Studies On Age-related Functional Declines And Morphological Changes Of Primary Visual Cortex Cells In Cat

There have been a mass of research documents showing that many visual functions deteriorate with age both for human beings and for animals. Age-related visual function decline could not be completely attributed to optical and retinal changes. Considerable visual impairments in the old might result from morphological(such as cell loss, dendritic change and synaptic density decrease) or functional(such as weakened GABA inhibition) degeneration of visual pathways.In this article we made preliminary studies on the mechnisam underlying visual function degradation by electrophysiological and morphological methods.1. The stimulus selectivity and pattern adaptation of neurons in the primary visual cortex (area 17) of young adult and old cats was, for the first time, comparatively examined using single-neuron in vivo recording techniques. Neurons of old cats, compared with those of the young, showed a significant decrease in orientation and direction selectivity accompanied by an increase in peak response as well as increased spontaneous activity. Decreased stimulus selectivity and increased peak response indicated that more V1 neurons of aged cats can respond well to stimuli with previously optimal or non-optimal orientations and directions. The increase in peak response and spontaneous activity resulted in a significant decrease in signal-to-noise ratio for old cats. This result is well consistent with that found in senescent macaque monkeys, indicating a universal neural change with age. Further, pattern adaptation of V1 neurons in old cats was significantly stronger than that in young adult cats, which provided once more an evidence that lowered GABAergic inhibition effect in the elderly might be a chief pathway mechanism underlying a series of visual function declines during aging.2. Silver staining and immunohistochemical methods were applied to explore the difference of GABAergic neurons and astrocytes(specifically indicated by antibody of GFAP) in the primary visual cortex between young adult and old cats. No considerable neuron loss was found during senescence ;compared with those of young there was a significant increase in the number and staining density of GFAP-IR cells in old cats;but as for GABA-IR cells,it was just the opposite:immunoreactive cells of old cats were characterized by decreased number and weakened staining. The result presented a direct morphological evidence for weakness of GABAergic intracortical inhibition ,which is considered to be a possible candidate for neural basis of age-related visual function decline, and further suggested that GABAergic neuron loss and(or) decreased GABA synthesis might be responsible for above degradation.