Neurotrophin-4 is more potent than brain-derived neurotrophic factor in promoting, attracting and suppressing geniculate ganglion neurite outgrowth.

The geniculate ganglion, which provides innervation to taste buds in the anterior tongue and palate, is unique among sensory ganglia in that its neurons depend on both neurotrophin-4 (NT4) and brain-derived neurotrophic factor (BDNF) for survival. Whereas BDNF is additionally implicated in taste axon guidance at targeting stages, much less is known about the guidance role of NT4 during targeting, or about either neurotrophin during initial pathfinding. NT4 and BDNF have distinct expression patterns in vivo, raising the possibility of distinct roles. We characterized the influence of NT4 and BDNF on geniculate neurites in collagen I gels at early embryonic through postnatal stages. During early pathfinding to the tongue (embryonic days 12-13; E12-13), NT4 and BDNF promote significantly longer outgrowth than during intralingual targeting (E15-18). NT4 is more potent than BDNF at stimulating neurite outgrowth and both factors exhibit concentration optima, i.e. intermediate concentrations (0.25 ng/ml NT4 or 25 ng/ml BDNF) promote maximal neurite extension and high concentrations (10 ng/ml NT4 or 200 ng/ml BDNF) suppress it. Only partial suppression was seen at E12 (when axons first emerge from the ganglion in vivo) and postnatally, but nearly complete suppression occurred from E13 to E18. We show that cell death is not responsible for suppression. Although blocking the p75 receptor reduces outgrowth at the optimum concentrations of NT4 and BDNF, it did not reduce suppression of outgrowth. We also report that NT4, like BDNF, can act as a chemoattractant for geniculate neurites, and that the tropic influence is strongest during intralingual targeting (E15-18). NT4 does not appear to act as an attractant in vivo, but it may prevent premature invasion of the epithelium by suppressing axon growth.