Lin X Y, Glanzman D L
Department of Physiological Science, University of California, Los Angeles 90095-1568, USA.
J Neurophysiol. 1997 Feb;77(2):667-74. doi: 10.1152/jn.1997.77.2.667.
Long-term potentiation of Aplysia sensorimotor synapses (apLTP) can be induced in Hebbian fashion by pairing brief tetanic stimulation of the sensory neuron with depolarization of the motor neuron. It has been proposed that Hebbian apLTP plays a significant role in classical conditioning of the defensive withdrawal reflex of Aplysia. However, as originally demonstrated, Hebbian apLTP is induced by simultaneous pairing of sensory neuron stimulation and motor neuron depolarization, whereas in the Aplysia classical conditioning paradigm the onset of the conditioned stimulus (CS) precedes the onset of the unconditioned stimulus (US) by 0.5 s. Therefore, if Hebbian apLTP does indeed mediate classical conditioning in Aplysia, temporally offset delivery of presynaptic stimulation and postsynaptic depolarization must be able to support apLTP. To ascertain whether temporally offset pre- and postsynaptic stimuli can support apLTP, we varied the interstimulus interval (ISI) between the onset of presynaptic tetanus and the onset of postsynaptic depolarization. In the first set of experiments we determined the amount of potentiation that results from varying the temporal interval between the onset of a single presynaptic tetanus and the onset of a single bout of postsynaptic depolarization. The ISI between the onset of the two stimuli ranged from 0.0 to 5.0 s. Significant apLTP was obtained with ISIs of 0.0 and 0.5 s. but the amount of potentiation was independent of the order in which the presynaptic and postsynaptic stimuli were delivered. Because classical conditioning of the withdrawal reflex in Aplysia is dependent on the temporal order of the CS and US, in a second set of experiments we compared the efficacy of forward and backward pairing of pre- and postsynaptic stimulation with the use of a conditioning-like protocol. Forward pairing and backward pairing (0.5-s ISI) yielded equal amounts of apLTP. These data raise questions for the hypothesis that Hebbian apLTP mediates classical conditioning of the withdrawal reflex in Aplysia. Our results indicate that Hebbian apLTP alone cannot fully account for classical conditioning in Aplysia. An additional cellular mechanism is required to explain the temporal specificity present in the behavioral results.
海兔感觉运动突触的长期增强（apLTP）可以通过将感觉神经元的短暂强直刺激与运动神经元的去极化配对，以赫布方式诱导产生。有人提出，赫布式apLTP在海兔防御性退缩反射的经典条件反射中起重要作用。然而，如最初所证明的，赫布式apLTP是由感觉神经元刺激和运动神经元去极化同时配对诱导产生的，而在海兔经典条件反射范式中，条件刺激（CS）的开始比无条件刺激（US）的开始提前0.5秒。因此，如果赫布式apLTP确实介导了海兔的经典条件反射，那么突触前刺激和突触后去极化的时间偏移传递必须能够支持apLTP。为了确定时间偏移的突触前和突触后刺激是否能支持apLTP，我们改变了突触前强直开始与突触后去极化开始之间的刺激间隔（ISI）。在第一组实验中，我们确定了改变单个突触前强直开始与单个突触后去极化发作之间的时间间隔所产生的增强量。两种刺激开始之间的ISI范围为0.0至5.0秒。在ISI为0.0和0.5秒时获得了显著的apLTP，但增强量与突触前和突触后刺激的传递顺序无关。因为海兔退缩反射的经典条件反射取决于CS和US的时间顺序，在第二组实验中，我们使用类似条件反射的方案比较了突触前和突触后刺激的正向和反向配对的效果。正向配对和反向配对（0.5秒ISI）产生了等量的apLTP。这些数据对赫布式apLTP介导海兔退缩反射经典条件反射的假说提出了疑问。我们的结果表明，仅赫布式apLTP不能完全解释海兔的经典条件反射。需要一种额外的细胞机制来解释行为结果中存在的时间特异性。
