Attila Losonczy

Regulation of neuronal input transformations by tunable dendritic inhibition

Transforming synaptic input into action potential output is a fundamental function of neurons. The pattern of action potential output from principal cells of the mammalian hippocampus encodes spatial and nonspatial information, but the cellular and circuit mechanisms by which neurons transform their synaptic input into a given output are unknown. Using a combination of optical activation and cell type–specific pharmacogenetic silencing in vitro, we found that dendritic inhibition is the primary regulator of input-output transformations in mouse hippocampal CA1 pyramidal cells, and acts by gating the dendritic electrogenesis driving burst spiking. Dendrite-targeting interneurons are themselves modulated by interneurons targeting pyramidal cell somata, providing a synaptic substrate for tuning pyramidal cell output through interactions in the local inhibitory network. These results provide evidence for a division of labor in cortical circuits, where distinct computational functions are implemented by subtypes of local inhibitory neurons.

Dendritic inhibition in the hippocampus supports fear learning

Fear, Memory, and Place Contextual fear conditioning (CFC) is widely used as a hippocampal-dependent classical conditioning task to model human episodic memory. Lovett-Barron et al. (p. 857) combined in vivo imaging with pharmacology, pharmacogenetics, and optogenetics and they found that somatostatin-expressing, dendrite-targeting γ-aminobutyric acid–releasing interneurons in hippocampal area CA1 are required for CFC. During CFC, sensory features of the aversive event reach hippocampal output neurons through excitatory cortical afferents and require active inhibitory filtering to ensure that the hippocampus exclusively encodes the conditioned stimulus. Cholinergic activation of somatostatin-positive hippocampal CA1 interneurons promotes fear-context associations. Fear memories guide adaptive behavior in contexts associated with aversive events. The hippocampus forms a neural representation of the context that predicts aversive events. Representations of context incorporate multisensory features of the environment, but must somehow exclude sensory features of the aversive event itself. We investigated this selectivity using cell type–specific imaging and inactivation in hippocampal area CA1 of behaving mice. Aversive stimuli activated CA1 dendrite-targeting interneurons via cholinergic input, leading to inhibition of pyramidal cell distal dendrites receiving aversive sensory excitation from the entorhinal cortex. Inactivating dendrite-targeting interneurons during aversive stimuli increased CA1 pyramidal cell population responses and prevented fear learning. We propose subcortical activation of dendritic inhibition as a mechanism for exclusion of aversive stimuli from hippocampal contextual representations during fear learning.

Multi-array silicon probes with integrated optical fibers: light-assisted perturbation and recording of local neural circuits in the behaving animal

Recordings of large neuronal ensembles and neural stimulation of high spatial and temporal precision are important requisites for studying the real‐time dynamics of neural networks. Multiple‐shank silicon probes enable large‐scale monitoring of individual neurons. Optical stimulation of genetically targeted neurons expressing light‐sensitive channels or other fast (milliseconds) actuators offers the means for controlled perturbation of local circuits. Here we describe a method to equip the shanks of silicon probes with micron‐scale light guides for allowing the simultaneous use of the two approaches. We then show illustrative examples of how these compact hybrid electrodes can be used in probing local circuits in behaving rats and mice. A key advantage of these devices is the enhanced spatial precision of stimulation that is achieved by delivering light close to the recording sites of the probe. When paired with the expression of light‐sensitive actuators within genetically specified neuronal populations, these devices allow the relatively straightforward and interpretable manipulation of network activity.

