JCJC SVSE 4 - JCJC : Sciences de la vie, de la santé et des écosystèmes : Neurosciences

Presubicular head direction cells - cellular properties and synaptic connectivity – Head direction

Small neuronal networks : Cells and circuits for signaling head direction.

The presubicular cortex is part of a neuronal circuit coding for spatial information. We dissect the neuro-anatomy and physiology of this small network: we examine how specific cellular currents and firing patterns of presubicular neurons contribute to its functional role, and how inhibitory and excitatory synaptic connections are organized.

Cellular and network mechanisms underlying spatial information processing

The coding of space involves an extended network of several brain regions. This project focuses on presubiculum, a small neuronal network that signals spatial information and in particular the orientation of the head. Understanding cognitive brain function is one of the most important questions in neuroscience. It is likely that small neuronal networks are the building blocks of information processing, and the presubicular microcircuit has not been studied at the cellular level. Presubiculum is a transitional cortical region between hippocampus and neocortex, and my goal will be to investigate its cellular and network elements. Knowledge of ionic currents, firing patterns and morphology of single neurons will lay the basis for understanding how this microcircuit operates. It will be crucial to address functional connectivity of different neurons and neuron types. Because cytoarchitectonis impact on function, different cell types in different layers will be distinguished and characterized. Chains of synaptically connected neurons may be organized following vertical columnar type structures. Specifically, the spread of excitation within presubiculum and from afferent regions will be examined. Inhibitory control may be important to sharpen the head directional signal in presubiculum. Intense directional activity could give rise to short term plasticity, while synaptic function may not be modified in the long term.<br />The dissection of the presubicular microcircuit will allow advances towards understanding its neuronal computation. The overall aim will be the description of how electrical cellular and network activity in the brain may encode information about space. It is hoped that such insight will help to better understand human neuropathological conditions.<br />

This project relies on several recent techniques, to explore the functional neuroanatomy of the presubicular cortex. Single neuron electrophysiology in vitro is combined with anatomical reconstruction of recorded cells, while state-of-the art optical methods give a high level of spatial and temporal precision for neuronal stimulation.
The rodent presubicular microcircuit is studied in the in vitro slice preparation. Cells are visually identified under the microscope, and electrical recordings using the patch clamp technique let us study intrinsic properties of single neurons. Recorded neurons are filled with biocytin for intracellular staining, and dendritic trees and axons are reconstructed post hoc in three dimensions. Data on firing patterns, adaptation, the expression of specific cellular currents and their pharmacology are correlated with neuronal form, localization and cell type.
Focal glutamate uncaging is combined with whole cell records from single neurons to map the sites generating excitatory and inhibitory inputs. Laser scanning photostimulation can rapidly cover a large portion of the presubiculum as we see it in the slice, and therefore a complete connectivity data set can be acquired during one recording session. Excitatory or inhibitory connections may be confirmed in paired recordings, and these will also inform about synapse dynamics and plasticity. Specific interneurons are targeted using genetically modified mouse strains. The distribution of interneuron subtypes is quantified with immunohistochemistry. Long-range afferents to presubiculum that provide inputs for head direction signaling are identified using retrograde tracing techniques.

One major aim of the Head Direction project has been the elucidation of the cellular neuroanatomy of presubiculum. Presubicular principal cells have been classified, using an unsupervised cluster analysis based on cellular position, form and physiology obtained from in vitro records. Presubicular cell types tend to correspond to neuronal types in other cortical regions, their spatial arrangement is closer to those of neighboring entorhinal cortex, than the cloud like architecture of the subiculum. Groups of neurons with comparable properties are arranged in superficial, intermediate and deep layers of presubiculum. Two specific features of presubicular neurons are of interest. First, layer IV cells, of the sparsely populated lamina dissecans, fired in bursts of action potentials, whereas principal cells from other superficial and deep layers discharged regularly. Second, repetitively firing principal neurons showed very little frequency adaptation consistent with a possible role in a maintained signaling of head direction.
The functional role of the previously described TTX-insensitive sodium current may be important for signal integration in presubicular neurons. We show it can be turned on in an all-or-none fashion by local glutamate application and give rise to bursts of action potentials. We now ask how many synchronous excitatory inputs are needed to activate this boosting current. Research on synaptic connections is ongoing. Spontaneous inhibitory activity dominates the local presubicular network, and synchronous inhibitory inputs can be detected in pairs of recorded principal neurons.

