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

The distribution of osteopontin-immunoreactive (OPN-ir) primary sensory neu rons was investigated in the spinal and trigeminal nervous systems. The dor sal root ganglion (DRG)I trigeminal ganglion (TG) and mesencephalic trigemi nal tract nucleus (Mes5) contained abundant OPN-ir neuronal cell bodies and their axons. Twenty-five and 31% of neurons were immunoreactive for this p rotein in the DRG and TG, respectively. These neurons were mostly large. A half (54%) of DRG neurons >1200 mu m(2) and 9% of those in the range 600-12 00 mu m(2) showed the immunoreactivity (ir). DRG neurons <600 mu m(2) were mostly devoid of OPN-ir (2%); 31% of TG neurons exhibited the ir. In the TG , a half (49%) of neurons >800 mu m(2) showed the ir and 21% of those in th e range 400-800 mu m(2) were immunoreactive for this protein. TG neurons <4 00 mu m(2) were mostly devoid of OPN-ir (2%). Virtually all (99%) Mes5 prim ary sensory neurons exhibited the ir. Muscle spindles in the soleus and mas seter muscles contained OPN-ir spiral axon terminals. In the hard palate an d incisor periodontal ligament, unencapsulated corpuscular endings exhibite d the ir. The co-expression of OPN with parvalbumin and calcitonin gene-rel ated peptide (CGRP) was also examined in the DRG and TG. In the DRG, virtua lly all (97%) OPN-ir neurons exhibited parvalbumin-ir. Conversely, 66% of p arvalbumin-ir DRG neurons co-expressed OPN-ir. In the TG, 81% of OPN-ir neu rons 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 presen t study indicates that OPN-ir primary sensory neurons in the DRG and Mes5 a re spinal and trigeminal proprioceptors. OPN-ir TG neurons appear to includ e low-threshold mechanoreceptors. (C) 2000 Elsevier Science B.V. All rights reserved.