Electrophysiology of visual pathways as a screening tool for neurodegenerative diseases : evidence from mouse disease models

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Electrophysiology of Visual Pathways as a Screening Tool for Neurodegenerative

Diseases

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HENRI OLAVI LEINONEN

Electrophysiology of Visual Pathways as a Screening Tool for Neurodegenerative

Diseases

Evidence from Mouse Disease Models

To be presented by permission of the Faculty of Health Sciences, University of Eastern Finland for public examination in Tietoteknia Auditorium, Kuopio Campus, on Friday,

September 9^th, 2016, at 12 noon

Publications of the University of Eastern Finland Dissertations in Health Sciences

Number 364

Department of Neurobiology, A.I. Virtanen Institute, Faculty of Health Sciences, University of Eastern Finland

Kuopio 2016

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Juvenes Print – Suomen Yliopistopaino Oy Tampere, 2016

Series Editors:

Professor Tomi Laitinen, M.D.

Institute of Clinical Medicine, Pathology Faculty of Health Sciences

Professor Hannele Turunen, Ph.D.

Department of Nursing Science Faculty of Health Sciences

Professor Kai Kaarniranta, M.D., Ph.D.

Institute of Clinical Medicine, Ophthalmology Faculty of Health Sciences

Associate Professor (Tenure Track) Tarja Malm, Ph.D.

A.I. Virtanen Institute for Molecular Sciences Faculty of Health Sciences

Lecturer Veli‐Pekka Ranta, Ph.D. (pharmacy) School of Pharmacy

Faculty of Health Sciences

Distributor:

University of Eastern Finland Kuopio Campus Library

P.O.Box 1627 FI‐70211 Kuopio, Finland http://www.uef.fi/kirjasto

ISBN (print): 978‐952‐61‐2199‐4 ISBN (pdf): 978‐952‐61‐2200‐7

ISSN (print): 1798‐5706 ISSN (pdf): 1798‐5714

ISSN‐L: 1798‐5706

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Author’s address: Department of Neurobiology, A.I. Virtanen Institute for Molecular Sciences, Doctoral Program of Molecular Medicine

University of Eastern Finland KUOPIO

FINLAND

Supervisors: Professor Heikki Tanila, M.D., Ph.D.

Department of Neurobiology, A.I. Virtanen Institute for Molecular Sciences Doctoral Program of Molecular Medicine

FINLAND

Dr. Kestutis Gurevicius, Ph.D.

Department of Neurobiology, A.I. Virtanen Institute for Molecular Sciences Doctoral Program of Molecular Medicine

FINLAND

Reviewers: Professor Hannu Uusitalo, M.D.

Department of Ophthalmology University of Tampere

TAMPERE FINLAND

Dr. Frans Vinberg, Ph.D. (Tech.)

Department of Ophthalmology & Visual Sciences Washington University

ST. LOUIS, MISSOURI

UNITED STATES OF AMERICA

Opponent: Adjunct Professor Simo Vanni, M.D., Ph.D.

Departments of Clinical Neurosciences & Neurology University of Helsinki & Helsinki University Hospital HELSINKI

FINLAND

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Leinonen, Henri

Electrophysiology of Visual Pathways as a Screening Tool for Neurodegnerative Diseases – Evidence from Mouse Disease Models

University of Eastern Finland, Faculty of Health Sciences

Publications of the University of Eastern Finland. Dissertations in Health Sciences 364. 2016. 92 p.

ISBN (print): 978‐952‐61‐2199‐4 ISBN (pdf): 978‐952‐61‐2200‐7 ISSN (print): 1798‐5706 ISSN (pdf): 1798‐5714 ISSN‐L: 1798‐5706

ABSTRACT

This thesis work provides evidence of the utility of electrophysiological visual tests as screening tools for neurodegenerative diseases using animal models of human diseases.

The idea is based on the fact that the retina of the eye is an integral part of the central nervous system (CNS) sharing a similar neurotransmitter system as the brain. Furthermore, the eye can be studied comprehensively in a noninvasive fashion.

The project was started by setting up electroretinography and visually evoked potential techniques that were spesifically tailored for studies in genetically modified mice.

We then tested four different mouse mutant strains, modeling human diseases, for their visual capabilities and compared functional findings to anatomical hallmarks.

In an R6/2 mouse model of Hungtington´s Disease (HD) we found early onset (1 month) defect in cone‐mediated retinal function affecting later (2 months) also rod‐

pathway, which would ultimately lead to blindness. Although mutated toxic huntingtin protein was prevalent in the retinas of R6/2 mice, no prominent anatomical abnormalities were found even in the late stage of the disease. Our findings in R6/2 mice are consistent with reports on HD patients showing abnormality in color vision, but no anatomical pathology in retinal samples post mortem. In the APPswe/PS1dE9 mouse model of Alzheimer´s Disease we observed hastened rod‐mediated inner retinal responses in specific conditions. The finding may be related to impaired cholinergic function, although we do not have data of cholinergic innervation in the APPswe/PS1dE mouse retinas yet.

Importantly, APPswe/PS1dE9 mouse cortical vision seemed preserved supporting the use of this mouse strain in behavioral testing. In the third study, we characterized a novel gene defect affecting vision in mice already at a young age (5 months) and manifesting age‐

related macular degeneration (AMD) type of pathology with advancing age. Young mice deficient in the transmembrane form of prolyl‐4‐hydroxylase (P4H‐TM) exhibited impairment in cone‐mediated retinal function that did not seem to be progressive in nature.

However, with age rod‐function showed impairment typical of AMD pathology. We also found several other hallmarks of AMD‐like pathology by light and electron‐microscopy in P4H‐TM null mouse retinal samples. In the final study, we characterized the retinal phenotype of CLN5 mice modeling the Finnish variant of Neuronal Ceroid Lipofuscinosis.

The first functional defect (1 month) in CLN5 mice was found to arise from the retinal pigment epithelium. Later, a full‐blown retinal pathology was observed that would finally lead to blindness. The findings from CLN5 mice are in line with those of human patients.

In all of the neurodegenerative disease models studied, we found retinal pathology that shares commonalities with corresponding human condition. Our findings support the idea that for neurodegenerative diseases, which span from the retina to the rest of the CNS, the eye examination serve as a potential screening tool.

