Vasoactive intestinal polypeptide-immunoreactive interneurons within circuits of the mouse basolateral amygdala

In cortical structures, principal cell activity is tightly regulated by different GABAergic interneurons (INs). In particular, vasoactive intestinal polypeptide-expressing (VIP+) INs innervate preferentially other INs, providing a structural basis for temporal disinhibition of principal cells. However, relatively little is known about VIP+ INs in the amygdaloid basolateral complex (BLA). In this study, we report that VIP+ INs have a variable density in the distinct subdivisions of the mouse BLA. Based on different anatomical, neurochemical and electrophysiological criteria, VIP+ INs could be identified as interneuron-selective INs and basket cells expressing CB1 cannabinoid receptors. Whole-cell recordings of VIP+ interneuron-selective INs revealed 3 different spiking patterns, which did not associate with the expression of calretinin. Genetic targeting combined with optogenetics and in vitro recordings allowed us to identify several types of BLA INs innervated by VIP+ INs, including other interneuron-selective INs, basket and neurogliaform cells. Moreover, light stimulation of VIP+ basket cell axon terminals, characterized by CB1 sensitivity, evoked IPSPs in ∼20% of principal neurons. Finally, we show that VIP+ INs receive a dense innervation from both GABAergic, although only 10% from other VIP+ INs, and distinct glutamatergic inputs, identified by their expression of different vesicular glutamate transporters. In conclusion, our study provides a wide-range analysis of single-cell properties of VIP+ INs in the mouse BLA and of their intrinsic and extrinsic connectivity. Our results reinforce the knowledge that VIP+ INs are structurally and functionally heterogeneous and that this heterogeneity could mediate different roles in amygdala-dependent functions. Significance statement: We provide the first comprehensive analysis of the distribution of VIP+ interneurons across the entire mouse BLA, as well as of their morphological and physiological properties. VIP+ interneurons in the neocortex preferentially target other interneurons to form a disinhibitory network that facilitates principal cell firing. Our study is the first to demonstrate the presence of such a disinhibitory circuitry in the BLA. We observed structural and functional heterogeneity of these INs and characterized their input/output connectivity. We also identified several types of BLA interneurons postsynaptic to VIP+ INs, whose inhibition may provide a temporal window for principal cell firing and facilitate associative plasticity, e.g. in fear learning. Disinhibition, thus, is emerging as a general mechanism, not limited to the neocortex.