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Richness of environment affects the number of contacts formed by boutons containing flat vesicles but does not alter the number of these boutons per neuron

Beaulieu, C.; Colonnier, M.

Journal of Comparative Neurology 274(3): 347-356

1988

A recent quantitative analysis of cat visual cortex has demonstrated that the numerical density (Nv) of symmetrical synaptic contacts formed by boutons containing flat vesicles (FS synapses) is nearly twice as large in animals raised in isolation (impoverished condition: IC) as in animals raised in a colony (enriched condition: EC). Although some FS synapses have been shown to be cholinergic there is evidence that many, indeed the vast majority, are GABAergic. In order to estimate whether the change in the Nv of FS contacts was accompanied by a change in the number of boutons containing GABA, we have incubated sections of tissue from both groups of animals in an antiserum for GAD. In spite of the large increase in the number of FS contacts in impoverished cortex, we saw no obvious change in the apparent amount of labelled GAD terminals. In retrospect we realized that though the amount of labelled GAD terminals might reasonably be expected to reflect the number of F-boutons, it might not correspond so closely to the number of contacts formed by these boutons (which is what we had measured in the previous study): The richness of the environment could conceivably affect the number of contacts formed by the F-boutons without affecting the number of boutons! We thus extended our study by estimating the number of F-boutons in the two conditions. For the total cortical thickness, the Nv of F-boutons is only 17% lower (P less than .05) in enriched than in impoverished cats. The diameter of the boutons is 6% larger (P less than .001) in the enriched cortex. Because the F-boutons become fewer in number as they become larger in size, the total percentage volume occupied by these boutons does not change between the two experimental conditions. We conclude that this is the reason why there appears to be no change in the general amount of GAD label between the two groups of cats. More importantly, since the Nv of neurons is also 17% lower in enriched cortex, the number of F-boutons per neuron (and presumably the total number of F-boutons in the visual area) actually remains unchanged. In contrast, the previous study showed that the number of FS contacts per neuron is significantly decreased in enriched cortex. It follows that the number of contacts formed by each bouton must be altered.(ABSTRACT TRUNCATED AT 400 WORDS)

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