National Library of Medicine Classification: QT 34, WB 141, WL 307, WL 358.5, WW 103, WW 270

Medical Subject Headings: Electrodiagnosis; Electroretinography; Evoked Potentials, Visual; Vision, Ocular;

Visual Pathways; Visual Cortex; Eye; Retina; Disease Models, Animal; Mice; Central Nervous System/pathology; Neurodegenerative Diseases/diagnosis; Huntington Disease; Alzheimer Disease; Neuronal Ceroid‐Lipofuscinoses

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Leinonen Henri

Näköjärjestelmän sähköfysiologia seulontamenetelmänä hermostorappeumasairauksissa – todistusaineistoa hiiritautimalleista

Itä‐Suomen yliopisto, terveystieteiden tiedekunta

Publications of the University of Eastern Finland. Dissertations in Health Sciences 364. 2016. 92 s.

ISBN (print): 978‐952‐61‐2199‐4 ISBN (pdf): 978‐952‐61‐2200‐7 ISSN (print): 1798‐5706 ISSN (pdf): 1798‐5714 ISSN‐L: 1798‐5706

TIIVISTELMÄ

Tämä väitöskirjatyö selvittää sähköfysiologisten näkötestien soveltuvuutta hermoston rappeumasairauksien diagnostiikassa käyttäen apuna sairauksien muuntogeenisiä hiirimalleja. Johtoajatuksenamme oli se, että silmän verkkokalvo on kehityksellisesti osa keskushermostoa. Merkittävää on myös, että verkkokalvoa voidaan tutkia kajoamattomin keinoin.

Tutkimusprojektimme alussa pystytimme elektroretinografia‐ ja näköherätevastemenetelmät, jotka räätälöimme erityisesti hermoston rappeumasairauksien hiirimalleilla tehtävään tutkimukseen. Testasimme näköjärjestelmän toimintaa neljällä ihmisen keskushermoston sairautta mallintavalla muuntogeenisellä hiirilinjalla ja vertasimme tuloksia anatomisiin löydöksiin.

Totesimme Huntingtonin taudin (HT) R6/2 hiirimallissa varhaisen tappisoluvälitteisen verkkokalvon toimintahäiriön, joka eteni myöhemmin myös sauvasolujärjestelmään. Vaikka haitallista, mutatoitunutta huntingtiini‐proteiinia löytyi R6/2 hiirten verkkokalvoilta runsaasti, emme havainneet selviä anatomisia muutoksia edes taudin myöhäisvaiheessa. Tuloksemme ovat linjassa HT‐potilailla saatujen tutkimustulosten kanssa, joissa on raportoitu värinäön poikkeavuuksia, mutta ei patologisia verkkokalvon rakenteellisia muutoksia kuoleman jälkeen. Alzheimerin taudin APP/PS1 hiirimallissa havaitsimme nopeutuneet sauvasoluvälitteiset sisemmän verkkokalvon toiminnalliset vasteet. Löydös voi liittyä heikentyneeseen kolinergiseen hermovälitykseen, vaikka meillä ei vielä ole näyttöä mahdollisista kolinergisen hermotuksen muutoksista APP/PS1 hiirten verkkokalvossa. APP/PS1 hiirten näköaivokuoren toiminta ei kuitenkaan ollut heikentynyt. Kolmannessa osatyössä karakterisoimme uuden geenipoikkeaman, joka vaikutti hiirten näköön jo nuorena ja johti verkkokalvon ikärappeumaa (AMD) muistuttavaan tilaan hiirten ikääntyessä. P4H‐TM:n suhteen poistogeenisillä hiirillä havaitsimme 5 kk iässä heikentyneet tappisoluvasteet, mutta tämä poikkeavuus ei kuitenkaan näyttänyt etenevän hiirten ikääntyessä. Sen sijaan sauvasolujen toiminta heikkeni hiirillä iän myötä. Lisäksi löysimmme useita AMD‐

patologialle tyypillisiä muutoksia valo‐ ja elektronimikroskopialla P4H‐TM hiirten verkkokalvonäytteistä. Viimeisessä osatyössä selvitimme neuronaalisen seroidilipofuskinoosin suomalaisvarianttia mallintavan CLN5 hiirilinjan verkkokalvon ilmiasua. Alkuvaiheessa verkkokalvon toimintahäiriö CLN5 hiirillä rajoittui verkkokalvon pigmenttiepiteeliin. Myöhemmin ilmeni täysimittainen verkkokalvotauti, joka johti sokeuteen. CLN5 hiirillä saadut tulokset ovat siten linjassa potilashavaintojen kanssa.

Löysimme verkkokalvomuutoksia kaikista tutkimistamme ihmisen hermoston rappeumasairauksien hiirimalleista. Tutkimustuloksemme tukevat ajatusta siitä, että silmäntutkimukset ovat lupaavia diagnostisia seulontatyökaluja hermostorappeumasairauksissa, jotka ilmenevät muun keskushermoston lisäksi myös verkkokalvolla.

Luokitus: QT 34, WB 141, WL 307, WL 358.5, WW 103, WW 270

Yleinen Suomalainen asiasanasto: elektrofysiologia; diagnostiikka; näkö; aivot; aivokuori; silmät;

verkkokalvo; koe‐eläinmallit; hiiret; keskushermosto; neurodegeneratiiviset sairaudet; Huntingtonin tauti;

Alzheimerin tauti; NCL‐taudit

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“An animal´s eyes have the power to speak a great language”

(Martin Buber, 1878‐1965)

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Acknowledgements

This research project was conducted in the Department of Neurobiology in A.I.

Virtanen Institute in University of Eastern Finland (UEF). The Doctoral Program of Molecular Medicine (DPMM at UEF) provided main a majority of funding during the years 2012‐2015. The project was partially funded by the Eye and Tissue Bank Foundation (Finland), Kuopio University Foundation and Finnish Cultural Foundation through personal research grants. The Finnish Pharmacists Association, Finnish Pharmacological Society, Brain Research Society of Finland, DPMM and Faculty of Health Sciences at UEF

provided traveling grant support.

I express my sincere gratitude and respect to my principal supervisor Professor Heikki Tanila, for offering me the opportunity to join his research team. You provided the knowledge about how the scientific world works to me, inside and out. You always supported me when I needed help. You never suppressed my ideas. Instead, you gave me the opportunity to explore and be creative. I sincerely thank my second supervisor Dr.

Kestutis Gurevicius for support, especially in the initial phase of this project. Without you setting up the primary methodology of this thesis would have been a long and rocky road and I would not be able to graduate in time.

