DIFFERENTIAL EXPRESSION OF MAG, MBP AND L1 IN THE DEVELOPING LATERAL SUPERIOR OLIVE

The aim of this study was to investigate whether glial-associated mole cules exhibit a pattern of expression that could influence oriented de ndrite outgrowth in the gerbil lateral superior olive (LSO). In partic ular, we have previously noted that axon fascicles are oriented parall el to isofrequency laminae in the medial limb of the LSO, as are LSO d endrites, a phenotype that emerges postnatally. Therefore, we examined the immunocytochemical staining pattern of antibodies directed agains t three proteins that are found along axons: myelin basic protein (MBP ), myelin-associated glycoprotein (MAG), and neuron-glia cell adhesion molecule (L1). MAG staining was first observed at postnatal day (P) 4 on the axon fibers surrounding the LSO. By P7 there was a differentia l pattern of MAG staining within the LSO, and immunopositive fibers we re observed solely in the medial limb (e.g., high frequency projection region). Between P7 and P12, MAG-staining was restricted largely to f ascicles in the medial limb, and these were oriented parallel to the i sofrequency axes. Few positive fibers of irregular orientation were ob served in the lateral limb (e.g., low frequency projection region). Si gnificant MAG-staining was not observed in the lateral limb until P15. The MAG immunoreactivity extended throughout the LSO by P21, although it was no longer restricted to axon fascicles. In contrast, MBP-posit ive fibers were uniformly distributed within the LSO by P12. Finally, L1 was found on oriented axon fascicles at PO, but became sparsely dis tributed throughout the LSO neuropil after P7, and was restricted to n euron cell bodies in the adult. Taken together, the results suggest th at oriented axon fascicles bearing MAG and L1 may contribute to the de velopmental refinement of dendrite and axon arbors within the LSO.