Rapid structural remodeling of peripheral taste fibers is independent of taste cell turnover

Taste bud cells are constantly replaced in taste buds as old cells die and new cells migrate into the bud. The perception of taste relies on new taste bud cells integrating with existing neural circuitry, yet how these new cells connect with a taste ganglion neuron is unknown. Do taste ganglion neurons remodel to accommodate taste bud cell renewal? If so, how much of the structure of taste axons is fixed and how much remodels? Here we measured the motility and branching of individual taste arbors (the portion of the axon innervating taste buds) over time with two-photon in vivo microscopy. Terminal branches of taste arbors continuously and rapidly remodel within the taste bud. This remodeling is faster than predicted by taste bud cell renewal, with terminal branches added and lost concurrently. Surprisingly, blocking entry of new taste bud cells with chemotherapeutic agents revealed that remodeling of the terminal branches on taste arbors does not rely on the renewal of taste bud cells. Although terminal branch remodeling was fast and intrinsically controlled, no new arbors were added to taste buds, and few were lost over 100 days. Taste ganglion neurons maintain a stable number of arbors that are each capable of high-speed remodeling. We propose that terminal branch plasticity permits arbors to locate new taste bud cells, while stability of arbor number supports constancy in the degree of connectivity and function for each neuron over time.

味蕾内的味细胞会随着衰老细胞死亡、新生细胞迁移至味蕾内而持续更新更替。味觉感知依赖于新生味细胞与现有神经环路的整合，但这些新生细胞如何与味觉神经节神经元（taste ganglion neuron）建立连接仍未明确。味觉神经节神经元是否会发生结构重塑以适配味细胞更新？若是如此，味觉轴突的结构有多少是固定不变的，又有多少会发生重塑？
本研究通过双光子活体显微镜成像（two-photon in vivo microscopy）技术，长期实时观测了单个味觉神经末梢分支（taste arbors，即支配味蕾的轴突部分）的运动特性与分支形态变化。结果显示，味蕾内的味觉神经末梢分支会持续且快速地发生结构重塑。该重塑速度快于基于味细胞更新的预期速率，且末梢分支的新增与丢失同步发生。
令人意外的是，利用化疗药物阻断新生味细胞进入味蕾后，研究发现味觉神经末梢分支的重塑并不依赖味细胞的更新。尽管末梢分支的重塑速度快且受内在机制调控，但在100天的观测周期内，并未有新的味觉神经末梢分支添加至味蕾，且仅有极少数分支发生丢失。
味觉神经节神经元维持着稳定的末梢分支数量，每个分支均可发生高速结构重塑。我们提出假说：味觉末梢分支的可塑性允许神经末梢定位新生味细胞，而末梢分支数量的稳定性则保障了每一个神经元在长期内的连接程度与功能维持恒定。