trkA modulation of developing somatosensory neurons in oro-facial tissues:Tooth pulp fibers are absent in trkA knockout mice

To investigate the nerve growth factor requirement of developing oro-facial somatosensory afferents, we have studied the survival of sensory fibers su bserving nociception, mechanoreception or proprioception in receptor tyrosi ne kinase (trkA) knockout mice using immunohistochemistry. trkA receptor nu ll mutant mice lack nerve fibers in tooth pulp, including sympathetic fiber s, and showed only sparse innervation of the periodontal ligament. Ruffini endings were formed definitively in the periodontal ligament of the trkA kn ockout mice, although calcitonin gene-related peptide- and substance P-immu noreactive fibers were reduced in number or had disappeared completely. trk A gene deletion had also no obvious effect on the formation of Meissner cor puscles in the palate. In the vibrissal follicle, however, some mechanorece ptive afferents were sensitive for trkA gene deletion, confirming a previou s report [Fundin et al. (1997) Dev. Biol. 190, 94-116]. Moreover, calretini n-positive fibers innervating longitudinal lanceolate endings were complete ly lost in trkA knockout mice, as were the calretinin-containing parent cel ls in the trigeminal ganglion. These results indicate that trkA is indispensable for developing nociceptiv e neurons innervating oral tissues, but not for developing mechanoreceptive neurons innervating oral tissues (Ruffini endings and Meissner corpuscles) , and that calretinin-containing, trkA dependent neurons in the trigeminal ganglion normally participate in mechanoreception through longitudinal lanc eolate endings of the vibrissal follicle. (C) 2001 IBRO. Published by Elsev ier Science Ltd. All rights reserved.