Glial cell line-derived neurotrophic factor (GDNF) from adult rat tooth serves a distinct population of large-sized trigeminal neurons

Glial cell line-derived neurotrophic factor (GDNF) mediates trophic effects for specific classes of sensory neurons. The adult tooth pulp is a well-defined target of sensory trigeminal innervation. Here we investigated potential roles of GDNF in the regulation of adult trigeminal neurons and the dental pulp nerve supply of the rat maxillary first molar. Western blot analysis and radioactive 35S-UTP in situ hybridization revealed that GDNF in the dental pulp and its mRNAs were localized with Ngf in the coronal pulp periphery, in particular in the highly innervated subodontoblast layer. Retrograde neuronal transport of iodinated GDNF and Fluorogold (FG) from the dental pulp indicated that GDNF was transported in about one third of all the trigeminal dental neurons. Of the GDNF-labelled neurons, nearly all (97%) were large-sized (> or =35 microm in diameter). Analysis of FG-labelled neurons revealed that, of the trigeminal neurons supporting the adult dental pulp, approximately 20% were small-sized, lacked isolectin B4 binding and did not transport GDNF. Of the large-sized dental trigeminal neurons approximately 40% transported GDNF. About 90% of the GDNF-accumulating neurons were negative for the high-temperature nociceptive marker VRL-1. Our results show that a subclass of large adult trigeminal neurons are potentially dependent on dental pulp-derived GDNF while small dental trigeminal neurons seems not to require GDNF. This suggests that GDNF may function as a neurotrophic factor for subsets of nerves in the tooth, which apparently mediate mechanosensitive stimuli. As in dorsal root ganglia both small- and large-sized neurons are known to be GDNF-dependent; these data provide molecular evidence that the sensory supply in the adult tooth differs, in some aspects, from the cutaneous sensory system.

Distribution of central sensory axons in transgenic mice overexpressing nerve growth factor and lacking functional p75 neurotrophin receptor expression

This study examined the roles of nerve growth factor (NGF) and the p75 neurotrophin receptor (p75NTR) in the growth of dorsal root ganglion (DRG) central processes in the dorsal horn. Two genetically modified mouse strains were used: transgenic mice that overexpress NGF in the CNS under the control of the glial fibrillary acidic protein promoter, and p75NTR exon III null mutant mice that express a hypomorphic form of this receptor. In both NGF transgenic and nontransgenic mice with hypomorphic expression of p75NTR, there is a significant loss of DRG neurons compared to mice with normal p75NTR expression. This reduction in neuron number has been shown to underlie a corresponding decrease in peripheral nociceptive sensory innervation. Within the CNS, however, nociceptive innervation of the dorsal horn appears to be unaffected by hypomorphic expression of p75NTR. Comparisons of calcitonin gene-related peptide immunoreactivity in the dorsal horn revealed that the area occupied by DRG central processes was not significantly different between p75NTR hypomorphic mice and wild-type siblings, or between NGF transgenic mice with either hypomorphic or normal expression of p75NTR. We propose that DRG central processes arborize extensively in both NGF-transgenic and nontransgenic p75NTR hypomorphic mice in order to compensate for the loss of DRG neurons and restore dorsal horn innervation to normal levels. We also present evidence suggesting that NGF plays only a minor role in the growth of DRG central processes.