Changes in the associational afferents to the dentate gyrus in the absence of its commissural input

摘要

Following the injection of a small amount of a labeled amino acid into the hilar region of the dentate gyrus in normal rats, the numbers of silver grains over the supra- and infrapyramidal blades of the dentate gyrus seen in autoradiograms taken at more rostral levels (due to axonal transport in the ipsilateral hippocampo-dentate associational fibers to the inner part of the molecular layer) show a fairly consistent ratio of about 2:1. If the commissural fibers to the same zone (from the hilar region of the contralateral side) are prevented from reaching the dentate gyrus by ablating the contralateral hippocampus and fimbria on the day after birth the corresponding ratio of silver grains is close to 1:1. On the reasonable assumption that the relative numbers of silver grains observed in this type of experiment bear some constant relationship to the relative numbers of labeled axon terminals, these findings suggest that in the absence of commissural afferents, the associational fibers are distributed more-or-less uniformly throughout the “hippocampal zone” of the molecular layer. The simplest interpretation of this is that the associational and commissural afferents normally compete with each other for the available synaptic sites in the hippocampal zone (and especially in the infrapyramidal blade of the dentate gyrus) as postulated by Gottlieb and Cowan (1972), and that when the associational afferents do not have to compete with the commissural fibers, they occupy all the available synaptic si