Lingual BDNF and NT-3 mRNA expression patterns and their relation to innervation in the human tongue: Similarities and differences compared with rodents

Brain-derived neurotrophic factor (BDNF) and neurotrophin 3 (NT-3) mRNAs ar e expressed in developing and adult rodent tongue and are important for the proper development of lingual gustatory and somatosensory innervation in r odents. Here, we wished to determine whether the findings in rodents apply to humans. By using in situ hybridization histochemistry, distinct, specifi c, and in some instances overlapping patterns of BDNF and NT-3 mRNA express ion were found in the developing and adult human tongue, gustatory papillae , and taste buds. BDNF mRNA was expressed in the superior surface epitheliu m of the developing fungiform papillae (i.e., developing taste buds), in th e epithelium covering the circumvallate papillae, and in the subepithelial mesenchyme. Interestingly, BDNF mRNA was expressed in the lingual epitheliu m before nerve fibers reached the epithelium, indicating a prespecializatio n of the gustatory epithelium before the arrival of nerves. In the adult fu ngiform papillae, BDNF mRNA labeling was found in taste buds and in restric ted areas in the non-gustatory lingual epithelium. NT-3 mRNA was found in t he developing lingual epithelium and gustatory papillae. NT-3 mRNA labeling was observed in the adult fungiform taste buds, overlapping with BDNF mRNA labeling, in contrast to what was seen in rodents. NT-3 mRNA was additiona lly found in restricted areas in filiform papillae. Protein gene product 9. 5 (PGP) antibodies were used to investigate a possible correlation between lingual innervation and sites of neurotrophin gene activity. Adult human to ngue innervation differed from that of rodents, possibly in part due to a d ifferent neurotrophin expression pattern in the human tongue. Based on thes e findings, we suggest that BDNF and NT-3 are important for the initiation and maintenance of the gustatory and somatosensory innervation also in huma ns. The broader and somewhat overlapping expression patterns of BDNF and NT -3 mRNAs, compared with rodents, suggest additional and possibly somewhat o verlapping roles for BDNF and NT-3 in the human tongue and also indicate di fferences between species. It is important that interspecies differences be taken into consideration. J. Comp. Neurol. 417: 133-152, 2000. (C) 2000 Wi ley-Liss, Inc.