Selective synaptic remodeling of amygdalocortical connections associated with fear memory

Abstract

Neural circuits underlying auditory fear conditioning have been extensively studied. Here we identified a previously unexplored pathway from the lateral amygdala (LA) to the auditory cortex (ACx), and found that selective silencing of this pathway using chemo-and optogenetic approaches impairs fear memory retrieval. Dual-color in vivo two-photon imaging of mouse ACx showed pathwayspecific increases in the formation of LA axon boutons, dendritic spines of ACx layer-5 pyramidal cells, and putative LA-ACx synaptic pairs after auditory fear conditioning. Furthermore, co-imaging of pre-and postsynaptic structures showed that essentially all new synaptic contacts were made by adding new partners to existing synaptic elements. Together, these findings identify an amygdalocortical projection that plays an important role in fear memory expression and is selectively modified by associative fear learning, and unravel a distinct architectural rule for synapse formation in the adult brain. 2 IntroductionAssociative learning enables an animal to adapt to and survive in a complex environment. In classical auditory fear conditioning, animals learn to associate a neutral stimulus (a sound) with a foot-shock, and exhibit fear responses to the sound presentation. Amygdala is critical for the formation of auditory fear memory 1 .Previous studies have also shown that the auditory cortex (ACx) is required for auditory fear learning [2][3][4] , and that fear conditioning could induce rapid and long-term changes in neuronal responses and spine dynamics in ACx. However, which synapses in ACx underwent modification remain unclear.Long-term in vivo two-photon imaging has been used to monitor structural remodeling of synaptic connectivity, as was shown by changes in presynaptic boutons or postsynaptic spines that represent the formation or elimination of synapses.Previous studies have shown that sensory experience and learning can induce changes in the turnover of presynaptic axon boutons 9 and postsynaptic dendritic spines. To further explore synaptic dynamics in specific pathways, concurrent imaging of preand post-synaptic structures in identified connections is required. This approach, although successfully applied in studying synaptic dynamics in hippocampal slices 16 , has not been used for in vivo imaging of the neocortex.In this study, combining tracing methods with electron microscopy (EM), we identified a projection to ACx that originates from lateral amygdala (LA), the major 3 input region of the amygdala. Chemo-and optogenetic silencing of LA axons in ACx during fear recall test greatly reduced animals' fear responses, suggesting that the LAACx pathway plays an important role in the expression of fear memory. By coimaging labeled axons originating from brain regions projecting to ACx and apical dendrites of pyramidal neurons in ACx, we were able to monitor the dynamics of putative synaptic pairs in specific pathways in vivo. We observed a selective increase in bouton and spine formation at LA-ACx connections after fear con...

Pyramidal Neurons in Different Cortical Layers Exhibit Distinct Dynamics and Plasticity of Apical Dendritic Spines

“…This result is consistent with a previous study showing that monocular deprivation increases spine formation and leads to higher spine density on the apical tufts of L5, but not L2/3, PNs in the binocular region of the mouse visual cortex (Hofer et al, 2009). A more recent work revealed pathway-specific increases in the formation of lateral amygdala axon boutons and dendritic spines of L5 PNs in the auditory cortex during fear conditioning, but no change in spine dynamics of L2/3 PNs (Yang et al, 2016). Given their high baseline spine dynamics, L2/3 PNs may have already reached the metabolic ceiling under baseline conditions, so cannot support higher spine dynamics.…”

“…Furthermore, the presynaptic partners of these spines remain elusive. Thus, a comprehensive understanding of the reorganization of synaptic circuits requires concurrent imaging of pre- and post-synaptic elements as illustrated by a recent study on the amygdalocortical circuit (Yang et al, 2016), or correlative light and electron microscopy (Knott et al, 2006). …”

“…Specifically, we investigated whether and how their spine dynamics differ through postnatal development into adulthood, during adolescent forelimb-specific motor skill learning, and in response to neonatal sensory deprivation. Despite a handful of articles directly comparing L2/3 and L5 PN apical dendritic spine dynamics (Holtmaat et al, 2005; Hofer et al, 2009; Schubert et al, 2013; Ma et al, 2016; Yang et al, 2016), the behavior of spines under the conditions above has never been investigated systematically. Given the importance of motor skill learning and early sensory experience for brain development, such data will improve our knowledge on how brain circuits change in response to early experience.…”

Reversible Inactivation of the Higher Order Auditory Cortex during Fear Memory Consolidation Prevents Memory-Related Activity in the Basolateral Amygdala during Remote Memory Retrieval

“…Many studies have shown that the auditory cortex, whether primary or higher order, is essential for associating complex tones to emotional events (LeDoux, 2000; Letzkus et al, 2011; Yang et al, 2016). Conversely, the involvement of these cortices in the association of simple tones with US is more controversial (LeDoux, 2000).…”

Functional consequences of pre- and postsynaptic expression of synaptic plasticity

“…Recently, Yang et al . [101] imaged layer-5 pyramidal cell synapses and found that boutons tend to grow more often than spines after an auditory fear conditioning task.…”