I would like to thank the official pre‐examiners of this thesis, Professor Hannu Uusitalo and Dr. Frans Vinberg, for their excellent comments and legitimate critique to the

manuscript.

I owe many thanks to Dr. Arto Lipponen who was never my official thesis supervisor but still used a lot of his time to teach me how to work in an electrophysiology lab. You also taught me many ´secrets´ of grant proposal writing. Many thanks to the whole team contributing to our ´grant writing clinic´ meetings. I actually was awarded by a couple

of personal grants, finally.

I wish to thank the former and current members of Neurobiology of Memory group for all your friendship and support. Especially, Dr. Irina Gureviciene, Pasi Miettinen and Henna Koivisto, who provided excellent technical assistance and guidance during my

PhD studies.

Completing this project would not have been possible without outstanding collaborations with other research groups. First, I would like to thank Dr. Giedrius Kalesnykas, CEO of Experimentica Ltd, for providing me the opportunity to join forces in the Publication I of this thesis project as well as enabling the commercial use of the ERG and VEP methodology, which were set up during this project. Symantas Ragauskas, with whom I shared main authorship in Publication I, and the other members of Dr. Kalesnykas´

gang, have also had a crucial role in my PhD project especially in guiding in immunohistochemical assays. Many thanks belong to all of them.

I am extremely grateful for being accepted to contribute to research led by Professors Peppi Karppinen and Johanna Myllyharju (University of Oulu). Publication III of this thesis project was our joint‐project, and there will be other great publication(s) to come.

I would also like to thank Professor Ari Koskelainen (Aalto University) for accepting me for a laboratory visit in his lab, and Marja Pitkänen for ex vivo ERG recordings, which

complemented Publication III.

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I am thankful to Professor Jari Koistinaho for letting me to conduct research in his laboratory and I am especially grateful for Dr. Katja Kanninen for leading the work on Publication IV of this project, Velta Keksa‐Goldsteine for the crucial Western Blot experiments and Mirka Tikkanen for excellent technical guidance. I also thank Mikko Huuskonen for personal friendship and scientific help, and Dr. Tarja Malm for journalist´s

work and scientific advice.

I owe thanks to all the co‐authors in my publications and manuscripts. Three out of four of the publications contributing to my PhD thesis were joint‐projects with other research groups and would not have been completed without everyone´s valuable work.

During my PhD studies I was privileged to supervise several excellent Bachelor and Master of Science students. Henna‐Riikka Lipponen, Okko Alitalo, Petteri Stenroos, Philip Kohlmann, Mari Puurula, Elina Moilanen and Anna Matilainen, I am convinced that you will have successful careers whether it is within academic world or beyond.

I wish to express my warm gratitude to all my friends and colleagues. You helped me to remain sane even in the darkest times of my PhD studies providing support and solace. I would like to thank Sakke and Mikko (the three of us comprising “The Keyot”

group) for all the sauna and beer nights, which were excellent in decoupling from work stress. I express my warmest gratitude to Emilia for her love, support and understanding when I was struggling with the rugged, last steps of my PhD studies.

Finally, I thank my mom Anneli and dad Erkki for providing the support that I needed. When I was growing‐up you never questioned my life‐choices and you never judged my decisions. You let me to choose my own path and here I am. Similar thanks goes

to my big sisters Henna and Heidi.

Kuopio, August 2016

Henri Leinonen

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List of the original publications

This dissertation is based on the following original publications:

I Ragauskas S, Leinonen H, Puranen J, Rönkkö S, Nymark S, Gurevicius K, Lipponen A, Kontkanen O, Puoliväli J, Tanila H, Kalesnykas G. Early retinal function deficit without prominent morphological changes in the R6/2 mouse model of Huntington´s Disease. PLoS One 9(12):e113317, 2014.

II Leinonen H, Lipponen A, Gurevicius K, Tanila H. Normal amplitude of electroretinography and visual evoked potential responses in AΒPP/PS1 mice.

Journal of Alzheimer´s Disease 51(1):21‐16, 2016.

III Leinonen H, Rossi M, Salo AM, Tiainen P, Hyvärinen J, Pitkänen M, Sormunen R, Miinalainen I, Zhang C, Soininen R, Kivirikko KI, Koskelainen A, Tanila H, Myllyharju J, Koivunen P. P4H‐TM deficiency in mice results in age‐related retinal and renal alterations in mice. Human Molecular Genetics, July 27, 2016. Doi:

1093/hmg/ddw228 [epub ahead of print].

IV Leinonen H, Keksa‐Goldsteine V, Ragauskas S, Kohlmann P, Singh Y, Savchenko E, Puranen J, Malm T, Kalesnykas G, Koistinaho J, Tanila H, Kanninen K. Retinal degeneration in a mouse model of late infantile neuronal ceroid lipofuscinosis is associated with compromised autophagy. Submitted.

The publications were adapted with the permission of the copyright owners.

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Abbreviations

Aβ Amyloid‐beta protein AD Alzheimer´s Disease AMD Age‐related macular

degeneration

APP Amyloid precursor protein APP/PS1 APPswe/PS1dE9 mouse

model of AD

CLN5 Protein associated to Finnish variant type of NCL

cGMP Cyclic guanosine monophosphate

CNS Central nervous system CPD Cycles per degree EEG Electroencephalogram ERG Electoretinogram FERG Flash ERG

FVEP Flash VEP

GM Genetically modified HD Huntington´s Disease Htt Huntingtin protein INL Inner nuclear layer IPL Inner plexiform layer IS Inner segment

ISI Inter‐stimulus interval KO Knock‐out

LGN Lateral geniculate nucleus LFP Local field potential

mHtt Mutated huntingtin protein

NCL Neuronal ceroid lipofuscinosis OCT Optical coherence

tomography ONC Optic nerve crush ONH Optic nerve head ONL Outer nuclear layer OPL Outer plexiform layer OP Oscillatory potential OS Outer segment PDE Phosphodiesterase PERG Pattern ERG

POS Photoreceptor outer segment PhNR Photopic negative response PVEP Pattern VEP

P4H‐TM Transmembrane form of prolyl‐4‐hydroxylase

RGC Retinal ganglion cell RNFL Retinal nerve fiber layer RPE Retinal pigment epithelium R6/2 Mouse model of HD

SC Superior colliculus

STR Scotopic threshold response VA Visual acuity

VEP Visually evoked potential V1 Primary visual cortex WT Wild‐type

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1 Introduction

Two millenniums ago a philosopher took up a matter that has attracted life scientists now only for decades. A famous quote by Cicero, a roman philosopher (106– 34 B.C.), went “Ut imago est animi voltus sic indices oculi” —the face is a picture of the mind as the eyes are its interpreter. A better‐known quote likely drawn from Cicero´s says, “The eyes are the window to the soul“. Currently some researchers are on expedition to find out if the popular quote could be extended: the eyes are the window to the soul – and to the brain.

