Myelin does not influence the choice behaviour of entorhinal axons but strongly inhibits their outgrowth length in vitro

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Abstract
Myelin is crucial for the stabilization of the entorhinohippocampal projection during late development and is a non‐permissive substrate for regrowing axons after lesion in the adult brain. We used two in vitro assays to analyse the impact of myelin on rat entorhinohippocampal projection neurons. A stripe assay was used to study the impact of myelin on the choice behaviour of axons from the entorhinal cortex (EC). Given a choice between alternating hippocampal membrane lanes from developmental stages ranging from early postnatal to adult, EC axons preferred to extend on early postnatal hippocampal membranes. Neither the neutralization of myelin‐associated factors by a specific antibody (IN‐1) nor the separation of myelin from membranes interfered with the axons' choice behaviour. The entorhinal axons showed no preference in the membrane combination of adult and myelin‐free adult hippocampal membranes. These stripe assay experiments demonstrate that support for EC axon choice in the developing hippocampus is maturation‐dependent and is not influenced by myelin. The application of IN‐1 in the outgrowth assay and the separation of myelin from membranes, enhanced elongation of outgrowing entorhinal axons on adult hippocampal membranes, whereas a control antibody did not. This shows that myelin‐associated factors have a strong inhibitory effect on the outgrowth length of entorhinal axons. In conclusion, we suggest that axonal elongation in the entorhinohippocampal system during development is strongly influenced by myelin‐associated growth inhibition factors and that specific target finding of entorhinal axons is regulated by a different mechanism.