Conti Rossella, Lisman John
CNRS, Laboratoire de Physiologie Cérébrale, Paris, France.
Hippocampus. 2002;12(5):667-79. doi: 10.1002/hipo.10096.
Synapses in the CA1 region of the hippocampus undergo bidirectional synaptic modification in response to different patterns of activity. Postsynaptic Ca2+ elevation can trigger either synaptic strengthening or weakening, depending on the properties of the local Ca2+ signal. During the pairing protocol for long-term potentiation (LTP) induction, the cell is depolarized under voltage-clamp and is given low-frequency synaptic stimulation. As an initial step toward understanding the Ca2+ dynamics during this process, we used confocal microscopy to study the Ca2+ signals in spines evoked by the depolarization itself. This depolarization activates voltage-dependent Ca2+ channels (VDCC), but whether these channels inactivate rapidly or remain functional throughout the long depolarizations used in the pairing protocol remains unknown. Cells were depolarized to 0 mV for 2-3 min. This depolarization led to a large initial elevation of Ca2+ in spines that never decayed back to resting levels. The maintained signal was close to the Kd of the low-affinity (5 microM) Ca2+ dye, Magnesium Green. We attempted to determine the functional role of this elevation, using the Ca2+-channel blocker D-890. The addition of D-890 in the internal solution produced a nearly complete abolition of the Ca2+ elevation during depolarization. Under these conditions, the NMDA conductance was normal, but LTP was almost completely blocked. This might suggest the importance of VDCC in LTP; however, we found that high concentrations of D-890 can directly inhibit calmodulin protein kinase II (CaMKII), an enzyme required for LTP induction. Thus, whereas D-890 is a useful tool for blocking postsynaptic VDCC, it cannot be used to study the contribution of these channels to plasticity. We conclude that the activation of VDCC produces a large and persistent elevation of Ca2+ in all spines, but does not produce either LTP or long-term depression (LTD) in the absence of synaptic stimulation. The possible reasons for this are discussed.
海马体CA1区域的突触会根据不同的活动模式进行双向突触修饰。突触后Ca2+升高可触发突触增强或减弱，这取决于局部Ca2+信号的特性。在长期增强（LTP）诱导的配对方案中，细胞在电压钳制下被去极化，并给予低频突触刺激。作为理解此过程中Ca2+动态变化的第一步，我们使用共聚焦显微镜研究去极化本身诱发的棘突中的Ca2+信号。这种去极化激活了电压依赖性Ca2+通道（VDCC），但这些通道在配对方案中使用的长时间去极化过程中是迅速失活还是保持功能仍然未知。细胞被去极化至0 mV持续2 - 3分钟。这种去极化导致棘突中Ca2+出现大幅初始升高，且从未恢复到静息水平。维持的信号接近低亲和力（5 microM）Ca2+染料镁绿的解离常数（Kd）。我们试图使用Ca2+通道阻滞剂D - 890来确定这种升高的功能作用。在内部溶液中添加D - 890几乎完全消除了去极化期间的Ca2+升高。在这些条件下，NMDA电导正常，但LTP几乎完全被阻断。这可能表明VDCC在LTP中的重要性；然而，我们发现高浓度的D - 890可直接抑制钙调蛋白依赖性蛋白激酶II（CaMKII），这是LTP诱导所需的一种酶。因此，虽然D - 890是阻断突触后VDCC的有用工具，但它不能用于研究这些通道对可塑性的贡献。我们得出结论，VDCC的激活在所有棘突中产生大量且持续的Ca2+升高，但在没有突触刺激的情况下不会产生LTP或长期抑制（LTD）。文中讨论了可能的原因。
