Development and physiology of GABAergic feedback excitation in parvalbumin expressing interneurons of the mouse basolateral amygdala.

We have previously shown that in the basolateral amygdala (BLA), action potentials in one type of parvalbumin (PV)-expressing GABAergic interneuron can evoke a disynaptic feedback excitatory postsynaptic potential (fbEPSP) onto the same presynaptic interneuron. Here, using whole-cell recordings from PV-expressing interneurons in acute brain slices we expand on this finding to show that this response is first detectable at 2-week postnatal, and is most prevalent in animals beyond 3 weeks of age (>P21). This circuit has a very high fidelity, and single action potential evoked fbEPSPs display few failures. Reconstruction of filled neurons, and electron microscopy show that interneurons that receive feedback excitation make symmetrical synapses on both the axon initial segments (AIS), as well as the soma and proximal dendrites of local pyramidal neurons, suggesting fbEPSP interneurons are morphologically distinct from the highly specialized chandelier neurons that selectively target the axon initial segment of pyramidal neurons. Single PV interneurons could trigger very large (~ 1 nA) feedback excitatory postsynaptic currents (fbEPSCs) suggesting that these neurons are heavily reciprocally connected to local glutamatergic principal cells. We conclude that in the BLA, a subpopulation of PV interneurons forms a distinct neural circuit in which a single action potential can recruit multiple pyramidal neurons to discharge near simultaneously and feed back onto the presynaptic interneuron.