In the fetal development, the eyes develop from diencephalon and can be thus considered as a part of central nervous system. This leads the retina of the eye and the brain to share similar cell types and neurotransmitter system. Like the cerebral and cerebellar cortices, the neural retina develops into a layered array of different neuronal types. Retinal ganglion cells (RGCs) are neuronal cells that are the output station of the retina. They form the optic nerve that sends visual information from the eye to the brain. Canonical lesson from textbooks is that there are two important information flows within the retina: 1. a vertically oriented photoreceptor  bipolar cell  RGC pathway where visual signals are hierarchically relayed inwards to the retina, and 2. a horizontally oriented pathway where horizontal cells and RGCs interplay to form visual contours. Today, however, the retina is not considered that simple. The RGCs are cabable for complex computation e.g. for direction and orientation selectivity (1). Even the primary visual cortex (V1) was traditionally believed to be merely a simple feature detector, a dogma that was overturned lately. The mouse V1 is cabable of higher cortical functions, even forms of learning and

memory (2).

Since Hubel and Wiesel published their Nobel winning discoveries from the structure of primary visual cortex in cats and its plastic changes upon monocular deprivation first in 1959 and in the 1960s, much of vision research had been conducted in cats. For a long time, it was thought that nocturnal rodents barely use their vision in navigation and their vision was believed to be very distinct from humans. This belief rendered rodent vision as useless for translational research. Since the year 2000, the paradigm has completely shifted (Fig. 1).

Fig 1. A graph of Pubmed hits with search combinations: “Cat AND vision” or “Mouse AND vision” illustrates the complete paradigm shift in vision research at around millennium change.

Since the year 2000 the amount of vision research reported in cats has slowly decreased whereas vision research conducted in mice has reached ~10-fold increase.

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Although the mouse visual system lacks many of the fundamentals of the human visual system, like the macula, and mouse visual acuity is admittedly poor, the similarities between mouse and human systems outweigh the differences (3). Secondly, genetic engineering of mouse genome is routine in molecular biology laboratories today, and mouse mutant banks are being held and even mouse phenotyping clinics. This makes it possible to investigate the effect of individual genes and gene mutants on mammalian

vision in health and disease.

The use of genetically modified (GM) mice in vision and ophthalmology research is not a new concept anymore. However, only a handful of academic research laboratories worldwide are using GM mice to seek if the eye could be used as a window into diseased brain, to the best of my knowledge. Most brain diseases; including Alzheimer´s and Parkinson´s diseases, have now been shown to manifest in the eye (4). In fact, it seems a rule of thumb that if the brain homeostasis is altered, the retina will manifest it. A concept of using vision research to facilitate brain disease screening and diagnosis is not a new one either. For example, after Hinton and coworkers published a research article representing dramatically degenerated optic nerves from Alzheimer´s Disease (AD) patients´ post mortem samples (5), a considerable amount of research was directed to vision research among AD. However, after three decades, we are still awaiting a break‐through. It is notable that when vision research among AD was a hot topic a couple decades ago, parallel translational research using GM mice was not yet plausible.

Nowadays, parallel vision research in mice and humans is highly feasible.

Visual function of both man and mouse can be tested with similar methods and with virtually the same protocols using electroretinogram (ERG) and visual evoked potential (VEP) recordings. The former measures electrical signals from the corneal surface that are generated in the retina whereas the latter measures the signals from the skull showing the activation of the brain upon photic stimulation. Both methods are minimally invasive and fairly economical. Moreover, inspection of fine‐anatomy of the human retina is not restricted to post mortem investigation as it is for the brain. The transparency of the eye makes the imaging of the living retina superlative. All major hospitals in developed countries are starting to obtain an optical coherence tomography device capable of computing anatomical parameters from the layered retina noninvasively. The safety of two‐

photon imaging of the living retina is being investigated (6), a technique that could enable single‐cell image resolution. Most strikingly, whoever owns a smartphone, and an easily available add‐on objective, can make a fundoscopy image of his/her own retina. Needless to say, imaging of the retina will increase whether it is for diagnostic, research or leisure

purposes.

At the center of this doctoral thesis is the concept of using electrophysiology of visual pathways as a tool to study neurodegerative diseases. To provide background, the anatomy and function of the visual system will be shortly reviewed and ERG and VEP methodology will be briefly described. A brief review of key diseases to this thesis will be provided as well. In the research section, the methodology that was set up in this thesis project will be presented with some key findings as well as a summary of results from original Publications I‐IV. Finally, the scientific evidence of the concept that the eye could work as a window to diseased brain will be discussed with some future directions.

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2 Visual pathways and visual processing

The mouse as nocturnal mammal does not utilize vision to the same extent as humans and thus the evolution has shaped the visual system of these species different in many aspects.

Vision research in mice, and nocturnal rodents in general, was considered pointless until around late 1990s (3). However, to be able to monitor and manipulate neurons precisely the use of mice eventually became inescacable. Today, research in mice unveils basic mechanisms of vision and even more mechanisms of disease states affecting it (7).

However, the gaps between mouse and human visual systems need to be understood and taken into account when applied to translational research. The next sub‐chapters will give a comparative overview of mouse and human visual systems and a brief introduction to functional architecture of the mammalian visual system, first at the level of the eye and then the brain.

2.1. MOUSE EYE IN COMPARISON TO HUMANS

The sensory visual experience starts in the eye where photons of visible light wavelengths are transformed into electrical signals in the retina at the back of the eye. Light enters the eye through cornea and lens that refract it. Ciliary muscle that can adjust the curvature of the lens surround the lens. This process, accommodation, allows to adjust our visual focus at different distances and to obtain clear images on the retina. In mice, the cornea is relatively thick and the lens large, and accommodation does not play as important of a role (8) (Fig. 2). The optical depth of field in mice is very large and aberrations of few diopters, a nuisance in large population of humans, are unmeaningful. Contact lenses up to +/‐10 diopters in power in front of the eye have not been shown to alter visual pattern discrimination in C57BL mouse (9,10).

