Center for Learning and Memory and Department of Psychology, University of Texas at Austin, Austin, Texas 78712, USA.
J Neurosci. 2013 May 8;33(19):8301-7. doi: 10.1523/JNEUROSCI.3792-12.2013.
Presynaptic axonal varicosities, like postsynaptic spines, are dynamically added and eliminated even in mature neuronal circuitry. To study the role of this axonal structural plasticity in behavioral learning, we performed two-photon in vivo imaging of cerebellar parallel fibers (PFs) in adult mice. PFs make excitatory synapses on Purkinje cells (PCs) in the cerebellar cortex, and long-term potentiation and depression at PF-PC synapses are thought to play crucial roles in cerebellar-dependent learning. Time-lapse vital imaging of PFs revealed that, under a control condition (no behavioral training), ∼10% of PF varicosities appeared and disappeared over a period of 2 weeks without changing the total number of varicosities. The fraction of dynamic PF varicosities significantly diminished during training on an acrobatic motor skill learning task, largely because of reduced addition of new varicosities. Thus, this form of motor learning was associated with greater structural stability of PFs and a slight decrease in the total number of varicosities. Together with prior findings that the number of PF-PC synapses increases during similar training, our results suggest that acrobatic motor skill learning involves a reduction of some PF inputs and a strengthening of others, probably via the conversion of some preexisting PF varicosities into multisynaptic terminals.
突触前轴突末梢，如突触后棘突，即使在成熟的神经元回路中也在不断地增加和消除。为了研究这种轴突结构可塑性在行为学习中的作用，我们在成年小鼠中进行了双光子在体成像的小脑平行纤维（PFs）。PFs 在小脑皮层上与浦肯野细胞（PCs）形成兴奋性突触，而 PF-PC 突触的长时程增强和长时程抑制被认为在小脑依赖的学习中起着关键作用。PFs 的延时活体成像显示，在对照条件下（无行为训练），在两周的时间内，约有 10%的 PF 末梢出现和消失，而末梢总数没有变化。在杂技运动技能学习任务的训练过程中，动态 PF 末梢的分数显著减少，这主要是由于新末梢的添加减少。因此，这种形式的运动学习与 PFs 的结构稳定性更大和末梢总数略有减少有关。与之前的研究结果一致，即在类似的训练过程中 PF-PC 突触的数量增加，我们的结果表明，杂技运动技能学习涉及到一些 PF 输入的减少和其他输入的增强，可能是通过一些预先存在的 PF 末梢转化为多突触末端。
