Brain-derived neurotrophic factor-immunoreactive primary sensory neurons in the rat trigeminal ganglion and trigeminal sensory nuclei

Immunohistochemistry for brain-derived neurotrophic factor (BDNF) was performed on the rat trigeminal ganglion (TG). The immunoreactivity (IR) was detected in 46% of TG neurons. These neurons were mostly small- or medium-sized (range, 149.7-1246.3 μm²; mean ± SD = 373.4 ± 151.6 μm²). A double immunofluorescence method also revealed that 54% of BDNF-immunoreactive (IR) neurons were immunoreactive for calcitonin-gene-related peptide. In addition, 93% of BDNF-IR TG neurons contained vanilloid receptor subtype 1. However, the co-expression of BDNF and vanilloid receptor 1-like receptor was very rare (less than 1%). In the trigeminal sensory nuclei, laminae II of the medullary dorsal horn was abundant in presumed BDNF-IR axon terminals. Such profiles were also detected in the dorsolateral part of the subnucleus oralis. The retrograde tracing and immunohistochemical methods demonstrated that BDNF-IR was common among cutaneous TG neurons (47%) but not tooth pulp TG neurons (13%). The present study indicates that BDNF-IR TG neurons have unmyelinated axons and project to the superficial medullary dorsal horn. It is likely that BDNF-containing neurons in both the trigeminal and spinal sensory systems have similarities in morphology and function. However, the content of BDNF in TG neurons probably depends on their peripheral targets. BDNF seems to convey nociceptive cutaneous input to the trigeminal sensory nuclei.