Immunocytochemical evidence that most sensory neurons of the rat molar pulp express receptors for both glial cell line-derived neurotrophic factor and nerve growth factor

SummaryMost pulpal afferent neurons have cytochemical features in common with the class of nociceptors that express neuropeptides and respond to NGF, while very few bind the plant lectin IB4, a widely used marker for the class of nociceptors that respond to the GDNF family of neurotrophic factors. The present study was undertaken to determine whether the GDNF receptor, GFRα-1, is expressed by pulpal afferents, and, further, to determine whether tooth injury evokes changes in expression of the GDNF and NGF receptors among pulpal afferents.The tracer, fluoro-gold (FG), was applied to shallow cavities in dentin of first and second maxillary molars. After 4 weeks, the molars of one side received a test injury consisting of a deeper cavity that exposed pulp horns. Animals were perfusion fixed 2 days later, and sections of the trigeminal ganglia were double immunostained with combinations of antibodies against GFRα-1, and TrkA.Under control conditions, GFRα-1 immunostaining was observed in 72% of neurons that projected to the molar pulp, TrkA in 78%, and immunostaining for both receptors was observed in 65% of pulpal afferents. Tooth injury evoked up-regulation of GFRα-1 expression (to 93%) and a slight down-regulation of TrkA expression (67%) among tooth afferents, while there was no discernable change in the proportion of pulpal afferents that expressed both TrkA and GFRα-1 (to 61%).These findings indicate that pulpal sensory neurons express a combination of properties that is relatively rare among somatosensory primary afferents and that pulpal afferents exhibit some plasticity of cytochemical phenotype in response to tooth injury.