Simon Chamberland

Inhibitory control of hippocampal inhibitory neurons

Information processing within neuronal networks is determined by a dynamic partnership between principal neurons and local circuit inhibitory interneurons. The population of GABAergic interneurons is extremely heterogeneous and comprises, in many brain regions, cells with divergent morphological and physiological properties, distinct molecular expression profiles, and highly specialized functions. GABAergic interneurons have been studied extensively during the past two decades, especially in the hippocampus, which is a relatively simple cortical structure. Different types of hippocampal inhibitory interneurons control spike initiation [e.g., axo-axonic and basket cells (BCs)] and synaptic integration (e.g., bistratified and oriens–lacunosum moleculare interneurons) within pyramidal neurons and synchronize local network activity, providing a means for functional segregation of neuronal ensembles and proper routing of hippocampal information. Thus, it is thought that, at least in the hippocampus, GABAergic inhibitory interneurons represent critical regulating elements at all stages of information processing, from synaptic integration and spike generation to large-scale network activity. However, this raises an important question: if inhibitory interneurons are fundamental for network computations, what are the mechanisms that control the activity of the interneurons themselves? Given the essential role of synaptic inhibition in the regulation of neuronal activity, it would be logical to expect that specific inhibitory mechanisms have evolved to control the operation of interneurons. Here, we review the mechanisms of synaptic inhibition of interneurons and discuss their role in the operation of hippocampal inhibitory circuits.

Dendritic Inhibition Provided by Interneuron-Specific Cells Controls the Firing Rate and Timing of the Hippocampal Feedback Inhibitory Circuitry

In cortical networks, different types of inhibitory interneurons control the activity of glutamatergic principal cells and GABAergic interneurons. Principal neurons represent the major postsynaptic target of most interneurons; however, a population of interneurons that is dedicated to the selective innervation of GABAergic cells exists in the CA1 area of the hippocampus. The physiological properties of these cells and their functional relevance for network computations remain unknown. Here, we used a combination of dual simultaneous patch-clamp recordings and targeted optogenetic stimulation in acute mouse hippocampal slices to examine how one class of interneuron-specific (IS) cells controls the activity of its GABAergic targets. We found that type 3 IS (IS3) cells that coexpress the vasoactive intestinal polypeptide (VIP) and calretinin contact several distinct types of interneurons within the hippocampal CA1 stratum oriens/alveus (O/A), with preferential innervation of oriens-lacunosum moleculare cells (OLMs) through dendritic synapses. In contrast, VIP-positive basket cells provided perisomatic inhibition to CA1 pyramidal neurons with the asynchronous GABA release and were not connected with O/A interneurons. Furthermore, unitary IPSCs recorded at IS3–OLM synapses had a small amplitude and low release probability but summated efficiently during high-frequency firing of IS3 interneurons. Moreover, the synchronous generation of a single spike in several IS cells that converged onto a single OLM controlled the firing rate and timing of OLM interneurons. Therefore, dendritic inhibition originating from IS cells is needed for the flexible activity-dependent recruitment of OLM interneurons for feedback inhibition.

Activity-Dependent Compartmentalized Regulation of Dendritic Ca2+ Signaling in Hippocampal Interneurons

Activity-dependent regulation of synaptic inputs in neurons is controlled by highly compartmentalized and dynamic dendritic calcium signaling. Among multiple Ca2+ mechanisms operating in neuronal dendrites, voltage-sensitive Ca2+ channels (VSCCs) represent a major source of Ca2+ influx; however, their use-dependent implication, regulation, and function in different types of central neurons remain widely unknown. Using two-photon microscopy to probe Ca2+ signaling in dendrites of hippocampal oriens/alveus interneurons, we found that intense synaptic activity or local activation of mGluR5 induced long-lasting potentiation of action potential evoked Ca2+ transients. This potentiation of dendritic Ca2+ signaling required mGluR5-induced intracellular Ca2+ release and PKC activation and was expressed as a selective compartmentalized potentiation of L-type VSCCs. Thus, in addition to mGluR1a-dependent synaptic plasticity, hippocampal interneurons in the feedback inhibitory circuit demonstrate a novel form of mGluR5-induced dendritic plasticity. Given an implication of L-type VSCCs in the induction of Hebbian LTP at interneuron excitatory synapses, their activity-dependent regulation may represent a powerful mechanism for regulating synaptic plasticity.

