Plasticity in the Rat Hippocampal Formation following Ibotenic Acid Lesion of the Septal Region: A Quantitative [14C]Deoxyglucose and Acetylcholinesterase Study

Abstract

The local cerebral glucose utilization was measured in the hippocampal formation 3, 21, and 90 days after bilateral lesions of the medial septal nucleus and the nucleus of the diagonal band of Broca by multiple ibotenic acid injections. The CMRglc was determined in hippocampal areas and layers and various limbic and visual regions by quantitative [14C]2-deoxyglucose autoradiography using a computerized image-processing system. Three days after lesion, CMRglc was significantly decreased in 26 of the 38 structures examined. The most pronounced reductions were found in CA2 and CA3, the subiculum, and the parasubiculum. The CMRglc values of the 21- and 90-day postlesion groups did not differ significantly from control data when univariate statistics were used. However, by means of a factor analysis and subsequently a discriminant analysis as a multivariate test for group differences, significant lesion-induced CMRglc changes could be detected between the control group, the 3-day group, and the 90-day group. The 21-day group did not differ significantly from the controls. The data indicate that 90 days after lesion of the medial septum/diagonal band complex (MSDB), a considerable recovery of the mean CMRglc was found in the hippocampal region, although a normal level was not reached. In a parallel series, processing of sections for acetylcholinesterase (AChE) histochemistry revealed a severe destruction of AChE-positive fibers in the hippocampus at 3 days after lesion and a conspicuous recovery in the amount of stain-able fibers and their staining intensity at 21 days postlesion. In the 90-day group, the AChE fibers recovered even further but did not reach the values of unlesioned sham-operated controls. The present study indicates that sprouting of surviving cholinergic afferents might be an important morphological substrate for CMRglc recovery in the hippocampus after MSDB lesion.