Figure 1. Network investigated in the present study. A, Summary of connections investigated. Blue and red lines indicate glutamatergic and GABAergic connections, respectively. The basomedial nucleus of the amygdala (BM) sends parallel projections to the ventromedial hypothalamic nucleus (VMH) and to the anterior portion of the bed nucleus of the stria terminalis (BNSTa). In turn, BNSTa sends projections to the VMH, which has been implicated in the regulation of defensive and social behaviors. Because prior studies have reported that most extrinsic projections of BM and BNST respectively originate from glutamatergic and GABAergic neurons, arrows making these connections are color-coded accordingly. However, this remains to be established, hence the question marks. B, BM and BNSTa send non-overlapping projections to the VMH. BM projects to the core of the VMH, where projection cells are located, whereas BNSTa projects to the shell of VMH and surrounding region, where GABAergic cells are found. The core of VMH is divided in sectors (DM, dorsomedial; VL, ventrolateral). It should be noted that the shell also contains a small contingent of glutamatergic neurons. However, in contrast to the GABA cells, which are homogeneously distributed in the shell, the glutamatergic cells occur in small but dense clusters that correspond to the “cell bridges” described in Fig. 2. The properties of these glutamatergic neurons are not investigated in the present study. C, Distribution of glutamatergic cells in Vglut2-Cre-IRES-knock-in mice crossed with Ai6 reporter mice. D, Distribution of GABAergic cells in Vgat-Cre-IRES-knock-in mice crossed with Ai6 reporter mice. Thal, thalamus; V, ventricle; ZI, zona incerta.