trial. Goal representations in the OFC are maintained by destination-specific neural ensemble dynamics, and their brief perturbation at the onset of a journey led to a navigational error. These findings suggest that the OFC is part of the internal goal map of the brain, enabling animals to navigate precisely to a chosen destination that is beyond the range of sensory perception.

Hypothalamic Supramammillary Control of Cognition and Motivation.

The supramammillary nucleus (SuM) is a small region in the ventromedial posterior hypothalamus. The SuM has been relatively understudied with much of the prior focus being on its connection with septo-hippocampal circuitry. Thus, most studies conducted until the 21st century examined its role in hippocampal processes, such as theta rhythm and learning/memory. In recent years, the SuM has been "rediscovered" as a crucial hub for several behavioral and cognitive processes, including reward-seeking, exploration, and social memory. Additionally, it has been shown to play significant roles in hippocampal plasticity and adult neurogenesis. This review highlights findings from recent studies using cutting-edge systems neuroscience tools that have shed light on these fascinating roles for the SuM.

A prefrontal-thalamo-hippocampal circuit for goal-directed spatial navigation.

Spatial navigation requires information about the relationship between current and future positions. The activity of hippocampal neurons appears to reflect such a relationship, representing not only instantaneous position but also the path towards a goal location. However, how the hippocampus obtains information about goal direction is poorly understood. Here we report a prefrontal-thalamic neural circuit that is required for hippocampal representation of routes or trajectories through the environment. Trajectory-dependent firing was observed in medial prefrontal cortex, the nucleus reuniens of the thalamus, and the CA1 region of the hippocampus in rats. Lesioning or optogenetic silencing of the nucleus reuniens substantially reduced trajectory-dependent CA1 firing. Trajectory-dependent activity was almost absent in CA3, which does not receive nucleus reuniens input. The data suggest that projections from medial prefrontal cortex, via the nucleus reuniens, are crucial for representation of the future path during goal-directed behaviour and point to the thalamus as a key node in networks for long-range communication between cortical regions involved in navigation.

The nucleus reuniens of the thalamus sits at the nexus of a hippocampus and medial prefrontal cortex circuit enabling memory and behavior.

The nucleus reuniens of the thalamus (RE) is a key component of an extensive network of hippocampal and cortical structures and is a fundamental substrate for cognition. A common misconception is that RE is a simple relay structure. Instead, a better conceptualization is that RE is a critical component of a canonical higher-order cortico-thalamo-cortical circuit that supports communication between the medial prefrontal cortex (mPFC) and the hippocampus (HC). RE dysfunction is implicated in several clinical disorders including, but not limited to Alzheimer's disease, schizophrenia, and epilepsy. Here, we review key anatomical and physiological features of the RE based primarily on studies in rodents. We present a conceptual model of RE circuitry within the mPFC-RE-HC system and speculate on the computations RE enables. We review the rapidly growing literature demonstrating that RE is critical to, and its neurons represent, aspects of behavioral tasks that place demands on memory focusing on its role in navigation, spatial working memory, the temporal organization of memory, and executive functions.

Functional division of hippocampal area CA1 via modulatory gating of entorhinal cortical inputs.

The hippocampus receives two streams of information, spatial and nonspatial, via major afferent inputs from the medial (MEC) and lateral entorhinal cortexes (LEC). The MEC and LEC projections in the temporoammonic pathway are topographically organized along the transverse-axis of area CA1. The potential for functional segregation of area CA1, however, remains relatively unexplored. Here, we demonstrated differential novelty-induced c-Fos expression along the transverse-axis of area CA1 corresponding to topographic projections of MEC and LEC inputs. We found that, while novel place exposure induced a uniform c-Fos expression along the transverse-axis of area CA1, novel object exposure primarily activated the distal half of CA1 neurons. In hippocampal slices, we observed distinct presynaptic properties between LEC and MEC terminals, and application of either DA or NE produced a largely selective influence on one set of inputs (LEC). Finally, we demonstrated that differential c-Fos expression along the transverse axis of area CA1 was largely abolished by an antagonist of neuromodulatory receptors, clozapine. Our results suggest that neuromodulators can control topographic TA projections allowing the hippocampus to differentially encode new information along the transverse axis of area CA1.

Postsynaptic decoding of neural activity: eEF2 as a biochemical sensor coupling miniature synaptic transmission to local protein synthesis.

