Fear learning induces structural and functional plasticity at GABAergic synapses in the basolateral amygdala
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KeywordsGABAA Receptor Fear Conditioning Extinction Training Fear Extinction GABAergic Synapse
Previous work has suggested that alterations in GABAergic function within the amygdala underlie fear learning. In particular, it has been shown that Pavlovian fear conditioning induces a downregulation of benzodiazepine binding sites as well as transcripts for gephyrin and some GABAA receptor subunits in the basal nucleus of the amygdala (BA), which were restored to control levels after fear extinction.
We have undertaken a combined anatomical and physiological approach to examine whether these alterations distinctively involve GABAA receptors in synaptic or extrasynaptic areas. Specifically, we analyzed – in the BA of mice that underwent fear conditioning as well as extinction – miniature inhibitory postsynaptic currents (mIPSCs), mRNA levels by in situ hybridization, and the density for the GABAA γ2 subunit by means of the freeze-fracture replica immunolabelling technique (SDS-FRL). SDS-FRL also allowed to precisely measure the size of GABAergic synapses.
A significant decrease in labelling density for the GABAA γ2 subunit could be detected in the synaptic area in fear-conditioned mice as compared to the control group and mice that had undergone extinction (p < 0.01; Kruskal-Wallis and Dunn’s multiple comparison tests). Conversely, GABAA γ2 extrasynaptic density was lower in the extinction group when compared to both the control and fear-conditioned mice (p < 0.005). The average size of GABAergic synapses in control mice was 0.034 ± 0.001 µm2 (n = 227 full synapses from 3 animals; CV = 0.62). Fear-conditioned animals showed a significantly (p < 0.01) larger average synaptic size (0.040 ± 0.001 µm2; n = 249; CV = 0.59), whereas in fear extinction mice it was similar to controls (0.031 ± 0.001 µm2; n = 290; CV = 0.59). Alterations in synapse size upon fear conditioning and extinction were associated with functional changes. In neurons recorded from acute slices obtained from fear-conditioned animals mIPSCs were larger (increased charge transfer/mIPSC) compared to recordings obtained from slices of control mice and animals subjected to extinction training. In situ hybridization analysis of the mRNA content for GABAA γ2 subunits revealed highly similar levels among the 3 groups in the BA (p = 31, one-way ANOVA) and central nucleus (p = 41).
Our results indicate that, in the BA, fear conditioning produces a reversible enlargement of GABAergic synapses and an increase in mIPSC charge transfer with no change in the overall number of synaptic GABAA receptors.
This work was supported by the FWF grant S10720 to F.F.
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