Dorsal root ganglion neurons projecting to the dorsal column nuclei of rats.

Dorsal root ganglion (DRG) neurons may give origin to ascending branches that terminate in the dorsal column nuclei (DCN); uncertainties still exist with regard to the proportion of these neurons in different DRGs and to the type of these neurons. The percentage and size of neurons that project to the DCN were determined in a large number of DRGs by means of the retrograde transport of colloidal gold-labeled wheat germ agglutinin conjugated to enzymatically inactive horseradish peroxidase (WGAapoHRP-AU). A total of 16,239 neurons was tallied in 80 DRGs from nine rats; 3,240 (20%) of these were retrogradely labeled by the tracer injected in the DCN. Percentages of DCN projecting neurons vary considerably at different segmental levels: they are higher in cervical (up to 63%) than in thoracic (up to 31% for T1, up to 12% for thoracic DRGs below T1) or lumbar DRGs (up to 15%). At cervical levels highest percentages were encountered in C6, C7, and C8 and lowest percentages in C2-C4. At lumbar levels highest percentages were encountered in L4 and lowest in L1 and L6. When considering the soma size of DRG neurons it appears that: 1) there are more large cells, labeled and unlabeled, at cervical (38%) than at lumbar levels (30%) and more at lumbar than at thoracic levels (23%); 2) at every level, most labeled, i.e., projecting, neurons are large; and 3) DRGs with the highest proportions of large vs. small cells contain the highest percentages of DCN projecting neurons. These results represent the first attempt at establishing the percentages and soma size of DCN projecting neurons from a large number of DRGs and at comparing the contribution to these nuclei from cervical, thoracic, and lumbar DRGs. Some of the differences in the ratio of projecting neurons at different levels may be explained on the basis of well-known anatomical features, e.g., the projections to the Clarke's column of many DRG neurons in lumbar ganglia. The contribution of virtually exclusively large DRG neurons to the DCN, suggested by indirect or incomplete evidence, is demonstrated by the present retrograde labeling and soma size measurements. The results relate to the functional component of peripheral receptors that relay their input via the dorsal columns and do not seem to support a recent suggestion that a sizeable fraction of unmyelinated primary afferents ascend in the dorsal columns to terminate in the DCN.