Fast Synaptic Subcortical Control of Hippocampal Circuits

Subcortical Network Regulation Subcortical neuromodulatory centers dominate the motivational and emotional state–dependent control of cortical functions. Control of cortical circuits has been thought to involve a slow, diffuse neuromodulation that affects the excitability of large numbers of neurons relatively indiscriminately. Varga et al. (p. 449) describe a form of subcortical control of cortical information processing whereby strong, spatiotemporally precise excitatory input from midbrain serotonergic neurons produces a robust activation of hippocampal interneurons. This effect is mediated by a synaptic release of both serotonin and glutamate and impacts network activity patterns. A form of subcortical control of cortical information processing is mediated by a synaptic release of serotonin and glutamate. Cortical information processing is under state-dependent control of subcortical neuromodulatory systems. Although this modulatory effect is thought to be mediated mainly by slow nonsynaptic metabotropic receptors, other mechanisms, such as direct synaptic transmission, are possible. Yet, it is currently unknown if any such form of subcortical control exists. Here, we present direct evidence of a strong, spatiotemporally precise excitatory input from an ascending neuromodulatory center. Selective stimulation of serotonergic median raphe neurons produced a rapid activation of hippocampal interneurons. At the network level, this subcortical drive was manifested as a pattern of effective disynaptic GABAergic inhibition that spread throughout the circuit. This form of subcortical network regulation should be incorporated into current concepts of normal and pathological cortical function.

Hippocampal Memory Traces Are Differentially Modulated by Experience, Time, and Adult Neurogenesis

Memory traces are believed to be ensembles of cells used to store memories. To visualize memory traces, we created a transgenic line that allows for the comparison between cells activated during encoding and expression of a memory. Mice re-exposed to a fear-inducing context froze more and had a greater percentage of reactivated cells in the dentate gyrus (DG) and CA3 than mice exposed to a novel context. Over time, these differences disappeared, in keeping with the observation that memories become generalized. Optogenetically silencing DG or CA3 cells that were recruited during encoding of a fear-inducing context prevented expression of the corresponding memory. Mice with reduced neurogenesis displayed less contextual memory and less reactivation in CA3 but, surprisingly, normal reactivation in the DG. These studies suggest that distinct memory traces are located in the DG and in CA3 but that the strength of the memory is related to reactivation in CA3.

Parvalbumin-positive basket cells differentiate among hippocampal pyramidal cells.

CA1 pyramidal cells (PCs) are not homogeneous but rather can be grouped by molecular, morphological, and functional properties. However, less is known about synaptic sources differentiating PCs. Using paired recordings in vitro, two-photon Ca(2+) imaging in vivo, and computational modeling, we found that parvalbumin-expressing basket cells (PVBCs) evoked greater inhibition in CA1 PCs located in the deep compared to superficial layer of stratum pyramidale. In turn, analysis of reciprocal connectivity revealed more frequent excitatory inputs to PVBCs by superficial PCs, demonstrating bias in target selection by both the excitatory and inhibitory local connections in CA1. Additionally, PVBCs further segregated among deep PCs, preferentially innervating the amygdala-projecting PCs but receiving preferential excitation from the prefrontal cortex-projecting PCs, thus revealing distinct perisomatic inhibitory interactions between separate output channels. These results demonstrate the presence of heterogeneous PVBC-PC microcircuits, potentially contributing to the sparse and distributed structure of hippocampal network activity.

Network mechanisms of theta related neuronal activity in hippocampal CA1 pyramidal neurons

Although hippocampal theta oscillations represent a prime example of temporal coding in the mammalian brain, little is known about the specific biophysical mechanisms. Intracellular recordings support a particular abstract oscillatory interference model of hippocampal theta activity, the soma-dendrite interference model. To gain insight into the cellular and circuit level mechanisms of theta activity, we implemented a similar form of interference using the actual hippocampal network in mice in vitro. We found that pairing increasing levels of phasic dendritic excitation with phasic stimulation of perisomatic projecting inhibitory interneurons induced a somatic polarization and action potential timing profile that reproduced most common features. Alterations in the temporal profile of inhibition were required to fully capture all features. These data suggest that theta-related place cell activity is generated through an interaction between a phasic dendritic excitation and a phasic perisomatic shunting inhibition delivered by interneurons, a subset of which undergo activity-dependent presynaptic modulation.

