An investigation regarding cellular, synaptic and glial processing in the inferior colliculi

Webb, Samuel (2020) An investigation regarding cellular, synaptic and glial processing in the inferior colliculi. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

The inferior colliculus (IC) is an integral centre of auditory processing. Neuronal processing of sound within IC is well established, as are its afferent and efferent projections. The IC is a complex structure that can be divided into sub-regions that undertake different aspects of processing. The central nucleus (CNIC) - a primary central auditory processing sub-region, the lateral cortex (LCIC) which is involved in polysensory integration, and the dorsal cortex (DCIC) which is involved in plasticity and processing descending signals from the auditory cortex. How processing is regulated within these sub-regions, and how non-neuronal cells, such as glia, contribute to local processing, is unclear. In other regions of the brain, astrocytes and microglia contribute to synaptic processing, but little is known of their morphology or variability throughout IC. Therefore, this thesis combines anatomical, physiological and molecular biological approaches to investigate astrocytic and microglial interrelationships with neuronal somata and synapses. Specifically, the morphology and colocalisation of microglial and astrocytic processes have been contrasted between sub-regions of IC in young guinea pig. Interestingly, analysis revealed that Iba1+ microglia, not GFAP+ astrocytes, make numerous abutments with GAD67+ somata (putative inhibitory neurons) throughout the IC parenchyma, and microglia were more ramified and interacted with more putative excitatory synapses (colocalised with synaptophysin and Homer1) in DCIC than other sub regions. As the IC was hypothesised to be an unexplored locus of pathology in neurodegeneration, a measurement of how markers of microglia, astrocytes and neurons were modified in theTgF344-AD (Tg) rat model of Alzheimer’s disease, from 6- to 18-months was conducted. This produced evidence for little neuronal loss up to 18-months in CNIC and other brain regions, but there was a considerable increase in GFAP+ expression, which mirrored results observed in the CA3 region of hippocampus. However, astrocytes in hippocampus demonstrated agedependent increases in astrocyte ramification length and number, but this was not found in any sub-region of IC. Furthermore, there were increased maximal respiration rates in isolated synaptosomes from 15-month Tg IC, which matched findings from hippocampus. Scanning electron microscopy revealed putative synaptosomes were reduced in size in 15-month Tg IC and hippocampus. QRT-PCR found a trend for a reduction in the expression of GAD65, SV2a, Iba1 and synaptophysin mRNA in the 15- month Tg model, suggesting loss of synapses and microglia precedes loss of neurons in this model. These findings show that microglia differentially contribute to processing between sub regions of young adult IC, and multiple hallmarks of hippocampal dysfunction in Alzheimer’s disease are found at the same time points in the Tg model. Collectively, these findings reveal novel roles for glia in the IC and suggest potential links between IC dysfunction and Alzheimer’s disease.