Functional Consequences of Dendritic Inhibition in the Hippocampus

The ability to store and recall memories is an essential function of nervous systems, and at the core of subjective human experience. As such, neuropsychiatric conditions that impair our memory capacity are devastating. Learning and memory in mammals have long been known to depend on the hippocampus, which has motivated widespread research efforts that converge on two broad themes: determining how different cell types in the hippocampus interact to generate neural activity patterns (structure), and determining how neural activity patterns implement learning and memory (function). Central to both these pursuits are pyramidal cells (PCs) in CA1, the primary hippocampal output, which transform excitatory synaptic inputs into the action potential output patterns that encode information about locations or events relevant for memory. CA1 PCs are embedded in a network of diverse inhibitory (GABA-releasing) interneurons, which may play unique roles in sculpting the activity patterns of PCs that implement memory functions. As a consequence, investigating the functional impact of defined GABAergic interneurons can provide an experimental entry point for linking neural circuit structure to defined computations and behavioral functions in the hippocampal memory system. In this thesis I have applied a panel of novel methodologies to the mouse hippocampus in vitro and in vivo to link structure to function and behavior, and determine 1) how hippocampal inhibitory cell types shape distinct patterns of PC activity, and 2) how these inhibitory cell types contribute to the encoding of contextual fear memories. To first establish the means by which interneuron subtypes contribute to PC activity patterns, I used optogenetic techniques to activate spatiotemporally distributed synaptic excitation to CA1 in vitro, and recorded from PCs to quantify the frequency of output spikes relative to input levels. I subsequently used a dual viral and transgenic approach to combine this technique with selective pharmacogenetic inactivation of identified interneurons during synaptic excitation. I found that inactivating somatostatin-expressing (Som+) dendrite-targeting interneurons increased the gain of PC input-output transformations by causing more output spikes, while inactivating parvalbumin-expressing (Pvalb+) soma-targeting interneurons did not. Inactivating Som+ inhibitory interneurons allowed the dendrites of PCs to generate local NMDA receptor-mediated electrogenesis in response to synaptic input, resulting in high frequency bursts of output spikes. This discovery suggests neuronal coding via hippocampal burst spiking output can be regulated by Som+ dendrite-targeting interneurons in CA1. Specific types of neural codes are believed to have different functional roles. Neural coding with burst spikes is known to support hippocampal contributions to classical contextual fear conditioning (CFC). In CFC the hippocampus encodes the multisensory context as a conditioned stimulus (CS), whose burst spiking output is paired with the aversive unconditioned stimulus (US) in the amygdala, allowing for fear memory recall upon future exposure to the CS. To investigate the contribution of Som+ interneurons to this behavior, I designed a CFC task for head-fixed mice, allowing for optical recording and manipulation of activity in defined CA1 cell types during learning. Pharmacogenetic inactivation of CA1 Som+ interneurons, but not Pvalb+ interneurons, prevented the encoding of CFC. 2-photon Ca2+ imaging revealed that during CFC the US activated CA1 Som+ interneurons via cholinergic input from the medial septum, driving inhibition to the PC distal dendrites that receive coincident excitatory input from the entorhinal cortex. Inactivating Som+ interneurons increases PC population activity, and suppressing dendritic inhibition during the US alone is sufficient to prevent fear learning. These results suggest sensory features of the US reach CA1 PCs through entorhinal inputs, and thus require active inhibitory filtering by Som+ interneurons to ensure hippocampal output exclusively encodes the CS during CFC. In conclusion, I found that Som+ interneurons in CA1 are an effective regulator of PC burst spiking because they inhibit dendritic electrogenesis. This function is used by the hippocampus to prevent the US from influencing the burst spike output of PCs that encode the CS, ensuring successful CFC. This work bridges the gap between cells, circuits, and behavior, and provides mechanistic insight into one of our most essential cognitive functions - the ability to learn and remember

Parvalbumin-Positive Basket Cells Differentiate among Hippocampal Pyramidal Cells

