The mode of termination of the hypothalamic projection to the dentate gyrus: an EM autoradiographic study.

It has been suggested, on electrophysiological grounds, that the projection from the hypothalamus to the dentate gyrus constitutes a long-axon, monosynaptic, inhibitory pathway. To clarify the mode of termination of this projection we have examined in EM autoradiographs the distribution and form of labeled synapses in the rat dentate gyrus following the injection of [3H]proline of high specific activity into the supramammillary region of the hypothalamus. As suggested by previous light microscopic studies the hypothalamo-dentate projection has been found to terminate in a narrow zone that extends from about the superficial half of the layer of granule cell somata to the inner one-fifth of the overlying molecular layer. Within this zone more than 80% of the silver grains observed were associated with vesicle-containing profiles, most of which could be identified as forming Type I, asymmetric synapses upon large dendritic shafts. A smaller number of labeled synapses was found upon granule cell somata or upon the sessile dendritic spines that occur on the proximal parts of the granule cell dendrites. Since all of the labeled synapses showed distinct asymmetric membrane specializations and contained spheroidal vesicles it is difficult to reconcile these morphological findings with the view that the hypothalamic afferents to the dentate granule cells are inhibitory.