Welzel Georg, Schuster Stefan
Department of Animal Physiology, University of Bayreuth, Bayreuth, Germany.
Front Cell Neurosci. 2019 Feb 12;13:43. doi: 10.3389/fncel.2019.00043. eCollection 2019.
During the last decades it became increasingly evident that electrical synapses are capable of activity-dependent plasticity. However, measuring the actual strength of electrical transmission remains difficult. Usually changes in coupling strength can only be inferred indirectly from measures such as the coupling coefficient and the coupling conductance. Because these are affected by both junctional and non-junctional conductance, plastic changes can potentially be due to both components. Furthermore, these techniques also require the blocking of chemical transmission, so that processes that involve crosstalk between chemical and electrical synapses will be suppressed. To directly examine the magnitude of errors that can occur, we use dual whole-cell current- and voltage-clamp recordings from the soma of the pair of easily accessible, electrically coupled Retzius cells in the leech to simultaneously determine coupling coefficients, coupling conductances and directly measured gap junctional currents. We present the first direct and comparative analysis of gap junction conductance using all three methods and analyze how each method would characterize the response of gap junctions to serotonin. The traditional coupling coefficients showed severe deficits in assessing the symmetry and strength of electrical synapses. These were reduced when coupling conductances were determined and were absent in the direct method. Additionally, both coupling coefficient and coupling conductance caused large and systematic errors in assessing the size and time course of the serotonin-induced changes of gap junctional currents. Most importantly, both measurements can easily be misinterpreted as implying long-term gap junctional plasticity, although the direct measurements confirm its absence. We thus show directly that coupling coefficients and coupling conductances can severely confound plastic changes in membrane and junctional conductance. Wherever possible, voltage clamp measurements should be chosen to accurately characterize the timing and strength of plasticity of electrical synapses. However, we also demonstrate that coupling coefficients can still yield a qualitatively correct picture when amended by independent measurements of the course of membrane resistance during the experiments.
在过去几十年里，越来越明显的是，电突触具有活动依赖性可塑性。然而，测量电传递的实际强度仍然很困难。通常，耦合强度的变化只能从诸如耦合系数和耦合电导等测量值中间接推断出来。由于这些都受到连接电导和非连接电导的影响，可塑性变化可能是由这两个成分共同导致的。此外，这些技术还需要阻断化学传递，因此涉及化学突触和电突触之间串扰的过程将被抑制。为了直接检查可能出现的误差大小，我们使用双全细胞电流钳和电压钳记录来自水蛭中一对易于获取的、电耦合的Retzius细胞胞体的信号，以同时确定耦合系数、耦合电导并直接测量缝隙连接电流。我们首次使用所有三种方法对缝隙连接电导进行了直接和比较分析，并分析了每种方法如何表征缝隙连接对5-羟色胺的反应。传统的耦合系数在评估电突触的对称性和强度方面存在严重缺陷。在确定耦合电导时这些缺陷会减少，而在直接测量方法中则不存在。此外，耦合系数和耦合电导在评估5-羟色胺诱导的缝隙连接电流变化的大小和时间进程时都会导致巨大的系统性误差。最重要的是，尽管直接测量结果证实不存在长期的缝隙连接可塑性，但这两种测量结果都很容易被误解为意味着长期的缝隙连接可塑性。因此，我们直接表明耦合系数和耦合电导可能会严重混淆膜电导和连接电导的可塑性变化。只要有可能，就应选择电压钳测量来准确表征电突触可塑性的时间和强度。然而，我们也证明，当通过实验期间膜电阻变化过程的独立测量进行修正时，耦合系数仍可得出定性正确的结果。
