The response of the associational afferents to the dentate gyrus to simultaneous or sequential elimination of the commissural and entorhinal afferents.

Previous studies have shown that following the removal of the commissural afferents to the dentate gyrus, the ipsilateral association afferents, which are normally distributed within the same region of the molecular layer, sprout new collateral branches and in time occupy essentially all the vacated synaptic sites. It is also known that when the entorhinal afferents to the dentate gyrus are interrupted the associational and commissural fibers can both undergo a similar phase of reactive synaptogenesis and give rise to new collaterals which extend for some distance into the denervated zone. Since the associational fibers can sprout after the removal of either the commissural or entorhinal afferents experiments were designed to determine their capacity for sprouting in newborn and young adult rats when both groups of afferents were eliminated either simultaneously or sequentially (i.e., after an interval of 8 weeks). The resulting changes in the terminal field of the associational afferents were assessed, two months after the last deafferentation, by measuring in autoradiographs the width of the zone occupied by the associational afferents labeled with [3H]proline, and by estimating the volume of this region in Timm-stained sections. The results indicate that under these conditions the associational afferents are capable of expanding their terminal field not only to occupy essentially all of the synaptic sites made available by the elimination of commissural fibers, but also to occupy a significant proportion of the space vacated by the removal of the entorhinal afferents. This suggests that the capacity of the associational afferents for reactive synaptogenesis is greater than that expressed after either commissural of entorhinal lesions alone.