Graubard K, Raper J A, Hartline D K
J Neurophysiol. 1983 Aug;50(2):508-21. doi: 10.1152/jn.1983.50.2.508.
Graded synaptic transmission between spiking motoneurons of the pyloric group was studied in the stomatogastric ganglion of the spiny lobster, Panulirus interruptus. Intracellular microelectrodes were placed in the cell bodies of both pre- and postsynaptic neurons. Graded synaptic transmission was found between all tested cell pairs that were known to display spike-evoked synaptic transmission, including PD to LP, PD to PE, PD to PL, PL to LP, and LP to PD. Graded synaptic transmission was effective below the threshold for spikes. Thus, it was possible to study the influence of graded synaptic transmission in normally active ganglia without blockage of spikes by tetrodotoxin. PD and LP neurons that were known to produce spike-evoked inhibitory postsynaptic potentials (IPSPs) were also capable of producing inhibitory effects on postsynaptic cells below the threshold for spikes. When tetrodotoxin (TTX) was used to eliminate both spikes and endogenous membrane oscillations, depolarization of presynaptic neurons produced hyperpolarization of postsynaptic cells. The presynaptic response to a current step usually showed a small early peak and a maintained, slightly lower plateau. The postsynaptic response had a delay, then a rise to a pronounced peak, and a roughly exponential decline to a maintained plateau. There was a presynaptic voltage threshold for any postsynaptic response; beyond the threshold, both pre- and postsynaptic peak and plateau responses increased with increasing current. PD neurons normally are depolarized beyond their release threshold in tetrodotoxin and, thus, released transmitter tonically for the many-hour duration of these experiments. Chemical, tonic synaptic transmission, here called graded synaptic transmission, was demonstrated by the presence of the following criteria: 1) reversal in sign of the postsynaptic response, 2) synaptic delay, 3) reversal potential, 4) postsynaptic conductance increase, 5) graded and reversible block by reduction of external Ca2+, and 6) specific graded block of the LP-to-PD synapse without effect on the PD-to-LP synapse by less than 10 microM picrotoxin added to the bathing medium.
在多刺龙虾（Panulirus interruptus）的口胃神经节中，研究了幽门组峰电位运动神经元之间的分级突触传递。将细胞内微电极置于突触前和突触后神经元的细胞体中。在所有已知显示峰电位诱发突触传递的测试细胞对之间都发现了分级突触传递，包括从幽门扩张神经元（PD）到侧幽门神经元（LP）、从PD到幽门后神经元（PE）、从PD到幽门外侧神经元（PL）、从PL到LP以及从LP到PD。分级突触传递在峰电位阈值以下有效。因此，有可能在正常活动的神经节中研究分级突触传递的影响，而不会因河豚毒素而阻断峰电位。已知产生峰电位诱发抑制性突触后电位（IPSPs）的PD和LP神经元，在峰电位阈值以下也能够对突触后细胞产生抑制作用。当使用河豚毒素（TTX）消除峰电位和内源性膜振荡时，突触前神经元的去极化会导致突触后细胞的超极化。突触前对电流阶跃的反应通常显示一个小的早期峰值和一个持续的、略低的平台期。突触后反应有一个延迟，然后上升到一个明显的峰值，然后大致呈指数下降到一个持续的平台期。对于任何突触后反应都有一个突触前电压阈值；超过该阈值，突触前和突触后峰值及平台期反应都随电流增加而增加。在这些实验的数小时时间内，PD神经元在河豚毒素中通常会去极化超过其释放阈值，因此会持续释放递质。化学性、持续性突触传递，这里称为分级突触传递，通过以下标准得以证明：1）突触后反应的符号反转；2）突触延迟；3）反转电位；4）突触后电导增加；5）通过降低细胞外Ca2+实现分级且可逆的阻断；6）向灌流介质中添加小于10微摩尔的苦味毒，可特异性地分级阻断LP到PD突触，而对PD到LP突触无影响。
