Quantitative neurohistological correlates of visual deprivation in the visual system of the rat; and GABA immunocytochemistry of the rat cerebellum and dorsal lateral geniculate nucleus

Abstract

Part one of the thesis is a stereological investigation of the effects of light exposure on the morphology of the dorsal lateral geniculate nucleus (dLGN) and visual cortex (area 17) of young adult rats dark-reared from birth. Four experimental groups were studied: a) rats reared normally from birth until 52 days post natum (DPN) - Group 52dL, b) rats reared upto 21 DPN in darkness and subsequently light exposed for 31 days - Group 21/31, c) rats dark-reared until 52 DPN and light exposed for 3 days - Group 3dL, and d) rats totally dark-reared until 52 DPN - Group 52dD. The principal morphological correlates of dark-rearing and light exposure were: i) a significant reduction in the volume numerical density (N^) of astroglia and oligodendroglia in lower layer V of the visual cortex in groups 3dL and 52dD additionally, group 3dL showed a dramatic increase in microglia N^ in thalamorecipient layer IV. No differences were detected in neuronal N^ between groups, and few morphological differences were found between groups 52dL and 21/31. The neuron and glia cell populations in the dLGN were relatively unaffected by dark-rearing. ii) Although the number, spacing and cross-sectional area of dendritic bundles in layer IV of the visual cortex were similar between the experimental groups, the number of dendritic profiles and their areal density per cluster were significantly reduced in groups 3dL and 52dD compared to group 21/31. iii) The major ultrastructural differences concern marked deficits in the 'inhibitory' circuitry of layer IV of the visual cortex, and in the synaptic glomeruli of the dlGN in group 52dD compared to group 21/31. These results are considered in relation to previous studies investigating the morphological effects of visual deprivation on the mammalian visual system. Part two is an immunocytochemical study identifying the GABAergic elements of the rat cerebellum using a serum against the 'inhibitory' neurotransmitter GABA. The somata and processes of Golgi, basket, and stellate cells were invariably immunopositive, as were the axon terminals of Purkinje cells. Granule cells, parallel fibres, mossy and climbing fibres, and neuroglia were immunonegative. Subsequently, the anti-GABA serum was used to identify the GABA-ergic nature of neurons in the rat dLGN using a combined immunocytochemical and Golgi/EM approach. The results of this study indicate that the thalamocortical projection cells are GABA immunonegative whilst the 'interneurons' are GABA immunopositive. Finally, a method is described which allows the Golgi-impregnation of a 'single' histological section 60-200pm thick.