Presubiculum integrates distant afferent connections from thalamus and visual cortex. Novel optogenetics techniques may allow functional connectivity mapping experiments using photoactivation of neuron terminals that have been virally transfected to express light-sensitive ion channels. Subcellular connectivity may be examined using two photon or holographic light stimulation, for a better understanding of the network architecture underlying spatial information processing in the presubicular microcircuit. Knowledge of physiological functioning of that part of the brain and of an extended network of neurons signaling head direction may also help research on pathology and notably neurodegenerative disorders that affect the parahippocampal region at an early stage of the disease.

Fricker, D., Simonnet, J., Bendels, M., Eugène, E., Cohen, I. and Miles, R. (2011) Direct activation of glutamate receptors by local photolysis of caged glutamate in presubicular pyramidal cells and interneurons. Göttingen Meeting of the German Neuroscience Society. T7-11A. Calibration experiments for laser photostimulation. Glutamate uncaging evokes synaptic-like EPSPs with highest amplitudes close to the soma.
Simonnet, J., Bendels, M., Eugène, E., Cohen, I., Miles, R. and Fricker, D. (2011) Morphological and electrophysiological properties of rat presubicular neurons. Canaux Ioniques, 22ème Colloque. A TTX-insensitive sodium current can boost and amplify uncaging evoked gluEPSPs.
Simonnet, J., Eugène, E., Cohen, I., Miles, R. and Fricker, D. (2012) Cellular neuroanatomy of rat presubiculum. Manuscript submitted. The first description of cytoarchitectonics of this transitional cortical region. We have discovered pyramidal burst firing neurons in layer IV, also referred to as ‘lamina dissecans’.

The hippocampus and the parahippocampal regions are important for spatial navigation in both rodents and humans. This project will explore the biophysical properties and synaptic connectivity of pyramidal cells in a little studied region of the hippocampal formation, the presubiculum, in the context of their functional role. Cells in the presubiculum are head-direction cells, discharging without adaptation when the head of an animal is oriented in one specific direction. In this way they contribute as a sort of compass to hippocampal function in spatial behaviours.

I have discovered that these cells possess a voltage-dependent inward current that is suppressed by removal of external sodium ions but is not blocked by the Na-channel antagonist tetrodotoxin (TTX). I wish to uncover the molecular identity of this TTX-insensitive Na current and to explore the functional consequences of its probable dendritic expression site. I will explore how this slowly inactivating Na-current contributes, together with other cellular currents to the weak adaptation of firing frequency of these cells in response to maintained current injection. This low adaptation is unusual for cortical pyramidal cells, but may be crucial for head-direction cells if they are to discharge in a maintained way while the head of the animal points in the same direction. I will also examine the synaptic connectivity of local inputs to pyramidal cells in layer V of the pre-subiculum using glutamate uncaging techniques. I will test the hypothesis that excitatory inputs, organised in a primarily columnar fashion, show little or no frequency dependance in order to maintain an unchanged firing frequency in response to a maintained input. I will also test whether inhibitory inputs are strongest across columns as needed to assure a separation between cells that signal different directions.

The ANR funding will allow the acquisition of the necessary equipment for this project, as for example the fibre optic coupled UV uncaging system. The theme of the project is a new direction of research within the laboratory that I have begun to develop and that I wish to pursue in building my own independent research group.

Project coordination

Desdemona Fricker (INSTITUT NATIONAL DE LA SANTE ET DE LA RECHERCHE MEDICALE - DELEGATION PARIS VI) – desdemona.fricker@upmc.fr

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

INSERM INSTITUT NATIONAL DE LA SANTE ET DE LA RECHERCHE MEDICALE - DELEGATION PARIS VI

Help of the ANR 240,000 euros
Beginning and duration of the scientific project: - 48 Months

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