Brain Research Institute, University of Zurich, CH-8057 Zurich, Switzerland.
Proc Natl Acad Sci U S A. 2014 Mar 18;111(11):4303-8. doi: 10.1073/pnas.1317667111. Epub 2014 Feb 18.
Dentate granule cells exhibit exceptionally low levels of activity and rarely elicit action potentials in targeted CA3 pyramidal cells. It is thus unclear how such weak input from the granule cells sustains adequate levels of synaptic plasticity in the targeted CA3 network. We report that subthreshold potentials evoked by mossy fibers are sufficient to induce synaptic plasticity between CA3 pyramidal cells, thereby complementing the sparse action potential discharge. Repetitive pairing of a CA3-CA3 recurrent synaptic response with a subsequent subthreshold mossy fiber response induced long-term potentiation at CA3 recurrent synapses in rat hippocampus in vitro. Reversing the timing of the inputs induced long-term depression. The underlying mechanism depends on a passively conducted giant excitatory postsynaptic potential evoked by a mossy fiber that enhances NMDA receptor-mediated current at active CA3 recurrent synapses by relieving magnesium block. The resulting NMDA spike generates a supralinear depolarization that contributes to synaptic plasticity in hippocampal neuronal ensembles implicated in memory.
颗粒细胞表现出异常低的活动水平，很少在靶向 CA3 锥体神经元中引发动作电位。因此，尚不清楚来自颗粒细胞的如此微弱的输入如何维持靶向 CA3 网络中足够水平的突触可塑性。我们报告说，苔藓纤维诱发的阈下电位足以诱导 CA3 锥体神经元之间的突触可塑性，从而补充了稀疏的动作电位放电。在体外，CA3 苔藓纤维反应的重复配对与随后的阈下苔藓纤维反应一起诱导了大鼠海马 CA3 回传突触的长时程增强。反转输入的时间会诱导长时程压抑。这种潜在的机制取决于由苔藓纤维引起的被动传导的巨大兴奋性突触后电位，该电位通过减轻镁阻断来增强活跃 CA3 回传突触上 NMDA 受体介导的电流。由此产生的 NMDA 尖峰产生超线性去极化，有助于与记忆有关的海马神经元集合中的突触可塑性。