Septo-hippocampal GABAergic signaling across multiple modalities in awake mice

Hippocampal interneurons receive GABAergic input from the medial septum. Using two-photon Ca2+ imaging of axonal boutons in hippocampal CA1 of behaving mice, we found that populations of septo-hippocampal GABAergic boutons were activated during locomotion and salient sensory events; sensory responses scaled with stimulus intensity and were abolished by anesthesia. We found similar activity patterns among boutons with common putative postsynaptic targets, with low-dimensional bouton population dynamics being driven primarily by presynaptic spiking.

Gating of hippocampal activity, plasticity, and memory by entorhinal cortex long-range inhibition

Fine-tuned information flow in the brain In addition to providing well-characterized excitatory inputs, the entorhinal cortex also sends long-range inhibitory projections to the hippocampus. Basu et al. described this input in detail and characterized its role for learning and memory. Multimodal sensory stimuli activate long-range inhibitory input in vivo. This input enables precisely timed information transfer within the cortico-hippocampal circuit. In this way, long-range inhibitory projections play an important role in providing specificity of fear conditioning, and thus help prevent overgeneralization. Science, this issue p. 10.1126/science.aaa5694 Inhibitory inputs from the lateral entorhinal cortex help to make contextual memory associations specific. INTRODUCTION The precise association of contextual cues with a behavioral experience enables an animal to discriminate between salient (harmful or rewarding) versus neutral environments. What signaling mechanisms during learning help select specific contextual signals to be stored as long-term memories? Hippocampal CA1 pyramidal neurons integrate direct multisensory excitatory input from entorhinal cortex (EC) with indirect, mnemonic excitatory input from the upstream hippocampal CA3 area, and both pathways have been implicated in memory storage. Paired activation of the direct and indirect inputs at a precise timing interval that matches the dynamics of the cortico-hippocampal circuit induces a long-term enhancement of the activation of CA1 neurons by their CA3 inputs (input timing–dependent plasticity or ITDP). However, EC additionally sends long-range inhibitory projections (LRIPs) to CA1, the function of which is largely unknown. Here, we explore the role of the LRIPs in regulating hippocampal synaptic activity and memory. RATIONALE GABAergic neurons (which release the inhibitory transmitter γ-aminobutyric acid or GABA) in medial entorhinal cortex (MEC) were recently found to send to hippocampus LRIPs that form relatively weak and sparse synapses on CA1 GABAergic interneurons. As lateral entorhinal cortex (LEC) conveys important contextual and object-related information to hippocampus, we examined whether this region also sends LRIPs to CA1. We expressed channelrhodopsin-2 (ChR2) selectively in LEC inhibitory neurons and examined the synaptic effects of LRIP photostimulation. The behavioral impact of the LRIPs was determined by selectively silencing these inputs locally in CA1 during contextual fear conditioning (CFC) and novel object recognition (NOR) tasks. We also used in vivo Ca2+ imaging to assess how different sensory and behavioral stimuli that typically make up a contextual experience activate the LEC LRIPs. Finally, we examined how the LRIPs influence information flow through the cortico-hippocampal circuit and contribute to ITDP. RESULTS LRIPs from LEC produced strong inhibitory postsynaptic potentials in a large fraction of CA1 interneurons located in the region of the EC inputs. Although pharmacogenetic silencing of LRIPs in hippocampus did not prevent CFC or NOR memory, it caused mice to show an inappropriate fear response to a neutral context and a diminished ability to distinguish a novel object from a familiar object. Calcium imaging revealed that the LRIP axons and presynaptic terminals responded to various sensory stimuli. Moreover, pairing such signals with appetitive or aversive stimuli increased LRIP activity, consistent with a role of the LRIPs in memory specificity. Intracellular recordings demonstrated that the LRIPs powerfully suppressed the activity of a subclass of cholecystokinin-expressing interneurons (CCK+ INs). These interneurons were normally strongly excited by the CA3 