Effect of interstimulus interval on pairing-induced LTP of Aplysia sensorimotor synapses in cell culture

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.