Role of intercalated and NPY-expressing cells in neuronal circuit of the amygdala

Local inhibitory microcircuit of amygdala is an active component in processing emotional information. Despite prominent evidence of its importance, our understanding of GABAergic cell types, their connectivity and role in amygdala network is limited. The aim of this thesis is to understand connectivity and physiology of two specific components of GABAergic microcircuit of amygdala: so-called intercalated cells and neuropeptide Y (NPY) expressing interneurons.
Intercalated cells (ITCs) of the amygdala are clusters of GABAergic neurons that surround the basolateral complex of amygdala (BLA). There is growing evidence suggesting that ITCs are required for the expression of fear extinction. The main intercalated nucleus (Im) is the largest of the ITC clusters and could be also important for emotional processing. Using patch-clamp whole-cell recordings paired with subsequent anatomical analysis I described basic physiology and anatomy of neurons within the Im. I found that these neurons share common characteristics to earlier described neurons within the medial ITC cluster, yet they can be divided into three distinct groups. Next, I provided anatomical and functional evidence that Im neurons project to central and basal nucleus of amygdala and that they are reciprocally connected with medial and lateral ITCs clusters. I found that Im neurons receive excitatory inputs from BLA as well as cortex; next I verified that heterogeneous inputs do not interact with each other. I have shown that the Im neurons express both AMPA and NMDA receptors, suggesting that they may undergo NMDA-dependent plasticity. I have reported that dopamine hyperpolarizes Im neurons via dopamine receptor 1, therefore providing a cellular substrate for disinhibition of the amygdala at the systemic level. Thus, the Im is likely to be an additional site of integration of the distributed network underlying acquisition, expression and extinction of conditioned fear.
In another project, I report novel interneuron type of the BLA and call it neurogliaform cell (NGFC) of amygdala. I used a mouse line expressing green fluorescent protein (GFP) under NPY promoter and patch clamp technique combined with pharmacology and electron microscope analysis. I performed paired recordings between presynaptic NPY-GFP positive (+) cells and postsynaptic principal neurons (PNs). Presynaptic NPY-GFP+ neurons display small soma and short dendrites embedded in a cloud of highly arborized axon. I showed that NPY-GFP+ cells are source of GABAA receptor-mediated slow inhibitory postsynaptic currents (IPSCs, decay time constant > 30 ms) evoked in PNs and in themselves (autapses). These slow IPSCs are known in literature as GABAA,slow. My results indicate that the slow kinetics of these IPSCs was likely caused by the low concentration and spillover of extracellular GABA. Physiologically-relevant in vivo firing re-played in NPY+-NGFCs in vitro evoked a transient depression of the IPSCs. Presynaptic GABAB receptors controlled the strength of this short-term plasticity. Interestingly, synaptic contacts made by NGFCs showed close appositions, without identifiable classical synaptic structures, between presynaptic boutons of the recorded cells and postsynaptic profiles. Thus, volume transmission of GABA is likely to be generated by this interneuron of the amygdala. NPY+-NGFC is a novel interneuron type of the BLA. The peculiar functional mode of NGFCs makes them unique amongst all GABAergic cell types of the amygdala identified so far.

Keywords: GABA
Intercalated Cells
Amygdala
Interneurons
Advisor: Capogna, M