Feedback inhibition defines transverse processing modules in the lateral amygdala.

The lateral amygdaloid (LA) nucleus is the main input station of the amygdala for sensory afferents. However, it is unclear how the lateral nucleus transforms these inputs, because its intrinsic connectivity is poorly understood. Here, we took advantage of the fact that glutamatergic neurons of the lateral nucleus send a primarily unidirectional projection to the basomedial nucleus. Consequently, it was possible to infer the targets of their intranuclear axons (projection cells vs inhibitory interneurons) by backfiring some projection neurons from the basomedial nucleus and analyzing evoked responses in other LA projection cells. Basomedial stimuli evoked markedly different synaptic responses depending on the orientation of the slices. In coronal slices (intact and decorticated), the prevalent response of LA neurons was an inhibition, regardless of the stimulation intensity. This inhibition was sensitive to GABA(A) and non-NMDA receptor antagonists, suggesting that it was mediated by the activation of GABAergic cells of the LA. In contrast, basomedial stimuli primarily evoked EPSPs in horizontal slices, regardless of the position of recorded neurons. In light of these findings, we conclude that the prevalent target of the intrinsic axon collaterals of projection cells depend on the rostrocaudal position of target neurons with respect to the parent cell body: inhibitory interneurons at rostrocaudal proximity versus other projection cells at a distance. Thus, feedback interneurons effectively divide the lateral nucleus in transverse processing modules that prevent runaway excitation within each module but allow intermixing of sensory information in the rostrocaudal plane.