Author:
Bristol Adam S.,Carew Thomas J.
Abstract
Many studies of the neural mechanisms of learning have focused on
habituation, a simple form of learning in which a response decrements with
repeated stimulation. In the siphon-elicited siphon withdrawal reflex (S-SWR)
of the marine mollusk Aplysia, the prevailing view is that
homosynaptic depression of primary sensory afferents underlies short-term
habituation. Here we examined whether this mechanism is also utilized in
habituation of the tail-elicited siphon withdrawal reflex (T-SWR), which is
triggered by an independent, polysynaptic afferent pathway that converges onto
the same siphon motor neurons (MNs). By using semi-intact preparations in
which tail and/or siphon input to siphon MNs could be measured, we found that
repeated tail stimuli administered in the presence of a reversible conduction
block of the nerves downstream of the tail sensory neurons (SNs) completely
abolished the induction of habituation. Subsequent retraining revealed no
evidence of savings, indicating that the tail SNs and their immediate
interneuronal targets are not the locus of plasticity underlying T-SWR
habituation. The networks closely associated with the siphon MNs are modulated
by cholinergic inhibition. We next examined the effects of network
disinhibition on S-SWR and T-SWR habituation using an Ach receptor antagonist
d-tubocurarine. We found that the resulting network disinhibition disrupted
T-SWR, but not S-SWR, habituation. Indeed, repeated tail stimulation in the
presence of d-tubocurarine resulted in an initial enhancement in responding.
Lastly, we tested whether habituation of T-SWR generalized to S-SWR and found
that it did not. Collectively, these data indicate that (1) unlike S-SWR,
habituation of T-SWR does not involve homosynaptic depression of SNs; and (2)
the sensitivity of T-SWR habituation to network disinhibition is consistent
with an interneuronal plasticity mechanism that is unique to the T-SWR
circuit, since it does not alter S-SWR.
Publisher
Cold Spring Harbor Laboratory
Subject
Cellular and Molecular Neuroscience,Cognitive Neuroscience,Neuropsychology and Physiological Psychology
Cited by
19 articles.
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