Stimulating a NTS to LC circuit elicits anxiety-like behavior andstate-dependent autonomic output.

Understanding how the brain integrates internal bodily signals to shape behavior is essential for characterizing the neural basis of interoception. Here, we investigate whether the nucleus of the solitary tract (NTS), a brainstem hub for viscerosensory input, communicates with the locus coeruleus (LC), a noradrenergic nucleus involved in arousal and cognition, to influence anxiety-like behavior. We utilized viral-induced optogenetics combined with electrocardiography and diaphragmatic electromyography to monitor heart rate and respiratory frequency. Prior to behavioral testing we photoactivated transduced NTS neurons either at their somata, or at their axonal terminals in the LC. Anxiety-related behaviors were assessed using the elevated plus maze (EPM) and novelty-suppressed feeding test (NSFT). Stimulation of the NTS somata, or their terminals in the LC, increased aspects of anxiety-like behaviors in the EPM and NSFT, including freezing time during the EPM and latency to eat in the center of the NSFT. Importantly, only NTS somal activation suppressed post-NSFT feeding. This suggests that the anxiety-like behaviors observed from activating NTS projections to the LC are not secondary to activating feeding circuits. Additionally, activation of NTS terminals in the LC altered heart rate and its variability following food deprivation. These findings reveal that the NTS to LC circuit enhances anxiety-like behavior and modulates cardiorespiratory dynamics state dependently, thereby identifying a likely brainstem pathway through which viscerosensory signals influence interoception.