Cytoarchitecture of the extranuclear and commissural dendrites of hypoglossal nucleus neurons as revealed by conjugates of horseradish peroxidase with cholera toxin

The ipsilateral extranuclear dendrites of hypoglossal nucleus (HN) neurons of the rat, as revealed by conjugates of horseradish peroxidase (HRP) with cholera toxin (CTHRP), were distributed to four distinct regions: (i) to the reticular formation proper (“external dendrites” of Cajal); (ii) to the solitary nucleus and tract; (iii) to the medial longitudinal fasciculus; and (iv) to the nucleus raphe obscurus. The latter two distributions are described for the first time. They were best revealed by CTHRP; and less adequately by a wheat germ agglutinin HRP conjugate (WGHRP). They could not be demonstrated by free HRP. Furthermore, CTHRP best revealed the distribution of commissural dendrites of HN neurons, the tuftlike structures at the termination of some external dendrites of Cajal, and the few dorsal protoplasmic commissural dendrites coursing dorsal to the central canal and to the contralateral solitary nucleus and the area postrema. To further clarify the distribution of the commissural dendrites of the HN, cholera toxin HRP was injected into the tongue of rats after one hypoglossal nerve had been severed. We found that the retrogradely labeled commissural dendrites crossed the midline horizontally to enter the contralateral HN and the solitary nucleus and tract as well as the reticular formation and the medial longitudinal fasciculus. The method greatly expanded our knowledge of the course and terminations of the commissural dendrites of the “protoplasmic commissure” of Van Gehuchten. We conclude that the cholera toxin HRP conjugate was a sensitive probe for the demonstration of the dendritic arborization of retrogradely labeled neurons.