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*T. SIGURDSSON1, C. K. CAIN1, V. DOYERE1,2, J. E. LEDOUX1; Several lines of evidence suggest that auditory fear conditioning is
mediated by synaptic plasticity at auditory inputs to the lateral amygdala
(LA). One outstanding question, however, is the relative contribution
of long-term plasticity at the thalamic and cortical auditory inputs to
the LA. We have previously shown that the magnitude of long-term potentiation
(LTP) is greater at cortical compared to thalamic inputs in vivo. Here
we ask whether this difference is due to a more fundamental difference
in the synaptic properties of the two inputs. In urethane-anesthetized
rats, the two inputs were examined by recording field potential responses
in the LA evoked by electrical stimulation of the auditory thalamus (MGm/PIN)
and auditory association cortex (TE3). Extending our previous findings,
we first show that high-frequency stimulation (HFS) induces LTP of much
greater magnitude at cortical compared to thalamic inputs across a range
of induction frequencies and stimulation protocols. We next show that
the integrated field potential response during HFS is greater during stimulation
of cortical inputs, suggesting that these inputs are more effective at
depolarizing postsynaptic LA neurons during HFS. Finally, we asked whether
this might reflect differences in short-term plasticity at the two inputs.
Using paired-pulse stimulation, we found that thalamic inputs display
primarily depression whereas cortical inputs show more facilitation. This
difference was seen in both evoked field potential and single-unit responses.
These results suggest that short-term plasticity, by affecting responses
during HFS, may differentially regulate the induction of LTP at thalamic
and cortical auditory inputs to the LA. In particular, the induction of
LTP at thalamic inputs may be constrained by the short-term depression
observed at these inputs. Possible pharmacological mechanisms underlying
the differences in short-term and long-term plasticity at the thalamic
and cortical inputs are currently being investigated. Program No. 370.1/KK19 |
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