SEMAPHORIN-III CAN FUNCTION AS A SELECTIVE CHEMOREPELLENT TO PATTERN SENSORY PROJECTIONS IN THE SPINAL-CORD

Distinct classes of primary sensory neurons in dorsal root ganglia sub serve different sensory modalities, terminate in different dorsoventra l locations in the spinal cord, and display different neurotrophin res ponse profiles. Large diameter muscle afferents that terminate in the ventral spinal cord are NT-3 responsive, whereas small diameter affere nts subserving pain and temperature are NGF responsive and terminate i n the dorsal spinal cord. Previous in vitro studies showed that the de veloping ventral spinal cord secretes a diffusible factor that inhibit s the growth of sensory axons. Here we show that this factor repels NG F-responsive axons but has little effect on NT-3-responsive axons, We also provide evidence implicating semaphorin III/collapsin, a diffusib le guidance molecule expressed by ventral spinal cord cells, in mediat ing this effect. These results suggest that semaphorin III functions t o pattern sensory projections by selectively repelling axons that norm ally terminate dorsally.