Eliot L S, Hawkins R D, Kandel E R, Schacher S
Center for Neurobiology and Behavior, College of Physicians and Surgeons of Columbia University, New York, New York 10032.
J Neurosci. 1994 Jan;14(1):368-83. doi: 10.1523/JNEUROSCI.14-01-00368.1994.
Synapses made by Aplysia sensory neurons onto motor- and interneuron followers in the intact nervous system exhibit an associative form of synaptic facilitation that is thought to contribute to classical conditioning of the animal's gill and siphon withdrawal reflex (Hawkins et al., 1983; Walters and Byrne, 1983). Here we demonstrate that a similar associative facilitation can be induced between individual sensory and motor neurons isolated in culture. Pairing tetanic stimulation with either of two facilitatory transmitters, 5-HT or small cardioactive peptide, considerably prolongs facilitation compared to either tetanus or transmitter alone. When corrected for the depression that occurs simply in response to low-frequency testing, the facilitation produced by one pairing trial does not decay for more than 20 min after training. This facilitation requires the temporal pairing (0.5 sec forward interstimulus interval) of the two stimuli, tetanus and 5-HT. Delivering the same two stimuli in an unpaired fashion (1 min forward interval) fails to produce the long-lasting effect. Measurements of spontaneous transmitter release during either paired or unpaired training reveal no changes in unitary mEPSP or mEPSC ("mini") amplitude, indicating that the facilitation involves a presynaptic mechanism. While both forms of training dramatically increase the initial frequency of spontaneous release, mini frequency does not remain elevated as long as the evoked EPSP following paired training, nor does paired training specifically enhance spontaneous release frequency. Pairing-specific facilitation was not blocked by the protein kinase C inhibitor H7. In contrast, the same training procedure produced pairing-specific increases of sensory neuron excitability and action potential width, suggesting that cAMP-mediated processes are involved in the paired effect. Although Ca2+ influx is necessary for the associative effect (Abrams, 1985), we find that the facilitation does not require influx through L-type voltage-gated Ca2+ channels, since the effect was not blocked by the dihydropyridine antagonist nitrendipine. Together, these findings indicate that the mechanism underlying associative, activity-dependent facilitation is intrinsic to the sensory neuron synapse, that it is presynaptically mediated by processes unique to evoked synaptic transmission, and that it appears to involve a pairing-specific broadening of the presynaptic action potential, allowing enhanced Ca2+ influx through the dihydropyridine-insensitive channels responsible for release.
在完整的神经系统中，海兔感觉神经元与运动神经元及中间神经元形成的突触表现出一种联合形式的突触易化，这种易化被认为有助于动物鳃和虹吸管退缩反射的经典条件反射（霍金斯等人，1983年；沃尔特斯和伯恩，1983年）。在这里，我们证明在培养中分离的单个感觉神经元和运动神经元之间可以诱导出类似的联合易化。与单独的强直刺激或递质相比，将强直刺激与两种易化递质之一5-羟色胺（5-HT）或小的促心活性肽配对，可显著延长易化时间。当校正仅因低频测试而出现的抑制时，一次配对试验产生的易化在训练后20多分钟内不会衰减。这种易化需要两种刺激——强直刺激和5-HT——的时间配对（刺激间隔0.5秒）。以非配对方式（间隔1分钟）给予相同的两种刺激不能产生持久的效果。在配对或非配对训练期间对自发递质释放的测量显示，单个微小兴奋性突触后电位（mEPSP）或微小兴奋性突触后电流（mEPSC，即“微小”）的幅度没有变化，这表明易化涉及一种突触前机制。虽然两种形式的训练都显著增加了自发释放的初始频率，但微小频率不会像配对训练后诱发的兴奋性突触后电位（EPSP）那样长时间保持升高，配对训练也没有特异性地提高自发释放频率。配对特异性易化没有被蛋白激酶C抑制剂H7阻断。相反，相同的训练程序使感觉神经元的兴奋性和动作电位宽度产生了配对特异性增加，这表明环磷酸腺苷（cAMP）介导的过程参与了配对效应。虽然钙离子内流对于联合效应是必需的（艾布拉姆斯，1985年），但我们发现易化不需要通过L型电压门控钙离子通道内流，因为这种效应没有被二氢吡啶拮抗剂尼群地平阻断。总之，这些发现表明，联合的、活动依赖的易化的潜在机制是感觉神经元突触所固有的，它是由诱发突触传递特有的过程在突触前介导的，并且它似乎涉及突触前动作电位的配对特异性展宽，从而允许通过负责释放的对二氢吡啶不敏感的通道增强钙离子内流。
