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

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Abstract
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 terninals. 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 oniy 17% lower (P<.05) in enriched than in impoverished cats. The diameter of the boutons is 6% larger (P<.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. In the present study we demonstrate that there is an average of 2 contacts per F-bouton in enriched and of 3 contacts per F-bouton in impoverished visual cortex. Thus, the richness of the environment does not affect the number of F-boutons per neuron but only the number of contacts established by each bouton. This clearly demonstrates the importance of the active zones in the cortical circuitry, at least for the GABAergic system. The more numerous contacts in the impoverished cortex are smaller, so that the computed average area if active zones per bouton is not significantly different between the two groups of animals. This is consistent with Changeux and Danch's suggestion that during learning there is a redistribution of receptiors in favor of synaptic contacts which are more frequently utilized. The effect of the richeness of the environment would thus be to restructure the inhibitory GABAergic system, rather than to change the amount of GABA or the number of its receptors.