VR1-immunoreactive primary sensory neurons in the rat trigeminal ganglion

Immunohistochemistry for VR1, a nociceptive transducer for vanilloid compounds, protons and heat (>43°C), was performed on the rat trigeminal ganglion (TG). The immunoreactivity (IR) was detected in 20% of TG cells and these neurons were mostly small- to medium-sized (mean±S.D. 427±189 μm²). Twenty-six percent of the TG neurons retrogradely labeled from the facial skin exhibited VR1-IR, while the IR was detected in only 8% of those labeled from the tooth pulp. Co-expression of VR1 was common among the calcitonin gene-related peptide-immunoreactive cutaneous neurons (63%) but not among the similar tooth pulp neurons (20%). The present study indicates that primary nociceptive neurons which respond to vanilloid compounds, protons and heat are abundant in the facial skin but not in the tooth pulp.