Semaphorin D acts as a repulsive factor for entorhinal and hippocampal neurons

We analysed the effects of semaphorin D on axons from the developing rat en torhinal-hippocampal formation. Explants from superficial layers of the ent orhinal cortex and of the hippocampus anlage were obtained from various dev elopmental stages and co-cultured with cell aggregates expressing semaphori n D. Neurites extending from entorhinal explants that had been isolated fro m early embryonic stages (E16 and E17) were not affected by semaphorin D, b ut were repelled at later stages (E20 and E21). Axons from hippocampal neur ons explanted at E21 were also repelled by semaphorin D, In situ hybridizat ion studies revealed expression of the semaphorin D receptor neuropilin-1 i n the entorhinal cortex from stage E17 to stage P7, and in the dentate gyru s and CA1-3 regions between E17 and adulthood. These data suggest that sema phorin D is involved in the formation of the perforant pathway and acts, vi a the neuropilin-1 receptor, as a repulsive signal that prevents entorhinal fibres from growing into the granular layer of the dentate gyrus. These da ta also suggest a role for semaphorin D in the development of intrahippocam pal connections.