Effect of Interstimulus Interval on Pairing-Induced LTP of Aplysia
Sensorimotor Synapses in Cell Culture.
Lin, Xiang Y. and David L. Glanzman.
Department of Physiological Science, Brain Research Institute, University
of California, 405 Hilgard Avenue, Los Angeles, CA 90024-1568.
APStracts 3:0212N, 1996.
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 which
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 s
to 5.0 s. Significant apLTP was obtained with ISIs of 0.0 s 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
Aplysia 's withdrawal reflex is dependent upon the temporal order of the CS
and US, in a second set of experiments we compared the efficacy of forward
pairing and backward pairing of pre- and postsynaptic stimulation using 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
Received 22 May 1996; accepted in final form 28 August 1996.
APS Manuscript Number J406-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 7 October 1996