Roles of presynaptic calcium stores at hipporampal mossy fiber synapse

兰尼定受体介导细胞内钙离子的释放，调节神经细胞和非神经细胞的各种功能。为了探讨Ryanodine受体在突触前钙动力学活动依赖的调制中的作用，我们采用光学方法选择性地监测了海马苔藓纤维-CA3突触前终末的钙离子浓度。将荧光指示剂Rhod-2 AM局部注射到CA3区透明层轴突束苔藓纤维中，选择性标记苔藓纤维。监测突触部位的荧光强度以评估突触前终末的钙离子水平。应用Ryanodine受体阻断剂TMB-8选择性地抑制高频刺激苔藓纤维诱发的突触前Ca~(2+)和Gt~(2+)。我们还研究了咖啡因对苔藓纤维-CA3突触的突触传递的影响。咖啡因通过兰尼定受体诱导细胞内钙离子释放。咖啡因的应用引起了场EPSP的强烈增强，这种突触增强伴随着成对脉冲易化的显著降低。这些结果表明，兰尼定受体存在于海马苔藓纤维突触的突触前终末，细胞内钙离子释放在该突触的突触前钙动力学和突触可塑性的调节中起着关键作用。

Ryanodine receptors mediate Ca^<2+> release from intracellular stores and regulate various functions of neuronal and non-neuronal cells. To investigate the roles of ryanodine receptors in activity-dependent modulation of presynaptic Ca^<2+> dynamics, we adopted optical methods to selectively monitor Ca^<2+> concentrations in the presynaptic terminals of mossy fiber-CA3 synapse in the hippocampus. Fluorescent Ca^<2+> indicator rhod-2 AM was locally injected into the axon bundles mossy fibers at stratum lucidum of CA3 region, resulting in selective labeling of the mossy fibers. Fluorescence intencity at the synaptic sites was monitored to assess the Ca^<2+> levels in the presynaptic terminals. Application of ryanodine receptor blacker TMB-8 selectively suppressed the presynaptic Ca^<2+> transient evoked by high frequency stimulation of the mossy fibers. We also investigated the effect of caffeine, which induces Ca^<2+> release from intracellular store through ryanodine receptors, on the synaptic transmission at mossy fiber-CA3 synapse. Application of caffeine caused robust enhancement of the field EPSPs, and this synaptic enhancement was accompanied by a marked reduction of paired-pulse facilitation. These results suggested that ryanodine receptors are present in the presynaptic terminals of the hippocampal mossy fiber synapses, and Ca^<2+> release from the intracellular stores plays a pivotal role in regulation of presynaptic Ca^<2+> dynamics as well as synaptic plasticity at this synapse.