Combination of fluoxetine and extinction treatments forms a unique synaptic protein profile that correlates with long-term fear reduction in adult mice

The antidepressant fluoxetine induces synaptic plasticity in the visual and fear networks and promotes the structural remodeling of neuronal circuits, which is critical for experience-dependent plasticity in response to an environmental stimulus. We recently demonstrated that chronic fluoxetine administration together with extinction training in adult mice reduced fear in a context-independent manner. Fear conditioning and extinction alter excitatory and inhibitory transmissions within the fear circuitry. In this study, we investigated whether fluoxetine, extinction or their combination produced distinct long-lasting changes in the synaptic protein profile in the amygdala, hippocampus and prefrontal cortex of conditioned mice. We determined that extinction induced synaptophysin expression and down-regulated the GluA1:GluA2 ratio throughout the fear network in water- and fluoxetine-treated mice, suggesting a common fluoxetine-independent mechanism for increased synaptic transmission and re-arrangement of AMPA-receptors by extinction training. In contrast to common changes, the presynaptic vesicular neurotransmitter transporters VGAT and Vglut1 were upregulated after extinction in water- and fluoxetine-treated mice, respectively. The cortical levels of the GABA transporter Gat1 were reduced in high-freezing water-drinking mice, suggesting a maladaptive increase of GABA spillover at cortical inhibitory synapses. Fear conditioning decreased, and extinction induced the expression of GABA-receptor alpha1 and alpha2 subunits in water- and fluoxetine-treated mice, respectively. Only a combination of fluoxetine with extinction enhanced GluN2A expression in the amygdala and hippocampus, emphasizing the role of this NMDA-receptor subunit in the successful erasure of fear memories. Our finding provides novel data that may become helpful in developing beneficial pharmacological fear-reducing treatment strategies.