Plasticity of amygdala intercalated cell microcircuits in fear learning
Molecular Biology and Physiology of Neurons and Glial Cells
Final Report Abstract
The amygdala plays a crucial role in attaching emotional significance to environmental cues. Its intercalated cells (ITC) are tight clusters of spiny GABAergic neurons, which are distributed around the basolateral amygdala complex (BLA). Distinct ITC clusters are involved in the acquisition and extinction of conditioned fear responses. Previously, we have shown that fear memory retrieval reduces the AMPA/NMDA ratio at thalamic afferents to ITC neurons within the dorsomedial (dm)- ITC cluster. We investigated the molecular mechanisms underlying the fear-mediated reduction in the AMPA/NMDA ratio at these synapses and asked whether specific changes in the synaptic density of AMPA-Rs underlie the observed change. Using the freeze-fracture replica immunolabeling technique to visualize the spatial distribution of intra- and extrasynaptic AMPA-Rs at high resolution, we showed that, following auditory fear conditioning in mice, the formation and retrieval of fear memory is linked to a significant reduction in the density of AMPA-Rs, particularly at spine synapses formed by inputs of the posterior intralaminar thalamic and medial geniculate nuclei onto identified ITC neurons. These findings directly link the regulation of AMPA-R trafficking to memory processes in identified neuronal networks. Dopaminergic signaling plays an important role in associative learning including fear and extinction learning. Dopaminergic midbrain neurons encode prediction error-like signals when threats differ from expectations. Within the amygdala, ITC clusters receive the densest dopaminergic projections, but their physiological consequences were incompletely understood. In mice, we revealed two distinct novel mechanisms how mesencephalic dopaminergic afferents control ITCs. Firstly, they corelease GABA to mediate rapid, direct inhibition. Secondly, released dopamine directly hyperpolarizes ITCs, and suppresses inhibitory interactions between distinct ITC clusters via presynaptic D1-receptors. Early extinction training augments both, GABA co-release onto dm-ITCs and dopamine-mediated suppression of dm- to ventromedial (vm)- inhibition between ITC clusters. These findings provide novel insights into dopaminergic mechanisms shaping the activity balance between distinct ITC clusters that could support their opposing roles in fear behavior. To directly interrogate the activity and role of different ITC clusters in mediating high and low fear state, we teamed up with the Lüthi and Holmes labs. Using a combination of in vivo calcium imaging, and functional chemogenetic manipulations, our collaborators revealed that dm- and vm-ITCs exert diametrically opposing roles during the acquisition and retrieval of fear extinction memory. Using slice physiology, we demonstrated that dm- and vm-ITC clusters antagonize one another through mutual synaptic inhibition. Moreover, we revealed their differential connectivity to fear- and extinction promoting neurons in the central amygdala (CeA) and BLA, whereby dm- and vm-ITCs can access functionally distinct cortical- and midbrain-projecting amygdala output pathways. Our findings demonstrate that the balance of activity between these ITC clusters represents a unique regulatory motif that orchestrates a distributed neural circuitry to regulate the switch between high and low fear states.
Publications
- (2016) Combined optogenetic and freeze-fracture replica immunolabeling to examine input-specific arrangement of glutamate receptors in mouse amygdala. J Vis Exp 110: e53853
Schönherr S, Seewald A, Kasugai Y, Bosch D, Ehrlich I, Ferraguti F
(See online at https://dx.doi.org/10.3791/53853) - (2016) Ex vivo optogenetic dissection of fear circuits in brain slices. J Vis Exp 110: e53628
Bosch D, Asede D, Ehrlich I
(See online at https://dx.doi.org/10.3791/53628) - (2020) Studying neural function ex vivo using optogenetic stimulation and patch clamp. In: Niopek D (ed) Methods Mol Biol, 2173: 1-20
Aksoy-Aksel A, Genty J, Zeller M, Ehrlich I.
(See online at https://doi.org/10.1007/978-1-0716-0755-8_1) - (2021) Fear learning causes a reduction in AMPA receptors at thalamic to amygdala intercalated cell synapses. Front. Synaptic Neurosci. 13:634558
Seewald A, Schönherr S, Hörtnagl H, Ehrlich I, Schmuckermair C, Ferraguti F
(See online at https://dx.doi.org/10.3389%2Ffnsyn.2021.634558) - (2021) Intercalated amygdala clusters orchestrate a switch in fear state. Nature 594: 403-407
Hagihara KM, Bukalo O, Zeller M, Aksoy-Aksel A, Karalis N, Limoges A, Rigg T, Campbell T, Mendez A, Weinholtz C, Mahn M, Zweifel LS, Palmiter RD, Ehrlich I, Lüthi A, Holmes A
(See online at https://doi.org/10.1038/s41586-021-03593-1) - (2021) Midbrain dopaminergic inputs gate amygdala intercalated cell clusters by distinct and cooperative mechanisms in male mice. eLife 2021;10:e63708
Aksoy-Aksel A, Gall A, Seewald A, Ferraguti F, Ehrlich I
(See online at https://doi.org/10.7554/eLife.63708.sa2)