SummaryCA1 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 Ca2+ 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.Video Abstrac

Dual Photosensitive Polymers with Wavelength-Selective Photoresponse

Polyurethane thin films that photopolymerize and photodegrade upon exposure to light of different wavelengths are presented. The chromic response is based on two caged monomers with the ability to be activated or photocleaved with different wavelengths under single and two-photon excitation. This material represents a dual photoresist with "positive" and "negative" tone contained in a single resist formulation and with the ability to generate complex 2D and 3D patterns.The authors thank the DFG-ANR bilateral funding program for financial support (ANR- 09-BLAN-0426–01 and DFG CA880/3–1). Andreas Best, Dr. K. Koynov and Dr. F. Laquai (MPIP Mainz) are gratefully acknowledged for their help with the two-photon exposure

Cognitive deficits caused by prefrontal cortical and hippocampal neural disinhibition

We review recent evidence concerning the significance of inhibitory GABA transmission and of neural disinhibition, i.e. deficient GABA transmission, within prefrontal cortex and hippocampus for clinically relevant cognitive functions. Both regions support important cognitive functions, including attention and memory, and their dysfunction has been implicated in cognitive deficits characterizing neuropsychiatric disorders. GABAergic inhibition shapes cortico-hippocampal neural activity and, recently, prefrontal and hippocampal neural disinhibition has emerged as a pathophysiological feature of major neuropsychiatric disorders, especially schizophrenia and age-related cognitive decline. Regional neural disinhibition, disrupting spatio-temporal control of neural activity and causing aberrant drive of projections, may disrupt processing within the disinhibited region and efferent regions. Recent studies in rats showed that prefrontal and hippocampal neural disinhibition (by local GABA antagonist microinfusion) dysregulates burst firing, which has been associated with important aspects of neural information processing. Using translational tests of clinically-relevant cognitive functions, these studies showed that prefrontal and hippocampal neural disinhibition disrupts regional cognitive functions (including prefrontal attention and hippocampal memory function); moreover, hippocampal neural disinhibition disrupted attentional performance, which does not require the hippocampus, but requires prefrontal-striatal circuits modulated by the hippocampus. However, some prefrontal and hippocampal functions (including inhibitory response control) are spared by regional disinhibition. We consider conceptual implications of these findings, regarding the distinct relationships of distinct cognitive functions to prefrontal and hippocampal GABA tone and neural activity. Moreover, the findings support that prefrontal and hippocampal neural disinhibition contributes to clinically relevant cognitive deficits, and we consider pharmacological strategies for ameliorating cognitive deficits by rebalancing disinhibition-induced aberrant neural activity

Anatomically heterogeneous populations of CB cannabinoid receptor-expressing interneurons in the CA3 region of the hippocampus show homogeneous input-output characteristics

A subpopulation of GABAergic cells in cortical structures expresses CB1 cannabinoid receptors (CB1 ) on their axon terminals. To understand the function of these interneurons in information processing, it is necessary to uncover how they are embedded into neuronal circuits. Therefore, the proportion of GABAergic terminals expressing CB1 and the morphological and electrophysiological properties of CB1 -immunoreactive interneurons should be revealed. We investigated the ratio and the origin of CB1 -expressing inhibitory boutons in the CA3 region of the hippocampus. Using immunocytochemical techniques, we estimated that approximately 40% of GABAergic axon terminals in different layers of CA3 also expressed CB1 . To identify the inhibitory cell types expressing CB1 in this region, we recorded and intracellularly labeled interneurons in hippocampal slices. CB1 -expressing interneurons showed distinct axonal arborization, and were classified as basket cells, mossy-fiber-associated cells, dendritic-layer-innervating cells or perforant-path-associated cells. In each morphological category, a substantial variability in axonal projection was observed. In contrast to the diverse morphology, the active and passive membrane properties were found to be rather similar. Using paired recordings, we found that pyramidal cells displayed large and fast unitary postsynaptic currents in response to activating basket and mossy-fiber-associated cells, while they showed slower and smaller synaptic events in pairs originating from interneurons that innervate the dendritic layer, which may be due to dendritic filtering. In addition, CB1 activation significantly reduced the amplitude of the postsynaptic currents in each cell pair tested. Our data suggest that CB1 -expressing interneurons with different axonal projections have comparable physiological characteristics, contributing to a similar proportion of GABAergic inputs along the somato-dendritic axis of CA3 pyramidal cells. (c) 2014 Wiley Periodicals, Inc

