Osteocalcin-immunoreactive primary sensory neurons in the rat spinal and trigeminal nervous systems

Osteocalcin-immunoreactivity (OC-ir) was examined in spinal and trigeminal primary sensory neurons of the adult rat. Sixteen percent of dorsal root ganglion (DRG) neurons were immunoreactive (ir) for this protein. These neurons were mostly large and measured 594-4583 μm² (mean ± S.D. = 2243 ±748 μm²). Thirty-four percent of DRG neurons > 1200 μm² and 4% of those in the range 600-1200 μm² showed the ir. Virtually all DRG neurons < 600 μm² were devoid of OC-ir. In the trigeminal ganglion (TG), 25% of neurons exhibited the ir. Such neurons were of various sizes (range = 156- 2825 μm², mean ± S.D.= 1234 ± 543 ±m²). Forty-five percent of TG neurons > 800 μm² and 6% of those < 400 μm² were immunoreactive for this protein. Twelve percent of TG neurons in the range 400-800 μm² showed the it. In the mesencephalic trigeminal tract nucleus (Mes5), 63% of primary sensory neurons contained OC-ir. Virtually all OC-ir DRG and Mes5 neurons co- expressed parvalbumin-ir but not CGRP-ir. On the other hand, only 31% of OC- ir neurons co-expressed parvalbumin-ir and 10% co-expressed CGRP-ir in the TG. The present study indicates that DRG and Mes5 primary sensory neurons co- expressing OC- and parvalbumin-irs are spinal and trigeminal proprioceptors. OC-ir TG neurons which co-express parvalbumin- and CGRP-irs appear to include low-threshold mechanoreceptors and nociceptors, respectively.