Activity-dependent regulation of dendritic protein synthesis is critical for enduring changes in synaptic function, but how the unique features of distinct activity patterns are decoded by the dendritic translation machinery remains poorly understood. Here, we identify eukaryotic elongation factor-2 (eEF2), which catalyzes ribosomal translocation during protein synthesis, as a biochemical sensor in dendrites that is specifically and locally tuned to the quality of neurotransmission. We show that intrinsic action potential (AP)-mediated network activity in cultured hippocampal neurons maintains eEF2 in a relatively dephosphorylated (active) state, whereas spontaneous neurotransmitter release (i.e., miniature neurotransmission) strongly promotes the phosphorylation (and inactivation) of eEF2. The regulation of eEF2 phosphorylation is responsive to bidirectional changes in miniature neurotransmission and is controlled locally in dendrites. Finally, direct spatially controlled inhibition of eEF2 phosphorylation induces local translational activation, suggesting that eEF2 is a biochemical sensor that couples miniature synaptic events to local translational suppression in neuronal dendrites.

Maternal immune activation alters nonspatial information processing in the hippocampus of the adult offspring.

The observation that maternal infection increases the risk for schizophrenia in the offspring suggests that the maternal immune system plays a key role in the etiology of schizophrenia. In a mouse model, maternal immune activation (MIA) by injection of poly(I:C) yields adult offspring that display abnormalities in a variety of behaviors relevant to schizophrenia. As abnormalities in the hippocampus are a consistent observation in schizophrenia patients, we examined synaptic properties in hippocampal slices prepared from the offspring of poly(I:C)- and saline-treated mothers. Compared to controls, CA1 pyramidal neurons from adult offspring of MIA mothers display reduced frequency and increased amplitude of miniature excitatory postsynaptic currents. In addition, the specific component of the temporoammonic pathway that mediates object-related information displays increased sensitivity to dopamine. To assess hippocampal network function in vivo, we used expression of the immediate-early gene, c-Fos, as a surrogate measure of neuronal activity. Compared to controls, the offspring of poly(I:C)-treated mothers display a distinct c-Fos expression pattern in area CA1 following novel object, but not novel location, exposure. Thus, the offspring of MIA mothers may have an abnormality in modality-specific information processing. Indeed, the MIA offspring display enhanced discrimination in a novel object recognition, but not in an object location, task. Thus, analysis of object and spatial information processing at both synaptic and behavioral levels reveals a largely selective abnormality in object information processing in this mouse model. Our results suggest that altered processing of object-related information may be part of the pathogenesis of schizophrenia-like cognitive behaviors.

Entorhinal-retrosplenial circuits for allocentric-egocentric transformation of boundary coding.

Spatial navigation requires landmark coding from two perspectives, relying on viewpoint-invariant and self-referenced representations. The brain encodes information within each reference frame but their interactions and functional dependency remains unclear. Here we investigate the relationship between neurons in the rat's retrosplenial cortex (RSC) and entorhinal cortex (MEC) that increase firing near boundaries of space. Border cells in RSC specifically encode walls, but not objects, and are sensitive to the animal's direction to nearby borders. These egocentric representations are generated independent of visual or whisker sensation but are affected by inputs from MEC that contains allocentric spatial cells. Pharmaco- and optogenetic inhibition of MEC led to a disruption of border coding in RSC, but not vice versa, indicating allocentric-to-egocentric transformation. Finally, RSC border cells fire prospective to the animal's next motion, unlike those in MEC, revealing the MEC-RSC pathway as an extended border coding circuit that implements coordinate transformation to guide navigation behavior.

Prefrontal-hippocampal interactions for spatial navigation.

Animals have the ability to navigate to a desired location by making use of information about environmental landmarks and their own movements. While decades of neuroscience research have identified neurons in the hippocampus and parahippocampal structures that represent an animal's position in space, it is still largely unclear how an animal can choose the next movement direction to reach a desired goal. As the goal destination is typically located somewhere outside of the range of sensory perception, the animal is required to rely on the internal metric of space to estimate the direction and distance of the destination to plan a next action. Therefore, the hippocampal spatial map should interact with action-planning systems in other cortical regions. In accordance with this idea, several recent studies have indicated the importance of functional interactions between the hippocampus and the prefrontal cortex for goal-directed navigation. In this paper, I will review these studies and discuss how an animal can estimate its future positions correspond to a next movement. Investigation of the navigation problem may further provide general insights into internal models of the brain for action planning.

Supramammillary Nucleus Modulates Spike-Time Coordination in the Prefrontal-Thalamo-Hippocampal Circuit during Navigation.

