CONNECTIONAL SPECIFICATION OF REGENERATING ENTORHINAL PROJECTION NEURON CLASSES CANNOT BE OVERRIDDEN BY ALTERED TARGET AVAILABILITY IN POSTNATAL ORGANOTYPIC SLICE COCULTURE

Layer II neurons of the entorhinal cortex project to the dentate gyrus and field CA3 and also send collaterals to the subiculum; layer III n eurons project to the subiculum and field CA1, but not to the dentate gyrus; and layer IV neurons project to the perirhinal cortex. We have previously shown that these specific differences between the projectio ns of the layer II and III neurons are maintained and can regenerate i n organo typic slice culture. In the present experiments we have confr onted Postnatal Day 7 (P7) rat entorhinal cortex with P7 tissue select ed from restricted parts of the overall entorhinal projection field. ( 1) When entorhinal slices were co-cultured with target slices containi ng only dentate gyrus, extracellular uptake of biotin dextran from cry stals placed on the dentate gyrus retrogradely labeled neurons in laye r II, and those of layer III were not labeled. (2) When entorhinal sli ces were co-cultured with target slices that contained only the subicu lum and the hippocampal field CA1 (but not dentate gyrus), neurons in both layers II and III of the entorhinal area were labeled. (3) When e ntorhinal slices were co-cultured with target slices containing the pe rirhinal area and no hippocampal or dentate tissue, the neurons of ent orhinal layer IV were labeled, but (4) when co-cultured with control t arget slices taken from the rostral parietal neocortex, no entorhinal neurons were labeled. Thus the exclusive relationship of layer II ento rhinal neurons to the dentate gyrus has already been established by 1 week of age and is maintained by the regenerating entorhinal axons. La yer III entorhinal neurons cannot be induced to project to the dentate gyrus even when deprived of their own target, and layer IV neurons ar e specified to project to the perirhinal area and will not project to any part of the hippocampal complex or to the rostral parietal cortex. Thus, deprivation of the normal target tissue and presentation of an incorrect target tissue (even when it is the correct target for one of the other classes of entorhinal neurons) are not sufficient to overri de the specificity of the entorhinal projection neurons. (C) 1996 Acad emic Press, Inc.