Recovery of enzyme markers for cholinergic terminals in septo-temporal regions of the hippocampus following selective fimbrial lesions in adult rats.

The activities of choline actyltransferase and acetylcholinesterase were determined in five consecutive septo-temporal regions of the ipsilateral and contralateral hippocampus from unlesioned controls and lesioned animals at various times following lateral, medial or complete unilateral transection of the fimbrial bundle in rats. In control animals distribution of cholinergic enzymes suggests a relatively heavier innervation of the ventral hippocampus. In lesioned animals depletion of enzyme activities in septo-temporal regions of the ipsilateral hippocampus was consonant with the known topography of cholinergic innervation of the hippocampus via the dorsal and ventral pathways. After 4 and 8 week post-lesion survival, a substantial recovery of both enzyme activities was evident following either of the lesion paradigms employed. However, the extent and the pattern of enzyme restitution depended on the type of fimbrial transection and the hippocampal region under consideration. Significant enzyme alterations were also observed in the contralateral hippocampus following all three lesion types. We interpret the lesion-induced temporal consequences in cholinergic enzymes to indicate initial degeneration and subsequent regeneration of cholinergic terminals in the hippocampus. The present findings also suggest that homologous fimbrial fibres spared by the partial lesions are responsible for the ensuing recovery. Thus, partial lesions of well-defined efferents constitute a suitable experimental paradigm to demonstrate homotypic reconstruction in the adult mammalian central nervous system.