Differential rearing effects on rat visual cortex synapses. III. Neuronal and glial nuclei, boutons, dendrites, and capillaries.

Morphological measures of neurons, astrocytes, oligodendrocytes, presynaptic boutons, dendrites and capillaries were examined in the upper 4 layers of occipital cortex in rats reared for 30 days postweaning in complex (EC), social (SC) or individual cage (IC) environments. EC rats had a lower numerical density of neuronal nuclei with a comparable volume fraction to SC and IC rats. The volume fraction of astrocyte and oligodendrocyte nuclei was significantly greater for EC rats than IC littermates, and IC rats also had more synapses and neurons/micron3 of glial nuclei. Environmental groups did not differ in the numerical density of presynaptic boutons but the number of boutons per neuron was greater in EC than in IC or SC rats. This result parallels the findings that EC rats have more synapses per neuron than IC rats. Electron microscopic estimates of dendritic volume fraction confirmed estimates from Golgi-impregnated neurons that there is more dendrite per neuron in the occipital cortex of EC rats than IC or SC rats. EC rats also had a larger capillary volume than SC or IC and these capillaries were closer together and had fewer synapses/micron3 of capillary in ECs. Another indicator of metabolic activity, mitochondria volume per neuron, gave similar results with ECs having a greater volume than ICs and SCs intermediate. These results indicate that not only are there more synapses per neuron in the visual cortex of rats from more complex environments but also that the brain appears to adjust to the metabolic requirements of its synapses or neurons, in terms of vascular, mitochondrial and glial support.