Conjunctive input processing drives feature selectivity in hippocampal CA1 neurons

Feature-selective firing allows networks to produce representations of the external and internal environments. Despite its importance, the mechanisms generating neuronal feature selectivity are incompletely understood. In many cortical microcircuits the integration of two functionally distinct inputs occurs nonlinearly through generation of active dendritic signals that drive burst firing and robust plasticity. To examine the role of this processing in feature selectivity, we recorded CA1 pyramidal neuron membrane potential and local field potential in mice running on a linear treadmill. We found that dendritic plateau potentials were produced by an interaction between properly timed input from entorhinal cortex and hippocampal CA3. These conjunctive signals positively modulated the firing of previously established place fields and rapidly induced new place field formation to produce feature selectivity in CA1 that is a function of both entorhinal cortex and CA3 input. Such selectivity could allow mixed network level representations that support context-dependent spatial maps.Howard Hughes Medical InstituteRikagaku Kenkyūjo (Japan

Selective Reduction of AMPA Currents onto Hippocampal Interneurons Impairs Network Oscillatory Activity

Reduction of excitatory currents onto GABAergic interneurons in the forebrain results in impaired spatial working memory and altered oscillatory network patterns in the hippocampus. Whether this phenotype is caused by an alteration in hippocampal interneurons is not known because most studies employed genetic manipulations affecting several brain regions. Here we performed viral injections in genetically modified mice to ablate the GluA4 subunit of the AMPA receptor in the hippocampus (GluA4HC−/− mice), thereby selectively reducing AMPA receptor-mediated currents onto a subgroup of hippocampal interneurons expressing GluA4. This regionally selective manipulation led to a strong spatial working memory deficit while leaving reference memory unaffected. Ripples (125–250 Hz) in the CA1 region of GluA4HC−/− mice had larger amplitude, slower frequency and reduced rate of occurrence. These changes were associated with an increased firing rate of pyramidal cells during ripples. The spatial selectivity of hippocampal pyramidal cells was comparable to that of controls in many respects when assessed during open field exploration and zigzag maze running. However, GluA4 ablation caused altered modulation of firing rate by theta oscillations in both interneurons and pyramidal cells. Moreover, the correlation between the theta firing phase of pyramidal cells and position was weaker in GluA4HC−/− mice. These results establish the involvement of AMPA receptor-mediated currents onto hippocampal interneurons for ripples and theta oscillations, and highlight potential cellular and network alterations that could account for the altered working memory performance

Transcranial magnetic stimulation (TMS) inhibits cortical dendrites

One of the leading approaches to non-invasively treat a variety of brain disorders is transcranial magnetic stimulation (TMS). However, despite its clinical prevalence, very little is known about the action of TMS at the cellular level let alone what effect it might have at the subcellular level (e.g. dendrites). Here, we examine the effect of single-pulse TMS on dendritic activity in layer 5 pyramidal neurons of the somatosensory cortex using an optical fiber imaging approach. We find that TMS causes GABAB-mediated inhibition of sensory-evoked dendritic Ca(2+) activity. We conclude that TMS directly activates fibers within the upper cortical layers that leads to the activation of dendrite-targeting inhibitory neurons which in turn suppress dendritic Ca(2+) activity. This result implies a specificity of TMS at the dendritic level that could in principle be exploited for investigating these structures non-invasively