Lingual and palatal gustatory afferents each depend on both BDNF and NT-4, but the dependence is greater for lingual than palatal afferents.

Neurons of the geniculate ganglion innervate taste buds located in two spatially distinct targets, the tongue and palate. About 50% of these neurons die in Bdnf(-/-) mice and Ntf4/5(-/-) mice. Bdnf(-/-)/Ntf4/5(-/-) double mutants lose 90-95% of geniculate ganglion neurons. To determine whether different subpopulations are differentially influenced by neurotrophins, we quantified neurons from two ganglion subpopulations separately and remaining taste buds at birth within each target field in wild-type, Bdnf(-/-), Ntf4/5(-/-), and Bdnf(-/-)/Ntf4/5(-/-) mice. In wild-type mice the same number of neurons innervated the anterior tongue and soft palate and each target contained the same number of taste buds. Compared to wild-type mice, Bdnf(-/-) mice showed a 50% reduction in geniculate neurons innervating the tongue and a 28% loss in neurons innervating the soft palate. Ntf4/5(-/-) mice lost 58% of the neurons innervating the tongue and 41% of the neurons innervating the soft palate. Taste bud loss was not as profound in the NT-4 null mice compared to BDNF-null mice. Tongues of Bdnf(-/-)/Ntf4/5(-/-) mice were innervated by 0 to 4 gustatory neurons and contained 3 to 16 taste buds at birth, indicating that some taste buds remain even when all innervation is lost. Thus, gustatory neurons are equally dependent on BDNF and NT-4 expression for survival, regardless of what peripheral target they innervate. However, taste buds are more sensitive to BDNF than NT-4 removal.