Synapse-Specific Inhibitory Control of Hippocampal Feedback Inhibitory Circuit

Local circuit and long-range GABAergic projections provide powerful inhibitory control over the operation of hippocampal inhibitory circuits, yet little is known about the input- and target-specific organization of interacting inhibitory networks in relation to their specific functions. Using a combination of two-photon laser scanning photostimulation and whole-cell patch clamp recordings in mice hippocampal slices, we examined the properties of transmission at GABAergic synapses formed onto hippocampal CA1 stratum oriens – lacunosum moleculare (O–LM) interneurons by two major inhibitory inputs: local projection originating from stratum radiatum interneurons and septohippocampal GABAergic terminals. Optical mapping of local inhibitory inputs to O–LM interneurons revealed that vasoactive intestinal polypeptide- and calretinin-positive neurons, with anatomical properties typical of type III interneuron-specific interneurons, provided the major local source of inhibition to O–LM cells. Inhibitory postsynaptic currents evoked by minimal stimulation of this input exhibited small amplitude and significant paired-pulse and multiple-pulse depression during repetitive activity. Moreover, these synapses failed to show any form of long-term synaptic plasticity. In contrast, synapses formed by septohippocampal projection produced higher amplitude and persistent inhibition and exhibited long-term potentiation induced by theta-like activity. These results indicate the input and target-specific segregation in inhibitory control, exerted by two types of GABAergic projections and responsible for distinct dynamics of inhibition in O–LM interneurons. The two inputs are therefore likely to support the differential activity- and brain state-dependent recruitment of hippocampal feedback inhibitory circuits in vivo, crucial for dendritic disinhibition and computations in CA1 pyramidal cells.

Cell type‐specific and activity‐dependent dynamics of action potential‐evoked Ca2+ signals in dendrites of hippocampal inhibitory interneurons

Non‐technical summary  Action potentials generated at the level of the cell body can propagate back to neuronal dendrites, where they activate different types of voltage‐sensitive calcium channels and produce massive calcium influx. Although these calcium signals may control dendritic integration, their mechanisms, dynamic properties and role in different cell types remain largely unknown. We found that in dendrites of hippocampal interneurons, an inhibitory cell type involved in control of network excitability, specific types of calcium channels are present but are recruited in an activity‐dependent manner. Furthermore, their activation produces calcium rises spatially restricted to proximal dendritic sites, where they control the efficacy of transmission at inhibitory synapses. The pathway by which this happens appears to constitute a negative feedback loop – increased firing activity of interneurons potentiates the inhibitory drive that they receive, thus decreasing the activity of interneurons further. This may have a profound effect on the recruitment of interneurons and network activity.

Fast two-photon imaging of subcellular voltage dynamics in neuronal tissue with genetically encoded indicators

Monitoring voltage dynamics in defined neurons deep in the brain is critical for unraveling the function of neuronal circuits but is challenging due to the limited performance of existing tools. In particular, while genetically encoded voltage indicators have shown promise for optical detection of voltage transients, many indicators exhibit low sensitivity when imaged under two-photon illumination. Previous studies thus fell short of visualizing voltage dynamics in individual neurons in single trials. Here, we report ASAP2s, a novel voltage indicator with improved sensitivity. By imaging ASAP2s using random-access multi-photon microscopy, we demonstrate robust single-trial detection of action potentials in organotypic slice cultures. We also show that ASAP2s enables two-photon imaging of graded potentials in organotypic slice cultures and in Drosophila. These results demonstrate that the combination of ASAP2s and fast two-photon imaging methods enables detection of neural electrical activity with subcellular spatial resolution and millisecond-timescale precision. DOI: http://dx.doi.org/10.7554/eLife.25690.001

Vesicles derived via AP-3-dependent recycling contribute to asynchronous release and influence information transfer

Summary Action potentials trigger synchronous and asynchronous neurotransmitter release. Temporal properties of both types of release could be altered in an activity-dependent manner. While the effects of activity-dependent changes in synchronous release on postsynaptic signal integration have been studied, the contribution of asynchronous release to information transfer during natural stimulus patterns is unknown. Here we find that during trains of stimulations, asynchronous release contributes to the precision of action potential firing. Our data show that this form of release is selectively diminished in AP-3b2 KO animals, which lack functional neuronal AP-3, an adaptor protein regulating vesicle formation from endosomes generated during bulk endocytosis. We find that in the absence of neuronal AP-3, asynchronous release is attenuated and the activity-dependent increase in the precision of action potential timing is compromised. Lack of asynchronous release decreases the capacity of synaptic information transfer and renders synaptic communication less reliable in response to natural stimulus patterns.

