Lupica C R, Proctor W R, Dunwiddie T V
Department of Pharmacology, University of Colorado Health Sciences Center, Denver 80262.
J Neurosci. 1992 Oct;12(10):3753-64. doi: 10.1523/JNEUROSCI.12-10-03753.1992.
We have utilized the favorable signal-to-noise ratios provided by whole-cell recording, combined with variance analysis, to determine the pre- or postsynaptic actions of a variety of manipulations on unitary EPSPs evoked by low-intensity stimulation of afferents to CA1 pyramidal neurons in slices of hippocampus. Estimates of quantal content (mcv) were determined by calculating the ratio of the squared average unitary EPSP amplitude (determined from 150-275 responses) to the variance of these responses (M2/sigma 2), while quantal amplitudes (qcv) were estimated by calculating the ratio of the response variance to average EPSP size (sigma 2/M). Estimates of mcv were highly correlated with those determined using the method of failures (mf). With paired stimulation (50 msec interpulse interval) there was a significant facilitation of the second unitary EPSP, accompanied by an increase in mcv, but not qcv, suggesting that this facilitation was of presynaptic origin. Superfusion of hippocampal slices with various concentrations of adenosine, the A1-selective adenosine receptor agonist cyclohexyladenosine, or the Ca2+ channel blocker cadmium significantly reduced average unitary EPSP amplitudes and mcv, without significantly altering qcv, suggesting a presynaptic locus for this inhibition. The 50% effective concentration for the apparent presynaptic action of adenosine on mcv in the present study (5.7 microM; 95% confidence limits = 4.2-7.7 microM) was significantly lower than its EC50 for reducing conventional, large EPSPs (33 microM; recorded with high-resistance microelectrodes), or extracellular field EPSPs (29 microM), as previously reported by this laboratory. The glutamate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) reduced average unitary EPSP amplitudes; in contrast to the above manipulations, it had no effect on mcv, but significantly altered qcv, which is consistent with its presumed postsynaptic mechanism of action. We conclude from these data that adenosine presynaptically reduces synaptic strength at Schaffer collateral-commissural synapses in the hippocampus by diminishing the number of quanta released, not by reducing the size of these individual quanta or postsynaptic sensitivity to excitatory neurotransmitter. These results suggest that the mechanism by which adenosine inhibits synaptic transmission in the hippocampus is similar, if not identical, to the mechanism by which it inhibits synaptic transmission at the neuromuscular junction.
我们利用全细胞记录提供的良好信噪比，并结合方差分析，来确定各种操作对海马切片中CA1锥体神经元传入纤维低强度刺激诱发的单位兴奋性突触后电位（EPSP）的突触前或突触后作用。通过计算平均单位EPSP幅度平方（由150 - 275次反应确定）与这些反应的方差之比（M2 / sigma 2）来确定量子含量（mcv）的估计值，而通过计算反应方差与平均EPSP大小之比（sigma 2 / M）来估计量子幅度（qcv）。mcv的估计值与使用失败法（mf）确定的估计值高度相关。在配对刺激（脉冲间隔50毫秒）时，第二个单位EPSP有显著增强，同时mcv增加，但qcv未增加，这表明这种增强是突触前起源的。用不同浓度的腺苷、A1选择性腺苷受体激动剂环己基腺苷或Ca2 +通道阻滞剂镉灌流海马切片，可显著降低平均单位EPSP幅度和mcv，而不显著改变qcv，这表明这种抑制作用位于突触前。在本研究中，腺苷对mcv的明显突触前作用的50%有效浓度（5.7 microM；95%置信限 = 4.2 - 7.7 microM）显著低于其降低传统大EPSP（33 microM；用高电阻微电极记录）或细胞外场EPSP（29 microM）的EC50，如本实验室先前报道。谷氨酸受体拮抗剂6,7 - 二硝基喹喔啉 - 2,3 - 二酮（DNQX）降低了平均单位EPSP幅度；与上述操作相反，它对mcv无影响，但显著改变了qcv，这与其推测的突触后作用机制一致。从这些数据我们得出结论，腺苷通过减少释放的量子数量而不是通过减小这些单个量子的大小或突触后对兴奋性神经递质的敏感性，在海马中突触前降低了Schaffer侧支 - 连合突触的突触强度。这些结果表明，腺苷抑制海马突触传递的机制即使不完全相同，也与它抑制神经肌肉接头处突触传递的机制相似。