Fig. 2. The structure of the mouse eye as compared to human. A: A schematic cross-section of human and mouse eyes. From outside the eye the light is focused through optical elements (cornea and lens) to the retina at the back of the eye. Humans focus their gaze so that the image falls on the macular region of eye. Fovea is responsible of the highest resolution vision containing only cone-photoreceptors. The mouse retina lacks macula/fovea, like most mammals, but it comprises increasing photoreceptor cell gradient from the peripheral to central retina (11).

As is common in noctural animals, the size of the cornea and even more the lens is pronounced in the mouse compared to the human eye (8). B:

Mouse retinal cross-section showing the layered anatomy of the retina. Light coming from the vitreous humor enters the retina at the ganglion

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cell layer (GCL) side. Intuitively paradoxical, the electrical signals are generated at the back of the retina in the outer segments (OS) of photoreceptors. RPE, retinal pigment epithelium; IS, photoreceptor inner segments; ONL, outer nuclear layer (photoreceptor nuclei); OPL, outer plexiform layer; INL, inner nuclear layer; IPL, inner plexiform layer (Figure adapted from (12)).

Since the mouse is a nocturnal animal, its retina contains fewer cone (3 %) than rod (97 %) photoreceptors. Indeed, the cones are responsible for color and day‐vision whereas rods function in the dark. The spectral sensitivity of mouse rods peaks at around 500 nm (13). Mice have two types of cone photoreceptors that differ in their photopigments and absorption spectra. One cone type is maximally sensitive to UV‐light at 360 nm, while another detects better visible light at medium (M‐)wavelengths and has a peak sensitivity at 508 nm (14). On the other hand, in mice most cones are not genuinely UV‐ or M‐cones but express both UV and M photopigments (15). The dual cones likely broaden the spectral range to which mouse cones are sensitive and allow better vision in different spectral compositions of ambient light (16). Interestingly, in the dorsal retina the cones are maximally sensitive at 508 nm, while UV sensitive cones dominate in the ventral retina (15,17). An evolutionary explanation for this organization is that the UV sensitive cones in the ventral retina encode well contrasts in darkness and thus may be sky sensors for birds of prey (15,18). According to this explanation, the M‐cones (sometimes referred to as M/L‐

cones in mice) in the dorsal retina are used to see the ground. In mice the UV‐sensitive cones are often referred to as S‐cones. Since mice lack cones sensitive at long wavelengths they are considered to be blind under dim red light conditions. Humans have three cone photoreceptor types and different cone spectral sensitivities than mice (19). The human cone spectral sensitivities are at 420 nm (blue), 531 nm (green), and 588 nm (red).

Despite the relatively small amount of cones in the mouse retina, the cones appear to play an important role in mouse vision (20). In this respect, it should be noted that also the human eye is on average rod‐dominant having only around 5 % of cones and 95 % rods (21). However, in the center of the human macula, in the fovea, cone density increases almost by 200‐fold simultaneously as the rod density decreases. The centermost part of the fovea, called foveola, is completely rod‐free. By contrast, in the mouse retina this peripheral‐central cone gradient is mild (11). Nevertheless, compared to humans the average density of rods and cones is large throughout the mouse retina. In the mouse central retina the photoreceptor cell density is ~3‐4 times higher than in the human macula (11). This is worth noting when modeling human retinopathies. Given the high photoreceptor cell density in the mouse retina the phagocytic load per retinal pigment epithelium cell is higher in mice than in humans. Despite the obvious difference in the central vision of mice and humans, mouse models of age‐related macular degeneration (AMD), affecting primarily human central vision, are invaluable research tools today

(22,23).

In mice, a high convergence of photoreceptors to retinal ganglion cells (RGC) and the small eye size result in a rather small number of RGCs, around 45 000 cells per retina (24). However, the peak density of RGCs at 8000 cells/mm² in the mouse appears rather similar to the cat (25,26), a species that is traditionally used in vision research. In the human central retina the peak RGC density is at around 35 000 cells/mm²but the density declines quickly towards the peripheral retina (27). Around 50 % of the human RGCs are located within 4.5 mm of the foveal center (16 deg), a region that comprises only 7.3 % of the total retinal area. As a result, human visual acuity is high (~30 cycles of degree per

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visual angle, CPD) at the central retina but not so much better than that of rodents in the peripheral retina (at eccentricities greater than around 12 deg) (28). The highest limit of visual acuity set by the C57BL mouse retina is 50‐fold worse than in humans at 0.6 CPD (9,29). However, visual pattern discrimination is not only determined by visual acuity but also by contrast sensitivity. The peak contrast sensitivity of mice is surprisingly good at around ~2 % compared to that of humans at ~0.5 % (30,31). Despite the differences between the mouse and the human eye, the structure and function of the retina in mice and humans is similar, especially at periphery (7,31).

2.2 FUNCTIONAL ARCHITECTURE OF THE MAMMALIAN RETINA

The mammalian retina displays 10 distinct laminar layers under a light‐microscope (Figs. 2

& 3, note the internal limiting membrane is missing in both images, it limits the nerve fiber layer from the vitreous). Although the light enters from the RGC side of the retina, that side is referred to as inner retina (proximal retina) and photoreceptor side as outer retina (distal retina). The neural retina is circumscribed by retinal pigment epithelium (RPE) on the photoreceptor side (Fig. 3.), and nerve fiber layer (NFL) on the RGC side of the retina.

Energy and nutrient supplies of the retina reside immediately below the RPE and at the NFL (32). Choriocapillaries below the RPE, i.e. choroid, receive massive blood flow and they are vital for the high metabolic demand of the outer retina (33). The actual retinal blood flow enters from the central retinal artery from the optic nerve head forming capillaries that nourish the inner retina (32). The blood flow in the choroid is manifold compared to the blood flow inside the retina. The primary energy source to the retina is provided by glucose. The retina has a high rate of anaerobic glycolysis even under basal physiological conditions, but it can switch to oxidative metabolism on demand (19).

Photoreceptors are metabolically very active due to maintenance of dark current, i.e.

continuous repolarization after depolarization of cell membranes in the dark. In addition, continuous phagocytosis of photoreceptor outer segment (POS) discs by the RPE, and their renewal, uses a lot of energy. Because energy requirements are high, oxygen consumption is also high. The capillary blood flow in the retina has been measured at 60 ml/min/100 g of tissue in primates, being similar to the blood flow in the brain (34). In the choroid, the blood flow is very high at 2000 ml/min/100 g of tissue (35), because the oxygen must diffuse from here to the inner segments of photoreceptors where their mitochondria are located. Oxygen usage by photoreceptors is 3‐4 times higher than in other CNS neurons (19). Thus hypoxic state may impact the retina very quickly (36).

