Migration of LHRH neurons into the spinal cord: evidence for axon-dependent migration from the transplanted chick olfactory placode.

In the chick embryo, luteinizing hormone-releasing hormone (LHRH) neurons originate in the olfactory placode and migrate along the olfactory nerve to the forebrain. In previous studies, we demonstrated that LHRH neurons followed the trigeminal nerve when the olfactory nerve was physically interrupted. To examine whether LHRH neurons possess the capacity to migrate along the different type of axons, the olfactory placode was transplanted into the base of the forelimb. Three to five days after the transplantation, LHRH neurons were detectable in the spinal nerve, the dorsal root ganglion, the sympathetic ganglion and the spinal cord. Double or triple labelling studies for LHRH, somatostatin and/or axonin-1 showed that LHRH neurons entered the spinal nerve in contact with the olfactory axons, which are specifically immunoreactive to somatostatin. Migrating LHRH neurons continued to associate closely with the olfactory axons in the spinal nerve. However, some LHRH neurons often migrated along with the axonin-1 positive spinal sensory axons, maintaining a distance from the olfactory axons. Furthermore, a few LHRH neurons were observed in the ventral root and the ventral funiculus independent of olfactory axons. As LHRH neurons were observed in the motor component of the spinal nerve, it is probable that LHRH neurons also invaded the spinal cord using the motor axons as a guiding substrate for their migration. These results suggest that the migration mode of LHRH neurons is axon dependent in the peripheral region, however, chemical identity with regard to axonal substrate choice for migration was not specified in the present study.