During navigation, hippocampal spatial maps are thought to interact with action-planning systems in other regions of cortex. We here report a key role for spike-time coordination in functional coupling of the medial prefrontal cortex (mPFC) to the hippocampus through the thalamic nucleus reuniens (NR). When rats perform a T-maze alternation task, spikes of neurons in mPFC and NR exhibit enhanced coordination to the CA1 theta rhythm before the choice point on the maze. A similar coordination to CA1 theta rhythm was observed in neurons of the supramammillary nucleus (SUM). Optogenetic silencing of SUM neurons reduced the temporal coordination in the mPFC-NR-CA1 circuit. Following SUM inactivation, trajectory representations were impaired in both NR and CA1, but not in mPFC, indicating a failure in transmission of action plans from mPFC to the hippocampus. The findings identify theta-frequency spike-time coordination as a mechanism for gating of information flow in the mPFC-NR-CA1 circuit.

Network mechanisms of hippocampal laterality, place coding, and goal-directed navigation.

The hippocampus and associated structures are responsible for episodic memory in humans. In rodents, the most prominent behavioral correlate of hippocampal neural activity is place coding, which is thought to underlie spatial navigation. While episodic memory is considered to be unique to humans in a restricted context, it has been proposed that the same neural circuitry and algorithms that enable spatial coding and navigation also support episodic memory. Here we review the recent progress in neural circuit mechanisms of hippocampal activity by introducing several topics: (1) cooperation and specialization of the bilateral hippocampi, (2) the role of synaptic plasticity in gamma phase-locking of spikes and place cell formation, (3) impaired goal-related activity and oscillations in a mouse model of mental disorders, and (4) a prefrontal-thalamo-hippocampal circuit for goal-directed spatial navigation.

Hippocampal Remapping after Partial Inactivation of the Medial Entorhinal Cortex.

Hippocampal place cells undergo remapping when the environment is changed. The mechanism of hippocampal remapping remains elusive but spatially modulated cells in the medial entorhinal cortex (MEC) have been identified as a possible contributor. Using pharmacogenetic and optogenetic approaches, we tested the role of MEC cells by examining in mice whether partial inactivation in MEC shifts hippocampal activity to a different subset of place cells with different receptive fields. The pharmacologically selective designer Gi-protein-coupled muscarinic receptor hM4D or the light-responsive microbial proton pump archaerhodopsin (ArchT) was expressed in MEC, and place cells were recorded after application of the inert ligand clozapine-N-oxide (CNO) or light at appropriate wavelengths. CNO or light caused partial inactivation of the MEC. The inactivation was followed by substantial remapping in the hippocampus, without disruption of the spatial firing properties of individual neurons. The results point to MEC input as an element of the mechanism for remapping in place cells.

Functional diversity along the transverse axis of hippocampal area CA1.

Decades of neuroscience research have shed light on the hippocampus as a key structure for the formation of episodic memory. The hippocampus is divided into distinct subfields - CA1, CA2 and CA3. While accumulating evidence points to cellular and synaptic heterogeneity within each subfield, this heterogeneity has not received much attention in computational and behavioural studies and subfields have until recently been considered functionally uniform. However, a couple of recent studies have demonstrated prominent functional differences along the proximodistal axis of the CA1 subfield. Here, we review anatomical and physiological differences that might give rise to heterogeneity along the proximodistal axis of CA1 as well as the functional implications of such heterogeneity. We suggest that such heterogeneity in CA1 operates dynamically in the sense that the CA1 network alternates, on a subsecond scale, between a state where the network is primarily responsive to functionally segregated direct inputs from entorhinal cortex and a state where cells predominantly are controlled by more integrated inputs from CA3.

Microfluidic local perfusion chambers for the visualization and manipulation of synapses.

The polarized nature of neurons and the size and density of synapses complicates the manipulation and visualization of cell biological processes that control synaptic function. Here we developed a microfluidic local perfusion (microLP) chamber to access and manipulate synaptic regions and presynaptic and postsynaptic compartments in vitro. This chamber directs the formation of synapses in >100 parallel rows connecting separate neuron populations. A perfusion channel transects the parallel rows, allowing access with high spatial and temporal resolution to synaptic regions. We used this chamber to investigate synapse-to-nucleus signaling. Using the calcium indicator dye Fluo-4 NW, we measured changes in calcium at dendrites and somata, following local perfusion of glutamate. Exploiting the high temporal resolution of the chamber, we exposed synapses to "spaced" or "massed" application of glutamate and then examined levels of pCREB in somata. Lastly, we applied the metabotropic receptor agonist DHPG to dendrites and observed increases in Arc transcription and Arc transcript localization.

Distance-dependent homeostatic synaptic scaling mediated by a-type potassium channels.