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Fig. 3. Schematic image of the retina and rod and cone structures. A: The retinal pigment epithelium (RPE) has numerous supportive functions for the retina, one of the most important ones being continuous phagocytosis of photoreceptor outer segments. From the apical side of RPE start photoreceptors (rods and cones) that convert light to electrical signals. The photoreceptors synapse with bipolar and horizontal cells (modulatory cells providing lateral inhibition) at the outer plexiform layer, and the bipolar cells synaptically transmit the signal further to ganglion cells (or amacrine cells in primary rod-pathway and then to ganglion cells) at the inner plexiform layer. Ganglion cell axons run along the nerve fiber layer and finally form the optic nerve. Müller cells are radial glial cells that are present throughout the entire retina and have a crucial supportive function. B: The rod and cone photoreceptors consist of outer segment (OS), where the visual transduction (see Fig. 4) takes place, connected to inner segment via connecting cilium (CC). The inner segment contains the cell machinery of photoreceptors including mitochondria. The nuclei (N) of photoreceptors are situated at the outer nuclear layer of the retina. Ribbon synaptic terminals functionally connect the photoreceptors to the interneurons. Calyceal processes are found in primate photoreceptors but are absent in mice (37). BB: basal body of the connecting cilium (Figure adapted from (38)).

The RPE is the outermost retinal layer, consisting of a monolayer of pigmented hexagonal cells (19). The basal side of the RPE cell is adjacent to the choroid while the apical side faces the neural retina. The basal aspect of RPE contains numerous infoldings and is adherent to its basement membrane forming a part of Bruch´s membrane of the choroid.

Therefore, the attachment of RPE and the choroid is vivid. The apical side of the RPE comprises microvilli that extend into the photoreceptor outer segments (Fig. 3). However, this subretinal space is loose compared to RPE‐Bruch´s membrane interface and does not contain intercellular junctions. The RPE has many functions such as fostering the retina and choriocapillaries (reviewed in (39)). It forms part of the blood‐retinal barrier (notably similar to choroid plexus in the ventricles of the brain), which selectively controls movement of nutritients and metabolites into the retina, and on the other hand, allows waste products out of the retina (40). The most important functions of RPE (in regard to normal vision) are maintenance of visual cycle (visual cycle explained in Fig. 4) and phagocytosis of shed POS. In mice, 10 % of POS is shed daily and RPE needs to phagocytose and degrade it out of the way for renewal. This mission renders RPE cells one of the most active phagocytosing cell types in the body (41). POS phagocytosis by RPE follows the circadian rhythm, being most active at the beginning of circadian cycle at light‐

onset (42,43). POS phagocytosis can be divided into five different phases: 1. recognition and attachment; 2. ingestion; 3. phagosome formation; 4. phagosome fusion with lysosome; and

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finally 5. digestion of POS (44). It is well appreciated that failure at any phase of the POS phagocytosis may lead to retinal degeneration (45). In addition, it has been recently shown that phagocytosis of POS is crucial for the visual cycle at RPE via a novel noncanonical form of autophagy (46).

Fig 4. A graph of visual cycle i.e.

phototransduction. The visual cycle is responsible for the first-in-order visual reaction and regeneration of visual pigment for continuum. Reaction a: 11- cis-retinal (11cRAL) diffuses from the RPE to photoreceptor outer segments (OS) and couples with opsin to generate rhodopsin (Rh). Reaction b: absorption of light photons by rhodopsin leads to isomerization of the chromophore from the 11-cis to the all-trans form (atRAL).

Reaction c: atRAL is reduced to all-trans- retinol (atROL) by all-trans-retinal- spesific dehydrogenases (all-trans-RDH).

Reaction d: atROL diffures to RPE where it is esterified by retinol acyltransferase (LRAT, lecithin) to all-trans-retinyl-ester (atRE). Reaction e: RPE-spesific 65 kDA protein (RPE65) catalyzes the isomerization of atRE to 11cROL. Reaction f: when 11cROL is oxidized back to 11cRAL by 11- cis-RDH, the 11cRAL is ready for diffusion back to the OS and the visual cycle is complete.

Notably, a failure in any step of the visual cycle has been shown to induce retinal degeneration (47). IPM, interphotoreceptor matrix; IRBP, inter-photoreceptor retinol binding protein (Figure adapted from (48)).

The POS consists of membranous discs where visual pigment molecules are located within the disc membrane (19). The outer and inner segments of photoreceptors are connected via the cilium (Fig. 3b). The inner segment is the power plant of the photoreceptor containing lots of mitochondria. The inner segment contains the cell nucleus, and on the other side of the nucleus, synaptic terminals connect photoreceptors to interneurons (21). Adjustable sensivity of the photoreceptors enable vision to function within an impressive dynamic range of over 10‐log units (49). Mammalian and amphibian rods can detect even single photons but they saturate at bright light (50). Still, vision works even at very bright sunlight thanks to rapid regeneration of visual pigments and adaptational mechanisms at the cone pathway (51). Cones are 100 times less sensitive than rods (52), but on the other hand, they do not saturate easily due to their extremely fast pigment regeneration and dark‐adaptation (51).

Unlike most sensory systems where appropriate stimulation causes sensory receptors to depolarize, the photoreceptors act by hyperpolarization and subsequent change in neurotransmitter release onto postsynaptic terminals. Visual sensation commence once absorption of light changes the conformation of retinal and activates (rhod)opsin (see Fig. 4) (21). Activation of opsin stimulates the G‐protein transducin, which then activates phosphodiesterase enzyme (PDE). Once activated, PDE hydrozylases cyclic guanosine monophosphate (cGMP). The decrease in cytosolic cGMP leads to closure of cyclic nucleotide gated ion‐channels preventing influx of Na⁺ and Ca²⁺ ions, and thereby hyperpolarizes photoreceptors. The G‐protein cascade substantially potentiates the photoresponse: many transducin molecules are activated with a single 11‐cis‐retinal

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photoisomerization and each PDE enzyme hydrozylases more than one cGMP molecule. As a result, absorption of a single photon by a rod leads to closure of approximately 200 ion channels corresponding to about 2 % of all the channels open in each rod in darkness.

