Houck Brenda D, Person Abigail L
Department of Physiology & Biophysics, University of Colorado School of Medicine, Aurora, Colorado, 80045.
J Comp Neurol. 2015 Oct 15;523(15):2254-71. doi: 10.1002/cne.23787. Epub 2015 May 12.
Motor commands computed by the cerebellum are hypothesized to use corollary discharge, or copies of outgoing commands, to accelerate motor corrections. Identifying sources of corollary discharge, therefore, is critical for testing this hypothesis. Here we verified that the pathway from the cerebellar nuclei to the cerebellar cortex in mice includes collaterals of cerebellar premotor output neurons, mapped this collateral pathway, and identified its postsynaptic targets. Following bidirectional tracer injections into a distal target of the cerebellar nuclei, the ventrolateral thalamus, we observed retrogradely labeled somata in the cerebellar nuclei and mossy fiber terminals in the cerebellar granule layer, consistent with collateral branching. Corroborating these observations, bidirectional tracer injections into the cerebellar cortex retrogradely labeled somata in the cerebellar nuclei and boutons in the ventrolateral thalamus. To test whether nuclear output neurons projecting to the red nucleus also collateralize to the cerebellar cortex, we used a Cre-dependent viral approach, avoiding potential confounds of direct red nucleus-to-cerebellum projections. Injections of a Cre-dependent GFP-expressing virus into Ntsr1-Cre mice, which express Cre selectively in the cerebellar nuclei, retrogradely labeled somata in the interposed nucleus, and putative collateral branches terminating as mossy fibers in the cerebellar cortex. Postsynaptic targets of all labeled mossy fiber terminals were identified using immunohistochemical Golgi cell markers and electron microscopic profiles of granule cells, indicating that the collaterals of nuclear output neurons contact both Golgi and granule cells. These results clarify the organization of a subset of nucleocortical projections that constitute an experimentally accessible corollary discharge pathway within the cerebellum.
据推测，小脑计算出的运动指令会利用伴随放电，即传出指令的副本，来加速运动校正。因此，确定伴随放电的来源对于验证这一假设至关重要。在这里，我们证实小鼠从小脑核到小脑皮质的通路包括小脑运动前输出神经元的侧支，绘制了这条侧支通路，并确定了其突触后靶点。在向小脑核的一个远端靶点——腹外侧丘脑注射双向示踪剂后，我们在小脑核中观察到逆行标记的胞体以及在小脑颗粒层中的苔藓纤维终末，这与侧支分支一致。为证实这些观察结果，向小脑皮质注射双向示踪剂后，在小脑核中逆行标记了胞体，在腹外侧丘脑中标记了轴突终扣。为了测试投射到红核的核输出神经元是否也会向小脑皮质发出侧支，我们采用了一种依赖于Cre的病毒方法，避免了直接的红核到小脑投射可能产生的混淆。将一种依赖于Cre的绿色荧光蛋白表达病毒注射到Ntsr1-Cre小鼠（在小脑核中选择性表达Cre）中，在间位核中逆行标记了胞体，并且在小脑皮质中发现了作为苔藓纤维终末的假定侧支。使用免疫组织化学的高尔基细胞标记物和颗粒细胞的电子显微镜图像确定了所有标记的苔藓纤维终末的突触后靶点，表明核输出神经元的侧支与高尔基细胞和颗粒细胞都有接触。这些结果阐明了核皮质投射的一个子集的组织结构，这些投射构成了小脑内一个实验上可及的伴随放电通路。
