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

Abstract

The distribution of osteopontin-immunoreactive (OPN-ir) primary sensory neurons was investigated in the spinal and trigeminal nervous systems. The dorsal root ganglion (DRG), trigeminal ganglion (TG) and mesencephalic trigeminal tract nucleus (Mes5) contained abundant OPN-ir neuronal cell bodies and their axons. Twenty-five and 31% of neurons were immunoreactive for this protein in the DRG and TG, respectively. These neurons were mostly large. A half (54%) of DRG neurons >1200 μm 2 and 9% of those in the range 600–1200 μm 2 showed the immunoreactivity (ir). DRG neurons <600 μm 2 were mostly devoid of OPN-ir (2%); 31% of TG neurons exhibited the ir. In the TG, a half (49%) of neurons >800 μm 2 showed the ir and 21% of those in the range 400–800 μm 2 were immunoreactive for this protein. TG neurons <400 μm 2 were mostly devoid of OPN-ir (2%). Virtually all (99%) Mes5 primary sensory neurons exhibited the ir. Muscle spindles in the soleus and masseter muscles contained OPN-ir spiral axon terminals. In the hard palate and incisor periodontal ligament, unencapsulated corpuscular endings exhibited the ir. The co-expression of OPN with parvalbumin and calcitonin gene-related peptide (CGRP) was also examined in the DRG and TG. In the DRG, virtually all (97%) OPN-ir neurons exhibited parvalbumin-ir. Conversely, 66% of parvalbumin-ir DRG neurons co-expressed OPN-ir. In the TG, 81% of OPN-ir neurons exhibited parvalbumin-ir and 69% of parvalbumin-ir ones showed OPN-ir. Virtually all OPN-ir DRG and TG neurons were devoid of CGRP-ir. The present study indicates that OPN-ir primary sensory neurons in the DRG and Mes5 are spinal and trigeminal proprioceptors. OPN-ir TG neurons appear to include low-threshold mechanoreceptors.