Ultrastructural evidence for increased contact between astrocytes and synapses in rats reared in a complex environment.

Rats raised from weaning in a complex environment have an increased number of synapses per neuron in the visual cortex in comparison to animals housed in standard laboratory cages. Previous research has suggested that experience-dependent synaptic changes may be coordinated with changes in astrocytes. The present study used electron microscopy to examine astrocytic processes in the visual cortex of rats raised in a complex environment (EC) or in standard laboratory cages, either individually (IC) or in pairs (social condition, SC). Measurements of the surface density of astrocytic membrane in direct apposition to synaptic elements revealed that astrocytic processes have increased contact with synaptic elements within the visual cortex of EC rats in comparison to SC and IC animals. In contrast, other astrocytic size variables revealed no significant change in astrocytic processes per unit volume of tissue. Previous work has indicated no significant differences in synaptic density in these subjects. The specific increase in the contact between astrocytes and synapses suggests an experience-related enhancement of the astrocytic involvement in synaptic activity.