Age-dependent remodelling of inhibitory synapses onto hippocampal CA1 oriens-lacunosum moleculare interneurons

Non‐Technical Summary  The main function of the inhibitory synapse is to provide the membrane hyperpolarization and, thereby, to control the level of activity of its target cell. Extensively studied in pyramidal neurons, the properties of inhibitory synapses that target inhibitory interneurons remain largely unknown. We studied the properties of inhibitory synapses formed onto interneurons involved in the hippocampal feedback inhibitory circuit. Our data revealed a significant, age‐dependent strengthening of inhibition of interneurons due to the synaptic incorporation of the α5 subunit of the GABAA receptor. This novel mechanism of age‐dependent refinement of local circuit inhibition may have a direct impact on the hippocampal network activity and performance during development.

Functionally heterogeneous synaptic vesicle pools support diverse synaptic signalling

Synaptic communication between neurons is a highly dynamic process involving specialized structures. At the level of the presynaptic terminal, neurotransmission is ensured by fusion of vesicles to the membrane, which releases neurotransmitter in the synaptic cleft. Depending on the level of activity experienced by the terminal, the spatiotemporal properties of calcium invasion will dictate the timing and the number of vesicles that need to be released. Diverse presynaptic firing patterns are translated to neurotransmitter release with a distinct temporal feature. Complex patterns of neurotransmitter release can be achieved when different vesicles respond to distinct calcium dynamics in the presynaptic terminal. Specific vesicles from different pools are recruited during various modes of release as the particular molecular composition of their membrane proteins define their functional properties. Such diversity endows the presynaptic terminal with the ability to respond to distinct physiological signals via the mobilization of specific subpopulation of vesicles. There are several mechanisms by which a diverse vesicle population could be generated in single presynaptic terminals, including distinct recycling pathways that utilize various adaptor proteins. Several additional factors could potentially contribute to the development of a heterogeneous vesicle pool such as specialized release sites, spatial segregation within the terminal and specialized delivery pathways. Among these factors molecular heterogeneity plays a central role in defining the functional properties of different subpopulations of vesicles.

Short-Term Facilitation at a Detonator Synapse Requires the Distinct Contribution of Multiple Types of Voltage-Gated Calcium Channels

Neuronal calcium elevations are shaped by several key parameters, including the properties, density, and the spatial location of voltage-gated calcium channels (VGCCs). These features allow presynaptic terminals to translate complex firing frequencies and tune the amount of neurotransmitter released. Although synchronous neurotransmitter release relies on both P/Q- and N-type VGCCs at hippocampal mossy fiber–CA3 synapses, the specific contribution of VGCCs to calcium dynamics, neurotransmitter release, and short-term facilitation remains unknown. Here, we used random-access two-photon calcium imaging together with electrophysiology in acute mouse hippocampal slices to dissect the roles of P/Q- and N-type VGCCs. Our results show that N-type VGCCs control glutamate release at a limited number of release sites through highly localized Ca2+ elevations and support short-term facilitation by enhancing multivesicular release. In contrast, Ca2+ entry via P/Q-type VGCCs promotes the recruitment of additional release sites through spatially homogeneous Ca2+ elevations. Altogether, our results highlight the specialized contribution of P/Q- and N-types VGCCs to neurotransmitter release. SIGNIFICANCE STATEMENT In presynaptic terminals, neurotransmitter release is dynamically regulated by the transient opening of different types of voltage-gated calcium channels. Hippocampal giant mossy fiber terminals display extensive short-term facilitation during repetitive activity, with a large several fold postsynaptic response increase. Though, how giant mossy fiber terminals leverage distinct types of voltage-gated calcium channels to mediate short-term facilitation remains unexplored. Here, we find that P/Q- and N-type VGCCs generate different spatial patterns of calcium elevations in giant mossy fiber terminals and support short-term facilitation through specific participation in two mechanisms. Whereas N-type VGCCs contribute only to the synchronization of multivesicular release, P/Q-type VGCCs act through microdomain signaling to recruit additional release sites.