The neurofilament triplet protein and other neurochemical markers in rat neocortex
Creators: name: Kirkcaldie, Matthew
Publisher: name: University Of Tasmania
Year: 2023

description: Evidence for intrinsic modularity in the mammalian neocortex is difficult to reconcile in a single coherent scheme of organisation. Instead, it appears that certain characteristics of neuronal morphology and interconnection produce a cortex with an intrinsic columnar bias, formed and regionally differentiated under the influence of subcortical afferents, and that the cortex as a whole is more homogeneous than may have been expected. Against this background, a growing set of characteristic biochemical markers expressed by individual neurons has emerged as an important prospect for recognising the mix of neuronal subgroups in individual regions, and several studies have made progress in associating these \chemical phenotypes\" with the morphologies connectional preferences distributions and electrophysiological qualities of the neurons exhibiting them. Most chemical phenotypes are expressed by subgroups of GABAergic neurons. Immunohistochemical labelling of the neurofilament triplet (NF) intermediate filament proteins allows subpopulations of the largest neuronal group the pyramidal cells to be identified throughout the brain. Characterisation of the properties associated with cellular expression of NFs would permit phenotypic groupings of most cortical neurons enabling the evaluation of neuronal populations in wide regions of the brain using simple techniques. This thesis comprises studies of NF-associated properties in the context of several key chemical phenotypes. The overall distribution of immunolabelling produced by SMI32 an antibody recognising perikaryal neurofilament was examined and characterised in the rat cortex. Retrograde tracing studies evaluated the possibility that the presence of NF enables longer projections in corticocortical neurons. Ultrastructural examination of cortical axons was used to examine the relationship between axonal neurofilaments myelination and axon calibre in the rat cortex. The developmental emergence of SMI32 labelling was examined and comparisons with the appearance of key GABAergic phenotypes was used to correlate NF expression with the changing environment of developing cortex. Finally the usefulness of cultured embryonic rat neurons for studying the development and intracellular distribution of NFs and GABAergic phenotypes was examined using several timepoints during a three week development in vitro. SMI32-labelled NFs were found in around 10% of neurons in several regions; characteristic laminar patterns of labelling were observed in the cortex as a whole consistently appearing in particular cortical areas. SMI32 labelling in retrogradely labelled cells was found not to correlate with estimates of the projection distances involved although some evidence for regionally specific differences in NF proportions of projection neurons was observed. Presumed NFs in cortical axons of the corpus callosum exhibited a strong correlation with axon calibre and myelination while differences observed in regional NF distribution were found to correlate with obvious ultrastructural differences in the cortex. The emergence of SMI32 labelling correlated with changes in the expression of other chemical markers in vivo whereas the environmental differences of in vitro culture produced significant alterations in the observable chemical phenotypes notably the early absence of NFs recognised by the SMI32 antibody. The role played by neurofilaments in the cells which express them remains mysterious but it is clear that there are major variations in their regional and intracellular expression and modification. They remain a strong prospect for subtyping pyramidal cells since they are probably expressed by neurons whose axons are myelinated."