The transcriptomic profile and synaptic excitability of vasoactive intestinal peptide-expressing interneurons in the mouse hippocampus

Authors: Luo, Xiao
Advisor: Topolnik, Lisa
Abstract: Neurons are the building blocks of nervous system. In the cortex, neurons can be divided into principal cells that perform excitatory computations through local and long-range synaptic connections and interneurons that controls all subcellular domains of principal cells. Interneurons inhibit principal cells by hyperpolarizing the postsynaptic membrane via GABA receptors. In addition to controlling the level of excitability of single cells via transient or long-lasting inhibition, they coordinate the firing of principal cell ensembles to generate network oscillations that travel across brain areas. The malfunction of interneurons leads to severe brain disorders such as schizophrenia, autism and epilepsy. In contrast to principal cells, interneurons display high level of diversity, consistant with their various functional roles in the brain circuitry. To understand their network functions, neuroscientists have developed several criteria to classify interneurons, including cytomorphology, connectivity, electrophysiological properties, andmolecular markers. In general, three interneuron types account for the majority of cortical interneurons: dendrite-targeting somatostatin (SOM)+ cells, soma-targeting parvalbumin (PV)+ cells, and interneuron-specific vasoactive intestinal peptide (VIP)+ cells. In the hippocampus, VIP+ cells (excluding VIP+ basket cell) play a unique role in the network, since they preferentially innervate interneurons but avoid principal cells. However, their taxonomy and physiological properties are less clearcompared to other interneuron types. My PhD project focused on two subtype of VIP cells: VIP+ long- projecting cell and type 3 interneuron-specific (IS3) cell. A novel long-projecting VIP+cell (VIP-LRP) has been identified in the hippocampal CA1 Oriens/Alveus (Francavilla et al., 2018). These cells selectively target interneurons in the hippocampal CA1 area, but also project to the neighbouring area subiculum. In addition, they are more active during the stationary period of wakefulness, and silent during theta or ripple oscillations. However, the molecular markers they express were unclear. To examine their molecular markers and develop cell-type specific mouselines using combinatorial genetic approach, I first performed immunohistochemistry to profile commonly expressed markers including muscarinic receptor 2 (M2R), cholecystokinin (CCK), calbidin (CB), neuronal nitric oxide synthase (nNOS), calretinin (CR), andSOM in VIP cells in the striatum oriens(SO) of CA1. We found that VIP-LRP cells were negative or SOM and nNOS but half of them expressed M2R. Moreover, a small fraction of GFP cells express CCK, CB, and CR. The proportion of M2R+ VIP-LRP cells was different between different mouse strains. Next, we performed transcriptomic profiling of anatomically identified VIP-LRPs using single-cell RNA sequencing. I identified several molecular markers, such as proenkephalin, neuropeptide Y and netrin G1, as well as many other genes that belong to several important gene families including ion channels, neurotransmitter receptors, neuromodulators, cell adhesion and myelination molecules. In addition, VIP-LRPs share common genes when comparing with VIP; CR and VIP; CCK cell types in the neocortex. Together, these data suggest that although VIP-LRPs represent an intermediate group within the VIP subtype, they may express genes related to specific features that allow for long-distance coordination of neuronal activities in the CA1 and the subiculum. After that, I examined the excitatory synaptic properties of another VIP+ subtype-the IS3 cell. Previous studies showed that they make synapses on interneurons in SO, which in turn control the integration of excitatory inputs received by the proximal and distal dendrites of pyramidal cells. However, the properties of excitatory inputs conveying on IS3 cells remain unknown. Using patch clamp recording and two-photon glutamate uncaging, we evaluated the synaptic properties of two excitatory inputs formed on the IS3 cells by the Schaffer collaterals (SC) from CA3 and the Temporoammonic (TA) pathway from entorhinal cortex. The results showed that the excitatory postsynaptic currents(EPSCs) evoked in IS3 cells by electrical stimulation of the TA pathway had a smaller amplitude, slower rise and decay time compared to that of the SC synapses. In addition, TA-IS3 synaptic transmission was mediated by AMPA and NMDA receptors. Furthermore, both TA and SC-EPSCs showed short-term synaptic facilitation in response to repetitive stimulation. Finally, TA and SC pathways displayed similar degree of spatial integration. When these synaptic properties were incorporated into the in vivo-like IS3 computational model (Guet-McCreight et al., 2016), The activation of IS3 cells can be driven by these excitatory inputs during hippocampal theta and ripple oscillations. In vivo two-photon imaging in awake mice showed that the firing of IS3 cells increased during theta rhythm, whereas their activites were not associated with ripples. Together, these data shows that while excitatory inputs are able to drive the firing of IS3 cells during theta, additional mechanisms, such as local inhibition and subcortical modulation may account for the silence of IS3 cells during ripples.
Document Type: Thèse de doctorat
Issue Date: 2018
Open Access Date: 29 January 2019
Grantor: Université Laval
Collection:Thèses et mémoires

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