Subicular Lesions Cause Dendritic Atrophy in CA1 and CA3 Pyramidal Neurons of the Rat Hippocampus

The subiculum is a major source of output projections from hippocampus to cortical and subcortical regions. Our previous studies have demonstrated the selective loss of CA1 pyramidal neurons of the hippocampus, and operant and spatial learning impairment in subicular lesioned rats [Govindaiah et al. (1997) Brain Res. 745, 121-126; Laxmi et al. (1999) Brain Res. 816, 245-148]. In the present study, the effect of ibotenate lesions of the subiculum on the dendritic morphology of CA1 and CA3 pyramidal neurons of the hippocampus was investigated in 30-day-old male Wistar rats. The ventral subiculum was lesioned bilaterally with multiple injections of ibotenic acid, stereotaxically. The dendritic branching points and intersections were studied in apical and basal dendrites up to 320 and 160 microm, respectively, in Golgi-impregnated CA1 and CA3 pyramidal neurons of the hippocampus. The results revealed a significant (P<0.001) decrease in the number of dendritic branching points, intersections and total number of dendrites in both apical and basal dendrites of CA1, as well as CA3 pyramidal neurons of the hippocampus. It is surprising that the subicular lesions caused dendritic atrophy of CA3 neurons without affecting the cell density. The results of the present study demonstrate the dendritic atrophy of hippocampal neurons following selective subicular lesions. This might be responsible for the impairments in operant and spatial learning tasks in these rats as observed in our earlier studies. In addition, hippocampal damage is also associated with an impairment in the process of the active monitoring of movements in space, rather than place learning per se [Whishaw (1998) Neurosci. biobeh. Rev. 22, 209-220]. Accordingly, further studies are required to correlate the differential effect of subicular lesions on impairments in learning and movement in space in rats.