Many lines of evidence suggest that the efficacy of synapses on CA1 pyramidal neuron dendrites increases as a function of distance from the cell body. The strength of an individual synapse is also dynamically modulated by activity-dependent synaptic plasticity, which raises the question as to how a neuron can reconcile individual synaptic changes with the maintenance of the proximal-to-distal gradient of synaptic strength along the dendrites. As the density of A-type potassium channels exhibits a similar gradient from proximal (low)-to-distal (high) dendrites, the A-current may play a role in coordinating local synaptic changes with the global synaptic strength gradient. Here we describe a form of homeostatic plasticity elicited by conventional activity blockade (with tetrodotoxin) coupled with a block of the A-type potassium channel. Following A-type potassium channel inhibition for 12 h, recordings from CA1 somata revealed a significantly higher miniature excitatory postsynaptic current (mEPSC) frequency, whereas in dendritic recordings, there was no change in mEPSC frequency. Consistent with mEPSC recordings, we observed a significant increase in AMPA receptor density in stratum pyramidale but not stratum radiatum. Based on these data, we propose that the differential distribution of A-type potassium channels along the apical dendrites may create a proximal-to-distal membrane potential gradient. This gradient may regulate AMPA receptor distribution along the same axis. Taken together, our results indicate that A-type potassium channels play an important role in controlling synaptic strength along the dendrites, which may help to maintain the computational capacity of the neuron.

Frequency-dependent signal transmission and modulation by neuromodulators.

The brain uses a strategy of labor division, which may allow it to accomplish more elaborate and complicated tasks, but in turn, imposes a requirement for central control to integrate information among different brain areas. Anatomically, the divergence of long-range neuromodulator projections appears well-suited to coordinate communication between brain areas. Oscillatory brain activity is a prominent feature of neural transmission. Thus, the ability of neuromodulators to modulate signal transmission in a frequency-dependent manner adds an additional level of regulation. Here, we review the significance of frequency-dependent signal modulation in brain function and how a neuronal network can possess such properties. We also describe how a neuromodulator, dopamine, changes frequency-dependent signal transmission, controlling information flow from the entorhinal cortex to the hippocampus.

Frequency-dependent gating of synaptic transmission and plasticity by dopamine.

The neurotransmitter dopamine (DA) plays an important role in learning by enhancing the saliency of behaviorally relevant stimuli. How this stimulus selection is achieved on the cellular level, however, is not known. Here, in recordings from hippocampal slices, we show that DA acts specifically at the direct cortical input to hippocampal area CA1 (the temporoammonic (TA) pathway) to filter the excitatory drive onto pyramidal neurons based on the input frequency. During low-frequency patterns of stimulation, DA depressed excitatory TA inputs to both CA1 pyramidal neurons and local inhibitory GABAergic interneurons via presynaptic inhibition. In contrast, during high-frequency patterns of stimulation, DA potently facilitated the TA excitatory drive onto CA1 pyramidal neurons, owing to diminished feedforward inhibition. Analysis of DA's effects over a broad range of stimulus frequencies indicates that it acts as a high-pass filter, augmenting the response to high-frequency inputs while diminishing the impact of low-frequency inputs. These modulatory effects of DA exert a profound influence on activity-dependent forms of synaptic plasticity at both TA-CA1 and Schaffer-collateral (SC)-CA1 synapses. Taken together, our data demonstrate that DA acts as a gate on the direct cortical input to the hippocampus, modulating information flow and synaptic plasticity in a frequency-dependent manner.

Miniature neurotransmission stabilizes synaptic function via tonic suppression of local dendritic protein synthesis.

Activity deprivation in neurons induces a slow compensatory scaling up of synaptic strength, reflecting a homeostatic mechanism for stabilizing neuronal activity. Prior studies have focused on the loss of action potential (AP) driven neurotransmission in synaptic homeostasis. Here, we show that the miniature synaptic transmission that persists during AP blockade profoundly shapes the time course and mechanism of homeostatic scaling. A brief blockade of NMDA receptor (NMDAR) mediated miniature synaptic events ("minis") rapidly scales up synaptic strength, over an order of magnitude faster than with AP blockade alone. The rapid scaling induced by NMDAR mini blockade is mediated by increased synaptic expression of surface GluR1 and the transient incorporation of Ca2+-permeable AMPA receptors at synapses; both of these changes are implemented locally within dendrites and require dendritic protein synthesis. These results indicate that NMDAR signaling during miniature synaptic transmission serves to stabilize synaptic function through active suppression of dendritic protein synthesis.