Effects of postoperative environments following dorsal hippocampal lesions on dendritic branching and spines in rat occipital cortex.

Male Long-Evans rats sustained bilateral dorsal hippocampal lesions or were sham-operated when 31 days old. They were reared thereafter in either an 'enriched' (EC) or an 'impoverished' (IC) environment for one month. The effects of lesion and rearing conditions were measured on dendritic branching and spines in layer V pyramidal cells of area 17 by using a concentric ring analysis and by counting the number of spines on 50 micron segments of basilar dendrites. Hippocampal lesions significantly decreased the branching and the number of spines in both EC and IC rats. In contrast to what was observed in most behavioral studies, in which the effects of the postoperative environment were even larger in animals with lesions than in control animals, the present experiment showed that the cytological measures were affected by postoperative rearing conditions only in sham-operated rats (EC greater than IC). It is suggested, therefore, that the morphologic processes underlying the effects of the environment on behavioral recovery after hippocampal lesions can hardly be located in layer V pyramidal cells of area 17, which is considered as one of the most sensitive to environment in intact animals.