Local and global interneuron function in the retina

Brain regions consist of intricate neuronal circuits with diverse interneuron types. In order to gain mechanistic insights into brain function, it is essential to understand the computational purpose of the different types of interneurons. How does a single interneuron type shape the input-output transformation of a given brain region? Here I investigated how different interneuron types of the retina contribute to retinal computations. I developed approaches to systematically and quantitatively investigate the function of retinal interneurons by combining precise circuit perturbations with a system-wide read-out of activity. I studied the functional roles of a locally acting interneuron type, starburst amacrine cells, and of a globally acting type, horizontal cells. In Chapter 1, I show how a defined genetic perturbation in starburst amacrine cells, the mutation of the FRMD7 gene, leads to specific effects in the direction-selective output channels of the retina. Our findings provide a link between a specific neuronal computation and a human disease, and present an entry point for understanding the molecular pathways responsible for generating neuronal circuit asymmetries. Chapter 2 addresses how mutated FRMD7 in starburst cells and the genetic ablation of starburst cells affect the computation of visual motion in the retina and in primary visual cortex. Chapter 3 addresses how horizontal cells mediate rod depolarization under bright daylight conditions. In Chapter 4, I combined the precise, yet retina-wide, perturbation of horizontal cells with a system-level readout of the retinal output. I uncovered that horizontal cells can differentially shape the response dynamics of individual retinal output channels. Our combined experimental and theoretical work shows how the inhibitory feedback at the first visual synapse shapes functional diversity in the retina

A combined experimental and computational approach to investigate emergent network dynamics based on large-scale neuronal recordings

Sviluppo di un approccio integrato computazionale-sperimentale per lo studio di reti neuronali mediante registrazioni elettrofisiologich

Developing a new generation of neuro-prosthetic interfaces: structure-function correlates of viable retina-CNT biohybrids

PhD ThesisOne of the many challenges in the development of neural prosthetic devices is the choice of electrode material. Electrodes must be biocompatible, and at the same time, they must be able to sustain repetitive current injections in a highly corrosive physiological environment. We investigated the suitability of carbon nanotube (CNT) electrodes for retinal prosthetics by studying prolonged exposure to retinal tissue and repetitive electrical stimulation of retinal ganglion cells (RGCs). Experiments were performed on retinal wholemounts isolated from the Cone rod homeobox (CRX) knockout mouse, a model of Leber congenital amaurosis. Retinas were interfaced at the vitreo-retinal juncture with CNT assemblies and maintained in physiological conditions for up to three days to investigate any anatomical (immunohistochemistry and electron microscopy) and electrophysiological changes (multielectrode array stimulation and recordings; electrodes were made of CNTs or commercial titanium nitride). Anatomical characterisation of the inner retina, including RGCs, astrocytes and Müller cells as well as cellular matrix and inner retinal vasculature, provide strong evidence of a gradual remodelling of the retina to incorporate CNT assemblies, with very little indication of an immune response. Prolonged electrophysiological recordings, performed over the course of three days, demonstrate a gradual increase in signal amplitudes, lowering of stimulation thresholds and an increase in cellular recruitment for RGCs interfaced with CNT electrodes, but not with titanium nitride electrodes. These results provide for the first time electrophysiological, ultrastructural and cellular evidence of the time-dependent formation of strong and viable bio-hybrids between the RGC layer and CNT arrays in intact retinas. We conclude that CNTs are a promising material for inclusion in retinal prosthetic devices

Functional Fluorescent Ca(2+) Indicator Proteins in Transgenic Mice under TET Control

Genetically encoded fluorescent calcium indicator proteins (FCIPs) are promising tools to study calcium dynamics in many activity-dependent molecular and cellular processes. Great hopes—for the measurement of population activity, in particular—have therefore been placed on calcium indicators derived from the green fluorescent protein and their expression in (selected) neuronal populations. Calcium transients can rise within milliseconds, making them suitable as reporters of fast neuronal activity. We here report the production of stable transgenic mouse lines with two different functional calcium indicators, inverse pericam and camgaroo-2, under the control of the tetracycline-inducible promoter. Using a variety of in vitro and in vivo assays, we find that stimuli known to increase intracellular calcium concentration (somatically triggered action potentials (APs) and synaptic and sensory stimulation) can cause substantial and rapid changes in FCIP fluorescence of inverse pericam and camgaroo-2

