Effects of deprivation of neonatal nerve growth factor on the expression of neurotrophin receptors and brain-derived neurotrophic factor by dental pulp afferents of the adult rat

The dental pulp is richly innervated by peptidergic nociceptive neurones th at are of special interest because of their central role in dental pain and because they have some features that are not typical of other somatic noci ceptors. Here, S-35-riboprobes were used to determine whether pulpal affere nts of adult (2-month-old) rats express the nerve growth-factor (NGF) recep tors, p75(NTR) and trkA, which are characteristic of peptidergic nociceptor s, and additionally, whether these cells express receptors (trkB and trkC) for other members of the neurotrophin family. In order to begin characteriz ing the postnatal role of NGF in regulating these neurones, the susceptibil ity of pulpal afferents to antiserum-mediated early postnatal NGF depletion spanning the period of pulpal innervation development was also examined. I n control animals, about 200 trigeminal ganglion cells were labelled after application of the retrograde tracer Fluoro-gold to the first maxillary mol ar. Among the labelled cells, 79% had positive hybridization signals for p7 5(NTR), 72% for trkA, 34% for trkB, 1% for trkC, and 77% for BDNF. Neonatal NGF depletion reduced the number of retrogradely labelled pulpal afferents by 33%: with numbers of smaller neurones being most strikingly subnormal. This reduction could be attributed to a partial depletion of the neurone po pulation that expressed p75(NTR) and trkA. Consistent with reports that NGF -responsive neurones also express BDNF: NGF deprivation resulted in a reduc tion in the number of pulpal afferents that expressed BDNF to an extent sim ilar to that seen for trkA. In contrast, anti-NGF exposure had little effec t on the number of pulpal afferents that expressed trkB. These findings ind icate that most pulpal afferents in the adult express the NGF receptors p75 (NTR) and trkA, and thus have a continuing potential susceptibility to NGF- mediated regulation of functions such as neuropeptide and BDNF synthesis. H owever, only a subpopulation of this group of neurones requires NGF in orde r to develop connections to the pulp during the neonatal period. Few, if an y, pulpal afferents express the high-affinity neurotrophin-3 (NT3) receptor trkC, although many have large cell bodies typical of NT3-responsive senso ry neurones. A small subpopulation of pulpal afferents seems to express no neurotrophin receptors.