A quantitative EM autoradiographic study of the commissural and associational connections of the dentate gyrus in the rat.

The distribution of anterogradely transported radioactivity in the ipsilateral associational and commissural afferents to the dentate gyrus, after injections of 3H-proline into the hilar region, has been analyzed in light and electron microscopic autoradiographs of adjoining semi-thin and ultra-thin sections, respectively. In the inner third of the molecular layer, in which the two classes of afferents terminate, between 57% and 69% of the label was found over fine preterminal axons and presynaptic profiles; the remaining radioactivity was about equally distributed between dendritic profiles and other tissue components (mainly glial processes). Estimates of the relative numbers of grains within the peak region of labeling over the molecular layer of the supra- and infrapyramidal blades in the LM and EM autoradiographs (what we have termed the supra:infrapyramidal ratio) are surprisingly close despite appreciable differences in the absolute number of grains in different experiments. This suggests that in selected systems grain density estimates of this type can provide an accurate indication of the relative number of axons and axon terminals derived from the labeled projection systems, at least if the analyses are carried out on the same autoradiographs. Our results provide evidence for the assumption underlying the so-called temporal competition hypothesis (Gottlieb and Cowan 1972) and the report of a re-organization that occurs in the distribution of the associational afferents after hippocampal commissurotomy (O'Leary et al. 1980).