THE RETICULOSPINAL GLUTAMATE SYNAPSE IN LAMPREY - PLASTICITY AND PRESYNAPTIC VARIABILITY
THE RETICULOSPINAL GLUTAMATE SYNAPSE IN LAMPREY - PLASTICITY AND PRESYNAPTIC VARIABILITY
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DOI：
10.1152/jn.1994.72.2.592
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发表时间：
1994-08-01
影响因子：
2.5
通讯作者：
HILL, RH
中科院分区：
文献类型：
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作者：
BRODIN, L;SHUPLIAKOV, O;HILL, RH
1.七鳃鳗的巨大网状脊髓轴突与脊髓神经元之间形成的突触能突触提供了一种中枢脊椎动物突触，其中突触前元件可以用一个或多个微电极刺穿，可以用于记录以及显微注射不同的物质。为了提供一个基础，使用这种突触的释放机制的研究，我们已经研究了使用依赖性调制的突触反应的条件下，常规的细胞体刺激，并在直接刺激突触前轴突。为了检查混合的电紧张性和化学网状脊髓兴奋性突触后电位（EPSP）随时间的稳定性，以1Hz的速率诱发动作电位800-1000次试验。在7个突触中的3个中，化学成分保持在相似的幅度，而在4个病例中发生了进行性降低（高达35%）。电紧张成分在所有情况下保持在相似的幅度。在成对脉冲刺激网状脊髓细胞体期间，（脉冲间隔65 ms）化学EPSP成分在所有测试病例中均显示净易化[从0.64 +/- 0.35至0.89 +/- 0.48（SD）mV，n = 13]，而电紧张性成分的峰值幅度不变（分别为1.37 +/- 0.68和1.36 +/- 0.66 mV）。在成对脉冲刺激期间记录轴突动作电位显示第一和第二动作电位的宽度没有差异[1/2宽度（分别为2.48 +/- 0.39 ms和2.48 +/- 0.42 ms; n = 8）]。不同突触之间的易化程度有显著差异，从百分之几的增加到两倍的增加（总EPSP振幅的平均变化为24 +/- 16%，对应于化学EPSP振幅的平均变化为44 +/- 26%）。这种类型的变异性也可以在由相同的无分支网状脊髓轴突到不同的突触后细胞上形成的突触中观察到。当成对的脉冲刺激施加到突触区附近的网状脊髓轴突时，（0.1-1 mm）的化学成分的净抑制发生在19例中的11例中，并且在其余情况下，净易化水平与细胞体刺激相比较低（范围在总EPSP振幅的+17和-23%变化之间;平均值-5%; n = 19）。为了测试局部刺激过程中EPSP可塑性的变化是否与递质释放增加相关，将两个微电极放置在同一网状脊髓轴突中距突触区不同的距离处。在突触附近（0.1-1 mm）施加去极化电流脉冲（1-2 ms）触发动作电位时，总EPSP振幅始终大于在较远距离处触发的动作电位。在局部刺激过程中，EPSP的振幅随去极化刺激脉冲的大小而增加。然而，如果轴突被刺入距离突触区域小于0.1 mm，则化学EPSP会受到抑制，但在电极被移除后会发生逆转。在局部刺激时EPSP总振幅的增加伴随着初始电紧张成分的峰值振幅的增加，表明突触前动作电位的振幅增加。轴突动作电位的记录证实，当用相邻微电极施加的去极化电流脉冲触发时，其幅度和持续时间都增加。为了研究给定的网状脊髓轴突运动神经元EPSP与1.0 mV振幅的初始电紧张成分和0.25 mV振幅的化学成分（远距离刺激）的解剖学相关性，细胞内标记这两个元素，并进行突触连接的完整重建。在距离运动神经元索马体310-340 μ m的远侧树突上观察到4个轴突-运动神经元接触，共包含7个活性区，但只有2个间隙连接。连续切片的分析表明，不同的活动区是由胶质过程彼此分开的。目前的研究结果表明，使用依赖性调制的单一的神经元能EPSP可以不同的单个突触之间沿着一个无分支的轴突，其中没有活动依赖性的变化的形状或传播的突触前动作电位可以被检测到。然而，如果刺激电极被放置在离突触区域1 mm的距离内，则突触前锋电位的形状以及EPSP的幅度和使用依赖性调制可以显著改变。当大型脊椎动物轴突被用于研究中枢神经系统中突触谷氨酸释放的机制时，这些数据应该被考虑在内。
1. The glutamatergic synapses formed between the unbranched giant reticulospinal axons onto spinal neurons in lamprey offer a central vertebrate synapse in which the presynaptic element can be impaled with one or several microelectrodes, which may be used for recording as well as microinjection of different substances. To provide a basis for the use of this synapse in studies of release mechanisms, we have examined the use-dependent modulation of the synaptic response under conditions of conventional cell body stimulation, and during direct stimulation of the presynaptic axon.2. To examine the stability of the mixed electrotonic and chemical reticulospinal excitatory postsynaptic potential (EPSP) over time, action potentials were evoked at a rate of 1 Hz for 800-1000 trials. In three out of seven synapses the chemical component remained at a similar amplitude, while in four cases a progressive decrease (up to 35%) occurred. The electrotonic component remained at a similar amplitude in all cases.3. During paired pulse stimulation of the reticulospinal cell body (pulse interval 65 ms) the chemical EPSP component showed a net facilitation in all cases tested [from 0.64 +/- 0.35 to 0.89 +/- 0.48 (SD) mV, n = 13], while the peak amplitude of the electrotonic component was unchanged (1.37 +/- 0.68 and 1.36 +/- 0.66 mV, respectively). Recording of the axonal action potential during paired pulse stimulation showed that the width of the first