Environmental Influence on Neuronal and Dendritic Spine Plasticity After Permanent Focal Brain Ischemia

Abstract

The aim of the study was to compare the effect of housing conditions on the intact and lesioned brain. Focal brain ischemia was induced by ligation of the right middle cerebral artery distal to the striatal branches in male, spontaneously hypertensive rats. The rats were postoperatively housed in individual cages for 24 h, then housed either in an enriched environment, i.e. larger cages allowing various activities or in standard laboratory cages, four to five animals in each cage. Nonlesioned rats were housed in the same way. Three weeks later the rats were anesthetized and the brains perfused with formaldehyde in situ. The dendritic geometry was studied with Three-Dimensional confocal laser scanning microscopy after microinjection of Lucifer Yellow in individual pyramidal neurons in layers II/III and V/VI in the left parietal cortex. Nissl-stained sections were used to visualize layers and area borders of the cortex. Nonlesioned rats in the enriched environment had significantly more dendritic spines than rats in standard cages, in all layers studied. Individual spine morphology was significantly different, with more branched spines and an increase of spine head and neck diameter, in enriched rats. In lesioned rats housed in the standard environment, pyramidal neurons in cortical layers II/III contralateral to the infarct were more heterogeneous then in nonlesioned rats in the same environment, and the number of spines was reduced. Neurons in rats postoperatively housed in an enriched environment were less heterogeneous and had significantly more spines than rats in standard housing (P<0.05), reaching the same number as in intact rats in a standard environment. In layers V/VI the number of dendritic spines on pyramidal neurons was significantly lower than in intact rats, in both lesioned groups. We conclude that housing in an enriched environment increases neuronal plasticity in the intact brain, and in addition, significantly improves dendritic morphology of contralateral pyramidal neurons in layers II/III after permanent focal brain ischemia.