The Insula Cortex Contacts Distinct Output Streams of the Central Amygdala

Abstract

The emergence of genetic tools has provided new means of mapping functionality in central amygdala (CeA) neuron populations based on their molecular profiles, response properties, and importantly, connectivity patterns. While abundant evidence indicates that neuronal signals arrive in the CeA eliciting both aversive and appetitive behaviors, our understanding of the anatomy of the underlying long-range CeA network remains fragmentary. In this study, we combine viral tracings, electrophysiological, and optogenetic approaches to establish in male mice, a wiring chart between the insula cortex (IC), a major sensory input region of the lateral and capsular part of the CeA (CeL/C), and four principal output streams of this nucleus. We found that retrogradely labeled output neurons occupy discrete and likely strategic locations in the CeL/C, and that they are disproportionally controlled by the IC. We identified a direct line of connection between the IC and the lateral hypothalamus (LH), which engages numerous LH-projecting CeL/C cells whose activity can be strongly upregulated on firing of IC neurons. In comparison, CeL/C neurons projecting to the bed nucleus of the stria terminalis (BNST) are also frequently contacted by incoming IC axons, but the strength of this connection is weak. Our results provide a link between long-range inputs and outputs of the CeA and pave the way to a better understanding of how internal, external, and experience dependent information may impinge on action selection by the CeA.SIGNIFICANCE STATEMENT Our current knowledge of the circuit organization within the central amygdala (CeA), a critical regulator of emotional states, includes independent information about its long-range efferents and afferents. We do not know how incoming sensory information is appraised and routed through the CeA to the different output channels. We address this issue by using three different techniques to investigate how a sensory region, the insula cortex (IC), connects with the motor, physiological and autonomic output centers of the CeA. We uncover a strong connection between the IC and the lateral hypothalamus (LH) with a monosynaptic relay in the CeA and shed new light on the previously described functions of IC and CeA through direct projections to the LH.

Keywords: central amygdala; input-output relationship; insula cortex; lateral hypothalamus; monosynaptic tracing; optogenetics.