inputs, which results in pronounced feedforward inhibition (FFI) of CA1 pyramidal neuron dendrites. By transiently and maximally suppressing the INs in a 15- to 20-ms temporal window, the LRIPs enhanced CA3 inputs onto CA1 pyramidal neurons that arrived within that timing interval. This disinhibition enabled temporally precise, paired activation of EC–Schaffer collateral (EC-SC) inputs (15 to 20 ms apart) to trigger dendritic spikes in the distal dendrites of CA1 PNs and to induce ITDP. CONCLUSION LRIPs from EC act as a powerful, temporally precise disinhibitory gate of intrahippocampal information flow and enable the induction of plasticity when cortical and hippocampal inputs arrive onto CA1 PNs at a precise 20-ms interval. We propose that the LRIPs increase the specificity of hippocampal-based long-term memory by assessing the salience of mnemonic information relayed by CA3 to the immediate sensory context conveyed by direct excitatory EC inputs. Long-range inhibitory projections gate cortico-hippocampal information flow in the short and long term. (Top) The cortico-hippocampal circuit. Inputs from EC arrive at CA1 directly through excitatory perforant path (PP) and LRIPs and indirectly through SCs of the trisynaptic path [dentate gyrus (DG)→CA3→CA1]. (Bottom) Recordings from different EC LRIP→CA1 circuit elements. (Top left) A CA1 IN that normally inhibits the pyramidal neuron (PN) dendrite is inhibited maximally by LRIP (blue, LRIP intact) 20 ms after EC stimulation (dotted guide lines). (Bottom left) This disinhibits the PN dendritic depolarization evoked by a SC input arriving 20 ms after EC input. Multiple EC-SC pairings result in more disinhibition (middle), which triggers dendritic Ca2+ spikes (10× pairings for 10 s) and (right) induces somatic long-term plasticity (90× pairings for 90 s) in the CA1 PN, where SC responses are potentiated for >1 hour. LRIP silencing (red) decreases dendritic depolarization and spike probability and blocks somatic plasticity. [Background from a plate by C. Golgi et al.1886, text translated and republished with plates in Brain Res. Bull. 54, 461–483 (2001)] The cortico-hippocampal circuit is critical for storage of associational memories. Most studies have focused on the role in memory storage of the excitatory projections from entorhinal cortex to hippocampus. However, entorhinal cortex also sends inhibitory projections, whose role in memory storage and cortico-hippocampal activity remains largely unexplored. We found that these long-range inhibitory projections enhance the specificity of contextual and object memory encoding. At the circuit level, these γ-aminobutyric acid (GABA)–releasing projections target hippocampal inhibitory neurons and thus act as a disinhibitory gate that transiently promotes the excitation of hippocampal CA1 pyramidal neurons by suppressing feedforward inhibition. This enhances the ability of CA1 pyramidal neurons to fire synaptically evoked dendritic spikes and to generate a temporally precise form of heterosynaptic plasticity. Long-range inhibition from entorhinal cortex may thus increase the precision of hippocampal-based long-term memory associations by assessing the salience of mnemonic information to the immediate sensory input.

Sublayer-Specific Coding Dynamics during Spatial Navigation and Learning in Hippocampal Area CA1

The mammalian hippocampus is critical for spatial information processing and episodic memory. Its primary output cells, CA1 pyramidal cells (CA1 PCs), vary in genetics, morphology, connectivity, and electrophysiological properties. It is therefore possible that distinct CA1 PC subpopulations encode different features of the environment and differentially contribute to learning. To test this hypothesis, we optically monitored activity in deep and superficial CA1 PCs segregated along the radial axis of the mouse hippocampus and assessed the relationship between sublayer dynamics and learning. Superficial place maps were more stable than deep during head-fixed exploration. Deep maps, however, were preferentially stabilized during goal-oriented learning, and representation of the reward zone by deep cells predicted task performance. These findings demonstrate that superficial CA1 PCs provide a more stable map of an environment, while their counterparts in the deep sublayer provide a more flexible representation that is shaped by learning about salient features in the environment. VIDEO ABSTRACT.