Ultrahigh Field Functional Magnetic Resonance Electrical Impedance Tomography (fMREIT) in Neural Activity Imaging

abstract: A direct Magnetic Resonance (MR)-based neural activity mapping technique with high spatial and temporal resolution may accelerate studies of brain functional organization. The most widely used technique for brain functional imaging is functional Magnetic Resonance Image (fMRI). The spatial resolution of fMRI is high. However, fMRI signals are highly influenced by the vasculature in each voxel and can be affected by capillary orientation and vessel size. Functional MRI analysis may, therefore, produce misleading results when voxels are nearby large vessels. Another problem in fMRI is that hemodynamic responses are slower than the neuronal activity. Therefore, temporal resolution is limited in fMRI. Furthermore, the correlation between neural activity and the hemodynamic response is not fully understood. fMRI can only be considered an indirect method of functional brain imaging. Another MR-based method of functional brain mapping is neuronal current magnetic resonance imaging (ncMRI), which has been studied over several years. However, the amplitude of these neuronal current signals is an order of magnitude smaller than the physiological noise. Works on ncMRI include simulation, phantom experiments, and studies in tissue including isolated ganglia, optic nerves, and human brains. However, ncMRI development has been hampered due to the extremely small signal amplitude, as well as the presence of confounding signals from hemodynamic changes and other physiological noise. Magnetic Resonance Electrical Impedance Tomography (MREIT) methods could have the potential for the detection of neuronal activity. In this technique, small external currents are applied to a body during MR scans. This current flow produces a magnetic field as well as an electric field. The altered magnetic flux density along the main magnetic field direction caused by this current flow can be obtained from phase images. When there is neural activity, the conductivity of the neural cell membrane changes and the current paths around the neurons change consequently. Neural spiking activity during external current injection, therefore, causes differential phase accumulation in MR data. Statistical analysis methods can be used to identify neuronal-current-induced magnetic field changes.Dissertation/ThesisDoctoral Dissertation Biomedical Engineering 201

Toward a comprehensive account of orientation selectivity in the retina.

Retinal Ganglion Cells (RGCs) form functionally distinct signaling channels that selectively encode features of the visual input including direction of motion, contrast polarity, size, and color. A highly conserved visual channel amongst vertebrates conveys orientation selectivity, i.e., the selective firing of neuronal cells in response to elongated stimuli along a preferred orientation. Orientation selectivity is an apparent critical computation and several studies have reported aspects of it, including cell type identity in anatomical reconstructions, and functional characterization of at least four different identified RGC types. But how cell types in the different studies relate is not well resolved; the mechanisms that generate the orientation selective responses in mice remain incompletely understood; and the retinofugal projections of OS RGC types are unknown. The goal of this study was to comprehensively characterize Orientation Selective (OS) RGC types in the mouse retina, and to elucidate the mechanisms that contribute to their tuning properties. We used population calcium imaging and hierarchical clustering to identify orientation selective RCGs in retinal explants. We then targeted these cells for detailed morphological and electrophysiological study. Our survey of RGC populations and subsequent morphological analysis distinguished 10 morphological types with apparent OS tuning. Electrophysiological analysis of 5 types identified specific tuning mechanisms, including a type with tuned excitation and inhibition, and a type with just tuned inhibition. Retrograde tracing from dLGN indicates that OS cells project to the shell region of the dorsal Lateral Geniculate Nucleus (dLGN), indicating that at least some OS RGC types contribute to dLGN OS tuning. This work provides new insight into the morphology and function of RGC types that exhibit OS properties. Additional studies will be necessary to further solidify the full complement of OS types in the retina and resolve their detailed circuit-level mechanisms, synaptic partners, molecular profiles, and retinofugal projections