Monoclonal antibodies reveal molecular differences between terminal fields in the rat dentate gyrus

We have derived a number of monoclonal antibodies which detect molecular differences correlating with the afferent inputs to the molecular layer of the adult rat hippocampal dentate gyrus. One group, dubbed OM-1 to OM-4, strongly stain the outer zone of the molecular layer, which receives its major innervation from the ipsilateral entorhinal cortex. A second group, IM-1 and IM-2, show a complementary pattern and preferentially stain the inner molecular layer, which receives inputs from the ipsilateral and contralateral hippocampus. These antigens are not, however, restricted to these layers, being found outside the hippocampus in several other areas of neuropil in the adult brain. In the developing brain the IM-1 antigen appears ubiquitously from the earliest age studied, embryonic day 12. Within the dentate gyrus, its restriction to the inner terminal field of the molecular layer only occurs during the second postnatal week. In contrast, OM staining appears only sparsely and late in the prenatal brain, appearing in developing cortical white matter between embryonic days 18 and 20. The outer dentate molecular layer becomes OM-positive from birth onwards, corresponding to the time of arrival of entorhinal axons during the first postnatal week. These two groups of monoclonal antibodies recognize a number of different glycoproteins. Ultrastructural immunohistochemistry shows they are cell surface molecules, and as such may be involved in the recognition events required for the establishment of specific patterns of neuronal connectivity.