The effects of a lengthy period of environmental diversity on well-fed and previously undernourished rats. II. Synapse-to-neuron ratios.

Rats were undernourished from the 16th day of gestation until 25 postnatal days of age and then weaned on to an ad libitum diet. Around 35 days of age, 12 previously undernourished male rats were assigned to an enriched environmental condition (EC) and their littermates to an isolated environmental condition (IC). A parallel set of well-fed rats was similarly assigned. After 80 days in these environmental conditions, all rats were killed by perfusion with 2% phosphate buffered glutaraldehyde. Small pieces of tissue containing the entire depth of the right visual cortex were embedded in Spurr's resin. Semithin (0.5 micron) sections were cut from these blocks and stained with toluidine blue. Photomicrographs of these sections were used to estimate the numerical density of neurons in cortical layers II to III. Ultrathin sections (approximately 70 nm) of the same region of cortex were then prepared for electron microscopy. These were used to estimate the mean synaptic disc diameter and synaptic numerical density. From these estimates of neuronal and synaptic numerical density, synapse-to-neuron ratios were calculated. The results of a two-way analysis of variance test revealed that environment had significant effects on neuronal numerical density, mean synaptic disc diameter, and synapse-to-neuron ratios. Neither nutrition nor its interaction with environment had significant effects on any of the parameters analyzed. These results suggest that environmental diversity can produce alterations in certain neuronal and synaptic characteristics in the visual cortex of both well-fed and previously undernourished rats.