Action potential (AP)-independent (miniature) neurotransmission occurs at all chemical synapses but remains poorly understood, particularly in pathologic contexts. Axonal endoplasmic reticulum (ER) Ca²⁺ stores are thought to influence miniature neurotransmission, and aberrant ER Ca²⁺ handling is implicated in progression of Huntington disease (HD). Here, we report elevated mEPSC frequencies in recordings from YAC128 mouse (HD-model) neurons (from cortical cultures and striatum-containing brain slices, both from male and female animals). Pharmacological experiments suggest that this is mediated indirectly by enhanced tonic ER Ca²⁺ release. Calcium imaging, using an axon-localized sensor, revealed slow AP-independent ER Ca²⁺ release waves in both YAC128 and WT cultures. These Ca²⁺ waves occurred at similar frequencies in both genotypes but spread less extensively and were of lower amplitude in YAC128 axons, consistent with axonal ER Ca²⁺ store depletion. Surprisingly, basal cytosolic Ca²⁺ levels were lower in YAC128 boutons and YAC128 mEPSCs were less sensitive to intracellular Ca²⁺ chelation. Together, these data suggest that elevated miniature glutamate release in YAC128 cultures is associated with axonal ER Ca²⁺ depletion but not directly mediated by ER Ca²⁺ release into the cytoplasm. In contrast to increased mEPSC frequencies, cultured YAC128 cortical neurons showed less frequent AP-dependent (spontaneous) Ca²⁺ events in soma and axons, although evoked glutamate release detected by an intensity-based glutamate-sensing fluorescence reporter in brain slices was similar between genotypes. Our results indicate that axonal ER dysfunction selectively elevates miniature glutamate release from cortical terminals in HD. This, together with reduced spontaneous cortical neuron firing, may cause a shift from activity-dependent to -independent glutamate release in HD, with potential implications for fidelity and plasticity of cortical excitatory signaling.

SIGNIFICANCE STATEMENT Miniature neurotransmitter release persists at all chemical neuronal synapses in the absence of action potential firing but remains poorly understood, particularly in disease states. We show enhanced miniature glutamate release from cortical neurons in the YAC128 mouse Huntington disease model. This effect is mediated by axonal ER Ca²⁺ store depletion, but is not obviously due to elevated ER-to-cytosol Ca²⁺ release. Conversely, YAC128 cortical pyramidal neurons fired fewer action potentials and evoked cortical glutamate release was similar between WT an YAC128 preparations, indicating axonal ER depletion selectively enhances miniature glutamate release in YAC128 mice. These results extend our understanding of action potential independent neurotransmission and highlight a potential involvement of elevated miniature glutamate release in Huntington disease pathology.

与动作电位 (AP) 无关的（微型）神经传递发生在所有化学突触中，但人们对它知之甚少，特别是在病理情况下。轴突内质网 (ER) Ca ²⁺储存被认为影响微型神经传递，异常的 ER Ca ²⁺处理与亨廷顿病 (HD) 的进展有关。在这里，我们报告了 YAC128 小鼠（HD 模型）神经元（来自雄性和雌性动物的皮层培养物和含有纹状体的脑切片）记录中 mEPSC 频率升高。药理学实验表明，这是通过增强补品 ER Ca ²⁺释放间接介导的。使用轴突定位传感器进行的钙成像显示，在 YAC128 和 WT 培养物中，缓慢的不依赖于 AP 的 ER Ca ^{2+释放波。}这些 Ca ²⁺波在两种基因型中以相似的频率发生，但在 YAC128 轴突中传播范围较小且幅度较低，这与轴突 ER Ca ²⁺储备耗尽一致。令人惊讶的是，YAC128 boutons 中的基础胞质 Ca ²⁺水平较低，并且 YAC128 mEPSC 对细胞内 Ca ²⁺螯合不太敏感。^{总之，这些数据表明，YAC128 培养物中微型谷氨酸释放的升高与轴突 ER Ca 2+}耗竭相关，但并非由 ER Ca ²⁺释放到细胞质中直接介导。与增加的 mEPSC 频率相反，培养的 YAC128 皮质神经元在体细胞和轴突中显示出较少频率的 AP 依赖性（自发）Ca ²⁺事件，尽管脑切片中基于强度的谷氨酸感应荧光报告器检测到的诱发谷氨酸释放在基因型。我们的结果表明，HD 患者中，轴突 ER 功能障碍选择性地增加了皮质末梢的微量谷氨酸释放。这与皮质神经元自发放电的减少一起，可能会导致 HD 中谷氨酸释放从活动依赖性转变为独立性，这对皮质兴奋信号的保真度和可塑性具有潜在影响。

意义声明在没有动作电位放电的情况下，所有化学神经元突触都会持续释放微型神经递质，但人们对此仍知之甚少，特别是在疾病状态下。我们在 YAC128 小鼠亨廷顿病模型中发现皮质神经元的微型谷氨酸释放增强。这种效应是由轴突 ER Ca ²⁺储备耗尽介导的，但并不明显是由于 ER 向细胞质 Ca ²⁺释放增加所致。相反，YAC128 皮质锥体神经元激发较少的动作电位，并且在 WT 和 YAC128 制剂之间诱发的皮质谷氨酸释放相似，表明轴突 ER 消耗选择性增强 YAC128 小鼠中的微型谷氨酸释放。这些结果扩展了我们对动作电位独立神经传递的理解，并强调了微型谷氨酸释放升高在亨廷顿病病理学中的潜在参与。