The retina modifies the photoresponse amplification magnitude at prevailing levels of illumination (21). This phenomenon is known as light adaptation. As levels of illumination increase, the photoreceptor sensitivity decreases preventing the receptors from saturating, and thereby they increase the range of light intensities at which they may operate. The concentration of Ca²⁺in POS is a key player in the light‐induced modulation of photoreceptor sensitivity. Light‐induced closure of ion channels leads to a net decrease of POS Ca²⁺concentration that causes many changes in the phototransduction cascade. These changes tend to reduce the receptor sensitivity to light. For instance, the lowered Ca²⁺

concentration increases activity of guanylate cyclase that synthetizes cGMP, leading to an elevated levels of cGMP. Likewise, the decreased levels of Ca²⁺increase the affinity of cGMP‐channels for cGMP diminishing the impact of light‐induced reduction in cGMP levels. The light adaptation mechanism driven by the Ca²⁺concentration is not the only light adaptational mechanism in the retina. Retinal sensitivity at background light levels is also neurally modulated. A key neurotransmitter here is dopamine (53). Many dopamine driven physiological mechanisms lead to an increased signal flow through cone circuits and reduced signal flow through rods. In Parkinson´s disease, a reduction in retinal dopamine levels may result in reduced visual contrast sensitivity. In a healhy retina, the light adaptation is a rather fast process: the retina habituates from complete darkness to bright light levels in 5‐10 minutes in humans (19). Vice versa, adaptation from bright sunlight to complete darkness (dark adaptation) is slower and may take 30 minutes.

The inverted mechanism by which photoreceptors act continues at photoreceptor–interneuron synapse. When a photoreceptor is hyperpolarized, activated, the release of its neurotransmitter glutamate into the synaptic cleft at the outer plexiform layer decreases (54). This modulates activity at the interneuron level, in bipolar and horizontal cells (52). Bipolar cells are second‐order visual transmitting cells, whereas horizontal cells regulate the signal transmission between photoreceptors and bipolar cells by lateral inhibition (Fig. 5). Bipolar cells can be divided into two major classes: rod bipolar and cone bipolar cells. Cone bipolar cells form subclasses: ON bipolar cells that depolarize and OFF bipolar cells that hyperpolarize to increments in light levels. Rod bipolar cells are always ON cells. On the other hand, rods have been shown to signal through OFF cone bipolar cells (55). The canonical mechanism by which bipolar, horizontal and ganglion cells make retina possible to detect differences in light increments, contours, is illustrated in Fig.

5.

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Fig. 5. A schematic drawing of bipolar-ganglion cell center-surround organization. Figure A.

represents ON-center cell activation and figure B. OFF-center cell activation (Figure adapted from (56)).

Bipolar cells contact RGCs and amacrine cells at the inner plexiform layer (IPL) (52). RGCs are the only output neurons of the retina and they mimic typical CNS neurons composed of a cell body, dendrites and an axon, that is unmyelinated (4). Immediately posterior to the eye globe (in fact already posterior to lamina cribrosa in humans), the RGC axons form the optic nerve that is myelinated by oligodendrocytes, similarly to other nerve fiber tracts in the CNS. Excitation of RGCs at the IPL is modulated in two ways by amacrine cells: 1.

feedforward inhibition from amacrine cell synapses directly onto RGCs dendrites, or 2.

feedback inbihition where amacrine cells contact axon terminals of bipolar cells (52).

Amacrine cells exert their action largely by two fast neurotransmitters: gamma‐

aminobutyric acid (GABA) and glycine. A fundamental difference between cone and rod bipolar cells signaling is that cone bipolars cells make direct synaptic connections to RGCs whereas rod bipolar cells synapse first with AII amacrine cells that then synapse with RGC (55). The visual transmission cascade discussed above is a mere simplification of reality.

Although there are only five classes of neuronal cells in the retina (photoreceptors, bipolar, horizontal, amacrine and RGCs), there are numerous subclasses of bipolar, amacrine and RGCs. Recent estimates suggest that there are at least 11 different subtypes of cone bipolar cells (57), 40 amacrine cell subtypes (58), and 32 RGC subtypes in the mouse retina (1).

Indeed, even the mouse retina is capable of high computation of visual stimuli ranging from direction selectivity to polarity sensitivity.

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2.2.1 Retinal neurotransmitters

Neurotransmitter composition seems to be similar in the retina and in the brain. Glutamate is the primary excitatory neurotransmitter throughout the vertical pathways of the retina, whereas GABA is the main inhibitory neurotransmitter (32). ON‐center bipolar cells signal via metabotropic glutamate channel (mGluR6) whereas OFF‐center bipolar cells signal via ionotropic AMPA and kainate receptors (59). Amacrine cells and horizontal cells in most vertebrate retinas exert their inhibitory action mostly via GABA (60). Another typical inhibitory neurotransmitter, glycine, is found in most small‐field types of amacrine cells (32). Modulatory monoamine neurotransmitters, dopamine and serotonin, are also found in amacrine cells (32,61,62), as well as acetylcholine (63). In fact, the two classes of dopamine receptors, D1‐ and D2‐class, are both found in mammalian retina fairly abundantly. D1‐

receptors are expressed in bipolar, ganglion and horizontal cells (64), whereas D2‐receptors are found in the synapse between horizontal cells, bipolar cells, and photoreceptors (65).

Thus, maybe not so surprisingly, drugs acting on monoamine and cholinergic neurotransmitter systems alter retinal function (66,67), and several psychiatric disorders are associated with abnormal ERG responses (66). In addition to above mentioned neurotransmitters, adenosine (68), nitric oxide (69) and several neuropeptides (32,70‐72) contribute more or less to neurotransmission within the retina.

2.2.2 Retinal glial cells

There are three main classes of glial cells in the mammalian retina: Müller cells, astrocytes and microglial cells (73). Müller cells are the most abundant glial cells in the retina, comprising 90 % of total retinal glia (73). Müller cells are radially oriented such that they pass through the retina from its inner vitreal border all the way to distal end of outer nuclear layer. Astrocytes are mainly located horizontally in the nerve fiber layer and they colocalize with the blood vessels in the inner nuclear layer. Indeed, the distribution of retinal astrocytes is always correlated with distribution of blood vessels and thereby astrocytes have a role in forming retina‐blood barrier. All retinal glial cell classes are integrated with the retinal neurons, allowing correct functioning of the retina and providing structural support. They are all phagocytic cells and thereby respond to immunological insult, and they interact with neurons and modulate the synapses. They contribute to neurotransmission by releasing certain neurotransmitters and trophic factors, and they are important buffers for K⁺ ions. In addition, the surrounding glial cells have an imperative role in supplying different nutritients to retinal neurons as the neurons in the retina are highly specialized and their metabolic demands are highly specific. A specific role of Müller cells is maintenance of cone‐spesific visual cycle, which enables much more rapid chromophore supply and pigment regeneration than the canonical RPE based visual cycle and enables cones to function rapidly even under extremely bright light conditions

(51).