and second action potential did not differ [1/2 width (2.48 +/- 0.39 ms and 2.48 +/- 0.42 ms, respectively; n = 8)].4. The degree of facilitation varied markedly between different synapses, ranging from an increase of a few percent to a two-fold increase (24 +/- 16% mean change of total EPSP amplitude, corresponding to 44 +/- 26% mean change of chemical EPSP amplitude). This type of variability was also observed in synapses made from the same unbranched reticulospinal axon onto different postsynaptic cells.5. When paired pulse stimulation was applied to the reticulospinal axon in the very vicinity of the synaptic area (0.1-1 mm) a net depression of the chemical component occurred in 11 out of 19 cases, and in the remaining cases the level of net facilitation was lower as compared with cell body stimulation (range between +17 and -23% change of total EPSP amplitude; mean -5%; n = 19).6. To test if the change of the EPSP plasticity during local stimulation correlated with an increased transmitter release, two microelectrodes were placed in the same reticulospinal axon at different distances from the synaptic area. The total EPSP amplitude was consistently larger, if the action potential was triggered with a depolarizing current pulse (1-2 ms) applied close to the synapse (0.1-1 mm), than at a longer distance. During local stimulation the EPSP amplitude increased with the magnitude of the depolarizing stimulus pulse. If the axon was impaled closer than 0.1 mm from the synaptic area, however, the chemical EPSP became depressed, but a reversal occurred after the electrode had been removed.7. The increase of the total EPSP amplitude during local stimulation was accompanied by an increase of the peak amplitude of the initial electrotonic component, indicating that the amplitude of the presynaptic action potential was increased. Recording of the axonal action potential confirmed that both its amplitude and duration increased when it was triggered with depolarizing current pulses applied from an adjacent microelectrode.8. To examine the anatomic correlate of a given reticulospinal axon-motoneuron EPSP with an initial electrotonic component of 1.0 mV amplitude and a chemical component of 0.25 mV amplitude (distant stimulation), both elements were labeled intracellularly and a complete reconstruction of the synaptic connections was performed. Four axon-motoneuron contacts, located on distal dendrites at 310-340 mu m distance from the motoneuron soma were observed, which altogether contained seven active zones, but only two gap junctions. Analysis of serial ultrathin sections showed that the different active zones were separated from each other by glial processes.9. The present results show that the use-dependent modulation of unitary glutamatergic EPSPs can vary between individual synapses along an unbranched axon, in which no activity-dependent changes in the shape or propagation of the presynaptic action potential can be detected. If the stimulation electrode is placed within a distance of 1 mm from the synaptic area, however, the shape of the presynaptic spike, as well as the amplitude and use-dependent modulation of the EPSP, can be markedly altered. These data should be taken into account when large vertebrate axons are employed in studies of the mechanisms underlying synaptic glutamate release in the CNS.