Santos-Valencia Fernando, Lackey Elizabeth P, Norton Aliya, Wardak Asem, Gaynor Cole S, Ediger Sean, Hemelt Marie E, Nguyen Tri M, Allen Lee Wei-Chung, Brunel Nicolas, Hull Court A, Regehr Wade G
Department of Neurobiology, Duke University Medical School, Durham, NC, USA.
Department of Neurobiology, Harvard Medical School, Boston, MA, USA.
bioRxiv. 2025 Jun 23:2025.06.22.660768. doi: 10.1101/2025.06.22.660768.
Climbing fiber (CF) inputs to Purkinje cells (PCs) instruct plasticity and learning in the cerebellum. Paradoxically, CFs also excite molecular layer interneurons (MLIs), a cell-type that inhibits PCs and can restrict plasticity and learning. However, two types of MLIs with opposing influences have recently been identified: MLI1s inhibit PCs, reduce dendritic calcium signals, and suppress plasticity of granule cell to PC synapses, whereas MLI2s inhibit MLI1s and disinhibit PCs. To determine how CFs can activate MLIs without also suppressing the PC calcium signals necessary for plasticity and learning, we investigated the specificity of CF inputs onto MLIs. Serial EM reconstructions indicate that CFs contact both MLI subtypes without making conventional synapses, but more CFs contact each MLI2 via more sites with larger contact areas. Slice experiments indicate that CFs preferentially excite MLI2s via glutamate spillover. In agreement with these anatomical and slice experiments, Neuropixels recordings show that spontaneous CF activity excites MLI2s, inhibits MLI1s, and disinhibits PCs. In contrast, learning-related sensory stimulation produced more complex responses, driving convergent CF and granule cell inputs that could either activate or suppress MLI1s. This balance was robustly shifted toward MLI1 suppression when CFs were synchronously active, in turn elevating the PC dendritic calcium signals necessary for LTD. These data provide mechanistic insight into why CF synchrony can be highly effective at inducing cerebellar learning by revealing a critical disinhibitory circuit that allows CFs to act through MLIs to enhance PC dendritic calcium signals necessary for plasticity.
攀缘纤维（CF）对浦肯野细胞（PC）的输入指导着小脑的可塑性和学习。矛盾的是，CFs也会兴奋分子层中间神经元（MLIs），这是一种抑制PCs并可能限制可塑性和学习的细胞类型。然而，最近已经鉴定出两种具有相反影响的MLIs：MLI1s抑制PCs，减少树突钙信号，并抑制颗粒细胞到PC突触的可塑性，而MLI2s抑制MLI1s并解除对PCs的抑制。为了确定CFs如何在不抑制可塑性和学习所需的PC钙信号的情况下激活MLIs，我们研究了CF对MLIs输入的特异性。连续电子显微镜重建表明，CFs与两种MLI亚型都有接触，但没有形成传统突触，但更多的CFs通过更多具有更大接触面积的位点与每个MLI2接触。切片实验表明，CFs通过谷氨酸溢出优先兴奋MLI2s。与这些解剖学和切片实验一致，神经像素记录显示，自发的CF活动兴奋MLI2s，抑制MLI1s，并解除对PCs的抑制。相比之下，与学习相关的感觉刺激产生了更复杂的反应，驱动了汇聚的CF和颗粒细胞输入，这些输入可以激活或抑制MLI1s。当CFs同步活动时，这种平衡强烈地向抑制MLI1s的方向转变，进而提高了长时程抑制（LTD）所需的PC树突钙信号。这些数据通过揭示一个关键的去抑制回路，为CF同步在诱导小脑学习方面为何能高效发挥作用提供了机制性见解，该回路使CFs能够通过MLIs发挥作用，增强可塑性所需的PC树突钙信号。
