Peptide 19 in the dorsal root ganglion and the mesencephalic trigeminal tract nucleus of the adult rat

Peptide 19-immunoreactivity (PEP 19-ir) was examined in the dorsal root ganglion (DRG) and the mesencephalic trigeminal tract nucleus (Mes5) of the adult rat. Thirty-eight percent of DRG cells were immunoreactive (ir) for PEP 19. These neurons were small to large and measured 167-4583 μm² (mean ± S.D. = 2048 ± 913 μm²). Seventy-five percent of DRG cells > 2000 μm² and 15% of those <1000 μm² exhibited PEP 19-ir. Thirty-six percent of DRG neurons in the range 1000-2000 μm² showed the ir. In the Mes5, 87% of primary neurons were ir for this peptide. Muscle spindles in the soleus and masseter muscles contained PEP 19-ir spiral axon terminals. Double immunofluorescence methods revealed the co-expression of PEP 19 and calcium binding proteins. Eighty-six percent of parvalbumin-ir neurons exhibited PEP 19-ir. Conversely, 60% of PEP 19-ir neurons showed parvalbumin-ir. The cell size analysis revealed that 55% of PEP 19-ir neurons > 600 μm² showed parvalbumin-ir and that all PEP 19-ir neurons <600 μm² were devoid of it. Ninety percent of PEP 19-ir DRG neurons showed S100-ir, whereas 60% of S100- ir ones co-expressed PEP 19-ir. In the Mes5, virtually all PEP 19-ir primary neurons exhibited parvalbumin-ir. The co-expression of PEP 19 and S100 could not be observed in the nucleus. The present study indicates that PEP 19-ir neurons which co-express parvalbumin-ir are proprioceptors in the spinal and the trigeminal systems. PEP 19-ir small DRG neurons without S100-ir are probably exteroceptors and may include unmyelinated nociceptors.