A few decades ago, it was still believed that the inverted orientation of the retina, where light would traverse through many cell layers before hitting the target, was an example of poor evolutional design (74). More recently the claim was proven wrong. It has been shown that Müller cells are wavelength‐dependent guides, guiding green‐red spectrum of the visible light onto cones and blue‐purple part of the spectrum onto nearby rods (75). Thus, Müller cells, which span vertically through the retina, enhance wavelength

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targeting in the retina and are crucial for visual optics. That is why the inverted structure of the retina is necessary.

2.3 FUNCTIONAL ORGANIZATION OF THE PRIMARY VISUAL PATHWAYS IN RODENTS AND PRIMATES

Optic nerve axons radiate to the optic chiasm where the majority cross to the opposite side, meaning that projections from the right eye mainly activate the left visual cortex and vice versa. The extent of crossing optic nerve axons varies between species and is greater in animals with laterally oriented eyes. In the mouse, around 97 % of optic nerve axons radiate to the contralateral side of the brain (76) whereas in humans the corresponding percentage is only 60 % (21). Once the optic nerve axons pass the optic chiasm, they form bilateral optic tracts that contain fibers from both eyes. The main targets of the optic tracts are superior colliculus (SC) and lateral geniculate nucleus (LGN) (Fig. 6).

Fig. 6. A schematic picture of central visual pathways in mouse.

Majority of the RGC axons (70 %, see text below) terminate in the superior colliculus (SC) or lateral geniculate nucleus (LGN), which is a thalamic relay station to the primary visual cortex (V1) (Figure adapted from (77)).

The dorsal LGN (dLGN) is a relay station from where the optic tracts innervate the striate (visual) cortex. In primates, virtually all optic nerve axons target the dLGN (78) but in mice 70 % of optic nerve axons target the superior colliculus (SC) (79,80). The SC is responsible for coordinating the head and eye movements with sensory targets (21). In mice, the superficial layers of SC are innervated by optic nerve axons whereas neurons in the deeper layers are not generally visually responsive but respond to somatosensory or auditory stimuli (81). It is thought that the superficial part of SC contains a map that represents the visual field whereas the underlying part of SC contains a vector map of the saccades (a quick reflexive movement of both eyes to fixate in one direction) that shift the direction of gaze (82). Interestingly, despite the large innervation from RGCs, lesioning SC in rats does not permanently affect visual acuity (83). Similarly, a lesion in primary visual cortex (V1) decreases visual acuity as determined by learned visual task but does not affect visual acuity in optokinetic tracking test, which is subcortically driven, likely from SC

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(84,85). It seems intuitively rational that rodents’ vision is heavily wired for reflexive

behavior.

In addition, optic nerve axons directly innervate the pretectum between thalamus and midbrain and the suprachiasmatic nucleus (SCN) of the hypothalamus (21).

Pretectum is mainly responsible for the pupillary light reflex that occurs upon a change in luminance level, but also intergeniculate leaflet (IGL) and ventral lateral geniculate (vLGN) nucleus contribute to this (20,86,87). Instead, the SCN is associated with circadian response to light, together with the IGL and vLGN (20,88). The pretectum, SCN, vLGN and IGL all receive direct but proportionally weak retinal input (89,90). These nuclei receive the majority of their retinal input from melanopsin‐expressing RGCs (known as intrinsically photosensitive RGCs, ipRGCs), which function as luminosity detectors (91‐93). The ipRGCs can activate independently from rods and cones (91), which may explain why light‐

dependent circadian rhythms may be preserved in diseases that kill rods and cones (89).

The spatial relationship between RGCs is presented as orderly representations or “maps” of visual space at their primary brain targets (SC and dLGN) (21). Often the targets receive input from both eyes, which requires these inputs be integrated to create a coherent map of individual points in visual space. For descriptive purposes retinas and their corresponding visual field are divided into quadrants: nasal and temporal, superior and inferior. In mammals, almost all ipsilaterally projecting RGCs are located at the temporal retina (20). Virtually all ipsilaterally projecting RGCs in the mouse are located at peripheral ventrotemporal region of the retina (known as ventrotemporal crescent, VTC).

However, only a minority of the RGCs (~10‐15 %) located at the VTC projects ipsilaterally and the rest contralaterally (94). An image seen in the mouse binocular field is generated in the RGCs at VTC areas of both eyes (95,96). The mouse binocular visual field is rather small covering around the central 30 % of the visual field whereas in humans it is around 60 % (97) (Fig. 7). In primates, virtually all RGCs in the temporal retina project ipsilaterally (98).

Fig. 7. Rodent and primate ocular orientations and horizontal visual fields compared. Left: schematic drawing of the rodent visual system. Rodent’s eyes are positioned laterally and their monocular visual fields are rather large whereas the binocular visual field is narrow. Retinal ganglion cells with receptive fields in the binocular zone from the ipsilateral eye send a minor projection to a restricted part of the dorsal lateral geniculate nucleus (dLGN). Instead, the contralateral eye sends predominant projections to the dLGN. Thalamocortical projections forward the visual information to the binocular zone of the primary visual cortex (V1). Right: corresponding schematics of the primate (e.g.

human) visual system. Forward-oriented eyes provide broader binocular visual field. RGCs from both eyes send rather similar projections to the eye-specific laminar structures in the dLGN.

Thalamocortical projections relay the visual information to the V1 that has the corresponding

Electrophysiology of visual pathways as a screening tool for neurodegenerative diseases : evidence from mouse disease models

Electrophysiology of Visual Pathways as a Screening Tool for Neurodegenerative

Diseases

HENRI OLAVI LEINONEN

Electrophysiology of Visual Pathways as a Screening Tool for Neurodegenerative

Diseases

Evidence from Mouse Disease Models

“An animal´s eyes have the power to speak a great language”

Acknowledgements

List of the original publications

Contents

Abbreviations

1 Introduction

2 Visual pathways and visual processing