An Olfactory Memory Circuit
Episodic memory is defined simply as memory for what happened, when, and where. The hippocampus mediates episodic memory and represents contextual information using the parameters of space and time, including where an event unfolded and the sequential order of related events. Episodic recollections are characterized by rich multisensory details, yet the mechanisms underlying the reinstatement of these non-spatiotemporal aspects of experience are unknown. In this thesis, we identified direct, topographically organized hippocampal projections to a poorly understood ring-like structure known as the anterior olfactory nucleus (AON). We demonstrated that manipulation of the hippocampal-AON pathway can influence odour perception and odour-guided behaviours. Selective inhibition of hippocampal-AON projections impaired mice in their ability to recognize odours associated with the spatial and/or temporal aspects of their environment. We also revealed that AON activity is generated by coincident olfactory and contextual inputs arriving from the olfactory bulb and hippocampus, respectively. Thus, we hypothesized that the AON acts as the physical repository for populations of neurons representing previously encountered odours within the context in which they occurred. The precise pattern of activity produced by the AON therefore composes the olfactory memory trace, or “odour engram”. To this end, we used a tamoxifen-inducible Cre recombinase system to control the timing of gene expression in the AON. In combination with chemo- and optogenetic tools, we manipulated tagged AON neuronal populations in a carefully designed set of behavioural paradigms. We found that AON activity is necessary and sufficient for driving the behavioural expression of specific odour memories, thereby establishing the AON as the long-term storage site for contextually-based odour engrams. This thesis represents the first demonstration of the neural substrate of odour memory in vertebrates, satisfying all criteria used for defining an engram. The ease and suitability of using olfaction will undoubtedly position the hippocampal-AON pathway as an ideal circuit model for investigating fundamental mnemonic and cognitive principles. Indeed, this model can become particularly important in translational research that may yet lead to the development of therapeutic targets for disorders of memory, such as Alzheimer’s disease.
PhD Exit Seminar
Tuesday, August 7, 2018 at 12:10pm, DV 3130 – University of Toronto at Mississauga
Samantha Mahabir (Gerlai Lab)
“The effect of embryonic alcohol exposure on brain function and behavior in zebrafish strains”
The biological mechanisms that underlie fetal alcohol spectrum disorder (FASD) are complex and poorly understood. This thesis aims to investigate potential underlying mechanisms of FASD by using zebrafish as a model organism. The research question asked is how does embryonic alcohol exposure alter brain function and behavior in different zebrafish strains? My first experiment explored the influence of environmental factors salinity and olfactory cues on zebrafish behavior. This was conducted to reduce experimental error variation and create more sensitive behavioral paradigms. My second experiment focused on characterizing the development of shoaling behavior and correlated neurochemicals in the absence of embryonic alcohol in order to establish baseline behavior. Next, I examined the effect of embryonic alcohol exposure on neurochemicals dopamine, serotonin and their metabolites and found embryonic alcohol exposure to disrupt the dopaminergic and serotonergic systems in the developing fish; as well I discovered these effects to be strain- dependent. I found that the specific development time point, concentration and short duration of alcohol exposure used in my experiments do not alter amino acid neurotransmitters glutamate, glycine, aspartate, taurine and GABA. Lastly, I have investigated apoptosis and have adapted the labeling TUNEL assay, for zebrafish. I found that mild alcohol exposure during development results in an increase in apoptosis and that these early responses result in long-lasting changes in neuronal markers and number of cells in specific brain areas. I included results on different zebrafish strains in some of my studies. Strain differences will facilitate the discovery of molecular mechanisms underlying changes in alcohol-related genes and will also allow researchers to choose the more appropriate strain for drug or mutation screening all of which will facilitate a better understanding of FASD.
PhD Exit Seminar
Tuesday, August 7, 2018 at 9:30am, Ramsay Wright Building, Room 432
Nihar Bhattacharya (Chang Lab)
“CHARACTERIZING VERTEBRATE RHODOPSIN NATURAL VARIATION IN EVOLUTION, FUNCTION, AND DISEASE”
Vertebrate dim light vision is mediated by the rod visual pigment, rhodopsin, a member of the G protein-coupled receptor (GPCR) superfamily of proteins. In the dark, rhodopsin is covalently bound to a vitamin A-derived 11-cis chromophore, which acts as an inverse agonist to stabilize the inactive state of rhodopsin. When exposed to light of a maximal wavelength (λmax), the 11-cis retinal chromophore isomerizes to an all-trans conformation, initiating a series of structural shifts to the light-activated state of rhodopsin. This results in a signalling cascade within the rod photoreceptor cell and, ultimately, the perception of light. The goal of this thesis is to investigate natural variation in rhodopsin function in the context of evolutionary adaptation, chromophore usage, and disease mutations. Following a general introduction, in Chapter II, I characterize the visual system of the diurnal colubrid snake Pituophis melanoleucus using immunohistochemistry of retinal sections and spectroscopy of purified visual pigments expressed in vitro, revealing an unusual rhodopsin with cone opsin properties found in cone-like rod photoreceptors. In Chapter III, I investigate the effects of the rare vertebrate chromophore, 11-cis 3,4 dehydroretinal (A2), on the spectral and non-spectral properties of rhodopsin. In Chapters IV and V, I study the effects of pathogenic mutations in rhodopsin that cause the retinal degenerative disease retinitis pigmentosa (RP). In Chapter IV of my thesis, I identify the phenotype of RP mutations found in the extracellular loop 2 of rhodopsin and assess the effects of functional rescue using two different approaches. Finally, in Chapter V, I characterize three novel RP mutations to investigate the relationship between the in vitro and clinical disease phenotypes. The investigations in this thesis expand our understanding of snake retinal evolution, the role of the chromophore in rhodopsin function, and the effect of pathogenic mutations on rhodopsin structure and function. This thesis combines data from non-model organisms, non-mammalian chromophores, and non-wildtype pathogenic mutations to significantly increase our understanding of the scope of rhodopsin functionality.